Sriram-

Like a large portion of the questions on this list, your question can be
answered by reading any basic microfabrication textbook.  For example, this
can be found on page 176-177 of "Fundamentals of Microfabrication" by Marc
Madou (1st ed).  In fact, if you had even bothered to do a simple search of
the mems-talk archives you would have found that this question has been asked
so many times that a compilation of answers has been posted.

I don't mean to single you out, but as a MEMS researcher trying to participate
in a useful forum I am sick of laziness in research. If anyone has any
comments to add, I welcome an open discussion of this issue.

Just so this message isn't a complete waste of your time Sriram, here is a
link to the mems-talk search page.

http://www.memsnet.org/search/

-Jon Engel
MASS Group, UIUC
http://mass.micro.uiuc.edu