Optical system, in particular for microlithography, and method for operating optical system

Abstract

To provide an optical system, in particular for microlithography, and a method for operating the optical system.SOLUTION: The invention relates to an optical system, in particular for microlithography, and a method for operating the optical system. The optical system comprises a laser light source for generating multiple light pulses, and a control unit configured to control the laser light source such that, for a light pulse sequence generated by the laser light source, time intervals between individual successive light pulses vary across the light pulse sequence.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical system, in particular for microlithography, and to a method for operating an optical system.The invention is advantageously applicable in particular in laser light sources used in microlithography projection exposure apparatus. [Background technology]

[0002] Microlithography is used to produce microstructured electronic components. The microlithography process is carried out in what is known as a projection exposure apparatus, which has an illumination device and a projection lens. In order to transfer the mask structure into a photosensitive coating on the substrate, the image of the mask (= reticle), illuminated by the illumination device, is projected by the projection lens onto a substrate (e.g., a silicon wafer) that is covered with a photosensitive layer (photoresist) and placed in the image plane of the projection lens.

[0003] Projection exposure apparatus designed for operation in the DUV range (e.g. at operating wavelengths below 250 nm, in particular below 200 nm) typically use laser light sources in the form of excimer lasers, in particular krypton fluoride excimer lasers at an operating wavelength of 248 nm or argon fluoride excimer lasers at an operating wavelength of 193 nm.

[0004] Challenges that exist during the development of projection exposure apparatus relate, firstly, to the increasing precision requirements for producing ever smaller structures in microlithography and for positioning them on the wafer, and secondly, also to the increase in throughput that can be achieved with the respective projection exposure apparatus, which increase is desirable in terms of cost considerations.

[0005] A significant effect, particularly with regard to the precision requirements to be met during operation of the above-mentioned laser light sources, is the occurrence of what are known as speckle patterns due to the spatial and temporal coherence of the generated light, which results in, among other things, a reduction in the overlay precision of structures produced in different lithographic steps, which ultimately leads to a degradation of the optical performance of the projection exposure apparatus.

[0006] As for the aforementioned throughput increases achieved in the respective projection exposure apparatuses, one problem is the fact that limitations are imposed on increasing the pulse energy and / or repetition rate, each of which is desirable for increasing the output power of the laser source. In this regard, increasing the pulse energy leads to higher radiation loads on the optical components in the laser source and also to greater stress on the electronic assembly. One known approach in this regard involves stretching the pulses generated by the laser source (e.g., having a pulse length of 20 ns) to longer durations, e.g., 100-450 ns, by using an optical pulse stretcher circuit, thus correspondingly reducing the degradation of downstream optical components. However, limitations are also imposed on such pulse length stretching due to the structural space constraints that exist for the optical pulse stretcher circuit, as well as on the output power, which decreases as the number of reflections increases. Furthermore, there is still a risk of degradation of the aforementioned optical components located upstream of the optical pulse stretcher circuit in the optical beam path.

[0007] On the other hand, an increase in the repetition rate, and therefore an increase in the frequency of the discharge within the resonator of the laser source, which is also considered due to an increase in the output power of the laser source, may lead to acoustic resonances and ultimately to a spectral bandwidth that exceeds the acceptable measurement limits.

[0008] With regard to the prior art, reference is made, by way of example only, to WO2018 / 132198, U.S. Pat. No. 10,451,890, U.S. Pat. No. 11,054,665, U.S. Pat. No. 7,782,922, and the publication by H. Miyamoto et al., "Next generation ArF laser technologies for multiple-patterning immersion lithography supporting leading-edge processes", Optical Microlithography XXXI, Proc. of SPIE Vol. 10587 (2018), 1058710-1 to 1058710-8, and the publication by K. Kakizaki et al., "Ultra-high-repetition-rate ArF excimer laser with long pulse duration for 193-nm lithography", Optical Microlithography XIV, Proc. of SPIE Vol. 4346 (2001), pages 1210-1218. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] WO2018 / 132198 [Patent Document 2] U.S. Patent No. 10,451,890 [Patent Document 3] U.S. Patent No. 11,054,665 [Patent Document 4] U.S. Patent No. 7,782,922 [Non-patent literature]

[0010] [Non-Patent Document 1] H. Miyamoto et al., "Next generation ArF laser technologies for multiple-patterning immersion lithography supporting leading-edge processes," Optical Microlithography XXXI, Proc. of SPIE Vol.10587 (2018), 1058710-1 to 1058710-8 [Non-patent document 2] K. Kakizaki et al., "Ultra-high-repetition-rate ArF excimer laser with long pulse duration for 193-nm lithography," Optical Microlithography XIV, Proc. of SPIE Vol. 4346 (2001), pages 1210-1218 Summary of the Invention

[0011] The object of the present invention is to provide an optical system, in particular for microlithography, comprising a laser light source, which reduces the occurrence of speckle patterns and makes it possible to increase the output power of the laser light source, while at least partially avoiding the problems mentioned above, as well as a method for operating the optical system.

