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