Apparatus including an electrostatic chuck and method for operating the apparatus

Abstract

An apparatus includes an electrostatic chuck for clamping a component and a mechanism for generating free charge adjacent to the electrostatic chuck. The mechanism for generating free charge is configured to generate free charge adjacent to the electrostatic chuck during a transition from a first excited state of the electrostatic chuck to a second excited state of the electrostatic chuck.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from EP application 18166955.7 filed on April 12, 2018, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to an apparatus including an electrostatic chuck and a method of operating the same. More particularly, but not exclusively, the apparatus may include a photolithography tool, the electrostatic chuck being configured to chuck a component such as a patterning device during photolithographic patterning. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern from a patterning device (e.g., a mask or reticle) onto a layer of radiation-sensitive material (resist) disposed on a substrate.

[0005] To project a pattern onto a substrate, a lithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Compared to lithography apparatuses that use radiation with a wavelength of, for example, 193 nm, lithography apparatuses that use extreme ultraviolet (EUV) radiation with a wavelength in the 4-20 nm range (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate.

[0006] Lithographic apparatuses often utilize high-voltage electrostatic clamps to, for example, clamp the patterning device during patterning operations. The electrostatic clamp and patterning device are typically maintained in a low-pressure, hydrogen-rich environment. This environment is non-conductive. Therefore, it is understood that charge can accumulate on dielectric or ungrounded surfaces. For example, during operation, charge may accumulate on dielectric or ungrounded surfaces through contact with components (e.g., a mask clamp) or through particle collisions during gas flow.

[0007] It should also be understood that EUV radiation may cause the hydrogen-rich environment to become conductive due to the generation of EUV-induced hydrogen plasma. Free charges generated in the EUV-induced hydrogen plasma may be attracted (or repelled) by the electric field generated by the electrostatic chuck. On the other hand, in the absence of EUV-induced plasma, or in areas that are far away from or well-shielded from any EUV-induced plasma, charges may accumulate on dielectric surfaces or ungrounded surfaces and may persist after any electric field is removed.

[0008] In addition to the accumulation of charge, very strong electrostatic fields (e.g., in the range of ~1-100 kV / cm) may be generated between the components of the electrostatic clamp and other system components. In particular, the high voltage applied to the electrodes of the electrostatic clamp causes nearby conductors (e.g., a conductive coating that may be present on the surface of the mask) to become polarized. In this way, strong electrostatic fields are generated, especially at sharp features (e.g., the edges of the conductive mask coating). The voltage applied to the electrostatic clamp electrodes may switch polarity frequently to avoid breakdown in the insulation of the electrostatic clamp. During such transitions, the electrostatic field in the area surrounding the clamp may change rapidly. Summary of the Invention

[0009] It is an object of the present invention to obviate or mitigate one or more problems associated with charge accumulation within a lithographic apparatus and / or the generation and switching of electrostatic fields within a lithographic apparatus.

[0010] According to a first aspect of the present invention, an apparatus is provided, comprising an electrostatic chuck for clamping a component and a mechanism for generating free charge adjacent to the electrostatic chuck. The mechanism for generating free charge is configured to generate free charge adjacent to the electrostatic chuck during a transition from a first excited state of the electrostatic chuck to a second excited state of the electrostatic chuck.

[0011] By providing free charge around the electrostatic chuck during repolarization (e.g., via EUV-induced H plasma), a relatively conductive medium will be provided near the chuck and any clamped patterning device. Thus, when the chuck is repolarized, the large amount of free charge will effectively shield any external fields generated by the chuck, especially any external fields generated by areas of the chuck that extend beyond the clamped patterning device (e.g., leads and contacts), thereby reducing the likelihood of particles being released from surfaces in these areas of the chuck.

[0012] It will be appreciated that, in use, when a component is clamped by an electrostatic clamp, the component will shield parts of the electrostatic clamp from the effects of the free charges that are generated. Thus, although the mechanism for generating free charges may be configured to generate free charges adjacent to the electrostatic clamp, in use, such charges will typically be prevented from reaching areas of the clamp that are directly shielded by the clamped component. That is, in use, the free charges that are generated will provide shielding of areas of the clamp that do not contribute to the clamping force. Indeed, it will be appreciated that, during clamping, the clamped component will be clamped by the magnetic field generated between the clamp and the component, and that the free charges present in the vicinity of the clamp and the clamped component will not interfere with this clamping effect. Rather, the free charges will typically not extend into the area between the clamp and the clamped component (e.g., the maximum spacing in some places is about 10 μm, while in other places they may be in direct contact).

[0013] The electrostatic clamp may include a clamping region configured to clamp the component. When the component is clamped, a clamping electric field may be generated between the clamping region and the component.

[0014] The electrostatic clamp may further include a non-clamping region. When the component is clamped by the clamping region, a secondary electric field may be generated around the non-clamping region.

[0015] For example, in the absence of a grounded conductive medium surrounding the non-clamped area, a secondary electric field may be generated between the non-clamped area and one or more of a portion of the device and / or a portion of the clamped component.

[0016] The fixture may include a first region configured to support a patterning device and a second region not configured to support the patterning device. The first region may include one or more clamping electrodes. The second region may include one or more secondary electrodes. Each secondary electrode may correspond to a respective clamping electrode. The second region may include a plurality of discrete sub-regions. For example, the second region may include protrusions extending on either side of the first region.

[0017] Each of the above electrodes (clamping electrode and secondary electrode) may be coated with a dielectric material. The dielectric material may have a thickness of about 100 μm.

[0018] The first region may include a clamping region. The second region may include a non-clamping region. Of course, it should be understood that in some embodiments, the clamped component may have dimensions substantially similar to those of the first region. However, in alternative embodiments, the clamped component may have dimensions smaller than the first region, such that, when clamped, some portions of the first region are covered by the clamped component, while other portions of the first region are not. In this arrangement, the uncovered portion of the first region may be considered to comprise the non-clamping region.

[0019] The apparatus may include a mechanism for generating free charge adjacent to the unclamped region. The mechanism for generating free charge may be configured to generate free charge adjacent to the unclamped region during a transition from the first excited state of the electrostatic chuck to the second excited state of the electrostatic chuck.

[0020] The electrostatic chuck includes at least one electrode, wherein when the component is clamped by the electrostatic chuck, a clamping voltage is applied to the at least one electrode such that a clamping electric field is generated between the clamping region and the component.

[0021] The first region may include a plurality of clamping electrodes. In the first excited state of the electrostatic clamp, a first clamping voltage may be applied to a first clamping electrode of the plurality of clamping electrodes, and a second clamping voltage may be applied to a second clamping electrode of the plurality of clamping electrodes. The first clamping voltage and the second clamping voltage may have opposite polarities.

[0022] The device may further include a voltage source. The voltage source may be configured to supply the clamping voltage. The clamping voltage may be, for example, a voltage of approximately ±1 to 10 kV. The clamping voltage may be, for example, a voltage of approximately ±2 kV.

[0023] The electrostatic chuck may further include at least one contact configured to provide an electrical connection to the at least one electrode. The mechanism for generating free charge may be configured to generate free charge adjacent to the at least one contact during the transition from the first excited state of the electrostatic chuck to the second excited state of the electrostatic chuck.

[0024] In the first excited state, a voltage having a first polarity may be applied to the at least one electrode. In the second excited state, a voltage having a second polarity opposite to the first polarity may be applied to the at least one electrode.

[0025] The voltage source may be configured to supply the voltage having the first polarity and / or the voltage having the second polarity.

[0026] The electrostatic chuck may include at least two electrodes. In a first excited state, a voltage having a first polarity may be applied to a first electrode of the electrodes, and a voltage having a second polarity may be applied to a second electrode of the electrodes. In a second excited state, a voltage having a second polarity may be applied to the first electrode of the electrodes, and a voltage having a first polarity may be applied to the second electrode of the electrodes.

[0027] The electrostatic chuck may further include at least two secondary electrodes. In a first excited state, the voltage having the first polarity may be applied to a first secondary electrode among the secondary electrodes, and the voltage having the second polarity may be applied to a second secondary electrode among the secondary electrodes. In a second excited state, the voltage having the second polarity may be applied to the first secondary electrode among the secondary electrodes, and the voltage having the first polarity may be applied to the second secondary electrode among the secondary electrodes.

[0028] A first electrode among the electrodes may be electrically connected to a first secondary electrode among the secondary electrodes. A second electrode among the electrodes may be electrically connected to a second secondary electrode among the secondary electrodes.

[0029] The clamp may be configured such that in each of the first excited state and the second excited state, the component may be clamped by the electrostatic clamp.

[0030] The mechanism for generating free charge adjacent the electrostatic chuck may include a gas source, and an ionizing radiation source configured to ionize a gas provided by the gas source.

[0031] The ionizing radiation source may include a source selected from the group consisting of: an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

[0032] There may also be provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate. The lithographic apparatus may comprise an apparatus according to the first aspect of the invention. The patterning device may comprise the above-described component to be clamped.

[0033] The lithographic apparatus may further include an illumination system configured to condition the radiation beam. The electrostatic clamp may be configured to clamp the patterning device. The patterning device may be capable of imparting a pattern to the radiation beam in its cross-section to form a patterned radiation beam. The lithographic apparatus may further include a substrate stage configured to hold a substrate. The lithographic apparatus may further include a projection system configured to project the patterned radiation beam onto the substrate.

[0034] The lithographic apparatus may be configured to perform a plurality of imaging exposures, during which a radiation beam is incident on a patterning device and a patterned radiation beam is projected onto a substrate. The electrostatic chuck may be configured to clamp the patterning device during the imaging exposures. The electrostatic chuck may be configured to transition from the first excited state to the second excited state between successive imaging exposures in the plurality of imaging exposures.

[0035] Each imaging exposure may include exposure of a plurality of dies on the wafer.The fixture may be repolarized during a time period between exposures of the first wafer and the second wafer.

[0036] A lithography system including the lithography apparatus may also be provided.

[0037] The lithography system may further comprise a radiation source configured to generate the above-mentioned radiation beam.The mechanism for generating free charges may comprise a secondary ionizing radiation source selected from the group consisting of: an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

[0038] The lithography system may further comprise a radiation source configured to generate the above-mentioned radiation beam, wherein the mechanism for generating free charges comprises the above-mentioned radiation source. The above-mentioned radiation source may be an EUV source.

[0039] The lithography system can also be configured to perform at least one non-imaging exposure, during which the radiation beam is incident on the pattern forming device and during which no radiation is projected onto the substrate; the above-mentioned non-imaging exposure is performed between consecutive imaging exposures in the above-mentioned multiple imaging exposures.

[0040] During a series of imaging exposures, the lithographic apparatus may further be configured to perform a non-imaging exposure between each successive imaging exposure.

[0041] The transition from the first excited state of the electrostatic chuck to the second excited state of the electrostatic chuck may be performed during the non-imagewise exposure.

[0042] By providing a radiation beam at the patterning device during non-imaging exposure, a source of free charge can be provided by means of a plasma, which will be generated by the ionization of gas molecules (e.g., hydrogen) present around the patterning device. Plasma will be generated in the area directly illuminated by the radiation beam as well as in adjacent areas (e.g., due to diffusion and secondary electrons). In this way, a single radiation beam (e.g., an EUV radiation beam) can be used both for imaging purposes and to provide a source of free charge during repolarization of the electrostatic chuck.

[0043] The lithographic system may be controlled such that the amount of radiation incident on the patterning device is greater during each imaging exposure than during the aforementioned non-imaging exposures.

[0044] It will be appreciated that the amount of radiation incident on the patterning device can be controlled in a variety of ways. For example, the amount of radiation can be controlled by varying the intensity of the radiation generated by the radiation source. Alternatively or additionally, the amount of radiation can be controlled by varying the spatial extent of the radiation beam incident on the patterning device (e.g., by using a shield or shielding blade). Alternatively or additionally, the amount of radiation can be controlled by varying the number or frequency of radiation pulses generated by the radiation source.

[0045] For example, the radiation dose required to provide sufficient free charge to provide useful shielding (over a predetermined period of time) may be less than the radiation dose required to perform an imaging exposure.

[0046] The amount of radiation incident on the patterning device may be gradually increased from a non-imagewise exposure to an imagewise exposure of the plurality of imagewise exposures.

[0047] In the above described embodiments, the shield or shield blade or shield blade arrangement may thus be used to control the amount of free charge generated around the electrostatic chuck, for example by partially blocking the radiation beam.

[0048] It is worth mentioning that the portion of the radiation beam incident on the shield or shield blade arrangement may also result in the generation of free charge. Thus, an alternative way to generate free charge adjacent to the electrostatic clamp is to provide the radiation beam at a surface different from the pattern forming device (e.g., the surface of the shield or shield blade). As described above, the shield or shield blade arrangement can be used to block the radiation beam from reaching the pattern forming device, or to control the spatial extent of the radiation beam incident on the pattern forming device. Applying the radiation beam or a portion thereof to the above-mentioned shield or shield blade may also result in the generation of free charge due to the interaction of the radiation beam with gas molecules (e.g., hydrogen) present or provided around, at or near the shield or shield blade arrangement. Free charge will be generated both in the area directly irradiated by the radiation beam and in adjacent areas (e.g., due to diffusion and secondary electrons). Since the shield or shield blade arrangement is typically relatively close to the pattern forming device and therefore relatively close to the electrostatic clamp, the free charge generated by irradiating the shield or shield blade arrangement may also generate free charge adjacent to the electrostatic clamp. Therefore, the generation of free charge by illuminating the shield or shield blade arrangement may also be considered as a mechanism for generating free charge adjacent to the electrostatic chuck.

[0049] Regarding this way of generating free charges adjacent to the electrostatic chuck, it may be pointed out that if the shield or shield blade arrangement is completely closed during the generation of the free charges, the risk of any unwanted radiation reaching the substrate may be avoided.

[0050] According to a second aspect of the present invention, a method of operating an apparatus comprising an electrostatic chuck and a mechanism for generating free charge adjacent to the electrostatic chuck is provided. The method comprises: controlling the electrostatic chuck to have a first excited state; controlling the electrostatic chuck to have a second excited state; and, during a transition from the first excited state to the second excited state, controlling the mechanism for generating free charge to generate free charge adjacent to the electrostatic chuck.

[0051] The method may further include providing the component adjacent to the electrostatic chuck. The component may be clamped by the electrostatic chuck when the electrostatic chuck is in one or both of the first excited state and / or the second excited state.

[0052] Controlling the mechanism for generating free charge to generate free charge adjacent the electrostatic chuck and / or the component may include providing a gas adjacent the electrostatic chuck and / or the component; and controlling an ionizing radiation source to provide ionizing radiation adjacent the electrostatic chuck and / or the component such that the gas is ionized.

[0053] Of course, it will be appreciated that any of the features described above in conjunction with the apparatus of the first aspect of the invention may be combined with features of the method of the second aspect of the invention.

[0054] According to a third aspect of the present invention, there is provided a lithographic apparatus comprising: an irradiation system configured to adjust a radiation beam; a support structure configured to support a pattern forming device, the pattern forming device being capable of imparting a pattern to the radiation beam in a cross section of the radiation beam to form a patterned radiation beam, the support structure comprising an electrostatic clamp configured to clamp the pattern forming device; a substrate stage configured to hold a substrate; and a projection system configured to project the patterned radiation beam onto the substrate. The lithographic apparatus is configured to perform pre-imaging exposure, during which the radiation beam is incident on the pattern forming device, and during which no radiation is projected onto the substrate. During the pre-imaging exposure, the amount of radiation incident on the pattern forming device gradually increases. The lithographic apparatus is configured to perform imaging exposure, in which the radiation beam patterned by the pattern forming device is projected onto the substrate.

[0055] The gradual or soft ramp-up of EUV power causes the conductivity of the area surrounding the patterning device and electrostatic clamp to gradually increase. This gradual increase in dielectric conductivity does not cause a sudden collapse of the electrostatic field, but instead allows charge to leak to various surfaces according to pre-existing field lines. Such a process allows for the neutralization of any surfaces that have become charged due to the previous clamp polarization state. Similarly, any charged particles on the surface of the clamp or patterning device can be neutralized. This gradual increase in EUV power can significantly reduce the variation in the occurrence of electrostatic discharge and therefore can reduce the generation rate of particles (typically generated by discharge events).

[0056] The imaging exposure may comprise a burst comprising a plurality of radiation pulses. Each of the pulses may comprise a substantially constant radiation dose. It will be appreciated that during the imaging exposure, the radiation beam may be pulsed such that the instantaneous radiation intensity is not uniform at all times. However, during the imaging exposure, the pulse rate may be sufficiently high such that the radiation incident on the patterning device and the patterned substrate is substantially uniform within a reference frame comprising tens, hundreds, or thousands of pulses. Furthermore, the pulse rate may be sufficiently high such that any plasma generated by the EUV radiation will likely persist for a time longer than the interval between adjacent pulses, such that once established, an equilibrium plasma density is achieved and maintained by the ongoing pulses.

[0057] The pre-imagewise exposure may immediately precede the imagewise exposure.

[0058] The pre-imaging exposure may comprise a burst comprising a plurality of radiation pulses.

[0059] Each pulse of the imaging exposure and / or pre-imaging exposure may, for example, have a duration of about 100 ns and a pulse spacing of about 20 μs (ie, a pulse frequency of about 50 kHz).

[0060] The pulse rate during the pre-imaging exposure can be high enough so that the hydrogen plasma generated (directly or indirectly) by the incident EUV photons in each pulse of the pre-imaging exposure persists longer than the gap between adjacent pulses. The burst can include multiple mini-bursts, each mini-burst including multiple (e.g., up to 10) radiation pulses.

[0061] The gradual increase in the amount of radiation during said pre-imaging exposure may be configured to be provided over a plurality of said radiation pulses.

[0062] In this way, as the amount of radiation incident on the patterning device is gradually increased during pre-image exposure, the plasma density can be gradually increased, thereby resulting in a desired gradual increase in the conductivity of the area surrounding the patterning device. This gradual increase can be configured to be provided over a plurality of the aforementioned microbursts.

[0063] The gradual increase in radiation during the above-mentioned pre-imaging exposure may be configured to be provided over at least 1000 radiation pulses.

[0064] In a system with a pulse frequency of about 50 kHz, 1000 radiation pulses will be delivered in about 20 ms. The gradual increase in radiation during the pre-imaging exposure can be configured to be provided over up to about 50,000 radiation pulses (i.e., up to about 1 second). Preferably, the gradual increase in radiation during the pre-imaging exposure can be configured to be provided over up to about 10,000 radiation pulses (i.e., up to about 0.2 seconds).

[0065] It will be appreciated that there is a tradeoff between the time required to increase the radiation intensity to the full intensity required for imaging (which, if too long, can reduce wafer throughput) and the benefits of reducing the likelihood of electrical discharges and / or particle generation or release.

[0066] The gradual increase in radiation during said pre-imaging exposure may be configured to be provided substantially linearly over a plurality of said radiation pulses.

[0067] For each of the plurality of consecutive radiation pulses, the radiation intensity may be increased by a predetermined amount such that the intensity of radiation incident on the patterning device gradually increases in a substantially linear manner.

