I am curious as to your thinking for #3.  How would the ambient temp affect
the actuator, be it metal or poly?  As far as I know the traditional hot arm,
cold arm lateral actuator is a single material, and would be unaffected by
changes in ambient temp as long as they are slow enough to allow equilibrium
between the two arms.  From the small size of such structures this time is
less than a second.  I would submit that you could substitute the word
"aluminum" for your mentions of "Polysilicon" and "Poly" in #1 and #2 below.
A disadvantage of Al would be the lower melting temp.

My thought is that the main reason for using poly over metals is that standard
processes such as MUMPS for surface micromachining have made this type of
structure easy to make, and the fabrication can be outsourced.

-Jon

>----- Forwarded Message -----
>From: "Kirt & Erika Zipf-Williams" 
>To: General MEMS discussion 
>Subject: RE: [mems-talk] Materail for Thermal laternal actuator
>
>Here are some of the reasons:
>
>1. Polysilicon is process-compatible and has been available for a long time.
>2. Poly forms a protective oxide of limited thickness, preventing it from
>"burning."
>3. By using the same material (silicon) for the substrate and the actuator,
>the actuator is not unintentionally acutated as the ambient temperature
>changes.
>
>    --Kirt Williams, Ph.D.    consultant