Dear Leon,

     I don't know how to get rid of the aluminum oxide and have it
stay off for the bonding process. I'm not sure there's a simple way to
do that since you would need to etch it off (pretty difficult as far
as I know) and passivate the bare aluminum surface (not sure if this
can be done easily either).

Here's what I would try first (in a clean room if possible):  clean
your wafers using a RCA1 clean (HF dip for the Si, RCA1 @70C for 5-10
minutes, HF dip again for Si (assuming that the RCA1 does not do
anything to the aluminum). Rinse in DIW (you may want to try with and
without DIW rinse to see if the surface chemistry is significantly
changed). At this point, your wafer surfaces should be particulate
free provided your chemicals and DIW are clean. I've also found that
the "typical" clean room cloths will introduce some particulates on
your wafer so you have to be careful about that. If you have a
Nomarski microscope, you can try inspecting the surfaces to see if
they are clean. Next, stick your wafers into an RIE with an argon or
nitrogen  plasma. Try a low pressure at roughly 100-200 W power for
about 30 seconds. Take the samples out and press them together
starting from the center and pushing out radially or from one side to
another. Anneal the bonded pair on a hot plate at 200 - 300 C for
20-60 minutes. Let me know if it bonds!


On Jul 7, 2009, at 10:04 PM, Leon Nathaniel Maurer wrote:

> 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

Felix Lu
felix_lu@yahoo.com