[0068] During a first portion of the pre-imaging exposure having a predetermined duration, the radiation beam can be controlled to deliver a first dose of radiation to the pattern forming device, wherein the first dose includes a dose that is less than about 10% of the imaging radiation dose delivered to the pattern forming device during the first portion of the imaging exposure, wherein the first portion of the pre-imaging exposure has the predetermined duration.

[0069] It will be appreciated that the actual duration of the first portion of the pre-imaging exposure and the first portion of the imaging exposure is not important. Rather, the apparatus is controlled such that the radiation dose delivered during the first portion of the pre-imaging exposure within a predetermined time period (e.g., 200 μs) is less than the radiation dose delivered during the corresponding predetermined time period (e.g., 200 μs).

[0070] The first dose may comprise a non-zero dose.The first dose may comprise approximately 5% of the imaging radiation dose delivered to the patterning device during the first portion of the imaging exposure.

[0071] The first portion of the pre-imaging exposure may include a plurality of pulses. Thus, the first dose may include the total dose delivered by the plurality of pulses in the first portion of the pre-imaging exposure. The first portion of the imaging exposure may also include a plurality of pulses. Thus, the imaging radiation dose delivered to the patterning device during the first portion of the imaging exposure may include the total dose delivered by the plurality of pulses in the first portion of the imaging exposure.

[0072] Prior to the first portion of the pre-imagewise exposure, the radiation intensity delivered to the patterning device by the radiation beam may be substantially zero. Thus, the first portion of the pre-imagewise exposure may constitute a jump from about 0% of the imaging dose to about 5% of the imaging dose.

[0073] The amount of radiation incident on the patterning device may be gradually increased from said first portion of pre-imagewise exposure to said imagewise exposure.

[0074] During a second portion of the pre-imaging exposure having the above-mentioned predetermined duration, wherein the second portion follows the above-mentioned first portion, the radiation beam can be controlled to deliver a second dose of radiation to the pattern forming device, wherein the above-mentioned second dose is greater than the above-mentioned first dose and less than the above-mentioned imaging radiation dose delivered to the pattern forming device during the above-mentioned first portion of the imaging exposure.

[0075] At least 1000 radiation pulses may be delivered between the start of said first portion of the pre-imagewise exposure and the start of said imaging exposure.

[0076] In this manner, the radiation dose may be gradually increased from a first portion of the pre-imaging exposure to the imaging exposure over a period of time spanning at least 1000 radiation pulses.

[0077] The lithographic apparatus can also be configured to perform a post-imaging exposure, during which the radiation beam is incident on the pattern forming device; and during which no radiation is projected onto the substrate, wherein the amount of radiation incident on the pattern forming device gradually decreases during the post-imaging exposure, and wherein the above-mentioned post-imaging exposure is after the imaging exposure.

[0078] The lithographic apparatus may be further configured to perform a first imaging exposure and a second imaging exposure. The pre-imaging exposure may immediately precede the second imaging exposure. Between the first imaging exposure and the pre-imaging exposure, the lithographic apparatus may be further configured to perform a non-imaging exposure, during which the radiation beam is incident on the patterning device and during which no radiation is projected onto the substrate.

[0079] During a first portion of the non-imagewise exposure having the predetermined duration, the radiation beam may be controlled to deliver a third dose of radiation to the patterning device, the third dose comprising approximately 10% of the imaging radiation dose. The device may be configured to cause the electrostatic chuck to transition from a first excited state to a second excited state during the non-imagewise exposure.

[0080] That is, the fixture may be repolarized during the non-imaging exposures between multiple imaging exposures.

[0081] The lithographic apparatus may be further configured to cause the electrostatic chuck to transition from the first excited state to the second excited state during the above-mentioned pre-imaging exposure.

[0082] That is, the fixture can be repolarized during the EUV power ramp-up period prior to the imaging exposure. This allows a low level of radiation (and therefore a relatively low plasma density) to be provided during fixture repolarization, while providing full radiation intensity only during the imaging exposure. The lower level of radiation can be sufficient to mitigate certain negative consequences associated with fixture repolarization (e.g., particle release) and can reduce the burden on the radiation source. More generally, a low level of radiation can also be advantageously applied during any voltage change applied to the fixture. As an example, a low level of radiation can be advantageously applied when an object is being loaded onto or unloaded from the fixture.

[0083] During a series of imaging exposures, the lithographic apparatus may further be configured to perform a non-imaging exposure between each successive imaging exposure.

[0084] It will be appreciated that the amount of radiation incident on the patterning device can be controlled in a variety of ways. For example, the amount of radiation can be controlled by varying the intensity of the radiation generated by the radiation source. Alternatively or additionally, the amount of radiation can be controlled by varying the spatial extent of the radiation beam incident on the patterning device (e.g., by using a shield or shielding blade). Alternatively or additionally, the amount of radiation can be controlled by varying the number or frequency of the pulses of radiation generated by the radiation source.

[0085] For example, the radiation dose required to provide sufficient free charge to provide useful shielding may be less than the radiation dose required to perform an imaging exposure.

[0086] A lithography system may also be provided, comprising the lithography apparatus according to the third aspect of the present invention. The lithography system may further comprise a radiation source configured to generate the radiation beam. The radiation source may be an EUV source.

[0087] According to a fourth aspect of the present invention, a method for operating a lithographic apparatus is provided. The lithographic apparatus comprises: an irradiation system configured to adjust a radiation beam; a support structure configured to support a pattern forming device, the pattern forming device being capable of imparting a pattern to the radiation beam in a cross section of the radiation beam to form a patterned radiation beam, the support structure comprising an electrostatic clamp configured to clamp the pattern forming device; a substrate table configured to hold a substrate; and a projection system configured to project the patterned radiation beam onto the substrate. The method comprises: causing the lithographic apparatus to perform pre-imaging exposure, during which the radiation beam is incident on the pattern forming device, and during which no radiation is projected onto the substrate, wherein during the pre-imaging exposure, the amount of radiation incident on the pattern forming device gradually increases. The method further comprises: causing the lithographic apparatus to perform imaging exposure, in which the radiation beam patterned by the pattern forming device is projected onto the substrate.

[0088] Of course, it will be appreciated that any of the features described above in conjunction with the apparatus of the third aspect of the invention may be combined with features of the method of the fourth aspect of the invention.

[0089] According to a fifth aspect of the present invention, there is provided an apparatus comprising an electrostatic clamp for clamping a component and a mechanism for generating free charge adjacent to the electrostatic clamp, the apparatus having a first configuration in which the component is clamped by the electrostatic clamp and a second configuration in which the component is spaced apart from the electrostatic clamp. The apparatus is configured to be in the first configuration at a first time point and in the second configuration at a second time point after the first time point. The mechanism for generating free charge is configured to generate free charge adjacent to the electrostatic clamp and / or the component at a third time point between the first time point and the second time point.

[0090] During processing operations, such as during removal of a patterning device in a lithographic apparatus, the free charge can be used within the apparatus to prevent negative effects associated with voltage amplification due to capacitance changes associated with increased separation between various electrically isolated system components. In particular, when a clamped component is removed from a fixture, the capacitance between them decreases (inversely proportional to the separation). The generated free charge can be used to transfer charge to reduce the effects of voltage amplification, thereby reducing the likelihood of discharge (e.g., via hydrogen breakdown if the Paschen limit is exceeded).

[0091] The mechanism for generating free charges may be configured to generate free charges adjacent to the electrostatic chuck and / or the component to prevent a potential difference between the electrostatic chuck and the component from exceeding a predetermined threshold.

[0092] It will be appreciated that both the timing and extent of free charge generation are somewhat flexible. Specifically, if a change in capacitance (associated with increasing distance between the electrostatic chuck and the component) causes the potential difference between the electrostatic chuck and the component to exceed a threshold (e.g., the hydrogen breakdown voltage according to Paschen's law), then discharge may occur. However, it will also be appreciated that the minimum discharge voltage will be a function of both distance and gas pressure. Thus, the voltage threshold will vary from one configuration to another.

[0093] The predetermined threshold is determined based on the pressure in the device. The pressure may be the pressure of hydrogen.

[0094] The third time point may be selected to prevent the potential difference between the electrostatic chuck and the component from exceeding the predetermined threshold.

[0095] Free charges may be generated at a selected time (ie, a third time point) to provide charges to reduce (or limit) the potential difference between the electrostatic chuck and the component before the potential difference exceeds a threshold value.

[0096] The predetermined threshold may be approximately 250V or lower, for example, approximately 130V.

[0097] The apparatus may be configured to be in the aforementioned second configuration at the aforementioned third point in time.

[0098] In particular, free charges can be generated shortly after the clamping voltage is removed and the component begins to separate from the clamp. In this way, the generated free charges will easily reach the surface of the clamp and the clamped component.

[0099] At the third point in time, a minimum separation between the surface of the fixture and the surface of the component may be greater than about 10 microns.

[0100] The surface of the clamp may be the surface of the clamp that contacts the component when the component is clamped. It will be appreciated that the clamp may include a generally flat surface that is provided with protrusions (which may be referred to as burls). The protrusions may ensure that even during clamping, the spacing between the generally flat surface of the clamp and the clamped surface of the component exceeds a minimum value (e.g., 10 microns). However, during clamping, it will be appreciated that the surface of the protrusion will contact the clamped component, and therefore, the minimum spacing between the surface of the clamp and the surface of the component during clamping is zero.

[0101] At the third point in time, a minimum spacing between the surface of the clamp and the surface of the component may be greater than or equal to about 100 microns.

[0102] At the third time point, the minimum distance between the surface of the clamp and the surface of the component may be smaller than a predetermined distance, which may be about 200 micrometers.

[0103] The mechanism for generating free charges may be configured to generate free charges adjacent the electrostatic chuck and / or the component when the apparatus is configured in the first configuration described above.

[0104] The apparatus may be configured to be in the aforementioned first configuration at the aforementioned third point in time.

[0105] It will be appreciated that, in use, when a component is clamped by an electrostatic clamp, the component will shield portions of the electrostatic clamp from the effects of the free charges that are generated. Thus, while the mechanism for generating free charge may be configured to generate free charge adjacent to the electrostatic clamp, in use, such charge will generally be prevented from reaching areas of the clamp that are directly shielded by the clamped component. However, as the component is separated from the clamp (i.e., after the clamping voltage is removed), it will be appreciated that the free charges may diffuse to the surfaces of the clamp and the clamped component, thereby compensating for the voltage amplification effect. Thus, when the component is clamped, the free charges generated adjacent to the clamping device and / or the component may effectively reduce the voltage amplification.

[0106] The electrostatic clamp may include a clamping region configured to clamp the component. When the component is clamped, a clamping electric field may be generated between the clamping region and the component.

[0107] The electrostatic chuck may include at least one electrode. When the component is clamped by the electrostatic chuck, a clamping voltage may be applied to the at least one electrode such that a clamping electric field is generated between the clamping region and the component.

[0108] The apparatus may further comprise a voltage source.

[0109] The mechanism for generating free charge adjacent the electrostatic chuck may include a gas source, and an ionizing radiation source configured to ionize a gas provided by the gas source.

[0110] The ionizing radiation source may include a source selected from the group consisting of: an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

[0111] The apparatus may further include a component replacement assembly configured to remove the component from the electrostatic chuck.

[0112] The component replacement assembly may be configured to control the spacing between the component and the electrostatic chuck.

[0113] There may also be provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate. The lithographic apparatus may comprise an apparatus according to the fifth aspect of the invention. The patterning device may comprise the above-described component to be clamped.

[0114] The lithographic apparatus may further include an illumination system configured to condition the radiation beam. The electrostatic clamp may be configured to clamp the patterning device. The patterning device may be capable of imparting a pattern to the radiation beam in its cross-section to form a patterned radiation beam. The lithographic apparatus may further include a substrate stage configured to hold a substrate and a projection system configured to project the patterned radiation beam onto the substrate.

[0115] A lithographic system comprising the lithographic apparatus may also be provided. The lithographic system may further comprise a radiation source configured to generate the radiation beam.

[0116] The means for generating free charges may include a secondary ionizing radiation source selected from the group consisting of: an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

[0117] The mechanism for generating free charges may include the above-mentioned radiation source. The above-mentioned radiation source may be an EUV source.

[0118] According to a sixth aspect of the present invention, a method for operating an apparatus is provided. The apparatus comprises an electrostatic clamp and a mechanism for generating free charge adjacent to the electrostatic clamp. The method comprises: providing a component adjacent to the electrostatic clamp; controlling the electrostatic clamp to have a first configuration, in which the component is clamped by the electrostatic clamp at a first time point; controlling the electrostatic clamp to have a second configuration, in which the component is spaced apart from the electrostatic clamp at a second time point after the first time point; and controlling the mechanism for generating free charge to generate free charge adjacent to the electrostatic clamp and / or the component at a third time point between the first time point and the second time point.

[0119] Of course, it will be appreciated that any of the features described above in conjunction with the apparatus of the fifth aspect of the invention may be combined with features of the method of the sixth aspect of the invention.

[0120] According to a seventh aspect of the present invention, there is provided a device comprising an electrostatic clamp for clamping a component and a mechanism for generating free charges adjacent to the electrostatic clamp. The device has a first configuration and a second configuration, wherein a voltage having a first polarity is applied to at least one clamp electrode and no component is clamped by the electrostatic clamp, and wherein a voltage having a second polarity opposite to the first polarity is applied to at least one clamp electrode. The device is configured to be in the first configuration at a first time point and to be in the second configuration at a second time point after the first time point. The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the first configuration and not to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the second configuration.

[0121] By providing free charge adjacent to an electrostatic clamp while the clamp is polarized in a first excited state, any particles trapped on the clamp surface can be charged. Then, when the polarization is reversed and free charge is no longer present, the charged particles can be released from the clamp surface by electrostatic repulsion. This cleaning process of the electrostatic clamp can avoid or at least reduce the negative consequences associated with particles being trapped on the clamp surface.

[0122] The device may be a device for cleaning an electrostatic chuck.

[0123] During the transition from the first configuration to the second configuration, a component may be provided adjacent to the electrostatic chuck.

[0124] By providing a component adjacent to the electrostatic clamp during the transition described above, particles released from the clamp due to the change in polarization can be captured by the component without contaminating other surfaces of the device. The component can be referred to as a sacrificial component or a clean component. The component can include a patterning device. The dimensions of the component can be substantially equal to the dimensions of the component intended to be clamped by the electrostatic clamp during the lithographic operation.

[0125] When the clamp is in the second configuration, the component may be clamped by the electrostatic clamp. That is, the electrostatic clamp may be configured to clamp the component when the clamp is in the second configuration.

[0126] The apparatus may have a third configuration in which no voltage is applied to the at least one chuck electrode and no component is clamped by the electrostatic chuck. The apparatus may be configured to be in the third configuration at a third time point after the second time point.

[0127] Once the particles are released (during the second configuration) by applying reverse polarization, the clamp can be returned to the neutral configuration (ie, no voltage applied to the clamp electrodes).

[0128] The mechanism for generating free charge may be configured to generate the free charge adjacent the electrostatic chuck when the electrostatic chuck is in the third configuration.

[0129] Once the particles are released by applying reverse polarization (during the second configuration), the clamp can be returned to the neutral configuration (i.e., no voltage applied to the clamp electrodes) and any residual charge on the clamp surface is removed by providing free charge to the adjacent clamp.

[0130] In the third configuration, no component may be provided adjacent to the electrostatic chuck. That is, the apparatus may be controlled such that in the third configuration no component is provided adjacent to the electrostatic chuck.

[0131] The apparatus may further include a component replacement assembly adjacent the electrostatic clamp configured to support the component.

[0132] The component replacement assembly may be configured to control the spacing between the component and the electrostatic chuck.

[0133] The component replacement assembly may be configured to provide the component adjacent to the electrostatic chuck between the first time point and the second time point.

[0134] The component replacement assembly may be configured to remove the component from the vicinity of the electrostatic chuck between the second time point and the third time point.

[0135] The electrostatic clamp may include a clamping region configured to clamp the component. When the component is clamped, a clamping electric field may be generated between the clamping region and the component.

[0136] When the clamping voltage is applied to the at least one electrode, the clamping electric field may be generated. The apparatus may further include a voltage source.

[0137] The mechanism for generating free charge adjacent the electrostatic chuck may include a gas source, and an ionizing radiation source configured to ionize a gas provided by the gas source.

[0138] The ionizing radiation source includes a source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

[0139] A lithographic apparatus may also be provided, the apparatus being arranged to project a pattern from a patterning device onto a substrate. The lithographic apparatus may comprise an apparatus according to the seventh aspect of the invention. The electrostatic clamp may be configured to clamp the patterning device during a lithographic operation.

[0140] The lithographic apparatus may further include an illumination system configured to condition the radiation beam. The electrostatic clamp may be configured to clamp the patterning device, which is capable of imparting a pattern to the radiation beam in its cross-section to form a patterned radiation beam. The lithographic apparatus may further include a substrate stage configured to hold a substrate, and a projection system configured to project the patterned radiation beam onto the substrate.

[0141] A lithographic system comprising the lithographic apparatus may also be provided. The lithographic system may further comprise a radiation source configured to generate the radiation beam.

[0142] The means for generating free charges may include a secondary ionizing radiation source selected from the group consisting of: an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

[0143] The mechanism for generating free charges may include the above-mentioned radiation source. The above-mentioned radiation source may be an EUV source.

[0144] According to an eighth aspect of the present invention, there is provided a method for operating an apparatus comprising: an electrostatic chuck comprising at least one chuck electrode, and a mechanism for generating free charge adjacent to the electrostatic chuck. The method comprises: controlling the electrostatic chuck to have a first configuration in which, at a first time point, a voltage having a first polarity is applied to the at least one chuck electrode and no component is clamped by the electrostatic chuck; controlling the electrostatic chuck to have a second configuration in which, at a second time point after the first time point, a voltage having a second polarity opposite to the first polarity is applied to the at least one chuck electrode; controlling the mechanism for generating free charge to generate free charge adjacent to the electrostatic chuck when the chuck is in the first configuration; and controlling the mechanism for generating free charge to not generate free charge adjacent to the electrostatic chuck when the electrostatic chuck is in the second configuration.

[0145] The method may further include providing a component adjacent to the electrostatic chuck while the electrostatic chuck is in the second configuration.

[0146] When the clamp is in the second configuration, the component may be clamped by the electrostatic clamp.

[0147] The method may further include controlling the apparatus to have a third configuration at a third time point after the second time point, in which third configuration no voltage is applied to the at least one clamp electrode.

[0148] The method may further include: removing the component from the vicinity of the electrostatic chuck between the second time point and the third time point.

[0149] The method may further include controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic chuck when the electrostatic chuck is in the third configuration.

[0150] Of course, it will be appreciated that any of the features described above in conjunction with the apparatus of the seventh aspect of the invention may be combined with features of the method of the eighth aspect of the invention.

[0151] According to a ninth aspect of the present invention, there is provided a method of operating an apparatus comprising an electrostatic clamp having a first electrode, the method comprising: a) providing a component adjacent to the electrostatic clamp; b) controlling the electrostatic clamp to provide a first clamping voltage to the first electrode so that the component is clamped by the electrostatic clamp; c) measuring a voltage associated with a portion of the component; and d) determining an adjustment to the first clamping voltage based on the measured voltage.

