Chapter 8Chapter 8Ion ImplantationIon Implantation

ION IMPLANTATION SYSTEMION IMPLANTATION SYSTEM Ion implanter is a high-voltage accelerator Ion implanter is a high-voltage accelerator

of high-energy impurity ionsof high-energy impurity ions Major components are:Major components are:

– Ion source Ion source (gases such as AsH(gases such as AsH33 , PH , PH33 , B , B22HH66))

– Mass Spectrometer Mass Spectrometer (selects the ion of (selects the ion of interest)interest)

– HV Accelerator HV Accelerator (voltage > 1 MeV)(voltage > 1 MeV)– Scanning System Scanning System (x-y deflection plates for (x-y deflection plates for

electronic control)electronic control)– Target Chamber Target Chamber (vacuum)(vacuum)

ION IMPLANTATION SYSTEMION IMPLANTATION SYSTEM Cross-section of an ion implanterCross-section of an ion implanter

2

1

3

90o

analyzingmagnet

25 kVIon

source

Resolving aperture

R R R

C C C

0 to 175kV

Accelerationtube

Focus

Neutral beam trapand beam gate

Neutral beamBeam trap

y-axisscanner

x-axisscanner

4

5

Wafer in process chamber

Integrator

Q

m/q=(B2R2)/(2V)

Or Faraday cup

Acceleration energy = voltage x charge

on ion

http://www.bpc.edu/mathscience/chemistry/images/periodic_table_of_elements.jpg

ION IMPLANTATIONION IMPLANTATION High energy ion enters crystal lattice High energy ion enters crystal lattice

and collides with atoms and interacts and collides with atoms and interacts with electronswith electrons– Types of collisions: Nuclear and electronTypes of collisions: Nuclear and electron

Each collision or interaction reduces Each collision or interaction reduces energy of ion until it comes to restenergy of ion until it comes to rest– Amount of energy loss is dependent on Amount of energy loss is dependent on

ion, the energy it has at the time of the ion, the energy it has at the time of the scattering event, and the type of scattering event, and the type of scattering.scattering.

From Handbook of Semiconductor Manufacturing Technology by Yoshio Nishi and Robert Doering

From Handbook of Semiconductor Manufacturing Technology by Yoshio Nishi and Robert Doering

ChannelingChanneling Deep penetration by the ion because it Deep penetration by the ion because it

traveled along a path where no traveled along a path where no semiconductor atoms are situatedsemiconductor atoms are situated– Process is used for materials characterization: Process is used for materials characterization:

Rutherford backscatteringRutherford backscattering To prevent channelingTo prevent channeling

– Implantation is performed at an angle of about Implantation is performed at an angle of about 8° off the normal to the wafer surface.8° off the normal to the wafer surface.

– The wafer surface is amorphorized by a high The wafer surface is amorphorized by a high dose, low energy implantation of a dose, low energy implantation of a nonelectrically active ion.nonelectrically active ion. Hydrogen, helium, and silicon are common ions used Hydrogen, helium, and silicon are common ions used

Determining the DoseDetermining the Dose

The implanted dose can be The implanted dose can be accurately measured by monitoring accurately measured by monitoring the ion beam current using a Faraday the ion beam current using a Faraday cupcup– The integrated current during the The integrated current during the

implant divided by the charge on the ion implant divided by the charge on the ion is the dose.is the dose.

Post Implantation AnnealsPost Implantation Anneals

An annealing step is required to repair An annealing step is required to repair crystal damage (recrystallization) and crystal damage (recrystallization) and to electrically activated the dopants.to electrically activated the dopants.– Dislocations will form during the anneal Dislocations will form during the anneal

so times and temperatures must be so times and temperatures must be chosen to force dislocations disappear.chosen to force dislocations disappear.

– If the anneal time is long and the If the anneal time is long and the temperature is high, a drive of the temperature is high, a drive of the implanted ions may occur.implanted ions may occur.

ION IMPLANTATIONION IMPLANTATION Projected range (RProjected range (RPP): the average distance an ion travels before it ): the average distance an ion travels before it

stops.stops. Projected straggle (Projected straggle (RRPP): deviation from the projected range due to ): deviation from the projected range due to

multiple collisions. multiple collisions.

http://eserver.bell.ac.uk

MODEL FOR ION MODEL FOR ION IMPLANTATIONIMPLANTATION

Distribution is GaussianDistribution is GaussianCCpp = peak concentration = peak concentrationRRpp = range = rangeRRp p = straggle= straggle

2

2

2

)(

)( p

p

R

Rx

PeCxC

MODEL FOR ION MODEL FOR ION IMPLANTATIONIMPLANTATION

For an implant contained within silicon, the For an implant contained within silicon, the dose isdose is

ppCRQ 2

ION IMPLANTATION MODEL ION IMPLANTATION MODEL Model developed by Lindhard, Model developed by Lindhard,

