soft breakdown in ultra-thin gate...

Salvatore CiminoLegnaro, May 20, 2002

Salvatore Cimino

Dipartimento di Elettronica e InformaticaUniversità di Padova

via Gradenigo 6a, 35131 Padova, Italy

Soft Breakdown in UltraSoft Breakdown in Ultra--Thin Thin Gate OxidesGate Oxides


Outline• Introduction: radiation effects on thin oxide

– Radiation Induced Leakage Current (RILC) – Radiation Soft Breakdown (RSB)

• Experimental and devices

• Main RSB characteristics: – Random Telegraph Noise (RTN) in RSB current– Quantum Point Contact model for RSB

• Impact of Radiation Damage on Lifetime

• Conclusions and New perspectives


Work done in collaboration with:

- ST Microelectronics, via Olivetti 2, 20041 Agrate Brianza, Italy

- Laboratori Nazionali di Legnaro (LNL) INFN

Many thanks to:M. Ceschia, A. Cester, L. Bandiera (DEI)J. Wyss, A. Candelori, A. Kaminsky, D. Pantano (LNL)

Collaborations


Introduction 1/2

• Low LET (Linear Energy Transfer) ion irradiation can produce RILC in thin gate oxides (tox< 7 nm)

• High LET heavy ion irradiation can produce RSB in thin gate oxides (tox< 5 nm)

(1) M. Ceschia, et al., IEEE Trans. Nucl. Sci. (45) 1998. (2) M. Ceschia et al., Proc. of MRS (592) 2000(3) F.W. Sexton, et al., IEEE Trans. Nucl. Sci. (45) 1998.

Contemporary CMOS devices feature gate oxides with tox< 3 nm

After irradiation, thin gate oxides may show:

• Radiation Induced Leakage Current (RILC)(1)

• Radiation Soft Breakdown (RSB)(2)

• Single Event Gate Rupture (SEGR)(3)

Observed 1st time by M. Ceschia after irradiation at LNL


Introduction 2/2

When Hard Breakdownoccurs the gate current increases by 3-4 orders of magnitude with respect to the fresh curve

RILC is only slightly larger than the fresh current.

Radiation Soft Breakdownappears with a large increase of the gate current (more than 1-2 order of magnitude)

0 1 2 30.5 1.5 2.5

Vg [V]

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-12

I g[A

]

Hard Breakdown

Fresh

Radiation Soft Breakdown

107 I ioni/cm2

RILC 1011 Si ioni/cm2


RILC conduction mechanisms

• RILC is associated to a trap-assisted tunnelling through neutral defects generated by irradiation(1)

• Electron looses part of its energy during the tunnelling process (inelastic tunnel)

��

∆�

φ

��

Irradiation: γ rays, X-rays,8 MeV electrons, heavy ions with LET<10-20 MeV⋅cm2/mg (LNL)

(1) M. Ceschia, et al., IEEE Trans. Nucl. Sci. (45) 1998.


Radiation Soft Breakdown

The Radiation Soft Breakdown current flows through region with high trap concentration produced by a single ion hit(1)

RILC

Radiation Soft Breakdown Spot

Electron

Defects

SubstrateSubstrate

Oxi

deO

xide

GateGate

(1) M. Ceschia et al., Proc. of MRS (592) 2000


Devices: square MOS capacitors with- area = 10-2cm2 - 10-6cm2

- tox = 2.6nm - 7nm

Measurements: - Gate Current vs. Gate Voltage- Gate Current vs.Time

Radiation sources: Si, Ni, Br, Ag, I and Au ions(at Tandem accelerator, LNL INFN)

Devices and experimental


Random Telegraph Noise in RSB Current

8 9 10 11 1210

15

20

25

30

35

I g[n

A]

Time [s]

• area = 10-2 cm2

• tox = 4 nm

• Ion Fluence = 7·106 I ions/cm2

• measured at Vg= -2.7V

• sampling frequency FS = 4 kHz

RSB Current shows large high→low and low→hightransitions (RTN noise)

A small noise is superimposed to each current level.

A.Cester et al. IEEE-Trans. Nucl. Sci. (48) 2001


Quantum Point Contact for RSB

We assume :

• Electron transport in the oxide isballistic

• Electron energy is quantized alongpath transverse direction

trap in the oxide

do

z

y

Poly-Si SiOxide

z

Ey

do=RSB path minimum transverse dimension

E


current across the QPC path:

Quantum Point Contact analitic model

QPC Energy Diagram Band

( ) ( ) ( ) ( )( )[ ] dEVg�1EEfVg�EEfEThe2

VgIVg�

ff

2

QPC ⋅⋅−+−−⋅−−⋅⋅⋅= ∫∞

⋅

Ef

Efφ

E

(1-β)Vg

βVg

oxide

Si

poli-Si

A. Cester, et al., IEEE-IEDM 2001


��

• A single Soft Breakdown (SB) path is generated across the oxide after only 1800 s

• This SB path drives more current than the RSB leakage current path

CVS@Vgstress = -2V on fresh devices No SB appears

4000

20

15

10

50 1000 2000 3000

Time [s]

I g[n

A]

SB onset

tox=2.8 nmArea = 10-3 cm2

Fluence = 107 I ions/cm2

E = 257 MeV

After irradiation: Constant Voltage Stress (CVS)@Vgstress = -2V(Eox=2.3 MV/cm )


��

• 2nd CVS @ Vgstress= -3.5 V: Hard Breakdown in only 360 s

• CVS @ Vgstress= -3.5 V may produce Breakdown in these oxides (but over very long time periods )

The radiation induced wear out must be taken into account in evaluating the actual impact of ionising radiation on device lifetime

400

Time [s]

I g[n

A]

500

400

300

2000 100 200 300

HB onset

tox=2.8 nmArea = 10-3 cm2

Fluence =107 I ions/cm2

E = 257 MeV


Conclusions

• After irradiation, gate oxides with tox< 3nm show RSB

• RSB is due to localised weak spots

• The RTN is a peculiar characteristic of the RSB

• RSB limits the life-time of devices

• A good model for RSB is the QPC

• New perspectives : Influence of irradiation on MOSFET parameters

soft breakdown in ultra-thin gate...

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