Recent advances in an excimer laser source for microlithography

This paper describes the advances made in a KrF excimer laser source designed specifically for the deep-UV reduction stepper. Data is presented on key performance aspects that are required for the laser to be used in the next generation of excimer steppers. Major topics discussed will be spectral bandwidth reduction, center wavelength control, pulse-to-pulse energy stability, higher power performance, and the production worthiness of the system. The first generation excimer laser (CX-2LS) for lithography operated at 200 Hz and 3 W, with a spectral bandwidth of 3 pm and a wavelength stability of +/- 0.5 pm. The new laser (ELS-4000) is designed and engineered to perform at 400 Hz and 4 W with a narrower spectral bandwidth (2 pm) and a wavelength stability of better than +/- 0.25 pm. In addition, the pulse-to-pulse stability has been improved in stepper-mode operation. The ELS-4000 also employs a new microprocessor controlled gas injection system that permits uninterrupted operation for 10-20 h (7-10 million pulses). This level of performance assures the compatibility of the laser with the requirements of production excimer steppers being developed with higher numerical aperature lenses, larger fields, and higher wafer throughputs.