direct experimental evidence linking silicon dangling bond defects to oxide leakage currents

1

Direct Experimental Evidence Linking Silicon Dangling Bond Defects to Oxide

Leakage Currents

P.M. Lenahan, J.J. Mele, A.Y. Kang, J. P. CampbellPenn State University,

University Park, PA 16802 

S.T. LiuHoneywell Corp.

Plymouth, MN 55441 

R.K. Lowry and D. WoodburyIntersil Corp.

Melbourne, FL 32902

R. Weimer, Micron TechnologiesBoise, Idaho 83707-0006

2

Ec

Ev

VB Edge

CB Edge

Si SiO2

CB Edge

VB Edge

EF Inelastic tunneling of silicon

conduction band electrons through oxide defects near

the Si/SiO2 boundary.

Trap

Stress Induced Leakage (SILC)

S.Takagi, et al. Trans.Electron.Dev 46, 348 (1999)E.Rosenbaum and L.F.Register, IEEE Trans.Electron.Dev. 44, 317(1997)

3

Literature suggests oxygen vacancy centers (E’ centers):

•J.H.Suehle, et al. IRPS (1994)

•J.H.McPherson and H.Mogul, J.Appl.Phys, 84, 1513 (1998)

•B.Schlund, et al. IRPS (1996)

•D.J.Dumin and J.Maddux, IEEE Trans.Electron.Dev., 40, 886 (1993)

•S.Takagi, et al. IEEE Trans.Electron.Dev., 46, 348 (1999)

•A.Yokozawa, et al. IEDM (1997)

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5

At least two independent studies indicate that E’ centers are generated when oxides are subjected to high electric fields. ** So, if we could link E’ center density to leakage current, we could establish an important role for the centers in oxide leakage phenomena.

* W. L. Warren and P.M. Lenahan, JAP 62, 4305 (1987), IEEE Trans Nucl. Sci 34, 1355 (1987)* H. Hazama, et al. Proceedings of the Workshop on Ultra Thin Oxides Jap.Soc. Appl. Phys. Tokyo 1998. Cat. No. Ap 982204 pp201-212.

6

Ec

Ev

Si SiO2

+ + + + + + + ++ + + + + + + ++ + + + + + + ++ + + + + + + ++ + + + + + + ++ + + + + + + +

Ev

Ec

Si SiO2

Ev

Ev

Ec

Ec

(Any net space charge near the Si/SiO2 boundary will decrease the tunneling barrier and increase oxide current for any gate potential.)

Test E’ hypothesis with neutral E’ centers

7

Approach: (a) Generate neutral E’ centers in a wide variety of oxides, annihilate the E’ centers by various means.

(b) Compare generation and annihilation of the E’ centers with generation and annihilation of oxide leakage current. (Are they strongly correlated?)

(c) Compare experimental results and “theoretical work on inelastic tunneling and SILC. (Are the defect densities “reasonable” in terms of the (very crude) theory available.)

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Oxides Utilized in the Study:  

3.3nm (forming gas) 3.3nm (no forming gas) 45 nm (forming gas) 

(all thermally grown)

9

10

3.3nm Oxide (forming gas)

3440 3445 3450 3455 3460 3465 3470 3475 3480 3485

Magnetic Field (Gauss)

Arb

itra

ry U

nit

s

As Processed

Post VUV

Post Anneal

Pb0 E’

11

I-V Characteristics of 3.3nm Oxide (forming gas)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3

Voltage

Cu

rre

nt

De

ns

ity

(n

A/c

m2 )

As Processed

90mVUV

90mAnneal

12

0.00

2.00

4.00

6.00

0 20 40 60 80 100VUV Illumination Time (min)

E' C

en

ter

De

ns

ity

(101

1 /cm

2)

0

5

10

15

0 20 40 60 80 100

VUV Illumination (minutes)

Cu

rre

nt

De

ns

ity

(nA

/cm

2)

E’ and Leakage Current Generation 3.3nm oxide (forming gas)

13

0

5

10

15

0 20 40 60 80 100

Anneal Time (minutes)

Cu

rre

nt

De

ns

ity

(nA

/cm

2)

0.00

2.00

4.00

6.00

0 20 40 60 80 100

Annealing Time (minutes)

E' C

en

ter

De

nsi

tie

s

(101

1 /cm

2 )

E’ and Leakage Current Anneal 3.3nm oxide (forming gas)

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3.3nm Oxide (no forming gas)

3440 3450 3460 3470 3480Magnetic Field (G)

ES

R A

mp

litu

de

(A

rb. U

nit

s)

90 MIN ANNEAL

90 MIN VUV

Virgin

Pb0 E’

