4. (Thermal) Oxidation

4. (Thermal) Oxidation
1. Mask against implant or diffusion of dopant into Si
2. Electrical isolation for device isolation
3. Gate oxide in MOS structures
4. Surface passivation (corrosion, impurity, stress etc.)
Gate
4. Surface Passivation
from corrosion, stress, etc.
ILD
Al
IMD
n +
n-well
N-Well
p + n +
p +
LOCOS
(2. Isolation)
p-substrate
3. Gate Oxide

Applications of Oxide
5. Diffusion Source
6. Dielectric Materials for Capacitor in DRAM
PSG on poly-Si → phosphorous drive-in
PSG
N-Well
n-well
poly-Si for Gate
p-substrate

Prof. Nathan Cheung, U.C. Berkeley
Properties of Thermal SiO 2

Prof. Nathan Cheung, U.C. Berkeley

Techniques for Oxide Formation
• RT ~ 200 o C
Ø wet anodization, sputtering
• 250 ~ 600 o C
Ø CVD (SiH 4 + O 2 → SiO 2 + 2H 2 )
• 600 ~ 900 o C
Ø CVD (pyrolysis of Si(OC 2 H 5 ) 4 , SiH 4 , SiCl 4 )
• 900 ~ 1200 o C
Ø THERMAL OXIDATION

Thermal Oxidation Process
1. Thermal oxidation of silicon is achieved by heating the wafer to a high
temperature, typically 800 to 1200 o C, in an atmosphere containing either
pure oxygen or water vapor.
- Dry Oxidation : Si(s) + O 2 (g) → SiO 2 (s)
- Wet Oxidation : Si(s) + 2H 2 O(v) → SiO 2 (s) + H 2 (g)
- Wet oxidation is used to grow thick oxides, and widely used for an impurity mask.
- Dry oxidation occurs slowly but results in a higher-density oxide than wet oxidation.
è Higher density results in a higher breakdown voltage (5-10 MV/cm)
è Thin gate oxide (

How does oxidation occur
1 st step: At high temperatures, both water vapor and oxygen move (diffuse)
easily through SiO 2 .
2 nd step: Oxygen arriving at the Si surface can combine with Si.
3 rd step: Chemical reaction occurs at the Si surface to form SiO 2 .
• Si is consumed as the oxide grows, and the resulting oxide expands
during growth (Fig. 3.2).
• The final oxide layer is approximately 54% above the original surface
of the silicon and 46% below it.

2. Modeling Oxidation
1. Oxidation occurs at the Si-SiO 2 interface.
2. As the oxide grows, oxygen must pass through more and more oxide.
3. Oxidation rate decreases as time goes on.
F 1 F 2 F 3
F 1 = gas phase
F 2 = through oxide
F 3 = reaction flux
Steady State Requires:
F 1 = F 2 = F 3
Figure 3.3
Model for thermal oxidation of silicon. X 0 is the thickness of the silicon dioxide layer at any time t. J
is the constant flux of oxygen diffusing through the layer, and N 0 and N i represent the oxygen
concentration at the oxide surface and silicon dioxide-silicon interface, respectively. Note that the
oxide growth occurs at the silicon interface.

1. Fick’s first law of diffusion : The particle flow per unit area, J (called
particle flux), is directly proportional to concentration gradient of the
particle.
( x,
t) ( N N )
N
i
-
J = F2
= -D
× = -D
x
X
D : diffusion coefficient, N : particle concentration
(-) : particles tend to move from a region of high concentration to
a region of low concentration
X 0 : thickness of the oxide at a given time
N 0 , N i : concentration of the oxidizing species in the oxide at the
oxide surface and SiO 2 -Si interface.
0
0
J = F = k s
× N
3 i
é
ê
ë
number of particles
2
cm ×sec
2. The oxidation rate at the SiO 2 -Si interface is proportional to the
concentration of the oxidizing species.
The flux at the interface:
(3.5)
k s : rate constant for the reaction at the SiO 2 -Si interface.
ù
ú
û
F 1 F 2 F 3

Eliminating N i using F 2 and F 3 ;
\
J
=
æ
ç X
è
D × N
0
+
0
D ö
÷
k s ø
(3.6)
3. The rate of change of thickness of the oxide layer with time is given by the
oxidizing flux divided by the number of molecules M of the oxidizing
species that are incorporated into a unit volume of the resulting oxide:
t
dX
dt
o
=
J
M
=
æ D × N0
ö
ç ÷
è M ø
æ D ö
ç X
0
+
÷
è ks
ø
(3.7)
This differential equation is easily solved by using separation of variables
method with the boundary condition X 0 (t = 0) = X i , which yields
=
X
B
2
o
+
æ
ç
è
X
B
A
o
ö
÷
ø
-t
(3.8)
M :
A =
2D
k s
number of molecules of oxidizing
species in a unit volume of oxide
2DN
M
0
B =
t =
X
B
2
i
X
i
+
æ B ö
ç ÷
è A ø

