Experimentation and modeling of organic photocontamination on lithographic optics

Photodeposition of organic films on transparent substrates irradiated in the presence of trace levels (ppb to ppm) of hydrocarbons has been experimentally investigated and a model is presented that describes the film growth behavior. The efficacy of a given organic precursor at forming a deposit is proportional to the product of its surface coverage (as governed by its partial pressure relative to its saturation partial pressure) and by its photon absorption cross section. These measurements are important in predicting the transmission characteristics of lithographic optics operating at 157-, 193-, and 248-nm wavelength. For example, a lens element irradiated continuously for one year (1 kHz, 0.1 mJ/cm(2) per pulse) in the presence of 1 part per billion of t-butyl benzene would exhibit a transmission of similar to 87% at 193 nm. The effects of oxygen-containing ambients are also documented, and methods for elimination and/or prevention of organic contamination are suggested.