You could try electrochemical discharge machining (also known as spark
erosion).
I believe two Japanese researchers, Shoji and Esashi, published work on
this a while back, although I can no longer find the reference.


Hui Ju wrote:
> Dear Everybody,
> Hello!
> Does anyone know how to make holes with a diameter of about 200
> microns on
> pyrex glass (7740, thickness of 200 microns)? I appreciate your help
> and any
> kind of adivce.
> Thank you.