Gas discharge excimer lasers require three or more part, mixtures of high purity gases to operate efficiently. Operation of excimer lasers is expensive, because they require a gas mixture composed of two or more costly, high-purity noble gases, e.g. Ne, Ar, Xe, Kr, and a highly reactive halogen gas, e.g. F, Cl. During operation of the excimer laser, the halogen gas component reacts with materials inside the laser, e.g. C, H, and is depleted from the gas mixture requiring periodic replacement. Halogen depletion coincides with the formation of impurities within the laser chamber, which impairs laser operation reducing the laser output power. The impurities reduce laser output through a combination of UV light absorption, scattering and discharge kinetics degradation. The output of the laser can be degraded by 50% when concentrations of impurities as low as 0.1% (1000 ppm) of the gas mixture are reached.
In order to maintain a constant power from the laser, the voltage applied to the laser's electrodes can be increased to overcome the reduction in output power caused by the contaminants and the depleted halogen. Unfortunately, the higher voltages lead to a more rapid deterioration of the electrode materials in the laser, and a large increase in maintenance costs. A portion of the laser output energy can be recovered by simply replacing the depleted halogen in the laser chamber; however, without a means to remove the impurities, the laser gas mixture must be eventually replaced to return the laser back to full output.
Accordingly, a significant portion of the operating cost of an excimer laser is therefore related to the contamination of costly, high purity, noble gases, e.g. argon, krypton, xenon and neon.
A conventional solution to extend gas lifetime or recover the expensive noble gas is disclosed in U.S. Pat. No. 6,735,233 issued May 11, 2004 to Osmanous et al, illustrated in FIG. 1, in which the use of a heated refractory metal, e.g. zirconium, filament for removal of impurities, such as H2O, N2 and O2, to extend the life of a gas discharge laser.
With reference to FIG. 2, Saito et al, discloses, in a paper entitled “Long Lifetime Operation of an ArF-Excimer Laser” in the Applied Physics B 62 (Laser and Optics) 1996, Pages 229-235, a dual purifier unit in which a liquid nitrogen-cooled condenser is in parallel with a halogen (F2) gas remover for performing separate filtration processes.
FIG. 3 illustrates a purification system disclosed in U.S. Pat. No. 4,629,611 issued Dec. 16, 1986 to Fan, in which a multi-stage filtration process is followed by a complicated gas replenishment system involving at least three separate source bottles and an equal number of flow controllers.
U.S. Pat. No. 6,215,806 issued Apr. 10, 2001 to Ohmi et al, illustrated in FIG. 4, discloses the use of premix gas supply bottles, but unfortunately, a plurality of supply bottles is still required along with a complicated valve and pressure control systems. Moreover, a single premix bottle cannot be used for both initial fill up and continuous replenishment of laser gases.
An object of the present invention is to overcome the shortcomings of the prior art by providing an apparatus and method, which incorporates a unique change to the design concept of an excimer laser, whereby the expensive noble gases are reclaimed, the halogen gas is sacrificed, and the impurities developed during operation of the excimer laser are removed. In the approach disclosed, a multi-stage gas purifier is used with a single, premix halogen-rich gas bottle containing noble, buffer and halogen gas, to maintain the optimum halogen content of the laser while also maintaining a consistent ratio of noble, buffer and halogen gases in the laser chamber.