14

15

I-V Characteristics of 3.3nm Oxide (no forming gas)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3

Voltage (V)

As P rocessed

90mVUV

90mAnneal

I-V Characteristics of 3.3nm Oxide (no forming gas)

16

0

2

4

6

0 20 40 60 80 100VUV Illumination Time (minutes)

E' D

en

sit

ies

(1

011

/cm

2 )

0

2

4

6

8

10

0 20 40 60 80 100

VUV Illumination (minutes)

Cu

rre

nt

De

ns

ity

(nA

)

E’ and Leakage Current Generation 3.3nm oxide (no forming gas)

17

0

2

4

6

0 20 40 60 80 100

Annealing Time (minutes)

E' D

ensi

ty (

1011

/cm

2 )

E’ and Leakage Current Anneal 3.3nm oxide (no forming gas)

0

2

4

6

8

10

0 20 40 60 80 100

Anneal Time (minutes)

Cu

rren

t D

en

sity

(nA

)

18

3440.5 3445.5 3450.5 3455.5 3460.5 3465.5 3470.5 3475.5 3480.5

Magnetic Field (G)

Arb

itra

ry U

nit

s

Pbo (g=2.0058) E` [g(z.c.)=2.0005]

As Processed

Post VUV

Post VUV and Anneal

45nm Oxides

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45nm Oxide

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

0 10 20 30Voltage (V)

Cu

rren

t D

ensi

ty (

Am

ps

/ cm

2)

10-8

10-9

10-10

10-11

Post VUV

Post VUV and

Anneal

AsProcessed

20

00.5

11.5

2

0 50 100 150VUV Illumination Time (min)C

urr

en

t D

en

sit

y(1

0-8A

/cm

2)

0

0.5

1

1.5

0 50 100 150VUV Illumination Time (min)

E

De

ns

ity

101

2 /cm

2

E’ and Leakage Current Generation 3.3nm oxide (forming gas)

20

21

0

100

200

300

0 20 40 60 80Anneal Time (min)

Cu

rren

t D

ensi

ty

(10

-8 A

/ cm

2)

0

0.5

1

1.5

0 20 40 60 80Anneal Time (min)

E' D

ensi

ty

101

2/c

m2

E’ and Leakage Current Anneal 45nm oxide (forming gas)

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Conclusions

In several quite different oxides we find that:

(1) Generation of E’ centers is accompanied by generation of oxide leakage currents.

(2) A brief 200oC anneal in air annihilates most of the E’ centers and most of the leakage current.

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Since (A) earlier work by two independent groups show that E’ centers are generated by high field stressing oxides,

And (B) recent theoretical and experimental studies indicate that E’ centers are good candidates for leakage current defects, we conclude that E’ centers are important, probablydominating defects in SILC (and RILC) in a widerange of oxides on silicon.

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Many studies report generation of interface states in conjunction with the creation of stress induced leakage currents.

Several studies also report a strong correlation between SILC and interface state generation.

Why is this so?

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Before Stressing

Si

H

After Stressing

Si Si Si Si Si Si Si

Si

H

Si

H

Si

H

Si

H

Si

H

Si

H

Si

H

Si

Si Si Si Si

Oxide

Oxide

Si/SiO2

Si/SiO2

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Consider Statistical MechanicsThe system will approach the lowest Gibbs Free Energy:

G = H-TS

Si

O

O

O

E’

Oxide

Si

O

O

O

E’HH

H

Si

SiSi

Si

Si

SiSi

Si

Si/SiO2

PbH Pb

h 0 (The oxide and interface Si-H bond enthalpies will be about equal)

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Entropy: S = k ln (Ω)Suppose all E’ dbs are unpassivated

OO

O

OO

O

OO

O

Contribution to configurational entropy of N E’ sites: S = k ln(1)

Suppose all Pb dbs are passivated

H

Si

H

Si

H

Si

H

Si

Contribution to configurational entropy of M PbH sites: S = k ln(1)

(Total of N sites)

(Total of M sites)

Si Si Si …

…

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Suppose we remove one H from the M PbH sites; the configurational entropy changes: ∆S = k ln (M)

H

Si

H

Si Si

H

Si

Suppose we add one H to the N E’ sites; the configurational entropy changes: ∆S = k ln (N)

OO

O

OO

O

OO

O

H

The Gibbs free energy of the system will be lowered by the transfer of some hydrogen from Pb sites to E’ sites.

(If kinetics allows it)

…

…Si Si Si

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The process will occur to some extent.

To how great an extent?

PbH + E’ + H2 Pb + E’H + H2

[Pb] [E’H]

[PbH] [E’]= exp ( - ∆G / kT) = K ~ 1