Excimer Lasers for Super-High NA 193 nm Lithography

Transcription

1 Excimer Lasers for Super-High NA 193 nm Lithography Rainer Paetzel, Hans Stephan Albrecht, Peter Lokai, Wolfgang Zschocke, Thomas Schmidt, Igor Bragin, Thomas Schroeder, Christian Reusch, Stefan Spratte Lambda Physik AG, Hans-Boeckler-Straße 12, D-3779 Goettingen, Germany Tel: Fax: paetzel@lambdaphysik.com ABSTRACT Excimer lasers are widely used as the light source for microlithography scanners. The volume shipment of scanner systems using 193nm is projected to begin in year 23. Such tools will directly start with super high numerical aperture (NA) in order to take full advantage of the 193 nm wavelength over the advanced 248 nm systems. Reliable high repetition rate laser light sources enabling high illumination power and wafer throughput are one of the fundamental prerequisites. In addition these light sources must support a very high NA imaging lens of more than.8 which determines the output spectrum of the laser to be less than.3 pm FWHM. In this paper we report on our recent progress in the development of high repetition rate ultra-narrow band lasers for high NA 193 nm microlithography scanners. The laser, NovaLine A43, is based on a Single Oscillator Ultra Linenarrowed (SOUL ) design which yields a bandwidth of less than.3 pm FWHM. The SOUL laser enables superior optical performance without adding complexity or cost up to the 4 khz maximum repetition rate. The A43 s high precision line-narrowing optics used in combination with the high repetition rate of 4 khz yields an output power of 2 W at an extremely narrow spectral bandwidth of less than.3 pm FWHM and highest spectral purity of less than.75 pm for the 95% energy content. We present performance and reliability data and discuss the key laser parameters. Improvements in the laser-internal metrology and faster regulation control result in better energy stability and improved overall operation behavior. The design considerations for line narrowing and stable laser operation at high repetition rates are discussed. Keywords: excimer laser, optical microlithography, 193 nm, ultra-narrow line, high spectral purity 1. INTRODUCTION The two main performance requirements of excimer laser for microlithograhy are high output power in combination with a ultra-narrow emission spectrum. Power is required to utilize the scanner cost efficiently. The spectral requirements get tighter in order to support high contrast imaging with super-high numerical aperture (NA) of >.8. These requirements present a general conflict in the optimization of the excimer laser. Optical elements in the lasers resonator in order to reduce the spectral width present losses, which cause a drop in power. Operation at high power levels again causes stress to these optical elements and a deterioration of the output spectrum may occur. For 193nm requirements with output power of 2 W and a bandwidth of less than.3 pm, FWHM we have developed the NovaLine A43 laser. The NovaLine A43 is the perfect match for next generation high NA 193nm scanners. This laser is based on a single laser chamber and therefore avoids the complexity of dual chamber strategies such as MOPA (master oscillator / power amplifier) or MOPO (master oscillator / power oscillator).

