Euv radiation generation method

Abstract

A method of generating extreme ultraviolet (EUV) radiation includes directing laser pulses of a laser beam onto a tin target to generate an EUV emitting plasma, wherein the laser pulses have a duration of at least 150 ns and have a wavelength between 1.6 micrometers and 2.5 micrometers.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to U.S. Application 63 / 569,362, filed March 25, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to a method for generating extreme ultraviolet (EUV) radiation. Background Technology

[0004] Light generated by a radiation source can be used by exposure equipment in semiconductor manufacturing processes. Examples of such exposure equipment are photolithography equipment, measurement or inspection equipment, more specifically, mask inspection equipment, and even more specifically, photochemical mask inspection equipment.

[0005] A lithography apparatus is a machine configured to apply a desired pattern onto a substrate. Lithography apparatuses can be used, for example, in the fabrication of integrated circuits (ICs). A lithography apparatus can project a pattern from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist) disposed on a substrate. To project the pattern onto the substrate, the lithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Compared to a lithography apparatus using radiation with a wavelength, for example, 193 nm, a lithography apparatus using EUV radiation with wavelengths in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0006] (Photochemical) mask inspection equipment is a device configured to measure the dimensions of a mask or mask blank or to detect defects in a mask or mask blank. EUV lithography uses mirrors instead of lenses as optical components. Mask blanks used in EUV lithography typically have a multi-layered structure that acts as a Bragg reflector. The multilayers can be alternating layers of molybdenum and silicon. If defects are present in this structure, the projected pattern will be distorted during the lithography process. Therefore, mask inspection to check for defects is considered a requirement of high-volume production processes. EUV mask inspection can be used for several purposes and at several different stages. First, the EUV mask inspection can be used to detect phase defects that may occur in the mask blank. Such phase defects may occur during the fabrication of the multi-layer stack of the mask blank. If not detected, these phase defects are printed on all chips printed using portions of the mask that include the phase defects. Such phase defects can be correctly detected by using the same or similar (13.5 nm) photochemical EUV wavelength as the lithography tool. Second, mask inspection can be used for the inspection of patterned masks and can be performed for the quality control of EUV patterned masks. For example, mask inspection can be used to measure critical dimensions on mask blanks. In addition to phase defects, absorber pattern defects on the surface can also be detected. Third, mask inspection can be used to simulate exposure and determine the degradation of optical contrast of defects detected in photochemical inspection. Fourth, mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair processes to improve pattern transfer fidelity. Furthermore, mask inspection can be used to check optical contrast after defect repair. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0007] EUV radiation can be generated using a laser-generated plasma (LPP) radiation source. Within the LPP radiation source, a laser beam can be used to irradiate fuel droplets to generate plasma that will emit EUV radiation.

[0008] The problem associated with LPP radiation sources is that the power of EUV radiation provided by LPP sources is relatively low. This limits the throughput of lithography equipment.

[0009] It may be desirable to provide an LPP radiation source that can produce higher power EUV radiation compared to at least some existing LPP radiation sources. Summary of the Invention

[0010] According to a first aspect of the invention, a method for generating extreme ultraviolet (EUV) radiation is provided, comprising directing a laser pulse of a laser beam onto a tin target to generate EUV emitting plasma, wherein the laser pulse has a duration of at least 150 ns and a wavelength between 1.6 micrometers and 2.5 micrometers.

[0011] Advantageously, using laser pulses with a duration of at least 150 ns and a wavelength between 1.6 micrometers and 2.5 micrometers provides EUV radiation with higher power than that achievable using at least some conventional LPP radiation sources. Additionally or alternatively, using laser pulses with a duration of at least 150 ns and a wavelength between 1.6 micrometers and 2.5 micrometers can provide greater efficiency and / or lower complexity compared to conventional LPP radiation sources.

[0012] The tin target can be a droplet with a diameter of at least 40 micrometers.

[0013] The EUV emission plasma can emit at least 0.05 J of EUV radiation collected by the collector within a solid angle.

[0014] The tin target can be a droplet with a diameter of at least 45 micrometers.

[0015] The EUV emission plasma can emit at least 0.07 J of EUV radiation collected by the collector within a solid angle.

[0016] The tin target can be provided at a repetition rate of 40 kHz or less.

[0017] The laser pulses can have a roughly flat time distribution.

[0018] The laser pulses can have a top-hat-shaped spatial distribution spanning a diameter of at least 100 micrometers.

[0019] In this embodiment, only one laser pulse is incident on each tin target.

[0020] Each laser pulse can have an energy of at least 1 J.

[0021] Each laser pulse can have an energy of up to 5 J.

[0022] The laser beam incident on the tin target may have a minimum density of 0.7 × 10⁻⁶. 11 W / cm 2 The intensity.

[0023] Each laser pulse can have a gradually rising edge with a duration of at least 5 ns.

[0024] The laser beam may have a diameter of at least 100 micrometers along an axial distance of at least 400 micrometers at the plasma formation site.