[0012] This object is achieved by an optical system according to the features of independent patent claim 1 and additionally by a method according to the features of independent patent claim 14.

[0013] According to the invention, an optical system, in particular for microlithography, comprises: a laser light source for generating a number of light pulses; a control unit configured to control the laser light source such that the time division between successive light pulses of a train of light pulses generated by the laser light source varies over the train of light pulses; Equipped with.

[0014] The concept underlying the present invention, particularly in an optical system comprising a laser light source for generating a large number of optical pulses, is to vary the time interval between two consecutive optical pulses in a pulse train, rather than selecting it to be constant as in the past. In this regard, the present invention particularly involves a deliberate detuning of the laser light source (in the sense of moving away from an operating state with a single, "optimized" time interval between consecutive optical pulses) with the aim of firstly achieving a reduction in speckle contrast (and thus improving the overlay performance of the optical system, which is crucially determined by the speckle contrast) and secondly, enabling an increase in the repetition rate of the laser light source (and thus increasing the throughput during operation of the optical system), while avoiding the excitation of unwanted resonances (chamber resonances) in the laser light source. In particular, the inventive variation of the time interval between two consecutive optical pulses in a pulse train or optical pulse train has the effect that the light field is different for each associated optical pulse, resulting in the loss of temporal and spatial coherence, which is crucial for speckle contrast.

[0015] According to one embodiment, the control unit is configured to control at least one actuator for manipulating the position of at least one optical component of the laser source.

[0016] According to one embodiment, the optical system comprises an optical pulse stretcher with a plurality of mirrors, in which case the optical components controlled in the actuators may in particular be mirrors of said optical pulse stretcher.

[0017] According to one embodiment, the control unit is configured to variably adjust the time delay of a trigger signal generated to trigger the supply of energy to the laser medium of the laser source. This embodiment is advantageous, in particular, insofar as any detrimental occurrence of acoustic resonances as described in the introduction can already be efficiently prevented before the light generated by the laser source reaches further components, such as an optical pulse stretcher. The laser medium may be the laser medium of a main oscillator of the laser source and / or the laser medium of one or more power amplification stages ("power amplifiers") of the laser source.

[0018] According to one embodiment, the optical system further comprises a first measurement unit for measuring a variable that is characteristic of the speckle contrast of the light generated by the laser light source, wherein the control unit can in particular be configured to vary the temporal pulse sequence of the light pulses depending on the output signal of said first measurement unit.

[0019] According to one embodiment, the optical system further comprises a second measurement unit for measuring a variable characteristic of the bandwidth of the light generated by the laser light source, the control unit being particularly here configured to vary the temporal pulse sequence of the light pulses depending on the output signal of said second measurement unit.

[0020] According to one embodiment, the control unit comprises a random number generator for the random variation of the time interval present between each two successive light pulses.

[0021] According to one embodiment, the control unit is configured to continuously increase or continuously decrease the time division existing between each two consecutive light pulses between a predefined lower limit value and a predefined upper limit value.

[0022] According to one embodiment, the laser source is designed to generate light pulses at a repetition rate of at least 7 kHz.

[0023] According to one embodiment, the optical system is designed for an operating wavelength of less than 250 nm, in particular for an operating wavelength of less than 200 nm.

[0024] The invention further relates to a method for operating an optical system, in particular for microlithography, which comprises a laser light source for generating a number of light pulses, and which is controlled such that for a train of light pulses generated by the laser light source, the time division existing between each successive light pulse varies over the train of light pulses.

[0025] According to one embodiment, the change is adjusted based on a measurement of a variable that is characteristic of the speckle contrast of the light produced by the laser light source.

[0026] According to one embodiment, the variation is adjusted based on a measurement of a respective bandwidth for at least one light pulse generated by the laser light source.

