Experimentation and modeling of organic photocontamination on lithographicoptics

Photodeposition of organic films on transparent substrates irradiated in th e presence of trace levels (ppb to ppm) of hydrocarbons has been experiment ally investigated and a model is presented that describes the film growth b ehavior. The efficacy of a given organic precursor at forming a deposit is proportional to the product of its surface coverage (as governed by its par tial pressure relative to its saturation partial pressure) and by its photo n absorption cross section. These measurements are important in predicting the transmission characteristics of lithographic optics operating at 157, 1 93, and 248 nm wavelength. For example, a lens element irradiated continuou sly for 1 yr (1 kHz, 0.1 mJ/cm(2)/pulse) in the presence of 1 ppb of t-buty l benzene would exhibit a transmission of similar to 87% at 193 nm. The eff ects of oxygen-containing ambients are also documented, and methods for eli mination and/or prevention of organic contamination are suggested. (C) 2000 American Vacuum Society. [S0734-211X(00)13003-3].