[0152] The electrostatic clamp includes a first electrode to which a first clamping voltage is applied. The first clamping voltage generates an electric field between the clamp and the component, enabling the component to be clamped at its back side to a clamping region of the clamp. The component also includes a front side disposed opposite the back side. The component can accumulate charge due to electrostatic attraction of particles (i.e., electrons or ions), particularly at the back side adjacent to the clamp, and thus be at a different voltage than the rest of the system. By measuring the voltage associated with a portion of the component (e.g., the back side of the component), an adjustment to the first clamping voltage can be determined such that the voltage associated with that portion of the component becomes zero. It will be appreciated that the adjustment can be determined as an additional voltage to be added to or subtracted from the first clamping voltage, as a percentage change in the first clamping voltage, or any other suitable term apparent to a skilled artisan. By determining and applying the adjustment to the first clamping voltage, a virtual grounding of the component relative to the surrounding system can be achieved. As a result, electrostatic attraction of charge (and charged particles) to the component can be reduced or eliminated. The component can be a patterning device (e.g., a reticle) clamped by the electrostatic clamp. However, it should be understood that the component may be another component to be clamped by the clamp.

[0153] The method may further include: b1) exposing the component to radiation while the component is clamped by the electrostatic chuck; b2) controlling the electrostatic chuck so that the component is released from the electrostatic chuck; and b3) removing the component from the vicinity of the electrostatic chuck.

[0154] In this way, voltage measurements can be performed in an offline process. In this implementation, the component is clamped by an electrostatic clamp and exposed to radiation. During exposure, charge can be deposited on the back side of the component (i.e., the side of the component clamped to the clamp). When the clamping voltage is removed to release the component, the accumulated charge remains on the surface of the component. The component can be removed from the vicinity of the clamp so that the associated voltage can be subsequently measured. Any adjustment to the first clamping voltage can then be determined based on the measured voltage.

[0155] The method may further include: e) adjusting the first clamping voltage according to the determined adjustment; and f) repeating steps a) to c) to verify the adjustment.

[0156] Once the adjustment to the first clamping voltage is confirmed, steps a) through c) can be repeated to verify that the adjustment successfully reduced the voltage associated with the component to zero volts. If a voltage associated with that portion of the component is still found, further adjustment and verification steps can be performed. As a result, since no net charge will remain on the virtually grounded component after exposure, problems associated with voltage amplification during unloading of a previously clamped component can be alleviated.

[0157] The method may further include, after repeating steps a) through c), determining a further adjustment to the first clamping voltage based on the measured voltage. The measurement and adjustment process may be repeated multiple times. The measurement and adjustment process may be repeated until the voltage associated with the portion of the assembly meets a predetermined criterion (e.g., is within a predetermined tolerance of zero volts).

[0158] The repetition of steps a) to c) can be performed using another component. In particular, in some implementations, the verification step can be performed using the same component as the component used for the initial measurement. However, in other implementations, it may be necessary to use another component that was not used for the initial measurement.

[0159] The method may further include, after determining the adjustment to the first clamping voltage, adjusting the first clamping voltage according to the determined adjustment; and measuring a voltage associated with the portion of the assembly.

[0160] As an alternative to an offline measurement process, the measurement can be performed online, i.e., while the component is clamped. In this implementation of the method, the adjustment is determined immediately after the measurement step, without releasing and removing the component from the fixture. It may be necessary to perform the adjustment and subsequent measurement steps while the component remains clamped in place. In this way, processing time can be reduced because the component does not need to be removed from the fixture to measure the associated voltage. In addition, the method can be repeated periodically. Alternatively or additionally, the method can be implemented as a continuous feedback loop in which the voltage associated with the component is monitored and the clamping voltage is automatically adjusted accordingly.

[0161] The electrostatic chuck may further include a second electrode, and the method may further include determining an adjustment to a second clamping voltage to be provided to the second electrode based on the measured voltage.

[0162] The first clamping voltage and the second clamping voltage may have different values. In particular, the first clamping voltage and the second clamping voltage may have different polarities. For example, the first clamping voltage may be approximately +1 kV to 10 kV, and the second clamping voltage may be approximately -1 kV to 10 kV. In particular, the first clamping voltage may be approximately +2 kV and the second clamping voltage may be approximately -2 kV. Of course, it will be understood that the absolute values ​​of the first clamping voltage and the second clamping voltage may also differ. Furthermore, it will be understood that the determined adjustment of the first clamping voltage and / or the second clamping voltage may be an adjustment of the first clamping voltage, or an adjustment of the second clamping voltage, or an adjustment of both the first clamping voltage and the second clamping voltage. In addition, the determined adjustment may be, for example, an adjustment of the difference between the first clamping voltage and the second clamping voltage, or an adjustment of the average value of the first clamping voltage and the second clamping voltage.

[0163] According to a tenth aspect of the present invention, a system for virtually grounding a component is provided, the system comprising: an apparatus comprising an electrostatic clamp configured to clamp the component, the electrostatic clamp comprising a first electrode configured to receive a first clamping voltage; a voltage monitor configured to measure a voltage associated with a portion of the component; and a computing unit configured to determine an adjustment to the first clamping voltage based on the measured voltage.

[0164] The system may further include a support assembly configured to support the component, wherein the support assembly includes a voltage monitor.The support assembly may be, for example, a replacement assembly configured to transport the component toward or away from the vicinity of the fixture.

[0165] The electrostatic chuck may include a second electrode configured to receive a second clamping voltage, and the computing unit may be further configured to determine an adjustment to the second clamping voltage based on the measured voltage.

[0166] The first clamping voltage and the second clamping voltage may have different values.

[0167] The voltage monitor may be an electrostatic voltmeter.

[0168] Of course, it will be appreciated that any of the features described above in conjunction with the method of the ninth aspect of the invention may be combined with features of the system of the tenth aspect of the invention.

[0169] Furthermore, it will be appreciated that any feature described above in conjunction with any of the first to tenth aspects of the invention may be combined with features described in the context of a different one of the aforementioned aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[0170] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0171] - Figure 1 A lithographic system including a lithographic apparatus and a radiation source is depicted;

[0172] - Figures 2a-2c The cross-sectional view, plan view and cross-sectional view are respectively shown in FIG. Figure 1 An electrostatic chuck used in a photolithography apparatus is shown;

[0173] - Figure 3 depicts a simulation of plasma density around the electrostatic chuck of FIG2 during exposure to EUV radiation;

[0174] - Figure 4a and 4b The exposure sequence and the fixture polarization sequence of the prior art lithography apparatus are respectively shown;

[0175] - Figure 5a and 5b The exposure sequence and the clamp polarization sequence of the lithography apparatus according to the embodiment of the present invention are respectively shown;

[0176] Figure 6a and 6b respectively showing an alternative exposure sequence and a clamp polarization sequence for a lithographic apparatus according to an embodiment of the present invention;

[0177] Figure 7a and 7b Another alternative exposure sequence and a clamp polarization sequence of a lithographic apparatus according to an embodiment of the present invention are respectively shown;

[0178] Figures 8a-8c An electrostatic chuck used in a lithographic apparatus according to an alternative embodiment of the present invention is shown in cross-sectional view, plan view and cross-sectional view respectively;

[0179] Figure 9a and 9b depicts an electrostatic chuck and patterning device used within a lithographic apparatus according to an embodiment of the present invention during a patterning device unloading process;

[0180] Figure 10 depicts a movement sequence of an electrostatic chuck and a patterning device used within a lithographic apparatus according to an embodiment of the present invention during a patterning device unloading process;

[0181] Figure 11 An equivalent circuit model of an electrostatic chuck and a patterning device used in a lithography apparatus according to an embodiment of the present invention is shown;

[0182] Figures 12a-12c Depicted in Figure 10 Simulation characteristics of the equivalent circuit of FIG9 during the movement sequence;

[0183] Figure 13 An equivalent circuit model of an electrostatic chuck and a patterning device used in a lithography apparatus according to an embodiment of the present invention is shown;

[0184] Figure 14 An electrostatic chuck and a patterning device used in a lithography apparatus according to an embodiment of the present invention are shown; Figures 15a-15e Depicting a cleaning process for an electrostatic chuck used in a lithographic apparatus according to an embodiment of the present invention;

[0185] Figure 16 depicts a flow chart of a method for providing a virtual ground for a patterning device according to one embodiment;

[0186] Figure 17 A flow chart illustrating an alternative method for providing a virtual ground for a patterning device according to another embodiment; and

[0187] Figure 18 An electrostatic chuck and patterning device having a support assembly including a voltage monitor according to embodiments of the present invention are depicted. DETAILED DESCRIPTION

[0188] Figure 1 A lithographic system is shown comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and provide the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0189] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Furthermore, the illumination system IL may comprise a faceted field mirror arrangement 10 and a faceted pupil mirror arrangement 11. Together, the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 provide a desired cross-sectional shape and a desired intensity distribution for the EUV radiation beam B. The illumination system IL may comprise other mirrors or arrangements in addition to or instead of the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11.

[0190] After being so conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To this end, the projection system PS may comprise a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B' so as to form an image having features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although in Figure 1 The projection system PS is shown with only two mirrors 13, 14, but the projection system PS may comprise a different number of mirrors (eg 6 or 8 mirrors).

[0191] The substrate W may include a previously formed pattern. In this case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B′ with the pattern previously formed on the substrate W.

[0192] A relative vacuum, ie, a small amount of gas (eg, hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL and / or in the projection system PS.

[0193] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL), or any other radiation source capable of generating EUV radiation.

[0194] Figure 2a A cross-section of the support structure MT is shown in greater detail. The cross-section is in the x-plane and extends vertically in the z-direction and horizontally in the y-direction in the direction shown. The support structure MT includes an electrostatic clamp 100 configured to clamp a patterning device MA during a photolithography operation. The clamp 100 includes a clamp body 102 and clamp electrodes 104A-104D disposed within the clamp body 102. The electrodes 104A-104D are separated from the generally flat clamp surface of the clamp 100 by a dielectric coating. Burls 106 protrude from the clamp surface of the clamp body 102 and serve to separate the clamped patterning device MA from the clamp body 102. The burls 106 may have a height of, for example, approximately 10 μm and may collectively cover approximately 1% of the surface of the clamp 100. It will be appreciated that many features of the clamp 100 (e.g., wiring, additional electrodes) have been omitted for simplicity.

[0195] The patterning device MA includes a substrate 120, which can typically be made of a material having an ultra-low coefficient of thermal expansion (e.g., a material manufactured by Corning). or manufactured by Schott AG ) is formed. The substrate 120 is generally planar and has a first plane 122 and a second plane 124 opposite to each other. In use (e.g., as Figure 1 ), the first surface 122 is configured to reflect the radiation beam B and cause a pattern to be imparted to the beam B. In particular, an area of ​​the first surface 122 may be patterned, thereby causing a patterned radiation beam B. The patterned area of ​​the first surface is provided with a conductive coating 126.

[0196] In order to enable the electrostatic chuck 100 to clamp the patterning device MA, the second surface 124 is provided with a conductive coating 128 , which typically covers a majority of the second surface 124 .

[0197] It will be appreciated that the electrostatic clamp 100 can utilize voltages of approximately several kilovolts to clamp the patterning device MA. For example, the clamp 100 can be a bipolar electrostatic clamp, wherein a first subset 102A, 102C of electrodes 104A-104D is connected to a voltage source (not shown) of approximately +1...10 kV (e.g., +2 kV), and a second subset 102B, 102D of electrodes 104A-104D is connected to a voltage source of approximately -1...10 kV (e.g., -2 kV). In this manner, a high electric field can be established between the clamp 100 and the patterning device MA, causing the patterning device MA to be attracted to the clamp 100. In particular, a charge having an opposite sign to the applied voltage is induced in regions of the conductive coating 128 adjacent to the electrodes 104A-104D, and an attractive force is established between the opposite charges at various locations across the clamp 100 and the patterning device MA. The region of the clamp 100 configured to support the patterning device MA can be referred to as a support region. Furthermore, when the clamp is operated to clamp the patterning device MA, a region of the clamp configured to generate a clamping force may be referred to as a clamping region.

[0198] Adjacent the patterning device MA is provided a shielding blade 140, 142. The shielding blade 140, 142 is configured to selectively shield the patterning device MA from the radiation beam B during an exposure sequence. In particular, the shielding blades 140, 142 may be provided in the y-direction (ie, Figure 2a The shielding blades 140, 142 are moved in the y-direction (left and right in FIG) to scan the radiation beam B across the surface of the patterning device MA during exposure. Furthermore, the shielding blades 140, 142 can be moved toward and away from each other in the y-direction to provide different degrees of shielding for the patterning device MA. For example, the shielding blades 140, 142 can be closed to shield the entire patterning device MA, or partially closed to allow radiation to pass through a narrow slit.

[0199] In summary, it will be appreciated that the shielding blades 140, 142 may be used to modulate the total dose of radiation incident on the patterning device MA. The blades 140, 142 may be spaced from the patterning device in the z-direction by approximately 5-10 mm (eg, 10 mm), for example.

[0200] Figure 2b The clamp 100 is shown in plan view (with Figure 2a ). The plan view is in the z-plane, which, in the orientation shown, extends vertically in the x-direction and horizontally in the y-direction. Figure 2a The cross section is along Figure 2b The shown line AA' is taken.

[0201] The electrodes 104A-104D each have a rectangular shape and are arranged generally parallel to one another. In this arrangement, the four electrodes shown each span the width of the clamped patterning device MA in the x-direction, and each electrode covers approximately one quarter of the length of the patterning device MA in the y-direction. Figure 2b , the position of the patterning device MA is indicated by dashed lines. The masking blades 140, 142 are shown in dotted outline.

[0202] The electrostatic chuck 100 may further include regions 108 and 110. Figure 2b In the orientation shown, the chuck protrudes from both the upper and lower sides (although in Figure 2a The protrusions 108, 110 are generally in the plane of the electrostatic chuck 100 and are therefore not as visible in the cross-sectional view shown. Figure 2a The protrusions 108, 110 are shown protruding further above or below the electrostatic chuck body 102. The protrusions 108, 110 may be referred to as chuck "ears."

[0203] Protrusion 108 includes secondary electrodes 114A and 114B. Protrusion 110 includes secondary electrodes 114C and 114D. Each of electrodes 104A-104D has a corresponding secondary electrode 114A-114D. Electrodes 104A-104D and each of the corresponding secondary electrodes 114A-114D are electrically connected together. That is, electrode 104A is electrically connected to 114A, and so on (although these connections are not shown). However, as in Figure 2b As can be seen in FIG, the protrusions 108, 110 extend beyond the periphery of the clamped patterning device MA and therefore do not contribute to the clamping force between the electrostatic clamp 100 and the patterning device MA. Therefore, the protrusions 108, 110 may be referred to as non-clamping regions of the clamp.

[0204] The secondary electrodes 114A-114D provide a convenient way to connect a voltage source to the primary electrodes 104A-104D. It will be appreciated that maintaining the flatness of the fixture 100 is desirable in order to improve imaging performance. However, in some cases, providing an external electrical connection to the fixture electrodes 104A-104D can cause distortion in the fixture flatness. Thus, by providing an internal connection between the electrodes within the fixture body (as described above), an external connection (not shown) to the secondary electrodes 114A-114D can be made without interfering with the key areas of the support pattern forming device MA of the fixture 100. This can improve the overall flatness of the pattern forming device MA during imaging operations. The external connection can include leads (not shown) connected to contacts provided by the secondary electrodes 114A-114D. The leads can be connected to a voltage source (not shown) configured to provide a clamping voltage as needed.

[0205] Figure 2c A side view of the fixture 100 is shown in cross section in the y plane. The illustrated view extends vertically in the x direction and horizontally in the z direction. The cross section is taken along Figure 2b The diagram is taken along line BB'. Thus, only electrode 104B and secondary electrodes 114B and 114C are visible. It can be seen that protrusions 108, 110 extend further in the x-direction than patterning device MA.

[0206] Additional shielding blades 144, 146 are positioned adjacent to the patterning device MA. The shielding blades 144, 146 are configured to selectively shield the patterning device MA from the radiation beam B during an exposure sequence. In particular, the shielding blades 144, 146 can be moved in the x-direction to control the width of the radiation beam B. This control can be used, for example, to adapt the lithographic apparatus to expose dies of different sizes and, as with the blades 140, 142, to adjust the total dose of radiation incident on the patterning device MA. The blades 144, 146 can be spaced, for example, by approximately 5-10 mm (e.g., 6 mm) from the patterning device in the z-direction.

[0207] It will be appreciated that in normal use, the electrodes 104A-104D are shielded by the patterning device MA above. However, the secondary electrodes 114A-114D are not shielded in the same manner. This is because the patterning device MA does not extend over the secondary electrodes 114A-114D (e.g., Figure 2b and 2c Of course, it should be understood that the secondary electrodes 114A-114D are insulated from the fixture surface in the same manner as the primary electrodes 104A-104D.

[0208] During use, the polarity of the voltage applied to electrodes 104A-104D is regularly switched. For example, the switching may occur between each wafer exposure (e.g., after about 100 individual dies have been exposed), or at longer intervals (e.g., after every 10 or 50 wafers have been exposed). This switching reduces the likelihood of cracking of the insulating layer around and between the electrodes during extended use. This switching of the polarity of the voltage applied to the electrodes may be referred to as repolarization. Each polarization state may be referred to as an excitation state of the fixture. For example, in a first excitation state, a positive voltage may be applied to electrode 104A, and in a second excitation state, a negative voltage may be applied to electrode 104A. Alternatively, the excitation state of the fixture may generally be referred to as the configuration of the fixture. A fixture configuration may include the excitation state and, optionally, other configuration details (e.g., whether the component is clamped by the device).

[0209] As a result of the lack of shielding of the secondary electrodes 114A-114D by the patterning device MA, external electric fields may be established between adjacent and oppositely polarized electrodes of the secondary fixture electrodes (e.g., between electrodes 114A-114B) or between the secondary electrodes and other nearby components. For example, electric fields may be established between the secondary electrodes 114A-114D and the conductive coatings 126, 128 on either side of the patterning device MA or between the secondary electrodes 114A-114D and the shielding blades 140, 142, 144, 146. Such field lines may be formed between the secondary electrodes 114A-114D and the conductive coatings 126, 128 on either side of the patterning device MA. Figure 2b and Figure 2c It is schematically shown in FIG.

[0210] Of course, it will be understood that during normal operation of the fixture 100, an electric field will be established between the surface of the fixture 100 and the surface of the pattern forming device MA. In addition, electrostatic discharge can occur due to the close separation between the various charged surfaces (including other components within the lithographic apparatus, such as the masking blades 140, 142, 144, 146). That is, electrostatic discharge can occur between any charged surfaces, and the likelihood of discharge increases as the strength of the electric field increases. Electrostatic discharge can generate particles from the surface and can also release particles that were previously attached to the surface within the lithographic apparatus. It will be understood that the release of such particles is undesirable in a lithographic apparatus because the particles can land on critical areas of the apparatus, which can cause pattern defects to form in the processed substrate.