[0027] According to one embodiment, the variation is adjusted so that the average bandwidth of the light pulses produced by the laser source is reduced compared to the average bandwidth that would be obtained without this variation.

[0028] According to one embodiment, the variation is adjusted so that the average bandwidth difference between successive light pulses is reduced compared to the average bandwidth difference obtained without this variation.

[0029] According to one embodiment, the variation is at least partly caused by manipulating the position of at least one optical component of the laser source, in particular the position of at least one mirror of the optical pulse stretcher circuit.

[0030] According to one embodiment, the variation is at least partially effected by variably adjusting a time delay of a trigger signal generated to trigger the delivery of energy to the laser medium of the laser source.

[0031] Further embodiments of the invention are evident from the present description and the dependent claims.

[0032] The invention is explained in more detail below on the basis of exemplary embodiments illustrated in the accompanying drawings. [Brief explanation of the drawings]

[0033] [Figure 1] 3A and 3B are diagrams for explaining the change according to the present invention of the time interval existing between successive light pulses in the optical system according to the first embodiment. [Figure 2] 10A and 10B are diagrams for explaining the change according to the present invention in the time interval between successive light pulses in the optical system according to the second embodiment. [Figure 3] 10A and 10B are diagrams for explaining the change according to the present invention of the time interval existing between successive light pulses in the optical system according to the third embodiment. [Figure 4] 1 is a schematic diagram illustrating a possible configuration of an optical system according to the present invention; [Figure 5] 1 is a block diagram for further illustrating a possible function of the optical system according to the present invention; [Figure 6] 1A and 1B are schematic diagrams illustrating possible configurations of a microlithographic projection exposure apparatus designed for operation in the DUV; DETAILED DESCRIPTION OF THE INVENTION

[0034] Embodiments of the present invention are described below with reference to the diagrams in FIGS. 1-3 and the schematic diagrams in FIGS.

[0035] These embodiments have in common that in an optical system comprising a laser light source generating a large number of light pulses, the time interval existing between two successive light pulses in a pulse train or light pulse train is not selected to be constant as in the conventional case, but rather is varied, with the aim of firstly achieving a reduction in speckle contrast (thereby improving the overlay performance of the optical system) and secondly making it possible to increase the repetition rate of the laser light source (thereby increasing the throughput during operation of the optical system), while at the same time avoiding the excitation of unwanted resonances (chamber resonances).

[0036] 1, in a first embodiment, the time interval between two consecutive light pulses emitted by the laser light source can be varied randomly throughout the light pulse train. In this case, lower and upper limits can be predefined within which the random variation can be implemented. The vertical dashed lines represent equidistant lines for illustrative purposes.

[0037] In a second embodiment according to FIG. 2, the time interval between two consecutive light pulses emitted by the laser light source can also be continuously increased or continuously decreased over the train of light pulses, in which case again lower and upper limit values ​​can be predefined.

[0038] In a third embodiment according to FIG. 3, suitable pulse sequences for avoiding unwanted chamber resonances can also be targeted and then predefined for further operation of the laser source.

[0039] According to FIG. 3 , first, for a specific configuration of the laser source, a currently generated pulse train 310 (as a temporal profile of the laser output power) is determined. Then, by means of a fast Fourier transform (FFT), an amplitude spectrum 320 for said pulse train 310 is determined. Furthermore, the resulting laser bandwidth of the laser source as a function of the laser repetition rate is determined; an exemplary profile is also shown in FIG. 3 and is labeled “330.” In this laser bandwidth profile 330, frequency bands associated with occurring resonances (illustrated as “331” and “332” in FIG. 3 by way of example) are then identified. Based on this, an “optimized” amplitude spectrum 320 suitable for eliminating these resonances 331 and 332 is determined by means of a fast Fourier transform (FFT), which amplitude spectrum is labeled “340.” The resulting laser bandwidth of the laser source as a function of the laser repetition rate is labeled “350.” From this, a sequence of pulses 360 (corresponding to the temporal profile of the laser output power) is then determined which, in the present merely exemplary scenario according to FIG. 3, moves the temporal position of the optical pulse from "361" to "362".

[0040] FIG. 4 is a purely schematic diagram showing a possible configuration in principle of a gas discharge laser system in the form of an excimer laser in which the present invention can be implemented.

[0041] The gas discharge laser system according to FIG. 4 comprises, among other things, a seed laser 410, a relay optical unit 420, an amplifier stage 430, and a laser output subsystem 440.

