Hello,

When you oxidize single crystal silicon you end up with Si, a thin layer
of Si02, and then "oxide."  When you oxidize Poly you end up with more
Si02 at the oxide/poly interface than when you oxidize single crystal
Si.  I'm not sure if the oxidation rate is higher, it may be that more
poly is being consumed (compared to single crystal Si) to create Si02
and looks like oxide.  Do you notice your poly layer is thinner?  How
are you measuring this increase in oxidation?

Regards,

Mike

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Michael Larsson
Sent: Thursday, January 25, 2007 12:38 PM
To: [email protected]
Subject: [mems-talk] Re: Why oxidation rate of polysilicon is much
higherthan that of single-crystal silicon(100)


Hi Jay,

At a guess, I would say this is predominantly due to the presence of
grain boundaries. These are regions of high(er) energy, compared to the
rest of the xtal structure, meaning GBs are more active/reactive, and
more prone to oxidation (along with other chemical reactions with Si).
O2 diffusion rates are probably also higher along GBs, due to the
disorder in the regular xtal lattice, promoting more rapid oxidation. I
would say that increased diffusion rates at GBs are the main reason for
the increased oxidation rates

Following the thermodynamic argument: GBs introduce disorder to the xtal
structure, increasing its energetic state.  Thermodynamics shows that
all things in nature tend towards a state of lower energy, thus the
poly-Si will strive to do this at any opportunity (and more so than SC
Si). Oxidation is a mechanism by which the energy state of many systems
are reduced (e.g. rust in metals).