Lithographic apparatus and related methods

Abstract

A lithographic apparatus includes: a plurality of optical elements defining an optical path; a first body; a second body; and a voltage source. The lithographic apparatus may be an extreme ultraviolet (EUV) lithographic apparatus. A plurality of optical elements defining an optical path are arranged to receive a beam of radiation, project the beam of radiation onto a reticle in order to pattern the beam of radiation and form an image of the reticle on a substrate. The first body and the second body are proximate the optical path. The voltage source is arranged to apply a potential difference between the first and second bodies. In particular, the first body, the second body and / or the voltage source are arranged to control the flux and / or energy distribution of ions incident on the first body by a plasma formed by the radiation beam in the optical path.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22172837.1 filed on May 11, 2022 and EP application 22197338.1 filed on September 23, 2022, and the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a lithographic apparatus and related methods of operating the lithographic apparatus. In particular, the present invention relates to an extreme ultraviolet (EUV) lithographic apparatus that can form a hydrogen plasma. The present invention relates to an apparatus and method for controlling the flux and / or energy distribution of plasma ions incident on a body within the lithographic apparatus. Background Art

[0004] A lithographic apparatus is a machine for applying a desired pattern onto a substrate. A lithographic apparatus can be used to manufacture integrated circuits (ICs), etc. A lithographic apparatus can, for example, project a pattern from a pattern forming device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0005] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of a feature that can be formed on that substrate. A lithographic apparatus that uses EUV radiation (electromagnetic radiation having a wavelength in the range of 4-20 nm) can be used to form smaller features on a substrate than a conventional lithographic apparatus (which can use electromagnetic radiation having a wavelength of 193 nm, for example).

[0006] In use, the parts of the EUV lithography apparatus that propagate EUV radiation can be maintained at a pressure far below atmospheric pressure, for example, using a container evacuated by a vacuum pump. It is known to provide hydrogen (at low pressure) in these parts of the EUV lithography apparatus. The EUV radiation beam is typically a pulsed radiation beam. As each radiation pulse propagates through an optical path in the lithography apparatus, gas molecules in the optical path tend to ionize, thereby forming a plasma in the optical path. The plasma can diffuse away from the optical path so that the plasma extends slightly outside the volume through which the radiation beam propagates.

[0007] It may be desirable to provide an apparatus that obviates or mitigates one or more problems associated with the prior art. Summary of the invention

[0008] According to a first aspect of the present invention, there is provided a lithography apparatus, comprising: a plurality of optical elements defining an optical path, the optical path being arranged to receive a radiation beam, project the radiation beam onto a mask, thereby patterning the radiation beam and forming an image of the mask on a substrate; a first body close to the optical path; a second body close to the optical path; and a voltage source, the voltage source being arranged to apply a potential difference across the first and second bodies; wherein the first body, the second body and / or the voltage source are arranged to control the flux and / or energy distribution of ions incident on the first body by a plasma formed by the radiation beam in the optical path.

[0009] The lithographic apparatus may be an extreme ultraviolet (EUV) lithographic apparatus. In use, the optical path may be maintained at a pressure well below atmospheric pressure, for example, using a container evacuated using a vacuum pump. It is known to provide hydrogen (at low pressure) in the optical path. The radiation beam is typically a pulsed radiation beam. As each radiation pulse propagates through the optical path, gas molecules in the optical path tend to ionize, thereby forming a plasma in the optical path. The plasma may diffuse away from the optical path so that the plasma extends slightly beyond the volume through which the radiation beam propagates.

[0010] The first object may be a sensitive object, so it may be necessary to control the flux and / or energy distribution of ions incident on the first object from the plasma. For example, the first object may be a mirror or a sensor in a lithography device.

[0011] The second body may comprise one or more walls of the lithographic apparatus (e.g., it may form part of a housing for components of the lithographic apparatus, such as mirrors, etc.). Typically, the second body may be further from the optical path than the first body. It will be appreciated that the second body may comprise a plurality of separate parts (e.g., walls of the lithographic apparatus).

[0012] It may be necessary to reduce the flux and energy of ions incident on the first body. For example, ions with high energy may pose a risk of sputtering, resulting in undesirable degradation of the first body. In addition, ions (e.g., hydrogen ions) may react with components of the first body (e.g., silicon from glass and stainless steel components and magnesium from aluminum components) to form volatile hydrides. Such volatile hydrides present in the lithographic equipment may be incident on the surface of the reflector in the lithographic equipment, causing the components etched from the first body to be deposited on the reflector. Such deposits may absorb EUV radiation, which is undesirable. In addition, such deposits may be oxidized and may therefore absorb more EUV radiation. Typically, hydrogen ions do not etch the outer native oxide layer of a material (e.g., silicon). However, ions with sufficient kinetic energy can penetrate these oxide layers to reach the underlying bulk material, and the ions can react with the bulk material to form volatile hydrides (e.g., silane). If the energy of the hydrogen ions is below a threshold kinetic energy, so that they cannot penetrate the native oxide layer outside the material, the formation of such volatile hydrides can be stopped.

[0013] In order to control the flux and energy distribution of ions incident on the first body, one might naively try to apply a bias voltage or potential to the first body. For example, if one wishes to prevent ions in the plasma from striking the first body, a positive voltage may be applied to the first body to repel the ions. Similarly, if one wishes to increase the energy or flux of ions from the plasma striking the first body, a negative voltage may be applied to the first body to attract the ions. However, for an object close to the plasma, the object is in electrical contact with the plasma (through the plasma sheath). Therefore, applying a bias voltage to only one object near the plasma tends to pull the potential of the plasma up to (a small fraction of) the bias potential. The result is that the bias potential does not result in a desired or desired potential difference between the first body and the plasma, and therefore has little effect on the flux and energy distribution of ions incident on the first body.

[0014] The lithographic apparatus according to the first aspect comprises two bodies (a first body and a second body) both of which are close to an optical path, and a voltage source is arranged to apply a potential difference across the first body and the second body. Advantageously, by changing the configuration of the first body and the second body relative to the optical path (where the plasma is formed), the lithographic apparatus according to the first aspect allows a potential difference to be maintained between the first body and the plasma. This, in turn, provides some control over the flux and energy distribution of ions incident on the first body.

[0015] It should be understood that, as used herein, an object being proximate to an optical path is intended to mean that the object is located near the optical path such that a plasma formed in the optical path is or can be connected to the object.

[0016] In some embodiments, the first body and the second body can be arranged such that the conductance between the first body and the plasma is significantly less than the conductance between the second body and the plasma, as now described.

[0017] The first body and the second body may be arranged such that conductance between the first body and the plasma is significantly smaller than conductance between the second body and the plasma.

[0018] That is, the resistance between the plasma and the second body is significantly less than the resistance between the plasma and the first body. For example, the first body and the second body can be arranged so that the conductance between the first body and the plasma is less than half of the conductance between the second body and the plasma. For example, the first body and the second body can be arranged so that the conductance between the first body and the plasma is less than one-tenth of the conductance between the second body and the plasma.

[0019] It is understood that there are several ways to achieve the conductance difference between the two bodies and the plasma. In general, the ideal method is to: (a) reduce the surface area of ​​the first body relative to the second body; (b) increase the plasma density between the second body and the optical path; (c) reduce the distance between the second body and the optical path.

[0020] The second body may define a textured surface facing the optical path.

[0021] The textured surface provided on the second body can increase the surface area of ​​the second body (compared to a flat surface). Advantageously, such increased surface area can increase conductance (reduce resistance) between the plasma and the second body.

[0022] The textured surface may be a corrugated surface.

[0023] The lithographic apparatus may further comprise means for creating a conductive medium between the optical path and the second body.

[0024] The means for generating a conductive medium between the optical path and the second body may comprise a voltage source arranged to generate a plasma between the optical path and the second body.

[0025] The plasma may be a radio frequency (RF) plasma.

[0026] Additionally or alternatively, the means for creating a conductive medium between the optical path and the second body may include any source of ionizing radiation.

[0027] The means for creating a conductive medium between the optical path and the second body may comprise an electron source arranged to increase an electron density between the optical path and the second body.

[0028] The means for generating a conductive medium between the optical path and the second body may comprise a radiation source arranged to generate radiation propagating between the optical path and the second body.

[0029] Such a radiation source may, for example, comprise an ultraviolet (UV) radiation source.Alternatively, the radiation source may, for example, comprise an electron beam source.

[0030] The second body may include a portion adjacent to the optical path.

[0031] For example, the second body may include not only a wall forming part of a housing for parts of the lithographic apparatus (such as a reflector etc.), but may also include an additional portion adjacent to the optical path. The portion may extend from the wall of the lithographic apparatus towards the optical path. Advantageously, this may reduce the distance between the plasma generated by the radiation beam and the second body (compared to an arrangement without the portion adjacent to the optical path). Advantageously, the reduced distance from the plasma to the second body increases the conductance (reduces the resistance) between the plasma and the second body.

