Hi Stefan,

   Due to SOI fabrication techniques, the device-SIO2 interface is has the
weaker bond interface and contains more voids and thus have a faster HF etch
velocity.  In addition, the device-SIO2 layer will always be overetched
because it is the first interface reached by the HF solution and will have a
3 um head start or whatever thickness of SIO2 is being used. Of course this
is assuming that there is no backside etching.

   Depending on the type of structures, you might have to deal with a lot of
surface tension delaying the etch of the bulk silicon and SIO2 interface.
In most cases most people use a surfactant in order to reduce those surface
tensions and get a more uniform etch.  For a temporary solution, I would
recommend using a common surfactant like Triton X100.  I have to warn you
that most surfactants come with an added complication of contamination.  For
a more consistent solution I would recommend contacting Honeywell or any
other chemical vendor that sells an HF solution containing a surfactant if
you are going into production mode.

Phil Tabada





>From: "Blunier, Stefan" 
>Reply-To: General MEMS discussion 
>To: 
>Subject: [mems-talk] Underetch of SOI
>Date: Thu, 16 Jan 2003 13:14:17 +0100
>
>Dear collegues
>I'm working on SOI wafers developing free hanging structures. During
>underetch with HF (48%) I noticed that the etch velocity of the two
>Si-SiO2 interfaces is different. SiO2 thickness is 3 microns. Production
>is bonding of thick device layer and lapping polishing to a thickness of
>50 microns.
>The etch velocity at the interface SiO2-device layer is faster than the
>etch velocity at the interface SupportSi-SiO2. To etch all the oxide
>under a 50 micron beam I have to overetch to long so my anchors
>underetch to much.
>Does anybody know this problem and how to avoid it?
>Thanks
>Stefan
>
>
>_____________________________________________________
>Dr. Stefan Blunier
>ETH Zuerich, Zentrum CLA G 21.2
>Institute of Mechanical Systemes
>Tannenstrasse 3
>CH - 8092 Zuerich
>Switzerland
>
>Phone:   +41 1 632 77 64
>Fax:  +41 1 632 11 45
>e-mail: blunier@imes.mavt.ethz.ch
>__________________________________________________
>
>
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