Manual of compact models for
Ferroelectric tunnel memristor (FTM)
SPINLIB: Model FTM
Z. H. Wang, W. S. Zhao, J-O. Klein, D. Ravelosona and C. Chappert
IEF (Université Paris Sud/CNRS UMR8622)
Contact: weisheng.zhao@u-psud.fr
Table of contents
I. General Introduction
II. Files Provided
III. Parameters
III.A CDF
III.B Technology Parameters
III.C Device Parameters
III.D Simulation environment Parameter
I.
General Introduction
A new type of memristive behavior was recently observed in Co/BTO/LSMO
ferroelectric tunnel junction (FTJ), defining a novel ferroelectric tunnel memristor
(FTM) (see Chanthbouala A et al 2012 Nature Mater. 11 860). This FTM shows large
OFF/ON resistance ratio (>100) and high operation speed (~10ns), promising to be
used for multilevel memories and synaptic-like circuits. In this manual, we present a
compact spice-compatible model of FTM for multilevel storage and neuromorphic
circuit simulation.
Programmed with Verilog-A language
Validated in Cadence 6.1.5 Spectre, CMOS Design Kit 40nm.
The objective of this guide is to provide an easy way to start the simulation of
FTM-based circuits.
Fig.1. (a) Schematic viewgraph of Co/BTO/LSMO ferroelectric tunnel memristor
(FTM). Ferroelectric ultrathin film BaTiO3 is sandwiched between Co and
La0.67Sr0.33MnO3. NdGaO3 is used for substrate. (b) Equivalent model of FTM based
on ferroelectric switching dynamics: Once a programming voltage is applied the
ferroelectric film with fully up-polarized domain (orange), the down-polarized
domains (red) nucleate and propagate, and the tunnel resistance is calculated by the
parallel resistance model of two FTJs.
II. Files Provided
Decompress the compressed file ModelBTOFTM.rar which you have downloaded.
There are three files included in the file decompressed:
One file named “model_BTOFTM” includes a file of the type of veriloga which is
the source code of this model, and a symbol file for this model;
The second file named “cell_BTOFTM” includes a package schematic of FTM and a
symbol file of FTM.
Fig.2 Symbol of the model FTM
This model of FTM contains three pins:
A virtual pin “State” is used to test the volume fraction of down-polarized domain
(corresponding to high resistance state). It is reflected by the amplitude of voltage at
this pin.
Another two pins “T1, T2” are the real pins of the junction. These two pins aren’t
symmetric: a voltage from T1 to T2 can switch the state from up-polarized to
down-polarized.
Another file named “simu_BTOFTM” is a simple test simulation with the model
which demonstrates how it works. The schematic of the test simulation is shown in
Fig.3. The initiate volume fraction of down-polarized domain is set to 90%. A pulse
shown in Fig.4a is applied to FTM. Here two pulses of -4V and 3.75V are firstly
applied to program the FTM, and then two programming pulses of -3.25V followed
by reading pulses of 0.1V are applied to program the FTM toward up-polarized state.
Finally similar pulses (but 3.25V) sequence is applied to program the FTM toward
opposite polarization.
The volume fraction of down-polarized domain is reflected by the level of virtual pin
”state”, as shown in Fig.4b. Evidently, the nucleation and growth of domain induced
by the applied pulse can be observed. Moreover, in Fig.4c the current through the
FTM measured at 100mV changes with the programming pulses, demonstrating that
the resistance of FTM can be continuously tuned by the applied voltage, and hence
the memristive behavior is validated.
Fig.3 Schematic of the test simulation
Fig.4 Result of the simulation with the model FTM
III.
Parameters
III.A Component Description Format (CDF)
In order to describe the parameters and the attributes of the parameters of individual
component and libraries of component, we use the Component Description Format
(CDF) tool. It gives us the independence of from applications and cellviews, and a
graphical user interface (the Edit Component CDF form) for entering and editing
component information.
We use CDF to define four frequently-used parameters shown in Fig.5, i.e. r (surface
radius), t_bto (barrier thickness), sim_step (simulation time step) and HRS_volume
(initial volume fraction of down-polarized domain). These parameters can be easily
modified depending on specific simulation.
You can also define other parameters for this library. For this purpose you click Tools
-> CDF -> Edit, enter “BTOFTM” as the Library Name and “cell_BTOFTM” as the
Cell Name. Select “Base” as the CDF Type. Then click “Add” under Component
Parameters. (see Fig.6)
Fill out the form as shown in Fig.6. You need to select the type of the parameter and
enter the name and defValue of the parameter. Then click “OK”.
Fig.5 Modify the CDF parameters
Fig.6 Edit the CDF parameters
III.B Technology Parameters
Parameter
PhiBasH
PhiBasL
Description
Average potential barrier height of HRS
Average potential barrier height of LRS
Potential height difference between two
PhiDeltaH
boundaries of BTO barrier for HRS
Potential height difference between two
PhiDeltaL
boundaries of BTO barrier for LRS
Factor calculated from
Fx
resistance-area product
FyH
FyL
Fitting factors determined by experimental
measurement
FzH
FzL
Scaling constant for calculation of coercive
C_JKD
voltage
Ps
Spontaneous polarization
tau0n
Attempt time of nucleation
tau0P
Attempt time of propagation
Un
Creep energy barrier for nucleation
Up
Creep energy barrier for propagation
Unit
eV
eV
Default value
0.94
0.45
eV
0.15
eV
0.1
191000
0.385
0.989
1.68
6.7
V/m1/3 4500
C/m2
s
s
eV
eV
0.26
2.8e-15
9e-14
0.67
0.52
These technology parameters are determined by the experimental measurement. They
are mainly dependent on the material and fabrication technology, so it is
recommended to keep their default value.
III.C Device Parameters
Parameter
t_bto
r
Description
Barrier thickness
Surface radius
Initial volume fraction of down-polarized
HRS_volume
domain
Unit
m
m
Default value
2e-9
175e-9
0.9
These device parameters depend mainly on the process and mask design and the
designers can change them to adapt their requirements.
III.D Simulation environment Parameter
Parameter
sim_step
Description
Simulation time step
Unit
s
Default value
1e-10
Sim_step is simulation time step set for simulator, and it can decide the simulation
speed. It can be modified according to the profile of applied pulse and value of tau0n
and tau0p. For example, if a rectangular pulse with a period of 100ns is applied to
FTM whose tau0n and tau0p are ~10ns, the sim_step can be set to 1ns to improve the
simulation speed while keeping results accurate.