Kris-

I am not very familiar with spin-on dopants, but given your info, here are
my thoughts:

* Did you sufficiently bake the wafers after dopant spin-on to drive off
solvents and densify the film?
* Did you RCA clean the wafers prior to drive-in?  If so, you may have had
non-uniform removal of the dopant film in the HF dip stage of the clean.
* How was your polysilicon deposited (and are you sure that the thickness
is uniform).  Oxidation rate is much higher for doped polysilicon, so you
could have widely varying oxidation rates across the wafer dependent on
both dopant and grain-size variation.
* If as above, you had widely different oxidation rates, then after oxide
removal, you will then have a large thickness variation of the polysilicon
which will change the cross sectional area of your resistor.

One obvious test to help diagnose the problem, is to pattern the resistors
after polysilicon deposition and measure the resistance change.  This will
give you a baseline comparison - if you see a large resistance change, then
you need to look at polysilicon thickness variation in deposition and your
photo tolerances.  You can then take this wafer and perform your dopant
spin-on, complete the experiment and re-measure.  Even if your polysilicon
step heights will affect the spin-on quality, this should still help you
see the specific change in resistance from your dopant process (spin-on,
bake, drive-in, oxide strip, etc.).

Good Luck!
Kevin Stark

Kevin C. Stark, Ph.D.
Senior Program Manager
NineSigma, Inc.
21945 Chagrin Blvd.
Cleveland, OH  44122
216-295-4806
216-295-4825 (fax)
[email protected]
www.ninesigma.com


-----Original Message-----
Message: 1
Date: Thu, 13 Mar 2003 10:01:47 -0800 (PST)
From: krishna 
Subject: [mems-talk] Piezoresistors

Dear Members,

we have fabricated Polysilicon Piezoresistors on Si(100) surface.The
polysilicon was doped with spin on dopant(phosphourous dopant) and drive
in and oxidation was done at same time at 1050C. The poly resistors are
barely 5 microns distance apart.when i measured the resistance value
the value of the resistance differed alot more than 300% change among the
adjacent resistors..

Can any one suggest me why this happened?

Thanks

kris