Isn't this a great e-mail exchange?

Great exposure.
Cheers,
Helge




                -----Original Message-----
                From:   Brubaker Chad [mailto:[email protected]]
                Sent:   Monday, December 17, 2001 1:23 PM
                To:     [email protected]
                Subject:        RE: [mems-talk] fixture for bonding two
wafers at 1000 deg C (fai rly lengthy)

                Please pardon my lengthy response

                Actually, in direct bonding applications, there is generally
no need to use
                a "fixture" to hold the two wafers together.  Of course,
this does depend on
                the makeup of the substrates being bonded.

                In the case of silicon, there is something that can be
performed prior to
                the placement into the 1000 deg C furnace called a
"prebond".  Basically,
                this prebond is the fixing of two mirror polished (roughness
less than 0.5
                nm for best results) substrates to each other via  Van der
Waals forces.
                This can be performed with either hydrophobic or hydrophilic
surfaces,
                although the hydrophilic case provides the strongest
prebond, with a bond
                strength of up to 150 mJ/m2 (by contrast, to fully annealed
directly bonded
                silicon wafers have a bond strength of 2 J/m2).  This
prebond can be
                observed by merely removing two prime wafers from their
cassette, and
                pressing the polished sides together (although the bond
strength is much
                weaker).  This is not stiction, as stiction affects will
typically still
                allow shear forces to easily move one wafer across the
other.  If the
                prebond has occurred, the wafers will not be able to slide
against one
                another, but instead remain fixed in place (thus,
maintaining any kind of
                alignment that has been performed).

                Several cleaning methods can be used to increase the surface
energy of the
                substrates, to increase the strength of the prebond.  The
purpose of these
                cleaning methods is to create a surface terminated in
hydroxyl (-OH) groups,
                which will promote hydrogen bonding between the two
substrates.  RCA clean,
                Piranha, megasonically activated DI rinse, reverse RCA, and
boiling nitric
                acid have all been used to increase bond strength.  It is
very important
                that no HF dip process is used at any point prior to forming
the pre-bond,
                as this will strip away the hydrophilic surface we are
attempting to
                achieve. In our studies, the boiling nitric produced the
strongest prebond,
                followed very closely by the reversed RCA process.  Because
of safety
                issues, we prefer the revered RCA process (listed below).
In addition to
                wet chemical activation methods, some dry methods can be
used, such as
                oxygen plasma, or UV/Ozone cleaning.  A final DI flush is
recommended just
                prior to bonding, regardless of the method used for
cleaning.

                Once the surfaces have been activated, they can be bonded by
simply placing
                one substrate face up on a table, placing the other face
down directly over
                (a thin layer of air will prevent direct contact between
them, assuming the
                second is laid down directly parallel to the first), and
forcing them into
                contact at one point.  The bond front that begins at that
point will extend
                to the full interface in a few seconds.  This bond front can
be observed
                using any sort of full wafer IR inspection set up.  Please
note that, since
                this method relies on the surfaces of the wafers coming into
direct contact,
                ant particle contamination will seriously hinder the
prebond, creating voids
                up to 4 orders of magnitude greater than the size of the
particle.  For
                instance, a 1 um particle can cause a 1 cm wide void along
bond interface.
                Because of the annealing temperature (above 1000 deg C), any
air trapped in
                these voids may cause delamination, as the overall pressure
in the void will
                increase from 1 atm to approximately 5 atm.

                For the reason mentioned above, please note that substrates
can only be
                bonded in atmosphere if they contain no cavities (except
through holes).  In
                order to bond wafers to create sealed cavities, the bond
must be performed
                in some level of vacuum (typically, 200 mbar absolute would
be sufficient).


                Once the prebond is performed, assuming there is no trapped
air, the wafers
                will remain fully fixed in place during the anneal process.
Once its done,
                the anneal can be performed by simply placing the wafer
stack into a furnace
                in the same manner as you would place a single wafer into
the furncae.  No
                clamping fixtures are necessary.

                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

                Best Regards,

                Chad Brubaker



                EV Group-Technology, Tel: (602) 437 9492 x 111, Fax: (602)
437 9435
                E-mail: [email protected]
, Web:
                www.EVGroup.com



                -----Original Message-----
                From: shay kaplan [mailto:[email protected]]
                Sent: Monday, December 10, 2001 7:46 AM
                To: [email protected]
                Subject: Re: [mems-talk] fixture for bonding two wafers at
1000 deg C


                try two quartz (fused silica) photomasks with the chrome
stripped off.

                shay

                [email protected] wrote:

                > Sir/Madam,
                >          In the abscense of a proper bonding equipment;
Can anyone tell
                > me , "How two wafers can be held together in a
furnace/oven", to perform
                > a "Direct bonding process".
                >    To be more precise, what kind of fixture materials can
be used for
                > temperatures in the range of 1000 deg.C.  If anyone knows
any company that
                > provides these kind of fixtures , please forward the
information.
                >
                > Regards,
                > javeed shaikh mohammed.
                > _______________________________________________
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