SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
SEMICONDUCTOR PROCESS TECHNOLOGY
LAB MODULE 4: OXIDATION PROCESS
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
THEORY
Introduction
One of the advantages of silicon substrate over other semiconductor substrate is the hightemperature process capability of the silicon. Silicon wafer processing always involves many
high-temperature (700 ºC to 1200 ºC) processes such diffusion, oxidation, deposition and
annealing.
The natural oxide of silicon, silicon dioxide is a very stable and strong dielectric mataerial, easily
formed by a high-temperature process. This is one of the important reasons that silicon dominates
the IC industry as the semiconductor substrate material. Normally, IC fabrication process starts
with an oxidation process, which grows a layer of silicon dioxide to protect the silicon surface.
Silicon wafers then go through thermal processes in high-temperature furnaces and RTP tools
many times during the wafer process flow. Figure 1 illustrate shows the typical IC fabrication
process flow.
Figure 1: Typical IC Fabrication Process Flow
Oxidation Process Technology
Oxidation Kinetic: Deal Grove Model
In thermal oxidation process, silicon reacts with either oxygen or water vapor to form silicon
dioxide. The oxidation reaction may be represented by the following reactions:
Si  O2  SiO2 ….......................................dry oxidation
Si  2H 2 O  SiO2  2H 2 ...............................wet oxidation
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
From previous experiment have demonstrated that oxidation proceeds by the diffusion of either
an oxygen or water species through the oxide already formed up which than reacts with the
silicon at the Si-SiO2 interface. As the oxidation continues, the interface moves into the silicon
and a new, clean silicon surface is produced. As a result, original silicon surface states
(unsatisfied bonds) and contamination are consumed and optimized device passivation is
achieved.
Thermal oxidation of silicon is normally carried out in a fused quartz tube in resistance heated
furnace. The silicon wafers are placed vertically in slot in a flat quartz boat that can accommodate
up to 200 four to six inches diameter wafers. There was a difference between wet and dry
oxidation. For dry oxidation, high purity oxygen from liquid source is transported into the furnace
tube through suitable regulators, valves, traps, filters and flowmeters. Water or steam oxidation
was carried out by bubbling O2 or N2 through a flask of deionized water maintained at a particular
temperature.
Silicon oxidation data are obtained by determining oxide thickness (Xo) as a function of oxidation
time (t) and other variables such as oxidation temperature and silicon orientation.
Rate of Oxide Growth
The rate of oxide growth describes how fast the oxide grows on the wafer. It depends on
parameter such as temperature, pressure, oxidizing condition (dry or wet), silicon crystal
orientation, and doping levels. The model for oxide growth on silicon is referred to as a linearparabolic model developed by Deal and Grove and accurately represents oxide growth in wide
range thickness (300 to 20,000Å). Oxide growth is described by two growth stages: the linear
stage and parabolic stage. The linear stage of oxide growth is valid up to 150Å of oxide thickness.
The linear equation is described by:
 B 
X 
t
 Ahr 
X= the thickness of growing oxide
(B/A) = the linear rate constant
t = time taken to grow oxide
In linear stage, oxidation varies linearly with time. Oxidation is reaction-rate controlled because
the limiting factor for oxidation growth is the reaction occurring at the Si/SiO2 interface.
The parabolic stage of oxidation growth is the second phase of oxidation growth and starts after
150Å of oxide thickness. The equation describes the parabolic stage is:
X  Bt  2
1
X = the thickness of growing oxide
B = parabolic rate constant
t = time taken to grow oxide
Oxide growth in the parabolic stage is much slower than in linear stage. As oxide layer become
thicker, the oxygen must diffuse through a larger distance to arrive at the Si/SiO2 interface. The
reaction is limited by the rate at which the oxygen diffuses through the oxide. The parabolic stage
of oxide growth is said to be diffusion controlled.
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
Growth of Thin Oxide
The Deal-Grove model provides excellent agreement with experimental data except for thin
(<20nm) SiO2 grown in O2.
