INTRODUCTION:
Spin on glass (SOG) is a mixture of SiO2 and dopants (either boron or phosphorous) that is suspended in a solvent solution. (Actually, our boron source is based on a boron polymer and therefore slightly different, but the procedures are the same). The SOG is applied to a clean silicon wafer by spin-coating just like photoresist. The SOG is our dopant source. To get the dopant out of the SOG and into the silicon wafer typically requires 2 steps (although it can be done in one). The first is a "pre-dep" in which the dopant leaves the SOG and saturates the silicon surface. After this the SOG is removed in an HF dip. Finally a "drive-in" step is performed inorder to diffuse the dopant into the silicon.
SAFETY:
SOG is classified as a flammable, corrosive liquid - similar to organic solvents. The MSDS sheets for all of these chemicals can be found in the yellow MSDS binder outside of Meyer 425. Materials that are contaminated with SOG are disposed of in the solid waste containers. Standard precautions involved with operating the spinner are also involved.
PROCEDURE:
I. Clean Sample.
1. Follow the RCA clean procedure. Follow ionic clean and water rinse with an additional HF dip followed by a final quick DI water rinse to create a hydrophobic surface.
II. Spin-Coat Procedure
This step leaves a uniform coating of SOG approximately 5000 Å thick across the wafer.
A. To begin, turn on the vacuum pump power switch. (Power switch is located
on the power cord of the vacuum pump.)
B. Turn the Spin-Coater "Power" switch on.
C. Press the "Control" button. Red light above "Control"
button should illuminate.
D. Next, set the spin cycle times and spin speeds.
E. Set "Speed I" at ~ 500 RPM by adjusting 'clock' position.
F. Set "Timer I" for 9 seconds. Step 1 clears excess material.
G. Set "Speed II" at 3000 rpm 'clock' position using the inside tick
marks.
H. Set "Timer II" to 20 seconds (remember, the timers may be
off, so its best to check this)..
I. Place and center a test wafer onto the spin chuck.
J. Once the wafer is centered, press "Vacuum" button to initiate vacuum.
Again, check to see that the red light above "Vacuum" is illuminated.
Press start and test your speeds and times with this wafer.
K.Now load your sample wafer and depress "Vacuum" botton.
L. Using a disposable pipette dispense enough SOG to coat ~ 95% of the wafer,
place the lid on the Spin-Coater and pull down the hood shield, then push the
"Start" button, .
M. Allow the Spin Coater to complete its entire cycle. Once both the "Timer
I" and "Timer II" lights turn off, lift the hood shield.
N. Press the "Vacuum" button (red light should turn off) to release
the wafer.
O. Dispose of pipette in 'Solid Waste' container. Wipe up any spilled SOG with
Kimwipes and dispose of similarly.
III. "Pre-dep" Thermal Treatment
A. Load samples into quartz boat in the diffusion furnace.
B. Pre-dep is performed in a flowing mixture that is 98% N2 and 2%
O2. To achieve this set the N2 MFC at 100% of full scale
(500 sccm) and the O2 MFC at 5% of full scale (200 sccm).
C. Program the furnace to ramp to the desired 'pre-dep' temperature and hold for 30
minutes.
IV. Strip SOG
A. Remove samples from oven after it has returned to room temperature.
B. Immerse samples in 10% HF solution (this will be prepared for you in advance.
Remember to use proper safety precautions with HF) to strip SOG (1-5 minutes).
(How do you tell when it's stripped)
C. Rinse in DI water, blow dry with nitrogen, and return to furnace for drive-in.
V. Grow Oxide/Drive in
A. This is performed in a flowing mixture of pure O2 so set the O2
MFC at 100% of full scale (200 sccm).
B.Program the furnace to ramp to the desired 'drive-in' temperature, and then
decrease to 850 ºC and over a period of from 20-80 minutes.
VI. Characterization
A. Remove samples from oven after it has returned to room temperature.
B. Immerse samples in 10% HF solution to strip oxide (1-5 minutes)
C. Rinse in DI water, blow dry with nitrogen.
D. Measure the sheet resistance.