INTRODUCTION:
Spin on glass (SOG) is a mixture of SiO2 and dopants (either boron or phosphorous) taht is suspended in a solvent solution. 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 requires 2 steps. 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 with an additional HF dip 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 the ~ 500 RPM by adjusting 'clock' position.
F. Set "Timer I" for 9 seconds. Step 1 clears excess resist.
G. Set "Speed II" at 3000 rpm 'clock' position using the inside tick marks.
H. Set "Timer II" 20 seconds.
I. Place and center a test wafer onto the spin chuck.
I. Once the wafer is centered, press "Vacuum" button to initiate vacuum. Again,
check to see that the red light above "Vacuum" is illuminated..
J. Using a disposable pipette dispense enough SOG to coat ~ 90% of the wafer, push the
"Start" button, place the lid on the Spin-Coater and pull down the hood shield.
K. Allow the Spin Coater to complete its entire cycle. Once both the "Timer I"
and "Timer II" lights turn off, lift the hood shield.
L. Press the "Vacuum" button (red light should turn off) to release the wafer.
M. Dispense 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 into the diffusion furnace.
B. This 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 to strip SOG (1-5 minutes)
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 'pre-dep' temperature, and then decrease to
850 ºC and over a period of 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.