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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
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Etienne Nowak et al. – SSDM 2009
Outline
�Motivation
�Methodology
�Program operation
�Retention
�Conclusion
3
2007
Etienne Nowak et al. – SSDM 2009
Outline
�Motivation
�Methodology
�Program operation
�Retention
�Conclusion
4
2007
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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2007
Etienne Nowak et al. – SSDM 2009
Outline
�Motivation
�Methodology
�Program operation
�Retention
�Conclusion
11
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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