157 nm 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 removed. A degenerate threshold model that assumes preferential adsorption at surface defects was employed to quantitatively analyze the LID data. Desorption thresholds for water and hydrocarbons were obtained from this model.

We have built and tested a laser induced desorption (LID), electron impact ionization, time-of-flight (TOF) mass spectrometer (MS) designed to nondestructively identify and measure adsorbed contaminants on critical surfaces for the microelectronics and optics industries. The LID-TOFMS combines the capability of a TOF mass spectrometer to measure all the desorbed molecules from a single laser shot with an infrared Er:YAG laser (2.94 micron), which is not strongly absorbed by many transparent optical materials but is strongly absorbed by water, the most common adsorbed surface contaminant, to yield surface composition as a function of position on the sample. The LID-TOFMS was calibrated using an oxalic acid film on a polished stainless steel plate, which also contained adsorbed water. Contaminants on CaF 2 surfaces measured by LID-TOFMS include water and hydrocarbons. Desorbed molecules decrease with increasing irradiations at a fixed laser fluence, suggesting that the surface is being cleaned.