Title: Dopant and Self-Diffusion in Silicon, Germanium and Silicon-Germanium

1
FLCC Seminar

• Title Dopant and Self-Diffusion in Silicon,
  Germanium and Silicon-Germanium
• Faculty E. E. Haller
• GSRA Chris Liao
• Department MSE
• University UC Berkeley

2
Dopant and Self-Diffusion in Silicon, Germanium
and Silicon-Germanium

• Eugene E. Haller and Chris Liao
• Materials Science and Engineering, UC Berkeley
• Materials Sciences Division, LBNL
• FLCC Seminar
• 12/4/06

3
Outline

• Motivation
• Background
• Ficks Laws
• Diffusion Mechanisms
• Non-equilibrium effects
• Experimental Techniques for Solid State Diffusion
• Isotopically enriched structure
• SIMS and Spreading Resistance Profiling
• Diffusion in Si
• Diffusion in Ge
• Diffusion in SiGe
• Conclusions and Future Work

4
Motivation

• Why is diffusion 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
5
Semiconductor Technology Roadmap
(International Technology Roadmap for
Semiconductors, 2005)
Difficult Challenges Modeling and Simulation
Difficult Challenges 32nm Summary of Issues
Front-end Process modeling for nanometer structures Diffusion/activation/damage/stress models and parameters including SPER and low thermal budget processes in Si-based substrate, that is, Si, SiGeC, Ge), SOI, and ultra-thin body devices
Front-end Process modeling for nanometer structures Modeling of epitaxially grown layers Shape, morphology, stress
Front-end Process modeling for nanometer structures Characterization tools/methodologies for ultra shallow geometries/junctions and low dopant level
Front-end Process modeling for nanometer structures Front-end processing impact on reliability
6
Ficks Laws (1855)
Ficks 1st Law Flux of atoms
2nd Law
Diffusion equations do not take into account
interactions with defects!
Jout
Jin
-RS
GS
7
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
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
10
Vacancy-assisted Diffusion Mechanisms
(native defects required)
Vacancy mechanism
(Sb in Si)
Dissociative mechanism
(Cu in Ge)
11
Interstitial-assisted Diffusion Mechanisms
(native defects required)
Interstitialcy mechanism
(P in Si)
Kick-out mechanism
(B in Si)
12
Why are Diffusion Mechanisms Important?

• Device processing can create non-equilibrium
  native defect concentrations for Si devices
• Implantation excess interstitials
• Oxidation excess interstitials
• Nitridation excess vacancies
• High doping Fermi level shift
• The non-equilibrium defects can lead to enhanced
  or retarded diffusion (Transient Enhanced
  Diffusion)

13
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).)
14
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)

15
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)
16
Experimental Techniques for Diffusion
Introduction of Diffusion 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.

17
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

18
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

30Si 75As
19
Diffusion Parameters found via Stable Isotope
Heterostructures

• Charge states of dopant and native defects during
  diffusion
• Contributions of native defects to self-diffusion
• Enhancement of extrinsic dopant and
  self-diffusion
• Mechanisms which mediate self- and dopant
  diffusion

20
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

21
Spreading Resistance Profiling
22
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)
23
Si and Dopant Diffusion
Arsenic doped sample annealed 950 C for 122 hrs
extrinsic
intrinsic
Io I- I--
24
Si and Dopant Diffusion
Arsenic doped sample annealed 950 C for 122 hrs
IoI-I--
25
Si and Dopant Diffusion
Arsenic doped sample annealed 950 C for 122 hrs
IoI-I--
26
Si and Dopant Diffusion
27
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)
28
Germanium Reenters Device Technology

• Diffusion in Ge (self- and dopant diffusion)
• Diffusion in Si stressed by Ge
• Diffusion in SiGe alloys
• SiGe isotope superlattices

29
Diffusion in Ge Self-Diffusion
Annealed 586 C for 55.55 hours
Ge self-diffusion coefficient determined from
74Ge/70Ge isotope structure
Fuchs, et al., Phys. Rev B 51 16817 (1995)
30
Si Diffusion in Ge
Annealed at 550 C for 30 days
H. H. Silvestri, H. Bracht, J. L. Hansen, A. N.
Larsen, and E. E. Haller, "Diffusion of silicon
in crystalline germanium," Semiconductor Science
and Technology, vol. 21, pp. 758-62, 2006.
31
B Diffusion in Ge

• Diffusion data for Boron in Ge are very limited
• Activation energy is very high for B diffusion in
  Ge
• Non-equilibrium transient effects appear to be
  small

32
As Diffusion in Ge

• As diffuses by vacancy mechanism
• Diffuses as singly negatively charged As-V pair

erfc fit
intrinsic regime concentration independent
diffusion
H. Bracht and S. Brotzmann, Materials Science in
Semiconductor Processing In Press, Corrected
Proof, 2006.
33
Self- and Dopant Diffusion in Ge
Impurity Publication Year
Cu 1991, 2004
H 1956
Ag 1991
Fe 1963
Au 1991
Sb 1967
As 2006
Zn 1997
P 1978
Al 1982
Ge 1985, 1995
Si 2006
B 2004
H. Bracht and S. Brotzmann, Materials Science in
Semiconductor Processing In Press, Corrected
Proof, 2006.
34
Effect of Strain on Diffusion in Si
P. R. Chidambaram, et al. IEEE Transactions on
Electron Devices, vol. 53, pp. 944-64, 2006.
Tensile
Compressive
35
Simultaneous Si and Ge Self-Diffusion in Si1-xGex
x 0.05
x 0.25
T 1100 C t 30 min
T 1050 C t 45min
28Si70Ge
28Si70Ge
36
Si and Ge Self-Diffusion in Si1-xGex
H. Bracht et al. unpublished, 2006
A. Strohm, et al. Physica B, vol. 308-310, pp.
542-545, 2001
37
Dopant Diffusion in Si1-xGex
(1996)
(2001)
(2003)
(2001)
(2003)
38
B Diffusion in Si1-xGex

• Boron diffusivity in strained as well as relaxed
  SiGe alloys decreases with increasing Ge content
• Unique among common group III or V dopants in
  SiGe
• Lever et al. suggest the formation of Ge-B pairs
  to explain the retardation
• Wang et al. and Delugas and Fiorentini report an
  increase in migration energy with increasing Ge
  content due to local strain
• True retardation mechanism still debatable

P. R. Chidambaram, et al. IEEE Transactions on
Electron Devices, vol. 53, pp. 944-64, 2006. and
references therein
39
As Diffusion in Si1-xGex
P. Laitinen, et al. Physical Review B-Condensed
Matter, vol. 68, pp. 155209-1-6, 2003
40
Conclusions

• Diffusion in semiconductors is increasingly
  important to device design as feature level size
  decreases.
• Self-diffusion coefficients and dopant
  diffusivities can be relatively easily obtained
  in Si, Ge, and SiGe, however, diffusion
  mechanisms are largely unknown.
• Diffusion using stable isotope multilayer
  structures will yield important diffusion
  parameters and diffusion mechanisms.
• The fundamental understanding of diffusion
  mechanisms will greatly help the device
  processing modeling and simulation

41
Future Work

• Use SiGe isotope multilayer structures to study
  simultaneous self- and intrinsic dopant diffusion
• Determine not only the diffusivities, but also
  the diffusion mechanisms
• Effect of strain will be studied by varying nat
  SiGe composition

x 0.05
42
Future Work Si1-xGex Multilayer Structures

• Five alternating 28Si1-x70Gex 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 with intrinsic and extrinsic dopant
  concentration

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