1

2007

Etienne Nowak et al. – SSDM 2009

Charge Localization During Program and Retention in NROM-like Non Volatile

Memory Devices

Etienne Nowak, Elisa Vianello*, Luca Perniola, Marc Bocquet, Gabriel Molas, Rabah Kies, Marc Gely, Gerard Ghibaudo+, Barbara De Salvo, Gilles Reimbold, Fabien

BoulangerCEA/LETI-Minatec, 38054 Grenoble, France

* DIEGM, University of Udine, Italy +IMEP/INPG Grenoble, France

etienne.nowak@cea.fr

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Etienne Nowak et al. – SSDM 2009

Outline

�Motivation

�Methodology

�Program operation

�Retention

�Conclusion

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2007

Etienne Nowak et al. – SSDM 2009

Outline

�Motivation

�Methodology

�Program operation

�Retention

�Conclusion

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Etienne Nowak et al. – SSDM 2009

Motivation (1/2)

e-

0V 4.5V

12V

h+

-14V

7V0V

�Purpose of the work:Extract information on pocket of trapped charges in alternative trapping materials for NROM devices

NROM

Write (CHE) Erase (HHI) 4bit/cell 8Gbit productR.Sahar et al, ISSCC 2008

�Benefit:Higher information density thanks to physically separated bits

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Etienne Nowak et al. – SSDM 2009

�Retention of cycled and uncycledSi3N4 devices has been well studiedM. Janai et al., IEEE IRPS Tech. Dig., 2008, pp417-4 23

�Few works have been done on different trapping layers T.Sugizaki et al., VLSI Tech Dig., 2003, pp.27-28.

� Intrinsic trapping properties of Si 3N4, HfO2, Al2O3 still not well understood� Maximum amount of trapped charge� Localization of the trapped charge � ∆∆∆∆Vt loss mechanisms on different material

Motivation (2/2)

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Etienne Nowak et al. – SSDM 2009

Outline

�Motivation

�Methodology

�Program operation

�Retention

�Conclusion

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Etienne Nowak et al. – SSDM 2009

Devices under analysis

�Three different trapping layers are comparedLPCVD Si3N4 / ALCVD HfO 2 / ALCVD Al 2O3

HTO

Si3N4

SiO2

10nm

6nm

5nm

N+ Poly

HTO

HfO2

SiO2

10nm

6nm

5nm

HTO

Al 2O3

SiO2

10nm

6nm

5nm

Blockingoxide

Trappinglayer

Tunneloxide

ControlGateN+ Poly N+ Poly

W/L=10/0.27 µm

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Etienne Nowak et al. – SSDM 2009

VS VD

VG

LLcharged

QchargedSiO2

Si3N4/HfO2/Al2O3

SiO2

xyVS VD

VG

LLcharged

QchargedSiO2

Si3N4/HfO2/Al2O3

SiO2

xy

Method to extract trapped charges information

1 - Measure ∆VtR and ∆VtF from the experimental results

L. Perniola et al., IEEE TNANO, 2005

0 2 4 6 8 101E-14

1E-12

1E-10

1E-8

1E-6

1E-4

∆∆∆∆VtF

∆∆∆∆VtR

Virgin Written V

S=1.5V Reverse Read

VD=1.5V Forward Read

Sou

rce

Cur

rent

I S

[A/u

m]

Gate Voltage VG [V]

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Etienne Nowak et al. – SSDM 2009

Method to extract trapped charges information

2 - Extrapolate the values of Lcharged and Qcharged from an analytical map calculated through the ψS approach

L. Perniola et al., IEEE TNANO, 2005

40 60 80 100 120 140

Charged Length L2 [nm]Effective charged Length Lcharged [nm]Cha

rge

Den

sity

Qch

arge

d[1

012

cm-2

]

40 60 80 100 120 140

Charged Length L2 [nm]Effective charged Length Lcharged [nm]Cha

rge

Den

sity

Qch

arge

d[1

012

cm-2

]