[0042] The seed laser 410 comprises, inter alia, a linewidth reduction module 411, a master oscillator chamber (MO) 412, a master oscillator output coupling unit (MO OC) 413 and a line center analysis module 414.

[0043] The relay optical unit 420 has, among other things, the effect of adapting or aligning the output signal of the seed laser 410 with respect to the amplifier stage 430 and may in particular comprise a beam expansion unit with an appropriate prism arrangement as well as an appropriate optical delay path.

[0044] The amplifier stage 430 comprises in particular a chamber 432 with a laser medium, a beam reversal module 431 and a unit 433 for coupling out a portion of the laser beam.

[0045] The laser output subsystem 440 comprises, inter alia, a bandwidth analysis module 441, an optical pulse stretcher circuit 442, and a further module 443. The optical pulse stretcher circuit 442 serves to stretch the pulse length of the optical pulses generated by the laser source to longer durations of the order of magnitude of (100-450) ns, for example by deflecting the electromagnetic radiation via a circulating path, in order to avoid degradation of downstream optical components. The further module 443 first serves to adjust the output laser radiation (e.g., regarding the degree of polarization, near-field or far-field characteristics, Poynting vector, etc.). According to the invention, the functionality of this module 443 is now extended so that it is also designed to measure the speckle contrast. Furthermore, according to the invention, the module 443 can also be designed to measure the time length of the optical pulses.

[0046] According to the invention, based on the signal provided by module 443 (sensor signal), the optical pulse stretcher circuit 442 can now be detuned to achieve the required reduction in speckle contrast.

[0047] 5 shows, only schematically, a block diagram for illustrating the corresponding closed-loop control concept. In this case, in FIG. 5, the optical pulse stretcher circuit is designated as "510" and the further module is designated as "520". Based on the sensor signal provided by module 520, closed-loop control unit 530 controls actuator 540, thereby causing the above-mentioned detuning of optical pulse stretcher circuit 510 suitable for reducing speckle contrast. For example, one or more mirrors in optical pulse stretcher circuit 510 can be mechanically adjusted by actuator 530.

[0048] The present invention is not limited to the above-mentioned control or detuning of the optical pulse stretcher circuit to achieve the desired speckle contrast reduction. In a further embodiment, the generation of the trigger signal can also be controlled in time, for example, for triggering a gas discharge based on the signal (sensor signal) provided by the bandwidth analysis module 441 or the line center analysis module 414, so that the trigger signal can be generated early or late by a few nanoseconds (ns) or microseconds (μs).

[0049] FIG. 6 shows a possible configuration in principle of a microlithography projection exposure apparatus 600 designed for operation in the DUV.

[0050] The projection exposure apparatus 600 according to Fig. 6 comprises an illumination device 610 and a projection lens 620. The illumination device 610 serves to illuminate a structure-bearing mask (reticle) 615 with light from a light source unit 605, which comprises a laser light source, for example in the form of an ArF excimer laser for an operating wavelength of 193 nm (or in the form of a KrF excimer laser for an operating wavelength of 248 nm), and a beam-shaping optical unit for generating a collimated light beam. In this case, the laser light source can be designed in a manner according to the invention.

[0051] The illumination device 610 comprises an optical unit 611, which in the example shown comprises, inter alia, a deflection mirror 612. The optical unit 611 may comprise, for example, a diffractive optical element (DOE) and a zoom axicon system for generating different illumination settings (i.e., intensity distributions in the pupil plane of the illumination device 610). Downstream of the optical unit 611 in the direction of light propagation, a light mixing device (not shown) is arranged in the beam path, which may, for example, in a manner known per se, comprise an arrangement of micro-optical elements suitable for realizing light mixing and a group of lens elements 613, downstream of which a field plane is arranged by a reticle masking system, which field plane is imaged onto a structure-bearing mask (reticle) 615, which is arranged in a further field plane by a lens 614 arranged downstream in the direction of light propagation, thereby delimiting the illumination field on the reticle. The structure-bearing mask 615 is imaged by a projection lens 620 onto a substrate or wafer 630 provided with a photosensitive layer (photoresist). In particular, the projection lens 620 can be designed for immersion operation, in which case, with respect to the light propagation direction, an immersion medium is arranged upstream of the wafer or its photosensitive layer. Furthermore, the projection lens 620 can have, for example, a numerical aperture NA greater than 0.85, in particular a numerical aperture NA greater than 1.1.