2 2. DESIGN ASPECTS Excimer lasers for microlithography are operating with a repetition rate of 4 khz and 2 W output power as a standard. Optimization of the laser discharge unit over years has lead to stable operation with good energy stability at 4 khz repetition rate and in our experimental lasers even up to 6 khz. When combining this laser discharge unit with an optical resonator to achieve an ultra-narrow bandwidth of i.e..3 pm, FWHM it becomes apparent that there is strong influence from the laser gain medium on the spectral performance. At low repetition rate and low power level the spectrum is mainly determined by the quality of the line narrowing module; wave front distortions are getting minimized by using high quality CaF 2 prism beam expander with perfect surface finish and coating. In order to maintain the spectrum at higher power levels and higher repetition rates more aspects must be covered in the design. 2.1 Mechanical Stability It was found to be critical to address the mechanical stability of the laser system in order to minimize the effect on the laser parameter, pre-dominantly wavelength stability, bandwidth and energy stability. The mechanical stability determines the reproducibility of the laser after certain maintenance, such as LDU (Laser Discharge Unit) exchange. Mechanical stability is also required to isolate mechanical vibrations from the optical resonator in order to achieve a stable beam in pointing and position. The source of mechanical vibrations is mainly found in the required gas circulation, which is employed to clear the gas discharge volume between consecutive laser pulses. For this a highspeed transversal blower system is used which operates with circulation speed of about 3 rpm. The complete gas circulation system consists of the blower within the gas, a magnetic coupler, a 3-phase asynchrony motor and a frequency inverter to control the ramp-up, speed and ramp-down of the blower system. Careful mechanical balancing of the blower together with coupler and motor minimizes the generation of mechanical vibrations. There remains another main source for mechanical vibrations, which is the gas discharge in the laser tube operated at 4kHz. Acoustic shock waves are generated with the laser trigger rate and the design must ensure that the resulting mechanical vibrations are not translated into other module like the line narrowing optics. For this task Lambda Physik has developed a passive damping system, which comprises in the so-called LDU suspension. For the NovaLine A43 laser the LDU suspension has been greatly improved in comparison to former models. Requirement for the LDU suspension is that it fixes the LDU in its position, enables accurate adjustment for laser optimization, leads to high position reproducibility after exchange of the LDU and minimizes the mechanical coupling to the laser chassis and in particular laser optics. Relative movement of the LDU against the resonator optics influences the laser parameters like the wavelength. In the laser design the dispersion of the line narrowing optics acts on the horizontal axis (y), perpendicular to the laser beam propagation. wavelength /pm laser pulse # Wavelength /pm laser pulse # Figure 1 Wavelength stability - previous LDU suspension without active stabilization Figure 2 Wavelength stability - new LDU suspension without active stabilization Relative movement on this axis leads to a direct influence of the laser parameter. The vertical axis (z) also influences the laser performance and must get tightly controlled. Movement in the direction of the beam propagation axis is less

3 critical. The new design comprises of a new end stop fixation and improved vibration isolation. Movement in the horizontal and vertical axis is reduced by a factor 3 6 and as a result vibration coupling into the chassis and laser optics is avoided. Figure 1 and Figure 2 compare results on wavelength stability achieved with the old and with the improved LDU suspension without active stabilization. The graphs show at same scale the single pulse wavelength stability (blue) and the moving average of 46 pulses (black). The inherent wavelength stability of the laser improved by more than factor 3. The active wavelength stabilization of the laser in combination with this excellent inherent stability ensures wavelength stability of better than ±.3 pm for a moving average window of 1 laser pulses. 2.2 Optical Stability Operation at 193nm wavelength requires careful selection of materials and perfect purging of the beam path. At a laser power level significant amount of energy is deposited in the optical elements and modules due to absorption losses of the elements. These losses finally lead to local heating of elements. The design must take care that the delicate optical elements are at preferred constant temperature and no temperature gradients occur. Such temperature gradients within these elements lead to wave front distortions and give direct impact on the wavelength selection and stability. When looking at ultra-narrow bandwidth of less than.3 pm, FWHM combined with high power the purge gas condition in the vicinity of the critical optical elements in the line-narrowing module is getting important. The purge gas is in direct contact with all optical surfaces and provides not only purging with clean gas but also cooling of the elements and surfaces. Careful selection of the purge gas type, pressure and purge flow characteristic are required to minimize influence of the high laser power on the bandwidth of the laser. The purge gas flow controls the convection and avoids local in-homogeneities, which would in return lead to wavefront distortion and degradation of the bandwidth. Norm. Intensity Rel. Wavelength [pm] bandwidth:.26pm, FWHM spectral purity:.6pm, E95% Figure 3 Bandwidth of NovaLine A43 For the NovaLine A43 we completed a study on these purge gas parameter, which has lead to a new optimization of the purge gas system. With this system influences of the laser power and operation cycle on the bandwidth and wavelength are avoided. Figure 3 shows the resulting bandwidth of the NovaLine A43 laser. The measured bandwidth is.26 pm, FWHM with a spectral purity of less than.6 pm for the 95% energy content. This result is obtained with the optimized purge gas system and re-presents an improvement of 15 2% when compared to high power operation with our former resonator design.

