Wet oxidation of silicon is typically used to rapidly grow thick oxides that are used as diffusion barriers. In order to obtain a defect free silicon/oxide interface, it is imperative that the wafers are cleaned before oxidation. This also prevents contamination of the furnaces. The furnace operation is controlled by a Process Controller which can be programmed with 8 step recipes. Gas flow is controlled by electronic mass flow controllers.
SAFETY:
The furnace gets very hot and takes a long time to cool down. Never try to open the furnace after you have started it. Processed wafers will be removed at a subsequent lab session.
PROCEDURE:
A. Fill the flask that sends steam into the furnace with about 250ml of distilled water. Turn on the hot plate under the flask and set the hot plate to about 5.5. It will take nearly an hour for the water to boil. It should be lightly boiling during the oxidation. As the water gets close to boiling, plug in the air pump which circulates air through the water and into the furnace. REMEMBER, THE HOT PLATE IS HOT!
B. Program the furnace controller for the desired time/temperature conditions. Turn the wet oxide oven on. On the front control panel there are four buttons: Page, Scroll, Down and Up buttons. The following procedure will program the furnace to ramp to your desired temperature, dwell at that temperarture for a given time, and turn itself off. The following procedure should provide you with a 20 degree/min ramp to the desired oxidation temperature, a dwell time, and turn the oven off.
|
Action |
Display Readout |
| Push the 'Page' button | Until you see 'RuN LiSt' |
| Push 'Scroll' button | Until you see top: 'StAt', bottom 'Off' |
| Push the 'Page' button | Until you see 'ProG LiSt' |
| Push 'Scroll' button | Top: 'Hb', Bottom 'Off' |
| Push 'Scroll' button | Top: ' Hb U ', Bottom ' 0 ' |
| Push 'Scroll' button | Top: ' rmP.U ', Bottom ' min ' |
| Push 'Scroll' button | Top: ' dwL.U ', Bottom ' min ' |
| Push 'Scroll' button | Top: ' CYC.n ', Bottom ' 1 ' |
| Push 'Scroll' button | Top: ' SEG.n ', Bottom ' 1 ' |
| Push 'Scroll' button | Top: ' tYPE ', Bottom ' rmP.t ' |
| Push 'Scroll' button | Top: ' tGt ', Bottom ' a number' |
| Use Down/UP buttons to . . . | Adjust value to desired temperature |
| Push 'Scroll' button | Top: ' dur ', Bottom ' a number' |
| Use Down/UP buttons to . . . | Adjust value to 20 |
| Push 'Scroll' button | Top: ' SEG.n ', Bottom ' 2 ' |
| Push 'Scroll' button | Top: ' tYPE ', Bottom ' dwEll' |
| Push 'Scroll' button | Top: ' dur ', Bottom ' a number' |
| Use Down/UP buttons to . . . | Adjust value to desired oxidation time |
| Push 'Scroll' button | Top: ' SEG.n ', Bottom ' 3 ' |
| Push 'Scroll' button | Top: ' tYPE ', Bottom ' End ' |
| Push 'Scroll' button | Top: ' End.t ', Bottom ' StOP ' |
| Push 'Scroll' button | 'ProG LiSt' |
| IMPORTANT: Before proceeding confirm that the program is correct by reviewing it with the instructor or TA. | |
| If the program is OK go ahead with steps B and C below | |
| Push the 'Page' button . . . | Until you see 'RuN LiSt' |
| Push 'Scroll' button | Top: ' StAt ', Bottom ' Hold ' |
| Push Up Arrow | Hold changes to run, blinks |
| Push the 'Page' button . . | Until you see 2 temperatures |
| The top value is true temp, bottom is setpoint. You can monitor the progress here. | |
B. Place clean silicon samples on quartz boat and place in the center of the
furnace. Close furnace door. Be careful to make sure the furnace door is completely
closed, if not the furnace will not turn on. *Note*: Treat the quartzware like
your silicon - it's expensive, fragile, should only be handled with gloves to
minimize contamination.
C. Once the flash is boiling and the air pump is on, turn on furnace as indicated
in the chart above. On the readout you will see both the setpoint(top) and the
actual temperature (bottom) which you can monitor to ensure that the program
is working properly.
E. After the dwell time has passed, monitor the temperatures to ensure that
the furnace is cooling. Allow the water to run for 30 minutes after the temperature
begins to decrease. Turn off hot plate and the air pump.
F. Turn the furnace off.
G. Go home. The oven will still be very hot. Do NOT open
it. Measuring thickness and uniformity at the subsequent lab section using the
Ellipsometer Compare ellipsometric measurments to
the following color chart:
Film Thickness (Microns) |
Color and Comments |
Film Thickness (Microns) |
Color and Comments |
0.05 |
Tan |
0.60 |
Carnation pink |
0.07 |
Brown |
0.58 |
Light orange or yellow to pink borderline |
0.10 |
Dark violet to red violet |
0.57 |
Yellow to "yellowish" (At times it appears to be light creamy gray or metallic) |
0.12 |
Royal blue |
0.56 |
Green yellow |
0.15 |
Light blue to metallic blue |
0.54 |
Yellow green |
0.17 |
Metallic to very light yellow green |
0.52 |
Green (broad) |
0.20 |
Light gold or yellow - slightly metallic |
0.50 |
Blue green |
0.22 |
Gold with slight yellow orange |
0.49 |
Blue |
0.25 |
Orange to melon |
0.48 |
Blue violet |
0.27 |
Red violet |
0.47 |
Violet |
0.30 |
Blue to violet blue |
0.46 |
Red violet |
0.31 |
Blue |
0.44 |
Violet red |
0.32 |
Blue to blue green |
0.42 |
Carnation pink |
0.34 |
Light green |
0.41 |
Light orange |
0.35 |
Green to yellow green |
0.39 |
Yellow |
0.36 |
Yellow green |
0.37 |
Green yellow |
IV. Color Chart for Thermally Grown SiO2 Films from S. Wolf and R.N. Tauber, Silicon Processing for the VLSI Era: Volume 1 - Process Technology, Lattice Press, 1986.