Solving eq. (3.8) for X 0 (t) yields:
1 1
é
êì
4×
B ü
\ t
2
X ( ) 1 ( ) 1 (3.9)
2
2
ú ú o
t = A í + t + ý -
êî
A þ
ë
û
2
A
t + >
4×
B
4
æ B ö
X o
( t) = ç ÷× ( t +t )
t = B × t
Short Times: ( t )
Long Times: ( t + t ) , t >> t
è
A ø
X o
ù
2
A
× B
( ) ( +t )
•Oxidation growth is proportional to time.
•The ratio B/A is called the linear
(growth) rate constant.
•In this region, growth rate is limited by
the reaction at the Si interface.
•Oxidation growth is proportional to the
square root of time.
•B is called the parabolic rate constant.
•In this region, the oxidation rate is
diffusion limited.

Oxidation Rate (summary)
• Oxide thickness after an oxidizing time t is
A æ t + t ö
x ç
÷
0
= 1+
-1
2
2
è A / 4B
ø
I. For short time (t + t ≪ A 2 /4B)
x 0 = B/A (t + t) : Linear Growth Law
linear rate constant B/A
B/A = C * /N : independent of D
II. For long time (t + t ≫ A 2 /4B)
x 02 = B(t + t) : Parabolic Growth Law
parabolic rate constant B
B = 2DC * /N
→ proportional to D
(diffusion controlled)
At initial stage of oxide growth, when surface reaction is the rate-limiting factor, the oxide
thickness varies linearly with time. As the oxide layer becomes thicker, the oxidant must
diffuse through the oxide layer to react at the Si-SiO 2 interface, and the reaction becomes
diffusion limited. The oxide growth then becomes proportional to the square root of the
oxidizing time, which results in a parabolic growth rate.

*******************************************************************************
참고자료: Solution of differential equation dX 0
/dt = J/M = (DN 0
/M)/(X 0
+ D/k s
)
*******************************************************************************
- Si 표면에서 산화되는 속도는 표면에 존재하는 산소전자의 밀도에 비례 또한 산화가
빨리 될수록 (산소가 빨리 소요될수록) flux도 증가
- 두식으로부터
.
- 산화막 두께의 시간에 따른 변화는

- 변수 분리법에 의해 다음을 얻고,
.
- 양변을 각 변수에 관해 적분하면 다음을 얻는다.
- 경계조건: t = 0에서 X o
= X i
이므로,
∴ 원래의 식은 다음과 같이 된다.

- X o
에 관한 이차 방정식의 근의 공식을 이용하면 다음을 얻는다.
- 위의 최종 식을 oxide 두께의 계산에 사용
경계조건: X o
(t = 0) = X i
: initial oxide thickness (native oxide 혹은 열산화 공정 이전의
oxide 두께)

*******************************************************************************

3. Factors which affect oxidation rate
- Temperature:
is strongly proportional to the temperature during the oxidation process.
- Oxidant species (wet and dry oxidation):
Water vapor has a much higher solubility than O 2
in SiO 2
, which
accounts for much higher oxide growth rate in a wet atmosphere. Slower
growth results in a denser, higher-quality oxide and is usually used for MOS
gate oxides.
- Pressure:
is proportional to the partial pressure of the oxidizing species, so
pressure can be used to control oxide growth rate. High-pressure is being
used to increase oxidation rates at low temperature (Fig. 3.8).
- Crystal orientation:
changes the number of Si bonds available at the Si surface, which
influences the oxide growth rate and quality of the Si-SiO 2 interface.
- Impurity doping:
Heavy doping of Si changes its oxidation characteristics: i.e.,
Phosphorus doping increases the linear rate constant without altering
parabolic rate constant. Boron doping increases the parabolic constant but has
little effect on the linear rate constant.
(100)
(111)
(011)
- Ambient gases:

7. Oxide Quality: Interface and Charges
Oxide
trapped
charge
Mobile ionic charge
K + Na +
+
+
- -
+ + + +
Fixed oxide charge
Interface trapped charge
SiO 2
SiO x
Si
• Interfacial trapped charge:
located at a SiO2-Si interface,
-depends on chemical composition of interface,
crystal orientation.
-typical density ~10 10 /cm 2 for
• Fixed oxide charge:
located in the oxide within 10 -35 Å of interface
-positive, depends on oxidation annealing
conditions, and silicon orientation
-typical density ~ 5x10 10 /cm 2 for
• Oxide trapped charge:
-distributed inside the oxide layer due to x-ray or
high-energy ‘e’ bombardment, cured by low-temp.
annealing
• Mobile Ionic Charge (Q m )
Ø due to contamination from ionized alkali metal atoms (Na + , Ka + )
Ø Drifts from gate/SiO 2 interface to Si/SiO 2 interface under positive gate field
Ø Changes threshold voltage V T in the device