[0025] According to a second aspect of the invention, a radiation source is provided, comprising: a nozzle configured to direct a tin target toward a plasma forming site; and a laser system configured to direct laser pulses having wavelengths between 1.6 micrometers and 2.5 micrometers toward the plasma forming site, wherein the radiation source further comprises a controller configured to control the laser system such that the laser pulses have a duration of at least 150 ns.

[0026] Advantageously, the radiation source provides EUV radiation with higher power than that achievable using at least some conventional LPP radiation sources. Additionally or alternatively, the radiation source can provide greater efficiency and / or lower complexity compared to conventional LPP radiation sources.

[0027] The nozzle can be configured to provide a tin target as a tin target having a diameter of at least 40 micrometers. The nozzle can also be configured to provide a tin target as a tin target having a diameter of at least 45 micrometers.

[0028] The controller can be configured to control the nozzle to deliver the tin target at a repetition rate of 40 kHz or less.

[0029] The controller can control the laser system to provide only one laser pulse per tin target.

[0030] The laser system can be configured to have a wavelength of at least 0.7 × 10⁻⁶ at the plasma formation site. 11 W / cm 2 The laser pulse is provided in a laser beam of a certain intensity.

[0031] According to a third aspect of the present invention, an exposure apparatus is provided, comprising the radiation source described in the second aspect of the present invention, and further comprising a lithography apparatus, a measurement apparatus, or an inspection apparatus.

[0032] Different aspects of the present invention can be combined together. Attached Figure Description

[0033] Embodiments of the invention will now be described by way of example only, with reference to the accompanying illustrative drawings, in which:

[0034] Figure 1 A lithography system including a lithography apparatus and a radiation source, according to an embodiment of the present invention, is schematically depicted.

[0035] Figure 2A system including a radiation source for (photochemical) mask inspection according to an embodiment of the present invention is schematically depicted;

[0036] Figure 3 A radiation source according to an embodiment of the present invention is schematically depicted;

[0037] Figure 4 The temporal distribution of laser pulses used by a radiation source according to an embodiment of the present invention is schematically depicted;

[0038] Figure 5 The spatial distribution of laser pulses used by a radiation source according to an embodiment of the present invention is schematically depicted; and

[0039] Figure 6 The waist of a laser beam is schematically depicted when it is incident on a tin droplet in the plasma formation region of a radiation source, according to an embodiment of the present invention. Detailed Implementation

[0040] Figure 1 A lithography system including a radiation source SO and a lithography apparatus LA is shown. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an irradiation system IL, a support structure MT configured to support a pattern forming apparatus MA (e.g., a mask), a projection system PS, and a substrate stage WT configured to support a substrate W.

[0041] The irradiation system IL is configured to adjust the EUV radiation beam B before it is incident on the pattern forming apparatus MA. Additionally, the irradiation system IL may include a faceted field mirror assembly 10 and a faceted pupil mirror assembly 11. The faceted field mirror assembly 10 and the faceted pupil mirror assembly 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. In addition to or in place of the faceted field mirror assembly 10 and the faceted pupil mirror assembly 11, the irradiation system IL may include other mirrors or devices.

[0042] After such adjustment, the EUV radiation beam B interacts with the patterning apparatus MA. Due to this interaction, a patterned EUV radiation beam B' is generated. A projection system PS is configured to project the patterned EUV radiation beam B' onto a substrate W. For this purpose, the projection system PS may include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto a substrate W held by a substrate stage WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thus forming an image with features smaller than those on the patterning apparatus MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS... Figure 1The diagram shows only two mirrors 13 and 14, but the projection system PS can include a different number of mirrors (e.g., six or eight mirrors).

[0043] The substrate W may include a previously formed pattern. In such a case, the photolithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0044] A relative vacuum can be provided in the radiation source SO, in the irradiation system IL, and / or in the projection system PS, i.e., a small amount of gas (e.g., hydrogen) at a pressure sufficiently below atmospheric pressure.

[0045] The lithography apparatus LA and radiation source SO described herein can be used in a method for performing a circuit layout patterning process. The circuit layout patterning method includes receiving a substrate having a resist layer. The method further includes directing EUV radiation from the radiation source to the resist layer to form a patterned resist layer. The method also includes developing and etching the patterned resist layer to form a circuit layout.

[0046] Figure 2 A schematic depiction of a system MS used for (photochemical) mask inspection. The mask inspection system MS can be used by means of... Figure 1 The photolithography apparatus described herein is used to identify or inspect defects in masks to be used in the photolithography process. The mask inspection system MS includes a radiation source SO, an illumination system ILM, an inspection system DS, and a mask platform ME.

[0047] The illumination system ILM is configured to adjust the EUV radiation beam B before it is incident on the mask MA. The illumination system ILM can provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system ILM may include a plurality of mirrors 22. The illumination system ILM may include one or more faceted mirror assemblies.