VS VD

VG

LLcharged

QchargedSiO2

Si3N4/HfO2/Al 2O3

SiO2

xyVS VD

VG

LLcharged

QchargedSiO2

Si3N4/HfO2/Al 2O3

SiO2

xy

∆VtR

∆VtF

0 2 4 6 8 101E-14

1E-12

1E-10

1E-8

1E-6

1E-4

∆∆∆∆VtF

∆∆∆∆VtR

Virgin Written V

S=1.5V Reverse Read

VD=1.5V Forward Read

Sou

rce

Cur

rent

I S

[A/u

m]

Gate Voltage VG [V]

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Etienne Nowak et al. – SSDM 2009

Outline

�Motivation

�Methodology

�Program operation

�Retention

�Conclusion

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Etienne Nowak et al. – SSDM 2009

Program

�Programming windows over 10 V for the 3 materials

10-6 10-4 10-2 100

0

2

4

6

8

10

12

10-6 10-4 10-2 100

0

2

4

6

8

10

12

Pro

gram

min

g W

indo

w

∆∆ ∆∆VtR

[V] HfO2 Al2O3 Si3N4

Stress Time t [s]

StressV

S=V

B=0V

VG=10V

VD=5V

StressV

S=V

B=0V

VG=12V

VD=5V

Pro

gram

min

g W

indo

w

∆∆ ∆∆VtR

[V]

Stress Time t [s]

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Etienne Nowak et al. – SSDM 2009

002468

1012141618

Cha

rge

Den

sity

Qch

arge

d[1

012cm

-2]

50 100 150 200 250Effective charged Length Lcharged [nm]

Charge localization

1. Charge “initially” localizes at ~40-60 nm next to dr ain2. After t~10 ms, Qcharged saturates, then Lcharged broadens3. Not significant difference between the trapping lay ers

Source Drain

Gate

Source Drain

Gate

Source Drain

Gate

Source Drain

GateVg=10V Vg=12V ; Vd=5V

HfO2Al2O3Si3N4

t ~ 0.01s

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Vd=5V Vg=12V Vd=5V Vg=12V

Source Drain

Gate

EY

Source Drain

Gate

EY

Etienne Nowak et al. – SSDM 2009

Effective Length Leff

Qcharged=17.5x1012cm-2

Lcharged

0.0

0.20.4

0.60.8

1.01.2

1.4

Nor

mal

Fie

ld E

Y[M

V/c

m]

-0.2 -0.1 0.0 0.1 0.2Source DrainPosition X [um]

Qcharged=0 to 20x1012cm-2

every 2.5x10 12cm-2

Effective Length Leff

Lcharged

Ey evolution during program

�Maximum Qcharged and subsequent Lchargedbroadening explained by:

�Decrease of Ey at the Si/SiO2 interface �Ey peak shift towards the source side

0.00.20.40.60.81.01.21.4

Nor

mal

Fie

ld E

Y[M

V/c

m]

-0.2 -0.1 0.0 0.1 0.2DrainSource Position X [um]

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Etienne Nowak et al. – SSDM 2009

Outline

�Motivation

�Methodology

�Program operation

�Retention

�Conclusion

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Etienne Nowak et al. – SSDM 2009

Retention operationSource Drain

Gate

Source Drain

Gate

100 101 102 103 104 105

0

1

2

3

4

5

T=25°C T=125°C HfO2 Al2O3 Si3N4

Pro

gram

min

g W

indo

w

∆∆ ∆∆VtR

[V]

Time t [s]

�Lateral charge migration is the main ∆∆∆∆VtR loss mechanism for the three materials at 25°C.�Charge loss is relevant only for Al 2O3 at 125°C

100 101 102 103 104 105

60

80

100

120

T=25°C T=125°C HfO2 Al2O3 Si3N4

Tota

l Cha

rge

Var

iatio

n Q

char

gedx

Lch

arge

d [%

]

Time t [s]

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Etienne Nowak et al. – SSDM 2009

Retention Model to extract the ∆∆∆∆Lcharged

�1D model of nitride�Drift-Diffusion of majority carriers�Effective mobility coefficient µeff

(((( )))) (((( )))) (((( ))))[[[[ ]]]] (((( ))))

(((( )))) (((( ))))

