Experiment 1: Dry Oxidation (SiO2) Process
Objective:
1.
To understand the basic principle of dry oxidation process.
2.
To determine the growth rate of the oxide.
Introduction
Oxidation is one of the most important thermal processes. It is an adding process, which adds
oxygen to a silicon wafer to form silicon dioxide to a silicon surface.
Oxidation reaction occurs on the surface of silicon when in contact with oxygen. Initially, the
reaction occurs quickly, however as the oxide layer on the surface grow, the oxygen must diffuse
through the oxide before it reaches the silicon and reacts. The diffusion of the oxygen (or water
vapor) is the oxidation rate-determining factor.
The oxidation rate is controlled by four conditions: temperature, pressure, humidity, and
crystal orientation. Generally, the crystal orientation of the surface determines how many silicon
atoms are exposed to oxygen in order for initial reaction to occur. The closest packed plane, in
case of silicon is (111) will react fastest because it has the most atoms on the surface. Increased
humidity increases the reaction rate. In this study, we used the <100> orientation silicon wafer.
The oxidation process can be carried out through wet or dry conditions. Dry oxidation
simply flow oxygen gas over the silicon and the reaction that occurs is:
Si + O2  SiO2
Wet condition produces the fastest reaction rate. Wet condition can be developed by
flowing oxygen through water so the oxygen carries it with water vapor. The reaction that occurs
is:
Si + 2H2O  SiO2 +
2H2
The reaction of pure oxygen occurs simultaneously with the reaction with water vapor;
however the latter occurs much more rapidly and causes the reaction with pure oxygen to
become insignificant.
Equipment / Chemicals
i.
Gas N2 and O2
ii.
Quartz boat
iii.
Quartz Rod
iv.
Timer
v.
Wet Oxidation Furnace
vi.
Spectrophotometer
vii.
Four point probe
viii.
Optical Microscope
Methodology
Parameter
List down all the parameters and settings
Temperature, T = 1000°C
N2 level = 1l/min
O2 level = 1l/min
Oxidation Time = 30 min
1.
Turn on furnace. Ramp up to T = 500°C
2.
Scribe wafer accordingly.
3.
Load 2 wafers into furnace tube.
4.
Start N2 flow for purging.
5.
Set the temperature.
6.
When T reaches the temperature, slowly open O2 valve and close N2 valve.
7.
Start the timer.
8.
Run oxidation process for desired time, e.g. t = 30 min.
9.
When oxidation complete, close O2 valve, open N2 valve.
10. Ramp down the furnace to T = 500°C
11. Unload and cool the wafer to the ambient temperature.
12. Close N2 valve.
13. Measure and record the oxide thickness in Table 2.3.1. Take at least 5 readings at
different point.
14. Observe wafer under optical microscope.
Characterization/Testing
1.
Thickness measurement
2.
Particle/defect/color inspection
Results
Table 2.3.1: Wet Oxidation
Oxidation time: 30 min
1
2
3
4
5
Thickness, A
Colour
Oxidation time,
t n = __________ min
Initial oxide thickness,
OxTi = __________ Å
After oxide thickness, O x Tn = __________ Å
Growth rate,
OGR =
OxTn  OxTi
= ___________ Å/min
tn
Discussion
1.
Calculate the growth rate of the oxidation process.
Avg
Std
Uni
2.
Why Hydrofluoric dip is necessary before oxide growth?
3.
What are the parameters affecting oxidation rate?
4.
Recognize the colour different of the oxide after each etching steps. Use colour code to
estimate the oxide thickness and compare the result with spectrophotometer
measurement.
5.
What are the advantages and disadvantages of wet oxidation over dry oxidation?
Conclusion
Based on your understanding, please conclude the experiment result.
Reference
1.
2.
http://www.guajara.com/wiki/en/wikipedia/e/el/electrical_resistivity.html
Peter Van Zant. 2000. Microchip Fabrication: A Practical Guide to Semiconductor
Processing 4th Edition. McGraw Hill Co.