8. Selective Oxidation
8.1 Local oxidation of silicon (LOCOS)
Fig 3.12 (a)
1 A thin layer of silicon dioxide is grown on the
wafer to protect the silicon surface.
2 A layer of silicon nitride is deposited over the
surface and patterned using photolithography.
3 The wafer goes through a thermal oxidation
step.
4 Some oxide growth occurs under the edges of
the nitride and causes the nitride to bend up at
the edges of the masked area.
This process results in the so-called semirecessed
oxide structure.
Fig. 3.12 (b)
1 A fully recessed oxide can be
formed by etching the silicon
prior to oxidation.
2 This process can yield a very
planer surface after the removal
of the nitride mask.
è subsequent processing reduces
the advantage of this process
over the semi-recessed version.

8.1.1 Deep Trench Isolation
To solve the problem of LOCOS, both the deep and shallow trench techniques
were developed.
Figure 3.13 (a) Deep trench isolation.
- Figure 3.13(a) depicts formation of deep trenches filled
with poly-silicon in combination with a LOCOS field
oxidation.
- A thin oxide pad is grown on Si followed by deposition
of a Si 3 N 4 layer.
- Lithography is used to define openings in the nitride
where trenches will be formed.
- The trenches are etched using reactive-ion etching and
can be deep with high aspect ratios.
- The surface of the trench is passivated with a thin layer
of thermally grown oxide, and then trench is refilled with
deposited poly-silicon.
- The final structure is produced by etching back any
excess poly-silicon, using a lithography step to remove the
nitride layer where oxidation is desired, and growing the
semi-recessed oxide layer.
- The poly-silicon may be doped, and similar structures
are used to form trench capacitors for use as storage
elements .
- The trenches may be as much as 5~10um in depth.

8.1.2 Shallow Trench Isolation
Shallow trench isolation is used to provide isolation between transistors in
MOS & bipolar technology with feature sizes below 0.5um.
- Figure 3.13(b) depicts one possible process flow for forming shallow trenches.
- A shallow trench with tapered sidewalls is etched in the Si following patterning of the nitride layer.
- The pad oxide may be etched away slightly to round the corners of the final structure.
- A thin oxide layer is grown as a liner on the trench walls, and the trench is then filled with an oxide
deposited using decomposition of TEOS or via a high-density plasma deposition.
- A process called chemical mechanical polishing (CMP) is used to remove the excess oxide and create a
highly planar surface.
- Finally, the nitride may be removed, leaving the shallow trench isolation between two Si regions.

8.2 Chemical Mechanical Polishing (CMP)
CMP is a technique to planarize the surface of the sample, and now widely used in both
bipolar & MOS processors to achieve the highly planar topologies required in deep
submicron lithography.
- The wafer is mounted on a rotating pattern.
- A liquid slurry is continuously dispensed onto the surface of the
polishing pad.
- A combination of the vertical force between the wafer and the
abrasive pad as well as the chemical action of the slurry is used to
polish the surface to a highly planar state.
- In the case of formation of the shallow trenches, the nitride layer
serves as a polishing stop. Polishing terminates when the nitride
layer is fully exposed.
Figure 3.15
Chemical mechanical polishing technique
Schematic drawing of a CMP polisher.

9. Oxide Thickness Characterization
H.W
One of the simplest methods for determining the thickness of an oxide:
To compare the color of the wafer with the reference color chart in Table 3.2

When a wafer is illuminated with white light perpendicular to the
surface, the light penetrates the oxide film and is reflected by the
underlying silicon wafer.
The color of the wafer correspond to the enhanced wavelength.
Constructive interference occurs when the path length in the oxide
(2X 0 ) is equal to an integer multiple of one wavelength of light in the
oxide.
2X 0
= kλ / n
k : any integer greater than zero
n : refractive index of the oxide (n = 1.46 for SiO 2
).

Accurate thickness measurement methods:
1) Ellipsometer:
- This instrument is often used to make an accurate reference color chart.
- Polarized monochromatic light is used to illuminate the wafer at an angle to
the surface.
- Light is reflected from both the oxide and silicon surfaces.
- The differences in polarization are measured and the oxide thickness can be
calculated.
2) A mechanical surface profiler can also be used to measure film thickness.
- The oxide is partially etched from the surface of a test wafer to expose a step
between the wafer and oxide surface.
- A stylus is mechanically scanned over the surface of the wafer, and thickness
variations are recorded on a strip-chart recorder. Measurable film thickness
is between 0.01μm and 5μm.
3) A light-interference effects in microscopy can also be used to measure film
thickness accurately.

