Optical system especially for microlithography and method for operating the 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 in particular to an optical system for microlithography and a method for operating the optical system. The present invention is advantageously applicable in particular to laser light sources used in microlithography projection exposure apparatuses.

Background Art

[0002] Microlithography is used to fabricate microstructured electronic components. The microlithography process is carried out in what is known as a projection exposure apparatus having an illumination device and a projection lens. In order to transfer the mask structure onto the photosensitive coating of the substrate, the image of the mask (= reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection lens.

[0003] In projection exposure apparatuses designed for operation in the DUV range (for example at operating wavelengths of less than 250 nm, in particular less than 200 nm), a laser light source in the form of an excimer laser is usually used, in particular a krypton fluoride excimer laser at an operating wavelength of 248 nm or an argon fluoride excimer laser at an operating wavelength of 193 nm.

[0004] Problems that exist during the development of projection exposure apparatuses relate, firstly, to the increasing accuracy requirements for producing ever more finely structured structures in microlithography and positioning them on the wafer, and secondly, to the increase in throughput achieved with each projection exposure apparatus, which increase is desirable from the perspective of cost considerations.

[0005] In particular, a significant impact that occurs regarding the accuracy requirements to be met during operation of the laser light source described above is the occurrence of what is known as a speckle pattern resulting from the spatial and temporal coherence of the generated light. Such a speckle pattern results in, among other things, a reduction in the overlay accuracy of structures fabricated in different lithography processes, thereby ultimately leading to a degradation in the optical performance of the projection exposure apparatus.

[0006] As far as the above-mentioned increase in throughput achieved with each projection exposure apparatus is concerned, one problem consists of the fact that limitations are imposed on the increase in pulse energy and / or the increase in the repetition rate, each of which is desirable to increase the output power of the laser light source. In this regard, an increase in pulse energy leads to a higher radiation load on the optical components within the laser light source and also to a greater stress on the electronic assembly. One approach known in this regard is that the pulses generated by the laser light source (for example, having a pulse length of 20 ns) are stretched to a longer duration, for example, up to (100 - 450) ns, by the use of an optical pulse stretching circuit, thereby correspondingly reducing the degradation of the downstream optical components. However, limitations are also imposed on the stretching of the pulse length as described above due to the structural space constraints existing for the optical pulse stretching circuit as well as on the output power that decreases as the number of reflections increases. Furthermore, the risk of degradation of the above-mentioned optical components arranged upstream of the optical pulse stretching circuit with respect to the optical beam path still exists.

[0007] On the other hand, an increase in the repetition rate, and thus an increase in the frequency of the discharge within the resonator of the laser light source, which is likewise considered for increasing the output power of the laser light source, may lead to acoustic resonance and ultimately to a spectral bandwidth exceeding the acceptable measurement values.

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

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Documents

[0010]

Non-Patent Document 1

[0011] An object of the present invention is to provide an optical system for microlithography, particularly comprising a laser light source, and a method for operating the optical system, which can reduce the generation of a speckle pattern and increase the output power of the laser light source while at least partially avoiding the above-described problems.

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

[0013] According to the present invention, an optical system for microlithography, in particular, comprises - a laser light source for generating a plurality of optical pulses, - a control unit configured to control the laser light source such that, for an optical pulse train generated by the laser light source, the time intervals existing between successive optical pulses vary over the optical pulse train and is provided with.

[0014] The concept underlying the present invention, particularly in an optical system comprising a laser light source for generating a plurality of optical pulses, is not to select the time interval existing between two consecutive optical pulses within a pulse train to be constant as in the prior art, but rather to vary it. In this regard, the present invention particularly includes a deliberate detuning of the laser light source (in the sense of deviating from an operating state having a single "optimized" time interval between consecutive optical pulses), and first, to achieve a reduction in the speckle contrast (and thus an improvement in the overlay performance of the optical system, which is critically determined by the speckle contrast), and second, to enable an increase in the repetition rate of the laser light source while avoiding the excitation of unwanted resonances (chamber resonances) in the laser light source (and thus increasing the throughput during operation of the optical system). In particular, the change according to the present invention in the time interval existing between two consecutive optical pulses within a pulse train or an optical pulse train has the effect that the light irradiation field is different for each associated optical pulse, and as a result, the temporal and spatial coherence, which is decisive for the speckle contrast, is lost.

