Dopant and Self-Diffusion in Silicon and Silicon Germanium

40
FLCC Dopant and Self-Diffusion in Silicon and Silicon Germanium Eugene Haller, Hughes Silvestri, and Chris Liao MS&E, UCB and LBNL FLCC Tutorial 4/18/05

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Dopant and Self-Diffusion in Silicon and Silicon Germanium. Eugene Haller, Hughes Silvestri, and Chris Liao MS&E, UCB and LBNL FLCC Tutorial 4/18/05. Outline. Motivation Background Fick’s Laws Diffusion Mechanisms Experimental Techniques for Solid State Diffusion - PowerPoint PPT Presentation

Transcript of Dopant and Self-Diffusion in Silicon and Silicon Germanium

Page 1: Dopant and Self-Diffusion in Silicon and Silicon Germanium

FLCC

Dopant and Self-Diffusion in Silicon and Silicon Germanium

Eugene Haller, Hughes Silvestri, and Chris Liao

MS&E, UCB and LBNL

FLCC Tutorial

4/18/05

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Outline• Motivation

• Background – Fick’s Laws– Diffusion Mechanisms

• Experimental Techniques for Solid State Diffusion

• Diffusion of Si in Stable Isotope Structures

• Future Work: Diffusion of SiGe in Stable Isotope Structures

• Conclusions

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Motivation

• Why diffusion is important for feature level control of device processing

– Nanometer size feature control: - any extraneous diffusion of dopant atoms may result in device performance degradation

• Drain extension Xj < 10 nm by 2008*• Extension lateral abruptness < 3 nm/decade by 2008*

– Accurate models of diffusion are required for dimensional control on the nanometer scale

*International Technology Roadmap for Semiconductors, 2004 Update

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Semiconductor Technology Roadmap(International Technology Roadmap for Semiconductors, 2004 Update)

Difficult Challenges ≥ 45nm Through 2010 Summary of Issues

Front-end Process modeling for nanometer structures

Diffusion/activation/damage models and parameters including SPE and low thermal budget processes in Si-based substrate, i.e., Si, SiGe:C, Ge (incl. strain), SOI, and ultra-thin body devices

Characterization tools/methodologies for these ultra shallow geometries/junctions and low dopant levels

Modeling hierarchy from atomistic to continuum for dopants and defects in bulk and at interfaces

Front-end processing impact on reliability

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MOSFET Scaling

Substrate

Gate

Source Drain

Substrate

Gate

Source DrainLeff

Nsub

Xj

LgTox

Planar Bulk-Si Structure Thin-Body Structure

So

urc

e

Dra

in

Gate 2Gate 2

Fin Width Wfin = TSi

Fin Height = Hfin = W

Gate Length = Lg

Current Flow

Gate 1Gate 1

So

urc

e

Dra

in

Gate 2Gate 2

Fin Width Wfin = TSi

Fin Height = Hfin = W

Gate Length = Lg

Current Flow

Gate 1Gate 1

scaling to Lg < 20nm

Si1-xGex in the S/D regions will be needed for thin-body PMOSFETs in order to

• enhance mobility via strain • lower parasitic resistance

– S/D series resistance– contact resistance

Si and Ge interdiffusion, as well as B diffusion in Si1-xGex and Si must be well understood and characterized

Silicon Substrate

Source Drain

SiO2

SOI

GateGate

Silicon Substrate

Source Drain

SiO2

SOI

GateGate

Courtesy of Pankaj Kalra and Prof. Tsu-Jae King

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Fick’s Laws (1855)

Diffusion equation does not take into account interactions with defects!

