Behraad,

Oxygen plasma would be an effective means for cleaning the substrate surface
prior to bonding.

With regards to the bonding process itself, EVG has developed the Low Temp Dry
Activated Plasma Bonding (LTDAPB) process which is specifically designed to
allow activation of the wafer surfaces in such a way as to increase initial bond
strength, and to reduce the overall thermal annealing necessary to complete the
bond.  Typical annealing temperatures for this process are 200 C, which should
have no impact to the metal layers at all.

The greatest critical factor in this is the roughness of the oxide surface,
especially considering the existence of topography (recesses) in the oxide.
Ideal roughness is ~0.5 nm (RMS), although the LTDAPB process allows an
extension in this (depending on surface material).

If you have any further questions, feel free to contact me (my information is
below), or visit the EVG website at www.evgroup.com.

Best Regards,

Chad Brubaker

EV Group       invent * innovate * implement
Technology - Tel: (602) 437-9492, Fax: (602)437-9435 e-mail:
[email protected], www.EVGroup.com

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 -----Original Message-----
From:   [email protected] [mailto:mems-talk-
[email protected]]  On Behalf Of Behraad Bahreyni
Sent:   Monday, January 26, 2004 4:53 PM
To:     [email protected]
Subject:        [mems-talk] Wafer Bonding Problem

Hello Everybody,

  I want to bond two Silicon wafers which are coated with a 2um layer of
CVD oxide. The oxide layer on one of the wafers is recessed and 1um of
metal (gold) is deposited (and patterned) inside the recessed region
(please check the illustration below). Since I have the metal layer on
the bottom wafer, I can not do an RCA clean on that wafer. This may
cause some photoresist to remain on the oxide layer, which as far as I
now, will cause bonding problems. Moreover, because of the metal layer,
I can not use high temperatures for bonding. An oxygen plasma cleaning
is possible, but I am not sure of its effect on bonding process and
quality.

  Does anybody have any experience with cases similar to this scenario?
I will highly appreciate any help and/or references for bonding this
kind of structures.

  Thank you in advance,
  Behraad


           _________________________________________________
          |                  Oxide                          |
           _________________________________________________
          |                                                 |
Top       |                  Silicon                        |
Wafer     |                                                 |
           _________________________________________________
          |                  Oxide                          |
           _________________________________________________

           ______________                        ___________
          |              \                      /           |
          |     Oxide     \                    /   Oxide    |
           ________________=======Metal========_____________
Bottom   |                                                 |
Wafer    |                  Silicon                        |
          |                                                 |
           _________________________________________________
          |                  Oxide                          |
           _________________________________________________



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