Common origin for enhanced low-dose-rate sensitivity and bias-temperature instability
under negative bias
Leonidas Tsetseris1
in collaboration with
R. D. Schrimpf1, D. M. Fleetwood1, R. L. Pease2, S. T. Pantelides1,3
1Vanderbilt University2RLP Research Inc.
3Oak Ridge National Lab
Work supported by AFOSR
Outline
Enhanced low-dose-rate sensitivity (ELDRS)
ELDRS under negative bias
Degradation under negative bias (NBTI)
Common origin for NBTI and ELDRS
Role of trapping, release, and reactions of hydrogen
Implications and conclusions
Previous models for zero or positive gate bias:
ELDRS
Degradation of many bipolar technologies enhanced at low dose-rates
Bimolecular mechanism model
(Hjalmarson et al., IEEE TNS’02)
Migration-reaction of hydrogen on the oxide side of
the Si-SiO2 interface
Space charge model
(Fleetwood et al., IEEE TNS’96, Rashkeev et al., IEEE TNS’02)
Density of interface traps in a gated LPNP transistor after irradiation at high (H) and low (L) dose-rates under gate biases 0, -50, and -100 v.
ELDRS observed under negative bias Vg
ELDRS: a new regime
Degradation under negative Vg:
Negative Bias Temperature Instability (NBTI)
I: Reaction-limited phase; II: Diffusion-limited phase
Typical NBTI conditions:
Oxide Electric field: 2-6 MV/cm
Temperature T: 75-200 ºC
Increase of interface traps-
oxide trapped charge at stress
time ts
Under negative bias protons in the oxide move
away from the interface
ELDRS-NBTI
Are there additional mechanisms for interface trap formation?
Is degradation at negative bias related to NBTI?
Why is there a dose-rate dependence for Vg < 0?
What are the effects of annealing, device history, polarity?
METHODOLOGY
First-principles (density-functional) calculations
Pseudopotentials, plane-wave basis, supercells
Study plausible and competing mechanisms
Study of charged states (trapping of carriers)
A
BEa
E
Reactions: A → B
E : Reaction Energy
Ea: Reaction Barrier
Barriers ↔ Activation Energy
NBTI: A (mostly!) HYDROGEN STORY
Interface traps: Si dangling bond (D)
Hydrogen
Passivates-Depassivates:
Si-H + H ↔ D + H2
D + H ↔ Si-H
= H
= Si
= O
REACTION (depassivation)
= H
= Si
= O
E ~ 0.5 eV Ea ~ 1 eV
SOURCE (of Hydrogen)
Dissociation of P-H complexes
A
B
E ≈ 0.2 eV
A
B
Ea ≈ 0.35 eV
With holes
E ≈ 0.6 eV
A
B
Ea ≈ 1.2 eV
No holes
Confirm experiments (Herring & Johnson, 1992, and others)
30%-40% PH complexes, Seager & Anderson, Sol. State Comm. ‘90
Dopant H charge state D-H binding energy (eV)
P H- 0.65
P H0 0.23
As H- 0.78
As H0 0.38
Sb H- 0.78
Sb H0 0.45
Minority carriers promote H release:
D+H- + hole → D+ + H0
Reduced binding energy + reduced diffusion barrier → enhanced dissociation for H0 from dopants
Release of H from dopants
Dose-rate dependence of degradation
Role of “bimolecular” reaction (Hjalmarson et al., TNS’02): H0 + H0 → H2
Sublinear dependenceSublinear dependence of concentration of Pb on dose rate g (ti: irradiation time):
HIGH DOSE-RATE EFFECT SUPPRESSED
C t Bg
rtP i
Hib
( ) ,
1 1
2
C t Bg r
DP iH
ib( ) ,
2
Steady-state condition for H: kg – kHH C2 – rH C = 0
H release rate, g dose rate Trapping of H
Depassivation: H+ + SiH → Pb + H2
Trapping site for hydrogen: oxygen clusters
Chain
Charge 0
Charge +
Charge ++
1.15 eV
0.9 eV
0.2 eV
0.46 eV
Ring
L. Tsetseris, Sanwu Wang, and S. T. Pantelides, APL’06
Oxygen
Silicon
Thermal donor
Trapping site for hydrogen: oxygen clusters
Oxygen
Silicon
Hydrogen
L. Tsetseris, Sanwu Wang, and S. T. Pantelides, APL’06
Efficient trapping with a release barrier of ~ 1.8 eV
CONCLUSIONS-IMPLICATIONS
ELDRS is possible for negative gate bias
H released in Si, creates interface traps
Common origin with NBTI
Degradation suppressed at high dose-rate
Interface roughness may play a role (MOSFET vs bipolar)
Dopant profile
Other trapping sites
L. Tsetseris et al., IEEE TNS’06