Dear Yahong,
There are a couple of major reasons for the use of Au-Si eutectic bonding
rather than Al-Si eutectic bonding.
The first, as you mentioned, is the temperature. Especially with wafer
level bonding, temperature becomes an extremely critical factor.
First of all, the Al-Si eutectic temperature (577 C) is right on (and in
some cases, beyond) the edge of the operational range of most commercial
bonding systems.
Additionally, one thing that should always be considered for wafer level
bonding is the TCE mismatch of all materials involved. Because the
temperature of bond is so much higher for the Al-Si eutectic, as opposed to
the Au-Si Eutectic (363 C), the effects of TCE mismatch become more
pronounced. Even if you are bonding like material to like material (two
silicon wafers, for instance), the intermediate later, Aluminum, has a
greatly different TCE from that of silicon. If the bond is performed at
~600 C, then the amount of stress in the film will be severe when the
material is returned to room temperature.
Another major reason for Au over Al has to do with the chemical properties
of the two materials.
When it comes to Au-Si eutectic bonding, our experience has been that oxide
(even native oxide) on the silicon surface is to be avoided, as it forms
something of a diffusion barrier to the migration of Au into the Si (or vice
versa). Our typical recommendation is to perform a brief BOE dip to strip
all native oxide prior to bonding. Then, we advise a 15 minute window
between the BOE dip and the removal of the silicon from an oxygen atmosphere
(by pumping down in vacuum, and replacing with inert atmosphere, for
instance). However, only the silicon surface is of concern, since gold will
not form an oxide.
Aluminum will actually form an oxide more readily than will silicon. This
makes the process more difficult to work as well. In cases where direct
Al-Al bonding has been performed, the substrates were initially cured in a
reducing atmosphere (forming gas) prior to bonding (in this case,
thermocompression bonding rather than eutectic).
Best Regards,
Chad Brubaker
EV Group-Technology, Tel: (602) 437 9492 x 119, Fax: (602) 437 9435
E-mail: [email protected], Web: www.EVGroup.com, 07/12/02
-----Original Message-----
From: Yahong Yao [mailto:[email protected]]
Sent: Thursday, July 11, 2002 10:13 PM
To: [email protected]
Subject: [mems-talk] Al-Si wafer level bonding?
Hello Folks,
Could anyone give comparison between Au-Si bonding and Al-Si bonding on
wafer level? I have seen many people do Au-Si bonding but not many do
Al-Si bonding. Besides Al-Si eutectic temperature is higher than Au-Si,
what else reason makes it not as popular as Au-Si? Bonding strength?
I would think Al is IC compatible and it is much easier to process in an IC
fab. Any comment is highly valued. If one can tell me some reference
papers talking about Al-Si bonding, that will be very much appreciated.
Regards,
Yahong
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