[0032] The portion adjacent to the optical path may extend at least partially around an optical axis of the optical path.

[0033] For example, the portion adjacent the optical path may comprise a generally cylindrical or frusto-conical hollow body through which the radiation beam propagates.

[0034] The voltage source may be arranged to apply a bias potential to the first body and the second body may be grounded.

[0035] For example, to reduce the flux and / or energy of ions from the plasma that strike the first body, a positive bias potential may be applied to the first body to repel the ions. Since the conductance between the second body and the plasma may be much greater than the conductance between the first body and the plasma, by grounding the second body, the (local) bias potential applied to the first body may have a negligible effect on the plasma potential.

[0036] Alternatively, a voltage source may be arranged to apply a bias potential to the second body and the first body may be grounded.

[0037] For example, to reduce the flux and / or energy of ions from the plasma that strike the first body, a negative bias potential may be applied to the second body to attract the ions. As described above, the conductance between the second body and the plasma may be much greater than the conductance between the first body and the plasma. Therefore, by applying a negative bias potential to the second body, the potential of the plasma may be significantly reduced. This, in turn, reduces the flux and energy of ions that strike the first body (grounded).

[0038] The second body may be formed of a material resistant to plasma etching.

[0039] Advantageously, this may reduce the amount of material etched from the second body (and which may subsequently be deposited on sensitive surfaces within the lithographic apparatus, such as surfaces of mirrors for example).It will be appreciated that the first and second bodies may be formed from materials compatible with the environment within the EUV lithographic apparatus.

[0040] The second body may be formed of tungsten.

[0041] Tungsten (W) is resistant to hydrogen plasma etching due to its quality. In addition, tungsten is compatible with the environment within EUV lithography equipment.

[0042] In other embodiments, the second body may be formed of another heavy noble metal, such as molybdenum (Mo), ruthenium (Ru), rhodium (Rh), silver (Ag), rhenium (Re), osmium (Os), iridium (Ir), or platinum (Pt). These materials may have at least some resistance to hydrogen plasma etching.

[0043] In some embodiments, the voltage source can be arranged so that the energy distribution of ions incident on the first body is within a range that results in etching contaminants on the surface of the first body but not in etching the bulk material of the first body, as now described.

[0044] The voltage source may be arranged to apply a potential difference across the first and second bodies such that an energy distribution of plasma ions incident on the first body is within a range that results in etching of contaminants on a surface of the first body and minimal etching of bulk material of the first body.

[0045] The voltage source may be arranged such that an alternating potential difference is applied across the first body and the second body.

[0046] The voltage source may be arranged so that the rate of change of potential across the plasma sheath of the plasma is small most of the time.

[0047] Advantageously, by reducing the amount of time that the rate of change of potential is above a threshold, the amount of time that the potential across the plasma sheath varies can be reduced. In some embodiments, the potential is constant or varies slowly for most of the time and oscillates (abruptly) between positive and negative potentials. In some embodiments, the potential oscillates (abruptly) between the positive portion of the duty cycle and the negative portion of the duty cycle, and in each of the positive and negative portions of the duty cycle, the potential is constant or varies slowly.

[0048] Advantageously, by reducing the amount of time over which the potential across the plasma sheath varies, the range or width of the energy distribution of ions striking the first member is reduced.

[0049] The potential difference may alternate between a positive portion, where the first body is positively biased, and a negative portion, where the first body is negatively biased.

[0050] The average value of the potential difference applied across the first body and the second body may be non-zero.

[0051] The non-zero average value of the potential difference applied to the first and second bodies is the direct current (DC) component of the AC potential difference. The average value of the potential difference applied across the first and second bodies depends on the value of the potential difference applied during the positive and negative portions and the duty cycle of the AC potential difference (i.e., the ratio of the duration of the negative portion to the duration of the positive portion). It will be appreciated that the average energy of the ions striking the first body depends on the average value of the potential difference across the first and second bodies.

[0052] While the average energy of ions striking the first body depends on the average value of the potential difference across the first and second bodies, the range or width of the energy distribution of ions striking the first member depends on the shape of the periodic potential difference applied across the first and second bodies.

[0053] The duty cycle of the potential difference applied across the first body and the second body may be such that a ratio of a duration of the negative portion to a duration of the positive portion is greater than 0.9.

[0054] Advantageously, this ensures that the first body is in a negative bias state at least 90% of the time, thereby attracting ions in the plasma to etch contaminants on the first body. The positive bias portion allows time to clear the accumulated charge on the first and second bodies.

[0055] The magnitude of the potential difference applied across the first body and the second body may increase during the negative portion.

[0056] For example, during the negative portion, the amplitude of the potential difference applied across the first and second bodies can be increased linearly. As described above, during the negative portion, the first body is negatively biased so as to attract ions from the plasma, thereby etching contaminants on the first body. During the negative portion, positive surface charge can accumulate on the first body. Note that the first body can be connected to a voltage source via a matcher or a capacitor. By increasing the amplitude of the potential difference applied across the first and second bodies during the negative portion, the effect of this surface charge can be at least partially explained. Similarly, this can reduce the range or width of the energy distribution of the ions that strike the first member, which is advantageous.

[0057] The frequency of the potential difference applied across the first body and the second body may be less than 400 kHz.

[0058] The first body and / or the second body may be connected to a voltage source via a matcher or a capacitor.

[0059] The first body and the second body are both connected to a voltage source via a matcher or a capacitor, and the net current averaged over a period of the AC potential difference is zero. Advantageously, since the net current averaged over a period of the AC potential difference is zero, there is no net charge accumulation on the surface within the lithographic apparatus LA over an integer number of periods of the AC potential difference. Therefore, it can be ensured that there is no net charge accumulation on the surface within the lithographic apparatus LA due to the AC potential difference. Such net charge accumulation on the surface within the lithographic apparatus LA is undesirable.

[0060] The lithographic apparatus may include a third body. The third body may be close to the optical path. The third body may be configured to be at ground potential or floating potential. Therefore, the third body may not have active control of the surface potential.

[0061] The lithographic apparatus may include a diode electrically connected to the second body. In this way, the surface of the second body is biased relative to ground in order to influence the interaction of the plasma with the first body. The relative potential may be established by a voltage source or a passive element (such as a diode).

[0062] The lithographic apparatus may be configured to place the first body at a floating potential, a ground potential, or another biased potential relative to ground.

[0063] According to a second aspect of the present disclosure, there is provided a method for controlling the flux and / or energy distribution of ions incident on a first body in a lithography device, the method comprising: guiding a radiation beam along an optical path in the lithography device, with the first body close to the optical path; and applying a potential difference across the first body and a second body also close to the optical path to control the flux and / or energy distribution of ions incident on the first body from a plasma formed by the radiation beam in the optical path.

[0064] The lithographic apparatus may be an extreme ultraviolet (EUV) lithographic apparatus. In use, the optical path may be maintained at a pressure well below atmospheric pressure, for example, using a container evacuated using a vacuum pump. It is known to provide hydrogen (at low pressure) in the optical path. The radiation beam is typically a pulsed radiation beam. As each radiation pulse propagates through the optical path, gas molecules in the optical path tend to ionize, thereby forming a plasma in the optical path. The plasma may diffuse away from the optical path so that the plasma extends slightly beyond the volume through which the radiation beam propagates.

[0065] The first object may be a sensitive object, and therefore the flux and / or energy distribution of ions incident on the first object in the plasma may need to be controlled. For example, the first object may be a mirror or a sensor in a lithography device.

[0066] The second body may include one or more walls of the lithographic apparatus (e.g., it may form part of a housing for components of the lithographic apparatus, such as mirrors, etc.). Typically, the second body may be further from the optical path than the first body. It will be appreciated that the second body may include a plurality of separate components (e.g., walls of the lithographic apparatus).

[0067] It may be necessary to reduce the flux and energy of ions incident on the first body. For example, ions with high energy may pose a risk of sputtering, resulting in undesirable degradation of the first body. In addition, ions (e.g., hydrogen ions) may react with components of the first body (e.g., silicon from glass and stainless steel components and magnesium from aluminum components) to form volatile hydrides. Such volatile hydrides present in the lithographic equipment may be incident on the surface of the reflector in the lithographic equipment, causing the components etched from the first body to be deposited on the reflector. Such deposits may absorb EUV radiation, which is undesirable. In addition, such deposits may be oxidized and may therefore absorb more EUV radiation. Typically, hydrogen ions do not etch the outer native oxide layer of a material (e.g., silicon). However, ions with sufficient kinetic energy can penetrate these oxide layers to reach the underlying bulk material, and the ions can react with the bulk material to form volatile hydrides (e.g., silane). If the energy of the hydrogen ions is below a threshold kinetic energy, so that they cannot penetrate the native oxide layer outside the material, the formation of such volatile hydrides can be stopped.

