Bruce,

If you just want to taper your sidewall to help your subsequent process
steps, such as better step coverage of the subsequently deposited thin
films, you want the slope to be more or less 45 deg. You can do it by
using a tapered photoresist (hardbaked at 125 deg for a few mins on
hotplate or 30 mins in oven) for masking your RIE process. With a good
process control, you can taper your silicon sidewall appreciably. Of
course the angle will vary but perhaps within the range of 30-60 deg is
okay and can be comfortably achieved. But for 85 deg? I don't know. It
looks "perfectly" vertical for me already. And for step coverage issue, 85
deg basically makes no difference from 90 deg.

Yours sincerely,

Isaac Chan, Ph.D.
University of Waterloo