[0211] In addition, when using C r In tests performed with the coated test pattern forming device, Lichtenberg patterns known to be associated with electrostatic discharge damage have been observed. Similarly, C-type patterns with morphologies indicative of generation by a high energy process have also been observed in these tests. rParticles (eg, nanospheres that have melted and resolidified). It will therefore be appreciated that electrostatic discharge can generate unwanted particles.

[0212] Of course, it should be understood that reducing the clamping voltage can reduce the electrostatic field and, therefore, reduce defects on the patterning device due to electrostatic discharge. This reduction has been observed in systems where the patterning device has a front-side coating comprising Mo. That is, it has been demonstrated that reducing the clamping voltage significantly reduces the number of defects on the reticle caused by Mo particles. However, such a reduction in the clamping voltage may not be feasible in some cases because it may result in a reduction in the clamping force.

[0213] In some cases, shielding around the fixture electrodes can be improved, thereby reducing the number of defects observed on the reticle. This shielding can significantly reduce the Mo and C r However, it should be understood that Figure 2b 、 2c The protrusions 108 and 110 are shown unshielded in normal use.

[0214] Electrostatic discharge can also be problematic for pellicles. Ultrathin pellicle films can be used to prevent particles from migrating between areas of a lithographic apparatus. However, it should be understood that ultrathin pellicle films (e.g., which may include a metal layer with a thickness on the order of a few nanometers) may overheat and rupture if any significant current flows through the metal layer. Of course, it should be understood that electrostatic discharge can cause significant current to flow through such a film. Therefore, electrostatic discharge also poses a risk to pellicles.

[0215] In addition, as described above, the electrostatic clamp is periodically repolarized. Each repolarization causes the sign of the electric field around any unshielded electrode to change. If a charged particle is attracted to that electrode during the first polarization state (e.g., when a negative voltage is applied), the charged particle will be repelled once the polarization state changes. In addition, when the clamp is in the first polarization state (e.g., when a negative voltage is applied to the electrode), particles on the clamp surface may become charged. However, once the polarization state changes, such charged particles may be repelled. That is, the strong electrostatic force can overcome the attractive force and cause any trapped particles to be released, which may result in more particles being incident on the pattern forming device or other system components.

[0216] In more detail, the high electric field generated by the electrostatic clamp strongly attracts any free charges. Free charges are charges that are not bound to a physical substrate but are free to move according to the electric field lines (positive ions (e.g., ions) or negative ions (e.g., electrons)). In addition, a large number of free charges are generated during EUV exposure. For example, electrons can be generated by photoemission or from EUV-induced plasma, which is typically generated in the presence of hydrogen gas (which is typically present in lithography tools). Positive ions can also be generated within the EUV plasma. Therefore, during lithography exposure, the unshielded clamp ears 108, 110 may attract free charges, causing any free space electric field to collapse (i.e., be compensated by the free charges), which means that the electric field is limited to the inner part of the clamp (i.e., between the clamp electrodes and the now charged clamp surface). The high charge density generated at the clamp surface may transfer charge to any trapped particles, which are then more strongly attracted by the electrostatic field.

[0217] Then, when the fixture polarity is reversed (with EUV plasma no longer present), the trapped charged particles will now have a charge of equal sign to the electrode, which will result in a strong repulsive force away from the electrode surface.

[0218] For example, it can be estimated that each secondary electrode (i.e., each electrode 114A-114D) will have a capacitance in the range of about 10 pF to about 500 pF (e.g., about 100 pF). The electrode capacitance can be calculated based on the (known) area of ​​each electrode, the (known) permittivity of the fixture dielectric, and the (known) dielectric layer thickness.

[0219] Assuming a clamping voltage of 2 kV, the charge on each ear is 200 nC (Q = CV), corresponding to approximately 720 μC / m 2 Such a charge density will generate a field strength E of about 4.107 V / m (according to E=σ / 2ε0, and assuming that the field radiates in two directions, resulting in the field strength being divided by a factor of 2).

[0220] The plasma generation process will now be discussed in more detail. It will be understood that the EUV photons within the beam will ionize the hydrogen molecules to generate H2 +Ions and free electrons. In the example of using 13.5nm EUV radiation, the energy of each photon is about 92eV, and the ionization energy of molecular hydrogen is about 15eV. Therefore, the free electrons generated can have sufficient energy (e.g., >75eV) and range to generate secondary plasma relatively far away from the initial ionization event. In addition, the electrons released in this way (i.e., with an energy of about 75eV) can ionize one, two or even three additional hydrogen molecules. Therefore, even if the primary plasma is generated only at the place where the EUV photons are incident (which generally does not include the clamp ears 108, 110), secondary plasma can also be generated near the clamp ears 108, 110. As described above, the shielding blades 140-146 are spaced about 5-10mm from the pattern forming device MA. Therefore, any plasma that reaches the clamp ears 108, 110 should diffuse through the gap formed between the blades 140-146 and the pattern forming device MA, or be generated as secondary plasma in situ.

[0221] Figure 3 Figure 2 shows the modeled plasma density near the patterning device during EUV exposure. The horizontal axis shows the distance R (in cm) from the center of the patterning device MA, and the vertical axis shows the distance z (in cm) from the fixture surface. The model assumes an EUV source with an output power of 40 W and a hydrogen pressure of 5 Pa. In the modeled environment, the patterning device MA is shown blocking the radiation path, and the shielding blades 140, 142 are opened by approximately 10 mm to allow EUV radiation to reach the central portion of the patterning device MA.

[0222] It can be seen that the plasma density in the central area near the pattern forming device MA reaches about 10 8 ions / cm 3 However, the density decreases to about 10 below the masking blade (where EUV photons do not reach directly) at about 1.5 cm from the center of the patterning device MA. 7 ions / cm 3 , and decreases to about 10 at about 4 cm from the center of the patterning device MA. 6 ions / cm 3 .

[0223] According to the above calculations, and assuming a particle size of about 100 nm at a density of 10 6 -10 7 ions / cm 3 In the case where the charged surface particles have already received about 10-100 charges in the plasma environment, the resulting electrostatic repulsion on the charged surface particles after repolarization of the clamp (in the absence of plasma) will be about 10 -8 -10 -7N. The attractive Van de Waal and other adhesion forces are expected to be about 10 -9 -10 -8 N.

[0224] Thus, based on the assumptions and approximate calculations presented above, particles trapped on the fixture surface may therefore experience strong repulsive forces, which will generally be able to overcome weaker adhesive forces. Any particles released in this manner will be accelerated along the electric field lines and may land on the masking blades 140-146, the patterning device MA, or other system components.

[0225] In addition, due to the generation of hydrogen plasma (such as Figure 3 As shown in FIG. 1 , shortly after EUV exposure begins, a significant and sudden increase in free charge density and a corresponding increase in the conductivity of the medium surrounding the fixture 100 and the clamped patterning device MA may cause high transient currents to flow between system components.

[0226] It should be understood that during clamping, an electric field may be established within the clamp dielectric and between the clamp surface and the clamped patterning device or other system components (the other system components may be several centimeters from the clamp surface). However, the sudden increase in conductivity in the area surrounding the clamp due to the generation of the hydrogen plasma will cause any established electric field extending beyond the clamp surface (e.g., between the clamp and the patterning device or other system components) to collapse because the conductive plasma will not be able to support the electric field. This may cause the electric field strength within the clamp dielectric to increase rapidly and may exceed the field emission threshold. In addition, once field emission occurs, the high current will cause the clamp electrodes to heat up. This heating will cause the threshold for field emission to lower, resulting in an increase in current. This in turn can cause the electrodes to be further heated and the field emission threshold to be further lowered. Therefore, in this way, shortly after the start of EUV exposure, the free charge density suddenly increases, which may cause high transient currents to flow between system components.

[0227] In some cases, such a sudden change in conductivity can preferentially cause field emission from sharp features of the clamped patterning device MA in the presence of such high electric fields. For example, it will be appreciated that the coating 128 can have sharp edges with a radius of curvature of, for example, about 100 nm. Similarly, field emission can occur from the surface of the clamp.

[0228] Furthermore, it will be appreciated that a dielectric coating having a substantially uniform thickness across the entire plane of a fixture electrode may decrease in thickness at the edges of the electrode. Consequently, the electric field strength may increase at the electrode edges, thereby increasing the risk of field emissions in that region.

[0229] The sensitivity of a particular component or feature to field emission can depend on the properties of the material and / or surface. Furthermore, the extent to which electrostatic discharge damages a surface will also depend on the material properties. For example, relatively poorly conductive coatings (e.g., hardened metals such as CrN, TaN, etc.) that can be used as coatings for patterning devices are susceptible to damage from electrostatic discharge.

[0230] Referring now to FIG. 4 , the Figure 1 and 2a to the operating mode of the lithography apparatus shown in 2c. Figure 4a The EUV dose received at the pattern forming device is shown. In particular, the magnitude of the EUV dose is schematically shown on the vertical axis, and time is shown on the horizontal axis.

[0231] As described above, in order to maintain the integrity of the electrical insulation, the voltage applied to the electrodes 104A-104D of the electrostatic chuck 100 is regularly reversed. Figure 4b The voltage applied to one of these electrodes is schematically shown in FIG, which shows voltage on the vertical axis and time on the horizontal axis. In particular, the voltage is ramped from 0 at time t0 to a clamping voltage +V at time t1. C The voltage is then held at +V during the exposure period. C , until time t2, after which the voltage drops to 0 at time t3 and then drops to -V at time t4 C After that, the voltage is kept at -V C At time t5, the voltage is then increased to zero at time t6. Thereafter, the cycle of positive and negative excitation of the clamp electrodes continues, providing periods of stable excitation between each repolarization event.

[0232] The photolithographic exposure is performed during the stable clamping period. However, it should be understood that the exposure may occur during the positive or negative polarization of the electrodes shown. Furthermore, it should be understood that different electrodes will follow different excitation periods. For example, for each positive to negative transition shown for electrode 104A, electrode 104B may follow the opposite transition. Furthermore, the other electrodes may be repolarized at different times (to ensure that some clamping force is always applied to the patterning device MA).

[0233] Reference again Figure 4a , in period T A 、T B 、T C and T D During this period, the EUV beam is mainly used for imaging. It should be noted that the time period T A 、T B 、T C and T DEach of T corresponds to a period in which the fixture polarization does not change. However, immediately following the period T A Before, there was a period T A ', during which EUV energy is incident on the pattern forming device MA. Similarly, immediately after the time period T A Afterwards, there is a short period T A During this period, EUV energy is incident on the pattern forming device MA. Time period T A It can be called an exposure burst. A ' and T A " can be called pre-exposure burst and post-exposure burst respectively. Exposure burst T A -T D corresponds to the EUV beam used for imaging (i.e., exposing the substrate) or metrology (e.g., alignment, mirror settings, etc.). Therefore, during this period, the radiation intensity reaching the patterning device MA (and provided to the wafer stage) is critical. However, before exposure, the burst T A ' and burst T after exposure A During this period, EUV power is still received at the patterning device, but is not used for imaging. Therefore, the radiation intensity reaching the patterning device MA (and provided to the wafer stage) is not critical and may vary.

[0234] It should be noted that in the example shown in FIG4 , the pre-exposure burst T A ' and burst T after exposure A Both are also included in the period of stable clamping (i.e., period T A '、T A and T A ” are all between t1 and t2), during which the fixture polarization does not change. That is, the entire EUV pulse is contained between the fixture repolarization events.

[0235] Furthermore, it should be understood that the illustrated illumination and poling sequences are schematic and not all-inclusive. For example, even during periods of apparently continuous EUV power, there may be pulses applied to the EUV source, such as between imaging of different dies or during wafer swapping.

[0236] In more detail, the burst T A '、T A and T A Each of the exposure bursts T may include many separate EUV pulses and may also include periods during which no pulses are provided. For example, the exposure burst T A may correspond to a full wafer exposure (eg, including about 100 die exposures), during which there may be a total of about 10 6 pulses (e.g., 10 exposures per die)4 pulses). Between each die exposure, there may be a wafer positioning period during which the EUV beam is not present. Such a period may last about 30-40 ms. In addition, before imaging begins, various metrology actions may be performed during which the beam B is incident on the patterning device MA and provided to the substrate table WT, but is not incident on the substrate itself (the substrate table WT may be moved so that the substrate W is not in the optical path).

[0237] Before exposure, burst T A ' and burst T after exposure A During this period, transient effects within the source SO can be monitored and controlled (e.g., to ensure that the wafer is exposed to the burst T A The radiation intensity experienced during the exposure is as uniform as possible). In addition, the burst T A ' and burst T after exposure A " can be used to perform calibration, alignment, or metrology operations instead of wafer exposure. However, regardless of their use and impact on the substrate, burst T A '、T A and T A ” all relate to EUV radiation incident on the patterning device MA.

[0238] It will be appreciated that shortly after time t1 (ie, once the patterning device has been properly clamped), the EUV power is rapidly increased to begin the pre-exposure burst T A ', the conductivity of the environment surrounding the electrostatic chuck will rapidly change from a non-conductive environment (containing low-density hydrogen) to a highly conductive environment (containing EUV-induced hydrogen plasma). As described above, this rapid change in conductivity can cause electrostatic discharge. Moreover, during the chuck repolarization event, particles trapped on the chuck protrusions 108, 110 (which were A '、T A and T A "(may have become charged in the plasma environment during the process) can be ejected from the fixture surface by a sudden change in the electric field.

[0239] FIG5 shows a modified illumination sequence according to an embodiment of the present invention. Figure 5b The polarization sequence of the electrostatic chuck is shown, which generally corresponds to the polarization sequence of the above reference Figure 4b The polarization sequence described. However, Figure 5a Shown with the above reference Figure 4a The irradiation sequence is compared to the modified irradiation sequence.

[0240] In particular, during the pre-exposure period T ADuring the exposure period T, the EUV power is gradually increased, rather than the EUV power suddenly switching from the OFF state to the ON state. That is, the EUV power incident on the pattern forming device MA is gradually increased when the clamp has been polarized (or repolarized). Similarly, in the post-exposure period T A During this period, EUV power can be gradually reduced instead of EUV power suddenly switching from ON state to OFF state. Note that in the pre-exposure period T A ' and the post-exposure period T A During this exposure, EUV power is applied to the patterning device, but not used for imaging. Within the meaning of the present invention, this may be referred to as non-imagewise exposure of the patterning device. Such non-imagewise exposure therefore refers to an exposure during which an EUV radiation beam is incident on the patterning device and during which no radiation is projected onto the substrate; the non-imagewise exposure may be performed, for example, between consecutive imaging exposures in the plurality of imaging exposures.

[0241] In an embodiment of the invention, this non-imagewise exposure is applied during poling or re-poling of the fixture.

[0242] A soft ramp-up of EUV power, such as that applied during non-imaging exposure, results in a gradual increase in the conductivity of the area surrounding the patterning device MA and electrostatic chuck 100. This gradual increase in dielectric conductivity does not cause the electrostatic field to collapse abruptly, but rather allows charge to leak toward various surfaces according to pre-existing field lines. This process allows compensation for any surfaces that may have become charged due to the previous chuck polarization state. Similarly, any charged particles on the surfaces of the chuck or patterning device can be compensated.

[0243] It may be desirable for the soft ramp to extend over multiple EUV pulses. For example, the soft ramp may take less than one thousand pulses, or less than about 20 milliseconds. Preferably, the soft ramp may take less than 10,000 pulses, or less than about 200 milliseconds. Generally, the ramp period will be small compared to the wafer exposure time. Wafer exposure may, for example, last about 30 seconds. Compared to a wafer exposure of about 30 seconds, a ramp time of less than about 1 second (or about 50,000 pulses) can be considered a soft ramp or gradual increase.

[0244] A soft ramp as described above may be beneficial in one or more of the following situations:

[0245] a. After the first startup of the EUV device;

[0246] b. After loading a new patterning device MA; and

[0247] c. After polarity reversal (repolarization) of the electrostatic chuck electrodes.

[0248] This soft ramping can be performed in several different ways. For example, the ramping can be performed by gradually ramping up the pulse energy of the source. Alternatively or additionally, the soft ramping can be achieved by slowly increasing the number of pulses in each of a plurality of micro-pulses that together contribute to the total radiation energy (or dose) delivered to the wafer. Each micro-pulse can, for example, include about 10 pulses, each having a duration of about 100 ns and delivered at a frequency of about 50 kHz. In the case of an LPP radiation source, the number of EUV pulses generated can be controlled by controlling the operation of the applied laser. In such an LPP radiation source, the EUV pulses are generated by irradiating a fuel target such as a tin (Sn) target with one or more laser beams. The amount of EUV radiation generated can, for example, be controlled by controlling the number of fuel targets irradiated. The applied pulsed laser beam can, for example, be controlled to irradiate only one-half or one-third of the fuel target, thereby reducing the amount of EUV radiation to 50% or 33%. Alternatively or additionally, a soft ramp-up may be achieved by inserting and slowly retracting one or more of the masking blades 140, 142, 144, 146. Of course, alternative mechanisms for such a soft ramp-up of EUV energy may also be implemented.

[0249] For example, a soft ramp can start with a small jump (e.g., to 5-10% of full imaging power) and then gradually increase from that level to 100% imaging power at the start of the imaging exposure. Alternatively, the radiation power can be increased linearly from 0% to 100%. During the gradual increase in radiation power, there may be several small steps, resulting in an overall linear increase. For example, the power can be increased in steps (e.g., 5% of 10%). This increase will still result in a gradual change in the conductivity of the medium surrounding the fixture and patterning device.

[0250] In summary, it will be appreciated that providing a gradual change in conductivity reduces the risk of discharge associated with a sudden increase in conductivity.

[0251] In some cases, EUV energy can be used at a rate similar to Figure 4a However, in other embodiments, the EUV energy may be stopped abruptly as is customary in the prior art.

[0252] As shown in Figure 5, the clamp electrode repolarization may occur during EUV ramp-up or ramp-down. Alternatively, the clamp electrode repolarization may occur during the EUV off period. It can also be noted that the EUV can be turned on during voltage ramp-down, for example, reducing the applied voltage to zero before unloading the object. Figure 5b The time period t shown 11 -t 12Such a voltage drop may be represented by .As mentioned above, it may also be advantageous to apply at least a low level of radiation during the above period.

[0253] FIG. 6 shows another embodiment in which the EUV power excitation is modified relative to the EUV power excitation as a conventional case. Figure 6b shows a fixture polarization sequence similar to that described above. Figure 4b However, in Figure 6a As can be seen in Figure 1 , between each EUV exposure burst T, the patterning device MA experiences constant EUV illumination. That is, EUV radiation is provided to the patterning device MA even during the period when the electrostatic chuck is repolarized. This requires maintaining the EUV energy output by source SO during the period between exposure bursts and controlling the shielding blades 140-146 to allow at least some EUV radiation to reach the patterning device MA. As described above, applying EUV power to the patterning device without using the power used for imaging is referred to as non-imaging exposure within the meaning of the present invention. Such non-imaging exposure may result in the generation of EUV-induced plasma.

