We also tried XeF2 at UCLA on a 4" wafer. Their system has control over etch rate either in regulated volumes released into the reaction chamber or as a continuous flow. I didn't find much difference. The range was also around .1-.2 u/min. We also tried placing an oxide coated wafer next to the backside to reduce the loading, but it didn't have a noticable effect. The only strange thing I noticed was a fine black powder which appeared around the edge of the wafer. ??? Otherwise, there was no appreciable etching of resist, oxide,nitride, or the metals we used other than the Si.