Message-Id:   <9610221607.AA27003@memcad3.mit.edu>


Hi Guenter,

I find that surface deposition of charges also occur on SiN, where
SiN is the dielectric sitting on top of the Si substrate.  Charges
can be deposited either after pull-in (as in your case) or while the
structure is electrostatically deflected towards the substrate w/o
touching it at a voltage below pull-in.  To see the latter effect
in the devices I test (MUMPs polysilicon process of MCNC), I need
to wait on the order of a few minutes.  This procedure deposits
charges of the opposite polarity on the SiN and will decrease the
subsequently measured pull-in voltage.

I have not calculated the surface charge density, but I see pull-in
voltage shifts of up to, for example, 1V on a sample with a pull-in
voltage near 25V, with a nominal electrostatic gap of 2.2um.

Seeing such charge deposition is not uncommon in dielectrics, and
are a result of dangling bonds on the surface.

Raj Gupta
MIT