[0254] By providing EUV radiation to the patterning device MA during repolarization (i.e., during the transition from the first excited state of the chuck to the second excited state), it will be appreciated that a relatively conductive medium (i.e., EUV induced hydrogen plasma) will always be provided in the vicinity of the clamped patterning device MA and the electrostatic chuck 100. Thus, when the chuck 100 is repolarized, the free space charge (e.g., Figure 3 The free charge will then be present around the clamp protrusions 108, 110 as shown) will allow for rapid redistribution of charge. The large number of free charges will effectively screen the field generated by the secondary electrodes 114A-114D and reduce the likelihood of particles being released from the surface. The free charge will also act to reduce dangerous high intensity fields. This will result in a reduction in high transient currents between the dielectric surface and the field concentrating features at the edge of the pattern forming device coating via the EUV induced plasma. That is, by providing a conductive dielectric at all times, the high transient currents and discharge risks associated with the sudden generation of EUV energy at the beginning of each exposure burst can be reduced or completely eliminated. It can also be noted that the EUV can be turned on during a voltage drop, for example, the reduction of the applied voltage to zero is completed before unloading of the object. As Figure 6b The time period t shown 11 -t 12 This voltage drop may be represented by As mentioned above, it may also be advantageous to apply at least a low level of radiation during the above-mentioned time period.

[0255] As shown in FIG7 , in yet another operation mode, the EUV power ( Figure 7a ) is modulated during repolarization ( Figure 7b ), such that each exposure burst T pulse is preceded by a gradually ramping up RU, followed by a gradually ramping down RD, and between each EUV burst T a low EUV power L is applied to the reticle. That is, instead of removing the EUV power completely between exposure bursts, the EUV power is gradually reduced during the repolarization and maintained at a low level. Compared to the arrangement described above with respect to FIG6 , this arrangement reduces the load on the EUV source, but also provides the advantage that the dielectric surrounding the electrostatic clamp is always kept in a conductive state, minimizing the effects of high transient currents that might be observed during a sudden EUV switch-on and allowing charge compensation to be performed during each repolarization event. It may also be noted that the EUV can be switched on during a voltage drop, for example, the reduction of the applied voltage to zero is completed before unloading of the object. As Figure 7b The time period t shown 11 -t 12 Such a voltage drop may be represented by .As mentioned above, it may also be advantageous to apply at least a low level of radiation during the above-mentioned time period.

[0256] It will be appreciated that in each of the arrangements described above with reference to Figures 5, 6, and 7, the performance of the lithographic apparatus can be improved by reducing defects caused by electrostatic discharge events and the emission of uncharged particles without any hardware changes. In particular, existing features of the EUV illumination system are used in a novel way to reduce the high free-space fields seen during and after repolarization events.

[0257] In each of the above embodiments, it will be appreciated that it is desirable to generate an EUV-induced plasma that provides a source of free charge in the vicinity of the patterning device MA, particularly in the vicinity of the fixture protrusions 108, 110. However, it will also be appreciated that sufficient free charge density will be required to effectively shield the electric field around the electrodes 114A-114D. Of course, the density of charge generated at any particular location will depend on the specific characteristics of any system, including EUV intensity, gas density, and system geometry. Furthermore, the amount of free charge required will also depend on management parameters (e.g., field strength, electrode geometry).

[0258] In view of the amount of charge expected to accumulate at the dielectric surfaces adjacent to the clamp secondary electrodes 114A-114D (as discussed in more detail above), it should be understood that charge must be supplied at at least a minimum rate to adequately prevent external fields from building up during a repolarization event.

[0259] For example, it can be expected that the time interval between about 200 ms and about 10 4The clamp repolarization occurs during each EUV pulse. Furthermore, the clamp repolarization switching rate can be, for example, about 16 kV / s. At this rate, the clamping voltage will change at a rate of about 0.3 V per EUV pulse. Furthermore, as described above, the number of charges required to shield the clamp ears at a clamping voltage of 2 kV is about 10 12 (200nC≈1.25×10 12 The magnitude of the charge).

[0260] It is assumed that both primary plasma ions and secondary remote plasma ions (caused by electrons removed during ionization) contribute to the charge present at the clamp ears (e.g. Figure 3 As shown), and assuming that the masking blades 140-146 are fully open (to allow the entire patterning device to be exposed to the radiation beam B), it is believed that 10 4 The EUV pulse will provide enough free charge to essentially prevent the buildup of external fields during the repolarization event. In this case, assuming a source power of 40 W, one can expect an ion density of about 10 at the fixture ear. 7 / cm 3 .

[0261] However, if the flux of free charges decreases, a higher number of EUV pulses may be required to provide sufficient rebalancing of the charge. For example, if the primary plasma cannot diffuse to the clamp ear, resulting in only the secondary plasma contributing free charges at the clamp ear, then the ion density at the clamp ear is expected to be about 10 6 / cm 3 This ion density may not provide enough free charges to fully compensate the repolarization field.

[0262] If this is determined to be the case, the rate at which repolarization occurs can be reduced. For example, if the repolarization rate is reduced by a factor of five (to 3.2 kV / s), the fixture repolarization will last for 5 × 10 4 EUV pulses are used, significantly increasing the free charge supply to compensate for the varying electric field. Of course, it should be understood that this adjustment may result in a reduction in productivity. However, as mentioned above, reducing the clamping switching speed by a factor of five results in an overall productivity reduction of <0.5%.

[0263] It will of course be understood by those skilled in the art that the amount of free charge required will depend on the clamping voltage and geometry, and that the proportion of free charge generated that can provide a shielding effect will also depend on a number of factors (some of which are discussed above). However, the skilled artisan will be able to modify various parameters (e.g., EUV intensity, EUV pulse duration, number of EUV pulses, H2 gas pressure, clamp voltage, switching frequency, shielding blade position, etc.) as needed to ensure that the clamp ears are adequately shielded. For example, empirical studies can be performed to establish the free charge levels required and / or provided in a particular arrangement. Alternatively, charge density modeling can be performed to determine the charge density levels that will be experienced at the clamp ear locations.

[0264] In the embodiments described with reference to Figures 6 to 8, EUV-induced plasma is generated by a so-called non-imaging exposure of the pattern forming device, i.e., during this non-imaging exposure, a radiation beam is incident on the pattern forming device and during this non-imaging exposure, no radiation is projected onto the substrate; the above-mentioned non-imaging exposure is performed between consecutive imaging exposures in the above-mentioned multiple imaging exposures.

[0265] By providing a radiation beam at the patterning device during non-imaging exposure, a source of free charge can be provided by means of a plasma, which will be generated by the ionization of gas molecules (e.g., hydrogen) present around the patterning device. Plasma will be generated in the area directly illuminated by the radiation beam as well as in adjacent areas (e.g., due to diffusion and secondary electrons). In this way, a single radiation beam (e.g., an EUV radiation beam) can be used both for imaging purposes and to provide a source of free charge during repolarization of the electrostatic chuck.

[0266] In an alternative embodiment, EUV power can be used as a source of free charge by causing it to be incident on a surface near the pattern forming device. Thus, an alternative way to generate free charge adjacent to the electrostatic clamp is to provide a radiation beam at a surface different from the pattern forming device (e.g., the surface of a shield or shielding blade). As described above, a shield or shielding blade arrangement can be used to block the radiation beam from reaching the pattern forming device or to control the spatial extent of the radiation beam incident on the pattern forming device. Applying a radiation beam or a portion thereof to the shield or shielding blade arrangement may also result in the generation of free charge due to the interaction of the radiation beam with gas molecules (e.g., hydrogen) present around, at, or near the shield or shielding blade arrangement or provided thereto. Free charge will be generated both in the area directly irradiated by the radiation beam and in adjacent areas (e.g., due to diffusion and secondary electrons). Since the shield or shielding blade arrangement is typically relatively close to the pattern forming device and therefore relatively close to the electrostatic clamp, the free charge generated by irradiating the shield or shielding blade arrangement may also result in the generation of free charge in the adjacent electrostatic clamp. Therefore, the free charge generated by illuminating the shield or shield blade arrangement may also be considered as a mechanism for generating free charge adjacent the electrostatic chuck.

[0267] In some embodiments, a secondary ionization source may be provided to allow plasma to be generated adjacent to the electrostatic chuck by means other than the EUV source SO. This arrangement may reduce the overall output load of the EUV source SO. It will be appreciated that the above embodiments may place greater demands on the EUV source SO by requiring more EUV output than is necessary for imaging. Furthermore, in some embodiments, it may not be possible for the EUV source to generate power continuously (e.g., Figure 6a ). Similarly, it may not be possible and / or desirable for an EUV source to provide arbitrary EUV pulse energies in the range of 0-100% of the nominal output power while also ensuring clean collector operation and pulse energy stability.

[0268] Thus, in some embodiments it may be preferable to provide an alternative mechanism for generating a region of higher gas conductivity than the primary EUV source.

[0269] For example, a source can be provided near the electrostatic clamp 100 and the clamped pattern forming device MA. The source can preferably be placed near the clamp protrusions 108, 110. A plurality of sources can be used. The source can be, for example, a soft X-ray source or a VUV light source that can operate at a pressure of less than one bar in a clean environment. The source can include a low-power ionizer with a power of about 0.1-1W. One such suitable device can be the VUV ionizer L12542 manufactured by Hamamatsu Photonics KK (Shizuoka, Hamamatsu City, Japan). In some embodiments, the source can include a radioactive source or an electron beam source.

[0270] like Figures 8a-8c shown (wherein the sub-figures and the parts shown generally correspond to Figures 2a-2c In one embodiment, the electrostatic chuck 100 and the patterning device MA are provided with a source S, such as those shown. In one embodiment, the source S can be activated, for example, during repolarization of the electrostatic chuck 100, thereby ensuring that the hydrogen gas in the region of the chuck 100 and the patterning device MA is ionized to produce a hydrogen plasma. As described above, providing free charge during repolarization effectively shields the field generated by the electrostatic chuck 100 (particularly around the secondary electrodes 114A-114D).

[0271] For example, Figures 8a-8c As shown in FIG. 1 (which shows the time when source S emits VUV radiation), a plasma P is generated due to the emitted VUV radiation. The plasma P results in a cloud of free charge being provided throughout the clamp and patterning device environment. Thus, although an electric field may exist between each of the electrodes 104A to 104D and the adjacent area of ​​the clamped patterning device MA, the unshielded electrodes 114A to 114D attract the charge cloud from the plasma, thereby forming shielding charges QA, QB, QC and QD. Each shielding charge QA-QD has an opposite sign to that of the corresponding secondary electrode 114A-114D. As a result, no significant and unwanted electric field is established around the electrostatic clamp 100, thereby reducing Figure 2b and 2c The effects of fields F1, F2 and F3 are shown.

[0272] Thus, instead of EUV radiation generated by source SO, source S can be used to provide free-space charge to shield unwanted electric fields, effectively causing the free-space field to collapse (by providing mobile charge that can compensate for any free-space field). This shielding results in the electric field generated by the chuck 100 being substantially confined to the interior of the insulator surrounding the electrostatic chuck electrodes 114A-D.

[0273] It will be appreciated that the secondary ionization source S may be particularly positioned in the vicinity of the protrusions 108, 110 of the fixture 100 in order to provide localized free charge to the areas where it is most needed. As such, it will be appreciated that the secondary source S may be used to provide an alternative to the illumination schemes described above with reference to Figures 6 and 7, in which EUV energy generated by the source SO is provided to the patterning device during repolarization.

[0274] In general, EUV sources SO and S (which may, for example, include soft X-ray sources or VUV ionizers) can be considered examples of ionizing radiation sources. Furthermore, these sources, combined with a hydrogen (or other) gas source, can be considered mechanisms for generating free charge. That is, a hydrogen plasma containing both positive ions and free electrons can be considered a cloud of free charge. Furthermore, this free charge includes both positive and negative free charges. This allows the free charges to compensate for and shield fields of both polarities. It should be understood that fields of both polarities are experienced at different electrodes in each fixture polarization state, and at each electrode in different polarization states.

[0275] Furthermore, as briefly described above, a significant voltage may be established between the jig 100 and the patterning device MA after the patterning device MA is removed from the jig 100. The removal process of the patterning device MA will now be described in more detail with reference to FIG.

[0276] Figure 9a A fixture 100 is shown to which a pattern forming device MA is clamped. The clamped pattern forming device MA is shown as being spaced apart from a replacement assembly 150. The replacement assembly comprises a replacement device 152 supporting a support structure 154. The support structure 154 comprises a small number of protrusions 156 extending from a surface of the support structure 154 towards the pattern forming device MA. The protrusions 156 may, for example, have a height of about 200 μm. The replacement assembly 150 is configured to move the pattern forming device MA towards and away from the fixture 100, for example so that the pattern forming device MA can be replaced with an alternative pattern forming device, or cleaned. The replacement device 152 may comprise a robotic arm that moves relative to the rest of the lithographic apparatus to move the pattern forming device MA towards and away from a loading lock (not shown). The support structure 154 and in particular the protrusions 156 support the pattern forming device MA during transport. From Figure 9a It can be seen that the protrusions 156 have a small area compared to the total area of ​​the patterning device MA. Thus, when the patterning device MA is supported by the support structure 154, there is only a small contact area. Figure 9a In the configuration shown, there is a small gap g between the electrostatic chuck 100 and the patterning device MA. This gap g may be, for example, about 10 μm (corresponding to the height of the burls 106 provided on the surface of the electrostatic body 100).

[0277] The surface of the patterning device MA is shown as being spaced apart by a gap b from the surface of the support structure 154. Figure 9a In the configuration shown, gap b exceeds 200 μm (the height of protrusion 156 ). Thus, when patterning device MA is spaced from the top surface of protrusion 156 , a gap of at least 200 μm must exist between the planar surface of patterning device MA and the planar surface of support structure 154 .

[0278] On the contrary, Figure 9b In the configuration shown (which includes Figure 9a 15), the patterning device MA is shown resting on top of the protrusion 156. Thus, the gap b is 200 μm, and the gap g (i.e., the gap between the electrostatic chuck 100 and the patterning device MA) is greater than 10 μm, indicating that there is no physical contact between the burl 106 (or indeed any part of the chuck 100) and the patterning device MA.

[0279] Figure 10 1 shows a time series illustrating the changes in gaps b and g caused by removal of patterning device MA. At time t20, gap g is initially 10 μm (corresponding to the height of burl 106, indicating that there is contact between the top of burl 106 and the surface of patterning device MA). Gap b at time t20 is well over approximately 900 μm, so there is no contact between any portion of replacement assembly 150 and patterning device MA.

[0280] At time t21, the replacement assembly 150 approaches the pattern forming device MA, as shown by the decrease in distance b. When the replacement assembly 150 is configured to hover below the pattern forming device MA, the movement stops at time t22, thereby maintaining a gap b of about 900 μm (i.e., the height of the protrusion 156 of 200 μm, plus the gap of 700 μm). Then at time t23, the electrostatic clamp 100 and the clamped pattern forming device MA gradually descend toward the replacement device 150. This movement continues slowly until about t24, at which time the distance b has decreased to 200 μm. That is, the electrostatic clamp 100 and the clamped pattern forming device MA are lowered until the lower surface of the pattern forming device MA (i.e., the lower surface of the pattern forming device MA) is at a distance b of 200 μm. Figure 8a and 8b 154 ) contacts the upper surface of the protrusion 156. This state is maintained until about time t25. During this period, the voltage applied to the clamp electrodes 104A-D, 114A-D is removed, so that the electrostatic clamp 100 no longer clamps the patterning device MA.

[0281] At time t25, the electrostatic clamp moves upward to move away from the patterning device MA (which is now fully supported by the protrusion 156 of the replacement assembly 150), causing the gap g to increase. This movement continues until time t26, at which time the gap g has increased to approximately 700 μm. At this point, the replacement assembly 150 supports the patterning device MA and hovers below the electrostatic clamp 100. At time t27, the replacement assembly 150 is caused to move the supported patterning device MA away from the electrostatic clamp 100, causing the gap g to increase from 700 μm to a larger distance. This movement continues until time t28 (after which the replacement assembly 150 can be moved to a patterning device replacement area, such as a load lock).

[0282] It should be understood that there is capacitance between the several system components mentioned above. Figure 11 As shown, the system can be modeled as a plurality of capacitors arranged in series. In particular, the replacement device 150 is connected to the lower surface of the pattern forming device MA (at Figure 9a 、 9b The capacitance between the two electrodes can be considered as a variable capacitance C b , which varies according to the gap b. Capacitance C b It can be calculated as follows:

[0283]

[0284] Where: ε0 is the dielectric constant of free space;

[0285] A is the area of ​​patterning device MA; and

[0286] b is the spacing between surfaces with b.

[0287] The capacitance of the patterning device MA itself can be considered as a fixed capacitance C r , which can be calculated as follows:

[0288]

[0289] Where: rr is the relative dielectric constant of substrate 120; and

[0290] r is the separation between the front and back surfaces of the patterning device MA.

[0291] The gap between the top side of the patterning device MA and the electrostatic chuck 100 can be considered as a variable capacitance C g , which can be calculated as follows:

[0292]

[0293] Where b is the spacing between the back side of the patterning device MA and the jig 100 .

[0294] Finally, a clamp capacitance C is provided between the electrodes of the clamp 100 (having a voltage of 0 V on average) and the surface of the electrostatic clamp. d The capacitor C d is fixed and can be calculated as follows:

[0295]

[0296] Where: rd is the relative permittivity of the fixture dielectric; and

[0297] d is the separation between the chuck electrodes 104A-D and the chuck surface.

[0298] Of course, it should be understood that the above expressions represent simplifications of actual capacitance and ignore various parasitic and additional components. Furthermore, the dielectric constant of the low-pressure hydrogen environment is assumed to be similar to that of free space (ε0). Similarly, the area of ​​each capacitor is assumed to be equal to A, and the effects of burls 106 and protrusions 156 are ignored. However, the described model is intended to illustrate general trends in capacitance and charge distribution. Those skilled in the art will appreciate that more precise modeling can be performed if desired.

[0299] like Figure 11 As shown, the capacitor C d 、C g 、C r and C b In addition, the capacitor C d and C r is fixed, and the capacitor C g and C b It will therefore be appreciated that in a closed system where no charge can enter or leave the system, and for a given initial state of charge, any change in the spacing g between the fixture 100 and the patterning device MA and any change in the spacing b between the patterning device MA and the replacement device 150 will result in a variable capacitance C g and C b Furthermore, this change in capacitance will also cause the voltage across the individual capacitors to vary significantly, potentially depending on the change in spacing.

[0300] In particular, for each capacitance, the relationship Q=CV must always hold (assuming no charge injection). Therefore, if the capacitance C is changed, and the amount of charge Q contained in this capacitance remains the same, the voltage V must change inversely proportional to the changed capacitance C. This can result in significant voltage amplification.

[0301] Of course, it will be appreciated that in some cases, charge may be injected into various nodes between the capacitors. In particular, the charge injected at the fixture surface may be modeled as charge Q s The charge injected into the back side of the patterning device can be modeled as charge Q b The charge injected into the front of the patterning device can be modeled as charge Q f . Charge source Q s , Q b and Q f like Figure 11 shown.

