Analytical and compact models of the ONO capacitance inembedded non-volatile flash devices

Davide Garetto*†, Erwan Dornel*, Denis Rideau§, William F. Clark*,Alexandre Schmid†, Saadia Hniki‡§, Clement Tavernier§, Hervé Jaouen§ and Yusuf Leblebici†

* IBM France, 850 rue Jean Monnet, Crolles, France† Microelectronic Systems Laboratory (LSM) - EPFL, Lausanne, Switzerland

§ STMicroelectronics, 850 rue Jean Monnet, Crolles, France‡ LAAS / CNRS, Université de Toulouse, 7 Avenue du C. Roche, Toulouse, France

PRINCIPLES

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THE STRUCTURE

MODEL DESCRIPTION

• The FG potential VFG in a non–volatile flash memory (NVM) device is the main parameter controlling the behavior of the cell; its calculation is required for compact

modeling purposes

• Common modeling technique for VFG based on the calculation of the coupling coefficients between all the terminals

• A model for the capacitance CCCFCF between the control gate (CG) and the FG, separated by an oxide-nitride-oxide (ONO) dielectric layer, is required

• Most compact models used in industry consider fringing or corner capacitances as fitting parameters not appropriate when technology scalability must be taken into

account

• Develop a physical–level model for CCCFCF supporting accurate modeling of cell layout scalability as well as process variations

• Integrate the model into an advanced compact model for flash devices

2D cross sections3D TCAD process simulations using

65 nm node process flow

D

S

CG

FG

LWfg

W/2

L = channel length of the cell

W = channel width of the cell

Wfg = floating gate wing: extension of the FG over the Shallow Trench isolation (STI) region

D

ONO layer sandwiched between control gate (CG) and floating gate (FG) polysilicon layers

1. Structure analysis and identification of the ONO capacitance components

2. Model definition

Apply the structure decomposition approach to the 3 cross-sections

Working principles of a floating gate memory cell:1) Information = charge on a floating gate (FG) node2) Read : quantify the charge on FG measuring VTH

3) Program: inject electrons in FG using channel hot electron injection (CHE)

4) Erase: discharge electrons on the substrate by Fowler-Nordheim tunneling

PROBLEMATICS / OBJECTIVES

3. Model validation

Scaling the ONO capacitance CCF and its different components

with respect to the width W and the length

L of the device.

Scaling CCF with respect to the active area of the cell (W * L)

• Excellent matching with the model has been found (average error < 3%)

• Model is also scalable with respect to the FG wing Wfg

For each capacitance component, integrate the Gauss law on the electrical field lines

Fringing capacitances

Analytical model : field lines approximated as semi-ellipses

Compact model : field lines approximated as straight lines with curvature correction

CONCLUSIONS

zxzxzxxl 3332

)(

2)(

ONO

SD

PO Tx

L

Tz

222)( zxxl 2)(

ONO

SD

PO Tx

L

Tz

)(

1

)(

1

xdlSdS

xdlC rr

ONOT

LTTC

)( ONOPO1

ONO0pl

Parallel plate capacitances

ONOT

LWWC

)( fg

ONO0pt

LC2

ONO0

crn

Corner capacitance

2

1 tl AA Fitting parameter representing the curvature of the field

lines in the compact model

2

)(

1

SD

POt D

T4

2ONOT

2

)(

1

SD

POl L

T)(SD

POONOl L

TT

)(SD

POONOt D

TT

(c) Cross-section BB’

Ccrn is the capacitance in the edge corner separating Cpt

from Cpl

Assumption on theelectrical field lines

crnflftplptCF CCCCCC Total ONOcapacitance

DD = maximum extension of the field lines of Cfl in

the spacer region

LD = maximum extension of the field lines of Cft in

the spacer region

Cpl = lateral parallel plate capacitance

between FG and CG in the STI region

Cfl = fringing capacitances of Cpl

Cpt = top parallel plate capacitance between

FG and CG

Cft = fringing capacitances of Cpt

• We have developed a fully physicalphysical and scalablescalable model to accurately estimate the FG–CG coupling ONO capacitance in embedded high density memory devices

• We have validated our compact model with 3D TCAD AC simulations3D TCAD AC simulations studying the dependence of CCF on critical layout dimensions

• We extracted the FG–CG coupling factor αcoupling factor αCC from DC TCAD simulations and demonstrated the importance of modeling 3D effects

• The proposed ONO capacitance model has been included into an accurate PSP–based compact model for eNVM devicescompact model for eNVM devices, comparing the DC characteristics of the flash device with the ones of a “dummy” device, where FG and CG are short–circuited

y

xz

Doping concentration [cm-3]

COUPLING EXTRACTION

FG – CG coupling coefficient αC extracted from DC TCAD simulations on the 3D structure vs. W, L and Wfg CG

FGC V

V

3D effects(parasitic hump effects and W

dependence) strongly influencing coupling and cell performance

2009