For what you are attempting to do, it sounds as if your best result
(assuming the materials allow for it) may be to perform a fusion bond
(typically, it would be known as a Silicon Direct, or Silicon fusion bond,
but the principles should work here). This process uses Van der Waals forces
to enact a low strength pre-bond, which is then annealed to yield a higher
strength permanent bond.

Basically, this is a three step process:

1. Wafer cleaning/surface activation:  Typically, new, clean wafers are
sufficient, but for best results, some kind of surface activation method
should be used.  We typically use a reverse RCA method (shown below).  After
the surface activation, the surface should be terminated in -OH groups.

Reverse RCA clean (RCA2 then RCA1 no HF dip)

        RCA2 (SC-2)
        HCl (37%): H2O2 (30%):H2O in a volume ratio of 1:1:6 at 700C for 5
minutes
        DI water rinse

        RCA1 (SC-1)
        NH4OH (29%):H2O2 (30%):H2O in a volume ratio of 1:1:5 at 700C for 5
minutes
        DI water rinse

2. Pre-bond - The prebond is created by placing the materials in direct
contact with each other.  Then, hydrogen bonding will occur between the
opposing -OH groups.  It is critical that the surfaces be extremely smooth
(<0.5 nm roughness, RMS).  If the substrates are compatible to this process,
a bond will form between them.  Note that this bond is NOT stiction.  If the
substrates are truly bonded, they will not slide against each other if a
shear force is applied, as would be the case with stiction.  With
transparent substrates, the quality of the bond can be directly viewed, as
Newton fringes (areas of constructive and destructive interference produced
by near contact of smooth, reflective surfaces) will appear in the
non-bonded areas.

3.  Anneal - Annealing the stack is performed by placing the substrates into
an annealing furnace, typically in excess of 1000 C (the specific
temperature will depend on substrate).  This dehydrates the pre-bond,
creating a permanent covalent bond.

Best Regards,

Chad Brubaker

 <<...OLE_Obj...>>

EV Group-Technology, Tel: (602) 437 9492 x 111, Fax: (602) 437 9435
E-mail: C.Brubaker@evgroup.com , Web:
www.EVGroup.com, 01/14/02

 <>

 -----Original Message-----
From:   Zigurts Majumdar [mailto:majumdar@uiuc.edu]
Sent:   Wednesday, December 26, 2001 7:11 PM
To:     mems-talk@memsnet.org
Subject:        [mems-talk] glass-glass bonding

HI, Does anyone have a detailed protocol for bonding glass to glass? We
have been trying to bond while maintaining an optical quality surface
(pure thermal bonding doesn't seem to work) and we haven't had luck with
applying a voltage across the glass with temp around 400 degrees. We're
using pyrex right now (borofloat, specifically). Any suggestion would be
appreciated.

Thanks,

Ziggy
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