8.1. microtech ion implant,1,2

Ion Implantation

Ion Implantation

Process by which energetic impurity atoms can be introduced into single crystal substrate in order to change its electronic properties.

Ion Implantation and Controlled Doping of Impurities

In this process the ions are accelerated to high energies and allowed to impact the silicon surfaces.

Because of inherent energy they penetrate into the lattice and are placed inside the silicon lattice.

Dr. G. Eranna Integrated Circuit Fabrication Technology © CEERI Pilani

Advantages

Extremely accurate dose control

Tailor made and well controlled doping profile

large range-of doses-1011 to 1016/cm2

Low Temperature process

Wide choice of masking materials (Oxide, PR, Metal)

Clean environment (Mass separation, vacuum)

Non-Equilibrium process (conc. Excess of S.S. limit)

Ion Implantation

Disadvantages

• Highly sophisticated and costly.

• Damage to semiconductor.

• Dopant redistribution during Annealing

• Charging of insulating layers.

• Photoresist heating and hard to strip.

Ion Implantation System

V

V

Ionsource

Analyzing magnet

Pump

Resolvingaperature

Accelerator

Focus

Neutralbeam gate

Neutraltrap X & Y

scanplates

Wafer

Faraday cup

Q0-30keV

0-200keV

ION IMPLANTATION

CEERI PILANI, INDIA


Ion Implanter

Implant Profiles

• At its heart ion implantation is a random process.

• High energy ions (1-1000 keV) bombard the substrate and lose energy through nuclear collisions and electronic drag forces.

Range R

Projected range RP

Vacuum Silicon

•The projected range describes the peak of the implanted profile

Implant Profiles

• Profiles can often be described by a Gaussian distribution, with a projected range and standard deviation. (200keV implants shown.)

€

C(x) =CP exp −x−RP( )2

2∆RP2

.

1021

1020

1017

1019

1018

Con

cent

ration

(cm

-3)

0 0.2 0.4 0.6 0.8 1Depth (µm)

SbAs P

B

0.606 CP∆RP

RP

€

Q = C x()−∞

∞∫ dx

€

Q=2π∆RP CP

(1)

(2)

or

where Q is the dose in ions cm-2 and is measured by the integrated beam current.

Implant Profiles

Dep

th (µ

m)

Implant Profiles

Stan

dard

Dev

iati

on (µ

m)

Implant Profiles

Ranges and standard deviation ∆Rp of dopants in randomly oriented silicon

Implant Profiles

• Monte Carlo simulations of the random trajectories of a group of ions implanted at a spot on the wafer show the 3-D spatial distribution of the ions. (1000 phosphorus ions at 35 keV.)

ImplantBeam

x

y

z

80

40

0

-40

050

-100-50

040

80120

Implant Profiles

• Side view shows depth distribution Rp and ∆Rp while the beam direction view shows the lateral straggle.

zBeam direction

80

40

0

-40

y

050100 -100-50

y

-40

0

40

80

xSide view

0 40 80 120

Implant Profiles

• The two-dimensional distribution near window edge is often assumed to be composed of just the product of the vertical and lateral distributions.

€

C x, y( )=Cvert x()exp − y2

2∆R⊥2

(3)

• Now consider what happens at a mask edge - if the mask is thick enough to block the implant, the lateral profile under the mask is determined by the lateral straggle.

• The description of the profile at the mask edge is given by a sum of point response functions with Gaussian form of Eq (3) above.

Masking Implants

• How thick does a mask have to be?

• For masking,

€

C* xm( )=CP* exp −

xm −RP*( )2

2∆RP* 2

≤CB

.

CP*

RP*

Depth

xm

C*(xm)CB

Con

centration

(4)

For an efficient masking, the thickness of the mask should be large enough that the tail of implant profile in Si is at some specified background conc.

Qp

Masking Implants

• Setting C*(xm)= CB and solving for the required mask thickness,

€

xm =RP* +∆RP

* 2ln CP*

CB

=RP

* +m∆RP* (5)

• The dose that penetrates the mask is given by

€

QP = Q

2π∆RP*

exp− x−RP*

2∆RP*

xm

∞∫

2

dx =Q2

erfc xm −RP*

2 ∆RP*

(6)

• Parameter m indicates that the mask thickness should be equal to the range + some multiple m times the standard deviation in masking material

Since the integral or sum of Gaussian function can be described as an error function

Profile : High tilt Angle

• Real structures may be even more complicated because mask edges are not vertical.• High tilt angle implant for halo doping to minimize short channel

effect

.

30 Degree Tilt

Dis

tanc

e (µ

m)

Distance (µm)

30Þ tilt

0 0.2 0.4 0.6 0.8 1.0

0.2

0

-0.2

-0.4

TSUPREM50Kev P

Profile Evolution During Annealing

• The only other profile we can calculate analytically is when the implanted Gaussian is shallow enough that it can be treated as a delta function and the subsequent anneal can be treated as a one-sided Gaussian. (Recall example in Dopant Diffusion )

Delta FunctionDose Q

(Initial Profile)

ImaginaryDelta Function

Dose Q

DiffusedGaussian

VirtualDiffusion

x0

€

C x,t( )= QπDt

exp − x2

4Dt

(8)


• Comparing Eqn. (1) with the Diffusion Gaussian profile, we see that ∆Rp is equivalent to . ThusDt2

€

C x,t( )= Q

2π∆RP2 +2Dt( )

exp −x−RP( )2

2 ∆RP2 +2Dt( )

(7)

²R P

Implanted

After Diffusion

2Dt

Evolution of a Gaussian Profile after annealing


• Real implanted profiles are more complex.

• Light ions backscatter to skew the profile up.

• Heavy ions scatter deeper.Co

ncen

trat

ion

(cm

-3)

Amorphous Silicon


• Gaussian distribution often match the central region of the implanted profiles .• B distribution is skewed toward surface resulting significant deviations in the tail of the distribution


• 4 moment descriptions of these profiles are often used (with tabulated values for these moments).

Range:

€

RP =1Q

xC x()−∞

∞∫ dx

Std. Dev:

€

∆RP = 1Q

x−RP( )−∞

∞∫

2

C x()dx

Skewness:

€

γ=x −RP( )3 C x( )dx

−∞

∞∫

Q ∆RP3

Kurtosis:

€

β=x −RP( )4 C x( )dx

−∞

∞∫

Q ∆RP4

(9)

(10)

(11)

(12)

• Channeling can produce unexpectedly deep profiles

Implants in Real Silicon - Channeling• Channeled profile is composed of two portions. Main peak+

channeled part. Accurate modeling requires additional parameter to specify the ratio of the dose in main &channeled profile

• Note that the channeling decreases in the high dose implant (green curve) because damage blocks the channels.

B Implant: <100> Zero Tilt 35KeV

ION IMPLANTATION

CEERI PILANI,INDIA

Non-Local and Local Electronic Stopping Dielectric Medium

Retarding E-field• Drag force caused by charged ion in “sea” of electrons (non-local electronic

stopping).

• Collisions with electrons around atoms transfers momentum and results in local electronic stopping.

CEERI PILANI, INDIA

Engineering

8.1. microtech ion implant,1,2