INTRODUCTION:

Spin-on glass (SOG) is a mixture of SiO2 forming chemicals and dopants (often boron or phosphorous) that is suspended in a solvent solution. The SOG is applied to a clean silicon wafer by spin-coating. 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 in order to diffuse the dopant into the silicon. Some processes only require a single step diffusion.

SAFETY:

Read MSDS before using SOG materials. SOG is classified as a flammable, corrosive liquid. Materials that are contaminated with SOG are disposed of in the solid waste containers. Standard precautions involved with operating the spinner are also involved. Use safety apparel as required (gloves, apron, etc.)

PROCEDURE:

I. Clean Sample. Use the RCA or piranha clean procedure.

II. Spin-Coat Procedure

This step leaves a uniform coating of SOG approximately 2000 Å thick across the wafer.

A. To begin, turn on the vacuum pump power switch. (Power switch may 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 at 3500 RPM.
F. Set Timer 30 seconds.
G. Place and center a test wafer onto the spin chuck.
H. Once the wafer is centered, apply Vacuum (depending upon the spinner, this can be automatic or manually activated) button to initiate vacuum.
I. Using a disposable pipette dispense enough SOG to coat ~ 80% of the wafer, push the “Start” button, place the lid on the Spin-Coater and pull down the hood shield.
J. Allow the Spin Coater to complete its entire cycle, lift the hood shield.
K. Press the “Vacuum” button (red light should turn off) to release the wafer (unless your spinner does this automatically).
M. Dispose of pipette in ‘Solid Waste’ container. Wipe up any spilled SOG with wipes and dispose of similarly.
N. bake at 200C in air or N2 for 5 minutes for track bake or hot plate (60 minutes for convection oven).

III. “Pre-dep” Thermal Treatment

A. Load samples into quartz boat and then into the diffusion furnace. Use slow push/pull or temperature ramping to prevent wafer warpage.
B. Predep 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). Larger diameter wafers and
furnace tubes may require higher flows in accordance to the increased tube volume). C. Program the furnace to ramp to the desired ‘pre-dep’ temperature and hold for 30 minutes. Then ramp down/or slow pull the wafers.

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) Use extreme caution when using etchants containing HF. Read MSDS for etchants to fully understand the hazards and observe all safety recommendations.
C. Rinse in DI water, blow dry with nitrogen, and return to furnace for drive-in.

V. Grow Oxide/Drive in

A. The push pull/ramping cycle is done in 2% O2 as above. The drive in is performed in a flowing mixture of pure O2 so set the O2 MFC at 100% of full scale (200 sccm) during drive in (or higher for large diameter wafers/tubes).
B. Program the furnace to ramp to the desired drive in temperature, dwell at drive in temp to achieve desired sheet resistance and junction depth, ramp down (or slow pull).

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 with the four point probe.

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