[0048] The mask platform ME can be configured to move the mask MA relative to the EUV radiation beam B, so that the EUV radiation beam is incident on different regions of the mask.

[0049] The detection system DS includes a detector 20 and may additionally include a plurality of mirrors 24. The plurality of mirrors 24 may be configured to collect EUV radiation BR reflected from the mask MA and form an image of the mask MA on the detector 20 (which may be an imaging array). A processor (not depicted) may receive signals output from the detector 20 and use those signals to locate defects in the mask MA.

[0050] Figure 1The radiation source SO shown belongs to the type that can be referred to as a laser-generated plasma (LPP) source, for example. The laser system 1 is arranged to deposit energy into a fuel (i.e., the target material) via a laser beam 2, such as tin (Sn) supplied, for example, from a fuel generator 3. Although tin is mentioned in the following description, any suitable fuel can be used. The fuel can be, for example, in liquid form and can be, for example, a metal or alloy. The fuel generator 3 may include a nozzle configured to guide fuel, for example, in droplet form, along a trajectory toward the plasma formation region 4. The laser beam 2 is incident on the fuel at the plasma formation region 4. The laser energy is deposited into the tin, generating plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from plasma 7 during the deexcitation of electrons and recombination with ions in plasma 7.

[0051] Collector 5 collects and focuses EUV radiation from plasma 7. Collector 5 includes, for example, a near-normal incident radiation collector 5 (sometimes more generally referred to as a normal incident radiation collector). Collector 5 may have a multi-layered mirror structure arranged to reflect EUV radiation (e.g., EUV radiation with a desired wavelength such as 13.5 nm). Collector 5 may have an ellipsoidal configuration with two foci. As discussed below, the first of the foci may be located at plasma formation region 4, and the second of the foci may be located at intermediate focal point 6.

[0052] Laser system 1 may be spatially spaced from radiation source SO. In such a case, laser beam 2 may be transmitted from laser system 1 to radiation source SO by means of a beam delivery system (not shown) including, for example, suitable directional mirrors and / or beam expanders and / or other optical devices. Laser system 1, radiation source SO, and beam delivery system (if present) may be considered together as a radiation system.

[0053] The radiation reflected by collector 5 forms an EUV radiation beam B. EUV radiation beam B is focused at intermediate focal point 6 to form an image of the plasma present in plasma formation region 4 at intermediate focal point 6. The image at intermediate focal point 6 serves as a virtual radiation source for irradiating system IL. Radiation source SO is arranged such that intermediate focal point 6 is located at or near opening 8 in the enclosure structure 9 of radiation source SO.

[0054] The radiation source SO of the mask inspection system MS can be related to... Figure 1 This corresponds to the radiation source SO depicted in the text. (In contrast to...) Figure 1 In the same manner described in the diagram, the radiation source SO of the mask inspection system MS can be focused on the EUV radiation beam B to form an intermediate focal point 6.

[0055] Figure 3A radiation source SO according to an embodiment of the present invention is schematically depicted. The radiation source SO includes a fuel generator 3 configured to guide molten tin droplets to a plasma formation region 4. Molten tin droplets 34 are depicted at the plasma formation region 4. The fuel generator 3 includes a nozzle 36 from which molten tin droplets are ejected. The diameter of the molten tin droplets is determined by the diameter of the nozzle 36. The nozzle includes an actuator (not depicted) for modulating the nozzle 36. The modulation frequency applied to the actuator determines the repetition rate of the molten tin droplets ejected from the nozzle 36. The liquid tin is held under pressure in the fuel generator 3. The pressure holding the liquid tin determines the velocity at which the molten tin droplets are ejected from the nozzle 36.

[0056] Figure 3 A laser system 1 for a radiation source SO is also depicted. Laser system 1 includes a laser 30, a pump laser 32, and a controller CT. The laser may be, for example, a Tm:YLF laser. Laser 30 is configured to provide a laser beam 2. Laser beam 2 comprises laser pulses (as schematically depicted) incident on a tin droplet 34.

[0057] The pump laser 32 can be, for example, a laser diode, such as a GaAs laser diode, and is configured to emit a pump laser beam 33 coupled to the gain medium of the laser 30. In this way, the pump laser 32 produces population inversion in the laser 30, causing the laser 30 to emit laser beam 2. Similar to the laser 30, the pump laser 32 can emit pulsed laser beams 36. Multiple pump lasers 32 can be provided.

[0058] Optical element 38 may be disposed between laser 30 and plasma forming region 4. Optical element 38 may include one or more focusing elements (e.g., one or more lenses). Optical element 38 may form the waist of laser beam 2 as plasma forming region 4.

[0059] The molten tin droplet 34 provided by the fuel generator 3 can be spherical. The diameter of the molten tin droplet 34 can be smaller than the diameter of the laser beam 2 at the plasma formation region 4 (i.e., the region where the laser beam is incident on the molten tin droplet). For example, the diameter of the molten tin droplet 34 can be less than half the diameter of the laser beam 2 at the plasma formation region 4.