CS

tx

DS

CS

x

JinJout

+GS

-RS

As V AV

RAV kr AV

CAV

tx

DAV

CAV

x

kr AV k f AS V

GAV k f AS V Example: Vacancy Mechanism

2nd Law

Jin Jout

dx

J DCS

x

Fick’s 1st Law: Flux of atoms

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Analytical Solutions to Fick’s Equations

D = constant

- Finite source of diffusing species:

CS

tx

DS

CS

x

DS

2CS

x 2

Solution: Gaussian

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

No

rma

lize

d C

on

cen

tra

tion

Depth

C x,t S

Dte

x 2

4 Dt

- Infinite source of diffusing species:Solution:

Complementary error function

C x, t Co 12

ez 2

0

y dz

, y

x

2 Dt

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

No

rma

lize

d C

on

cen

tra

tion

Depth

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Solutions to Fick’s Equations (cont.)D = f (C) Diffusion coefficient as a function of concentration

Concentration dependence can generate various profile shapes and penetration depths

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Solid-State Diffusion ProfilesExperimentally determined profiles can be much more complicated - no analytical solution

B implant and anneal in Si with and without Ge implant

Kennel, H.W.; Cea, S.M.; Lilak, A.D.; Keys, P.H.; Giles, M.D.; Hwang, J.; Sandford, J.S.; Corcoran, S.; Electron Devices Meeting, 2002. IEDM '02, 8-11 Dec. 2002

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Direct Diffusion Mechanisms in Crystalline Solids

Pure interstitial

Direct exchange

Elements in Si: Li, H, 3d transition metals

No experimental evidenceHigh activation energy → unlikely

(no native defects required)

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Vacancy-assisted Diffusion Mechanisms

As V AVDissociative mechanism

As Ai V

Vacancy mechanism

(Sb in Si)

(Cu in Ge)

(native defects required)

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Interstitial-assisted Diffusion Mechanisms

Interstitialcy mechanism

Kick-out mechanism

As I AI

As I Ai

(P in Si)

(B in Si)

(native defects required)

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Why are Diffusion Mechanisms Important?

• Device processing can create non-equilibrium native defect concentrations– Implantation: excess interstitials

– Oxidation: excess interstitials

– Nitridation: excess vacancies

– High doping: Fermi level shift

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Oxidation Effects on Diffusion

• Oxidation of Si surface causes injection of interstitials into Si bulk

• Increase in interstitial concentration causes enhanced diffusion of B, As, but retarded Sb diffusion

• Nitridation (vacancy injection) causes retarded B, P diffusion, enhanced Sb diffusion

(Fahey, et al., Rev. Mod. Phys. 61 289 (1989).)

Oxidation during device processing can lead to non-equilibrium diffusion

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Implantation Effects on DiffusionTransient Enhanced Diffusion (TED) - Eaglesham, et al., Appl. Phys. Lett. 65(18) 2305 (1994).

• Implantation damage generates excess interstitials

– Enhance the diffusion of dopants diffusing via interstitially-assisted mechanisms

– Transient effect - defect concentrations return to equilibrium values

• TED can be reduced by implantation into an amorphous layer or by carbon incorporation into Si surface layer

– Substitutional carbon acts as an interstitial sink

– Stolk, et al., Appl. Phys. Lett. 66 1371 (1995)

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Doping Effects on DiffusionHeavily doped semiconductors - extrinsic at diffusion temperatures

– Fermi level moves from mid-gap to near conduction (n-type) or valence (p-type) band.

– Fermi level shift changes the formation enthalpy, HF, of the charged native defect

– Increase of CI,V affects Si self-diffusion and dopant diffusion

CV ,Ieq CSi

o expSV ,I

F

kB

exp

HV ,IF

kBT

, H

V F H

V oF EF E

V

Ec

Ev

V--/-

V-/o

V+/++

Vo/+

Io/+

0.11 eV

0.57 eV

0.35 eV

0.13 eV0.05 eV

V states (review by Watkins, 1986)

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Doping Effects on Diffusion

10-19

10-18

10-17

10-16

10-15

10-14

10-13

10-12

0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86

D (

cm2s-1

)

1000/T (1000/K)

DSi

(ni)

DSi

(ext)

DAs

(ni)

DAs

(ext)

D ni D ext ni

CAseq

The change in native defect concentration with Fermi level position causes an increase in the self- and dopant diffusion coefficients

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Experimental Techniques for Diffusion

Creation of the Source- Diffusion from surface- Ion implantation- Sputter deposition- Buried layer (grown by MBE)

Annealing

Analysis of the Profile

- Radioactivity (sectioning)- SIMS- Neutron Activation Analysis- Spreading resistance- Electro-Chemical C/Voltage

Modeling of the Profile- Analytical fit- Coupled differential eq.