[0068] In order to control the flux and energy distribution of ions incident on the first body, one can naively try to apply a bias voltage or potential to the first body. For example, if one wishes to prevent ions in the plasma from impacting the first body, a positive voltage can be applied to the first body to repel the ions. Similarly, if one wishes to increase the energy or flux of ions in the plasma that impact the first body, a negative voltage can be applied to the first body to attract the ions. However, for an object close to the plasma, the object is in electrical contact with the plasma (via the plasma sheath). Therefore, applying a bias voltage to only one object near the plasma will tend to pull the potential of the plasma up to the bias potential. The result is that the bias potential does not cause a potential difference between the first body and the plasma, and therefore has little effect on the flux and energy distribution of ions incident on the first body.

[0069] The method according to the second aspect uses two objects (a first body and a second body) that are both close to the optical path and a potential difference is applied across the first and second bodies (e.g. using a voltage source). Advantageously, by controlling the potential difference applied across the first and second bodies, the method according to the second aspect allows a potential difference to be maintained between the first body and the plasma. This, in turn, provides some control over the flux and energy distribution of ions incident on the first body.

[0070] It should be understood that, as used herein, an object being proximate to an optical path is intended to mean that the object is located near the optical path such that a plasma formed in the optical path is or can be connected to the object.

[0071] In some embodiments, the first body and the second body can be arranged such that the conductance between the first body and the plasma is significantly less than the conductance between the second body and the plasma, as now described.

[0072] The first body and the second body may be arranged such that conductance between the first body and the plasma is significantly smaller than conductance between the second body and the plasma.

[0073] That is, the resistance between the plasma and the second body is significantly less than the resistance between the plasma and the first body. For example, the first body and the second body can be arranged so that the conductance between the first body and the plasma is less than half of the conductance between the second body and the plasma. For example, the first body and the second body can be arranged so that the conductance between the first body and the plasma is less than one-tenth of the conductance between the second body and the plasma.

[0074] It will be appreciated that there are several ways to achieve a conductance difference between two objects and the plasma. In general, the ideal method is to: (a) reduce the surface area of ​​the first body relative to the second body; (b) increase the plasma density between the second body and the optical path; (c) reduce the distance between the second body and the optical path.

[0075] The method may also include creating a conductive medium between the optical path and the second body.

[0076] Generating a conductive medium between the optical path and the second body may include generating plasma between the optical path and the second body.

[0077] The plasma may be a radio frequency (RF) plasma.

[0078] Additionally or alternatively, the mechanism for producing the conductive medium may include any source of ionizing radiation.

[0079] Generating a conductive medium between the optical path and the second body may include increasing an electron density between the optical path and the second body.

[0080] Generating a conductive medium between the optical path and the second body may include directing radiation to propagate between the optical path and the second body.

[0081] Such radiation may, for example, comprise ultraviolet (UV) radiation.Alternatively, the radiation may, for example, comprise an electron beam source.

[0082] Applying a potential difference across the first body and the second body may include applying a bias potential to the first body and grounding the second body.

[0083] For example, to reduce the flux and / or energy of ions from the plasma striking the first body, a positive bias potential may be applied to the first body to repel the ions. Since the conductance between the second body and the plasma may be much greater than the conductance between the first body and the plasma, by grounding the second body, the (local) bias potential applied to the first body may have a negligible effect on the plasma potential.

[0084] Applying a potential difference across the first body and the second body may include applying a bias potential to the second body and grounding the first body.

[0085] For example, to reduce the flux and / or energy of ions from the plasma that strike the first body, a negative bias potential may be applied to the second body to attract the ions. As described above, the conductance between the second body and the plasma may be much greater than the conductance between the first body and the plasma. Therefore, by applying a negative bias potential to the second body, the potential of the plasma may be significantly reduced. This, in turn, reduces the flux and energy of ions that strike the first body (grounded).

[0086] The second body may be formed of a material resistant to plasma etching.

[0087] Advantageously, this may reduce the amount of material etched from the second body (and which may subsequently be deposited on sensitive surfaces within the lithographic apparatus, such as surfaces of mirrors).It will be appreciated that the first and second bodies may be formed from materials compatible with the environment within the EUV lithographic apparatus.

[0088] The second body may be formed of tungsten.

[0089] Tungsten (W) is resistant to hydrogen plasma etching due to its quality. In addition, tungsten is compatible with the environment within EUV lithography equipment.

[0090] In other embodiments, the second body may be formed of another inert heavy metal, such as molybdenum (Mo), ruthenium (Ru), rhodium (Rh), silver (Ag), rhenium (Re), osmium (Os), iridium (Ir), or platinum (Pt). These materials may have at least some resistance to hydrogen plasma etching.

[0091] In some embodiments, the applied voltage can cause the energy distribution of ions incident on the first body to be within a range that results in etching contaminants on the surface of the first body but not in etching the bulk material of the first body, as now described.

[0092] Applying a potential difference across the first body and the second body may cause an energy distribution of ions from the plasma incident on the first body to be within a range that results in etching of contaminants on the surface of the first body and minimal etching of bulk material of the first body.

[0093] The potential difference applied across the first body and the second body may include an alternating potential difference applied across the first body and the second body.

[0094] The potential difference applied across the first body and the second body may be such that the rate of change of potential between the plasma sheaths of the plasma is small most of the time.

[0095] Advantageously, by reducing the amount of time that the rate of change of potential is above a threshold, the amount of time that the potential across the plasma sheath varies can be reduced. In some embodiments, the potential is constant or varies slowly for most of the time and oscillates (abruptly) between positive and negative potentials. In some embodiments, the potential oscillates (abruptly) between the positive portion of the duty cycle and the negative portion of the duty cycle, and in each of the positive and negative portions of the duty cycle, the potential is constant or varies slowly.

[0096] Advantageously, by reducing the amount of time over which the potential across the plasma sheath varies, the range or width of the energy distribution of ions striking the first member is reduced.

[0097] The potential difference may alternate between a positive potential portion where the first body is positively biased and a negative potential portion where the first body is negatively biased.

[0098] The average value of the potential difference applied across the first body and the second body may be non-zero.

[0099] The non-zero average value of the potential difference applied across the first and second bodies is the direct current (DC) component of the AC potential difference. The average value of the potential difference applied across the first and second bodies depends on the values ​​of the potential difference applied during the positive and negative portions and the duty cycle of the AC potential difference (i.e., the ratio of the duration of the negative portion to the duration of the positive portion). It will be appreciated that the average energy of the ions striking the first body depends on the average value of the potential difference applied across the first and second bodies.

[0100] While the average energy of ions striking the first body depends on the average value of the potential difference applied across the first and second bodies, the range or width of the energy distribution of ions striking the first member depends on the shape of the periodic potential difference applied across the first and second bodies.

[0101] The duty cycle of the potential difference applied across the first body and the second body may be such that a ratio of a duration of the negative portion to a duration of the positive portion is greater than 0.9.

[0102] Advantageously, this ensures that the first body is in a negative bias state at least 90% of the time, thereby attracting ions in the plasma to etch contaminants on the first body. The positive bias portion allows time to clear the accumulated charge on the first and second bodies.

[0103] The magnitude of the potential difference applied across the first body and the second body may increase during the negative portion.

[0104] For example, during the negative portion, the amplitude of the potential difference applied across the first and second bodies can be increased linearly. As described above, during the negative portion, the first body is negatively biased so as to attract ions from the plasma, thereby etching contaminants on the first body. During the negative portion, positive surface charge can accumulate on the first body. Note that the first body can be connected to a voltage source via a matcher or a capacitor. By increasing the amplitude of the potential difference applied across the first and second bodies during the negative portion, the effect of this surface charge can be at least partially explained. Similarly, this can reduce the range or width of the energy distribution of the ions that strike the first member, which is advantageous.

[0105] The frequency of the potential difference applied across the first body and the second body may be less than 400 kHz.

[0106] The first body and / or the second body may be connected to a voltage source via a matcher or a capacitor.

[0107] The first body and the second body are both connected to a voltage source via a matcher or a capacitor, and the net current averaged over a period of the AC potential difference is zero. Advantageously, since the net current averaged over a period of the AC potential difference is zero, there is no net charge accumulation on the surface within the lithographic apparatus LA over an integer number of periods of the AC potential difference. Therefore, it can be ensured that there is no net charge accumulation on the surface within the lithographic apparatus LA due to the AC potential difference. Such net charge accumulation on the surface within the lithographic apparatus LA is undesirable.

[0108] The method may further comprise providing the third body at ground potential or floating potential. In other words, there is no active control of the surface potential.

[0109] The method may further include providing a diode electrically connected to the second body to bias the second body with respect to ground.

[0110] The method may further include placing the first body at a floating potential, a ground potential, or another biased potential relative to ground.