[0060] Laser beam 2 may, for example, have a wavelength of at least 1.6 micrometers. Laser beam 2 may, for example, have a wavelength of up to 2.5 micrometers. Laser beam 2 may, for example, have a wavelength of about 1.9 micrometers. The gain medium of laser 30 can be selected to provide the desired wavelength of laser beam 2. For example, Tm:YLF laser 30 can provide laser beam 2 with a wavelength of about 1.9 micrometers (e.g., 1.88 micrometers). The gain medium for Tm:YLF may, for example, be a plate, disk, or fiber optic structure. Other laser gain media may be used.

[0061] As mentioned above, the pump laser 32 can be, for example, a laser diode or multiple laser diodes. The laser diode can, for example, provide a pump laser beam 33 at a wavelength of approximately 790 nanometers. The laser diode can, for example, be a GaAs laser diode.

[0062] The combination of a pump laser diode emitting a laser beam at a wavelength of 790 nm and a Tm:YLF laser emitting a laser beam at a wavelength of 1.88 μm can be particularly advantageous. High-power, high-efficiency laser diodes emitting at 790 nm are commercially available. Tm:YLF has an absorption peak at approximately 790 nm. Fast nonradiative cross-relaxation will fill the upper level of the Tm:YLF, thereby providing particularly efficient pumping for the Tm:YLF. Furthermore, the Tm:YLF emitting photons at 1.88 nm... 3 The F4 level has a relatively long lifetime of about 15 ms, which provides good energy storage and allows for efficient multi-pulse extraction (i.e., multiple pulses for complete extraction of population inversion).

[0063] Each tin droplet 34 can receive a single pulse of the laser beam 2 without also receiving any laser pre-pulse. That is, unlike conventional LPP radiation sources, embodiments of the present invention do not require a pre-pulse. Advantageously, this simplifies the LPP radiation source SO. A pre-pulse is not used to modify the tin droplets into a disk shape before the laser beam pulse (as is done in prior art LPP radiation sources). Modifying the tin droplets into a disk shape is highly sensitive to shock waves generated by the vaporization of previous droplets. Embodiments of the present invention do not use a pre-pulse and do not require the tin droplets to have a disk shape. Embodiments of the present invention are more tolerant of deformed tin droplets at plasma formation sites and therefore more tolerant of shock waves generated by previous droplets. This means that the spacing between consecutive tin droplets can be reduced (compared to prior art LPP radiation sources). For example, the spacing between droplets can be as small as 1 mm.

[0064] A single laser pulse converts a droplet 34 of molten tin into EUV emission plasma. The molten tin droplet transforms from a liquid state into vapor containing some liquid. The vapor is ionized to become plasma. The plasma cools and emits its own light. This process continues until all the liquid tin has been used up, at which point the plasma self-extinguishes.

[0065] Figure 4 A pulse 40 of the laser beam is schematically depicted. Pulse 40 is schematically depicted as an intensity I varying with time t. The duration of pulse 40 may be, for example, at least 150 ns, and may be, for example, up to 400 ns or greater.

[0066] Advantageously, a laser pulse 40 providing a duration of at least 150 ns provides more energy (for the same radiation intensity) than a shorter pulse would provide. This allows the pulse to convert a larger tin droplet 34 into EUV emission plasma. The tin droplet 34 can be spherical and can, for example, have a diameter of at least 40 micrometers, and at most 50 micrometers or larger. If tin droplets with a diameter greater than 50 micrometers are used, tin contamination may be difficult to manage. Furthermore, the plasma emitted by tin droplets with a diameter greater than 50 micrometers may be spatially too large for efficient use by the lithography apparatus.

[0067] Conventional LPP EUV radiation sources use molten tin droplets with a diameter of approximately 27 micrometers. When using molten tin droplets with a larger diameter, such as 40 micrometers, the volume of the molten tin droplets that can be converted into EUV radiation and the mass of tin increase significantly. For example, increasing the diameter from 27 micrometers to 40 micrometers increases the mass of tin by 3.4 times.

[0068] The use of tin droplets with a diameter of 40 micrometers and laser pulses with a duration of at least 150 ns provides a very significant increase in EUV energy from each tin droplet. This combination provides at least 0.05 J of EUV energy from each tin droplet when incident on collector 5. This energy is much higher than that provided by conventional EUV LPP sources and allows for a significant increase in the throughput of the lithography equipment.

[0069] Table 1 below shows the EUV energy provided by tin droplets with a diameter of 40 micrometers. Energy is indicated as E0. inband The indicated radiation is within the EUV wavelength range (e.g., 13.5 nm + / - 0.135 nm). The energy is generated by collector 5 (see [reference]). Figure 1 The amount of energy collected within the solid angle. The intensity of laser beam 2 is indicated as I. laser The energy of the laser beam 2 pulse (which is used before the vaporization of the tin droplet is complete) is indicated as E. laser And the vaporization time for the tin droplets is indicated as T. vap .

