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