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

[0016] According to one embodiment, the optical system comprises an optical pulse stretching circuit having a plurality of mirrors. In this case, the optical component controlled by the actuator can in particular be the mirror of the optical pulse stretching circuit.

[0017] According to one embodiment, the control unit is configured to variably adjust the time delay of a trigger signal generated to trigger the energy supply to the laser medium of the laser light source. In particular, as long as any adverse occurrence of acoustic resonance as described in the introduction part can already be efficiently prevented before the light generated by the laser light source reaches further components such as, for example, an optical pulse stretching circuit, this embodiment is beneficial. The laser medium can be the laser medium of the master oscillator of the laser light source and / or the laser medium of one or more power amplification stages ("power amplifiers") of the laser light source.

[0018] According to one embodiment, the optical system further comprises a first measurement unit for measuring a variable which is a characteristic of the speckle contrast of the light generated by the laser light source. In this case, the control unit can be specifically configured to change the temporal pulse sequence of the optical pulses in response to the output signal of the first measurement unit.

[0019] According to one embodiment, the optical system further comprises a second measurement unit for measuring a variable which is a characteristic of the bandwidth of the light generated by the laser light source. The control unit can be specifically configured here to change the temporal pulse sequence of the optical pulses in response to the output signal of the second measurement unit.

[0020] According to one embodiment, the control unit comprises a random number generator for the irregular variation of the time intervals existing between each two consecutive optical pulses.

[0021] According to one embodiment, the control unit is configured to continuously increase or continuously decrease the time intervals existing between each two consecutive optical pulses between a pre-defined lower limit value and a pre-defined upper limit value.

[0022] According to one embodiment, the laser light source is designed to generate optical 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 present invention further relates to a method for operating an optical system, particularly for microlithography, wherein the optical system comprises a laser light source for generating a number of optical pulses, and the laser light source is controlled such that the time intervals between successive optical pulses in the optical pulse train generated by the laser light source vary across the optical pulse train.

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

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

[0027] According to one embodiment, the change is adjusted such that the average bandwidth of the optical pulses generated by the laser light source is reduced compared to the average bandwidth obtained without this change.

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

[0029] According to one embodiment, the change is at least partially 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 the optical pulse stretching circuit.

[0030] According to one embodiment, the change is at least partially caused by variably adjusting the time delay of a trigger signal generated to trigger the energy supply to the laser medium of the laser light 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

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[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 are common in that in an optical system comprising a laser light source that generates a number of optical pulses, the time intervals existing between pulses in a pulse train or between two consecutive optical pulses in a train of optical pulses are not selected to be constant as in the prior art, but rather are varied. First, it is aimed at achieving a reduction in speckle contrast (and thereby an improvement in the overlay performance of the optical system), second, at making it possible to increase the repetition rate of the laser light source (and thereby increase the throughput during operation of the optical system), and at the same time avoiding the excitation of unwanted resonances (chamber resonances).

[0036] Referring first to FIG. 1, in a first embodiment, the time interval between two consecutive optical pulses emitted by the laser light source can be varied randomly over the train of optical pulses. In this case, lower and upper limit values can be defined in advance, and in particular the above-mentioned random variation can be carried out within that range. The vertical dashed lines represent equidistant lines for illustration purposes.

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

[0038] In a third embodiment according to FIG. 3, the succession of pulses suitable for avoiding unwanted chamber resonances is also determined to be subject, and can then be defined in advance for further operation of the laser light source.

