In Do German

Uma Parthavi M

Dept. of Electrical Engineering,

Indian Institute of Technology Delhi.

Tutor: Prof. N Dasgupta

Doping by Diffusion and Implantation

Contents

Doping by Diffusion and Implantation2

Doping Two step doping process

Diffusion equipment & sources

Diffusion-Microscopic & Macroscopic point of view

Fick‟s Laws – solutions

Diffusivity

Influence of Electric Field, Defects

Oxidation Enhanced Diffusion

Ion Implantation Implantation Basics

Ion implanter

Implantation profiles

Channeling

Damage annealing

Comparison between diffusion and ion implantation

References

Contents

Doping Silicon


Diffusion :

The spread of particles through random motion from regions of

higher concentration to regions of lower concentration

Ion implantation

Bombarding the substrate with ions accelerated to high

velocities

Introduction

Creating Doped regions


Step1 : Pre-deposition

Controllably introduce desired dopant atoms

Methods: Solid phase diffusion from glass

layers

Gas phase diffusions

Ion Implantation

Step2 : Drive-in

The introduced dopants are driven deeper into the wafer without further introduction of dopant atoms

Two step process for producing a junction

Diffusion

Diffusion sources

Doping by Diffusion and Implantation5 Diffusion

Diffusion- Equipment


Diffusion Equipment(showing predep. Of BSG)[2]

Diffusion

Diffusion


Microscopic Point of View :

Considers the motion of dopant at atomic scale

Computationally expensive and used in simulation tools

More accurate

Macroscopic Point of View :

Considers overall motion of dopant profile

Fick‟s Laws

Considering the macroscopic point of view is important

because it gives a sufficiently accurate first hand picture

Diffusion

Microscopic Point of View

Vacancy Assisted Diffusion Interstitial Assisted diffusion


Impurity atom

Diffusion

Fick’s First Law


Diffusive flux has a magnitude proportional to spatial

concentration gradient

F is flux(atoms/cm2sec); D is the diffusivity(cm2sec-1);

is the concentration gradient.

Flow is opposite to the direction of concentration gradientx

C

CDF

x

CDF

Diffusion

Fick’s Second Law


Increase in the concentration in a cross section of unit area with time is simply the difference between the flux into the volume and the flux out of the volume.

“what goes in and doesn’t go out stays there”

If D is a constant,

x

FF

x

F

t

C outin

Flux in and out of a volume element

).(. CDFt

C

Diffusion

Solutions to Fick’s Equations


Steady state – linear

Limited source in infinite medium - Gaussian

Limited source at surface - Gaussian

Infinite source – Error function

Diffusion

Steady state


Steady state – dopant concentration in constant with time

Solving for the above gives, C=a+bx

02

2

x

CD

Diffusion

Limited source in infinite medium


Boundary conditions:

0C

C

0t

0t

as

as

for

for

0x

0x

QdxtxC ),(

A constant dose of dopants introduced in an infinite

mediumDopants

+ve

-ve

Si Wafer

Diffusion

Consequences


Solution has an evolving Gaussian form

Symmetric about the origin

Peak concentration decreases by and is given by C(0,t)

Diffusion length =

It is an approximate measure of how much the dopant has diffused

t/1

Dt2

Time evolution of Gaussian profile[1]

Diffusion

Limited source at the surface


Dopant dose Q introduced at the surface

Can be treated as an effective dose of 2Q being

introduced in a virtual infinite medium

Dopants introduced at the surface

Si Wafer

Real Dopants

Virtual mediumVirtual Dopants

Diffusion

Infinite source


Consider series of slices,

each with thickness ,

having a dose of C .

The solution for this case is

simply the linear

superposition of Gaussian

solutions for thin slices

Boundary conditions:

C=0 at t=0 for x>0

C=C at t=0 for x<0

x

x

dDt

x

Dt

CtxC

0 2

4)(exp

2),(

Diffusion from an infinite source

Diffusion


Cs is the concentration at the surface and Cs=C/2

Surface conc. is constant

Total Dose

DtC

dxDt

xerfCQ s

s

2)]

2(1[

0

Time evolution of erfc profile[1]

Diffusion

Diffusivity


For common impurities in

silicon,

k is the Boltzmann

constant, EA is the

activation energy in eV and

T is the temperature in

degrees Kelvin.

)exp(0

kT

EDD A

Diffusivity for common dopants [3]

Diffusion

Solid solubility


Maximum Thermodynamic concentration of dopant that can be

dissolved in silicon without forming a separate phase

In reality, electrical solubility is less than the solid solubility

because of formation of neutral clusters with vacancies

Solid solubility plots for common dopants[1]

Diffusion

Influence of Electric field


Dominant when doping concentrations exceed intrinsic

carrier concentrations.

