dramatic reduction of gate leakage current of ultrathin

Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky

2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD

Dramatic Reduction of Gate Leakage Current of Ultrathin Oxides through Oxide Structure Modification

ZhiZhi ChenChen, Jun , Jun GuoGuo, and , and ChandanChandan B. B. SamantaraySamantaray

Department of Electrical and Computer Engineering Department of Electrical and Computer Engineering University of KentuckyUniversity of KentuckyLexington, KY 40506Lexington, KY 40506



OutlineOutline

•• Theory of Hydrogen/Deuterium Isotope Effect Theory of Hydrogen/Deuterium Isotope Effect

•• Experimental Evidence for Origin of Isotope EffectExperimental Evidence for Origin of Isotope Effect

•• Discovery of Phonon EnergyDiscovery of Phonon Energy--Coupling EnhancementCoupling Enhancement

•• Dramatic Improvement of Quality of Gate OxidesDramatic Improvement of Quality of Gate Oxides



e-e-

Si

H

H

SiSi

Si Si Si

Si

H

H

H

Si

SiSiSi Si

SiSi

SiO2

Sie- e-

Si

SiSi

Si

Si Si

Hot electr ons desorb hydr ogen, cr eating interface states which degrade device performance.

Hydrogen Passivated MOSFETHydrogen Passivated MOSFET

O

O

O O

O

O

O

O

O O

O

O

O Si Si

e-

SiO2SiO2 n+n+

p-Si substratep-Si substrate

Polysilicon Gate Deposited

Insulator

Metal Drain Contact

Metal Source Contact

Hot Electrons near Drain

n-n-



Van de Walle & Jackson Theory(Van de Walle et al., Appl. Phys. Lett. vol. 69, 2441 (1996))

Two competing processes:Two competing processes:

••Hot electron excitation causes Hot electron excitation causes SiSi--H/D bond breaking.H/D bond breaking.

••DeDe--excitation is due to energy coupling from excitation is due to energy coupling from SiSi--D to phonon. D to phonon.

Reason:Reason: νν ∝∝ 1/m based on1/m based on IR spectroscopy theoryIR spectroscopy theory

SiSi--H H vibrationalvibrational frequency frequency νν ~ 650 ~ 650 cm-1.SiSi--D D vibrationalvibrational frequency frequency ≈≈ 460 cm460 cm--11 ((Si-Si TO phonon mode)

DeDe--excitation is more efficient for excitation is more efficient for SiSi--D bonds than for D bonds than for SiSi--H bondsH bonds------This is why This is why SiSi--D bonds are stronger than D bonds are stronger than SiSi--H ones. H ones.



Schematic of Hydrogen/Deuterium Effect: Energy coupling from Si-D bending mode to Si-Si TO phonon mode

No coupling from Si-H bending mode to Si-Si TO phonon mode

OO

SiSi

SiSiSiSi

SiSi

SiSi

SiSi

SiSi

SiSiSiSi

SiSiOO

OO

OO

DD HHSiOSiO22

Energy CouplingEnergy Coupling

SiSi

No CouplingNo Coupling



Direct Measurement of the Vibrational Frequency of Si-H/D Bonds

No experimental data available for No experimental data available for SiSi--D D vibrationalvibrational frequency in the frequency in the SiOSiO22/Si interface/Si interface

Only in the deuterated amorphous Si (α-Si), the vibrationalfrequency (510 cm-1) was measured*. However, the chemical environment of the amorphous Si is very different from that of crystal Si.

*J.-H. Wei, M.-S. Sun, and S.-C. Lee, Appl. Phys. Lett. 71, 1498 (1997).



Origin of the Isotope Effect: No Energy Coupling from Si-H to Si-Si TO phonon

0.000

0.002

0.004

0.006

0.008

0.010

425475525575625675725775825875

Wavenumber (cm-1)

Abs

orba

nce

Si/oxide, hydrogen annealed

Si/oxide, unannealed

Si-OLO

Si-Si TOX4 point

Si-O TOrocking

Si-Si LO

Si-HBending

Si-O TObending

No difference between the H-annealed sample and the as-oxidized one, except for the Si-H bending vibration. (Chen et al. Appl. Phys. Lett. 83, 2151-2153, (2003))



Origin of the Isotope Effect: Energy Coupling from Si-D to Si-SiTO phonon & Si-O TO rocking mode (Chen et al. Appl. Phys. Lett. )

