Surface analysis by laser-induced desorption time-of-flight mass spectrometry

Abstract

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.