[0070]

[0071] Table 1 - EUV energy generated using 40-micrometer diameter tin droplets.

[0072] As can be seen from Table 1, the time T taken for the tin droplet to vaporize is... vapThe duration decreases as the intensity of the laser beam increases. The duration of the laser pulse incident on the molten tin droplet should be at least as long as the vaporization time to ensure complete vaporization of the tin (incomplete vaporization will cause significant tin contamination in the radiation source). As can be seen from Table 1, at least 0.05 J of energy E should be provided. inband The duration of the laser pulse is at least 150 ns and can be greater than 300 ns (e.g., up to 400 ns).

[0073] Table 2 below shows the EUV energy provided by tin droplets with a diameter of 45 micrometers.

[0074]

[0075] Table 2 - EUV energy generated using 45-micrometer diameter tin droplets.

[0076] For example, the time T required for the vaporization of a 45-micrometer diameter tin droplet is as follows: vap The duration decreases as the laser beam intensity increases. The duration of the laser pulse incident on the molten tin droplet should be at least as long as the vaporization time to ensure complete vaporization of the tin. As can be seen from Table 2, an energy of at least 0.07 J E should be provided. inband The duration of the laser pulse is at least 150 ns and can be greater than 300 ns (e.g., up to 400 ns).

[0077] like Figure 4 As schematically depicted, in this embodiment, the pulse 40 of the laser beam 2 does not have a Gaussian or other peak-time pattern. Instead, the pulse 40 has a generally flat time distribution. A gradually rising edge may be present at the leading edge of the pulse 40, and a gradually falling edge may be present at the trailing edge of the pulse (as schematically depicted). The gradually rising edge converts a portion of the tin droplet from liquid form to vapor. This is advantageous because tin vapor absorbs energy from the laser pulse more efficiently than liquid tin. Converting a portion of the tin droplet to vapor form before the laser pulse reaches its maximum intensity advantageously improves the efficiency of EUV radiation generation (compared to the case where the laser pulse has a sharp rising edge at its leading edge). This is because, without the gradually rising edge, the maximum intensity EUV radiation would immediately incident on the liquid tin and would be poorly absorbed by the liquid tin. The gradually rising edge may, for example, have a duration of up to 5 ns. The gradually rising edge may, for example, have a duration of up to 20 ns. A gradually rising edge of about 10 ns can provide good efficiency.

[0078] A gradually falling edge, as depicted at the trailing edge of the pulse, is not required. A sharp falling edge can be used. Preferably, the pulse should be stopped as quickly as possible once all the tin droplets have been converted into EUV emission plasma. This is because any subsequent laser radiation from the pulse will not contribute to EUV generation.

[0079] Although Figure 4 Pulse 40 is schematically depicted as having a constant intensity for most of its duration, but in practice, the intensity of pulse 40 may fluctuate to some extent. The term "generally flat time distribution" can be interpreted, for example, as meaning that the pulse intensity does not change by more than 10% (e.g., more than 5%) for most of its duration. The pulse intensity can be within 3 sigma standard deviations. Various methods can be used to obtain a generally flat time distribution, such as using a pi-shaper and / or a deformable mirror.

[0080] Advantageously, the generally flat time distribution of pulse 40, combined with a pulse duration of 150 ns or more, allows the pulse to gradually vaporize the tin droplet. This does not provide gradual vaporization, as opposed to vaporization caused, for example, by pulses that include a main peak and a tail, but instead provides an initial peak followed by a steep drop. Such an initial peak is undesirable because it heats the tin droplet to a temperature much higher than the optimal vaporization temperature, and thus provides poor conversion efficiency to EUV. Providing a pulse with a generally flat time distribution allows the pulse intensity to be selected as needed (e.g., to match better conversion efficiency to EUV).

[0081] In this embodiment, the pulses 40 of the laser beam 2 do not have a Gaussian spatial distribution or other peak forms of spatial distribution. Alternatively, the pulses 40 of the laser beam 2 may have a top-hat profile (e.g., a generally flat spatial distribution). Figure 5 The image shows an example of the top-hat-shaped spatial distribution of the laser beam pulse 40. Figure 5 The intensity I is schematically depicted as a function of radial position R. The pulses 40 of the laser beam 2 can have a generally flat spatial distribution spanning a diameter of at least 100 micrometers.

[0082] Advantageously, the flat spatial distribution of the laser beam pulses 40 can provide uniform heating across the entire molten tin droplet 34. This can provide better conversion efficiency than, for example, laser pulses with a Gaussian spatial distribution. This is because a Gaussian spatial distribution will provide much more heating at the center of the molten tin droplet than at its edges, and will thus heat the molten tin droplet to a temperature much higher than the optimal temperature for EUV conversion efficiency.

