Laser-Produced Plasma Light Sources for EUV Lithography
DAVID C. BRANDT, IGOR V. FOMENKOV, BRUNO M. LA FONTAINE and MICHAEL J. LERCEL, Cymer, Inc., San Diego, CA
LPP sources are the leading source technology for EUV lithography. Today's sources have an average power of 50W at intermediate focus, at 80% duty cycle using pre-pulse technology.
Laser-produced plasma (LPP) sources have been developed as the primary approach for EUV lithography for optical imaging of circuit features at sub-22nm design rules. EUV lithography is the front runner for next generation critical dimension imaging after 193nm immersion lithography for critical layer patterning. Leading device manufacturers have taken delivery of first generation EUV scanners in 2011 and are ramping those tools to pilot-line capability in 2012.
FIGURE 1. Scale drawing of laser produced plasma source.
The LPP source architecture is shown in Fig. 1. The three major subsystems of the source are the drive laser, the beam transport system (BTS) and the source vessel. The drive laser is a CO2 laser with multiple stages of amplification to reach the required power level of up to 30 kW. It is operated in pulsed mode at 50 kHz with radio-frequency (RF) pumping from generators (not shown) operating at 13.56 MHz. The laser is typically installed in the sub-fab along with its RF generators. The laser beam is expanded as it leaves the drive laser to lower the energy density on the BTS mirrors. The laser and BTS are completely enclosed and interlocked to meet laser Class 1 requirements. The BTS delivers the beam to a focusing optic where the CO2 light at 10.6 ??m wavelength is focused to a minimum spot size inside the vessel. A droplet generator delivers liquid tin droplets of 30 ??m diameter at 50 kHz repetition rate; both laser pulse and droplet are steered and timed to ensure proper targeting. The laser pulse vaporizes and heats the tin into a plasma cloud of critical temperature and density. The EUV light emitted by the plasma is collected and reflected with a multilayer-coated ellipsoidal mirror to the intermediate focus (IF) where it passes through a small aperture into the scanner volume that houses the illumination optics.
FIGURE 2a. 32W average power with better than ??0.5% dose stability.
FIGURE 2b. Dose stability histogram at 32W.
Recent test results are shown in Figs. 2a and 2b. The test configuration included upgrades to the drive laser which allow continuous operation of the source during the full die scan and operation at high duty cycle. 32W average power with better than ??0.5% 3?? dose stability is shown. During this test the source was running at 92% duty cycle with burst duration of 2 seconds.
The test source configuration also includes a pre-pulse capability, which can operate at the full repetition rate of the source (50kHz). The pre-pulse technology allows the droplet to be conditioned by a first laser pulse with energy less than that of the main heating pulse, which then strikes the target after it has developed into a tin vapor cloud of lower density which enables a higher conversion efficiency (CE) into EUV energy at ?? =13.5nm. Testing was conducted to characterize the pre-pulse operational mode; the results are shown in Fig. 3a and 3b. The duty cycle of the source was increased up to 80% where the source was operated at an average EUV power of ~50W.
FIGURE 3a. 50W average power at 80% duty cycle using pre-pulse.
FIGURE 3b. 90W burst power at 20% duty cycle (18W average power) using pre-pulse.
At 20% duty cycle the average power in the burst was approximately 90W (open-loop operation) using pre-pulse technology, as shown in Fig. 3b. This corresponds to 18W average power; however, it is a significant milestone that demonstrates the capability of the source to generate EUV power levels very close to the final 100W target.
Significant advancements in collector lifetime were made in 2011, as shown in Fig. 4. This improvement is primarily due to the increased robustness of the collector reflective coatings now being used. The open circles were the results of our original life test completed in 2010. The results of two new coatings on collectors installed on sources at chipmakers show the current performance and their associated significant lifetime improvement. Continued advancement of coatings and debris mitigation techniques are expected to enable further increases of the collector lifetime to meet the ultimate goal of one year.
FIGURE 4. Recent collector lifetime results for sources in the field.
LPP sources are the leading source technology for EUV lithography. Eight LPP sources have been built and are operational at leading device makers. An average power of 50W at intermediate focus at 80% duty cycle using pre-pulse technology has been shown. Collector lifetime of >30 billion pulses while maintaining high average reflectivity are being produced in volume. The combination of 10.6 ??m laser light and Sn droplet source element is proving to provide reliable operation, with the sources in the field having now reached pilot line capability.
DAVID C. BRANDT, is senior director EUV Marketing, IGOR V. FOMENKOV, is MTS Fellow EUV, BRUNO M. LA FONTAINE, is senior director EUV lithography applications, and MICHAEL J. LERCEL, is senior director EUV product marketing, at Cymer, Inc. San Diego, CA, www.cymer.com.
Solid State Technology, Volume 55, Issue 5, June 2012