[0052] While the present invention has also been described based on particular embodiments, many variations and alternative embodiments will be apparent to those skilled in the art, for example, by combining and / or substituting features of the particular embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the present invention is limited only in the sense of the appended claims and equivalents thereof. [Explanation of symbols]

[0053] 410 Seed Laser 411 Linewidth Reduction Module 412 Main Oscillator Chamber 413 Master Oscillator Output Coupling Unit 414 Line Center Analysis Module 420 Relay Optical Unit 430 Amplification stage 431 Beam Reversal Module 432 Chamber 433 units 440 Laser Output Subsystem 441 Bandwidth Analysis Module 442 Optical Pulse Enlarger Circuit 443 More Modules 510 Optical pulse stretcher circuit 520 More Modules 530 Closed-loop control unit, actuator 600 Microlithography Projection Exposure Tool 605 Light Source Unit 610 Lighting Devices 611 Optical Unit 612 Deflecting Mirror 613 Lens Element Group 614 Lens 615 Structure-bearing mask 620 projection lens 630 wafers

Claims

1. An optical system, in particular for microlithography, comprising: a laser light source for generating a number of light pulses; a control unit configured to control the laser light source such that, for a train of light pulses generated by the laser light source, the time division existing between successive light pulses varies over the train of light pulses; An optical system comprising:

2. 2. The optical system of claim 1, wherein the control unit is configured to control at least one actuator for manipulating a position of at least one optical component of the laser light source.

3. 3. The optical system according to claim 1, further comprising an optical pulse stretcher circuit having a plurality of mirrors.

4. 4. The optical system according to claim 2, wherein said optical component is a mirror of said optical pulse stretcher circuit.

5. The optical system according to any one of claims 1 to 4, characterized in that the control unit is configured to variably adjust a time delay of a trigger signal generated to trigger the supply of energy to a laser medium of the laser light source.

6. The optical system according to any one of claims 1 to 5, further comprising a first measuring unit for measuring a variable that is characteristic of the speckle contrast of the light generated by the laser light source.

7. 7. The optical system according to claim 6, characterized in that the control unit is configured to vary the temporal pulse sequence of the light pulses depending on the output signal of the first measurement unit.

8. The optical system according to any one of claims 1 to 7, further comprising a second measuring unit for measuring a variable that is characteristic of the bandwidth of the light generated by the laser light source.

9. 9. The optical system according to claim 8, characterized in that the control unit is configured to vary the temporal pulse sequence of the light pulses depending on the output signal of the second measurement unit.

10. 10. The optical system according to claim 1, wherein the control unit comprises a random number generator for the random variation of the time interval between each two successive light pulses.

11. 11. The optical system according to claim 1, wherein the control unit is configured to continuously increase or continuously decrease the time division existing between each two consecutive light pulses between a predefined lower limit value and a predefined upper limit value.

12. The optical system according to any one of claims 1 to 11, characterized in that the laser light source is designed to generate the light pulses at a repetition rate of at least 7 kHz.

13. Optical system according to any one of claims 1 to 12, characterized in that it is designed for an operating wavelength of less than 250 nm, in particular for an operating wavelength of less than 200 nm.

14. A method for operating an optical system, in particular for microlithography, comprising a laser light source for generating a number of light pulses, the laser light source being controlled such that for a train of light pulses generated by the laser light source, the time division existing between successive light pulses varies over the train of light pulses.

15. 15. The method of claim 14, wherein the change is adjusted based on a measurement of a variable that is characteristic of the speckle contrast of the light produced by the laser light source.

16. 16. The method of claim 14 or 15, wherein the variation is adjusted based on a measurement of a respective bandwidth for at least one light pulse generated by the laser light source.

17. 17. The method of claim 16, wherein the variation is adjusted such that the average bandwidth of the light pulses produced by the laser light source is reduced compared to the average bandwidth obtained without the variation.

18. 18. A method according to claim 16 or 17, characterized in that the variation is adjusted so that the average bandwidth difference between successive light pulses is reduced compared to the average bandwidth difference obtained without the variation.

19. 19. The method according to any one of claims 14 to 18, characterized in that the variation is at least partly caused by manipulating the position of at least one optical component of the laser light source, in particular the position of at least one mirror of an optical pulse stretcher circuit.

20. 20. The method according to any one of claims 14 to 19, characterized in that the variation is at least partly caused by variably adjusting a time delay of a trigger signal generated to trigger the supply of energy to the laser medium of the laser light source.

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