Ferroelectric capacitor modeling method for circuit simulation
Technical Field
The invention belongs to the technical field of ferroelectric capacitor (Fecap) modeling, and particularly relates to a circuit simulation-oriented ferroelectric capacitor (Fecap) modeling method.
Background
Ferroelectric capacitors (fecaps) and ferroelectric field effect transistors (fefets) are emerging electronic components with unique electrical properties such as non-volatile memory and circuit programmability. In addition, ferroelectric RAM (FeRAM) based on FeCap is receiving wide attention in the industry for its excellent performance and process compatibility. These features make them potential competitors to new memory Computing (CIM) for energy-efficient hardware acceleration.
With advances in process technology, electronic Design Automation (EDA) tools are increasingly needed to support the design and verification of fem circuits based on FeCap/fefets.
Currently, the mainstream industrial SPICE circuit simulation tools are all realized based on an improved node analysis (MNA) formula, and MNA is an analysis method based on node voltage, which combines Kirchhoff Voltage Law (KVL), kirchhoff Current Law (KCL) and constitutive equations of elements, solves unknown quantities in a circuit by establishing an equation set, expresses the circuit equation in a matrix form, and is convenient for computer program processing and solving.
In the prior art, an MNA model of ferroelectric capacitor applied to SPICE is realized by adopting MNA, if the MNA model is not available, a designer must manually create an equivalent circuit model for an emerging device, and the resolving speed of the circuit equivalent model in a simulation tool is also behind the MNA model.
Accordingly, the present invention addresses the simulation tool model problems and needs currently faced by the above-mentioned ferroelectric circuits.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention aims to provide a ferroelectric capacitor (Fecap) modeling method oriented to circuit simulation, which is based on MNA, and utilizes a physical model formula (L-K model) of Fecap, kirchhoff Current Law (KCL) and Kirchhoff Voltage Law (KVL) to represent unknown variables (node voltage, branch current and branch polarization) to form a model code of Fecap, so that the SPICE simulation of Fecap is more accurate and rapid.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
A modeling method of ferroelectric capacitor (Fecap) facing circuit simulation comprises the following steps;
step 1, introducing a modified node analysis Method (MNA) into MNA model modeling of Fecap;
step 2, MNA model forming process of Fecap element;
Based on a modified node analysis Method (MNA), a compensation equation is obtained by utilizing an L-K model formula of Fecap and is used for solving an equation of an unknown variable to obtain an admittance matrix G and a known quantity vector column RHS of a final Fecap element, and the admittance matrix G and the known quantity vector column RHS are combined to obtain an MNA model of Fecap.
In the step 1, the specific process of modeling the MNA model of the MNA and Fecap is as follows:
Solving unknown variables by using Kirchhoff Current Law (KCL), kirchhoff Voltage Law (KVL) and characteristic equations of the element itself, which is also called introducing a compensation equation, so as to obtain an admittance matrix G of the element, a variable column X and a known quantity column RHS, wherein the admittance matrix G and the known quantity column RHS are combined to obtain an MNA model of the element, and each time an unknown variable is added, a new row and a new column are generated in the admittance matrix G, and corresponding unknown quantity is generated at a corresponding position of the variable column X.
In combination with the physical characteristics of the ferroelectric capacitor (Fecap), when the ferroelectric capacitor is introduced into the circuit, the polarization state and current of the ferroelectric capacitor are new unknowns (the original unknowns are two node voltages), and an admittance matrix G of 4X4 and a variable column X of 1X4 (VN +(t),VN-(t),i(t),p(t))T and an RHS column of 1X 4) are obtained, and then the admittance matrix G and the known column RHS are combined to obtain an MNA model of FECAP.
The step 2 specifically comprises the following steps:
2.1 Based on the generalization of the L-K model Fecap, obtaining the general formula of the Fecap model, so as to facilitate the subsequent linearization process;
2.2 Utilizing a backward Euler method and an N.R. iteration method to compensate equations of Fecap (which are nonlinear devices) for solving equations of unknown variables, carrying out linearization processing based on MNA, and carrying out linearization processing on the compensating equations of Fecap so as to meet solving requirements of a Spice simulation tool and form a final MNA model;
2.3 Combining Fecap terminal voltages, polarization states, and model formulas to arrive at a specific expression of the functions F1 and F2 to form the final MNA model of Fecap.