Scharff and Schiott (LSS)Scharff and Schiott (LSS)– Range and straggle roughly proportional Range and straggle roughly proportional

to energy over wide rangeto energy over wide range

– Ranges in Si and SiORanges in Si and SiO22 roughly the same roughly the same

Computer models now availableComputer models now available

Range of impurities in SiRange of impurities in Si

10 100 1000Acceleration energy (keV)

Rp

0.01

0.1

1.0 B

P

As

Sb

Proj

ecte

d ra

nge

(m

)

Straggle of impurities in SiStraggle of impurities in Si

B Sb

AsP

Rp

R

0.10

0.01

0.00210 100 1000

Acceleration energy (keV)

Nor

mal

and

tran

sver

se s

trag

gle

(m

)

http://www.iue.tuwien.ac.at/phd/hoessinger/node22.html

Si SiO2

Si3N4

AZ-7500 resist

http://www.ensc.sfu.ca/~glennc/e495/e495l7j.pdf

http://www.ensc.sfu.ca/~glennc/e495/e495l7j.pdf

SiOSiO22 AS A BARRIER AS A BARRIER

The minimum oxide thickness for selective The minimum oxide thickness for selective implantation: implantation:

Xox = RP + RP (2 ln(10CP/CBulk))0.5

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

Other MaterialsOther Materials

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

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

Metals are of such a high density that even a Metals are of such a high density that even a very thin layer will mask most implantations.very thin layer will mask most implantations.– Nickel is one of the most commonly used metal Nickel is one of the most commonly used metal

masksmasks

ADVANTAGESADVANTAGES Low temperature processLow temperature process

– The wafer is cooled from the backside during The wafer is cooled from the backside during high energy, high current diffusions are high energy, high current diffusions are performedperformed

– Less change of stress-induced dislocations due Less change of stress-induced dislocations due to thermal expansion issuesto thermal expansion issues

Wider range of barrier materialsWider range of barrier materials– PhotoresistPhotoresist

Wider range of impuritiesWider range of impurities– No concern about solid solubility limitationsNo concern about solid solubility limitations– Implantation of ions such as oxygen, hydrogen, Implantation of ions such as oxygen, hydrogen,

helium, and other ions with low solid solubility helium, and other ions with low solid solubility is possible.is possible.

Advantages over DiffusionAdvantages over Diffusion

Better control and wider range of Better control and wider range of dose compared to predep diffusionsdose compared to predep diffusions– Impurity concentration profile controlled Impurity concentration profile controlled

by accelerating voltageby accelerating voltage– Very shallow layersVery shallow layers– Lateral scattering effects are smaller Lateral scattering effects are smaller

than lateral diffusion.than lateral diffusion.

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

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

RADIATION DAMAGERADIATION DAMAGE Critical dose to make layer amorphous Critical dose to make layer amorphous

varies with temperature and impurityvaries with temperature and impurity

1018

1017

1016

1015

1014

1013

Temperature, 1000/T (K-1)

B

P

SbCri

tica

l dos

e (a

tom

/cm

2 )

0 1 2 3 4 5 6 7 8 9 10

RecrystallizationRecrystallization

Radiation damage can be removed by Radiation damage can be removed by annealing at 800-1000annealing at 800-1000ooC for 30 min. C for 30 min. After annealing, a significant percentage After annealing, a significant percentage of the impurities become electronically of the impurities become electronically active.active.– Point defects coalesce into line dislocationsPoint defects coalesce into line dislocations– Line dislocations merge into loop dislocationsLine dislocations merge into loop dislocations– Loop dislocations slowly disintegrate as Loop dislocations slowly disintegrate as

interstitial Si atoms move on to lattice sitesinterstitial Si atoms move on to lattice sites

Ion ImplantationIon Implantation Implanting through a sacrificial oxide layer: Implanting through a sacrificial oxide layer:

– Large ions (arsenic) can be slowed down a little Large ions (arsenic) can be slowed down a little before penetrating into the silicon.before penetrating into the silicon.

– The crystal lattice damage is suppressed (at The crystal lattice damage is suppressed (at the expense of the depth achieved).the expense of the depth achieved).

– Collisions with the thin masking layer tends to Collisions with the thin masking layer tends to cause the dopant ions to change direction cause the dopant ions to change direction randomly, thereby suppressing channeling randomly, thereby suppressing channeling effect.effect.

– The concentration peak can be brought closer The concentration peak can be brought closer to the silicon surface. to the silicon surface.

Ion ImplantationIon Implantation For deep diffusion (>1µm), For deep diffusion (>1µm),

implantation is used to introduce a implantation is used to introduce a certain dose, and thermal diffusion is certain dose, and thermal diffusion is used to drive in the dopants.used to drive in the dopants.

The resulting profile after diffusion can The resulting profile after diffusion can be determined by:be determined by:

)2(2

)(

2

2

2

22

1),( DtR

Rx

p

p

p

eDtR

QtxC