I 5 Ion Implantation

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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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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Monte Carlo Simulation of 50keV Boron implanted into Si

5000ionssimulation


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[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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(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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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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Photographof the Eaton HE3High Energy

Implanter,showing theion beamhitting the300mm wafer

end-station


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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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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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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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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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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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Stopping Mechanisms

E1(keV) E2(keV)B into Si 3 17P into Si 17 140As into Si 73 800


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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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(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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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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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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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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( )

( )[ ] 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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(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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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

I 5 Ion Implantation

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