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Section 6: Ion Implantation

Jaeger Chapter 5

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Ion Implantation - Overview• Wafer is Target in High Energy Accelerator• Impurities “Shot” into Wafer• Preferred Method of Adding Impurities to Wafers

– Wide Range of Impurity Species (Almost Anything)– Tight Dose Control (A few % vs. 20-30% for high

temperature pre-deposition processes)– Low Temperature Process

• Expensive Systems• Vacuum System

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Equipment( )

( )dttInqA

Q

qrmV

T

∫=

=

=

0

2

1 Dose Implanted

2 Field Magnetic

x q particle chargedon Force

B

BvF

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Ion Implantation

x

Blocking maskSi

+ C(x) as-implantdepth profile

Depth xEqual-Concentrationcontours

Reminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects.

Reminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects.

y

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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 uniformity (< 1% variation across 12” wafer)

n+n+

Application example: self-aligned MOSFET source/drain regions

SiO2

p-Si

As+As+ As+

Poly Si Gate

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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

EXAMPLES Implanting into Si:

Ion Energy Loss Characteristics

H+

B+

As+

Electronic stoppingdominatesElectronic 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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Simulation of 50keV Boron implanted into Si

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Model for blanket implantation

( ) ( )

( ) pp

p

p

pp

RNdxxNQ

R

RRx

NxN

Δ=

=Δ

=⎥⎥⎦

⎤

⎢⎢⎣

⎡

Δ

−−=

∫∞

0

p

2

2

2= Dose

StraggleR

Range Projected

2exp

ProfileGaussian

π

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Rp and ΔRp values are given in tables or charts e.g. see pp. 113 of Jaeger

Note: this means 0.02 μm.

Projected Range and Straggle

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Selective Implantation

( ) ( ) ( )

( )

( ) solution ldimensiona-one is xNstraggle transverse

2221

,

=Δ⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛

Δ+

−⎟⎟⎠

⎞⎜⎜⎝

⎛

Δ−

=

=

⊥

⊥⊥

R

Rayerfc

RayerfcyF

yFxNyxN

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Transverse (or Lateral) Straggle (ΔRt or Δ R⊥)

ΔRt

ΔRtΔRp

>1

ΔRt

ΔRp

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y

Mask

C(y) at x=Rp

x = Rp

Implanted specieshas lateral distribution,larger than mask opening

Implanted specieshas lateral distribution,larger than mask opening

x

y

Higher concentrationLowerconcentration

Feature Enlargement due to lateral straggle

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(2) Projected Range:

(3) Longitudinal Straggle:

(1) Dose ( )φ = ∞∫ C x dx0

(4) Skewness:

(5) Kurtosis:

( )dxxCxRp ∫∞

⋅≡0

1φ

( ) ( ) ( )dxxCRxR pp ∫∞

⋅−≡Δ0

22 1φ

( ) ( ) 00,130

33 <>-≡ ∫

∞orMdxxCRpxM

φ

( ) ( )∫∞

−∝0

4 dxxCRx p

Rpx

C(x)

Kurtosis characterizes thecontributions of the “tail” regions

-describes asymmetry between left side and right side

Definitions of Profile Parameters

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Selective Implantation – Mask thickness

• Desire Implanted Impurity Level to be Much Less Than Wafer Doping

N(X0) << NBor

N(X0) < NB/10

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What fraction of dose gets into Si substrate?

x=0 x=d

C(x)Mask material (e.g. photoresist)

Si substrate

C(x) Mask material with d=∞

x=0 x=d

-

Transmission Factor of Implantation Mask

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( ) ( )

( )

( )( )

T C x dx C x dx

erfcd R

R

erfc x e dy

C x dC x R

d

p

p

yx

p

= −

=−⎧

⎨⎩

⎫⎬⎭

= −

==

∫∫

∫

∞

−

−

00

0

4

12 2

1 2

10

2

Δ

πRule of thumb Good masking thickness:

d R Rp p= + 4 3Δ. ~

are values of for ions intothe masking material

ΔRpRp ,

Transmitted Fraction

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Junction Depth

( )( )

⎟⎟⎠

⎞⎜⎜⎝

⎛Δ±=

=⎥⎥⎦

⎤

⎢⎢⎣

⎡

Δ

−−

=

B

pppj

Bp

pjp

Bj

NN

RRx

NRRx

N

NxN

ln2

2exp 2

2

The junction depth is calculated from the point at which the implant profile concentration = bulk concentration:

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Sheet Resistance RS of Implanted Layers

x

C(x) log scale

xj

CB

μ

1017 1019

μn

Total doping conc

μp

p-sub (CB)

n

( ) ( )[ ]∫ −μ⋅=

jx

0 B

SdxCxCxq

1R

Example:n-type dopants implantedinto p-type substrate

x =0

x =xj

x

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Approximate Value for RS

( )

[ ]

⇒ → ≅

≅

=

∫R

q C x dx q

Rq

R ohm

s x

s

s

j

1 1

10μ μφ

μφ

If C(x) >>CB for most depth x of interest and use approximation: μ(x) ~ constant

use the μ for the highestdoping region which carriesmost of the current

This expression assumes ALLimplanted dopants are 100%electrically activated

or ohm/square

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200 keV Phosphorus is implanted into a p-Si ( CB= 1016/cm3) with a dose of 1013/cm2 .

From graphs or tables , Rp =0.254 μm , ΔRp=0.0775μm

(a) Find peak concentration

Cp = (0.4 x 1013)/(0.0775 x10-4) = 5.2 x1017/cm3

(b) Find junction depths

(c) Find sheet resistance

(b) Cp exp[ -( xj-0.254)2/ 2 ΔRp2]= NB with xj in μm

∴ ( xj - 0.254)2 = 2 ×(0.0775)2 ln [ 5.2 ×1017/1016] or xj = 0.254 ± 0.22 μm ; xj1 = 0.032 μm and xj2 = 0.474 μm

From the mobility curve for electrons (using peak conc as impurity conc), μn= 350 cm2 /V-sec

Rs = 1

qμnφ = 1

1.6×10-19 ×350 ×1013 ≈ 1780 Ω/square.

PhosphorusImplant

p-substrate (1E16 /cm3)

x

x

j1

j2

Rp

CB

Example Calculations

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Channeling

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Use of tilt to reduce channeling

To minimize channeling, we tilt wafer by 7o with respect to ion beam.To minimize channeling, we tilt wafer by 7o with respect to ion beam.

Random component

channeledcomponent

C(x)

x

Axial ChannelingPlanar ChannelingRandom

Lucky ions fall into channel despite tilt

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Si+

1 E15/cm2

Si crystal

Disadvantage : Needs an additional high-dose implantation stepDisadvantage : Needs an additional high-dose implantation step

B+

Si crystalAmorphous Si

Step 1High dose Si+implantation to covertsurface layer intoamorphous Si

Step 2Implantation ofdesired dopantinto amorphous surface layer

Prevention of Channeling by Pre-amorphization

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With Accelerating Voltage = x kV

B+

P+

As+

Kinetic Energy = x · keV

B+++

B++ Kinetic Energy = 2x · keV

Kinetic Energy = 3x · keV

Singlycharged

Doublycharged

Triplycharged

Note: Kinetic energy is expressed in eV . An electronic charge q experiencing a voltage drop of 1 Volt will gain a kinetic energy of 1 eV

Kinetic Energy of Multiply Charged Ions

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BF2+

Kinetic Energy = x keV

BFF

B has 11 amuF has 19 amu

accelerating voltage= x kV+ -

%20191911

11BFof.E.K

Bof.E.K

vm21Fof.E.K

vm21Bof.E.K

vvvVelocity

2

2BF

2BB

FFB

=++

≈

⋅=

⋅=

==

+

Molecular ion will dissociate immediatelyinto atomic components after entering a solid.All atomic componentswill have same velocityafter dissociation.

SolidSurface

Molecular Ion Implantation

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Implantation Damage

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Amount and type of Crystalline Damage

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(1) Restore Si crystallinity.

(2) Place dopants into Si substitutional sitesfor electrical activation

After implantation, we need an annealing step.A typical anneal will:

Post-Implantation Annealing Summary

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Deviation from Gaussian Theory

• Curves deviate from Gaussian for deeper implants (> 200 keV)

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Shallow Implantation

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Rapid Thermal Annealing

•Rapid Heating•950-1050o C•>50o C/sec•Very low dopant diffusion

(b)

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Dose-Energy Application Space