[0039] According to FIG. 3, first, due to the specific configuration of the laser light source, the continuous 310 of the currently generated pulses (as the time profile of the laser output power) is determined here. Next, by means of a fast Fourier transform (FFT), the amplitude spectrum 320 for the continuous 310 of the above pulses is determined. Further, the resulting laser bandwidth of the laser light source according to the laser repetition rate is determined, and in FIG. 3, an exemplary profile is similarly illustrated and shown as "330". In this profile 330 of the laser bandwidth, the frequency bands (shown as "331" and "332" as an example in FIG. 3) associated with the generated resonances are then identified. Based on this, an "optimized" amplitude spectrum 320 suitable for removing these resonances 331 and 332 is determined according to the fast Fourier transform (FFT), and the said amplitude spectrum is shown as "340". The resulting laser bandwidth of the laser light source according to the laser repetition rate is shown as "350". From this, the continuous 360 of the pulses modified as the target (corresponding to the time profile of the laser output power) is then determined, and in this scenario shown merely as an example according to FIG. 3, the time position of the optical pulse moves from "361" to "362".

[0040] FIG. 4 is merely a schematic diagram and shows a theoretically possible configuration 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 particularly comprises a seed laser 410, a relay optical unit 420, an amplification stage 430, and a laser output subsystem 440.

[0042] The seed laser 410 particularly comprises a linewidth reduction module 411, a master oscillator chamber (MO), a master oscillator output coupler (MO OC), 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 amplification stage 430, and may in particular comprise a beam expansion unit having an appropriate prism configuration and an appropriate optical delay path.

[0044] The amplification stage 430 particularly comprises a chamber 432 having a laser medium, a beam inversion module 431, and a unit 433 that combines and outputs a part of the laser beam.

[0045] The laser output subsystem 440 particularly comprises a bandwidth analysis module 441, an optical pulse stretching circuit 442, and a further module 443. The optical pulse stretching circuit 442 functions to stretch the pulse length of the optical pulse generated by the laser light source to a longer duration on the order of (100 - 450) ns, for example by deflecting the electromagnetic radiation via a circulation path to avoid degradation of downstream optical components. The further module 443 functions first to optimize the output laser radiation (e.g., with respect to polarization, near-field or far-field characteristics, pointing vector, etc.). According to the present invention, the functionality of this module 443 is here extended such that the module 443 is also designed to measure the speckle contrast. Further, according to the present invention, the module 443 may also be designed to measure the temporal length of the optical pulse.

[0046] According to the present invention, based on the signal (sensor signal) supplied by the module 443, the optical pulse stretching circuit 442 may here be detuned to achieve the required reduction in speckle contrast.

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

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

[0049] FIG. 6 shows a possible configuration in principle of a microlithographic projection exposure apparatus 600 designed for operation in 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 and comprises, for example, a laser light source 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 parallel light beam. In this case, the laser light source can be designed according to the method according to the present 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 Inversion Module 432 Chamber 433 Unit 440 Laser Output Subsystem 441 Bandwidth Analysis Module 442 Optical Pulse Stretching Circuit 443 Further Module 510 Optical Pulse Stretching Circuit 520 Further Module 530 Closed-Loop Control Unit, Actuator 600 Micro Lithography Projection Exposure Device 605 Light Source Unit 610 Illumination Device 611 Optical Unit 612 Deflection Mirror 613 Lens Element Group 614 Lens 615 Structure-Supporting Mask 620 Projection Lens 630 Wafer

Claims

1. An optical system, particularly for microlithography, comprising: - a laser light source for generating a plurality of optical pulses; - an optical pulse stretching circuit for stretching the pulse length of the optical pulses generated by the laser light source; - a control unit configured to control the laser light source such that, for the train of optical pulses generated by the laser light source, the time intervals existing between successive optical pulses vary over the train of optical pulses; wherein: - the optical system comprises a first measurement unit for measuring a variable which is a characteristic of the speckle contrast of the light generated by the laser light source, and the control unit is configured to vary, over the train of optical pulses, the time intervals existing between successive optical pulses in response to the output signal of the first measurement unit; - the control unit is configured to detune the optical pulse stretching circuit so that the speckle contrast decreases and to vary the temporal succession of the optical pulses, based on the output signal of the first measurement unit; An optical system characterized by the above.