[0111] It should be understood that one or more aspects or features described above or mentioned in the following description may be combined with one or more other aspects or features. BRIEF DESCRIPTION OF THE DRAWINGS

[0112] Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0113] Figure 1 is a schematic diagram of a lithography system including a lithography apparatus and a radiation source;

[0114] Figure 2 is a schematic diagram of a lithographic apparatus according to an embodiment of the present disclosure, comprising two bodies proximate an optical path and a voltage source;

[0115] Figure 3 Shows that cross Figure 2 Example waveforms of alternating bias potentials applied by the first and second bodies of the illustrated arrangement; and

[0116] Figure 4 is a schematic diagram of a method according to an embodiment of the present disclosure, which method can be used Figure 2 The lithography apparatus shown is used for this purpose.

[0117] Figure 5 is a schematic diagram of a lithography apparatus according to an embodiment of the present invention, comprising three bodies close to an optical path and a voltage source. DETAILED DESCRIPTION

[0118] Figure 1 A lithography system is shown. The lithography system comprises a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithography apparatus LA comprises an illumination system IL, a support structure MT configured to support a reticle assembly 15 comprising a pattern forming device MA (e.g., a reticle or mask), a projection system PS and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident on the pattern forming device MA. The projection system is configured to project the radiation beam B (now patterned by the pattern forming device MA) onto a substrate W. The substrate W may include a previously formed pattern. In this case, the lithography apparatus aligns the patterned radiation beam B with the pattern previously formed on the substrate W.

[0119] The radiation source SO, the illumination system IL and the projection system PS may be constructed and arranged so as to be isolated from the external environment. A gas (e.g., hydrogen) having a pressure lower than atmospheric pressure may be provided in the radiation source SO. A vacuum may be provided in the illumination system IL and / or the projection system PS. A small amount of gas (e.g., hydrogen) having a pressure much lower than atmospheric pressure may be provided in the illumination system IL and / or the projection system PS.

[0120] Figure 1The radiation source SO shown in is of a type that can be referred to as a laser produced plasma (LPP) source. A laser 1 (which can be a CO2 laser, for example) is arranged to deposit energy into a fuel, such as tin (Sn) provided from a fuel emitter 3, via a laser beam 2. Although tin is mentioned in the following description, any suitable fuel can be used. The fuel can be in liquid form, for example, and can be, for example, a metal or an alloy. The fuel emitter 3 can include a nozzle configured to direct tin (for example, in the form of droplets) along a trajectory to a plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. The deposition of laser energy into the tin produces a plasma 7 at the plasma formation region 4. During de-excitation and recombination of ions in the plasma, radiation, including EUV radiation, is emitted from the plasma 7.

[0121] EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multilayer structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength (e.g., 13.5 nm)). The collector 5 may have an elliptical configuration with two elliptical foci. The first focus may be located at the plasma formation region 4 and the second focus may be located at the intermediate focus 6, as described below.

[0122] In other embodiments of the laser produced plasma (LPP) source, the collector 5 may be a so-called grazing incidence collector, which is configured to receive EUV radiation at a grazing incidence angle and focus the EUV radiation at an intermediate focus. The grazing incidence collector may, for example, be a nested collector, comprising a plurality of grazing incidence reflectors. The grazing incidence reflectors may be arranged axially symmetrically around the optical axis.

[0123] The radiation source SO may include one or more contamination traps (not shown). For example, the contamination trap may be located between the plasma formation region 4 and the radiation collector 5. The contamination trap may be, for example, a rotating foil trap, or may be any other suitable form of contamination trap.

[0124] The laser 1 may be separated from the radiation source SO. In this case, the laser beam 2 may be transferred from the laser 1 to the radiation source SO by means of a beam transmission system (not shown) comprising, for example, suitable guide mirrors and / or beam expanders and / or other optical devices. The laser 1 and the radiation source SO may be considered together as a radiation system.

[0125] The radiation reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at a point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as an intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near an opening 8 in an enclosure 9 of the radiation source SO.

[0126] The radiation beam B enters the illumination system IL from the radiation source SO, and the illumination system IL is configured to condition the radiation beam. The illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and the faceted pupil mirror device 11 together provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident on a mask assembly 15 held by a support structure MT. The mask assembly 15 includes a pattern forming device MA and a film 19. The film is mounted to the pattern forming device MA by a film frame 17. The mask assembly 15 may be referred to as a mask and film assembly 15. The pattern forming device MA reflects and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or in place of the faceted field mirror device 10 and the faceted pupil mirror device 11. Although the exemplary embodiment shows the film 19 (as part of the mask assembly 15), in some other embodiments, the film may not be present, in which case the pattern forming device MA is supported by the support structure MT.

[0127] The patterned radiation beam B is reflected from the patterning device MA and enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B onto a substrate W held by a substrate WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image having features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 may be applied. Although the projection system PS is Figure 1 There are two mirrors 13, 14 in the embodiment, but the projection system PS may include any number of mirrors (eg six mirrors).

[0128] For example, the lithographic apparatus can be used in a scanning mode, in which the support structure (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto the substrate W (i.e., dynamic exposure). The speed and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the demagnification and image reversal characteristics of the projection system PS. The patterned radiation beam incident on the substrate W may include a band of radiation. The band of radiation may be referred to as an exposure slit. During scanning exposure, movement of the substrate table WT and the support structure MT may cause the exposure slit to travel over an exposure field of the substrate W.

[0129] Figure 1The radiation source SO and / or the lithographic apparatus shown in may include components not shown. For example, a spectral filter may be provided in the radiation source SO. The spectral filter may substantially transmit EUV radiation but substantially block radiation of other wavelengths (such as infrared radiation).

[0130] In other embodiments of the lithography system, the radiation source SO may take other forms. For example, in alternative embodiments, the radiation source SO may include one or more free electron lasers. The one or more free electron lasers may be configured to emit EUV radiation, which may be provided to one or more lithography devices.

[0131] As briefly described above, the reticle assembly 15 includes a pellicle 19 disposed adjacent the pattern forming device MA. The pellicle 19 is disposed in the path of the radiation beam B so that the radiation beam B passes through the pellicle 19 both when approaching the pattern forming device MA from the illumination system IL and when reflected by the pattern forming device MA toward the projection system PS. The pellicle 19 comprises a film or membrane that is substantially transparent to EUV radiation (although it absorbs a small amount of EUV radiation). An EUV transparent pellicle or a film that is substantially transparent to EUV radiation is herein defined as a pellicle 19 that is transmissive to at least 65% of EUV radiation, preferably at least 80% and more preferably at least 90% of EUV radiation. The pellicle 19 is used to protect the pattern forming device MA from particle contamination.

[0132] Although efforts may be made to maintain a clean environment inside the lithographic apparatus LA, particles may still be present inside the lithographic apparatus LA. In the absence of the pellicle 19, particles may be deposited on the patterning device MA. Particles on the patterning device MA may adversely affect the pattern imparted to the radiation beam B, and thereby affect the pattern transferred to the substrate W. The pellicle 19 advantageously provides a barrier between the patterning device MA and the environment in the lithographic apparatus LA so as to prevent particles from being deposited on the patterning device MA.

[0133] The pellicle 19 is located at a distance from the patterning device MA sufficient to prevent any particles incident on the surface of the pellicle 19 from being in the field plane of the lithographic apparatus LA. This separation between the pellicle 19 and the patterning device MA serves to reduce the extent to which any particles on the surface of the pellicle 19 impart a pattern to the radiation beam B imaged onto the substrate W. It will be appreciated that when a particle is present in the radiation beam B but is not located in the field plane of the radiation beam B (e.g., not on the surface of the patterning device MA), any image of the particle will not be focused on the surface of the substrate W. In the absence of other considerations, it may be desirable to position the pellicle 19 at a considerable distance from the patterning device MA. However, in practice, the space available in the lithographic apparatus LA to accommodate the pellicle is limited due to the presence of other components. In some embodiments, the spacing between the pellicle 19 and the patterning device MA may be, for example, approximately between 1 mm and 10 mm, such as between 1 mm and 5 mm, such as between 2 mm and 2.5 mm.

[0134] Figure 1 The lithographic apparatus LA shown comprises a plurality of optical elements 10, 11, 13, 14 which define an optical path ( Figure 1 ). The optical elements 10, 11, 13, 14 are arranged to receive a radiation beam B from a radiation source SO, project the radiation beam B onto a mask MA, thereby patterning the radiation beam B and forming an image of the mask MA on a substrate W.

[0135] In use, the optical path of the lithographic apparatus LA may be maintained at a pressure well below atmospheric pressure, for example using a container evacuated using a vacuum pump. It is known to provide hydrogen gas (at low pressure) in the optical path. The radiation beam B is typically a pulsed radiation beam. As each radiation pulse propagates through the optical path, gas molecules in the optical path tend to ionize, thereby forming a plasma in the optical path. The plasma may diffuse away from the optical path so that the plasma extends slightly outside the volume through which the radiation beam B propagates.

