DRY SURFACE CLEANING USING CO2 SNOW

Controlled expansion of high purity carbon dioxide through a nozzle forms a high velocity "snow" stream that effectively removes both particulate and thin film contaminants from silicon wafer surfaces [W. Whitlock, Presented at the 20th Annual Meeting of the Fine Particle Society, Boston, MA, August 22, 1989 (unpublished); R. Sherman and W. Whitlock, J. Vac. Sci. Technol. B 8, 563 (1990)]. This process will clean surfaces leaving no detectable film residue as well as reduce adventitious (native) hydrocarbon surface content. This article discusses the performance of CO2 snow cleaning on a variety of materials typically encountered in a surface analysis laboratory. Cleaning tests were performed on metals (aluminum and copper sheets), semiconductors (Si and InP wafers), and insulators (ceramics, laser optics, glass plates, and polymers). Performance is judged primarily through x-ray photoelectron spectroscopy measurements with primary consideration given to reduction of surface hydrocarbons. These measurements are compared with samples which were cleaned with a conventional solvent process. The results indicate that CO2 snow cleaning is comparable to solvent cleaning in its effectiveness for removal of hydrocarbon films. Recommendations are made for the use of CO2 snow cleaning in both industrial and laboratory applications.