OPTICAL SYSTEM, IN PARTICULAR FOR MICROLITHOGRAPHY, AND METHOD FOR OPERATING AN OPTICAL SYSTEM

Abstract

An optical system, in particular for microlithography, comprises a laser light source for generating a multiplicity of light pulses, and a control unit configured to control the laser light source in such a way that, for a light pulse sequence generated by the laser light source, the time period between respectively successive light pulses varies across the light pulse sequence. A method comprises operating the optical system.

Claims

1. (canceled)

2. An optical system, comprising: a laser light source configured to generate a multiplicity of light pulses; and a control unit configured to control the laser light source so that, for a light pulse sequence generated by the laser light source, a time period between respectively successive light pulses varies across the light pulse sequence, wherein the control unit is configured to continuously increase or to continuously decrease the time period between each two successive light pulses.

3. The optical system of claim 2, further comprising a measuring unit configured to measure a variable that is characteristic of a speckle contrast of the light generated by the laser light source, wherein the control unit is configured to control the laser light source so that, for the light pulse sequence generated by the laser light source, the time period between respectively successive light pulses varies across the light pulse sequence depending on output signals the measuring unit.

4. The optical system of claim 2, further comprising a measuring unit configured to measure a variable that is characteristic of a bandwidth of the light generated by the laser light source, wherein the control unit is configured to control the laser light source so that, for the light pulse sequence generated by the laser light source, the time period between respectively successive light pulses varies across the light pulse sequence depending on output signals the measuring unit.

5. The optical system of claim 2, further comprising a first measuring unit and a second measuring unit, wherein: the first measuring unit configured to measure a variable that is characteristic of a speckle contrast of the light generated by the laser light source; the second measuring unit configured to measure a variable that is characteristic of a bandwidth of the light generated by the laser light source the control unit is configured to control the laser light source so that, for the light pulse sequence generated by the laser light source, the time period between respectively successive light pulses varies across the light pulse sequence depending on output signals the first measuring unit; and the control unit is configured to control the laser light source so that, for the light pulse sequence generated by the laser light source, the time period between respectively successive light pulses varies across the light pulse sequence depending on output signals the second measuring unit.

6. The optical system of claim 2, further comprising an actuator, wherein the laser light source comprises an optical component, the actuator is configured to manipulate the position of the optical component, and the control unit is configured to control the actuator.

7. The optical system of claim 6, further comprising an optical pulse stretcher comprising a plurality of mirrors, wherein the optical component comprises a mirror of the plurality of mirrors.

8. The optical system of claim 2, wherein: the laser light source comprises a laser medium; and the control unit is configured to variably adjust a temporal delay of a trigger signal generated to trigger an energy feed into the laser medium.

9. The optical system of claim 2, wherein the control unit comprises a random number generator configured to randomly vary a time period between each two successive light pulses.

10. The optical system of claim 2, wherein the laser light source is configured to generate the light pulses with a repetition rate of at least 7 kHz.

11. The optical system of claim 2, wherein the optical system is configured to operate at an operating wavelength of less than 250 nm.

12. An apparatus, comprising: an optical system according to claim 2; an illumination device; and a projection lens, wherein the apparatus is a microlithographic projection exposure apparatus.

13. A method of operating an optical system comprising a laser light source configured to generate a multiplicity of light pulses and a controller configured so that, for a light pulse sequence generated by the laser light source, a time period between respectively successive light pulses varies across the light pulse sequence, the method comprising: i) continuously increasing the time period between each two successive light pulses; or ii) continuously decreasing the time between each two successive light pulses.

14. The method of claim 13, wherein varying the time period between each two successive light pulses is based on a measurement of a variable that is characteristic of a speckle contrast of the light generated by the laser light source.

15. The method of claim 14, wherein varying the time period between each two successive light pulses is based on a measurement of a respective bandwidth for at least one light pulse generated by the laser light source.

16. The method of claim 13, wherein varying the time period between each two successive light pulses is based on a measurement of a respective bandwidth for at least one light pulse generated by the laser light source.

17. The method of claim 13, wherein varying the time period between each two successive light pulses reduces an average bandwidth of the light pulses generated by the laser light source.

18. The method of claim 17, wherein varying the time period between each two successive light pulses reduces an average bandwidth difference between successive light pulses.

19. The method of claim 13, wherein varying the time period between each two successive light pulses reduces an average bandwidth difference between successive light pulses.

20. The method of claim 13, wherein varying the time period between each two successive light pulses comprises manipulating a position of an optical component of the laser light source.