Massoud and Plummer have suggested that the initial stages of silicon oxidation in dry O2 may be
represented by the following equation:
 x
dx
B

 C1 exp  o
dt 2 xo  A
 L1

 x
  C 2 exp  o

 L2



In this expression, the first term on the right-hand side is the contribution from the original linearparabolic model.
Factor of Affecting Oxidation Rate
Generally, the oxidation rate is controlled by four conditions: temperature, pressure, humidity,
and crystal orientation. However, several factors in contributing to oxidation rate have been
observed for a number of years include:
a. Silicon Orientation
b. Dopant Concentration
c. Surface Preparation
d. Ambient Type
e. Chlorine Additions
f. Nitridation
g. Oxidant Pressure
Dry Oxidation Process
Dry oxidation has a lower growth rate than wet oxidation; however, the oxide film quality is
better than the wet oxide. Therefore, thin oxides such as screen oxide, pad oxide, and especially
gate oxide normally use the dry oxidation process.
Usually there are two nitrogen sources in an oxidation system, one for the process application,
with higher purity, and another with lower purity for the chamber purge. Because nitrogen is a
stable gas, it does not react with silicon, even at 1000 ºC. Therefore, nitrogen is always used in
the oxidation process during system idle time, wafer loading, temperature ramp and stabilization,
and wafer unloading steps. For dry oxidation, high-purity oxygen gas is used to oxidize silicon.
HCl is also commonly used during the oxidation step as a getter to remove mobile metallic ions,
especially sodium, by forming immobile chloride compound. This is very important, since a trace
amount of sodium can cause MOS transistor malfunction and affect IC chip performance and
reliability.
The quartz tube starts to sag when the temperature is above 1150 ºC, so oxidation processes
cannot operate at that temperature for too long. The dry oxidation process normally operates at
about 1000 ºC.
When the silicon dioxide is forming on the single-crystal silicon surface, there is an abrupt
change at the silicon-silicon dioxide interface. There are always some dangling bonds at the
interface because of the crystal structure mis-match. The dangling bonds induce so-called
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
interface state charge, which is a positive charge that strongly affects IC chip performnace and
reliability. Post oxidation annealing in N2/H2 ambient is a common technique to reduce the
interface state charge.
A typical dry oxidation process flow for gate oxide growth is as follows;
1) Idle with purge N2 flow
2) Idle with process N2 flow
3) Wafer boats push in with process N2 flow
4) Temperature ramp-up with process N2 flow
5) Temperature stabilization with process N2 flow
6) Oxidation with O2, HCl; stop N2 flow
7) Oxide annealing; stop O2, start process flow N2
8) Temperature cool down with process N2 flow
9) Wafer boats pull out with process N2 flow
10) Idle with proces N2 flow
During the system idle time, the furnace is always kept at a high temperature, such as 850 ºC, so
that it does not need too much time to ramp up the temperature to the required process
temperature. Before wafer loading, process N2 gas starts to flow into the process tube and fill it
with high purity nitrogen. When the wafer boats are placed in the process tube,the temperature
starts to ramp up, with ramp up rate about 10 ºC/min.
After the furnace reaches the setting temperature required by the process, a few minutes of
temperature-stabilizing with N2 flow are required to allow temperature oscillation, to die down
and make the furnace reach the steady state of the setting temperature.
Now the system is ready for oxidation. Turning on the oxygen and anhydrate hydrogen chloride
flows and turning of the nitrogen flow, makes oxygen react with silicon to form a thin layer of
silicon dioxide on the silicon wafer surface. After the required oxide thickness is reached, the O2
and HCl flows are stopped and N2 flow is resumed. The wafers stay at high temperature for a
while to anneal the oxide. This step improves the quality of silicon dioxide, makes it denser,
reduces the interface state, and increase the breakdown voltage.
Wet Oxidation Process
Using H2O instead of O2 as the oxygen source is called wet oxidation. At high temperature, H2O
can dissociate and form hydroxide, HO, which can diffuse in the silicon dioxide faster than O2.
