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PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Semiconductor doping
• Two Levels of Masks - photoresist, alignment • Etch and oxidation to isolate – thermal oxide, deposited oxide, wet etching, dry etching, isolation schemes • Doping - diffusion/ion implantation • Metallization - Materials deposition, PVD, CVD
Si solar Cell
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory What’s a metal, a semiconductor?
IV
How do we “dope” a semiconductor
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Electrons and holes
Valence Band
Conduction Band
Ev
ED Ec
EA
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Sheet Resistance, what is it?
L W t
R = ρ L/Wt We can rearrange to get a film dependent quantity called the Sheet Resistance Rs = ρ/t =R / (L/W)
Notice L/W is unit less, but gives us the number of “squares” in the length of the bar. The units of Rs are ohms, but they are often given as Ω / .
What is the Resistance of this bar of material with resistivity ρ?
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Sheet Resistance - Four Point Probe
If Probe spacing is: • Larger than film thickness • Smaller than distance to edge of film • Probe points are “small”
Rs=4.53 V/I and
ρ=Rst where t is thickness
Using a four point approach is a standard technique for eliminating the effects of contact resistance
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
How do we get the doping?
Rs and t give us ρ, which gives us doping (but we must know t)
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Another way to get doping - from C-V of a diode
Formation of a p-n junction
Formation of a Schottky junction
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
1/C2 vs V
Assumes an abrupt junction - Schottky, p+n or n+p
v
C-2
Slope gives carrier Concentration
x-intercept give Vbi
What if the line isn’t straight?
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
How about the thickness of our Oxide?
Again, C = εA/W, so we should have another way to measure W. In practice, we must be careful about what C we use.
Corresponds to oxide thickness
What about trapped charge?
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Inversion in an MOS structure
accumulation (negative bias)
no bias
inversion (positive bias)
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory What about I-V Characteristics?
Forward biased pn junction: Probability that carriers are over the barrier is like a Boltzmann factor
But, there is also an electric field pushing carriers back so at V = 0 there should be no current.
We can write this in a simpler form as:
What about when light is shining on the device?
Note, there is a sign difference with respect to the capacitance analysis
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory How can we tell the carrier type
Thermovoltage
Hall Effect • carrier type • mobility • sheet concentration
Hot Probe
e e e e e e e e
V
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Other methods of getting at the carriers
• SIMS • RBS – Rutherford Backscattering • Polaron profiler • Spreading Resistance • ...
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Doping - reminder
Goal of Doping: Substitution of atoms with excess or deficiency of valence electrons e.g. B or P substituting for Si
Diffusion doping (in fact most doping) is typically done in two steps: (Almost all doping is now ion implantation)
Predeposition - Use a source to create the desired dose
Drive in - Source at surface removed, additional diffusion to get desired distribution (in ion implantation the anneal also removes damage and activates the dopant).
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Generic Predeposition Process
Deliver Dopants to Partially Masked Substrates • Diffusion (Hot) • Ion Implantation (Cold)
Structure:
Mask: Oxide, Nitride, Photoresist
Silicon
Dopants
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Dopant delivery Options for Diffusion
Gas Source: • Nasty Gases: AsH3, PH3, B2H6 • Very similar to Deal –Grove Oxidation
Liquid Source: • SOG: Spin-On Glass • Doped SiO2 dissolved in solvents • Apply exactly like Photoresist
Solid Source: • Glass Discs (B2O3, P2O5) • Close-space Sublimation • Vapors sublime/diffuse/react
CB
Co
Cs
Ci
δ
xj
Which is Best?
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Drive-in - estimating the profile
Fick’s law - You need the PDE, but you also need the boundary conditions!
C(z,0) = 0, Z ≠ 0 dC(0,t)/dz = 0
C(∞,t) = 0
Solution: ⎟⎟⎠
⎞⎜⎜⎝
⎛
= Dt-z
T eDtQ 4
2
t)C(z,π
We can model the drive in step from our homework, here after a P predep with p8545 we had a sheet resistance of 12Ω/��� and depth of 1.1µm. This gave a carrier concentration of 5x1019/cm3 and a surface concentration of 5.5x1015/cm2
Characteristic Length Scale - Diffusion Length
€
C(z,t)dz =QT0
∞
∫ = constant
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
What about the diffusion Coefficient?
Use first three terms in Fair’s vacancy model.
−−⎥⎦
⎤⎢⎣
⎡++= 2
2
DnnD
nnDD
ii
o
From Campbell table 3.2 (1100C=1373K)
Do = 3.9cm2/s e-(3.66/k1373) = 1.43 x 10-13cm2/s D- = 4.4cm2/s e-(4.0/k1373) = 9.13 x 10-15cm2/s D2- = 44cm2/s e-(4.37/k1373) = 4.00 x 10-15cm2/s
D = 1.56 x 10-13cm2/s
I told you to assume n~ni ~1019/cm3
Is this reasonable?
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Simulations Suprem-IV is a process simulation tool developed at Stanford University
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
nanoHub TCAD tools
https://nanohub.org/tools
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Suprem simulation of boron predep and drive-in
Boron Diffusion
0
5
10
15
20
0 2 4
Depth in microns
Log1
0(B
oron
)
Boron Predep 1100C30 min.
Boron drivein 1100C30 min.
Boron drivein 1100C60 min.
Boron drivein 1100C60 min 200 angstromoxide cap
Boron Diffusion
18.0
18.5
19.0
19.5
20.0
20.5
21.0
0 0.5 1 1.5 2 2.5
Depth in microns
Log1
0(B
oron
)
Boron predep in gas at 5 x 1020/cm3 concentration followed by drive-ins.
Effect of oxide cap on profile near the surface
Why 5x1020/cm3? 1) Damage threshold 2) Solubility limit 3) B partial pressure 1) Dimensional argument
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory Solid Solubility, what is it?
5x1020/cm3
1100C
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Oxide is an effective anti-diffusion barrier for Si VLSI?
1) For boron but not for phosphorus 2) For phosphorus but not for boron 3) It works well for both 4) It depends
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory Final Topic on Diffusion: Oxide
How fast do dopants diffuse through oxide? Diffusivity important, Solubility important
Consider Do of Boron Si prefactor 0.37cm2/s Activation Energy 3.46eV SiO2 prefactor 0.0003cm2/s Activation Energy 3.53eV
Now Do of Phosphorous Si prefactor 3.9 cm2/s Activation Energy 3.66eV SiO2 prefactor 0.19 cm2/s Activation Energy 4.03eV
• Oxide is often used as a diffusion mask- how thick does it need to be? • Oxide is used for isolation - does it isolate? What is the thermal load? • Oxide is also a gate dielectric with heavily B doped polysilicon gates - diffusion through gate is an issue
Silicon
Oxide
Metal Doped polysiliconM
O
S
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Suprem-IV Wet Oxide then Diffusion
Effect of oxide cap on profile near the surface
Substrate is P doped at 1 x 1014/cm3, Wet oxide growth at atmospheric pressure for 60 minutes at 1000C, Boron predep from 30 minutes at 1100C in gas with a concentration of 5 x 1020/cc.
Oxide antidiffusion barrier
0
5
10
15
20
0 1 2 3 4
Depth in microns
Log1
0(Bo
ron) 60 min wet O2 at 1000C,
30 min boron predep at1100C30 minute boron predep at1100C
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Simulation of predep and drive-in to find junction depth
1000°C P predep in p-type wafer doped at 1x1017/cm3. 1100°C drive in. How long to get a 4.0µm deep junction?