Electric discharge laser with acoustic chirp correction

Abstract

Methods and structural changes in gas discharge lasers for minimizing wavelength chirp at high pulse rates. Applicants have identified the major cause of wavelength chirp in high pulse rate gas discharge lithography lasers as pressure waves from a discharge reflecting back to the discharge region coincident with a subsequent discharge. The timing of the arrival of the pressure wave is determined by the temperature of the laser gas through which the wave is traveling. During burst mode operation, the laser gas temperature in prior art lasers changes by several degrees over periods of a few milliseconds. These changing temperatures change the location of the coincident pressure waves from pulse to pulse within the discharge region causing a variation in the pressure of the laser gas which in turn affects the index of refraction of the discharge region causing the laser beam exiting the rear of the laser to slightly change direction. This change in beam direction causes the grating in the LNP to reflect back to the discharge region light at a slightly different wavelength causing the wavelength chirp. Solution to the problem is to include in the laser chamber structural elements to moderate or disperse the pressure waves and to maintain the laser gas temperature as close as feasible to constant values.