H2O2 cleaning stabilizes reference wafers
06/01/1998
H2O2 cleaning stabilizes reference wafers
New data suggest that hydrogen-peroxide (H2O2) cleaning of a bare-silicon reference wafer before use for ellipsometry calibration ensures a repeatable native-oxide surface and reliable measurement results. This conclusion comes from Rudolph Technologies, Flanders, NJ, and a one-year study that monitored the dynamics of the contaminant layer that naturally forms on bare wafers.
The study began with etching a silicon wafer to bare silicon. Immediately after, ellipsometric measurement detected a 12-? SiO2 film. Six months later, the measured film thickness had increased to approximately 17 ? and continued to grow about 0.2 ?/month. Cleaning with H2O2 repeatedly brought the measured oxide thickness back to 14 ? (see figure).
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Changes in measured oxide thickness over one year with periodic H202 cleaning.
Rudolph engineers concluded that the data indicate the underlying oxide film stabilized within one week of oxide formation, and it remained stable over the one-year duration of the study. In addition, the measured film thickness - the underlying oxide plus the adsorbed contaminants - demonstrated a rapid thickness increase after each cleaning, suggesting that cleaning can play a significant role in achieving repeatable film thickness measurements of reference wafers.
Rudolph stated, "This work indicates that the slow thickness increase is not solely due to diffusion-limited oxide growth, but is primarily due to what is believed to be airborne molecular contaminants adsorbing onto the oxide surface. This phenomena could be a particular problem when comparing ellipsometric data from different locations." The wafers used in this study were stored in a standard 25-lot cassette held in a cassette carrier in a Class 10 cleanroom.
Rudolph is continuing this work and will eventually document a safe, easy-to-use cleaning procedure as part of a film-thickness standard-test procedure that brings reference wafers back to a clean surface that is repeatable to ?0.1? or better. - P.B.