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Primary Experimental Approaches

• Radiotracer Diffusion– Implantation or diffusion from surface– Mechanical sectioning– Radioactivity analysis

• Stable Isotope Multilayers – new approach– Diffusion from buried enriched isotope layer– Secondary Ion Mass Spectrometry (SIMS)– Dopant and self-diffusion

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Radiotracer Diffusion

• Diffusion using radiotracers was first technique available to measure self-diffusion– Limited by existence of radioactive isotope

– Limited by isotope half-life (e.g. - 31Si: t1/2 = 2.6 h)

– Limited by sensitivity

– Radioactivity measurement

– Width of sections

Application of radio-isotopes to surface

annealing

Mechanical/Chemical sectioning

Measure radioactivity of each section

Depth (m)

Con

cen

trat

ion

(cm

-3)

Generate depth profile

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Diffusion Prior to Stable Isotope Multilayer StucturesWhat was known about Si, B, P, and As diffusion in Si

Si: self-diffusion: interstitials + vacanciesknown: interstitialcy + vacancy mechanism, QSD ~ 4.7 eVunknown: contributions of native defect charge states

B: interstitial mediated: from oxidation experiments known: diffusion coefficient unknown: interstitialcy or kick-out mechanism

P: interstitial mediated: from oxidation experiments known: diffusion coefficient unknown: mechanism for vacancy contribution

As: interstitial + vacancy mediated: from oxidation + nitridation experiments known: diffusion coefficient unknown: native defect charge states and mechanisms

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Stable Isotope Multilayers

• Diffusion using stable isotope structures allows for simultaneous measurements of self- and dopant diffusion– No half-life issues

– Ion beam sputtering rather than mechanical sectioning

– Mass spectrometry rather than radioactivity measurement

28Si enriched

FZ Si substrate

nat. Si

a-Si cap

1017

1018

1019

1020

1021

1022

0 500 1000 1500

con

cen

tra

tio

n (

cm-3

)

Depth (nm)

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Secondary Ion Mass Spectrometry

• Incident ion beam sputters sample surface - Cs+, O+

– Beam energy: ~1 kV

• Secondary ions ejected from surface (~10 eV) are mass analyzed using mass spectrometer

– Detection limit: ~1012 - 1016 cm-3

• Depth profile - ion detector counts vs. time – Depth resolution: 2 - 30 nm

Ion gun

Mass spectrometer

Ion detector

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Diffusion Parameters found via Stable Isotope Heterostructures

• Charge states of dopant and native defects in diffusion

• Contributions of native defects to self-diffusion

• Enhancement of extrinsic dopant and self-diffusion

• Mechanisms which mediate self- and dopant diffusion

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Si Self-Diffusion

• Enriched layer of 28Si epitaxially grown on natural Si

• Diffusion of 30Si monitored via SIMS from the natural substrate into the enriched cap (depleted of 30Si)

– 855 ºC < T < 1388 ºC– Previous work limited to short times

and high T due to radiotracers

• Accurate value of self-diffusion coefficient over wide temperature range:

(Bracht, et al., PRL 81 1998)

1095 ºC, 54.5 hrs

1153 ºC, 19.5 hrs

DSi 560 170240 exp

4.760.04 eV

kBT

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1017

1018

1019

1020

1021

0 200 400 600 800 1000 1200 1400 1600

co

nce

ntr

ati

on

(cm-3)

Depth (nm)

