i 5 ion implantation
TRANSCRIPT
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8/6/2019 I 5 Ion Implantation
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Ion Implantation
x
mask
Si
+ C(x)as-implant profile
* Concentration Profile is a single-peak functionof depth
x
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Advantages of Ion Implantation
Precise control of dose and depth profile Low-temp. process (can use photoresist as mask) Wide selection of masking materials
e.g. photoresist, oxide, poly-Si, metal
Less sensitive to surface cleaning procedures Excellent lateral dose uniformity (< 1% variation across 8 wafer)
n+n+
Application example: formation of self-aligned source/drain regions
SiO 2 p-Si
As +As + As +
Poly Si Gate
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Monte Carlo Simulation of 50keV Boron implanted into Si
5000ionssimulation
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
[Conc] = # of atoms/ cm 3
[dose] = # of atoms/ cm 2
Depth x in cm
dose ( ) = C x dx0
C(x) in #/cm 3
(1) Range and profile shape depends on the ion energy
(for a particular ion/substrate combination)(2) Height (i.e. Concentration) of profile depends onthe implantation dose
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8/6/2019 I 5 Ion Implantation
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
(3) Mask layer thickness can block ion penetration
Thin mask
Thick Mask
photoresistSiO
2 ,
Si 3 N 4 , or others
Completeblocking IncompleteBlockingSUBSTRATE
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Ion Implanter
e.g. AsH 3 As+ , AsH + , H + , AsH 2+
Magnetic Mass seperation
Ion source
Translational wafer holder
motion.
As+
AcceleratorColumn
Accelerator Voltage: 1-200kV Dose ~ 10 11-1016 /cm2 Accuracy of dose:
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8/6/2019 I 5 Ion Implantation
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
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8/6/2019 I 5 Ion Implantation
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Photographof the Eaton HE3High Energy
Implanter,showing theion beamhitting the300mm wafer
end-station
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Implantation Dose
[ ]
2
# cm
area Implant time Implant
q ampsinCurrent Beam Ion
=
=
Overscanning of beam across wafer is common.In general , Implant area > Wafer area
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8/6/2019 I 5 Ion Implantation
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
A
Secondaryelectroneffecteliminated
e
Faradaycup
ion +
+V
Practical Implantation Dosimetry
+ bias applied to
Faraday Cup to collectall secondary electrons.Cup current = Ion current
* Charge collected by integrating cup current / cup area = dose
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Dose [#/area] : looking downward, how many fishper unit area for ALL depths
Concentration [#/volume] :Looking at a particular location,how many fish per unit volume
Meaning of Dose and Concentration
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8/6/2019 I 5 Ion Implantation
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Ion Implantation Energy Loss Mechanisms
Si+
+
Si
Si
e e
+ +Electronicstopping
Nuclearstopping
Crystalline Si substrate damaged by collision
Electronic excitation creates heat
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Light ions/at higher energy more electronic stopping
Heavier ions/at lower energy more nuclear stopping
EXAMPLESImplanting into Si:
Energy Loss and Ion Properties
H+
B+
As +
Electronic stoppingdominates
Electronic stoppingdominates
Nuclear stoppingdominates
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Stopping Mechanisms
E1(keV) E2(keV)B into Si 3 17P into Si 17 140As into Si 73 800
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8/6/2019 I 5 Ion Implantation
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Substrate
Less crystallinedamageSe > S n
More crystallinedamage atend of rangeSn > S e
Surface
x ~ Rp
A+
Eo =incidentkineticenergy
Se
E ~ 0
Sn
E=E o
Se
Sn
Depth xSn dE/dx| nSe dE/dx| e
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
(1) Restore Si crystallinity.
(2) Put dopants into Si substitutional sitesfor electrical activation
After implantation, we need an annealing step.~900 oC, 30min to
Implantation Damage
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Gaussian Approximation of Implant ProfileC(x)
C p
0.61 C p
R p R px=0
x
Choose Gaussian functionas approximation
linear scale
log scale
x
x
( )
( )
( )
straggleallongitudin R
range projected ReCp xC
p
p
R
R x
p
p
==
=
2
2
2
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Rp and Rp values are given in tables or chartse.g. see pp. 93-94 of Jaeger
Projected Range and Straggle
Note: this means 0.02 m.
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Rp and Rp values from Monte Carlo simulation[see 143 Reader for other ions]
(both theoretical & expt values are well known for Si substrate)
10 100 1000100
1000
10000
Rp =185.34201 +6.5308 E -0.01745 E2 +2.098e-5 E 3 -8.884e-9 E 4
Rp =51.051+32.60883 E -0.03837 E2 +3.758e-5 E 3 -1.433e-8 E 4
Rp
Rp
B1 1 into Si
P r o j e c
t e d R a n g e
& S t r a g g
l e i n
A n g s
t r o m
Ion Energy E in keV
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
( )
( )[ ] p p RC
dx xC
dx xC
=
=
+
2
0x
p p
RC
=
2 p R4.0
Gaussian
Using Gaussian Approximation:
+negligible
Dose-Concentration Relationship
Dose =
Cp
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
(2) Projected Range :
(3) Longitudinal Straggle :
(1) Dose ( ) =
C x dx0
Definitions of Profile Parameters
(4) Skewness:-describes asymmetry between left side and right side
(5) Kurtosis:
( )dx xC x R p
0
1
( ) ( ) ( )dx xC R x R p p
0
22 1
( ) ( ) 00,1 303
3 or M dx xC R p x M ( ) ( )
0
4 dx xC R x p
R px
C(x)
Kurtosis characterizes thecontributions of the tailregions
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Professor Nathan Cheung, U.C. Berkeley EECS143 Lecture #7
Common Approximations used to describe implant profiles
1. Gaussian Distribution - SimpleBetter fit near peak regions
2. Pearson IV Distribution - 4 shape-parameters, messy algebraBetter fit even down to low concentrationregions. Default model used in CAD tools
In EE143, we use Guassian approximation for convenience. This discussion of Pearson IV is just for your reference