Direct Experimental Evidence Linking
Silicon Dangling Bond Defects to Oxide
Leakage Currents
P.M. Lenahan, J.J. Mele, A.Y. Kang, J. P. Campbell
Penn State University,
University Park, PA 16802
S.T. Liu
Honeywell Corp.
Plymouth, MN 55441
R.K. Lowry and D. Woodbury
Intersil Corp.
Melbourne, FL 32902
R. Weimer, Micron Technologies
Boise, Idaho 83707-0006
1
Ec
Stress Induced Leakage (SILC)
Trap
CB Edge
EF
VB Edge
CB Edge
Ev
Si
SiO2
Inelastic tunneling
of silicon
conduction band
electrons through
oxide defects near
the Si/SiO2
boundary.
VB Edge
S.Takagi, et al. Trans.Electron.Dev 46, 348 (1999)
E.Rosenbaum and L.F.Register, IEEE Trans.Electron.Dev.
2
44, 317(1997)
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)
3
4
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.
5
Test E’ hypothesis with neutral E’
Ec
Ec
centers
Ec
Ec
Ev
Ev
Ev
Si
Ev
SiO2
++++++++
++++++++
++++++++
++++++++
++++++++
++++++++
Si
SiO2
(Any net space charge near the Si/SiO2 boundary will
decrease the tunneling barrier and increase oxide current for
6
any gate potential.)
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.)
7
Oxides Utilized in the Study:
3.3nm (forming gas)
3.3nm (no forming gas)
45 nm (forming gas)
(all thermally grown)
8
9
Arbitrary Units
3.3nm Oxide (forming gas)
As Processed
Post VUV
Post Anneal
Pb0
3440
3445
3450
3455
E’
3460
3465
3470
3475
3480
Magnetic Field (Gauss)
3485
10
2
Current Density (nA/cm )
I-V Characteristics of 3.3nm Oxide (forming gas)
12.00
As Processed
90mVUV
90mAnneal
10.00
8.00
6.00
4.00
2.00
0.00
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
2.3
Voltage
11
E’ and Leakage Current Generation 3.3nm oxide (forming gas)
E' Center Density
(1011/cm 2)
6.00
4.00
2.00
0.00
Current Density
(nA/cm 2)
0
20
40
60
80
VUV Illum ination Tim e (m in)
100
15
10
5
0
0
20
40
60
80
100
VUV Illum ination (m inutes)
12
E' Center Densities
(1011/cm2)
E’ and Leakage Current Anneal 3.3nm oxide (forming gas)
6.00
4.00
2.00
0.00
0
20
40
60
80
100
Current Density
(nA/cm 2)
Annealing Time (minutes)
15
10
5
0
0
20
40
60
80
100
Anneal Tim e (m inutes)
13
ESR Amplitude (Arb. Units)
3.3nm Oxide (no forming gas)
3440
90 MIN ANNEAL
90 MIN VUV
Virgin
3450
Pb0
E’
3460
3470
Magnetic Field (G)
3480
14
I-V Characteristics of 3.3nm Oxide (no forming gas)
15
E' Densities (1011/cm2)
E’ and Leakage Current Generation 3.3nm oxide (no forming gas)
6
4
2
0
0
20
40
60
80
VUV Illumination Time (minutes)
100
Current Density
(nA)
10
8
6
4
2
0
0
20
40
60
VUV Illumination (minutes)
80
100
16
E' Density (1011/cm2)
E’ and Leakage Current Anneal 3.3nm oxide (no forming gas)
6
4
2
0
0
20
40
60
80
100
80
100
Current Density (nA)
Annealing Time (minutes)
10
8
6
4
2
0
0
20
40
60
Anneal Time (minutes)
17
45nm Oxides
Arbitrary Units
As Processed
Post VUV
Post VUV and Anneal
Pbo (g=2.0058)
E` [g(z.c.)=2.0005]
3440.5 3445.5 3450.5 3455.5 3460.5 3465.5 3470.5 3475.5 3480.5
Magnetic Field (G)
18
45nm Oxide
Current Density (Amps / cm2)
1.0E-07
10-8
1.0E-08
Post VUV
As
Processed
-9
10
1.0E-09
1.0E-10
10-10
Post VUV
and
Anneal
1.0E-11
10-11
0
10
20
Voltage (V)
30
19
Current Density
(10-8A/cm 2)
E’ and Leakage Current Generation 3.3nm oxide (forming gas)
2
1.5
1
0.5
0
0
50
100
150
VUV Illum ination Tim e (m in)
20
20
E’ and Leakage Current Anneal 45nm oxide (forming gas)
E' Density
1012/cm2
1.5
1
0.5
0
0
20
40
60
80
Current Density
(10-8 A / cm2)
Anneal Time (min)
300
200
100
0
0
20
40
60
Anneal Time (min)
80
21
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.
22
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, probably
dominating defects in SILC (and RILC) in a wide
range of oxides on silicon.
23
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?
24
Before Stressing
Oxide
Si
Si
Si/SiO2
Si
Si
Si
Si
Si
Si
H
H
H
H
Si
Si
Si
Si
After Stressing
Oxide
Si
Si/SiO2
Si
Si
Si
H
H
H
H
Si
Si
Si
Si
25
Consider Statistical Mechanics
The system will approach the lowest Gibbs Free
Energy:
G = H-TS
O
O
O
PbH
E’
Si
Oxide
O
O
E’H
Si H
O
Pb
H
Si
Si/SiO2
Si
Si
Si
h 0
Si
Si
Si
Si
(The oxide and interface Si-H bond
enthalpies will be about equal)
26
Entropy: S = k ln (Ω)
Suppose all E’ dbs are unpassivated (Total of N sites)
O
O
O
O
Si
O
O
O
Si
…
Si
O
O
Contribution to configurational entropy of N E’ sites:
Suppose all Pb dbs are passivated
S = k ln(1)
(Total of M sites)
H
H
H
H
Si
Si
Si
Si
…
Contribution to configurational entropy of M PbH sites: S = k ln(1)
27
Suppose we remove one H from the M PbH sites; the
configurational entropy changes: ∆S = k ln (M)
H
H
Si
Si
H
Si
Si
Suppose we add one H to the N E’ sites; the
configurational entropy changes: ∆S = k ln (N)
O
O
O
Si
O
O
O
Si H
O
O
Si
…
…
O
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) 28
The process will occur to some extent.
To how great an extent?
PbH + E’ + H2
Pb + E’H + H2
[Pb] [E’H]
= exp ( - ∆G / kT) = K ~ 1
[PbH] [E’]
29