Si and Dopant Diffusion

Arsenic doped sample annealed 950 ˚C for 122 hrs

AsV o Ass V o e

Vacancy mechanism

AsI o Ass I

Interstitialcy mechanism

ni

extrinsic intrinsic

Io

I-

I--

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1017

1018

1019

1020

1021

0 200 400 600 800 1000 1200 1400 1600

co

nce

ntr

ati

on

(cm

-3)

Depth (nm)

Si and Dopant Diffusion

Arsenic doped sample annealed 950 ˚C for 122 hrs

niIoI-I--

AsV o Ass V o e

Vacancy mechanism

AsI o Ass I

Interstitialcy mechanism

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1017

1018

1019

1020

1021

0 200 400 600 800 1000 1200 1400 1600

co

nce

ntr

ati

on

(cm

-3)

Depth (nm)

Si and Dopant Diffusion

Arsenic doped sample annealed 950 ˚C for 122 hrs

ni

IoI-I--

AsV o Ass V o e

Vacancy mechanism

AsI o Ass I

Interstitialcy mechanism

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Si and Dopant DiffusionSupersaturation of Io, I+ due to B diffusion

Io and I+ mediate Si and B diffusion

Enhancement due to Fermi level effectDiffusion mechanism: Kick-out

– Bi0 Bs

- + I 0 + h

– Bi0 Bs

- + I +

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Si and Dopant DiffusionPhosphorus Diffusion Model:Interstitialcy or Kick-out mechanism – Io, I-

Pair assisted recombination or dissociative mechanism – V0

PI0(Pi0) Ps

I0 e

PI0(Pi0) Ps

I

PI0(Pi0)V 0 Ps

e

PI(Pi) Ps

I0

PI(Pi)V 0 Ps

1016

1017

1018

1019

1020

1021

500 1000 1500

Co

nce

ntr

atio

n (

cm-3

)

Depth (nm)

Annealed 1100 ˚C for 30 min

1264.023.0P scm

kT

eV)11.033.3(exp36.0D

1240.32

8.3SD

Iscm

kT

eV)24.040.5(exp103.4D

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Native Defect Contributions to Si Diffusion

Diffusion coefficients of individual components add up accurately:

DSi(ni )tot fIo CI

o DIo fI

CI DI

fI CI DI DSi(ni )

(Bracht, et al., 1998)

(B diffusion) (As, P diffusion)(B, P diffusion)

10-19

10-18

10-17

10-16

10-15

10-14

0.7 0.8 0.9

D (

cm2s-1

)

1000/T (1000/K)

DSi

(I-)

DSi

(I+)

DSi

(Io)

DSi

(Io+I++I-)

DSi

(ni)

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Diffusion in Ge Stable Isotope Structure

Annealed 586 °C for 55.55 hours

Fuchs, et al., Phys. Rev B 51 1687 (1995)

Ge self-diffusion coefficient determined from 74Ge/70Ge isotope structure

DGe 1.210 3 exp 3.00.05 eV

kBT

cm2s 1

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Diffusion in Si1-xGex

• SiGe is used as new material to enhance electronic devices– Will face same device diffusion issues as Si

– Currently, limited knowledge of diffusion properties

compressive strain 30% Idsat increase

Intel’s 90nm CMOS TechnologySi1-xGex in PMOS S/D regions to enhance on-state drive current without increasing off-state leakage

Si1-xGex in the S/D regions will be needed for thin-body PMOSFETs in order to

• enhance mobility via strain • lower parasitic resistance

– S/D series resistance– contact resistance

Si and Ge interdiffusion, as well as B diffusion in Si1-xGex and Si must be well understood and characterized

Silicon Substrate

Source Drain

SiO2

SOI

GateGate

Silicon Substrate

Source Drain

SiO2

SOI

GateGate

T. Ghani et al., 2003 IEDM Technical Digest

Courtesy of Pankaj Kalra and Prof. Tsu-Jae King

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Diffusion in SiGe Isotope Structures