The step 2.1) is as follows:
selecting an LK model, the relationship between polarization P and unit energy G is described by the following Landau free energy equation;
wherein E is the electric field, and the L-K model is further derived:
Where ρ is the thermodynamic coefficient, T FE represents the thickness of the ferroelectric layer, v=et FE is the voltage across the ferroelectric layer, and α, β, γ are coefficients related to the material properties, combined with the analysis of the L-K model described above, and the general formula of the Fecap model is listed in terms of terminal voltage and polarization state of Fecap:
V(t)=F1(I(t),P(t)) (1)
wherein the current I and polarization P flowing through Fecap are internal state variables of Fecap, respectively, V represents terminal voltage, and F1 and F2 represent arbitrary functions;
To this end, equations (1) and (2) have been obtained for solving Fecap the unknown variables based on MNA, since current and polarization variables are introduced into column vector X, corresponding rows and columns are generated when generating admittance matrix G, and next, the model is subjected to linearization processing to extract admittance matrix G, variable column vector X, and known quantity column vector RHS.
In said 2.2), the semiconductor devices in SPICE can be generally classified into two types, i.e., linear devices and nonlinear devices;
The linear and nonlinear components of the device depend on the relation between the terminal voltage of the device and the current flowing through the device, and the nonlinear circuit containing nonlinear elements needs to be subjected to linearization processing on model formulas of the nonlinear circuit;
The general formula of Fecap model was linearized using the Newton. Raphson iteration and the backward Euler method.
The method comprises the following steps:
1) First, the terminal voltage is represented by the node voltage;
2) Using n.r. iteration, taylor first order expansion of the F1 function at the operating point (I (n),P(n)) can be obtained:
Wherein F 1 (n)(t)=F1(I(n)(t),P(n) (t)), further transforming formula (3) for facilitating extraction of matrix elements
Wherein VN+, VN-, I (t), P (t) are variables to be solved to be put into X column vectors, corresponding coefficients thereof are taken as elements of an admittance matrix, related terms of all n iterations on the right are taken as known quantities to be put into RHS column vectors, and for the subsequent model description, the known terms are equivalent to Irhs
For equation (2), linearizing dP/dt by using a backward Euler method, and performing F 2 function processing the same as F 1 function processing to obtain a first-order Taylor expansion at a working point (V (n)(t),P(n) (t)), wherein the first-order Taylor expansion is obtained:
Wherein h is a time step, F 2 (n)(t)=F2(V(n)(t),P(n)(t)),V(t)=VN+(t)-VN- (t), and performing transformation on formula (5)
Extracting the elements with the coefficients G before the variables I (t), P (t) and V N+(t)、VN- (t), and equally, for the subsequent model description, equating the known term to be Prhs and putting the element into RHS column vector
For currents flowing through nodes V N+ and V N- to flow in the positive direction, assuming the flow into node V N+ is in the positive direction, there are the variables V N+: I (t) and V N-: -I (t) with only I (t), the coefficients are 1 and-1, and no known term is provided.
According to the above description, coefficients before extracting unknown variables form an admittance matrix G:
column vector X formed by unknown variables:
Column vector RHS formed from a known quantity:
to this end, the admittance matrices G and RHS of composite Fecap MNA model have been obtained, and next, the F1 and F2 functions are obtained to write the model into the SPICE simulation program.
The step 2.3) specifically comprises the following steps:
For the F 1 function, the terminal voltage of Fecap is contributed by the potential difference of the parasitic resistance inside the ferroelectric and the ferroelectric layer potential difference, so the L-K model formula based on Fecap is available:
wherein AFE is the surface area of the ferroelectric capacitor, and for F 2 function, dp/dt in the L-K model formula of transformation Fecap is as follows:
combining the admittance matrices G and RHS, since the F1 and F2 functions are known, then an MNA model with the exact parameters Fecap is obtained;
The corresponding matrix parameters are only written into modeling codes specified by SPICE to obtain the built-in MNA model Fecap in SPICE.
The invention has the beneficial effects that:
the invention adopts the improved node analysis Method (MNA) to realize Fecap MNA model which can be applied to the industrial SPICE simulation tool, the step 2 is based on the MNA, and Fecap MNA model which can be analyzed and calculated by the SPICE simulator is obtained by utilizing the L-K model formula of Fecap, so that the ferroelectric capacitor can be directly simulated in the SPICE program to obtain more accurate electrical characteristics, and the potential and the competitiveness of the ferroelectric device in the design of a simulated memory circuit are greatly improved.