[0302] Figures 12a-12c The changes in various parameters of the model of the fixture environment during the unloading sequence are shown. Specifically, Figure 12a As shown, the patterning device MA is removed from the fixture 100, as described above with reference to FIG. Figure 10 The changes in intervals b and g during a series of movements similar to the movements described above are shown. The x-axis shows time in seconds, while the y-axis shows distance in meters (logarithmic scale). First, distance b decreases from about 100 mm to about 1 mm at time 3 s, and then distance b further decreases to about 200 μm at about 6 s (i.e., contact between protrusion 156 and patterning device MA). Distance g then increases from about 10 μm at time 8 s to about 700 μm at time 9 s, and then increases again from about time 11 s.

[0303] As mentioned above Figure 11 As described above, the capacitance associated with various system components also changes according to the spacings b and g. The changing capacitance associated with these various motions is shown in FIG. Figure 12b As shown, where the x-axis shows time in seconds and the y-axis (again on a logarithmic scale) shows the time per capacitor C d 、C g 、C r and C b As expected, it can be seen that the capacitance C d and C r There is no change during the entire time period. However, it can be seen that the capacitance C b The capacitance C increases by about three orders of magnitude between time 2 seconds and time 6 seconds (which corresponds to the movement of the replacement assembly 150 toward the patterning device MA). In contrast, from time 8 seconds to time 12 seconds, the capacitance C increases by about three orders of magnitude between time 2 seconds and time 6 seconds (which corresponds to the movement of the replacement assembly 150 toward the patterning device MA). g (which corresponds to the capacitance between the pattern forming device MA and the electrostatic clamp 100) is reduced by about 4 orders of magnitude in two stages. g First it decreases from about 20 nF to about 300 pF at time 9 s, and then decreases again to about 20 pF at time 12 s.

[0304] Figure 12c shows that during the removal sequence discussed above Figure 11 The voltages at various points within the equivalent circuit shown. In particular, the voltage V at the fixture dielectric surface is shown. d (which is also equal to the capacitor C d The voltage across ) varies only minimally during this sequence.

[0305] The voltage V at the front of the patterning device MA FS Can be understood as voltage V d 、V g and V r (respectively across capacitor C d 、C g and C r The voltage V FS It is also equal to the cross capacitor C b The voltage V b It can be seen that V FS It ramps up from a relatively small amount to approximately 500 volts at time 11 seconds. This corresponds to the time when the gap g between the patterning device MA and the electrostatic chuck 100 increases significantly.

[0306] However, by far the largest change in voltage occurs at the back of the patterning device MA, which is determined by the voltage V d and V g The sum (i.e., across capacitor C d and C g ) is represented by the sum of the voltages of the patterning device MA and the electrostatic chuck 100. It can be seen that during the initial small interval between the patterning device MA and the electrostatic chuck 100, the voltage increases to over 1000 volts (i.e., the interval reaches about 700 μm at about 8 seconds). However, then at about time 11 seconds, the voltage rises even more significantly to over 4300 volts, corresponding to a significant increase in the distance g from the patterning device MA to the electrostatic chuck 100.

[0307] It should be noted that the modeled voltage variation is based on the assumption that some charge will be injected from the electrostatic clamp 100 to the back of the patterning device MA. The modeled charge injection is as follows Figure 13 As shown, it is generally similar to the above reference Figure 11 The equivalent circuit is shown in FIG1 and includes a single charge source Q modeled as injecting approximately 500 nC of charge at approximately 1 second. This charge injection can also be Figure 11 c, which shows the voltage between time 1s and 8s with an expanded vertical scale. In particular, it can be seen that at time 1s, the injection of 500nC of charge results in a voltage V BS 、V FS Suddenly increased to about 80V.

[0308] In practice, instead of injecting charge precisely at time 1s, charge can accumulate on the back side of the patterning device MA during clamping. Furthermore, when the patterning device is clamped, field emission can occur due to sharp features on the clamp surface (e.g., trapped particles). This can cause the patterning device to become charged (typically negatively).

[0309] Assuming no further charge is introduced into the system, then Figure 11 and 13 The equivalent circuit model shown can be used to model the voltage evolution in response to the above capacitance changes. It can be understood that Figure 12c The resulting high voltage shown would significantly increase the risk of discharge due to decomposition of hydrogen in the vicinity of the electrostatic chuck 100 and patterning device MA (eg, because the voltage at the surface of the patterning device MA exceeds the lowest Paschen limit of hydrogen (about 250 V)).

[0310] Therefore, another challenge associated with electrostatic discharge within the lithographic apparatus is during unloading of the patterning device after clamping. As described above, charge can be trapped at the dielectric surface of the fixture 100. In addition, once the charge is released, residual charge can remain on the clamped patterning device MA. As the unclamped patterning device MA moves away from the fixture surface, the increased spacing between the fixture surface and the patterning device surface results in a decrease in capacitance and an amplification of voltage. That is, in a closed system, given the proportional relationship between charge and voltage (i.e., Q=CV), when the capacitance changes (inversely proportional to the spacing between the parallel plates), any decrease in capacitance will result in a proportional increase in voltage. Therefore, since the patterning device MA and the fixture 100 are separated, the voltage of the patterning device will increase sufficiently to cause electrical breakdown of the hydrogen. This discharge can result in the generation of particles, leading to subsequent defects.

[0311] However, it has been recognized that the effects of the varying capacitance can be mitigated to some extent by introducing free charges during the unloading process. For example, a separate ionization source S, or indeed an EUV source SO, can be used to generate a hydrogen plasma that provides free charges (as described above) and allows the electric fields established across the various dielectric components (and gaps) to be relaxed during the removal process.

[0312] Providing free charges can lead to a significant reduction in the voltages built up between the various system components. That is, the voltages generated by the Figure 12c The electric field created by the high voltage shown is Figure 11 As shown, it can be considered that such charge is generated by the charge source Q s , Q b and Q fThe charge source is provided to the surfaces of the patterning device and the fixture. These charge sources are effectively provided by the hydrogen plasma, which provides sufficient charge to each node in the equivalent circuit of the fixture 100 and the patterning device MA to ensure that they remain in a neutral state. In other words, the free charges in the plasma are driven by any electric fields as soon as they begin to build up, and cause them to collapse.

[0313] In this way, potential problems associated with a significant voltage buildup on the patterning device MA after removal from the electrostatic chuck 10 can be mitigated or avoided entirely. As mentioned above, it will be appreciated that this effect is not binary, and that some (reduced) field may still be established if insufficient charge is provided. However, it will be appreciated that even reducing voltage amplification (rather than avoiding it entirely) can be beneficial, particularly if the voltage is thus always kept below the lowest Paschen limit of hydrogen (approximately 250 V).

[0314] Furthermore, free charge can be provided at various times during the separation of the fixture 100 and the patterning device MA. In practice, it will be appreciated that when the patterning device MA is clamped, it may be difficult for free charge to penetrate between adjacent surfaces. Therefore, there may be an effective minimum spacing for optimal free charge provision. This spacing will depend on the desired penetration depth (i.e., the distance that free charge should penetrate between the separation surfaces of the fixture and patterning device to provide effective charge neutralization).

[0315] The minimum spacing can also be a function of voltage (which, as described above, increases as the spacing increases). For example, in the case where the voltage between the separating surfaces is small (e.g., zero), charge will not penetrate into the very small gap due to random diffusion and surface recombination. However, in the case where there is a significant voltage between the separating surfaces, an electric field will also be generated between the (neutral) environment around the fixture 100 and pattern forming device MA and the surfaces of the fixture 100 and pattern forming device MA with an increased voltage. Any such field may cause free charges of appropriate sign to be sucked into the volume between the separating surfaces. Typically, the effect of such a field is stronger than the random diffusion of free charge, resulting in deeper penetration of charge into the volume than in the absence of a field. For a penetration depth of about 1 mm, the minimum gap that allows charge to penetrate into the fixture area can be, for example, on the order of 100 microns.

[0316] It should be understood that there may be a distance between the physical edge of the patterning device MA (or the beveled edge of the patterning device) and the start of the conductive coating 128. This distance may be approximately 1 mm. Thus, a plasma penetration depth of less than 1 mm will only provide free charge to the non-conductive surface of the patterning device MA, and not to the conductive coating 128. The conductive coating 128 can act as a reservoir of charge that can be released locally in a very short time during the discharge process. Thus, it is beneficial to provide free charge to the conductive coating 128 during separation to neutralize any charge imbalance.

[0317] It should also be understood that it may be beneficial to provide free charge before the voltage difference between the separated components exceeds a dangerous level (e.g., 250 V). Therefore, there may be an effective maximum separation distance at which free charge is optimally provided. The maximum separation distance at which charge should be provided will vary depending on many factors.

[0318] For example, the initial spacing between the jig 100 and the parallel surfaces of the patterning device MA is one such factor. This initial spacing can be on the order of 10 microns (which corresponds to the height of the burl 106, indicating direct contact between the top of the burl 106 and the surface of the patterning device MA). It will be appreciated that the capacitance will vary inversely with the spacing between the jig 100 and the parallel surfaces of the patterning device MA (rather than the spacing between the burl and the patterning device MA, which is initially zero). Thus, the spacing corresponding to a hazardous voltage level will depend on the initial spacing level. That is, the ratio of the spacing between the jig 100 and the parallel surfaces of the patterning device MA when the burl is in contact with the patterning device (e.g., a "spacing" of 10 microns) and when the patterning device has left the jig will be substantially equal to the inverse of the capacitance ratio for these two configurations.

[0319] In addition, the interval corresponding to the dangerous voltage level will also depend on the voltage imbalance between the components of the clamp and the clamped pattern forming device. For example, it will be appreciated that clamping is typically achieved by multiple electrodes to which opposite voltages are applied. These voltages effectively cancel each other out, thereby maintaining the pattern forming device at a nominal total voltage of zero (even when a clamping voltage of ±1-10 kV is applied to each clamp electrode). However, it will also be appreciated that there may be variations between the precise clamping voltages of each electrode or in the capacitance of each electrode. Any of these factors may cause the entire pattern forming device to deviate from voltage neutrality. Alternatively or additionally, there may be charge transfer, for example, by micro-discharges from charged particles present between the protrusions of the clamp. Furthermore, various alternative charge transfer mechanisms may result in a net charge and associated voltage remaining on the pattern forming device once the clamping voltage has been removed from the clamp 100.

[0320] In one embodiment, assuming that any voltage imbalance is less than ~10V (or ~0.5% of the 2kV clamping voltage), and that the voltage between components should be prevented from exceeding 200V (to minimize the risk of discharge), the voltage amplification factor must not exceed 20. This would require providing compensatory free charge before the spacing (between parallel surfaces, not between burl tips) increases by a factor of 20. In such an example where the nominal spacing during clamping is 10μm, a spacing of approximately 200μm can be considered the effective maximum spacing at which free charge can be optimally provided.

[0321] Of course, it will be appreciated that the effective maximum and minimum intervals for providing free charge will depend on many characteristics and will vary between different device configurations and operating conditions. More generally, it will be appreciated that free charge can be generated at times selected to provide charge to reduce (or limit) the potential difference between the electrostatic clamp and a component previously clamped (and subsequently released) before that potential difference exceeds a threshold. In one embodiment, the free charge is provided from before the separation begins to increase until a time after the minimum separation has passed. In an alternative embodiment, the free charge is provided from before the separation has exceeded the maximum separation until a time after the maximum separation has passed. In an alternative embodiment, the free charge is provided from before the separation reaches the minimum separation until a time after the maximum separation has passed.

[0322] In addition to using the free charges associated with the hydrogen plasma as the charge source described above, in alternative embodiments, the free charges may be used to clean the electrostatic chuck 100 .

[0323] It will be appreciated that particles may deposit on the surface of the electrostatic clamp 100, for example, between the burls 106. Such particles may be detrimental to the performance of the clamp. For example, such particles may promote the transfer of charge between the insulator of the electrostatic clamp 100 and the surface of the clamped patterning device MA. This charge transfer may result in additional adhesion of the electrostatic clamp 100 even when the clamping voltage has been removed from the electrodes. For example, particles may deposit on the insulator, causing charge to remain at the clamp surface. This trapped charge may induce a corresponding mirror charge in the polarization coating of the clamped patterning device MA.

[0324] It will be appreciated that any particle trapped on the surface of the electrostatic chuck 100 may result in field emission from the tip of such particle (which may have sharp features), thereby causing a flow of electrons to be emitted toward the positively polarized coating 128 of the patterning device MA (assuming a negative bias is provided to the electrode adjacent to the particle). Alternatively, charge may be transferred by positive or negative hydrogen ions (or indeed any other gas supplied in the gap between the patterning device and the electrostatic chuck 100). It will be appreciated that tunneling ionization may occur in fields with strengths that may exceed 100 MV / m. The ions generated in this manner may then be attracted to the polarized coating 128 of the patterning device MA.

[0325] It should also be noted that any particles present at the surface of the conductive coating 128 of the patterning device MA may be transferred to the insulating surface of the electrostatic chuck 100. In particular, in the presence of an alternating electric field, particles will be understood to adhere more strongly to insulating surfaces than to conducting surfaces.

[0326] Figure 14 Some of the interactions between the particles and the charged surfaces of the electrostatic clamp 100 and the patterning device MA are shown in more detail. In particular, Figure 14 A cross section of a portion of an electrostatic chuck 100 and a portion of a patterning device MA is shown. Several particle-related events are illustrated in greater detail at different portions of the chuck / patterning device interface. In the embodiment shown, electrode 104A is negatively biased, while electrode 104B is positively biased. Field lines are indicated by solid arrows F.

[0327] In the first region X, particle X1 is first associated with the surface of the patterning device MA. Particle X1 is urged toward the body of the electrostatic chuck 100 by the electrostatic field generated between electrode 104A and the surface 208 of the patterning device MA. It will be appreciated that particle X1 is initially positively charged and is therefore attracted toward the negatively biased electrode 104A.

[0328] Similarly, another particle Y1 (this time negatively charged) is shown at region Y. Particle Y1 is initially associated with surface 128 of patterning device MA and is initially negatively charged. Under the influence of the positive voltage at electrode 104B, particle Y1 is pushed towards the insulating surface of electrostatic chuck 100.

[0329] At region W, field emission can be seen due to sharp features on particle W1, causing electrons to be emitted to the conductive surface 128 of patterning device MA. Such a process may result in regions of positive charge W2 remaining at the fixture surface and may also result in an attractive force being generated between the charged particles and the electrode.

[0330] At region V, particle V1 is located on the insulating surface of electrostatic chuck 100. Tunneling ionization may occur at particle V1, generating negative ions. Specifically, under the field generated by the polarization of electrode 104A, electrons can tunnel from the chuck surface through the particle's potential barrier. This process may result in the generation of negative ions, and positive charge V2 may remain on the chuck surface.

[0331] In another region Z, particles Z1 associated with the surface of the fixture 100 can undergo tunneling ionization (e.g., by electrons tunneling from the particles to the fixture surface), thereby generating positive ions, and negative charge Z2 remaining at the fixture surface. Under the influence of the electric field established between the electrode 104B and the patterning device MA, any such positive ions will be attracted to the patterning device MA. As a result, the positive ions (and associated charge) can be transferred to the patterning device.

[0332] It should be understood that the above reference Figure 14 The various scenarios described illustrate several different situations that may occur at the interface between the patterning device MA and the electrostatic chuck 100, but are not exhaustive. However, each of these situations (and others not described) can be mitigated to some extent by using a free charge source. For example, an ionization source (e.g., a VUV source S or an EUV source SO) can be used as a mobile charge source to compensate for some of the accumulated charge associated with the various scenarios described above. Additionally, charge compensation can also be used to remove trapped particles from the electrostatic chuck.

[0333] Now refer to Figures 15a to 15e Describes a procedure by which an electrostatic chuck can be cleaned. Figure 15a , a portion of the electrostatic clamp 100 is shown again. In this example, only two electrodes 104A, 104B are shown. However, it should be understood that the electrostatic clamp 100 may correspond to the above reference Figure 3 The electrostatic clamp described in Figure 15a In the initial state shown, the electrostatic chuck 100 is unbiased. Particles P1 and P2 are trapped on the surface of the chuck 100. Particle P1 is adjacent to electrode 104A, while particle P2 is adjacent to electrode 104B. The ionization source S is positioned adjacent to the chuck 100 and is initially de-energized so that no VUV light is emitted and no hydrogen plasma is generated.

[0334] Now refer to Figure 15b , which shows the first step in the cleaning process of the electrostatic chuck 100, the source S is energized to generate a plasma P. The plasma P extends across the surface of the electrostatic chuck 100, and the intensity decreases with increasing distance from the source. Simultaneously, the electrodes 104A and 104B are energized, such that the electrode 104A is negatively biased and the electrode 104B is positively biased.

[0335] The free charge of the plasma P, combined with the bias applied to the fixture 100, causes charge to be attracted to the surfaces of the fixture 100 adjacent to each of the electrodes 104A, 104B. A positive charge Q1 is formed adjacent to the electrode 104A, while a negative charge Q2 is formed adjacent to the electrode 104B. The trapped particles P1, P2 are charged by the charges Q1, Q2 attracted to the fixture surfaces.

[0336] In the above reference Figure 15b The voltage applied to the electrodes 104A, 104B during the described process may be lower than a typical clamping voltage. For example, while the clamping voltage may be on the order of 1 to 5 kV, the voltage supplied to the electrodes during cleaning operations may be, for example, between about 0.1 and 2 kV.

[0337] Figure 15c Another stage of the cleaning process of the electrostatic clamp 100 is shown. A clean reticle 160 is positioned adjacent to the electrostatic clamp 100. The clean reticle 160 includes a body 162 coated with a conductive layer 164 and finally an insulating layer 166. The insulating layer can be formed of an insulating material such as Kapton. To clamp the clean reticle 160 to the electrostatic clamp 100, zero voltage can be applied to the electrodes 104A and 104B. To release the particles P1, P2, a voltage can be briefly applied to the electrodes 104A, 104B, as shown in FIG. Figure 15c But please note that Figure 15c The configuration shown provides a bias voltage relative to Figure 15b The reverse is shown. That is, a positive voltage is applied to electrode 104A, while a negative voltage is applied to electrode 104B. Power supply S is no longer activated.

[0338] Once these new electric fields are established, any charge previously present at the surface of the fixture 100 will be repelled by the reverse bias voltage applied to the electrodes 104A, 104B. This will cause particles P1, P2 to be repelled from the fixture surface and deposited on the surface of the clean reticle 160. That is, in Figure 15b The charge accumulated on particles P1 and P2 during the charging process is used to cause the particles to Figure 15c The charges Q1, Q2 will also be repelled by the voltage applied to the electrodes 104A, 104B. However, in the absence of a conductive medium through which to move, the charges Q1, Q2 may remain on the surface of the fixture 100.

[0339] Now refer to Figure 15d , the clean mask 160 is removed from the electrostatic chuck 100 together with the particles P1 and P2 deposited on the surface of the clean mask 160. In order to remove, that is, to release the clean mask 160, a method such as Figure 15bThe voltage shown, that is, electrodes 104A and 104B are excited so that electrode 104A is negatively biased and electrode 104B is positively biased. Figure 15d As shown, the voltage applied to the electrodes 104A, 104B is removed. Note that trapped surface charges Q1, Q2 may still remain at the surface of the electrostatic chuck 100.