4 With the progress of 193 nm excimer lasers in terms of repetition rate, power and bandwidth the demand on the quality of optical materials has increased a lot. The high repetition rate operation of 193 nm lasers requires the discharge geometry to become smaller. Remaining with 5 mj pulse energy this leads to an energy density of about 5 mj/cm 2 respectively power level of 2 W / cm 2. To maintain with stable bandwidth of less than.3 pm, FWHM over multi-billion laser pulses the influence of the optical modules and laser windows on the optical laser performance must get minimized. For the NovaLine A43 we have addressed the laser windows. The windows not only see the highest energy density but also face pressure and temperature gradients, since they are in direct contact with the gas. Contamination of the window surface presents another possible problem, which for the NovaLine High 4 khz 8mJ U [kv] laser pulses [mio] Figure 4 Operating voltage over laser pulses of 193 nm laser A43 laser has been completely eliminated. For this the electrostatic gas cleaning system of the laser discharge unit has been improved significantly. Gas flow has been adapted for the 4 khz operation and the window purging with clean gas improved. As a result even after 3.5 billion pulses at an increased energy level there is virtually no sign of any contamination or degradation of the windows detectable. 2.3 Wavelength Monitoring With the narrower bandwidth a tighter control of the wavelength is required in order to take full advantage for the imaging. Two aspects of the wavelength are important which is the absolute wavelength accuracy and the relative wavelength and reproducibility. The relative wavelength is detected and stabilized by a monitor etalon system Signal 2 Signal Pixel Pixel Figure 5 Signal of standard monitor Figure 6 Signal of re-designed monitor

5 A built-in hollow-cathode lamp routinely calibrates the wavelength of the monitoring scale. The hollow cathode lamp gives reference to the atomic transition of iron at nm. For the NovaLine A43 laser the wavelength and bandwidth monitor system have been redesigned. The finesse of the monitoring optics has been increased to achieve a higher wavelength resolution of the monitor system. A new camera system is employed which gives improved signal to noise ratio and therefore in combination with the higher finesse monitor optics leads to an increased resolution of the monitor system. The camera system is based on a CCD line sensor; it has been tested to show very little degradation under 193 nm irradiation. With these design feature the new monitor system meets the demand on spectral resolution of <.1 pm, wavelength stability and lifetime of more than 5 billion laser pulses. The performance of the new monitor system can be seen in figure 5 and figure 6. A comparison of the detected signal with the standard and the re-designed system is shown. The reproducibility of the absolute calibration is improved by the increased resolution of the wavelength monitor system. To perform an absolute wavelength calibration of the monitor system the laser wavelength gets scanned and simultaneously the signal of the hollow-cathode lamp detected. For the advanced fast computer system of the NovaLine A43 the calibration procedure of the wavelength has been revised. The routine now uses more measurement points to fit the measured signal of the lamp and still allows re-calibration with minimum downtime of 3 seconds. Such re-calibration may be automatically performed during any overhead time of laser or scanner and therefore any reduction in system uptime is avoided. Figure 7 Reproducibility of absolute wavelength calibration 22 A43 wavelength calibration reproducibility # of events ,4 -,3 -,2 -,1,,1,2,3,4 wavelength deviation after wavelength check [pm] Figure 7 shows the result of 25 consecutive calibrations. The reproducibility of the absolute wavelength calibration is found to be ±.1 pm. This extremely small uncertainty in the absolute wavelength leads to reproducible imaging performance of the scanner and allows precise tweaking of the wavelength to minimize imaging aberrations and optimize contrast performance. 3. PERFORMANCE To confirm the module lifetimes and the stable performance over time the NovaLine A43 laser has been in a longterm endurance test. This so called IRONMAN (Improving Reliability of New Machinery at Night) simulates the various operating conditions of the laser and turned out to be an effective tool to identify improvements on modules and software. In the following some performance data of the NovaLine A43, taken from the IRONMAN test are shown. The most important performance aspect, which in the microlithography application takes direct influence on the imaging performance and critical dimension, is the bandwidth and energy stability. Figure 8 shows the bandwidth

