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12/7/2009 1
New insight on the charge trapping mechanisms of SiN-based memory
by atomistic simulations and electrical modeling
E.Vianelloa,b, L.Perniolaa, P.Blaisea, G.Molasa, J.P.Colonnaa, F.Driussib, P.Palestrib, D.Essenib, L.Selmib, N.Rochata, C.Licitraa, D.Lafonda, R.Kiesa, G.Reimbolda, B.De Salvoa, F.Boulangera
aCEA, LETI, MINATEC, France bDIEGM, Univ. of Udine-IUNET, Italy
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12/7/2009 ElisaVianello-IEDM2009 2
Charge Trapping (CT) memories Charge Trapping NAND Flash are based on 5-7 nm
Silicon Nitride (SiN) layer: • TANOS (TaN-Al2O3-SiN-SiO2-Si) [Lee, IEDM ‘03] • BE-SONOS (barrier eng. SONOS) [Lue, IEDM ‘05]
The performance of SONOS/TANOS can be improved by SiN engineering
[Sandya, EDL ‘09] [Goel, EDL ‘09] [Lin, IEDM ‘08]
Little knowledge about the physical mechanisms at the origin of the SiN trapping properties
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12/7/2009 3
Combined synergic use of: - physical and chemical characterization - electrical characterization - atomistic simulation - electrical modeling Aimed at: - understanding the physical properties of SiN layers with different stoichiometry - nature of the traps - physical process involved in charge trapping - impact on the electrical performance of cells
Objectives of the work
ElisaVianello-IEDM2009
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12/7/2009 4
Outline • Device description and characterization
– Electrical characterization – Physical and chemical characterization
• Atomistic simulations – SiN model definition from experiments – electrically active defects – trap charging properties
• Modeling of the electrical behavior • Conclusions
ElisaVianello-IEDM2009
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Device description
12/7/2009 5
std SiN Si-rich SiN type LPCVD LPCVD SiH2Cl2/NH3 [sccm] 0.1 5 S/D annealing 1050 °C, N2 amb 1050 °C, N2 amb
ElisaVianello-IEDM2009
std SiN Si-rich SiN
p substrate
TiN
HfAlO
trapping layer HTO
tunnel ox
8 nm
3.5 nm
5 nm
2.5 nm
n+ n+
same EOT verified by CV curves
same physical dimensions
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Electrical characterization
12/7/2009 6
Same EOT, fixed VG and same VT same electric field
ElisaVianello-IEDM2009
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Electrical characterization
12/7/2009 6
Same program, different erase and retention intrinsic differences in the trapping properties of std and Si-rich SiN
ElisaVianello-IEDM2009
Same EOT, fixed VG and same VT same electric field
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12/7/2009 7
Physico-Chemical Characterization
ElisaVianello-IEDM2009
SIMS
• confirm the excess Si in the Si-rich sample • show pile up of hydrogen in SiN layer
Si/N
~5-10 %
H
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12/7/2009 8
Hydrogen in the SiN layers MIR
• N-H: higher concentration in the std SiN (~1020 cm-3)
• Si-H: comparable in the two samples (~1019 cm-3) • total H content: ~2% in the std SiN ~1% in the Si-rich SiN
ElisaVianello-IEDM2009
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12/7/2009 9
Outline • Device description and characterization
– Electrical characterization – Physical and chemical characterization
• Atomistic simulations – SiN model definition from experiments – electrically active defects – trap charging properties
• Modeling of the electrical behavior • Conclusions
ElisaVianello-IEDM2009
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Simulation framework • Spin-polarized Density Functional Theory: - SIESTA code [Ordejón, Ph. Rev. B ‘96] - Local Spin Density Approximation (LSDA) - Trouillier-Martin pseudopotentials for the core electrons - supercells of 224 atoms, 1x1x1 k-sampling - DPZ basis, Energy Shift 50 meV, Mesh cut-off 100 Ha
• G0W0: - ABINIT code [Gonze, Comp. Mat. Sc. ‘02] - Trouillier-Martin pseudopotentials for the core electrons - supercells of 28 atoms - fully converged (1000 electron bands)
12/7/2009 ElisaVianello-IEDM2009 10
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Simulated vs. Experimental physical properties
11
EG [eV] mass density [g/cm3] sim
(GW) exp
(spectr. ellips.) sim
(GW+DFT) exp
(XRR) β-Si3N4 5.8 3.15 std SiN 5.3 5.3 ~3 ~2.85 Si-rich SiN 4.6 4.7 ~3.05 ~2.9
std SiN: Si-rich SiN:
~2% H (MIR) and ~1% excess Si (SIMS) ~1% H (MIR) and ~6% excess Si (SIMS)
Starting point: crystalline β-Si3N4
12/7/2009 ElisaVianello-IEDM2009
H and excess Si generate defects
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β-Si3N4
Families of studied defects
Si N
N Si 4-fold coordinated by four N
H N
+(n)H Si vacancy
(n)N-H n=1,…,4
12ElisaVianello-IEDM2009
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β-Si3N4
2.6 Å
Families of studied defects
Si
+Si
Si
+(n)H +(n)H
N
Si
N Si 4-fold coordinated by four N
N 3-fold coordinated by three Si
Si H
Si Si
N
Si vacancy N vacancy
Si dangling bond (dB) (n)Si-H
(n)N-H
ElisaVianello-IEDM2009 12
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Which are the thermodynamically favoured defects?