[0340] Now turn Figure 15e In the final stage of the cleaning process, any residual charge on the surface of the chuck 100 can be removed by again generating an ionization field through the source S to provide plasma P. When the plasma P is provided without any bias applied to the electrodes 104A and 104B, the residual charges Q1 and Q2 on the surface of the electrostatic chuck 100 can be neutralized by the free space charge of the plasma. In other words, any residual charge on the insulator of the chuck 100 can be removed.

[0341] The above cleaning process can also be performed with an EUV source SO that is also used for photolithography exposure. Figure 14 In the process described in sections a-14e, using a separate ionization source S rather than a primary EUV source SO can be particularly beneficial because this allows the generation of mobile charges (e.g., hydrogen plasma) to be independent of the operation of the EUV source SO. Furthermore, using a separate source S allows the generation of compensating mobile charges in a region where EUV is not present or in a volume isolated from the EUV source. This allows charge compensation to be performed at lower pressures (e.g., between 0.0001 and 1 Pa) than the typical pressure of the EUV source SO (e.g., which can operate at pressures of approximately 1 to 10 Pa). Thus, using a secondary ionization source can be inherently cleaner (due to the higher vacuum levels available). Furthermore, the power of the secondary ionization source can be easily adjusted and optimized to achieve clean performance.

[0342] It will also be appreciated that separating the cleaning of the electrostatic chuck from the operation of the EUV source SO allows the cleaning process to be performed in a smaller volume lithography apparatus, rather than in the projection system. This facilitates working in a lower vacuum environment, since the entire volume of the light source SO, illumination system IL, and projection system PS does not need to be evacuated, and only the area used to clean the chuck needs to be maintained at a lower vacuum level.

[0343] The cleaning process described above includes several steps. However, it should be appreciated that not all of these steps are required. For example, cleaning the reticle can be omitted. In the absence of a reticle, any trapped charge can be repelled by reversing the polarity of the fixture electrodes. This process may result in particles being thrown into the environment surrounding the fixture. Thus, the process can preferably be performed in an environment other than the operational lithography apparatus. This cleaning can be performed within a dedicated cleaning tool.

[0344] Furthermore, the clean reticle described above is described as including a metal surface layer covered by a thin dielectric layer facing the fixture. However, both the metal layer and the dielectric layer are optional. However, it should be understood that the use of a dielectric layer facing the fixture can provide maximum particle adhesion for any particles incident on the clean reticle. In particular, such charged particles will be attracted to the dielectric surface by Coulomb attraction and will retain their charge (due to the dielectric surface), resulting in the induction of a mirror charge in the metal surface below the dielectric layer.

[0345] In the foregoing description, various embodiments have been described in which EUV-induced plasmas, or plasmas generated by secondary ionization sources, can be used to reduce the extent to which high free-space fields in a lithography apparatus (or associated tools) can cause problems. Such free-space fields can become problematic during EUV power ramp-up and electrostatic chuck repolarization. However, as described above, by utilizing free charges generated at appropriate times, the negative effects associated with these events can be mitigated or avoided.

[0346] Furthermore, a gradual increase in EUV power supplied to the electrostatic chuck and patterning device can be used to allow any free-space fields to gradually collapse, rather than providing a sudden influx of charge carriers that could result in high transient currents and associated problems (e.g., discharges, particle generation). This effect (i.e., a gradual increase in EUV power at the chuck) can be achieved by gradually moving the masking blade to block EUV from being incident on the patterning device MA. This control can be used to modulate the free charge (i.e., ions or electrons) supplied to the chuck during polarization of the chuck or during repolarization of the chuck. Again, this allows the free-space fields to be removed or reduced in a smoothly controlled manner, rather than changing them abruptly.

[0347] Additionally, EUV-induced plasma or plasma generated by a secondary ionizer can be used during patterning device processing (e.g., patterning device removal) to prevent negative effects associated with voltage amplification due to capacitance changes associated with increased distance between various isolated system components.

[0348] Furthermore, the use of free-space charge has been demonstrated to provide a mechanism by which electrostatic clamps can be cleaned to remove trapped particles from the clamp's surface. This process can be performed in conjunction with specially designed sacrificial cleaning reticles.

[0349] It will be appreciated that the above-described embodiments include numerous significant advantages. Furthermore, in some embodiments, these advantages can be achieved without modifying the structure of existing lithography systems. That is, in some embodiments, existing EUV sources can be used in novel ways to deliver energy at different times during an exposure cycle than is typically the case. This arrangement can be implemented in existing devices without requiring significant hardware modifications, requiring only changes to the control process. Furthermore, the nature of these modifications to the operating protocol can be achieved without significantly impacting the throughput of existing devices. Thus, operating procedures can be changed between existing exposure cycles without significantly affecting cycle length.

[0350] Furthermore, the cleaning mechanism described above for cleaning electrostatic clamps can avoid or at least reduce the negative consequences associated with particles being trapped on surfaces of the clamp, namely, when particles are present between the clamped surfaces, trapped particles (and possibly also trapped charge) can cause unpredictable or drifting clamping forces.

[0351] Furthermore, the chuck cleaning sequence described above can be used to clean chucks when they are not in use within an active lithography apparatus. That is, using an external or secondary plasma source ensures that the chuck can be effectively cleaned without disrupting the normal operation of the lithography apparatus. Furthermore, since it does not rely on an EUV source to assist in the cleaning process, it also allows chuck cleaning to be performed while the lithography apparatus is offline for other purposes (e.g., routine maintenance).

[0352] As described above, the electrostatic clamp comprises electrodes that are provided with a clamping voltage so as to generate an electric field that allows the pattern forming device (i.e., the mask) to be clamped. The electrostatic clamp may comprise a pair of electrodes: a positive electrode and a negative electrode. The clamping voltage provided to the electrodes may be approximately plus or minus 1-10 kV, for example, plus or minus 2 kV. The pairing of the positive and negative electrodes means that the voltage at the surface of the pattern forming device is approximately zero, i.e., the surface is held at a voltage approximately halfway between the positive and negative electrode voltages. However, tolerances associated with various factors may mean that the voltage at the surface of the pattern forming device is not actually zero volts compared to the rest of the system. Instead, there may be a voltage imbalance, which may cause charge to accumulate on the pattern forming device. It will be understood that the pattern forming device may be more generally referred to as an assembly.

[0353] The electrodes are coated with a material having an ultra-low thermal expansion coefficient (e.g., Corning manufactured) with a thickness of about 100 μm. ). As described above, the clamp may include a substantially flat surface provided with protrusions (which may be referred to as burls). The protrusions may ensure that the spacing between the substantially flat surface of the clamp and the clamped surface of the patterning device exceeds a minimum value (e.g., 10 microns) even during clamping. However, it will be understood that during clamping, the surface of the protrusions will contact the clamped patterning device and, therefore, the minimum spacing between the surface of the clamp and the surface of the patterning device during clamping is zero. The spacing between the substantially flat surface of the clamp and the clamped surface of the patterning device determines the capacitive coupling of the electrode to the patterning device. Due to tolerances in the output of the high voltage amplifier to the electrodes or tolerances in the capacitance of the individual electrodes (e.g., due to thickness variations in the micrometer range of coating materials such as ULE), the surface potential of the patterning device may deviate from zero by up to 10 V. For example, a deviation within a tolerance of 0.1% of the 2 kV high voltage amplifier output to the first clamping electrode may result in a voltage of 4 V appearing on the surface of the patterning device. Similarly, a deviation of plus or minus 1 μm (ie, 1%) in the thickness of the coating material (100 μm) on the electrode will result in a more severe imbalance.

[0354] One issue associated with holding the patterning device at a non-zero voltage is that charged particles may be attracted to the surface of the patterning device, which can lead to imaging defects. Furthermore, holding the patterning device at a non-zero voltage during EUV exposure can cause charge to accumulate on the backside of the patterning device. For example, during EUV-ON, a plasma (e.g., a hydrogen plasma) can be generated by the interaction of EUV radiation and the gases present in the scanner. The plasma includes free charges (ions) that can be transferred to the surface of the patterning device, particularly the backside, due to the electric field generated by the clamp. That is, if the surface of the patterning device is at a non-zero voltage during EUV exposure, the potential field established between the grounded portion of the system and the non-zero patterning device will cause the free charges generated in the plasma to flow toward (or away from) the patterning device, causing it to become charged (positively or negatively). Subsequently, when the clamping voltage is removed (and when the plasma is no longer present), the residual charge accumulated on the surface of the patterning device can remain, resulting in a potential difference between the patterning device and the rest of the system (e.g., grounded components of the system).

[0355] Another problem associated with the patterning device being at a non-zero voltage is that, as described above, during unloading of the patterning device, any charge remaining on the surface of the patterning device induces a voltage therein that is significantly amplified (and the capacitance is therefore reduced) as the patterning device moves away from the fixture. This can lead to discharges between the back surface of the patterning device (i.e., the side facing the fixture) and grounded portions of the system nearby. Furthermore, particles present on the front surface of the patterning device may be ejected from the surface at high speed and cause damage within the system. It is therefore desirable to provide methods for alleviating one or more of the problems identified above.

[0356] To address the challenges of electrostatic attraction of particles to the patterning device (reticle) and / or voltage amplification and discharge during unloading, it is recommended to ground the patterning device (i.e., connect it to earth ground) so that the net voltage associated with the patterning device is zero volts. In particular, it may be preferable to provide a "virtual ground" for the patterning device. In other words, rather than providing a physical ground connection to the patterning device, it is desirable to balance the voltages induced on the patterning device by adjusting the individual voltages of the electrodes in the electrostatic chuck to produce a total of zero volts. In this way, no additional hardware is required, and there is no risk of losing the physical ground connection.

[0357] In a first method of providing a virtual ground, as Figure 16 As shown, in step 200, for example, using reference Figure 9a and 9b The replacement assembly provides a patterning device adjacent to an electrostatic chuck. The electrostatic chuck is then controlled at step 202 such that the patterning device is clamped to the electrostatic chuck. Specifically, a first clamping voltage may be applied to a first electrode of the chuck to induce an electric field that is used to clamp the patterning device to the chuck.

[0358] The patterning device is exposed to EUV radiation at step 204 and then released and removed from the vicinity of the electrostatic clamp at step 206. As described above, charge may accumulate on the back side of the patterning device during clamping and exposure. Figures 10 to 13 As described above, as the patterning device moves away from the fixture, the voltage associated with the surface of the patterning device (e.g., the back of the patterning device) can increase significantly. This voltage is measured at step 208. It should be understood that the voltage associated with a different portion of the patterning device can be measured instead. For example, the voltage associated with the front of the patterning device can be measured. As described above, particularly with reference to Figure 12c , the front voltage can be smaller than the back voltage. For example, due to the internal capacitance of the patterning device, the front voltage can be about five times smaller than the back voltage.

[0359] Based on the measured voltage, an adjustment to the first clamping voltage is determined at step 210. It will be appreciated that any measured offset voltage can be used to calculate the adjustment voltage. In the event that the determined adjustment is not zero, the first clamping voltage can be adjusted at step 212 based on the determined adjustment. For example, the output of a high-voltage amplifier connected to the first electrode can be adjusted. It will be appreciated that the adjustment can be determined based on a voltage to be added to or subtracted from the electrode voltage, a percentage change from the current voltage, a gain change of the high-voltage power amplifier, or any other suitable term as will be apparent to those skilled in the art. In this manner, it can be ensured that the voltage induced in the patterning device is balanced at zero volts.

[0360] It will be appreciated that the electrostatic chuck may include more than one electrode, and in particular, the chuck may include two to n electrodes. In this case, step 210 also includes determining adjustments to the second to nth clamping voltages, and step 212 may include adjusting any or all of the second to nth clamping voltages in accordance with the determined adjustments. For example, if it is determined that the reticle surface is induced to be maintained at a voltage of +4V during clamping, 4V may be subtracted from the clamping voltage of each of the positive and negative clamping electrodes. Alternatively, adjustments may be made only to the positive (or negative) clamping electrode configured to induce a voltage of a certain magnitude and direction at the reticle surface, as described above, at a voltage substantially between the potentials of the clamping electrodes, close to zero volts.

[0361] To verify that the correct adjustment has been made, the method can be repeated. It should be understood that the same patterning device can be used during the verification method. Alternatively, another patterning device can be used. Such a verification process can be repeated until a satisfactory result is determined (e.g., until the voltage induced in the patterning device during the clamping period is substantially zero, or below a predetermined threshold).

[0362] In some implementations, the measuring of the voltage is performed while the patterning device is clamped by a clamp. Figure 17 An example flow chart of this implementation is shown schematically in FIG. Figure 18 The system shown is executed.

[0363] Figure 18 The system shown corresponds largely to Figure 9a In particular, Figure 18The system includes a fixture 100 to which a patterning device MA is clamped. In this particular configuration, the fixture 100 includes four electrodes 104A, 104B, 104C, and 104D. The clamped patterning device MA is shown spaced apart from a replacement assembly 150. The patterning device MA includes a front side 126 facing the replacement assembly 150 and a back side 128 adjacent the fixture 100. The replacement assembly includes a replacement device 152 supported by a support structure 154. The replacement assembly 150 is configured to move the patterning device MA toward and away from the fixture 100, for example, to enable the patterning device MA to be replaced with a replacement patterning device, or to be cleaned.

[0364] Reference again Figure 17 , at step 300, a patterning device is provided adjacent to the fixture. Again, this can be accomplished using the replacement assembly 150. At a subsequent step 302, the fixture is controlled to clamp the patterning device. In particular, a first clamping voltage can be provided to a first electrode of the fixture, thereby inducing an electric field that is used to clamp the patterning device to the fixture. The system includes a voltage monitor 180 configured to measure a voltage associated with a portion of the patterning device. In some implementations of the method, the measured voltage associated with the portion of the patterning device is a front side voltage. In some implementations, the voltage monitor 180 can be a portion of the replacement assembly 150 that is in contact with the front side of the patterning device, such as Figure 18 As shown. However, the voltage monitor 180 can be located at any suitable location within the system. In some implementations, the voltage monitor 180 can be an electrostatic voltmeter. The electrostatic voltmeter can be arranged to measure the voltage associated with the front side of the patterning device without contacting the front side of the patterning device, thereby avoiding the transfer of charge to (or from) the patterning device during the measurement. A similar measurement arrangement (i.e., using an electrostatic voltmeter) can be used to measure the voltage at the front side or back side of the patterning device during the offline method described above. Of course, it should be understood that Figure 18 The diagrams shown are merely schematic. In particular, it should be understood that the illustrated connection from the voltage monitor 180 to the front face of the patterning device is used to illustrate that the voltage at the front face of the patterning device is being measured, but does not necessarily imply a physical connection from the voltage monitor 180 to the patterning device. As described above, the voltage monitor 180 can be an electrostatic voltmeter arranged to measure the voltage associated with the front face of the patterning device without physically contacting it.

[0365] In contrast to the offline method described above, in this implementation, a voltage measurement is performed while the patterning device is clamped by the fixture in step 304. Based on the measured voltage, an adjustment to the first clamping voltage is determined in step 306. If the determined adjustment is not zero, the first clamping voltage is adjusted in step 308. After the adjustment step 308, the method returns to the step of measuring the voltage associated with a portion of the patterning device. The voltage is measured and the adjustment value is determined. In this way, it can be verified whether the first clamping voltage has been properly adjusted. It should be understood that if the determined adjustment is zero, the method can end after step 306. Alternatively, further voltage measurements can be performed at appropriate intervals to ensure that the virtual ground is maintained correctly. For example, further measurements can be performed periodically between EUV exposures by providing a replacement assembly adjacent to the patterning device without removing the patterning device from the fixture.

[0366] It should be understood that if the electrostatic chuck has more than one electrode, for example, two to n electrodes, each with its own clamping voltage, then step 306 may also include determining adjustments to the second ... nth clamping voltages based on the measured voltages. Similarly, step 308 may include adjusting any or all of the second ... nth clamping voltages based on the determined adjustments.

[0367] It will be further appreciated that steps 300, 302, 304 and 306 correspond to Figure 16 The method shown is steps 200, 202, 208 and 210. Figure 17 The method shown is performed in real time, with Figure 16 Unlike the offline method shown, EUV exposure of the patterning device is not required in order to "pin" the charge in place on the patterning device for measurement, however, it will be appreciated that Figure 17 The method may include EUV exposure as an optional step. In this case, if the voltage monitor 180 forms part of the replacement assembly 150, the replacement assembly 150 will have to include an aperture to allow EUV radiation to reach the clamped patterning device. Alternatively, if the voltage monitor 180 forms part of another system component, the replacement assembly can be moved away from the patterning device so that EUV radiation can reach the patterning device without being obstructed by the replacement assembly. It will also be appreciated that free charge generated by alternative means other than EUV exposure (e.g., a secondary ionization source) can be used to provide charge to the clamped patterning device.

[0368] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Other possible applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0369] Although specific reference may be made herein to embodiments of the present invention in the context of a lithographic apparatus, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions. Indeed, embodiments of the present invention may form part of any apparatus that uses an electrostatic clamp.

[0370] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications such as imprint lithography where the context permits.

[0371] Where the context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read-only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), etc. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and that such actions are actually caused by a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., and that performing such actions may cause an actuator or other device to interact with the real world.

[0372] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in ways other than those described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. An apparatus comprising an electrostatic chuck for holding a component and a mechanism for generating free charge adjacent to the electrostatic chuck: wherein the mechanism for generating free charge is configured to generate free charge adjacent to the electrostatic chuck during a transition from a first excited state of the electrostatic chuck to a second excited state of the electrostatic chuck, The component is part of a patterning device in a lithographic apparatus.

2. The device according to claim 1, wherein: The electrostatic clamp comprises a clamping region configured to clamp the component; and When a component is clamped, a clamping electric field is generated between the clamping area and the component.

3. The device according to claim 2, wherein: The electrostatic chuck further includes a non-clamping region; and When the component is clamped by the clamping area, a secondary electric field is generated around the non-clamping area.

4. The apparatus of claim 2 or 3, wherein the electrostatic clamp comprises at least one electrode, wherein when a component is clamped by the electrostatic clamp, a clamping voltage is applied to the at least one electrode such that the clamping electric field is generated between the clamping region and the component.

5. The apparatus of claim 4 , wherein the electrostatic chuck further comprises at least one contact configured to provide an electrical connection to the at least one electrode, wherein the mechanism for generating free charge is configured to generate free charge adjacent to the at least one contact during the transition from the first excited state of the electrostatic chuck to the second excited state of the electrostatic chuck.

6. A device according to claim 4 or 5, wherein in the first excitation state, a voltage having a first polarity is applied to the at least one electrode, and in the second excitation state, a voltage having a second polarity opposite to the first polarity is applied to the at least one electrode.

7. The apparatus of claim 6, wherein the electrostatic chuck comprises at least two electrodes, and wherein: In the first excited state, a voltage having the first polarity is applied to a first electrode among the electrodes, and a voltage having the second polarity is applied to a second electrode among the electrodes, and In the second excited state, a voltage having the second polarity is applied to the first electrode among the electrodes, and a voltage having the first polarity is applied to the second electrode among the electrodes.

