Thermal doping review example

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Thermal doping review example
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Thermal Doping Example
Pattern a wafer and place an oxide film on top of
the exposed silicon.
Silicon wafer
This section was protected by the mask
Dopant will diffuse into the unprotected silicon
as function of time and temperature in the
furnace
Oxide film
Dopant containing film.
Cross section cut view that is not to scale
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Thermal Doping Example
Side of wafer that will have the functional device
Sources of dopants
Solid wafer made of the dopant material
Solid Source
Wafer side that will house the functioning device.
Wafer side that will house the functioning device.
Cross section view of oven rack to hold wafers
and solid dopant
sol-gel film
sol-gel film
Spin-on Dopant film
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Thermal Doping Example
Sources of dopants
Solid as vapor source
Solid dopant placed in platinum boat
for educational use only. From, R.C. Jaeger,
Introduction to Microelectronic Fabrication, 2nd
Ed., Prentice Hall, 2002
5
Thermal Doping Example
Sources of dopants
Liquid as vapor source
for educational use only. From, R.C. Jaeger,
Introduction to Microelectronic Fabrication, 2nd
Ed., Prentice Hall, 2002
6
Thermal Doping Example
Sources of dopants
Pure vapor source
for educational use only. From, R.C. Jaeger,
Introduction to Microelectronic Fabrication, 2nd
Ed., Prentice Hall, 2002
7
Thermal Doping Example
Pattern a wafer and place an oxide film on top of
the exposed silicon.
Place a dopant containing film on the wafer and
heat for some time.
Silicon wafer
Oxide film
Dopant containing film.
Cross section cut view that is not to scale
8
Thermal Doping Example
Region of interest
Silicon wafer
Dopant containing film.
Oxide film
Cross section cut view that is not to scale
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Oxide film
Silicon wafer
Dopant containing film.
Cross section cut view that is not to scale
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Cross section cut view that is not to scale
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DEGLAZE
then
CLEAN
Cross section cut view that is not to scale
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Cross section cut view that is not to scale
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Cross section cut view that is not to scale
14
Thermal Doping Example
Practical factors
How thick does the protective oxide have to be?
Oxide film needed to be thick enough to mask
diffusion process
1
If your furnace is at 1100 degrees C, it will be
at least 3.5 hrs before the boron gets through
the1 micron thick oxide protective cover.
for educational use only. Fig 3.7 p 53, R.C.
Jaeger, Introduction to Microelectronic
Fabrication, 2nd Ed., Prentice Hall, 2002
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Thermal Doping Example
Practical factors
How much dopant will dissolve in the silicon?
The real issue is how many dopant atoms will
replace silicon atom.
You can dissolve more P and As atoms into crystal
than can substitute for silicon atoms.
At 900 C and maximum Boron concentration
(solubility)at the surface is about
Therefore,

for educational use only. From, R.C. Jaeger,
Introduction to Microelectronic Fabrication, 2nd
Ed., Prentice Hall, 2002
16
Thermal Doping Example
Practical factors
How much does temperature influence the dopant
transport into the silicon?


-
e
D(T)
These plots can be modeled as exponential
functions
for educational use only. From, R.C. Jaeger,
Introduction to Microelectronic Fabrication, 2nd
Ed., Prentice Hall, 2002
10.5
3.69 ev
From the model, what is the diffusion coefficient
for P at 900 C?


-
e
D(1173)
(900 C equals 1173 K)


3.69
B and P
-
e
(1173)
10.5
As
Come on! Work it out, its good for you.
?
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Thermal Doping Example
Practical factors
What are the model equations for the diffusion
of dopant from an infinite source?
Concentration profile through the diffusion
region as a function of distance and time.
X 0 at the outside edge of the wafer
t 0 before the diffusion starts.
Total dopant that was added to substrate.
Distance into wafer were the concentration of the
n and p materials is identical.
Junction depth
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Thermal Doping Example
Practical factors
What are the model equations for the diffusion of
dopant from a constant or fixed source?
Concentration profile through the diffusion
region as a function of distance and time.
X 0 at the outside edge of the wafer
t 0 before the diffusion starts.
Concentration at the surface as a function of
time?
Put X 0 and solve for all values of time.
Distance into wafer were the concentration of the
n and p materials is identical.
Junction depth
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Thermal Doping Example
Practical factors
What is erfc and how do I use it.?
The error function and its complement are popular
functions because they are solutions to
differential equations that deal with diffusion
problems.
erfc( )
Values for the function are available from tables
or plots like this one,
or approximation functions like this one also
found in common mathematics software packages.
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The gaussian curve on the right is also often
used as a substitute for the erf complement. For
most of the model curves shown the plots have
similar shape and functional response.
Thermal Doping Example
Practical factors
for educational use only. From, R.C. Jaeger,
Introduction to Microelectronic Fabrication, 2nd
Ed., Prentice Hall, 2002
for educational use only. From, R.C. Jaeger,
Introduction to Microelectronic Fabrication, 2nd
Ed., Prentice Hall, 2002
21
Thermal Doping Example
Practical Problem
You have a n-type silicon wafer that has a
resistivity of 0.36 ohm-cm. You want to use
boron to form the base region in the wafer for an
npn transistor.
You perform a solid-solubility limited boron
predeposition at 900 C for 15 minutes followed
by (after deglaze and clean) a 5 hour drive-in
at 1100C.
Find the boron surface concentration , the
junction potential and the dose.
(I) just after the predeposition step.
(II) just after the drive in step.
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Thermal Doping Example
Practical Problem
Find the boron surface concentration, the
junction potential, and dose.
(I) just after the predeposition step.
(a) Boron surface concentration just after the
predeposition step.
1)
2)
Find the value for diffusion coefficient at 900 C.
900 C
1173 K
For Boron, B, the model becomes
,B


-
D(1173)
e
,B
B
3)
Find the number of boron atoms, N ( x, t ) when x
0 and t 15 minutes (900 seconds).
1/2


N ( x , t )
erfc
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Thermal Doping Example
Practical Problem
Find the boron surface concentration, the
junction potential, and dose.
(I) just after the predeposition step.
(b) Boron junction depth in the original
resistivity of 0.36 ohm-cm n doped wafer.
Determine the number of Boron atoms that
correspond to the same resisitivity. (dopant
concentration vs resistivity plot)
1)
2)
Use the concentration profile model as a function
of distance and time and solve for the junction
depth distance.
(c) Boron dose for this process.
Integrate the area under the concentration
profile model for the pre-deposition or the
drive-in process.
1)
(II) just after the drive-in step.
(a) Boron surface concentration just after the
drive-in step.
1)
Use the concentration profile model as a function
of distance and time and solve when x 0.
-
e
N ( x , t )
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Thermal Doping Example
Practical Problem
Find the boron surface concentration, the
junction potential, and dose.
(II) just after the drive-in step.
(a) Boron surface concentration just after the
drive-in step.
1)
Use the concentration profile model as a function
of distance and time and solve when x 0.
-
e
N ( x , t )
(b) Boron junction depth, just after drive in
step, in the original resistivity of 0.36
ohm-cm n doped wafer.
1)
Solve concentration profile model as a function
of distance and time for junction depth.
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Find the boron surface concentration, the
junction potential, and dose.
(II) just after the drive-in step.
(a) Boron surface concentration just after the
drive-in step.
1)
Use the concentration profile model as a function
of distance and time and solve when x 0.
-
e
N ( x , t )
(b) Boron junction depth, just after drive in
step, in the original resistivity of 0.36
ohm-cm n doped wafer.
1)
Solve concentration profile model as a function
of distance and time for junction depth.