2. The optical system according to claim 1, characterized in that the control unit is configured to control at least one actuator for operating the position of at least one optical component of the laser light source.

3. The optical system according to claim 1 or 2, characterized in that the optical pulse stretching circuit has a plurality of mirrors.

4. The optical system according to claim 2, characterized in that the optical component is a mirror of the optical pulse stretching circuit.

5. An optical system, particularly for microlithography, comprising: - a laser light source for generating a plurality of optical pulses; - a control unit configured to control the laser light source such that, for the train of optical pulses generated by the laser light source, the time intervals existing between successive optical pulses vary over the train of optical pulses; wherein the laser light source is a gas discharge laser light source. - The optical system includes a second measurement unit for measuring a variable that is a characteristic of the bandwidth of the light generated by the laser light source, and the control unit temporally controls the generation of a trigger signal for triggering a gas discharge based on the output signal of the second measurement unit, reduces the speckle contrast, and is configured to change the temporal pulse succession of the light pulses. An optical system characterized by this. **Claim 6** The optical system according to claim 1 or 5, characterized in that the control unit includes a random number generator for the irregular variation of the time interval existing between each two consecutive light pulses. **Claim 7** The optical system according to claim 1 or 5, characterized in that the control unit is configured to continuously increase or continuously decrease the time interval existing between each two consecutive light pulses. **Claim 8** The optical system according to claim 1 or 5, characterized in that the laser light source is designed to generate the light pulses at a repetition rate of at least 7 kHz. **Claim 9** The optical system according to claim 1 or 5, characterized in that it is designed for an operating wavelength of less than 250 nm. **Claim 10** The optical system according to claim 1 or 5, characterized in that it is designed for an operating wavelength of less than 200 nm. **Claim 11** A method for operating an optical system, particularly for microlithography, the optical system including a laser light source for generating a plurality of light pulses, an optical pulse stretching circuit for stretching the pulse length of the light pulses generated by the laser light source, and a first measurement unit for measuring a variable that is a characteristic of the speckle contrast of the light generated by the laser light source, and for the train of light pulses generated by the laser light source, the laser light source is controlled such that the time interval existing between consecutive light pulses varies over the train of light pulses. Based on the measured value of the variable that is a characteristic of the speckle contrast of the light generated by the laser light source, the variation is adjusted. Based on the output signal of the first measurement unit, the optical pulse stretching circuit is detuned such that the speckle contrast decreases, and the temporal pulse succession of the light pulses is changed. A method characterized by this. **Claim 12** A method for operating an optical system, particularly for microlithography, the optical system comprising a laser light source for generating a plurality of optical pulses and a second measurement unit for measuring a variable which is a characteristic of the bandwidth of the light generated by the laser light source, the laser light source being a gas discharge laser light source, and the laser light source being controlled such that for the train of optical pulses generated by the laser light source, the time intervals existing between successive optical pulses vary over the train of optical pulses, Based on the output signal of the second measurement unit, the generation of a trigger signal for triggering a gas discharge is temporally controlled such that the contrast of the speckle is reduced, and the temporal succession of the optical pulses is varied. A method characterized thereby.

13. The method according to claim 11 or 12, characterized in that the variation is adjusted such that the average bandwidth of the optical pulses generated by the laser light source is reduced as compared to the average bandwidth of the optical pulses obtained without such variation.

14. The method according to claim 11 or 12, characterized in that the variation is adjusted such that the average bandwidth difference is reduced as compared to the average bandwidth difference between successive optical pulses obtained without such variation.

15. The method according to claim 11, characterized in that the variation is at least partially caused by operating the position of at least one optical component of the laser light source.

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