F is the flux as discussed earlier, C is

the net doping concentration at x

h is upper bounded by 2

x

ChDF

22 41

inC

Ch

Diffusion

Effect of electric field on low

concentration regions[1]

Doping by Diffusion and Implantation21 Diffusion

Influence of defects


DA is the effective diffusivity ,DA* is the normal equilibrium diffusivity under inert

conditions, fI is the fraction of dopants diffusing with interstitial mechanism, fv is the

fraction of dopants diffusing with vacancy-type mechanism, CI is the interstitial

concentration, CV is the vacancy concentration, CI* is the interstitial concentration at

equilibrium, CV* is the vacancy concentration at equilibrium

Diffusion

Oxidation enhanced diffusion


P,B diffusion – enhanced ; Sb– retarded

Oxidation of Si to SiO2 causes volume to increase –induces stress which is relieved by the Si atoms moving to interstitial spaces

Oxidation injects interstitials ; P,B prefer interstitial type diffusion

Interstitials combine with vacancies – decrease in vacancies ; Sb prefers vacancy type diffusion

Plot showing effect of oxidation in diffusion of As and Sb

implants[1]

Diffusion

Ion Implantation Basics


Energetic and violent technique – Dominant doping

technique for past 20 yrs

Direct bombardment of accelerated dopant ions onto the

substrate

Cascade of damages created in the perfect Si lattice –

removed by annealing

Precise control on the amount and distribution of the dose

Energy of ions control the distribution

Ion beam current controls the dose

Implantation

Ion Implanter


Ion Sources: Gas : Arsine, Phosphine, Boron difluoride in a zeolite matrix ; allow rapid beam

tuning

Solid : elemental sources of As, P ; vaporized

Ion Implantation System[5]

Implantation

Ion Implanter


Gas from the source is ionized by electrons from a filament/plasma discharge

Ions are extracted by voltage and mass analyzed to select only one ion species

B is the magnetic field , proportional to the current I, V is the external voltage applied, m is the mass of a ion, v is the velocity of an ion, q is the charge of an ion

Different ions can be chosen by varying the external voltage and the current to the coils

Iq

mVr

qvBr

mv

mvqVEK

IB

12

21..

2

2

Beam of B11(top) and B10 separated

Courtesy: Albion Systems

Implantation

Ion Implanter


The radius of curvature is proportional to square root of the mass

Ions are further accelerated depending on the requirements and incident on the target

The implant dose is measured by locating the sample at the end of a „Faraday cup‟

I is the collected beam current, A is the implant area, t is the integration time and q is the charge on the ion

dtq

I

AQ

1

Range of Energy and Dose needed for different applications [6]Implantation



Range of an ion is the actual distance travelled by it before stopping

Projected Range Rp is the average distance travelled normal to the surface

ΔRp is the standard deviation of the projected range also called straggle

Heavy ions – Smaller Rp and ΔRp

Lighter ions – Greater Rp & ΔRp

Implantation profiles of commonly used dopant

atoms[6]

Implantation

Implantation profiles[7]

Doping by Diffusion and Implantation31 Implantation


Doping by Diffusion and

Implantation

32

Can be approximated to a Gaussian

C(x) is the concentration distribution, Rp is the range,ΔRp is the straggle, Cp is the peak concentration

The 2D distribution is usually assumed to be a product of vertical and lateral distribution

)2

)(exp()(2

Rp

RpxCpxC

RpCpQ 20

0,1

0,2

0,3

0,4

0,5

0,6

0 50 100 150 200 250

Ran

ge

(um

)

Energy(KeV)

As

P

B

0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,1

0 50 100 150 200 250

Stra

gg

le(u

m)

Energy(KeV)

As

P

B

Range and Straggle for As,P,B

Data from BYU’s Range and Straggle calculatorImplantation

Pearson Model


Channeling


Crystalline Si – planar and axial

channels

Once an ion enters a channel, it can be

steered along the channel until it comes

to rest either by drag or sharp collision

High doses – less channeling

Implantation

Impact of channeling on profiles


Impact of channeling on B profile[8]

Implantation

Avoiding channeling


Channeling can be reduced by –

Oxide screening

Tilting the wafer (ideally 7degrees)

Screening by amorphous Si

Implantation

Avoiding channeling

ImplantationDoping by Diffusion and Implantation37

From: http://www.silvaco.com/tech_lib_TCAD/simulationstandard/1996/dec/a1/a1.html

Ion stopping mechanism


Nuclear Stopping: Collision of ions with lattice atoms

Depends on Ion energy

Tends to dominate at the end of the stopping process when ions have lost much of their

energy

Produces damage

Electronic Stopping: Nonlocal electronic Stopping

Drag experienced by the ion in a dielectric medium; dissipative, does not alter the trajectory

Directly proportional to the ion velocity

Depends on ionization state of the ion

Local electronic Stopping

If the ion comes close enough to a lattice atom, momentum transfer due to e-transfer possible

Subtly alters the trajectory – minor compared to nuclear stopping

Depends on the ion velocity

Implantation

Stopping power for common ions


Total stopping power =

electron stopping power+

nuclear stopping power

Nuclear stopping

dominates at low energies

Electron stopping

dominates at higher

energies, for lighter atoms

Stopping powers of dopants[1]