13
DFT + chemical potentials (real fabrication conditions) Defect’s Gibbs free energy of formation
more favoured defects: 1SiH 4NH SidB
• std SiN 1Si-H and 4N-H
• Si-rich Si dB becomes more important
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12/7/2009 14
CB
VB
4N-H defect does not generate states in the band-gap
The 4N-H defect is not electrically active ElisaVianello-IEDM2009
H N
Which defects are electrically active? The 4N-H defect
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The 1Si-H defect, neutral state (D0)
12/7/2009 15
Fermi level
1Si-H defect generates states in the band-gap
The 1Si-H defect is electrically active ElisaVianello-IEDM2009
CB
VB
2.6 Å Si
Si
Si
[Peterson JAP ‘06]
H
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The Si dB defect, neutral state (D0)
12/7/2009 16
Fermi level
Si dangling bond generates states in the band-gap
The Si-dB defect is electrically active ElisaVianello-IEDM2009
CB
VB
Si
Si
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Charging of the defects
17
1Si-H
Si dB
12/7/2009 ElisaVianello-IEDM2009
D0
D0
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Charging of the defects (D-)
1712/7/2009 ElisaVianello-IEDM2009
1Si-H
Si dB
D-
D- D0
D0
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Charging of the defects (D+)
17
1Si-H
Si dB
Si-H and Si dB defects are amphoteric (D-,D0,D+) 12/7/2009 ElisaVianello-IEDM2009
D-
D- D0
D0
D+
D+
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Negatively charged defects (D-)
18
1Si-H Si dB
12/7/2009 ElisaVianello-IEDM2009
ET ET
Charge state D- (initial state for retention and erase):
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Negatively charged defects (D-)
18
1Si-H Si dB
12/7/2009 ElisaVianello-IEDM2009
ET ET
Charge state D- (initial state for retention and erase):
• similar trap energy depth
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Negatively charged defects (D-)
18
1Si-H Si dB
12/7/2009 ElisaVianello-IEDM2009
ET ET
Charge state D- (initial state for retention and erase):
• similar trap energy depth • available electrons: 1 e- in the 1Si-H 2 e- in the Si dB!
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12/7/2009 19
Outline • Device description and characterization
– Electrical characterization – Physical and chemical characterization
• Atomistic simulations – SiN model definition from experiments – electrically active defects – trap charging properties
• Modeling of the electrical behavior • Conclusions
ElisaVianello-IEDM2009
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Modeling of the defects
20
D0
D-
1Si-H
Si
dB
nT=0 QN=0
nT=1e-
nT=2e- nT=1e-
QN=0
QN(x,t) is the excess charge w.r.t. neutral SiN QN causes the VT =f(QN)
nT(x,t) is the electron concentration in the higher energy states nT influences the charge loss J =f(nT)
+1e-
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Electrical model
12/7/2009 21
Trap density QN nT @ neutral state std SiN NT
Si-H -qnT 0 Si-rich SiN NT
SidB+NTSi-H -q(nT-NT
SidB) NTSidB
nT
ElisaVianello-IEDM2009
electrostatic potential solved self consistently with
• tunnelling fluxes • transport in the SiN CB • SRH recombination
[Vianello, TED ‘09]
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Program characteristics
22
t=0 neutral device
• programming is driven by the e- injection from substrate (QN) • similar program transients in std and Si-rich SiN 12/7/2009 ElisaVianello-IEDM2009
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Retention characteristic
23
Same EOT and initial VT same electric field in the stack double occupation number (nT)larger charge loss in Si-rich SiN
t=0 programmed device
12/7/2009 ElisaVianello-IEDM2009
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Experimental Verification: Field accelerated charge loss
24
• starting from neutral state QN=0 • small nagative VG identical hole injection (if any)
ElisaVianello-IEDM2009
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Experimental Verification: Field accelerated charge loss
This behavior is explained by the different occupation number and it can not be explained with different trap energy depth
24
• starting from neutral state QN=0 • small negative VG identical hole injection (if any)
ElisaVianello-IEDM2009
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Conclusions IN SUMMARY WE • Investigated experimentally and by simulations
different SiN compositions linked to H and excess Si • Developed atomistic models for std and Si-rich SiN
consistent with: XRR, MIR, SIMS, spectr. ellips, etc.... • Implemented the defect models in an electrical
simulator MAIN RESULTS: • Defects of different nature dominate the trapping
properties of std and Si-rich SiN • The defect’s occupation number for the same charge
state can explain the behavior of std and Si-rich SiN • These results open new perspectives in the study of
other materials for Charge Trapping NVMs 12/7/2009 25ElisaVianello-IEDM2009
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Acknowledgments • Italian MIUR:
FIRB RBIP06YSJJ project
• French Public Authorities: NANO 2012 program
12/7/2009 27ElisaVianello-IEDM2009