HMDS is best done in a YES oven under controlled conditions. You might contact
Bill Moffet at Yield Engineering Services for better instruction.

It might be to no avail as the work we have done on our new silicon etch process
shows a selectivity of oxide to silicon of around 1:100. If the math is right
you will only be able to etch around 70 microns total before you run out of hard
mask material. You might need to thicken up the oxide and etch it with plasma to
insure proper wall angle before Silicon etch process. Bob Henderson