Yes as Pierre has suggested XeF2 is a viable option. The attack on copper is
very low or non existent depending on the nature of your device. The XeF2
silicon etch  is a dry gas phase etch so it is very easy to manage. Note that
unlike KOH it is truly isotropic, so the shape of the etch will be different. If
you are interested in how XeF2 might work please give me a call or send me
email. XACTIX is the overwhelming market leader in providing XeF2 etching
equipment so we can even help you try it out if it looks like it makes sense.

Regards
David Springer