Hello all,
I'm presently having some difficulty with doping my polysilicon. My
substrate structure is as follows (from bottom to top):
300 microns thick p-type Silicon
400 nm low Stress LPCVD SiN
150 nm undoped polysilicon.
Now I would like to reduce the sheet resistance of the polysilicon. To
do this I've used Emulsitone's Borosilicafilm spin on dopant called
"Borosilicafilm I" -data sheet available on Emulsitone website.
I had initial problems to do with removing the Borosilicafilm after the
diffusion (all to do with my lack of oxygen ambient).
I used a Jandel 4 point probe to try and measure the sheet resistance of
my undoped polysilicon. It was essentially immeasurable implying a sheet
resistivity of greater than 10 mega ohms /square! I was looking to get
the sheet resistance value to between 50-250 ohm persquare. I then would
like to dope the contact regions (I am fabricating polysilicon
resistors) to around 10 ohm per square.
I used the following protocol to dope my polysilicon:
1. Standard clean
2. Spin Borosilicafilm dopant
3. Bake 20 minutes 200oC (essentially to remove the solvent from the
dopant film)
4. Diffused for 15/30 minutes at 1050oC (this is the maximum temperature
of my furnace) in 95%/5% N2/O2
5. Borosilicafilm is etched off either using CHF3 dry etch or 49% HF
(90% of my tests I use wet etching rather than dry etching)
6. Measure sheet resistance.
After diffusion for 15 minutes the sheet resistance of my polysilicon
reduces to around 200 ohms pr square while diffusion for 30 minutes
results in a sheet resistance of 150 ohms per square. So far so good.
However when I doped the polysilicon film for a second time (recall I'd
like a sheet resistance of 10 ohms persquare in the contact regions),
following the exact same protocol as above, I found the sheet resistance
INCREASES to around 270 ohms per square after 15 minutes and 350 ohms
per square after 30 minutes.
My simple question is this: Can anyone shed on light on this phenomenon?
Many thanks,
James
--
Dr. James Paul Grant
Postdoctoral Research Associate
Complex Systems Design Group
76 Oakfield Avenue Room 3
University of Glasgow
Glasgow
Scotland
G12 8LS
Telephone: +44(0)141 330 3374
