Analysis of adsorbed contaminants of CaF2 surfaces by infrared laser induced desorption

157nm photolithography technologies are currently under development and have been accepted as the leading candidate for fabrication of the next generation semiconductor devices after 193nm. At this and shorter wavelengths, molecular contamination of surfaces becomes a serious problem as almost all molecules absorb at 157nm and below. The light transmitted by a photolithographic tool can be significantly decreased by the presence of a few monolayers adsorbed on its many optical surfaces. We have developed a laser induced desorption, electron impact ionization, time-of-flight mass spectrometer (LIDTOFMS) to study contaminants on 157nm and other ultraviolet optics, e.g., polished CaF2. The LID TOFMS of CaF2(100) samples showed water ions, hydrocarbon ions, oxygen-containing hydrocarbon ions, as well as alkali metal ions (Na+,K+). For multiple irradiations of one site at fixed laser fluence, the ion intensities decreased as the number of pulses increased, suggesting that surface contaminants were being remove...