Therefore, the wet oxidation process has a significantly higher oxidation rate than the dry
oxidation process.
It is used to grow thick oxides such as masking oxide, blanket filed oxide, and the LOCOS oxide.
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SEMICONDUCTOR PROCESS TECHNOLOGY
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Oxidation Thin-Film Process Characterization
1. Thickness Measurement
Monitoring the oxidation process means measuring oxide thickness and uniformity. Ellipsometry
is commonly used to measure dielectric thin-film refractivity and thickness. When a beam of light
is reflected from the film surface, the polarization status change, as schematically shown in
Figure 2. By measuring this change, one can get information about the fil refractive index and
thickness.
Figure 2: Ellipsometry Technique
Oxide thicknes also can be determined using the color chart. After the oxide is grown, the color
of the wafer surface changes. The color depends on film thickness, refractive index, and the angle
of the light. The reflected light from the oxide surface (light 1) and the reflected light from the
silicon-silicon dioxide interface (light 2) have the same frequencies, but with different phase,
since light 2 travels a longer distance inside the oxide film. The two reflected lights interfere with
each other and cause both constructive and destructive interference at different wavelengths,
since the refractive index is a function of wavelength. This concept is illustrated in Figure 3.
Color charts are convenient tool to measure film thickness. Although no longer used for thickness
measurement in advanced IC fabs, they are still usefull tools to estimate the oxide thickness
quickly and to detect obvious nonuniformity problems.
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
Figure 3: Reflectometry / Color Chart Concept
For a precise measurement of oxide thickness, spectroreflectometry is used. It measures the
reflected light intensity at different wavelengths, a the thin-film thickness can be calculated from
the relation of reflected light intensity and the wavelength of the light.
In this lab, spectrophotometer is used to measure oxide thickness. This system is a thin film
measurement system from Filmetrics, F20. This equipment is to measure the film thickness of
semiconductor and dielectric up to 450m at certain wavelength range. The thin film must be
smooth, translucent or lightly absorbing film or reflective substrate.
2. Process Uniformity
The uniformity of the oxide thickness is routinely measured during process development and for
statistical process control (SPC) monitoring.
The more measurement points are taken, the more accurate is the analysis. However, more
measurement points need longer measurement time, which means lower throughput and higher
cost.
The 49-point, 3σ standard deviation nonuniformity is the most common definition for process
qualification in the semiconductor industry. For production wafer, less time consuming 5-point
and 9-point measurement are commonly used for process control and monitoring. Figure 11
illustrate the standard mapping patterns for 5-point, 9-point and 49-point measurements
respectively.
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
Figure 2: Mapping Patterns of Uniformity Measurement
The most widely used non-uniformity measurement is based on the equation, called Max-Min
Uniformity;
Non-uniformity (%) = (Max Value – Min Value) / 2 x average
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
Experiment 1: Wet Oxidation (SiO2) Process
Objective:
In this experiment, student will carry out two kind of oxidation; dry and wet thermal oxidation.
The oxidation process will be at certain process temperature and time to determine and
differentiate dry and wet oxidation process.
1. To understand the basic principle of wet oxidation process.
2. To determine the growth rate of the wet oxide.
Equipment / Chemicals
i.
Steam (H20)
ii.
Gas N2 and O2
iii.
Quartz boat
iv.
Quartz Rod
v.
Timer
vi.
Wet Oxidation Furnace, WFM
vii.
Spectrophotometer, SPM
viii.
Four point probe, FPP
ix.
High Power Optical Microscope, HOM
Characterization/Testing
1. Thickness measurement
2. Sheet resistance
3. Particle/defect/color inspection
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
Process Run Card
Process Flow Run Card
Group:
Name:
Lot Number:
Exp No:
Orientation:
Size:
Resistivity:
Lot Start Date:
LP#
Equipment
Wet Oxidation
1
WOFM
Thickness:
Planner:
Process/Recipe
Time
Out
1. Preheat furnace.
T = 500°C
2. Start N2 flow for purging.
N2 @ level 7.5
Flow rate: 0.5L/mins
O2 @ level 0
3. Load wafer into furnace tube.
T = 500°C
4. Set temperature to 1000°C
T = 1000C
5. Slowly close N2 valve and
open O2 valve.