21. The method of claim 13, wherein varying the time period between each two successive light pulses comprises adjusting a temporal delay of a trigger signal generated to trigger an energy feed into a laser medium of the laser light source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In the figures:

[0032] FIG. 1 shows a diagram for elucidating a variation according to the disclosure of the time period existing between successive light pulses in an optical system in accordance with a first embodiment;

[0033] FIG. 2 shows a diagram for elucidating a variation according to the disclosure of the time period existing between successive light pulses in an optical system in accordance with a second embodiment;

[0034] FIG. 3 shows diagrams for elucidating a variation according to the disclosure of the time period existing between successive light pulses in an optical system in accordance with a third embodiment;

[0035] FIG. 4 shows a schematic illustration for elucidating the possible set-up of an optical system according to the disclosure;

[0036] FIG. 5 shows a block diagram for elucidating further the possible functioning of an optical system according to the disclosure; and

[0037] FIG. 6 shows a schematic illustration for elucidating the possible set-up of a microlithographic projection exposure apparatus designed for operation in the DUV.

DETAILED DESCRIPTION

[0038] Embodiments of the present disclosure are explained below with reference to the diagrams in FIGS. 1-3 and also the schematic illustrations in FIGS. 4-5.

[0039] What these embodiments have in common is that in an optical system comprising a laser light source for generating a multiplicity of light pulses, the time period existing between two successive light pulses within a pulse succession or a light pulse sequence is not to be chosen to be constant in a conventional way, but rather is varied, with the aim firstly of achieving a reduction of the speckle contrast (and thus an improvement in the overlay performance of the optical system) and secondly of making it possible to increase the repetition rate of the laser light source (and thus to increase the throughput during operation of the optical system) while at the same time avoiding an excitation of undesired (chamber) resonances.

[0040] Referring firstly to FIG. 1, in a first embodiment, the time period between two successive light pulses emitted by the laser light source can be varied randomly across a light pulse sequence. In this case, a lower limit value and an upper limit value can be predefined, in particular, within which this random variation can be carried out. The vertical dashed lines represent equidistant lines for illustration.

[0041] In a second embodiment in accordance with FIG. 2, the time period between two successive light pulses emitted by the laser light source can also be continuously increased or continuously reduced over a light pulse sequence, in which case once again a lower limit value and an upper limit value can be predefined.

[0042] In a third embodiment in accordance with FIG. 3, a pulse succession suitable for avoiding undesired chamber resonances can also be determined in a targeted manner and then be predefined for the further operation of the laser light source.

[0043] In accordance with FIG. 3, a currently generated pulse succession 310 (as temporal profile of the laser output power) is firstly determined here for a concrete configuration of the laser light source. An amplitude spectrum 320 is then determined for this pulse succession 310 in accordance with a fast Fourier transform (FFT). Furthermore, the resulting laser bandwidth of the laser light source as a function of the laser repetition rate is determined, an exemplary profile likewise being illustrated and designated by 330 in FIG. 3. In this profile 330 of the laser bandwidth, occurring resonances and the associated frequency bands (designated by way of example by 331, 332 in FIG. 3) are then identified. On the basis thereof, an optimized amplitude spectrum 320 suitable for eliminating these resonances 331, 332 is determined in accordance with a fast Fourier transform (FFT), the amplitude spectrum being designated by 340. The resulting laser bandwidth of the laser light source as a function of the laser repetition rate is designated by 350. From this a pulse succession 360 modified in a targeted manner (corresponding to the temporal profile of the laser output power) is then determined, wherein in the present scenario in accordance with FIG. 3 and merely by way of example the temporal position of a light pulse is shifted from 361 to 362.

[0044] FIG. 4 shows, in a merely schematic illustration, the set-up possible in principle of a gas discharge laser system in the form of an excimer laser in which the present disclosure can be realized.

[0045] The gas discharge laser system in accordance with FIG. 4 comprises, in particular, a seed laser 410, a relay optical unit 420, an amplifying stage 430 and a laser output subsystem 440.

[0046] The seed laser 410 comprises, in particular, a linewidth narrowing module 411, a master oscillator chamber (MO=master oscillator chamber) 412, a master oscillator output coupling unit (MO OC=master oscillator output coupler) 413 and a line centre analysis module 414.

[0047] The relay optical unit 420 has the effect, inter alia, of adapting or aligning the output signal of the seed laser 410 with respect to the amplifying stage 430 and can comprise, in particular, a beam expanding unit with a suitable prism arrangement and also a suitable optical retardation path.