[0083] Figure 6A laser beam 2 and a molten tin droplet 34 are schematically depicted at the plasma formation region 4. As schematically depicted, the laser beam 2 can be focused to form a waist at the plasma formation region 4. The diameter of the laser beam 2 is larger than the diameter of the molten tin droplet 34. Advantageously, this means that the molten tin droplet 34 can continue to be irradiated by the laser beam as it expands due to heating from the laser beam 2. This avoids the reduction in conversion efficiency to EUV radiation that would occur if a portion of the expanded molten tin droplet falls outside the laser beam 2. In other embodiments, the waist is not located at the plasma formation region 4. For example, if a laser beam diameter larger than the waist is desired, the waist can be located before or after the plasma formation region 4.

[0084] The molten tin droplet may have a trajectory perpendicular to the axis of the laser beam 2. The molten tin droplet may be propelled backward along the axis of the laser beam 2 upon vaporization. The distance the molten tin droplet travels may be, for example, 400 micrometers or more, e.g., up to 1.2 mm. In such a case, the plasma formation site may be described as elongated. The laser beam may be configured to have a desired diameter over the entire range of the elongated plasma formation site (e.g., a desired diameter along the length corresponding to the distance the molten tin droplet travels backward). The laser beam 2 may be configured to have a diameter of at least 100 micrometers at the plasma formation site along an axial distance of at least 400 micrometers (e.g., up to 1.2 mm). In such a context, the term "axial distance" may be interpreted as meaning in the direction of laser beam propagation.

[0085] As mentioned above, laser beam 2 (and therefore laser beam pulse 40) can have a top-hat-shaped spatial distribution. The diameter of laser beam 2 can be considered as the diameter of the generally flat top portion of laser beam 2.

[0086] The pulses 40 of the laser beam 2 can have a generally flat spatial distribution spanning a diameter of at least 100 micrometers (e.g., up to 150 micrometers).

[0087] The laser beam 2 can have a diameter at least twice the diameter of the tin droplet 34. During the combustion / vaporization of the tin droplet 34, the tin droplet, or some of the tin, can expand. Providing a laser beam 2 with a diameter at least twice the diameter of the tin droplet 34 ensures that the laser beam 2 is incident on the expanded tin, thereby converting the expanded tin into EUV radiation. The laser beam 2 can have a diameter up to three times the diameter of the tin droplet 34. The laser beam 2 can have a diameter greater than three times the diameter of the tin droplet 34 (e.g., up to four times the diameter of the tin droplet). However, such a large diameter may mean that the outer portion of the diameter does not interact with the tin and therefore does not contribute to the generation of EUV radiation (thus reducing the conversion efficiency to EUV radiation).

[0088] The pulse 40 of the laser beam 2 can, for example, have an energy of at least 1 J or greater. The pulse 40 of the laser beam can, for example, have an energy of 2 J or greater, such as up to 4 J. Pulses with energies greater than 4 J, such as up to 5 J, can only provide a small increase in the amount of EUV radiation produced. This can be applied to tin droplets, for example, with a diameter of 40 micrometers or greater, such as up to 50 micrometers. This is because additional energy is not desired to vaporize the tin, and it would also potentially cause the tin to be undesirably heated above the desired temperature for EUV radiation production.

[0089] The tin droplet 34 can be spherical with a diameter of at least about 40 micrometers, such as 45 micrometers or larger. Conventional LPP radiation sources can use spherical tin droplets with a diameter of about 30 micrometers. A spherical tin droplet with a diameter of 40 micrometers has up to 2.4 times the mass (and volume) of a 30-micrometer tin droplet. A spherical tin droplet with a diameter of 45 micrometers has up to 3.4 times the mass (and volume) of a 30-micrometer tin droplet. These larger droplets, compared to conventional tin droplets, are capable of producing significantly more EUV radiation than a 30-micrometer tin droplet. Advantageously, this allows the LPP radiation source SO to provide higher EUV radiation power than conventional LPP radiation sources.

[0090] When a laser pulse 40 with a duration of at least 150 ns and a wavelength between 1.6 μm and 2.5 μm is directed onto each molten tin droplet, effective EUV radiation generation can be achieved using only one laser pulse per droplet. This differs from conventional LPP arrangements, where one or two initial laser pulses are incident on the molten tin droplets before a laser pulse with a wavelength of approximately 10 μm. In conventional LPP arrangements, the initial laser pulse is used to alter the shape of the molten tin droplets and reduce their density to improve the efficiency of EUV generation. When using a laser pulse with a duration of at least 150 ns and a wavelength between 1.6 μm and 2.5 μm, it is not necessary to use an initial laser pulse to alter the shape of the molten tin droplets or reduce their density.

[0091] Laser beam 2 can have a minimum size of 0.7 × 10⁻⁶. 11 W / cm 2 For example, 1.4 × 10 11 W / cm 2 Or greater strength (1.4 × 10⁻⁶) 11 W / cm 2 Or provide more than 0.7×10 11 W / cm 2 Significantly higher EUV power). The intensity of the laser beam (which is higher than that in a conventional LPP radiation source) allows for the generation of more EUV radiation from molten tin droplets than in a conventional LPP radiation source.