6 of the laser plotted over 3 billion laser pulses of the IRONMAN test. Over this period no degradation of the bandwidth is observed. The square dots represent the bandwidth detection of the lasers built-in monitor system, whereas the diamond dots show the bandwidth value measured by an external high resolution grating spectrometer. The deviation between the internal bandwidth and the externally measured bandwidth is quite small over the whole period. The bandwidth shows a good margin of about 2% over this period and there is no indication of degradation. Bandwidth (Spectrum Grating, deconvoluted) Bandwidth [pm] Bandwidth, FWHM Grat. Spectr., FWHM Specification Pulse Counter [mio] Figure 8 Bandwidth of NovaLine A43 The energy stability of the laser takes direct influence on achieving the accurate dose on the wafer. For the process it is important to achieve a high repeatability of the dose. The dose repeatability is continuously monitored over the test. The target value is a dose stability of.3% for a moving average window of 112 laser pulses. Within the IRONMAN test the energy is controlled by lasers built-in energy regulation loop, which utilizes the feedback signal from the lasers built-in energy monitor. The performance of the NovaLine A43 laser is shown in figure 9. Dose Repeatability (MAVG Burst Dose Repeatibility [%] Dose Repeatibility Specs Laser Pulse [mio] Figure 9 Dose Stability of NovaLine A43

7 The dose stability of the NovaLine A43 laser was measured to be.15% at the 1 billion test point and is found to be.2% at the 3 billion laser pulses test point which still present a high margin versus the specification limit. 4. CONCLUSION The NovaLine A43 laser has been developed for 193 nm microlithography. Improvements in core technology development enables this laser product which delivers ultra-line-narrowed light essential for high contrast imaging at lens NA`s exceeding.8.the laser provides high power of 2 W and a bandwidth of less than.3 pm, FWHM. The design is based on a single oscillator (SOUL) and proves to deliver stable performance and good module lifetime. This design approach ensures a small footprint of the laser, favorable operating cost and avoids complexity of oscillator amplifier systems. Further, improvements in optical materials and coating technology have led to greatly increased lifetimes of the optical modules and reduced operating cost. Superior quality of the optical modules supports a high efficiency operation of the laser and extended component lifetime. 5. REFERENCES 1. Wolfgang Zschocke, Hans Stephan Albrecht, Thomas Schroeder, Igor Bragin, Martin Sprenger, Farid Seddighi, Christian Reusch, Anna Cortona, Kai Schmidt, Rainer Paetzel, Klaus Vogler, High Repetition Rate Excimer Lasers for 193nm Lithography ; SPIE Microlithography 22 Santa Clara, CA March Rainer Paetzel, Klaus Vogler, Hans Stephan Albrecht, Thomas Schroeder, Igor Bragin, Juergen Kleinschmidt, Wolfgang Zschocke, High power 193nm excimer lasers for DUV lithography, SPIE Microlithography 21, Santa Clara, CA, USA, 25 Feb. 2 Mar Ingo Klaft, Frank Voss, Igor Bragin, Elko Bergmann, Tamas Nagy, Norbert Niemoeller, Stefan Spratte, Klaus Vogler, Sergei Govorkov, Gongxue Hua, High-Power High-Repetition Rate F2-lasers for 157 nm Lithography, SPIE Microlithography 22 Santa Clara, CA March 22