Chapter 8 Ion Implantation Instructor: Prof. Masoud Agah

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Chapter 8Ion Implantation

Instructor: Prof. Masoud Agah

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ION IMPLANTATION

Two problems associated with diffusion especially for IC fabrication:– High-temperature process– Unable to provide shallow junction depths

Ion implantation is a relatively simple means to place a known number of atoms in a wafer.

Ion implantation process:– Ionization of the dopant source to form positive ions– Acceleration of ions through a high voltage field to reach the required

energy– Projection of high-energy ions towards the wafer surface (target)– Collision of ions with silicon atoms resulting in energy loss– End of penetration of ions in the substrate (coming to rest)

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SYSTEM REQUIREMENTS

May achieve better control of distribution of dopants

versus depth with ion implantation

Process can be faster

Process does not require as much thermal

processing

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ION IMPLANTATION SYSTEM

Ion implanter is a high-voltage accelerator of high-

energy impurity ions

Major components are:

– Ion source (gases such as AsH3 , PH3 , B2H6)

– Mass Spectrometer (selects the ion of interest.

Gives excellent purity control)

– HV Accelerator (voltage up to 1 MeV)

– Scanning System (x-y deflection plates for

electronic control)

– Target Chamber (vacuum)

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Cross-section of an ion implanter

2

1

3

90o

analyzingmagnet

25 kV

Ionsouce

Resolving aperture

R R R

C C C

0 to 175kV

Accelerationtube

Focus

Neutral beam trapand beam gate

Neutral beam

Beam trap

y-axisscanner x-axis

scanner4

5

Wafer in process chamber

Integrator

Q

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Cross-section of an

ion implanter

ELECTROSTATICDEFLECTION

TARGET

IONACCELERATION

MAGNET

IONSOURCE

IONEXTRACTION

IONSEPARATION

MAGNET

MAGNETANODE

ANODEFILAMENTCATHODE

SOURCE OFELEMENT

ELECTROSTATICDEFLECTION

TARGET

IONACCELERATION

MAGNET

IONSOURCE

IONEXTRACTION

IONSEPARATION

MAGNET

MAGNETANODE

ANODEFILAMENTCATHODE

SOURCE OFELEMENT

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ION IMPLANTATION

High energy ion enters crystal lattice and collides with

atoms and interacts with electrons

Each collision or interaction reduces energy of ion

until it comes to rest

Interactions are a complex distribution. Models have

been built and tested against observation

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ION IMPLANTATION

To prevent channeling, implantation is normally performed at an angle of about 8° off the normal to the wafer surface.

An annealing step is required to repair crystal damage and to electrically activated the dopants.

The implanted dose can be accurately measured by monitoring the ion beam current.

Complex-doping profiles can be produced by superimposing multiple implants having various ion energies and doses.

Lateral scattering effects are smaller than lateral diffusion.

Expensive $$$$$

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ION IMPLANTATION

Projected range (RP): the average distance an ion travels before it stops.

Projected straggle (RP): deviation from the projected range due to multiple collisions.

http://eserver.bell.ac.uk

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MODEL FOR ION IMPLANTATION

Distribution is Gaussian

Np = peak concentration

Rp = range

Rp = straggle

N x N ( x – Rp ) 2 Rp p( ) exp 22



The implanted impurity

profile can be

approximated by the

Gaussian distribution

function.

For an implant

contained within silicon:

Q = (2π)0.5

NP RP



Model developed by Lindhard, Scharff and Schiott

(LSS)

Range and straggle roughly proportional to energy

over wide range

Ranges in Si and SiO2 roughly the same

Computer models now available at low cost for PCs



Range of impurities in Si

10 100 1000Acceleration energy (keV)

Rp

0.01

0.1

1.0 B

P

As

Sb

Pro

ject

ed r

ange

(m

)



Straggle of impurities in Si

BSb

AsP

Rp

R

0.10

0.01

0.00210 100 1000

Acceleration energy (keV)

Nor

mal

and

tran

sver

se s

trag

gle

(m

)


SiO2 AS A BARRIER

SiO2 serves as an excellent barrier against ion-implantation

SiO2Silicon

Np

NB

N(X0)

0 Rp X0 Depth, x


SiO2 AS A BARRIER

The minimum oxide thickness for selective implantation:

Xox = RP + RP (2 ln(10NP/NBulk))0.5

An oxide thickness equal to the projected range plus six times the straggle should mask most ion implants.

A silicon nitride barrier layer needs only be 85% of the thickness of an oxide barrier layer.

A photoresist barrier must be 1.8 times the thickness of an oxide layer under the same implantation conditions.

Metals are of such a high density that even a very thin layer will mask most implantations.


ADVANTAGES

Advantages over diffusion:

– low temperature process

– allows wider range of barrier materials

– permits wider range of impurities

– better control of dose

– wider range of dose

– can control impurity concentration profile

– can introduce very shallow layers


PROFILE CONTROL

Various shapes of profiles can be created by varying the energy of the incident beam

200 KILOELECTRONVOLTS

FINAL PROFILE

100

50

2010

15

10

5

00 50 100 150 200 250 300 350

DEPTH (NANOMETERS)

NIT

RO

GE

N C

ON

CE

NT

RA

TIO

N (

AT

OM

IC P

ER

CE

NT

)


RADIATION DAMAGE

Impact of incident ions knocks atoms off lattice sites With sufficient dose, can make amorphous Si layer


RADIATION DAMAGE

Critical dose to make layer amorphous varies with temperature and impurity

Radiation damage can be removed by annealing at 800-1000oC for 30 min. After annealing, almost all impurities become electronically active.

1018

1017

1016

1015

1014

1013

Temperature, 1000/T (K-1)

B

P

SbCri

tical

dos

e (a

tom

/cm

2 )

0 1 2 3 4 5 6 7 8 9 10


Ion Implantation

Implanting through a sacrificial oxide layer:

– Large ions (arsenic) can be slowed down a little before

penetrating into the silicon.

– The crystal lattice damage is suppressed (at the expense

of the depth achieved).

– Collisions with the thin masking layer tends to cause the

dopant ions to change direction randomly, thereby

suppressing channeling effect.

– The concentration peak can be brought closer to the silicon

surface.


Ion Implantation

For deep diffusion (>1µm), implantation is used to

introduce a certain dose, and thermal diffusion is

used to drive in the dopants.

The resulting profile after diffusion can be

determined by:


Ion Implantation

A boron implantation is to be performed through a 50nm oxide so that the peak concentration is at the Si-SiO2 interface. The implant dose in silicon is to be

1013/cm2. What are the energy of the implant and the peak concentration at the interface?

– Peak at Si-oxide interface RP = 0.05µm Energy = 15keV (RP=0.023µm)

– Implanted dose in silicon = 1013 Q=2x1013 NP = Q/2.5RP = 3.5x1018/cm3

How thick should the oxide layer be to mask the implant if the background concentration is 1016/cm3?

– Xox = 0.05 + 0.023(2 ln(10 x 3.5 x 1018/1016))0.5 = 0.14µm

If the oxide layer is 50nm, how much photoresist is required on top of the oxide to completely mask the implant?

– PR thickness = 1.8 x (oxide thickness) = 1.8 x (0.14 – 0.05) = 0.16µm

Chapter 8 Ion Implantation Instructor: Prof. Masoud Agah

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