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: C.Brubaker@evgroup.com, Web: www.EVGroup.com, 07/12/02 -----Original Message----- From: Yahong Yao [mailto:hhelen@hotmail.com] Sent: Thursday, July 11, 2002 10:13 PM To: mems-talk@memsnet.org 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 _________________________________________________________________ Join the worlds largest e-mail service with MSN Hotmail. http://www.hotmail.com _______________________________________________ MEMS-talk@memsnet.org mailing list: to unsubscribe or change your list options, visit http://mail.mems-exchange.org/mailman/listinfo/mems-talk Hosted by the MEMS Exchange, providers of MEMS processing services. Visit us at http://www.memsnet.org/