Polysilicon is also a good masking material for HF. You can also try to
use a thick photoresist (e.g. AZ9260 10micron) for short processes.
Anisotropic etching of glass can only be done in a plasma process (e.g.
AOE); 70 micron is quite a lot for such a process and Nickel would be
required as a mask.

Marco