O2 @ level 4.5
Flow rate: 1.55L/mins
N2 @ level 0
6. Hold wafer in the furnace.
T : 1000°C
t : 90 mins
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INSEP
Date:
Substrate Type:
Start Wafer Quantity:
Authorized by:
Data
out
Remarks
SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
7. Close O2 valve, open N2 valve
N2 @ level 7.5
Flow rate: 0.5L/mins
O2 @ level 0
8. Unload the wafer.
9. Close N2 valve.
T = 500°C
Ox Thick
2
SPM
1. Measure oxide thickness
Ox Thick = ______________ A
Visual Inspect
3
HOM
1. Observe
particle/scratches/colour
Result and Discussion
1. Plot graph thickness vs. time for wet oxidation process.
2. Determine the growth rate of the wet oxide.
3. What is the typical range of temperature in thermal oxidation process?
4. Explain the principle of the dry oxidation process and describe the film growth
mechanism.
5. Why HF dip is necessary before oxide growth.
6. What are the parameters affecting oxidation rate?
7. 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
8. What are the advantages and disadvantages of wet oxidation over dry oxidation?
9. What types of oxide in CMOS devices are produce by dry oxidation process.
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
Experiment 2: Dry Oxidation (SiO2) Process
Objective
1. To understand the basic principle of dry oxidation process.
2. To determine the growth rate of the dry oxide.
Equipment / Chemicals
i.
Gas N2 and O2
ii.
Quartz boat
iii.
Quartz Rod
iv.
Timer
v.
Dry Oxidation Furnace, DFM
vi.
Spectrophotometer, SPM
vii.
High Power Optical Microscope, HOM
Characterization/Testing
1. Thickness measurement
2. Sheet resistance
3. Particle/defect/color inspection
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SEMICONDUCTOR PROCESS TECHNOLOGY
Process Flow Run Card
Group:
Name:
Lot Number:
Exp No:
Orientation:
Size:
Resistivity:
Lot Start Date:
LP#
Equipment
Dry Oxidation
1
DOFM
MODULE 4: WET/DRY OXIDATION
Thickness:
Planner:
Process/Recipe
Time
Out
1. Preheat furnace.
T = 500°C
2. Start N2 flow for purging.
N2 @ level 7.5
Flow rate: 0.5L/mins
O2 @ level 0
3. Load wafer into furnace tube.
T = 500°C
4. Set temperature to 1000°C
T = 1000C
5. Slowly close N2 valve and
open O2 valve.
O2 @ level 4.5
Flow rate: 1.55L/mins
N2 @ level 0
6. Hold wafer in the furnace.
T : 1000°C
t : 90 mins
7. Close O2 valve, open N2 valve
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INSEP
Date:
Substrate Type:
Start Wafer Quantity:
Authorized by:
Data
out
Remarks
SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
N2 @ level 7.5
Flow rate: 0.5L/mins
O2 @ level 0
8. Unload the wafer.
9. Close N2 valve.
T = 500°C
Ox Thick
2
SPM
1. Measure oxide thickness
Visual Inspect
3
HOM
1. Observe
particle/scratches/colour
Results and Discussions
10. Plot graph thickness vs. time for dry/wet oxidation process.
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
11. Determine the growth rate of the dry/wet oxide.
12. What is the typical range of temperature in thermal oxidation process?
13. Explain in your own words the principle of the dry oxidation process and describe the
film growth mechanism.
14. Why HF dip is necessary before oxide growth.
15. What are the parameters affecting oxidation rate?
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 4: WET/DRY OXIDATION
16. 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
17. What are the advantages and disadvantages of dry oxidation over wet oxidation?
18. What types of oxide in CMOS devices are produce by dry/wet oxidation process.
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