[0048] The amplifying stage 430 comprises, in particular, a chamber 432 having the laser medium, a beam reversing module 431 and a unit 433 for coupling out part of the laser beam.

[0049] The laser output subsystem 440 comprises, in particular, a bandwidth analysis module 441, an optical pulse stretcher 442 and a further module 443. The optical pulse stretcher 442 serves to stretch the pulse length of the light pulses generated by the laser light source to a longer time duration of the order of magnitude of (100-450) ns, for example, by deflecting the electromagnetic radiation by way of circulation paths, in order to avoid degradation of downstream optical components. The further module 443 serves firstly for qualifying the output laser radiation (e.g. with regard to degree of polarization, near-field or far-field properties, Poynting vector, etc.). According to the disclosure, the functionality of this module 443 is now extended in such a way that the module 443 is furthermore also designed for measuring the speckle contrast. Moreover, according to the disclosure, the module 443 can also be designed for measuring the temporal duration of the light pulses.

[0050] On the basis of the (sensor) signals supplied by the module 443, according to the disclosure, the optical pulse stretcher 442 can now be detuned so as to achieve the sought reduction of the speckle contrast.

[0051] FIG. 5 shows, merely schematically, a block diagram for elucidating a corresponding closed-loop control concept. In this case, in FIG. 5, the optical pulse stretcher is designated by 510 and the further module by 520. On the basis of the sensor signals supplied by the module 520, a closed-loop control unit 530 controls an actuator 540 and thereby brings about the aforementioned detuning of the optical pulse stretcher 510 that is suitable for reducing the speckle contrast. For example, one or more mirrors in the optical pulse stretcher 510 can be mechanically adjusted by way of the actuator 530.

[0052] The disclosure is not restricted to the above-described control or detuning of the optical pulse stretcher in order to realize the desired speckle contrast reduction. In further embodiments, on the basis of the (sensor) signals supplied by the bandwidth analysis module 441 or by the line centre analysis module 414, the generation of the trigger signal for triggering the gas discharge can also be controlled temporally, such that e.g. the trigger signal can be generated earlier or later by a few nanoseconds (ns) or microseconds (s).

[0053] FIG. 6 shows a set-up possible in principle of a microlithographic projection exposure apparatus 600 designed for operation in the DUV.

[0054] The projection exposure apparatus 600 in accordance with FIG. 6 comprises an illumination device 610 and a projection lens 620. The illumination device 610 serves for illuminating a structure-bearing mask (reticle) 615 with light from a light source unit 605 comprising a laser light source for example in the form of an ArF excimer laser for an operating wavelength of 193 nm (or else in the form of a KrF excimer laser for an operating wavelength of 248 nm) and also a beam shaping optical unit that generates a parallel light beam. In this case, the laser light source can be designed in the manner according to the disclosure.

[0055] The illumination device 610 comprises an optical unit 611 which, inter alia, comprises a deflection mirror 612 in the example illustrated. The optical unit 611 can comprise for example a diffractive optical element (DOE) and a zoom-axicon system for producing different illumination settings (i.e. intensity distributions in a pupil plane of the illumination device 610). Downstream of the optical unit 611 in the light propagation direction there are situated in the beam path a light mixing device (not illustrated), which in a manner known per se, for example, can comprise an arrangement of micro-optical elements suitable for achieving light mixing, and also a lens element group 613, downstream of which is situated a field plane with a reticle masking system, which is imaged by a lens 614 disposed downstream in the light propagation direction onto the structure-bearing mask (reticle) 615 arranged in a further field plane and thereby delimits the illuminated region on the reticle. The structure-bearing mask 615 is imaged by the projection lens 620 onto a substrate, or a wafer 630, provided with a light-sensitive layer (photoresist). In particular, the projection lens 620 can be designed for immersion operation, in which case an immersion medium is situated upstream of the wafer, or the light-sensitive layer thereof, in relation to the light propagation direction. Furthermore, it can have for example a numerical aperture NA greater than 0.85, in particular greater than 1.1.

[0056] Even though the disclosure has also been described on the basis of specific embodiments, numerous variations and alternative embodiments will be apparent to a person skilled in the art, e.g. through combination and/or exchange of features of individual embodiments. Accordingly, it goes out saying for a person skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present disclosure, and the scope of the disclosure is restricted only within the meaning of the appended patent claims and the equivalents thereof.