Dear all, I have a question about a - for us - surprising problem with using chromium as an etch mask in DRIE (the Bosch process). Short version: We believe that a thin layer of something (unknown to us) was formed on top of the chromium during the etching. Have anyone else on this list experienced this problem before (1) and would you know what species might have been formed? Longer version: Recently when we etched silicon (SOI wafer, the etch was stopped when all trenches had reached the buried oxide below 50mu device layer). The chromium seemed to be absolutely untouched after the etch (as it has always been previous times). Except chromium, silicon and buried oxide there was also CYTOP, (a polymer) gold and ordinary positive resist on the wafer. But after the etch the chromium wouldn't go away in regular chrome etchant. We then made the following two observations: A. When gold pads on top of the chromium were etched the chromium below these *could* be removed with Chromium etchant. B. When the chromium was slightly and very shallowly scratched with a tweezer these scratches *could* be etched by chromium etchant. Because of these two points we draw the conclusions that a some unknown has formed a thin layer on top of the chromium. It should also be added that we have used chromium as an etch mask before in the same machine without encountering this problem. But at earlier times we have always used lower platen power shorter etch times. This time - also a first - we used for a fraction of the total etch the SOI option that our STS machine provide. I can't see why this last would have an impact, but the etch overall changed quite dramatically and seemed much more aggressive while using the SOI option. I would be thankful for any answers. regards, Sven Holmström