Dopant and Self-Diffusion in Silicon and Silicon Germanium

1
Dopant and Self-Diffusion in Silicon and Silicon
Germanium

• Eugene Haller, Hughes Silvestri, and Chris Liao
• MSE, UCB and LBNL
• FLCC Tutorial
• 4/18/05

2
Outline

• Motivation
• Background
• Ficks 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

3
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 lt 10 nm by 2008
• Extension lateral abruptness lt 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
4
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, SiGeC, Ge (incl. strain), SOI, and ultra-thin body devices
Front-end Process modeling for nanometer structures Characterization tools/methodologies for these ultra shallow geometries/junctions and low dopant levels
Front-end Process modeling for nanometer structures Modeling hierarchy from atomistic to continuum for dopants and defects in bulk and at interfaces
Front-end Process modeling for nanometer structures Front-end processing impact on reliability
5
MOSFET Scaling

• 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

Courtesy of Pankaj Kalra and Prof. Tsu-Jae King
6
Ficks Laws (1855)
Ficks 1st Law Flux of atoms
2nd Law
Diffusion equation does not take into account
interactions with defects!
Jout
Jin
-RS
GS
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Analytical Solutions to Ficks Equations
D constant
- Finite source of diffusing species
Solution Gaussian
- Infinite source of diffusing species
Solution Complementary error function
8
Solutions to Ficks Equations (cont.)
D f (C) Diffusion coefficient as a function of
concentration
Concentration dependence can generate various
profile shapes and penetration depths
9
Solid-State Diffusion Profiles
Experimentally determined profiles can be much
more complicated - no analytical solution
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
B implant and anneal in Si with and without Ge
implant
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Direct Diffusion Mechanisms in Crystalline Solids
(no native defects required)
Pure interstitial
Elements in Si Li, H, 3d transition metals
Direct exchange
No experimental evidence High activation energy ?
unlikely
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Vacancy-assisted Diffusion Mechanisms
(native defects required)
Vacancy mechanism
(Sb in Si)
Dissociative mechanism
(Cu in Ge)
12
Interstitial-assisted Diffusion Mechanisms
(native defects required)
Interstitialcy mechanism
(P in Si)
Kick-out mechanism
(B in Si)
13
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

14
Oxidation Effects on Diffusion
Oxidation during device processing can lead to
non-equilibrium 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).)
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Implantation Effects on Diffusion

• Transient 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 Diffusion

• Heavily 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

V states (review by Watkins, 1986)
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Doping Effects on Diffusion
The change in native defect concentration with
Fermi level position causes an increase in the
self- and dopant diffusion coefficients
18
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

20
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

Mechanical/Chemical sectioning
Generate depth profile
Application of radio-isotopes to surface
Concentration (cm-3)
Measure radioactivity of each section
annealing
Depth (?m)
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Diffusion Prior to Stable Isotope Multilayer
Stuctures
What was known about Si, B, P, and As diffusion
in Si Si self-diffusion interstitials
vacancies known interstitialcy vacancy
mechanism, QSD 4.7 eV unknown 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

23
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

24
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 lt T lt 1388 ºC
• Previous work limited to short times and high T
  due to radiotracers
• Accurate value of self-diffusion coefficient over
  wide temperature range

1153 ºC, 19.5 hrs
1095 ºC, 54.5 hrs
(Bracht, et al., PRL 81 1998)
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Si and Dopant Diffusion
Arsenic doped sample annealed 950 C for 122 hrs
extrinsic
intrinsic
Io I- I--
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Si and Dopant Diffusion
Arsenic doped sample annealed 950 C for 122 hrs
IoI-I--
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Si and Dopant Diffusion
Arsenic doped sample annealed 950 C for 122 hrs
IoI-I--
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Si and Dopant Diffusion
Supersaturation of Io, I due to B diffusion Io
and I mediate Si and B diffusion
Enhancement due to Fermi level effect

• Diffusion mechanism Kick-out
• Bi0 ? Bs- I 0 h
• Bi0 ? Bs- I

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Si and Dopant Diffusion
Phosphorus Diffusion Model Interstitialcy or
Kick-out mechanism Io, I- Pair assisted
recombination or dissociative mechanism V0
Annealed 1100 C for 30 min
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Native Defect Contributions to Si Diffusion
(Bracht, et al., 1998)
Diffusion coefficients of individual components
add up accurately
(B diffusion)
(As, P diffusion)
(B, P diffusion)
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Diffusion in Ge Stable Isotope Structure
Annealed 586 C for 55.55 hours
Ge self-diffusion coefficient determined from
74Ge/70Ge isotope structure
Fuchs, et al., Phys. Rev B 51 1687 (1995)
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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

• 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

T. Ghani et al., 2003 IEDM Technical Digest
Courtesy of Pankaj Kalra and Prof. Tsu-Jae King
34
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

35
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)

36
Si Diffusion in Pure Ge
Annealed at 550 C for 30 days
37
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

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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 lt 0)
• Tensile strain (x - y gt 0)
• ?x - y? 0.05

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

Proposed isotope heterostructure MBE grown -
Group of Prof. Arne Nylandsted Larsen
40
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.