[0136] It may be necessary to reduce the flux and energy of ions incident on some components within the lithographic apparatus LA. For example, high-energy ions may pose a risk of sputtering, resulting in undesirable degradation of the bulk of such components. In addition, ions (e.g., hydrogen ions) may react with components of the bulk of the component (e.g., silicon from glass and stainless steel components and magnesium from aluminum components) to form volatile hydrides. Such volatile hydrides present within the lithographic apparatus LA may be incident on the surfaces of the mirrors 10, 11, 13, 14 within the lithographic apparatus LA, resulting in components etched from certain components being deposited on the mirrors 10, 11, 13, 14. Such deposits may absorb EUV radiation, which is undesirable. In addition, such deposits may be oxidized and may therefore absorb more EUV radiation. Typically, hydrogen ions do not etch the outer native oxide layer of a material (e.g., silicon, Si). However, ions with sufficient kinetic energy can penetrate such oxide layers to reach the underlying bulk material, and the ions may react with the bulk material to form volatile hydrides (e.g., silane, SiH4). The formation of such volatile hydrides can be stopped if the energy of the hydrogen ions is below a threshold kinetic energy so that they cannot penetrate the native oxide layer outside the material.

[0137] It may be desirable to provide ions near, or direct ions to be incident on, certain components within the lithographic apparatus LA. For example, the ions may react with contaminants, thereby preventing them from being deposited on surfaces of mirrors 10, 11, 13, 14 within the lithographic apparatus LA. Similarly, ions incident on surfaces of mirrors 10, 11, 13, 14 within the lithographic apparatus LA may etch these contaminants from the mirrors 10, 11, 13, 14.

[0138] In order to control the flux and energy distribution of ions incident on an object, one may naively try to apply a bias voltage or potential to the object. For example, if one wishes to prevent ions in a plasma from hitting an object, a positive voltage may be applied to the first body to repel ions. Similarly, if one wishes to increase the energy or flux of ions in the plasma that hit the first body, a negative voltage may be applied to the first body to attract ions. However, the inventors have realized that for an object close to the plasma, the object is in electrical contact with the plasma (through the plasma sheath). Therefore, applying a bias voltage only to an object near the plasma will tend to pull the potential of the plasma up to (a small part of) the bias potential. The result is that the bias potential does not result in a desired or desired potential difference between the object to which the bias is applied and the plasma, and therefore has little effect on the flux and energy distribution of ions incident on the first body.

[0139] According to an embodiment of the present disclosure, there is provided a Figure 1A lithographic apparatus LA of the type shown comprises a plurality of optical elements 10, 11, 13, 14 defining an optical path. The optical elements 10, 11, 13, 14 are arranged to receive a radiation beam B from a radiation source SO and project the radiation beam B onto a mask MA so as to pattern the radiation beam B and form an image of the mask MA on a substrate W. A lithographic apparatus LA according to an embodiment of the present disclosure comprises two bodies close to the optical path and a voltage source, as now referred to Figure 2 described.

[0140] exist Figure 2 , a plasma 100 formed in a lithographic apparatus LA is schematically shown when a radiation beam B propagates along an optical path. The lithographic apparatus LA includes a first body 102 and a second body 104. Within the lithographic apparatus LA, the first body 102 and the second body 104 are both close to the optical path and therefore close to the plasma 100. It should be understood that an object close to the optical path as used herein means that the object is located near the optical path so that the plasma 100 formed in the optical path is connected to the object or can be connected to the object. As will be understood by those skilled in the art, most of the plasma 100 is surrounded by a plasma sheath 106, which extends to surrounding objects including the first body 102 and the second body 104.

[0141] The lithographic apparatus LA further comprises a voltage source 108 arranged to apply a potential difference between the first body 102 and the second body 104. In the example shown, the voltage source 108 is arranged to apply a bias potential V to the first body 102. b , and the second body 104 is grounded. It should be understood that in alternative embodiments, the voltage source 108 may be arranged to apply a bias potential V to the second body 104. b， And the first body 102 may be grounded. Typically, the voltage source 108 is arranged to keep the first body 102 and the second body 104 at different potentials, so that there is a potential difference between the first body 102 and the second body 104.

[0142] In particular, for embodiments where the voltage source 108 is arranged to apply an alternating potential difference across the first body 102 and the second body 104 , each of the first body 102 and the second body 104 may be connected to a bias potential through a matcher or capacitor 110 .

[0143] The first body 102 , the second body 104 and / or the voltage source 108 are arranged to control the flux and / or energy distribution of ions of the plasma 100 formed in the optical path by the radiation beam B incident on the first body 102 .

[0144] For example, in order to reduce the flux and / or energy of ions from the plasma 100 that strike the first body 102, a positive bias potential V may be applied to the first body 102.b Since the conductance between the second body 104 and the plasma 100 may be much larger than the conductance between the first body 102 and the plasma 100, by grounding the second body 104, the (local) bias potential applied to the first body 102 may have a negligible effect on the potential of the plasma.

[0145] Alternatively, in other embodiments, in order to reduce the flux and / or energy of ions from the plasma 100 that strike the first body 102, a negative bias potential may be applied to the second body 104 to attract the ions. As described below, the conductance between the second body 104 and the plasma 100 may be much greater than the conductance between the first body 102 and the plasma 100. Therefore, by applying a negative bias potential to the second body 104, the potential of the plasma may be significantly reduced. This, in turn, reduces the flux and energy of ions that strike the (grounded) first body 102.

[0146] The first body 102 may be a sensitive object, and thus it may be desirable to control the flux and / or energy distribution of ions from the plasma 100 incident on the first body 102. For example, the first body 102 may be a mirror or a sensor within a lithographic apparatus, etc. Alternatively, the first body 102 may be any object within a lithographic apparatus (e.g., made of glass, stainless steel, or aluminum) from which volatile hydrides may be released if ions having sufficient energy to penetrate bulk materials are incident upon the body.

[0147] The second body 104 may include one or more walls of the lithographic apparatus LA (e.g., it may form part of a housing for components of the lithographic apparatus LA, such as mirrors, etc.). Typically, the second body 104 may be further away from the optical path than the first body 102. It will be appreciated that the second body 104 may include multiple separate components (e.g., walls of the lithographic apparatus LA).

[0148] The lithographic apparatus LA according to the present disclosure comprises two bodies (a first body 102 and a second body 104), both of which are close to the optical path (and therefore, in use, close to the plasma 100), and a voltage source 108 is arranged to apply a potential difference across the first and second bodies 102, 104. Advantageously, by changing the configuration of the first and second bodies 102, 104 relative to the optical path (where the plasma 100 is formed), the lithographic apparatus LA according to the present disclosure allows a potential difference to be maintained between the first body 102 and the plasma 100. This, in turn, provides some control over the flux and energy distribution of ions incident on the first body 102.

[0149] In some embodiments, the first body 102 and the second body 104 are arranged such that the conductance between the first body 102 and the plasma 100 is significantly less than the conductance between the second body 104 and the plasma 100, as described below. Figure 2 A simplified electrostatic model is shown in FIG. 1 , wherein the connection of the first body 102 to the plasma 100 via the plasma sheath 106 is schematically shown as having a first resistance R1, and the connection of the second body 104 to the plasma 100 via the plasma sheath 106 is schematically shown as having a second resistance R2. It is suggested that the first and second bodies 102, 104 should be arranged such that R2 < <R1。

[0150] That is, the resistance R2 between the plasma 100 and the second body 104 is significantly smaller than the resistance R2 between the plasma 100 and the first body 102. 1。 For example, the first body 102 and the second body 104 may be arranged such that the conductance between the first body 102 and the plasma 100 is less than half of the conductance between the second body 104 and the plasma 100. For example, the first body 102 and the second body 104 may be arranged such that the conductance between the first body 102 and the plasma 100 is less than one tenth of the conductance between the second body 104 and the plasma 100.

[0151] Note that in this arrangement the resistance R2 between the plasma 100 and the second body 104 is significantly less than the resistance R1 between the plasma 100 and the first body 102, which is not common in known lithographic apparatus. For example, typically the first body 102 may be a sensitive object, the flux and / or energy distribution of the ions received by it need to be controlled. Such a body 102 is typically very close to the plasma 100, so the resistance R1 may be relatively low. It is for this reason that biasing only the first body 102 will typically not have a significant effect on the flux and energy distribution of the ions (because such a low resistance R1 will mean that the plasma potential may be strongly affected by the bias potential). The second body 104 may include one or more walls of the lithographic apparatus LA (which may, for example, form part of the housing of a component of the lithographic apparatus LA (e.g., a reflector, etc.). Such a body 104 may typically be much farther from the plasma 100 than the first body 102, so the resistance R2 may be relatively high. In at least some embodiments of the present disclosure, the lithographic apparatus LA has been modified in some manner to ensure that the resistance R2 between the plasma 100 and the second body 104 is significantly less than the resistance R1 between the plasma 100 and the first body 102 .

[0152] It should be appreciated that there are various ways to achieve the conductance difference between the first and second bodies 102, 104 and the plasma 100 (i.e., the condition of R2 << R1). Generally, it is desirable to: (a) reduce the surface area of the first body 102 relative to the second body 104; (b) increase the density of the plasma 100 between the second body 104 and the optical path; (c) reduce the distance between the second body 104 and the optical path.

