An Optical Method for Monitoring Metal Contamination during Aqueous Processing of Silicon Wafers

An optical technique based on simple absorption spectroscopy has been demonstrated for monitoring metal contam-inant deposition from aqueous processing solutions widely employed during microelectronics fabrication. Cu deposition from 0.15 and 0.25 % HF solutions contaminated with 3.5 ppm Cu has been observed as a decrease in the absorption of a HeNe laser reflected at glancing incidence from a Si wafer. This is caused by the depletion of Cu2 from the mass-trans-fer boundary layer, providing direct evidence that Cu deposition is rate-limited by diffusion. This technique allows deter-mination of which metallic species can deposit onto Si wafers in diffusion-limited processes during various aqueous pro-cessing steps in microelectronics manufacture. In addition, Cu deposition is seen to coincide with the completion of Si07 dissolution, confirming that Cu2 is reduced by an electroless process involving simultaneous Si oxidation. The decrease in absorption is consistent with a mass-transfer boundary layer thickness of about 335(20) lim. The optical absorption returns to its original value about 2 mm after the onset of deposition, consistent with diffusion-limited deposition of one monolayer of Cu, which is followed by a transition to a slowe surface rate-limited process. Aqueous processing of semiconductors has been widely employed for removing residual metals, organics, and par-ticles from the silicon substrates on which microelectron