KrF lasers with up to 500 Hz repetition rate and a bandwidth of about 1 pm are in use for DUV-microlithography. Increasing resist sensitivity demands for even higher repetition rate are needed in order to allow precise dose control. In some cases step & scan exposure tools apply reflective optics instead of refractive ones. This diminishes the bandwidth requirements by about two orders of magnitude, but a high polarization degree of >= 98% is a basic requirement for the laser light source. Dose control and statistics define the second basic requirement for the laser. A dose accuracy of less than 2% demands small energy increments, i.e., for pulse energy in the 10 to 20 mJ range. Throughput requires 10 to 20 W of average laser power. Therefore, the repetition rate must be in the 1 kHz range.
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