[0153] In some embodiments, the second body 104 may define a textured surface facing the optical path. Providing a textured surface on the second body 104 can increase the surface area of the second body 104 (compared to a flat surface). Advantageously, this increased surface area can increase the conductance between the plasma 100 and the second body 104 (reducing the resistance R2). For example, the textured surface of the second body 104 can be a corrugated surface.

[0154] In some embodiments, the lithographic apparatus LA may further include means 112 for generating a conductive medium between the optical path and the second body 104.

[0155] The means 112 for generating a conductive medium between the optical path and the second body 104 may include a voltage source arranged to generate a plasma between the optical path and the second body 104. The plasma may be a radio frequency (RF) plasma.

[0156] Additionally or alternatively, the means 112 for generating a conductive medium between the optical path and the second body 104 may include any ionizing radiation source 114 (schematically shown in Figure 2 ).

[0157] For example, the means 112 for generating a conductive medium between the optical path and the second body 104 may include an electron source arranged to increase the electron density between the optical path and the second body 102.

[0158] In some embodiments, the means 112 for generating a conductive medium between the optical path and the second body 104 may include a radiation source arranged to generate radiation propagating between the optical path and the second body 104. Such a radiation source may include, for example, an ultraviolet (UV) radiation source. Alternatively, the radiation source may include, for example, an electron beam source.

[0159] In some embodiments, the second body 104 may include a portion adjacent to the optical path. For example, the second body 104 may include not only a wall that forms a portion of a housing of a component of the lithographic apparatus LA (e.g., a reflector, etc.), but may also include an additional portion adjacent to the optical path. Such a portion may extend from the wall of the lithographic apparatus toward the optical path. Advantageously, this may reduce the distance between the plasma 100 generated by the radiation beam and the second body 104 (compared to an arrangement without a portion adjacent to the optical path). Advantageously, this reduces the distance from the plasma 100 to the second body 104 and increases the conductance between the plasma 100 and the second body 104 (reducing the resistance R2).

[0160] In some embodiments, the portion of the second body 104 adjacent to the optical path extends at least partially around the optical axis of the optical path. For example, the portion of the second body 104 adjacent to the optical path may include a generally cylindrical or frustoconical hollow body through which the radiation beam B propagates.

[0161] In some embodiments, the second body 104 may be formed of a material that is resistant to etching by the plasma 100. Advantageously, this may reduce the amount of material etched from the second body 104 (and that may subsequently be deposited on sensitive surfaces within the lithography apparatus LA, such as the surfaces of mirrors). It will be appreciated that the first body 102 and the second body 104 may be formed of materials that are compatible with the environment within the EUV lithography apparatus LA.

[0162] In some embodiments, the second body 104 may be formed of tungsten. Tungsten (W) is resistant to etching by the hydrogen plasma 100 due to its quality. In addition, tungsten is compatible with the environment within the EUV lithography apparatus LA.

[0163] In other embodiments, the second body may be formed of another inert heavy metal, such as molybdenum (Mo), ruthenium (Ru), rhodium (Rh), silver (Ag), rhenium (Re), osmium (Os), iridium (Ir), or platinum (Pt). These materials may have at least some resistance to hydrogen plasma etching.

[0164] In some embodiments, voltage source 108 is arranged such that the energy distribution of ions incident on first body 102 is within a range that results in etching contaminants on the surface of first body 102 but not in etching the bulk material of first body 102 , as now described.

[0165] The voltage source 108 may be arranged to apply a potential difference V between the first and second bodies 102, 104. b , so that the energy distribution of ions from the plasma 100 incident on the first body 102 is within a range that results in etching of contaminants on the surface of the first body 102 and in minimal etching of the bulk material of the first body 102. Referring now to Figure 3 Describe an example potential difference V to achieve this b。

[0166] In some embodiments, the voltage source 108 is arranged so as to have Figure 3 The potential difference V shown in the waveform 200 b is applied on the first body 102 and the second body 104 .

[0167] Potential difference V b is an AC potential difference with a specific waveform of 200 V b。

[0168] When applied with Figure 3 The potential difference V of the waveform 200 is shown b When the potential V is reduced, the rate of change of the potential on the plasma sheath 106 of the plasma 100 is small for most of the time. Advantageously, by reducing the potential V b The amount of time that the rate of change of is above the threshold, the amount of time that the potential on the plasma sheath 106 changes is reduced. Figure 3 In the embodiment shown, the potential V b It is constant or changes slowly most of the time and oscillates (suddenly) between positive and negative potentials. Figure 3 In the embodiment shown, the potential V b The voltage V oscillates (suddenly) between the positive portion 202 of the duty cycle and the negative portion 204 of the duty cycle, and in each of the positive portion 202 and the negative portion 204 of the duty cycle, the voltage V b Is constant or slowly changing.

[0169] Advantageously, by reducing the amount of time over which the potential on the plasma sheath 106 varies, the range or width of the energy distribution of ions striking the first member 102 is reduced. b This allows for precise control of the energy of the ions striking the first member 102 .

[0170] exist Figure 3 In the embodiment shown, the potential difference V b Alternating between positive portions 202 where the first body 102 is positively biased and negative portions 204 where the first body 102 is negatively biased.

[0171] When applied with Figure 3 The potential difference V of the waveform 200 is shown b When the potential difference between the first body 102 and the second body 104 is <V b The average value of > is not zero. The potential difference applied to the first body 102 and the second body 104 <V b The non-zero average value of > is the AC potential difference V bThe potential difference applied to the first body 102 and the second body 104 is <V b > depends on the value of the potential difference applied during the positive portion 202 and the negative portion 204 and the duty cycle of the AC potential difference (i.e., the ratio of the duration of the negative portion 204 to the duration of the positive portion 202). It can be understood that the average energy of the ions that strike the first body 102 depends on the potential difference applied across the first body 102 and the second body 104. <V b > the average value.

[0172] Although the average energy of the ions striking the first body 102 depends on the potential difference applied across the first and second bodies 102, 104 <V b > average value, but the range or width of the distribution of the energy of the ions striking the first member 102 depends on the periodic potential difference V applied across the first and second bodies 102, 104 b shape.

[0173] If a sinusoidal waveform potential difference is applied to the first and second bodies 102, 104, the potential on the plasma sheath 106 changes continuously. As a result, the ion energy distribution of the ions incident on the first body 102 is wide. Therefore, it is impossible to ensure that all ions incident on the first body 102 have the following energies: (a) above the threshold for etching contaminants; (b) below the threshold for etching the bulk material of the first body 102. In contrast, when a sinusoidal waveform potential difference is applied to the first and second bodies 102, 104, the potential on the plasma sheath 106 changes continuously. As a result, the ion energy distribution of the ions incident on the first body 102 is wide. Therefore, it is impossible to ensure that all ions incident on the first body 102 have the following energies: (a) above the threshold for etching contaminants; (b) below the threshold for etching the bulk material of the first body 102. Figure 3 The potential difference V of the waveform 200 is shown b When the potential on the plasma sheath 106 is generally constant, the potential on the plasma sheath 106 is generally constant. This results in a very narrow ion energy distribution of ions incident on the first body 102. In addition, the stable potential on the plasma sheath 106 results in low electron heating. Therefore, when a potential with Figure 3 The potential difference V of the waveform 200 is shown b Advantageously, it may be possible to ensure that all ions incident on the first body 102 have energies that are: (a) above a threshold for etching contaminants; and (b) below a threshold for etching the bulk material of the first body 102.

[0174] In some embodiments, the potential difference V applied to the first and second bodies 102, 104 is b The duty cycle is such that the ratio of the duration of the negative portion 204 to the duration of the positive portion 202 is greater than 0.9. Advantageously, this ensures that the first body 102 is negatively biased at least 90% of the time, thereby attracting ions from the plasma 100 to etch contaminants on the first body 102. The positive portion 202 allows time to remove the charge accumulated on the first and second bodies 102, 104.

[0175] exist Figure 3 In the illustrated embodiment, during the negative portion 204, the potential difference |V b For example, in this embodiment, during the negative portion 204, the potential difference |V applied to the first and second bodies 102, 104 is b | linearly increases in magnitude. As described above, during the negative portion 204, the first body 102 is negatively biased so as to attract ions from the plasma 100, thereby etching contaminants from the first body 102. During the negative portion, positive surface charge may accumulate on the first body 102. Note that the first body 102 may be connected to the voltage source 108 via a matcher or capacitor 110. By increasing the potential difference V applied to the first and second bodies 102, 104 during the negative portion 204 b Again, this can reduce the range or width of the energy distribution of ions striking the first member 102, which is advantageous.

[0176] In some embodiments, a potential difference V applied across the first body 102 and the second body 104 b The frequency is less than 400kHz.