[0092] Without the need for an initial laser pulse for the tin droplets before generating the EUV emission plasma, the conversion efficiency to EUV radiation is good (e.g., >3%). This good conversion efficiency is due to the wavelength between 1.6 and 2.5 micrometers, which more effectively vaporizes the liquid tin droplets (compared to a conventional 10-micrometer laser pulse). Therefore, in embodiments of the invention, an initial pulse is not used. The tin receives radiation only from one laser pulse. Consequently, the next laser pulse is incident on the next fuel droplet.

[0093] The duration of pulse 40 of laser beam 2 can be selected based on the diameter of the tin droplet (and therefore also the mass of the tin droplet). The diameter of the tin droplet is a known parameter determined by the diameter of nozzle 36 of fuel generator 3, as further explained above. For a tin droplet of a given diameter, the pulse duration can be selected to provide a conversion that transforms the entire tin droplet into EUV emission plasma (and thereby maximizes EUV generation). The selection of the pulse duration takes into account the intensity of the laser beam. This is further reflected in Tables 1 and 2 above.

[0094] To increase the amount of EUV radiation from the LPP radiation source, the number of molten tin droplets converted into EUV radiation per second can be increased. This can be referred to as the molten tin droplet repetition rate. However, increasing the molten tin droplet repetition rate means that (for a given droplet velocity) the droplets travel closer to each other as they reach the plasma-forming region. When the molten tin droplets vaporize at the plasma-forming region, they generate shock waves (which can include debris from the plasma, such as ions). These shock waves can alter the path of the next molten tin droplet, preventing it from reaching the plasma-forming region precisely.

[0095] Embodiments of the present invention advantageously avoid this problem by using larger tin droplets (compared to conventional LPP radiation sources) instead of increasing the repetition rate of the tin droplets. Larger tin droplets produce more EUV radiation (compared to smaller tin droplets) without requiring an increased repetition rate. Therefore, embodiments of the present invention can provide tin droplets at a repetition rate of 40 kHz or less (e.g., 30 kHz or less).

[0096] The controller CT of laser system 1 can control the laser system to operate using any of the parameters mentioned above. The controller CT can also control the modulation of the nozzle 36 of fuel generator 3, and thus control the repetition rate of the molten tin droplets. The controller CT can control other parts of the radiation source, such as optics 38. The controller CT may include a processor.

[0097] Although embodiments of the invention have been described in conjunction with spherical molten tin droplets, molten tin droplets may have other shapes. For example, molten tin droplets may be non-spherical or deformed due to the forces exerted by the vaporization of the preceding molten tin droplet. For example, molten tin droplets may be ellipsoidal. In the case of non-spherical molten tin droplets, the diameter mentioned herein in conjunction with molten tin droplets can be interpreted as meaning the maximum size of the non-spherical droplet. Typically, molten tin droplets are examples of tin targets.

[0098] While specific reference may be made herein to embodiments of the invention within the context of a photolithography apparatus, embodiments of the invention can be used in other apparatuses. Embodiments of the invention can form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing objects such as wafers (or other substrates) or masks (or other patterning apparatuses). These apparatuses can generally be referred to as photolithography tools. Such photolithography tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0099] Where circumstances permit, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any means for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, machine-readable media include read-only memory (ROM); random access memory (RAM); disk storage media; optical storage media; flash memory devices; electrical, optical, acoustic, etc. Furthermore, firmware, software, routines, and instructions may be described herein as performing certain actions. However, it should be understood that these descriptions are merely for convenience, and these actions are actually caused by a computing device, processor, controller, or other means executing firmware, software, routines, instructions, etc., and such actions may cause actuators or other means to interact with the physical world.

[0100] The following aspects are also part of this disclosure.

[0101] 1. A method for generating extreme ultraviolet (EUV) radiation, comprising directing a laser pulse of a laser beam onto a tin target to generate EUV emitting plasma, wherein the laser pulse has a duration of at least 150 ns and a wavelength between 1.6 micrometers and 2.5 micrometers.

[0102] 2. The method according to aspect 1, wherein the tin target is a droplet having a diameter of at least 40 micrometers.

[0103] 3. The method according to aspect 2, wherein the EUV emitting plasma emits at least 0.05 J of EUV radiation collected by a collector within a solid angle.

[0104] 4. The method according to aspect 1, wherein the tin target is a droplet having a diameter of at least 45 micrometers.

[0105] 5. The method according to aspect 4, wherein the EUV emitting plasma emits at least 0.07 J of EUV radiation collected by a collector within a solid angle.

[0106] 6. The method according to any of the foregoing aspects, wherein the tin target is provided at a repetition rate of 40 kHz or less.

[0107] 7. The method according to any of the foregoing aspects, wherein the laser pulse has a substantially flat time distribution.

