Thanks for the advice everyone.

@Joe,

I knew nothing about that, so thanks for the heads up. I'll have to look in to
it. I did a test run with Si wafers without Al, and they bonded nicely, so
that's an encouraging sign (I'm just working with Si at the moment -- sapphire
will come later if needed).

@Bill,

What is plasma bonding? (Since I've started on this project, I hear about new
types of bonding every day!)

@Felix,

The substrates are ion milled before we sputter the Al on -- we haven't had any
problem with the Al not adhering. Right before bonding (after the Al is on) we
don't do much of a clean (the wafer bonding station I have access to has this
fancy cleaning system, but it's only setup with DI water... so that's what I
use) My understanding is that (given a relatively clean wafer) the native oxide
on the Al does more than anything else to make bonding difficult, but we don't
have a good way to get rid of it since it grows back quickly. Thoughts?

Thanks again.
-Leon