8. The apparatus of any preceding claim, wherein the clamp is configured such that in each of the first and second excited states, a component can be clamped by the electrostatic clamp.

9. The apparatus of any preceding claim, wherein the means for generating free charge adjacent to the electrostatic chuck comprises: a gas source, and a source of ionizing radiation configured to ionize gas provided by the gas source.

10. The apparatus of claim 9, wherein the ionizing radiation source comprises a source selected from the group consisting of: an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

11. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an apparatus according to any preceding claim, and wherein the patterning device comprises the component to be clamped.

12. The lithographic apparatus according to claim 11, further comprising: an irradiation system configured to condition the radiation beam; wherein the electrostatic clamp is configured to hold the patterning device, the patterning device being capable of imparting a pattern to the radiation beam in a cross-section of the radiation beam to form a patterned radiation beam; a substrate table configured to hold a substrate; as well as A projection system is configured to project the patterned radiation beam onto the substrate.

13. The lithographic apparatus according to claim 12, wherein: The lithographic apparatus is configured to perform a plurality of imaging exposures, during which the radiation beam is incident on the patterning device and during which the patterned radiation beam is projected onto the substrate, the electrostatic chuck being configured to hold the patterning device during the imaging exposures; as well as The electrostatic chuck is configured to transition from the first excited state to the second excited state between successive imaging exposures in the plurality of imaging exposures.

14. A lithographic system comprising the lithographic apparatus according to claim 13, the lithographic system further comprising a radiation source configured to generate the radiation beam, wherein the mechanism for generating free charges comprises a secondary ionizing radiation source, the secondary ionizing radiation source being selected from the group consisting of: an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

15. A lithographic system comprising the lithographic apparatus according to claim 13, the lithographic system further comprising a radiation source configured to generate the radiation beam, wherein the mechanism for generating free charges comprises the radiation source.

16. A lithography system according to claim 15, wherein the lithography system is further configured to perform at least one non-imaging exposure, during which the radiation beam is incident on the pattern forming device and no radiation is projected onto the substrate during the at least one non-imaging exposure; the non-imaging exposure is performed between consecutive imaging exposures in the multiple imaging exposures.

17. The lithographic system of claim 16, wherein the transition from the first excited state of the electrostatic chuck to the second excited state of the electrostatic chuck is performed during the non-imaging exposure.

18. The lithographic system of claim 17, wherein the lithographic system is controlled such that a greater amount of radiation is incident on the patterning device during each imaging exposure than during the non-imaging exposure.

19. The lithographic system of claim 18, wherein the amount of radiation incident on the patterning device increases gradually from a non-imagewise exposure to an imagewise exposure of the plurality of imagewise exposures.

20. A method of operating an apparatus comprising an electrostatic chuck for holding a component and a mechanism for generating free charge adjacent to the electrostatic chuck, the method comprising: controlling the electrostatic chuck to have a first excited state; controlling the electrostatic chuck to have a second excited state; as well as controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic chuck during a transition from the first excited state to the second excited state, The component is part of a patterning device in a lithographic apparatus.

21. A lithographic apparatus comprising: an irradiation system configured to condition the radiation beam; a support structure configured to support a patterning device capable of imparting a pattern to the radiation beam in a cross-section of the radiation beam to form a patterned radiation beam, the support structure comprising an electrostatic chuck configured to hold a component and a mechanism for generating free charge adjacent to the electrostatic chuck, wherein the mechanism for generating free charge is configured to generate free charge adjacent to the electrostatic chuck during a transition from a first excited state of the electrostatic chuck to a second excited state of the electrostatic chuck, wherein the component is part of the patterning device; a substrate table configured to hold a substrate; as well as a projection system configured to project the patterned radiation beam onto the substrate; in: the lithographic apparatus being configured to perform a pre-imaging exposure during which the radiation beam is incident on the patterning device and during which no radiation is projected onto the substrate, wherein an amount of radiation incident on the patterning device is gradually increased during the pre-imaging exposure; and The lithographic apparatus is configured to perform an imagewise exposure in which the radiation beam patterned by the patterning device is projected onto the substrate.

22. The lithographic apparatus of claim 21, wherein the pre-imaging exposure comprises a burst comprising a plurality of radiation pulses.

23. The lithographic apparatus of claim 22, wherein the gradual increase in the amount of radiation during the pre-imaging exposure is configured to be provided over a plurality of the radiation pulses.

24. The lithographic apparatus of claim 23, wherein the gradual increase in radiation during the pre-imaging exposure is configured to be provided over at least 1000 radiation pulses.

25. The lithographic apparatus of any one of claims 22 to 24, wherein the gradual increase in radiation during the pre-imaging exposure is configured to be provided substantially linearly over a plurality of the radiation pulses.

26. A lithographic device according to any one of claims 21 to 25, wherein during a first portion of the pre-imaging exposure having a predetermined duration, the radiation beam is controlled to deliver a first dose of radiation to the pattern forming device, the first dose comprising a dose that is less than about 10% of the imaging radiation dose delivered to the pattern forming device during the first portion of the imaging exposure, the first portion of the pre-imaging exposure having the predetermined duration.

27. The lithographic apparatus of claim 26, wherein the amount of radiation incident on the patterning device increases gradually from the first portion of the pre-imagewise exposure to the imagewise exposure.

28. The lithographic apparatus of claim 26 or 27, wherein at least 1000 radiation pulses are delivered between the start of the first part of the pre-imaging exposure and the start of the imaging exposure.

29. The lithographic apparatus according to any one of claims 21 to 28, further configured to perform a first imaging exposure and a second imaging exposure, wherein: The pre-imagewise exposure immediately precedes the second imagewise exposure; and Between the first imaging exposure and the pre-imaging exposure, the lithographic apparatus is further configured to perform a non-imaging exposure during which the radiation beam is incident on the patterning device and during which no radiation is projected onto the substrate.

30. A lithographic apparatus according to claim 29 as dependent on claim 26, wherein during a first portion of the non-imaging exposure having the predetermined duration, the radiation beam is controlled to deliver a third dose of radiation to the pattern forming device, the third dose comprising a dose that is less than about 10% of the imaging radiation dose, and the apparatus is configured to cause the electrostatic clamp to transition from a first excited state to a second excited state during the non-imaging exposure.

31. The lithographic apparatus of any one of claims 21 to 29, further configured to cause the electrostatic chuck to transition from a first excited state to a second excited state during the pre-image exposure.

32. A lithographic system comprising the lithographic apparatus according to any one of claims 21 to 31, the lithographic system further comprising a radiation source configured to generate the radiation beam.

33. A method of operating a lithographic apparatus, the lithographic apparatus comprising: an irradiation system configured to condition the radiation beam; a support structure configured to support a patterning device capable of imparting a pattern to the radiation beam in a cross-section of the radiation beam to form a patterned radiation beam, the support structure comprising an electrostatic chuck configured to hold a component and a mechanism for generating free charge adjacent to the electrostatic chuck, wherein the mechanism for generating free charge is configured to generate free charge adjacent to the electrostatic chuck during a transition from a first excited state of the electrostatic chuck to a second excited state of the electrostatic chuck, wherein the component is part of the patterning device; a substrate table configured to hold a substrate; as well as a projection system configured to project the patterned radiation beam onto the substrate; The method comprises: causing the lithographic apparatus to perform a pre-imaging exposure during which the radiation beam is incident on the patterning device and during which no radiation is projected onto the substrate, wherein an amount of radiation incident on the patterning device gradually increases during the pre-imaging exposure; as well as The lithographic apparatus is caused to perform an imagewise exposure in which the radiation beam patterned by the patterning device is projected onto the substrate.

34. An apparatus comprising an electrostatic chuck for holding a component and a mechanism for generating free charge adjacent the electrostatic chuck, the apparatus having a first configuration in which the component is held by the electrostatic chuck and a second configuration in which the component is spaced apart from the electrostatic chuck, wherein: the apparatus being configured to be in the first configuration at a first point in time and in the second configuration at a second point in time subsequent to the first point in time; as well as The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic chuck and / or the component at a third time point between the first time point and the second time point, The component is part of a patterning device in a lithographic apparatus.

35. The apparatus of claim 34, wherein the mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic chuck and / or the component to prevent a potential difference between the electrostatic chuck and the component from exceeding a predetermined threshold.

36. The device of claim 35, wherein the predetermined threshold is determined based on a pressure in the device.

37. The apparatus of claim 36, wherein the third time point is selected to prevent the potential difference between the electrostatic chuck and the component from exceeding the predetermined threshold.

38. The apparatus of any one of claims 35 to 37, wherein the predetermined threshold is approximately 250 volts.

39. An apparatus according to any one of claims 34 to 38, wherein the apparatus is configured to be in the second configuration at the third point in time.

40. The apparatus of any one of claims 34 to 39, wherein at the third point in time, a minimum separation between a surface of the fixture and a surface of the component is greater than about 10 microns.

41. The apparatus of any one of claims 34 to 40, wherein at the third point in time, a minimum spacing between a surface of the clamp and a surface of the component is less than a predetermined spacing.

42. The device of claim 41, wherein the predetermined spacing is approximately 200 microns.

43. The apparatus of any one of claims 34 to 38, wherein the means for generating free charges is configured to generate free charges adjacent to the electrostatic chuck and / or the component when the apparatus is configured in the first configuration. The apparatus of claim 43 , wherein the apparatus is configured to be in the first configuration at the third point in time.

45. The apparatus of any one of claims 34 to 44, wherein the electrostatic clamp comprises a clamping region configured to clamp the component, wherein a clamping electric field is generated between the clamping region and the component when the component is clamped.

46. ​​The apparatus of claim 45, wherein the electrostatic clamp comprises at least one electrode, wherein when a component is clamped by the electrostatic clamp, a clamping voltage is applied to the at least one electrode such that the clamping electric field is generated between the clamping region and the component.

47. The apparatus of any one of claims 34 to 46, wherein the means for generating free charge adjacent to the electrostatic chuck comprises: a gas source, and a source of ionizing radiation configured to ionize gas provided by the gas source.

48. The apparatus of claim 47, wherein the ionizing radiation source comprises a source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

49. The apparatus of any one of claims 34 to 48, further comprising a component replacement assembly configured to remove the component from the electrostatic chuck.

50. The apparatus of claim 49, wherein the component replacement assembly is configured to control a spacing between the component and the electrostatic chuck.

51. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an apparatus according to any one of claims 34 to 50.

52. The lithographic apparatus according to claim 51 , further comprising: an irradiation system configured to condition the radiation beam; wherein the electrostatic clamp is configured to hold the patterning device, the patterning device being capable of imparting a pattern to the radiation beam in a cross-section of the radiation beam to form a patterned radiation beam; a substrate table configured to hold a substrate; as well as A projection system is configured to project the patterned radiation beam onto the substrate.

53. A lithography system comprising a lithography apparatus according to claim 52, the lithography system further comprising a radiation source configured to generate the radiation beam, wherein the mechanism for generating free charges comprises a secondary ionizing radiation source, the secondary ionizing radiation source being selected from the group consisting of: an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

54. A lithographic system comprising the lithographic apparatus according to claim 52, the lithographic system further comprising a radiation source configured to generate the radiation beam, wherein the mechanism for generating free charges comprises the radiation source.

55. A method of operating an apparatus comprising an electrostatic chuck and a mechanism for generating free charge adjacent to the electrostatic chuck, the method comprising: providing an assembly adjacent to the electrostatic chuck; controlling the electrostatic chuck to have a first configuration in which the component is clamped by the electrostatic chuck at a first point in time; controlling the electrostatic chuck to have a second configuration in which the component is spaced apart from the electrostatic chuck at a second time point after the first time point; as well as controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic chuck and / or the component at a third time point between the first time point and the second time point, The component is part of a patterning device in a lithographic apparatus.

56. An apparatus comprising an electrostatic chuck for clamping a component and a mechanism for generating free charge adjacent the electrostatic chuck, the apparatus having a first configuration in which a voltage having a first polarity is applied to at least one clamping electrode and no component is clamped by the electrostatic chuck, and a second configuration in which a voltage having a second polarity opposite to the first polarity is applied to the at least one clamping electrode, wherein: the apparatus being configured to be in the first configuration at a first point in time and in the second configuration at a second point in time subsequent to the first point in time; as well as the mechanism for generating free charge being configured to generate the free charge adjacent to the electrostatic chuck when the electrostatic chuck is in the first configuration, and not to generate the free charge adjacent to the electrostatic chuck when the electrostatic chuck is in the second configuration, The component is part of a patterning device in a lithographic apparatus.

57. The apparatus of claim 56, wherein a component is provided adjacent to the electrostatic chuck during transition from the first configuration to the second configuration.

58. The apparatus of claim 57, wherein the component is clamped by the electrostatic clamp when the clamp is in the second configuration.

59. An apparatus according to any one of claims 56 to 58, wherein the apparatus has a third configuration in which no voltage is applied to the at least one clamping electrode and no component is clamped by the electrostatic clamp; and the apparatus is configured to: be in the third configuration at a third time point after the second time point.

60. The apparatus of claim 59, wherein the mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic chuck when the electrostatic chuck is in the third configuration.

61. An apparatus according to claim 59 or 60, wherein in the third configuration no component is provided adjacent the electrostatic chuck.

62. The apparatus of any one of claims 56 to 61, further comprising a component replacement assembly adjacent the electrostatic clamp configured to support a component.

63. The apparatus of claim 62, wherein the component replacement assembly is configured to control a spacing between the component and the electrostatic chuck.

64. The apparatus of claim 63, wherein the component replacement assembly is configured to provide the component adjacent to the electrostatic chuck between the first point in time and the second point in time.

65. The apparatus of claim 63 or 64 as appended to claim 59, wherein the component replacement assembly is configured to remove the component from the vicinity of the electrostatic chuck between the second point in time and the third point in time.

66. The apparatus of any one of claims 56 to 65, wherein the electrostatic clamp comprises a clamping region configured to clamp a component, wherein a clamping electric field is generated between the clamping region and the component when the component is clamped.

67. The apparatus of any one of claims 56 to 66, wherein the mechanism for generating free charges adjacent to the electrostatic chuck comprises: a gas source, and a source of ionizing radiation configured to ionize gas provided by the gas source.

68. The apparatus of claim 67, wherein the ionizing radiation source comprises a source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

69. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an apparatus according to any one of claims 56 to 68, and wherein the electrostatic clamp is configured to clamp the patterning device during a lithographic operation.

70. The lithographic apparatus according to claim 69, further comprising: an irradiation system configured to condition the radiation beam; wherein the electrostatic clamp is configured to hold the patterning device, the patterning device being capable of imparting a pattern to the radiation beam in a cross-section of the radiation beam to form a patterned radiation beam; a substrate table configured to hold a substrate; as well as A projection system is configured to project the patterned radiation beam onto the substrate.

71. A lithography system comprising a lithography apparatus according to claim 70, the lithography system further comprising a radiation source configured to generate the radiation beam, wherein the mechanism for generating free charges comprises a secondary ionizing radiation source, the secondary ionizing radiation source being selected from the group consisting of: an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

72. A lithographic system comprising the lithographic apparatus according to claim 70, the lithographic system further comprising a radiation source configured to generate the radiation beam, wherein the mechanism for generating free charges comprises the radiation source.

73. A method of operating a device, the device comprising: an electrostatic chuck for clamping a component, the electrostatic chuck comprising at least one clamping electrode; and a mechanism for generating free charge adjacent to the electrostatic chuck, the method comprising: controlling the electrostatic chuck to have a first configuration in which, at a first point in time, a voltage having a first polarity is applied to the at least one chucking electrode and no component is chucked by the electrostatic chuck; controlling the electrostatic chuck to have a second configuration in which, at a second time point after the first time point, a voltage having a second polarity opposite to the first polarity is applied to the at least one chucking electrode; controlling the mechanism for generating free charge to generate free charge adjacent the electrostatic chuck when the chuck is in the first configuration; and controlling the mechanism for generating free charges so as not to generate free charges adjacent to the electrostatic chuck when the electrostatic chuck is in the second configuration, The component is part of a patterning device in a lithographic apparatus.

74. The method of claim 73, further comprising: A component is provided adjacent to the electrostatic chuck when the electrostatic chuck is in the second configuration.

75. The method of claim 73 or 74, further comprising controlling the apparatus to have a third configuration in which no voltage is applied to the at least one clamping electrode at a third time point after the second time point.

76. The method of claim 75, further comprising: The component is removed from the vicinity of the electrostatic chuck between the second time point and the third time point.

77. The method of claim 75 or 76, further comprising controlling the mechanism for generating free charges to generate free charges adjacent the electrostatic chuck when the electrostatic chuck is in the third configuration.

78. A method of operating an apparatus according to any one of claims 1 to 10, the method comprising: a) providing the component adjacent to the electrostatic chuck; b) controlling the electrostatic chuck to provide a first clamping voltage to a first electrode of the electrostatic chuck, so that the component is clamped by the electrostatic chuck; c) measuring a voltage associated with a portion of the component; as well as d) determining an adjustment to the first clamping voltage based on the measured voltage.

79. The method of claim 78, further comprising: b1) exposing the assembly to radiation while the assembly is held by the electrostatic chuck; b2) controlling the electrostatic chuck so that the component is released from the electrostatic chuck; as well as b3) Removing the component from the vicinity of the electrostatic chuck.

80. The method of claim 79, further comprising: e) adjusting the first clamping voltage according to the determined adjustment; as well as f) Repeat steps a) to c) to verify the adjustment.

81. The method of claim 80, wherein repetitions of steps a) through c) are performed using additional components.

82. The method of claim 78, further comprising: After determining the adjustment to the first clamping voltage, adjusting the first clamping voltage according to the determined adjustment; as well as A voltage associated with the portion of the component is measured.

83. The method of any one of claims 78 to 82, wherein the electrostatic chuck further comprises a second electrode, the method further comprising: Based on the measured voltage, an adjustment to a second clamping voltage to be provided to the second electrode is determined.

84. The method of claim 83, wherein the first clamping voltage and the second clamping voltage have different values.

85. A system for virtually grounding a component, comprising: An apparatus comprising an electrostatic chuck configured to clamp a component and a mechanism for generating free charge adjacent to the electrostatic chuck, wherein the mechanism for generating free charge is configured to: generate free charge adjacent to the electrostatic chuck during a transition from a first excited state of the electrostatic chuck to a second excited state of the electrostatic chuck, the electrostatic chuck comprising a first electrode configured to receive a first clamping voltage; a voltage monitor configured to measure a voltage associated with a portion of the assembly; as well as a computing unit configured to determine an adjustment to the first clamping voltage based on the measured voltage, The component is part of a patterning device in a lithographic apparatus.

86. The system of claim 85, further comprising a support assembly configured to support the component, wherein the support assembly includes the voltage monitor.

87. The system of claim 85 or 86, wherein the electrostatic clamp comprises a second electrode configured to receive a second clamping voltage, and the computing unit is further configured to determine an adjustment to the second clamping voltage based on the measured voltage.

88. The system of claim 87, wherein the first clamping voltage and the second clamping voltage have different values.

89. The system of any one of claims 85 to 88, wherein the voltage monitor is an electrostatic voltmeter.

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