0 .0 0 0

0 .0 0 2

0 .0 0 4

0 .0 0 6

0 .0 0 8

0 .0 1 0

0 .0 1 2

4 1 54 6 55 1 55 6 56 1 56 6 57 1 57 6 58 1 58 6 5

W a v e n u m b e r (c m -1)

Abs

orba

nce

S i/o x id e , u n a n n e a le d

S i/o x id e , d e u te riu m

S i-O T Ob e n d in g

S i-O T Oro c k in g

S i-S i L O

S i-S i T OX 4 p o in t

S i-O L O

S i-D B e n d in g

The absorbance of the Si-Si TO phonon mode and the Si-O TO rocking mode are all enhanced significantly (>25%) after deuterium anneal. (Chen et al. Appl. Phys. Lett. 83, 2151-2153, (2003))



New finding: Energy is coupled from Si-D bending mode to Si-Si TO phonon mode and also to Si-O TO rocking mode

(Chen et al. Appl. Phys. Lett. 83, 2151-2153, (2003))

OO

SiSi

SiSiSiSi

SiSi

SiSi

SiSi

SiSi

SiSiSiSi

SiSiOO

OO

OO

DD HHSiOSiO22


SiSi

No CouplingNo Coupling




Challenge: How to further enhance the energy coupling?

Hypothesis:Hypothesis: Shift the Shift the SiSi--D D vibrationalvibrational mode toward mode toward SiSi--SiSi TO phonon mode.TO phonon mode.

Method 1: Mechanical stressjust a little shift (~6-8 cm-1)

Method 2: Electrical stress

Method 3: Thermal stress How?



Surprising Discovery: Phonon Energy-Coupling Enhancement

The absorbance of the Si-Si TO phonon mode,the Si-O TO rocking mode, and Si-Si LO mode are all enhanced significantly (>50%) after rapid thermal processing (RTP). There is further enhancement after deuterium annealing. Tox=23 nm.

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

415440465490515540565590615640Wavenumbers(cm-1)

Abs

orba

nce

1. Si/Oxide, Unannealed

2. Si/Oxide, RTP annealed

3. Si/Oxide, RTP plus D2annealed

Si-O TOrocking

Si-Si LO

Si-Si TOX4 point

Si-D Bending

1

2

3



The Enhancement is not due to the Surface Plasmon.It is well-known that the surface plasmon on the nanoscale metallic islands also produces strong surface-enhanced IR spectra. In order to avoid the metallic island-like surface, we used n- wafer (n=2×1014 cm-3 and ρ=20.8 Ω-cm) for experiments.

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

415435455475495515535555575595615635Wavenumbers(cm-1)

Abs

orba

nce 1. Si/Oxide, Unannealed

2. Si/Oxide, RTP annealed

1

2

Si-Si TOX4 point

Si-O TOrocking

Si-Si LO

Si-O LO



Dependence of Enhancement on the Oxide ThicknessFor thick oxide (Tox=80 nm), there is almost no enhancement except for the Si-Si LO mode after rapid thermal processing (RTP)---- implying stress-related phenomena. This also suggests that there should be no effect for the polysilicon/oxide stack.

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

410430450470490510530550570590610630650

Wavenumbers(cm-1)

Abs

orba

nce

Si/Oxide, UnannealedSi/Oxide, RTP annealed Si-O TO

rocking

Si-Si LO

Si-Si TOX4 point



Dependence of Enhancement on the Cooling Time

The enhancement is strongly dependent on the cooling time ----implying the stress-related phenomena.

0.0000.0020.0040.0060.0080.0100.0120.0140.0160.018

430450470490510530550570590610630650Wavenumbers(cm-1)

Abs

orba

nce

1. Si/Oxide, Unannealed

2. Si/Oxide, RTP annealed,cooling 10 minutes

3. Si/Oxide, RTP annealed,cooling 5 minutes

4. Si/Oxide, RTP annealed,cooling 20 seconds

1

2

3

4

Si-Si TOX4 point

Si-O TOrocking

Si-Si LOSi-O

LO



Phonon Energy-Coupling Enhancement: improvement of hot-electron degradation

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

100 101 102 103 104 105

L=2um & W=150 um, Stressed at Vg=4V and Vd=8V

tStress Time (s)

D2 anneal only

RTP and D2 anneal

200X

-10

0

10

20

30

40

50

100 101 102 103 104

D2 anneal only, stress Vg=4V Vd=8VRTP+D2 anneal, stress at Vg=9V Vd=18V

Time (s)

Z. Chen and J. Z. Chen and J. GuoGuo, presented at the 35th IEEE SISC, San Diego, CA, Dec. 9, presented at the 35th IEEE SISC, San Diego, CA, Dec. 9--11, 2004.11, 2004.