[0108] 8. The method according to any of the foregoing aspects, wherein the laser pulses have a top-hat-shaped spatial distribution spanning a diameter of at least 100 micrometers.

[0109] 9. The method according to any of the foregoing aspects, wherein only one laser pulse is incident on each tin target.

[0110] 10. The method according to any of the foregoing aspects, wherein each laser pulse has an energy of at least 1 J.

[0111] 11. The method according to any of the foregoing aspects, wherein each laser pulse has an energy of up to 5 J.

[0112] 12. The method according to any of the foregoing aspects, wherein the laser beam incident on the tin target has a density of at least 0.7 × 10⁻⁶. 11 W / cm 2 The intensity.

[0113] 13. The method according to any of the foregoing aspects, wherein the leading edge of each laser pulse has a gradually rising edge with a duration of at least 5 ns.

[0114] 14. The method according to any of the foregoing aspects, wherein the laser beam has a diameter of at least 100 micrometers along an axial distance of at least 400 micrometers at the plasma formation site.

[0115] 15. A radiation source comprising: a nozzle configured to direct a tin target toward a plasma forming site; and a laser system configured to direct laser pulses having wavelengths between 1.6 micrometers and 2.5 micrometers toward the plasma forming site, wherein the radiation source further comprises a controller configured to control the laser system such that the laser pulses have a duration of at least 150 ns.

[0116] 16. The radiation source according to aspect 15, wherein the nozzle is configured to provide a tin target having a diameter of at least 40 micrometers or at least 45 micrometers.

[0117] 17. The radiation source according to aspect 15 or aspect 16, wherein the controller is configured to control the nozzle to provide the tin target at a repetition rate of 40 kHz or less.

[0118] 18. The radiation source according to any one of aspects 15 to 17, wherein the controller controls the laser system to provide only one laser pulse per tin target.

[0119] 19. The radiation source according to any one of aspects 15 to 18, wherein the laser system is configured to provide the laser pulse in a laser beam, the laser beam having a radius of at least 0.7 × 10⁻⁶ at the plasma formation site. 11 W / cm 2 The intensity.

[0120] 20. An exposure apparatus comprising a radiation source according to any one of aspects 15 to 19, and further comprising a lithography apparatus, a measurement apparatus, or an inspection apparatus.

[0121] While specific embodiments of the invention have been described above, it will be understood that the invention can be practiced in other ways than those described. The above description is intended to be illustrative and not restrictive. Therefore, those skilled in the art will understand that modifications can be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A method for generating extreme ultraviolet (EUV) radiation, comprising directing a laser pulse of a laser beam onto a tin target to generate EUV emitting plasma, wherein the laser pulse has a duration of at least 150 ns and a wavelength between 1.6 micrometers and 2.5 micrometers.

2. The method according to claim 1, wherein, The tin target is a droplet with a diameter of at least 40 micrometers.

3. The method according to claim 2, wherein, The EUV emitting plasma emits at least 0.05 J of EUV radiation collected by the collector within a solid angle.

4. The method according to claim 1, wherein, The tin target is a droplet with a diameter of at least 45 micrometers.

5. The method according to claim 4, wherein, The EUV emitting plasma emits at least 0.07 J of EUV radiation within a solid angle, which is collected by the collector.

6. The method according to any of the preceding claims, wherein, The tin target is provided at a repetition rate of 40 kHz or less.

7. The method according to any of the preceding claims, wherein, The laser pulses have a roughly flat time distribution.

8. The method according to any of the preceding claims, wherein, The laser pulses have a top-hat-shaped spatial distribution spanning a diameter of at least 100 micrometers.

9. The method according to any of the preceding claims, wherein, Only one laser pulse is incident on each tin target.

10. The method according to any of the preceding claims, wherein, Each laser pulse has an energy of at least 1 J, and / or each laser pulse has an energy of up to 5 J.

11. The method according to any of the preceding claims, wherein, The laser beam incident on the tin target has a density of at least 0.7 × 10⁻⁶. 11 W / cm 2 The intensity.

12. The method according to any of the preceding claims, wherein, Each laser pulse has a gradually rising edge with a duration of at least 5 ns at its leading edge.

13. The method according to any of the preceding claims, wherein, The laser beam has a diameter of at least 100 micrometers along an axial distance of at least 400 micrometers at the plasma formation site.

14. A radiation source, comprising: A nozzle configured to guide a tin target toward a plasma formation site; The radiation source also includes a laser system configured to direct laser pulses having wavelengths between 1.6 micrometers and 2.5 micrometers to the plasma-forming site, wherein the radiation source further includes a controller configured to control the laser system such that the laser pulses have a duration of at least 150 ns.

15. The radiation source according to claim 14, wherein, The nozzle is configured to provide a tin target having a diameter of at least 40 micrometers or at least 45 micrometers.

16. An exposure apparatus comprising a radiation source according to any one of claims 14 to 15, and further comprising a lithography apparatus, a measurement apparatus, or an inspection apparatus.

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