====∂∂∂∂

∂∂∂∂∂∂∂∂

∂∂∂∂++++∂∂∂∂∂∂∂∂====

∂∂∂∂∂∂∂∂

0

2

2

,,

,,,

,

εε

txqnx

txEx

txnDtxEtxnµ

xttxn

r

eff

Source side

Drain side

Position

Charge density

Lcharged

Qcharged

( )0

, =∂

∂x

txn0=V

Shape1

Shape2Source side

Drain side

Position

Charge density

Lcharged

Qcharged

( )0

, =∂

∂x

txn0=V

Shape1

Shape2

Drift-Diffusion equations

Source Drain

Gate

Source Drain

Gate

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Etienne Nowak et al. – SSDM 2009

Retention Model to extract the ∆∆∆∆Lcharged

�Drift predominant over Diffusion�∆Lcharged independent of the shape of the trapped charges�Drift follows an empirical law: Lcharged =Lcharged0 +A*ln(t)

Source side

Drain side

Position

Charge density

Lcharged

Qcharged

( )0

, =∂

∂x

txn0=V

Shape1

Shape2Source side

Drain side

Position

Charge density

Lcharged

Qcharged

( )0

, =∂

∂x

txn0=V

Shape1

Shape2

Drift-Diffusion, Shape1Drift-Diffusion, Shape2Diffusion, Shape1

0

50

100

150

Effe

ctiv

e ch

arge

d Le

ngth

in

crea

se ∆∆ ∆∆

L char

ged

[nm

]

10-3 10-1 101 103 105 107

Time t [s]

∆∆∆∆Lcharged=A*ln(t)

A=ααααµµµµeffQtotal/εεεεrεεεε

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Etienne Nowak et al. – SSDM 2009

Data vs model: lateral charge migration

�Lateral migration for the three material follows a logarithmic law with different A coefficient�Lowest drift observed for Si3N4

103 104 105

0

5

10

15

20

25

A=1.7

A=0.55

A=3.7

HfO2 Al2O3 Si3N4

Effe

ctiv

e C

harg

ed L

engt

h

incr

ease

∆∆ ∆∆L ch

arge

d [n

m]

Time t [s]

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Etienne Nowak et al. – SSDM 2009

Outline

�Motivation

�Methodology

�Program operation

�Retention

�Conclusion

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Etienne Nowak et al. – SSDM 2009

Conclusion

� Comparative study of trapping properties in Si3N4, HfO2 and Al2O3 in program and retention conditions����Large window (~10 V) possible for all trapping material s����Maximum Qcharged is limited by electrostatics not by the

trapping layer properties

� Method allows separating vertical vs lateral charge migration����Lateral migration, due to charge drift, is the main Vt shift

mechanism in retention mode for the three materials at 25°C

����Log(t) dependence of lateral migration, and Si3N4 sho ws the lowest drift

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Etienne Nowak et al. – SSDM 2009

Thanks for your attention!

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Etienne Nowak et al. – SSDM 2009

Extraction Method

�Analytical model Based on Liu Surface Potential model �Calculate ∆VtR and ∆VtF for a given Qcharged and Lcharged

�Extract Qcharged and Lcharged from measured ∆VtR and ∆VtF

[L. Perniola et al., IEEE Trans. on Nanotech., Vol. 4, No. 3, pp. 360-368, May 2005]

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Etienne Nowak et al. – SSDM 2009

Retention Model

(((( )))) (((( )))) (((( ))))[[[[ ]]]] (((( ))))

(((( )))) (((( ))))

====∂∂∂∂

∂∂∂∂∂∂∂∂

∂∂∂∂++++∂∂∂∂∂∂∂∂====

∂∂∂∂∂∂∂∂

0

2

2

,,

,,,

,

εε

txqnx

txEx

txnDtxEtxnµ

xttxn

r

eff

(((( )))) (((( ))))2

2,,

xtxn

Dt

txn∂∂∂∂

∂∂∂∂====∂∂∂∂

∂∂∂∂ (((( )))) (((( ))))(((( ))))(((( ))))

∂∂∂∂∂∂∂∂====

∂∂∂∂∂∂∂∂

∫∫∫∫

∫∫∫∫∞∞∞∞

0

0*

,

,,

,

dxtxn

dxtxntxn

xA

ttxn

x

(((( ))))00

0*,

εε

Qµ

εε

dxtxnqµA

r

totaleff

r

eff======== ∫∫∫∫

∞∞∞∞

0====D 0====effµ

Diffusion equation Drift equation

Drift-Diffusion equation