• Diffusion of Si in pure Ge

• Si and Ge self-diffusion in relaxed Si1-xGex structures

• Si and Ge self-diffusion in strained Si1-xGex structures

• Simultaneous Si and Ge dopant and self-diffusion

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Si Diffusion in Pure Ge

• Before determination of Si and Ge self-diffusion in SiGe can be made must determine Si diffusion in Ge and Ge diffusion in Si– Large amounts of data on Ge diffusion in Si - used as a tracer for Si self-

diffusion due to longer half-life– Much less data on Si diffusion in Ge

• MBE grown Ge layer– 100 nm spike of Si (1020 cm-3)

Ge substrate

Ge epilayer

~ 1020 cm-3 Si

1017

1018

1019

1020

0 100 200 300 400 500 600 700 800

Con

cent

ratio

n (c

m-3

)

Depth (nm)

[Si] [C] - - -

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Si Diffusion in Pure Ge

1017

1018

1019

1020

150 200 250 300 350 400 450

Si C

on

cen

tra

tion

(cm

-3)

Depth (nm)

10-19

10-18

10-17

10-16

10-15

10-14

10-13

0.00085 0.00095 0.00105 0.00115

550650750850950

DS

i (cm

-3)

1/T (1/K)

[Strohm, 2002]

[Uppal, 2003]

[Södervall, 1997]

[Räisänen, 1981]

[Current Work]

T (°C)

DSi 38 27.471.5 exp

(3.320.1)eV

kT

cm2s 1

Annealed at 550 °C for 30 days

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Si and Ge Self-DiffusionRelaxed Si1-xGex Structures

• Use isotope heterostructure technique to study Si and Ge self-diffusion in relaxed Si1-xGex alloys. (0.05 ≤ x ≤ 0.85)

– No reported measurements of simultaneous Si and Ge diffusion in Si1-xGex alloys

• Proposed isotope heterostructure:– MBE grown - Group of Prof. Arne Nylandsted Larsen, Univ. of Aarhus, Denmark

Si substrate

SiGe graded buffer layer

200 nm nat. Si1-xGex

200 nm nat. Si1-xGex

400 nm 28Si1-x70Gex

1017

1018

1019

1020

1021

1022

0 100 200 300 400 500 600 700 800

Si0.95

Ge0.05

(850C 30days) simulation

30Si Conc.76 Ge Conc.

Con

cent

ratio

n (c

m-3

)

Depth (nm)

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Si and Ge Self-DiffusionStrained Si1-xGex Structures

• Study Si and Ge self-diffusion in strained Si1-xGex alloys. – 0.15 ≤ x ≤ 0.75

• Vary composition between layers to generate: – Compressive strain (x - y < 0)

– Tensile strain (x - y > 0) x - y≈ 0.05

Si substrate

SiGe graded buffer layer

100 nm nat. Si1-yGey

100 nm nat. Si1-yGey

200 nm 28Si1-x70Gex

Proposed isotope heterostructure: MBE grown - Group of Prof. Arne Nylandsted Larsen

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Simultaneous Dopant and Self-Diffusion Si1-xGex Multilayer Structures

• Five alternating 28Si1-x70Gex (0.05 ≤ x ≤ 1) and natural Si1-xGex

layers with amorphous cap• Implant dopants (B, P, As) into amorphous cap • Simultaneous Si and Ge self-diffusion and dopant diffusion

amorphous Si cap

Si substrate

SiGe graded buffer layer

100 nm nat. Si1-xGex

100 nm 28Si1-x70Gex

Proposed isotope heterostructure: MBE grown - Group of Prof. Arne Nylandsted Larsen

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Conclusions

• Diffusion in semiconductors is increasingly important to device design as feature level size decreases.

• Device processing can lead to non-equilibrium conditions which affect diffusion.

• Diffusion using stable isotopes yields important diffusion parameters which previously could not be determined experimentally.

• Technique will be extended to SiGe alloys with variation of composition, strain and doping level.