11. Summary
1. Silicon dioxide provides a high-quality insulating barrier on the surface of silicon wafer.
- This layer can serve as a barrier layer during subsequent impurity diffusion process steps.
2. Thicker layers of silicon dioxide are conveniently grown in high-temperature oxidation
furnaces using wet and dry oxygen.
- Oxidation occurs much more rapidly in wet oxygen than in dry oxygen.
- The dry-oxygen environment produces a higher quality oxide and is usually used for the growth
of MOS gate oxides.
- As the oxide becomes thicker, grow rate slows and becomes proportional to the square root of
time.
- These two growth regions are characterized by the linear and parabolic growth-rate constants.
3. Oxide cleanness is extremely important for MOS processes, and great care
is exercised to prevent sodium contamination of the oxide.
- The addition of chlorine during oxidation improves oxide quality.
- Oxidation alters the impurity distribution at the surface of the silicon wafer. Boron tends to be
depleted from the silicon surface, whereas phosphorous tends to pile up at the silicon surface.
4. Oxidation thickness can be accurately measured using ellipsometers,
interference microscopes, and mechanical surface profilers or can be estimated from the
apparent color of the oxide under vertical illumination with white light.

참고자료

Oxidation Rate: Temperature & Oxidant
Parabolic B = C 1 exp(-E 1 /kT)
Linear B/A = C 2 exp(-E 2 /kT)
Dry O 2
Wet H 2 O
(640 Torr)
C 1 = 7.72 x 10 2 mm 2 /h
C 2 = 3.71 x 10 6 mm 2 /h
C 1 = 3.86 x 10 2 mm 2 /h
C 2 = 0.97 x 10 8 mm 8 /h
E 1 = 1.23 eV
E 2 = 2.00 eV
E 1 = 0.78 eV
E 2 = 1.96 eV
- The rate-constant data follow straight line when plotted on a semilogarithmic scale vs. reciprocal T
- This type of behavior is referred to as an Arrhenius relationship. D = D 0 exp(-E A /kT ) (3.12)
The rate depends on the surface
bond structure of Si atoms,
à orientation dependence
Independent of crystal
orientation
Temperature dependence of both linear- and parabolic-rate constant

Oxidation Rate: Pressure
High pressure increases the oxide growth rate,
by increasing the linear and parabolic rate constants.
( The increase in the rate constants arises from the increased N o .)
B
A
B
kSC0
=
N
DC
=
N
kS
*
@ C
N
2D
@ C
N
2
0
*
kS
= K
H
PG
N
2D
= K
H
P
N
Trade off: DP = 1 atm Û DT = 30 o C
Ø Low temperature oxidation can be
achieved under high pressure for
the same oxidation rate.
G
Method
1. Pressurizing water-pumping
2. Producing water by pyrogenic system

Oxidation Rate: Crystal Orientation
(111)
(011)
(100)
• The parabolic rate depends on the D of oxidants through SiO 2 .
• On the other hand, the linear rate constant B/A depending on the reaction k S
at SiO 2 /Si interface is strongly related to the crystallographic orientation of Si.
• The growth rate ratio (v 111 /v 100 ) decreases at high temperatures and at long time
since the parabolic rate constant is predominant. (diffusion-limited)
Ø CO changes the # of Si bonds available at the surface
(100) silicon: the smallest number of unsatisfied Si bonds at the Si-SiO2
interface, and the choice of the (100) orientation yields the lowest number of
interface traps, leading to lowest oxidation rate.

Oxidation of Poly-Si

Oxidation Rate: Doping
• Heavy doping of silicon also changes its oxidation characteristics.
(i.e., Group III and V dopants enhance the oxidation rate when heavily doped.)
• Depending on the impurity redistributions, oxidation rate depends on
Ø the C B at SiO 2 for diffusion controlled oxidation (B dominates).
Ø the C B at Si surface for reaction controlled oxidation (B/A dominates).
Boron segregated in SiO 2
weakens the SiO 2 bond structures.
ØRapid diffusion of O 2 and H 2 O
Ø(B dominates the process)
Phosphorus piles up at Si surface.
Ø Enhanced oxidation rate in the
reaction controlled regime
(B/A dominates the process)

Oxidation Rate: Additional Gases
• Halogenic Oxidation:
The presence of chlorine mixed with O 2 gas during dry oxidation
1. Enhances the oxidation rate.
2. Improves device characteristics.
Chlorine-containing gases: Cl 2 , HCl, TCE, TCA