Removal of Thermally Grown Silicon Dioxide Films Using Water at Elevated Temperature and Pressure

Abstract

The removal of thermally grown silicon dioxide films from silicon wafer surfaces with water at elevated temperature and pressure was studied using ellipsometry and x‐ray photoelectron spectroscopy (XPS). Complete removal of 50 nm silicon dioxide layers was observed with XPS after exposure of the sample to deionized (18 MΩ cm) water at 280°C and 241 bar (3500 psi) for 30 min. To the detection limit of XPS (∼0.5 atomic percent), no metal contamination was deposited on the surface. Removal rates were determined at temperatures between 260 and 305°C at 138 bar (2000 psi), and increased from 2.9 to 11.2 nm/min over this temperature range. A surface reaction limited rate equation, used previously to describe quartz dissolution in water over a wide range of temperature, was used to estimate rate constants for silicon dioxide removal. Calculated rate constants were higher than reported values for quartz dissolution, presumably due to the amorphous structure of thermally grown silicon dioxide. An activation energy of 76.6 kJ/mol was calculated for the etching process, which is comparable to values reported for quartz dissolution under similar conditions (62.6 – 79.0 kJ/mol). The agreement of activation energy values indicates that the dissolution mechanism may be similar for thermal silicon dioxide and quartz. Surface roughening was examined for silicon and silicon dioxide samples via atomic force microscopy and scanning electron microscopy. Silicon dioxide surfaces showed low surface roughening, while silicon samples had significant amounts of silicon dioxide redeposited on the surface. Microbalance measurements indicated that silicon surfaces etched ∼25 times faster than thermally grown silicon dioxide, accounting for the silicon dioxide redeposition.