Hypothesis: Si-O bonds might be strengthened.This is because energy is also coupled from Si-O rocking mode to Si-Si TO

phonon mode and also to Si-D bending mode

OO

SiSi

SiSiSiSi

SiSi

SiSi

SiSi

SiSi

SiSiSiSi

SiSiOO

OO

OO

DDSiOSiO22


SiSi




Hydrogen/Deuterium Effect on Gate Oxide: No Effect

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

0 2 4 6 8 10 12

D2 anneal

D2 anneal

D2 anneal

H2 anneal

H2 anneal

Gate Voltage (V)

10-3

10-4

10-5

10-6

10-7

10-8

10-2

Tox

=10.2nm



Direct Rapid Thermal Processing Only: Improvement of Breakdown Voltage (15%) and Reduction of Leakage Current (10X)

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

0 2 4 6 8 10 12 14

Gate Voltage (V)

H2 anneal only

RTP+H2 anneal

10-3

10-4

10-5

10-6

10-7

10-8

10-2

Tox

=11.1nm



Direct Rapid Thermal Processing Plus D2 Annaeal: Improvement of Breakdown Voltage (30%) and Reduction of Leakage Current (100X)

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

0 5 10 15

Gate Voltage (V)

D2 anneal only

RTP+D2 anneal

10-3

10-4

10-5

10-6

10-7

10-8

10-2

Tox

=10.2nm



Direct Rapid Thermal Processing Plus D2 Anneal: Improvement of Breakdown Voltage (30%) and Reduction of Leakage Current (100X)

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

0 1 2 3 4 5 6

Gate Voltage (V)

D2 anneal only

RTP+D2 anneal

10-3

10-4

10-5

10-6

10-7

10-8

10-2 Tox

=3.7nm



Direct Rapid Thermal Processing Plus D2 Anneal of Thin Oxides: Reduction of leakage current (105X)

This is similar to that of HfSiON (Gusev et al., IEDM Technical Digest, 451-454 (2001))

10-10

10-8

10-6

10-4

10-2

100

0 0.2 0.4 0.6 0.8 1 1.2

Control sample: D

2 anneal (30 min)

RTP (1 min) plusD

2 anneal (30 min)

105 X

Tox

=2.2nm

Gate Voltage (V)



Capacitance-Voltage Curves and oxide thickness of oxide measured before and after RTP

-2 -1 0 1 2

Control-1MHzRTP1min-1MHz-S1RTP1min-1MHz-S2RTP1min-1MHz-S3

350

300

200

150

100

50

0

Tox

=3.0 nm

250

Gate Voltage (V)

Table: Comparison of oxide thickness measured Table: Comparison of oxide thickness measured using using ellipsometryellipsometry before and after RTPbefore and after RTP

There is only a slight flatThere is only a slight flat--band voltage shift and thickness remains unchanged after RTP.band voltage shift and thickness remains unchanged after RTP.

Sample No.

Oxidation Parameters

Tox before RTP

Tox after RTP

#1110051

N2 @1000sccm, O2 @20 sccm, 9000C for 20 s

22.4 Å

23.3 Å

#0628052 N2 @1000sccm,

O2 @20 sccm, 9000C for 10 s

19.8 Å

20.9 Å

#1110052 N2 @2000sccm, O2 @20 sccm, 9000C for 20 s

19.5 Å

20.09Å



SummarySummary

•• We discovered a new effect, phonon energyWe discovered a new effect, phonon energy--coupling coupling enhancement, i.e. the energy coupling from the enhancement, i.e. the energy coupling from the SiSi--D bond to the D bond to the SiSi--SiSi TO mode and the TO mode and the SiSi--O rocking mode is dramatically O rocking mode is dramatically enhanced after the RTP processing directly on the oxide.enhanced after the RTP processing directly on the oxide.

• In addition to strengthening Si-D bonds, Si-O bonds are also strengthened. The breakdown voltage of oxides after RTP processing is improved by 30%.

• The leakage current of thin oxide (2.2 nm) after direct RTP processing is reduced by 105 times, similar to that of high-k oxides.



AcknowledgementsAcknowledgements

•• This research is supported by National Science Foundation This research is supported by National Science Foundation ECS-0093156 and EPS- 0447479.

dramatic reduction of gate leakage current of ultrathin

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