Deepak

On page 135 of Senturia's "Microsystems Design" we see the formula

k = e*A*V/g3 where V = pull in voltage and g = pull in gap

A little manipulation of this equation gives
g = V*(C/k)1/2

 From this equation we see that the gap (distance from the top plate) is
directly proportional to the capacitance(C) and inversely proportional
to the stiffness(k).This formula assumes ideal everything, i.e, no
fringing fields, membrane and electrode are the same dimensions and
shape and that the membrane mechanics is linear. These conditions do not
hold good in your case therefore this formula and its conclusion about
the 1/3rd gap is not accurate.

We know that fringing increases capacitance. According to the formula
above, this should mean that the pull in gap will also increase. Now let
us look at stiffness. When the stiffness goes up the gap goes down This
makes intuitive sense.

Here is the paper quoted in the earlier reply.
http://www.nsti.org/publ/MSM98/M3406.pdf This paper also indicates that
pull in can be larger than the 1/3rd gap.

I hope this answers your questions

Shyam Venugopal
Applications Engineer
Coventor Inc.