[0177] In the Figure 3 The bias voltage V b In an embodiment applied to the first body 102 (or the second body 104), the first body 102 and / or the second body 104 may be connected to the voltage source 108 via a matcher or capacitor 110. By connecting both the first body 102 and the second body 104 to the voltage source 108 via the matcher or capacitor 110, the net current averaged over a period of the AC potential difference is zero. Advantageously, since the net current averaged over a period of the AC potential difference is zero, there is no net charge accumulation on the surface within the lithographic apparatus LA over an integer number of periods of the AC potential difference. Therefore, it can be ensured that there is no net charge accumulation on the surface within the lithographic apparatus LA due to the AC potential difference. Such net charge accumulation on the surface within the lithographic apparatus LA is undesirable.

[0178] According to some embodiments of the present disclosure, a method of controlling the flux and / or energy distribution of ions incident on a first body 102 in a lithographic apparatus LA is provided. Figure 4As shown, the method 300 includes step 302, i.e., directing a radiation beam B along an optical path in the lithographic apparatus LA, with the first body 102 being close to the optical path. The method 300 also includes step 304, i.e., applying a potential difference between the first body 102 and a second body 104 also close to the optical path to control the flux and / or energy distribution of ions incident on the first body 102 from the plasma 100 formed by the radiation beam B in the optical path.

[0179] It should be understood that method 300 may include the above-referenced Figures 1 to 3 Any or all features of the lithographic apparatus LA described.

[0180] The method 300 uses two bodies (a first body 102 and a second body 104) that are close to the optical path, and applies a potential difference (e.g., using a voltage source) between the first and second bodies 102, 104. Advantageously, by controlling the potential difference applied between the first and second bodies 102, 104, the method 300 allows a potential difference to be maintained between the first body 102 and the plasma 100. This, in turn, provides some control over the flux and energy distribution of ions incident on the first body 102.

[0181] It should be understood that the ion flux incident on an object as used herein is intended to mean the number of ions incident on the object per unit area per unit time.

[0182] according to Figure 5 In the embodiment shown, the figure schematically shows a plasma 100 formed in a lithographic apparatus LA when a radiation beam B propagates along an optical path. The lithographic apparatus LA includes a first body 102, a second body 104, and a third body 116. In the lithographic apparatus LA, the first body 102, the second body 104, and the third body 116 are all close to the optical path, and therefore close to the plasma 100. It should be understood that the object close to the optical path used in this article means that the object is located near the optical path, so that the plasma 100 formed in the optical path is connected to the object or can be connected to the object. It will be understood by those skilled in the art that most of the plasma 100 is surrounded by a plasma sheath 106, and the plasma sheath 106 extends to the surrounding objects including the first body 102, the second body 104, and the third body 116.

[0183] In the illustrated embodiment, the third body 116 is grounded, but may also be allowed to have a floating potential. Thus, there is no active control of the surface potential of the third body 116. The second body 104 is electrically biased relative to ground to affect the interaction of the plasma with the first body 102, which may be an optical element, such as a reflector. The potential relative to ground may be provided by a voltage source 108 or by a passive component, such as a diode. The first body 102 includes a critical surface that requires control of plasma interactions, which is achieved by biasing the second body 104. The first body 102 may be at a floating potential, connected to a ground potential, or at another bias potential relative to ground. The voltage source 108 may be configured to operate as described herein.

[0184] The first body 102 may be a sensitive object, and thus it may be desirable to control the flux and / or energy distribution of ions from the plasma 100 incident on the first body 102. For example, the first body 102 may be a mirror or a sensor within a lithographic apparatus, etc. Alternatively, the first body 102 may be any object within a lithographic apparatus (e.g., made of glass, stainless steel, or aluminum) from which volatile hydrides may be released if ions having sufficient energy to penetrate bulk materials are incident upon the body.

[0185] The second body 104 may include one or more walls of the lithographic apparatus LA (e.g., it may form part of a housing for components of the lithographic apparatus LA, such as mirrors, etc.). Typically, the second body 104 may be further away from the optical path than the first body 102. It will be appreciated that the second body 104 may include a plurality of separate components (e.g., walls of the lithographic apparatus LA). Unless otherwise noted, the terms voltage and potential are synonymous herein and may be used interchangeably.

[0186] The mask or reticle mentioned in this document can be interpreted as a pattern forming device (a mask or reticle is an example of a pattern forming device), and the two terms can be used interchangeably. In particular, the term "mask assembly" is synonymous with "reticle assembly" and "pattern forming device assembly".

[0187] Although embodiments of the invention may be specifically mentioned herein in the context of lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the 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 apparatus). These apparatus may generally be referred to as lithographic tools.

[0188] The term "EUV radiation" may be taken to encompass electromagnetic radiation having a wavelength in the range 4-20 nm, for example a wavelength in the range 13-14 nm. EUV radiation may have a wavelength less than 10 nm, for example a wavelength in the range 4-10 nm, such as 6.7 nm or 6.8 nm.

[0189] Although the application of lithography equipment in IC manufacturing may be specifically mentioned herein, it should be understood that the lithography equipment described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guidance and detection patterns of magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film-magnetic heads, etc.

[0190] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be implemented in a manner other than that described. The above description is intended to be illustrative, not restrictive. Therefore, it will be appreciated by those skilled in the art that modifications may be made to the present invention as described without departing from the scope of the following claims and clauses.

[0191] 1. A lithography device, comprising: a plurality of optical elements defining an optical path, arranged to receive a radiation beam, project the radiation beam onto a mask so as to pattern the radiation beam and form an image of the mask on a substrate; a first body close to the optical path; a second body close to the optical path; and a voltage source arranged to apply a potential difference between the first body and the second body; wherein the first body, the second body and / or the voltage source are arranged to control the flux and / or energy distribution of ions incident on the first body from a plasma formed by the radiation beam in the optical path.

[0192] 2. The lithographic apparatus of clause 1, wherein the first body and the second body are arranged such that a conductance between the first body and the plasma is significantly smaller than a conductance between the second body and the plasma.

[0193] 3. A lithographic apparatus according to clause 1 or clause 2, wherein the second body defines a textured surface facing the optical path.

[0194] 4. A lithographic apparatus according to clause 3, wherein the textured surface is a corrugated surface.

[0195] 5. The lithographic apparatus of any preceding clause, further comprising means for producing a conductive medium between the optical path and the second body.

[0196] 6. A lithographic apparatus according to clause 5, wherein the means for producing a conductive medium comprises a voltage source arranged to produce a plasma between the optical path and the second body.

[0197] 7. A lithographic apparatus according to clause 5 or clause 6, wherein the means for generating a conductive medium comprises an electron source arranged to increase an electron density between the optical path and the second body.

[0198] 8. The lithographic apparatus of any of clauses 5 to 7, wherein the means for producing a conductive medium comprises a radiation source arranged to produce radiation propagating between the optical path and the second body.

[0199] 9. The lithographic apparatus of any of clauses 5 to 8, wherein the second body comprises a portion adjacent to the optical path.

[0200] 10. The lithographic apparatus of clause 9, wherein the portion adjacent to the optical path extends at least partially around an optical axis of the optical path.

[0201] 11. A lithographic apparatus according to any preceding clause, wherein the voltage source is arranged to apply a bias potential to the first body and the second body is grounded.

[0202] 12. The lithographic apparatus of any of clauses 1 to 10, wherein the voltage source is arranged to apply a bias potential to the second body and the first body is grounded.

[0203] 13. The lithographic apparatus of any preceding clause, wherein the second body is formed of a material resistant to plasma etching.

[0204] 14. A lithographic apparatus according to any preceding clause, wherein the second body is formed from tungsten.

[0205] 15. A lithographic apparatus according to any of the preceding clauses, wherein the voltage source is arranged to apply a potential difference across the first body and the second body so that the energy distribution of ions of the plasma incident on the first body is within a range that results in etching of contaminants on the surface of the first body and minimal etching of bulk material of the first body.

[0206] 16. The lithographic apparatus of any preceding clause, wherein the voltage source is arranged such that an alternating potential difference is applied across the first body and the second body.

[0207] 17. A lithographic apparatus according to any preceding clause, wherein the voltage source is arranged such that the rate of change of potential across a plasma sheath of the plasma is small most of the time.

[0208] 18. A lithographic apparatus according to any of the preceding clauses, wherein the potential difference alternates between a positive portion in which the first body is positively biased and a negative portion in which the first body is negatively biased.

[0209] 19. The lithographic apparatus of clause 16 or clause 18, wherein an average value of the potential difference applied across the first body and the second body is non-zero.

[0210] 20. A lithographic apparatus according to any preceding clause, when directly or indirectly dependent upon clause 16 or clause 18, wherein the duty cycle of the potential difference applied across the first body and the second body is such that the ratio of the duration of the negative portion to the duration of the positive portion is greater than 0.9.

[0211] 21. A lithographic apparatus according to any preceding clause when directly or indirectly dependent on clause 18, wherein the amplitude of the potential difference applied across the first body and the second body increases during the negative portion.

