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Diffusion in Extrinsic Silicon

SFR Workshop & ReviewApril 17, 2002

Hughes Silvestri, Ian Sharp, Hartmut Bracht, and Eugene Haller

Berkeley, CA

2002 GOAL: Diffusion measurements on P doped Si to complete comprehensive model of Si diffusion by 9/30/2002.

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Motivation• Reduction of junction depth

in CMOS technology

• To maintain shallow profile:– Must control diffusion during further annealing steps– Understand the Fermi level effect on diffusion and its impact on native

defect formation

• Development of a comprehensive, predictive diffusion model that incorporates physically justified parameters

(P.A. Packan, MRS Bulletin, June 2000)

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Our Approach• Use of an isotopically enriched Si heterostructure allows

observation of Si and dopant diffusion simultaneously.

• Determination of dopant diffusion mechanism and charge state of point defects involved.

• Determine contribution of charged native defects to Si self-diffusion.

DSi = f Io CIo DIo + fI + CI+ DI+ + fV− CV − DV − +Kƒ = correlation factor

ƒv = 0.5, ƒI = 0.73

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Extrinsic p-type Extrinsic n-type

Fermi Level EffectHeavily 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 lowers the formation enthalpy, HF, of the charged native defect

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

( )−− −−=

−

=

VFF

VFV

B

FIV

B

FIVo

Sieq

IV EEHHTk

HkS

CC o,expexp ,,,

Dopant charge state Native defect charge state extrinsic n-type As

+ I -, V - extrinsic p-type As

- I +, V +

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Approach: Isotopically enriched Si heterostructure• Alternating layers of natural Si (92.2% 28Si, 4.7% 29Si, 3.1% 30Si) and 28Si (99.95% 28Si)

(UHV-CVD grown at Lawrence Semiconductor Research Laboratory, Tempe, AZ)

• Ion implanted amorphous natural Si cap layer used as dopant source to suppress transient enhanced diffusion (TED) (MBE grown at University of Aarhus, Denmark)

• (

• Stable isotope structure allows for simultaneous Si and dopant diffusion studies

120 nm 28Si enriched

FZ Si substrate

120 nmnatural Si

a-Si cap 260 nm

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Extrinsic p-type: B diffusion experiments

• Secondary Ion Mass Spectrometry (SIMS) used to determine concentration profiles of 11B, 28Si, and 30Si (Accurel Systems, CA)

• Computer modeling of 11B and 30Si concentration profiles and comparison to experimental results

Boron dopant source:

Boron ion implantation in amorphous Sicap layer:

30 keV, 0.7x1016 cm-2

37 keV, 1.0x1016 cm-2

Diffusion: samples sealed in silica ampoules and annealed between 850 oCand 1100 oC

11B30Si

30Si depleted layers

Natural Si layers

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Extrinsic p-type: B diffusion results

Si diffusion under intrinsic

conditions

30Si SIMSdata

30Si modelfit

11B SIMSdata

11B modelfit• B and Si diffusion are described

simultaneously• Diffusion mechanism (kick-out)

– Bi0ó Bs

- + I 0 + h

– Bi0ó Bs

- + I +

• Interstitialcy: (BI)0 ó Bs- + I 0,+

not very likely (f[BI] < 0.3 )

1000 oC for 4hr 55min

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Extrinsic p-type: B simulation results• Supersaturation of I0 and I+ due to

B diffusion

• I0 and I+ mediate Si and B diffusion

• Enhancement of DB(p) over DB(ni) under p-type doping

• Enhancement of Dsi(p) over Dsi(ni) due to Fermi level effect

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Extrinsic n-type: As diffusion results• Implant As into amorphous layer: 0.7 × 1016 cm-2 at 130 keV, 1.0 × 1016 cm-2 at 160 keV

• Anneal: 950 °C < T < 1100 °C• Secondary Ion Mass Spectrometry (SIMS)

10 17

10 18

10 19

10 20

10 21

0 200 400 600 800 1000 1200 1400 1600

conc

entr

atio

n (c

m-3

)

Depth (nm)

V2-

V0

V-

Arsenic sample annealed 950 °C for 122 hrs

Reactions for simulation:

AsV( )o ↔ Ass+ + V−

AsI( )o ↔ Ass+ + Io + e−

Vacancy mechanism

Interstitialcy mechanism

V- and Io control Si self-diffusion under n-type extrinsic doping.

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Extrinsic n-type: As simulation results

No supersaturation of Io or V-

during As diffusion

0.1

1

10

0 1000 2000

Cx/C

eq

Depth (nm)

V - Io

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)

Enhancement of As and Si self-diffusion under extrinsic conditions entirely due to Fermi level effect.

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Native defect contributions to Si self-diffusion

10 -19

10 -18

10 -17

10 -16

10 -15

10 -14

0.7 0.8 0.9

D (c

m2s-1

)

1000/T (1000/K)

DSi

(V-)

DSi

(I+)

DSi

(Io)

DSi

(Io+I++V-)

DSi

(ni)

Intrinsic components calculated from extrinsic data

Diffusion coefficients of individual components add up accurately:DSi(ni )tot = fIo CI o DI o + fI+ CI + DI+ + fV − CV− DV− = DSi(ni )

(Bracht, et al., 1998)

(B diffusion) (As diffusion)(As diffusion)

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Current / Future Developments• Isotope heterostructure allows for simultaneous analysis of Si self-diffusion

and dopant diffusion.• Boron diffusion via the kick-out mechanism: Bi

0 ⇔ Bs- + I 0,+

• As diffusion via interstitialcy and vacancy mechanisms:

• Able to determine contributions of various native defect charge states to intrinsic Si self-diffusion.

• Activation enthalpy for self-diffusion due to Io, I+, V-:

• Diffusion measurements on P doped Si to complete comprehensive model of Si diffusion by 9/30/2002.

• Apply technique to other materials systems (SiGe), by 9/30/2003.

2002 and 2003 Goals:

AsV( )o ↔ Ass+ + V−AsI( )o ↔ Ass

+ + Io + e−

DSi(I 0): Q=(4.32 ± 0.15) eV, DSi(I +): Q=(4.74 ± 0.13) eV, DSi(V -): Q=(5.18 ± 0.13) eV