You can make 100 um holes in 500um thick glass by laser machining or
mechanical drilling.
But if you need a number of holes, the methods is not proper because of
high cost.
If you don't have certain reasons to use pyrex glass, the photosensitive
glass process meets your need.
It is also bonded with Si by thermal bonding or anodic bonding.
And in some cases, it is wafer-level-bonded with silicon.

Sooje Cho from MEMSware