> Just wondering why people prefer using poly-si instead of
> other mat. such as metal for typical thermal lateral actuator?

Here are a few advantages of poly-Si:

 - The volumetric heating in an electrothermal actuator scales up
linearly with the resistivity of the material.  For a given current, a
silicon actuator will dissipate orders of magnitude more heat than a
similar metal actuator.
 - When using a silicon actuator, the circuit resistance is dominated by
the actuator, not the connections, wire bonds, metal contacts, etc.  For
a metal actuator, the reverse can be true: the connections may have a
higher resistance than the actuator itself.  This variation affects
reproducibility when a constant voltage is used for activation.
 - Most thermal actuators rely on the heat being confined to a certain
area of the beam.  The high thermal conductivity of metals can make this
difficult.  For example, gold is about an order of magnitude more
thermally conductive than poly-Si.
 - Unlike with metals, the resistivity of poly-Si is easily adjusted by
doping.

There are also some disadvantages: for example, residual stress can
cause out-of-plane bowing, and an extra step is required to fabricate
metal contacts to the silicon.  Hope this helps,

John Maloney
http://john.maloney.org
MEMS Engineer
MicroCHIPS, Inc.
http://www.mchips.com