Heavy Metal Contamination from Resists during Plasma Stripping

A new mode of heavy metal contamination has been investigated. Generally used commercial positive photoresists containing heavy metal less than 0.5 ppm decreased minority carrier lifetime of the silicon substrate under the resist layer during resist stripping with a plasma asher. Since this contamination is induced by direct plasma exposure, it can be avoided by using plasmaless resist removal methods, such as wet stripping or down flow ashing. However, even in such resist stripping methods, a wafer temperature higher than 300°C introduces this problem. This contamination is reduced by both covering the silicon surface with a silicon oxide layer and by purifying the resists. Neither of these methods are perfect.