The Fecap MNA model designed by the invention accords with the simulation solving principle of the SPICE single component, and can show the advantage of higher speed in the simulation solving of the equivalent circuit SPICE of Fecap.
Drawings
FIG. 1 is a flow chart of the invention for obtaining Fecap MNA Model the required admittance matrix G and column vector RHS based on MNA using the general formula of Fecap L-K model.
FIG. 2 is a simplified diagram of the present invention for obtaining F1 and F2 functions to form a concrete Fecap MNA Model.
Fig. 3 is a schematic diagram of the admittance matrix G and RHS combination.
Detailed Description
The invention is described in further detail below with reference to the accompanying drawings.
As shown in fig. 1 and 2, a method for modeling a ferroelectric capacitor (Fecap) for circuit simulation includes the following steps;
step 1, revising a node analysis Method (MNA) so that the MNA is connected with MNA model modeling of Fecap;
The step 1 specifically comprises the following steps:
first, a specific procedure of NA (node analysis method) is described:
when SPICE (simulation program with integrated circuit key) simulation is performed, firstly, the topology structure of a circuit is required to be described through a netlist file of SPICE, and the netlist file describes all nodes contained in the topology structure of the circuit and branches formed among all nodes and elements on the branches in a code form;
SPICE, after obtaining the topology structure of the circuit through analyzing the netlist, can list equations for the current-voltage relation in the circuit. Commonly used methods are Kirchhoff Current Law (KCL) and kirchhoff voltage law KVL. The solution of the equation set is relatively convenient in a computer in the form of a matrix and a vector, namely, the coefficients before the unknown variable (node voltage) is extracted form a matrix Y, also called an admittance matrix, the unknown variable is converted into a column vector V, and the rest (known quantity) is converted into a column vector I, so that the circuit analysis result is expressed as
Y*V=I(1.1)
Since the components in the circuit can describe the branch current according to the node voltage of the branch, the same form as the formula (1.1) can be written, and the same result as the result of (1.1) can be obtained when the admittance matrix of the components is added according to the same node, so that in the Spice simulation tool, the admittance matrix of the components is important, and the time for forming the circuit admittance matrix during simulation can be greatly reduced. However, when there is an unknown amount of an element in the circuit that cannot be represented by the node voltage, the circuit is not solved and the node analysis method is no longer applicable.
The MNA (modified node analysis) is different from NA in that more unknown variables can be introduced, then the unknown variables are solved by using characteristic equations of KCL, KVL and the element itself, which is also called introducing a compensation equation, so as to obtain an admittance matrix G of the element, a variable column X and a known quantity column RHS, where the G and RHS are combined to form an MNA model of the element, and it should be noted that each additional unknown variable generates a new row and a new column in the G, and at the same time, a corresponding unknown quantity is generated at a corresponding position of X.
In combination with the physical characteristics of the ferroelectric capacitor, when it is introduced into the circuit, the polarization state and current of the ferroelectric capacitor are new unknowns (the original unknowns are two node voltages), so that an admittance matrix G (fig. 1) of 4X4 is obtained, a variable column X (VN +(t),VN-(t),i(t),p(t))T and an RHS column of 1X4 (fig. 1) of 1X4 are obtained, and then G and RHS are combined to obtain an MNA model of FECAP (fig. 2).
Step 2, an MNA model forming process of the Fecap element is carried out, a compensation equation is obtained by utilizing an L-K model formula of Fecap based on MNA, and an equation of an unknown variable is solved, so that a final admittance matrix G of the Fecap element and an MNA model of a known quantity vector column RHS to Fecap are obtained;
The step 2 specifically comprises the following steps:
2.1 Fecap, obtaining a general formula of a Fecap model based on the general formula of the L-K model of Fecap, so as to facilitate subsequent linearization treatment;
the main FeCap models are the Preisach model and the Landau-Khalatnikov (L-K) model. Here, the LK model is selected for illustration because it has a clear physical meaning. The L-K model can accurately describe the polarization response of the ferroelectric material under an external electric field, and the relationship between the polarization P and the unit energy G can be described by the following Landau free energy equation;
wherein E is the electric field, and the L-K model is further derived:
Where ρ is the thermodynamic coefficient, T FE represents the thickness of the ferroelectric layer, v=et FE is the voltage across the ferroelectric layer, and α, β, γ are coefficients related to the material properties. Specific values of these model parameters can be determined by fitting experimental measurement data. Next, the general formula of the Fecap model is listed based on the terminal voltage and polarization state of Fecap in combination with the analysis of the L-K model described above:
V(t)=F1(I(t),P(t)) (1)
where I (current through Fecap) and P (polarization) are internal state variables of Fecap, respectively, V represents terminal voltage, F1, F2 represent arbitrary functions, and a specific L-K model is subsequently introduced in 2.2. To this end, equations (1) and (2) have been obtained for solving Fecap the unknown variables based on MNA (since current and polarization variables are introduced in column vector X, corresponding rows and columns are generated when generating admittance matrix G), and the next step is to linearize the model to extract admittance matrix G, variable column vector X, and known quantity column vector RHS.
