Performance Improvement of an ArF Excimer Laser for Microlithography by Means of Gaseous Impurity Control

Abstract

The effect of small amounts of gaseous additives and impurities on ArF laser performance has been studied. It has been found that the output characteristics of the ArF laser improves significantly at a xenon concentration of 10 ppm but strongly deteriorates at impurity concentrations above 10 ppm of e.g. O2, CF4 and HF, especially, at high laser repetition rates. A new laser discharge chamber has therefore been developed using metal seals and coating the inner chamber walls with thin fluoride layers. The layers prevent impurities that are otherwise generated through chemical reactions between the fluorine of the laser gas and the chamber wall. The output stability of the ArF excimer laser has been very successfully improved by the new chamber design for repetition rates up to 4 kHz.

Elucidation of Steplike Output Energy Decrease Observed in ArF Excimer Laser for Microlithography

“…The latter requires a custom-made laser chamber that suppresses the generation of impurity gases. [11][12][13] However, even for a high-purity gas containing Xe at 10 ppm inside the custom-made laser chamber, we observed a steplike decrease in the output energy of the ArF excimer laser under burst-mode operation. The steplike energy decrease occurred just after the first circulation of the laser gas in the chamber.…”

“…Various tests [12][13][14] concerning the steplike energy decrease of the ArF excimer laser confirmed that the energy recovers after a rest time of approximately 0.2 s during burst operation, and that the ratio of the decrease in the energy decreases with increasing gas temperature. Because the absorbing compound F x O y is unstable, its dissociation time and the dependences of its concentration on gas temperature and initial oxygen concentration were investigated.…”

“…Custom-made clean chambers and high-purity laser gases were used in order to exclude the influence of small amounts of gaseous impurities on ArF excimer laser performance. 13) We also exercise care to prevent the accumulation of impurity gases in the test chamber by laser discharges. The test chamber was refilled with fresh laser gas before the lasershot number reached one million so that the total amount of uncontrolled impurity gases should be well below 10 ppm.…”

“…ArF laser energy decrease Finally, the measured results of the absorption rates of the laser lights at 193 nm were compared with the steplike ArF laser energy decrease reported previously. 13) For this comparison, the absorption measured using two ArF excimer lasers 30 ms after the end of a burst of 1000 pulses at 4 kHz has been considered [Fig. 6(a)].…”

Investigation of a Step-Like Output Energy Decrease Observed in an ArF Excimer Laser for Microlithography

“…It is well known that both parameters are significantly improved by adding a small amount (typically 10 ppm) of xenon (Xe) 9,10) and by controlling concentration of impurity gases such as oxygen (O 2 ) and hydrogen fluoride (HF) to be below 10 ppm. 11,12) In order to maintain the concentration of impurities at the lowest level, we have developed a specially designed laser chamber 13) that can suppress the generation of impurity gases. However, even for a high-purity gas containing Xe at 10 ppm and the special laser chamber, we observed a steplike decrease of the output energies of the ArF excimer laser under the burst operation.…”