Lion,

As a practical matter, you cannot oxidize Si on GaAs.  At temperatures one
would use to thermally oxidize Si, the GaAs will dissociate, compromising
the crystal quality.

Assuming that your "GaAs" is more than bulk material i.e. it is epitaxial
layers capped with GaAs, you may do great damage to the layer structure.

I have evaporated SiO2 on GaAs-capped epilayers and used the SiO2 as a mask
for dry etching in the past.  I would suggest that you try a lower
deposition rate for the SiO2, to insure you get a good quality oxide.

An alternative might be to use PECVD SiO2, if you have access to it.  This
should give you a very good oxide, and good adhesion to GaAs.  The
deposition temperatures are typically <350 C, so you will not damage you
epilayers.

Good luck!

************************************
Philip D. Floyd, Ph.D.
Iridigm Display Corporation
2415 Third Street, Suite 235
San Francisco, CA 94107
ph:  (415) 626-8800 x138
fax: (415) 626-9775
email: [email protected]
web:  http://www.iridigm.com/
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-----Original Message-----
From: DARREN S GRAY [mailto:[email protected]]
Sent: Friday, February 13, 2004 9:46 AM
To: [email protected]
Subject: [mems-talk] Re: SiO2 on GaAs by ebeam evaporator

Dear Lion,

Regarding your question 1), silicion dioxide is a somewhat poor choice
for ebeam evaporation, since the evaporated material will have a
poorly characterized structure and ratio of silicon to oxygen.  Consider
evaporating pure silicon, which I've found to be easier than SiO2 from a
technical standpoint.  You may get a more stable layer this way, and you
can always oxidize the deposited layer to get a silicon oxide.  Also
consider a thinner layer, to reduce stresses, if your process permits.  If
you would prefer to continue using SiO2, consider thermal evaporation.


Darren Gray
Graduate Student
Johns Hopkins University


> Message: 2
> Date: Thu, 12 Feb 2004 12:29:00 -0500
> From: "Liang Chen" 
> Subject: [mems-talk] SiO2 on GaAs by ebeam evaporator
> To: 
> Message-ID: <001401c3f18d$b3ff94a0$0e870280@sissi>
> Content-Type: text/plain;     charset="iso-8859-1"
>
> Hi,
>
> I'm trying to use SiO2 as etching mask on GaAs, so I use ebeam evaporator
to deposit a thin layer (3000A) SiO2 on it. The deposition rate is 10A/s,
and very stable. But when I dip that sample in AZ400K developer, some part
of SiO2 lift off from the substrate. The substrate is very clean, because
when I took it out from a growth chamber (we're using MBE to grow structures
on GaAs substrate), I immediately put it into evaporator chamber.
>
> So does anyone have such experience and can give me some suggestion on the
following questions?
>
> 1) Is this ebeam evaporation of SiO2 totally unsuitable for etching mask
purpose?
> 2) Do I need special treatment on GaAs surface prior to loading it into
evaporator?
> 3) Do I need post deposition treatment, like thermal curing at, for
example, 400C?
>
> Thanks a lot
>
> Lion



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