2.2 Utilizing a backward Euler method and an N.R. iteration method to compensate equations of Fecap (nonlinear device) for solving equations of unknown variables, carrying out linearization processing based on MNA, and carrying out linearization processing on the compensating equations of Fecap so as to meet solving requirements of a Spice simulation tool and form a final MNA model;
In said 2.2), the semiconductor devices in SPICE can be generally classified into two types, i.e., linear devices and nonlinear devices;
The linearity and nonlinearity of the device depend on the relationship between the terminal voltage of the device and the current flowing through the device, however, if the nonlinear circuit containing nonlinear elements is not subjected to linearization processing on the model formula, SPICE is still obtained as a linear relationship between voltage and current when the circuit is analyzed and solved, and simulation of the elements is in error. Thus, the general formula of the Fecap model was linearized using the Newton. Raphson iteration method and the backward Euler method.
Firstly, terminal voltage is represented by node voltage;
2) Using n.r. iteration, taylor first order expansion of the F1 function at the operating point (I (n),P(n)) can be obtained:
Wherein F 1 (n)(t)=F1(I(n)(t),P(n) (t)), further transforming formula (3) for facilitating extraction of matrix elements
Wherein VN+, VN-, I (t), P (t) are variables to be solved to be put into X column vectors, corresponding coefficients thereof are taken as elements of an admittance matrix, related terms of all n iterations on the right are taken as known quantities to be put into RHS column vectors, and for the subsequent model description, the known terms are equivalent to Irhs
For equation (2), 1) linearizing dP/dt by using a backward euler method, and the F 2 function is the same as the F 1 function, and the first-order taylor expansion is performed at the working point (V (n)(t),P(n) (t)), so as to obtain:
Wherein h is a time step, F 2 (n)(t)=F2(V(n)(t),P(n)(t)),V(t)=VN+(t)-VN- (t), and performing transformation on formula (5)
Extracting the elements with the coefficients G before the variables I (t), P (t) and V N+(t)、VN- (t), and equally, for the subsequent model description, equating the known term to be Prhs and putting the element into RHS column vector
For currents flowing through nodes V N+ and V N- to flow in the positive direction, assuming the flow into node V N+ is in the positive direction, there are the variables V N+: I (t) and V N-: -I (t) with only I (t), the coefficients are 1 and-1, and no known term is provided.
According to the above description, coefficients before extracting unknown variables form an admittance matrix G:
column vector X formed by unknown variables:
Column vector RHS formed from a known quantity:
to this end, the admittance matrices G and RHS of composite Fecap MNA model have been obtained, and next, the F1 and F2 functions are obtained to write the model into the SPICE simulation program.
2.3 Combining Fecap terminal voltages, polarization states and model formulas to obtain specific expressions of functions F1 and F2 to form a final MNA model of Fecap;
the step 2.3) specifically comprises the following steps:
For the F 1 function, the terminal voltage of Fecap is contributed by the potential difference of the parasitic resistance inside the ferroelectric and the ferroelectric layer potential difference, so the L-K model formula based on Fecap is available:
Wherein AFE is the surface area of the ferroelectric capacitor. For the F 2 function, dp/dt in the L-K model formula of transform Fecap is:
combining the admittance matrices G and RHS, since the F1 and F2 functions are known, then an MNA model with the exact parameters Fecap is obtained;
As shown in fig. 3:
The corresponding matrix parameters are only written into modeling codes specified by SPICE to obtain the built-in MNA model Fecap in SPICE.