Implantation

Stopping mechanisms


Damage During implantation


Nuclear stopping – ions transfer energy to lattice atoms; crystalline structure damaged

Energy required to displace a Si atom to create a Frenkel pair (I +V) is 15eV

Damage to the crystal is in the following ways:

Creation of interstitials and vacancies

Creation of local zones of amorphous material

High dose implants might turn crystal to amorphous state

The above two types of damage are called Primary crystalline damage ; Repaired by thermal process known as annealing

But subjecting wafer to thermal process for a long time might cause diffusion of dopants - undesirable

Implantation

Annealing


Primary damage anneals at 400oC

Firstly I and V combine in the bulk ; this leaves only I‟s originating from introduction of extra atom

Later vacancies and interstitials recombine at the surface

Above 400oC extra I‟s condense into rod shaped defects – {311} planes

Upon annealing after 900oC, they start disappearing

Damage less than a critical value can be repaired.

For damage above critical value, {311} defects form stable dislocation loops –secondary damage

Steps in Annealing with time [1]

{311} Ribbon Defects[1]

Implantation

Annealing


Largest concentration @ interface between crystalline and

amorphous Si – EOR(End of Range) Defects

These EOR loops are known to disappear in some instances

after 60 sec anneal at 1100oC

EOR loops detrimental if present at junctions

Annealing cycles are chosen to cause enough dopant diffusion

so that the loops are contained in highly doped regions and

are shielded from any depletion regions

Implantation

Dopant Activation


Activation Dopants should occupy

substitutional sites Broken bonds should be repaired

to improve mobility

Low primary Damage: all damage anneals out

High primary Damage : Amorphization Solid Phase Epitaxy provides

nearly ideal soln

Partial Damage: Formation of secondary damage 950 -1050oC required

Fraction of atoms activated for boron implant [9]F

ract

ion

of

ato

ms

act

ive

Implantation

Annealing


Annealing can be done in two ways:

Furnace Annealing

Rapid Thermal Annealing

Implantation

Furnace Annealing


Inert ambient – Nitrogen or Argon

Oxide capping layer recommended to avoid evaporation of dopants

Temperature range – 750-1100oC

Time >30 mins

Problem of Diffusion of implanted dopants

Transient enhanced Diffusion – not suited for shallow junctions Typical Furnace used for annealing

Implantation

Rapid Thermal Annealing


Bank of lamps that rapidly

heat a wafer

Optical energy transfer

Ramp rate of 100oC/s

Wafer attains uniform

temperature in few ms

Annealing time: 1-100 s

No diffusion during anneal

RTA furnace Schematic

Implantation

Comparison of Diffusion and Ion

implantation

Diffusion Ion Implantation


Advantages: No damage created

Batch fabrication possible

Disadvantages: Limited to solid solubility

Low dose predeps difficult

High temperature process

Shallow junctions difficult

Advantages: Low temperature process

Precise dose and junction depth control

Implantations through thin layers of oxide/nitride possible

Short process times

Disadvantages: Implant damage enhances

diffusion

Additional cost of annealing

Dislocations may cause junction leakage

Channeling

Comparision

References


[1] J D Plummer, M D Deal and P B Griffin, “Silicon VLSI Technology: fundamentals, practice and modelling”,Pearson Edu. Inc.,2001

[2] John (2010, June 1), “Diffusion of impurities for IC fabrication” [online].Available:http://www.circuitstoday.com/diffusion-of-impurities-for-ic-fabrication

[3] H.Puchner , “Advanced Process Modelling for VLSI Technology,” Ph.D. dissertation, Dept. Elect. Eng., Technical Univ. of Vienna,Vienna,Austria, 1996

[4] National Technology Roadmap for Semiconductors (NTRS); SIA: San Jose, 1997.

[5] John (2010, June 2), “Ion Implantation” [online].Available:http://www.circuitstoday.com/ion-implantation

[6] L Rubin and J Poate(2010, Dec 2), “Ion Implantation in silicon technology” [online].Available: http://www.aip.org/tip/INPHFA/vol-9/iss-3/p12.html

[7] (2010, Dec 2),”Ion Implantation Processes in Semiconductor Manufacturing” [online].Available: http://www.leb.e-technik.uni erlangen.de/lehre/mm/html/implant.htm

References

http://www.circuitstoday.com/diffusion-of-impurities-for-ic-fabrication











http://www.circuitstoday.com/ion-implantation



http://www.aip.org/tip/INPHFA/vol-9/iss-3/p12.html





http://www.leb.e-technik.uni-erlangen.de/lehre/mm/html/implant.htm







References


[8] C Tian,S Gara,G Hobler and G Stingeder , “Boron Implantation in Si: Channeling

Effects Studied by SIMS and Simulation ,” Mikrochim. Acta, ser. D, vol. 107, pp.

161-169, 1992

[9] B. L. Crowder and F. F. Morehead, Jr, “Annealing characteristics of n-type dopants

in ion-implanted silicon,” Applied physics letters, ser. D, vol. 14, pp. 313-315,

May 1969

References

Thank You!!!!!

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