[0212] 22. A lithographic apparatus according to any preceding clause when directly or indirectly dependent upon clause 16 or clause 18, wherein the frequency of the potential difference applied across the first body and the second body is less than 400 kHz.

[0213] 23. The lithographic apparatus of any preceding clause, wherein the first body and / or the second body is connected to the voltage source via a matcher or a capacitor.

[0214] 24. A lithographic apparatus according to any preceding clause, wherein the apparatus comprises a third body proximate to the optical path, optionally wherein the third body is configured to be at ground potential or floating potential.

[0215] 25. A lithographic apparatus according to any preceding clause, wherein the apparatus comprises a diode electrically connected to the second body.

[0216] 26. A lithographic apparatus according to any preceding clause, wherein the apparatus is configured to place the first body at a floating potential, a ground potential or another biased potential relative to ground.

[0217] 27. A method for controlling the flux and / or energy distribution of ions incident on a first body in a lithography device, the method comprising: guiding a radiation beam along an optical path in the lithography device, the first body being close to the optical path; and applying a potential difference across the first body and a second body also close to the optical path to control the flux and / or energy distribution of ions incident on the first body from a plasma formed by the radiation beam in the optical path.

[0218] 28. The method of clause 27, wherein the first body and the second body are arranged such that conductance between the first body and the plasma is significantly less than conductance between the second body and the plasma.

[0219] 29. The method of clause 27 or 28, further comprising creating a conductive medium between the optical path and the second body.

[0220] 30. The method of clause 29, wherein generating a conductive medium comprises generating a plasma between the optical path and the second body.

[0221] 31. The method of clause 29 or 30, wherein generating a conductive medium comprises increasing an electron density between the optical path and the second body.

[0222] 32. A method according to any of clauses 29 to 31, wherein generating a conductive medium comprises directing radiation to propagate between the optical path and the second body.

[0223] 33. A method according to any of clauses 27 to 32, wherein applying a potential difference across the first and second bodies comprises applying a bias potential to the first body and grounding the second body.

[0224] 34. A method according to any of clauses 27 to 32, wherein applying a potential difference across the first body and the second body comprises applying a bias potential to the second body and grounding the first body.

[0225] 35. A method according to any of clauses 27 to 34, wherein the second body is formed of a material resistant to plasma etching.

[0226] 36. A method according to any of clauses 27 to 35, wherein the second body is formed from tungsten.

[0227] 37. A method according to any one of clauses 27 to 36, wherein a potential difference is applied across the first body and the second body so that the energy distribution of ions of the plasma incident on the first body is within the following range, which results in etching of contaminants on the surface of the first body and minimizes etching of the bulk material of the first body.

[0228] 38. A method according to any of clauses 27 to 37, wherein applying a potential difference across the first body and the second body comprises applying an alternating potential difference across the first body and the second body.

[0229] 39. A method according to any of clauses 27 to 38, wherein a potential difference is applied across the first and second bodies such that the rate of change of potential across a plasma sheath of the plasma is small most of the time.

[0230] 40. A method according to any one of clauses 27 to 39, wherein the potential difference alternates between positive potential portions and negative potential portions, wherein the first body is positively biased and wherein the first body is negatively biased.

[0231] 41. A method according to clause 38 or clause 40, wherein the average value of the potential difference applied across the first body and the second body is non-zero.

[0232] 42. A method according to any one of clauses 27 to 41, when directly or indirectly subordinate to clause 38 or clause 40, wherein the duty cycle of the potential difference applied across the first body and the second body is such that the ratio of the duration of the negative portion to the duration of the positive portion is greater than 0.9.

[0233] 43. A method according to any one of clauses 27 to 42 when directly or indirectly dependent on clause 37, wherein the amplitude of the potential difference applied across the first body and the second body increases during the negative portion.

[0234] 44. A method according to any one of clauses 27 to 43 when directly or indirectly dependent on clause 19 or clause 31, wherein the frequency of the potential difference applied across the first body and the second body is less than 400 kHz.

[0235] 45. The method according to any of clauses 27 to 44, wherein the first body and / or the second body is connected to the voltage source via a matcher or a capacitor.

[0236] 46. ​​A method according to any of clauses 27 to 45, wherein the method further comprises providing a third body at ground potential or floating potential.

[0237] 47. A method according to any of clauses 27 to 46, wherein the method further comprises providing a diode electrically connected to the second body so as to bias the second body relative to ground.

[0238] 48. A method according to any of clauses 27 to 47, wherein the method comprises setting the first body at a floating potential, a ground potential or another biased potential relative to ground.

Claims

1. A lithographic apparatus, comprising: a plurality of optical elements defining an optical path arranged to receive a radiation beam, project the radiation beam onto a reticle so as to pattern the radiation beam and form an image of the reticle on a substrate; a first body adjacent to the optical path; a second body adjacent to the optical path; as well as a voltage source arranged to apply a potential difference across the first body and the second body; The first body, the second body and / or the voltage source are arranged to control a flux and / or energy distribution of ions incident on the first body by a plasma formed by the radiation beam in the optical path. 2 . The lithographic apparatus according to claim 1 , wherein the first body and the second body are arranged such that a conductance between the first body and the plasma is significantly smaller than a conductance between the second body and the plasma.

3. The lithographic device according to any of the preceding claims further comprises a mechanism for producing a conductive medium between the optical path and the second body, the mechanism comprising a voltage source arranged to produce plasma between the optical path and the second body, and / or comprising an electron source arranged to increase the electron density between the optical path and the second body, and / or comprising a radiation source arranged to produce radiation propagating between the optical path and the second body. 4 . The lithographic apparatus of claim 3 , wherein the second body comprises a portion adjacent to the optical path, the portion extending at least partially around an optical axis of the optical path.

5. A lithographic apparatus according to any one of the preceding claims, wherein the voltage source is arranged to apply a bias potential to the first body and the second body is grounded, or wherein the voltage source is arranged to apply a bias potential to the second body and the first body is grounded.

6. The lithographic apparatus of any preceding claim, wherein the second body is formed of tungsten.

7. A lithographic apparatus according to any one of the preceding claims, wherein the voltage source is arranged to apply a potential difference across the first body and the second body so that the energy distribution of ions from the plasma incident on the first body is within the following range, which causes etching of contaminants on the surface of the first body and causes minimal etching of the bulk material of the first body.

8. The lithographic apparatus of any preceding claim, wherein the voltage source is arranged such that an alternating potential difference is applied across the first body and the second body.

9. A lithographic apparatus according to any one of the preceding claims when directly or indirectly dependent on claim 8, wherein the duty cycle of the potential difference applied across the first body and the second body is such that the ratio of the duration of the negative part to the duration of the positive part is greater than 0.

9.

10. A lithographic apparatus according to any preceding claim, wherein the apparatus comprises a third body adjacent the optical path, optionally wherein the third body is configured to be at ground potential or at a floating potential.

11. The lithographic apparatus of any preceding claim, wherein the apparatus comprises a diode electrically connected to the second body.

12. A method of controlling the flux and / or energy distribution of ions incident on a first body in a lithographic apparatus, the method comprising: directing a radiation beam along an optical path in the lithographic apparatus, the first body being adjacent to the optical path; as well as A potential difference is applied across the first body and a second body also adjacent the optical path so as to control the flux and / or energy distribution of ions incident on the first body by a plasma formed by the radiation beam in the optical path.

13. The method of claim 12, wherein the first body and the second body are arranged such that conductance between the first body and the plasma is significantly smaller than conductance between the second body and the plasma.

14. The method according to claim 12 or 13, further comprising creating a conductive medium between the optical path and the second body, wherein generating the conductive medium comprises generating plasma between the optical path and the second body, and / or wherein generating the conductive medium comprises increasing the electron density between the optical path and the second body, and / or Wherein generating the conductive medium comprises directing radiation to propagate between the optical path and the second body.

15. A method according to any one of claims 12 to 14, wherein applying a potential difference across the first body and the second body includes applying a bias potential to the first body and grounding the second body, or wherein applying a potential difference across the first body and the second body includes applying a bias potential to the second body and grounding the first body.

16. The method of any one of claims 12 to 15, wherein applying a potential difference across the first body and the second body comprises applying an alternating potential difference across the first body and the second body.

17. A method according to any one of claims 12 to 16, wherein the potential difference alternates between a positive portion in which the first body is positively biased and a negative potential in which the first body is negatively biased.

18. A method according to claim 16 or 17, wherein the average value of the potential difference applied across the first body and the second body is non-zero.

19. A method according to any one of claims 12 to 18 when directly or indirectly dependent on claim 16 or 17, wherein The duty cycle of the potential difference applied across the first body and the second body is such that a ratio of the duration of the negative portion to the duration of the positive portion is greater than 0.

9.

20. A method according to any one of claims 12 to 19, wherein the method further comprises providing a third body at ground potential or at floating potential.

Images