4 5 vianello 4.4

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

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

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

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


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


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

Electrical characterization

12/7/2009 6

Same EOT, fixed VG and same VT same electric field


Electrical characterization

12/7/2009 6

Same program, different erase and retention intrinsic differences in the trapping properties of std and Si-rich SiN


Same EOT, fixed VG and same VT same electric field

12/7/2009 7

Physico-Chemical Characterization


SIMS

•  confirm the excess Si in the Si-rich sample •  show pile up of hydrogen in SiN layer

Si/N

~5-10 %

H

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


12/7/2009 9






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

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

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

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

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


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

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

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

Charging of the defects

17

1Si-H

Si dB


D0

D0

Charging of the defects (D-)


1Si-H

Si dB

D-

D- D0

D0

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+

Negatively charged defects (D-)

18

1Si-H Si dB


ET ET

Charge state D- (initial state for retention and erase):


18

1Si-H Si dB


ET ET


• similar trap energy depth


18

1Si-H Si dB


ET ET


• similar trap energy depth • available electrons: 1 e- in the 1Si-H 2 e- in the Si dB!

12/7/2009 19






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-


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


electrostatic potential solved self consistently with

•  tunnelling fluxes •  transport in the SiN CB •  SRH recombination

[Vianello, TED ‘09]

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

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


Experimental Verification: Field accelerated charge loss

24

• starting from neutral state QN=0 • small nagative VG identical hole injection (if any)


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)


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

Acknowledgments •  Italian MIUR:

FIRB RBIP06YSJJ project

•  French Public Authorities: NANO 2012 program

12/7/2009 27ElisaVianello-IEDM2009

4 5 vianello 4.4

Documents