Hi Leon,

I haven't done any bonding of materials with mismatched thermal
expansion coefficients, but my understanding is that the trick is to
heat and cool VERY slowly. Maybe 1 C/min.

I haven't done Al-Si bonding either, but here's some general advice that
might apply to you. A critical issue I encountered in oxide-oxide
bonding was in regards to wafer specs. I was blindly using 100 mm Prime
grade wafers thinking Prime was Prime and that was as good as I could
hope for. But not all Prime grade wafers are created equal. The standard
specifies a range for specs like TTV & Bow/Warp. After several seemingly
inconsistent bond attempts, I took wafers from two different batches
(both Prime) and had them characterized. It turned out the wafer from
the batch that consistently bonded had low Bow & Warp (~3 um) and the
wafer from the batch that consistently gave me trouble had high Bow &
Warp (~ 25 um). Both were within spec for Prime (and even within
generally recommended specs for wafer bonding), but one worked well and
the other gave me a lot of trouble. It can be hard to distinguish
between a problem with your process and a problem with your samples. So
my advice is to make sure the substrate you are starting with is of the
highest quality so as to avoid difficult to diagnose problems down the
line. Surface roughness, TTV, and Bow/Warp are critical. If those specs
are good enough, you don't even need a bonding tool, simply place the
wafers together and they'll stick. Many wafer manufacturers will work
with you to produce wafers with tighter specs - that's the path I'm
pursuing now.

good luck,
Joe Grogan



Leon Maurer wrote:
> Howdy,
>
> I'm looking for information on Al-Si wafer bonding, and I saw a good
> thread here from 2002 about why I don't want to do it, so I was
> wondering if there was equally good information here about how to do
> it if I had to.
>
> I'm new to wafer bonding, and the papers I've found that mention it
> haven't been too informative (although perhaps I haven't found the
> right papers). One issue is that the wafer bonding setup I have access
> to only goes up to 550C, and for various reasons (including that we'd
> eventually like to bond sapphire to Si with an intermediate Al layer
> -- my understanding is that the differences in thermal expansion rates
> can cause trouble) we'd like to avoid temperatures that high.
>
> We don't need particularly strong bonding -- mostly it just has to
> survive (man)handing during the rest of fabrication (which will
> involve some lithography and etching). However, the bond or two I've
> made so far aren't cutting it (200C, 2.5MPa, 30min, they fail if you
> look at them funny). Anyone know what strengths can be achieved at
> different temperatures/pressures/times? Any tricks that could make
> things go more smoothly? Other advice?
>
> Thanks.
> -Leon

--
Joseph Grogan
Graduate Student
Department of Mechanical Engineering & Applied Mechanics
University of Pennsylvania
220 South 33rd st
Room 229, Towne Building
Philadelphia PA, 19104
Lab Phone: 215-898-1380