JPH06291270A - Simulation of floating gate device - Google Patents

Abstract

PURPOSE:To enable simulation in a short time by calculating a floating gate voltage based on data regarding floating gate device which carries out simulation and measurement values of a voltage applied to a control gate and a drain voltage. CONSTITUTION:In a floating gate device 1, prediction of boundary conditions between a floating gate 12 and a control gate 13 is complicated and a value to be obtained can not be calculated unless accurate boundary conditions are not set. Therefore, a voltage of the floating gate 12 is calculated based on data regarding the floating gate device 1 which carries out simulation and measurement values of a voltage applied to the control gate 13 and a drain voltage. Simulation is carried out by regarding the floating gate 12 and the control gate 13 as a single gate. The floating gate device 1 can be simulated in a short time in this way.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a floating gate device simulation method using a device simulator.

[0002]

2. Description of the Related Art A floating gate device is a non-volatile memory that stores data even when the power is turned off.
It has a structure in which another gate is formed between the gate electrode of a normal MOS type transistor and the substrate.

FIG. 2 shows a schematic sectional view of a floating gate device. As shown here, in the floating gate device 1, the floating gate 12 is formed above the substrate 11, and the control gate 13 is formed further above the floating gate 12. The periphery of each of the floating gate 12 and the control gate 13 is covered with an oxide film (not shown). A source 14 and a drain 15 are formed on the substrate 11 by diffusing impurities.

In the floating gate device 1 thus constructed, electrons are injected into the floating gate 12 by applying a voltage to the control gate 13 and the drain 15. Floating gate 1
Since the periphery of 2 is covered with an oxide film, the injected electrons are stored in the floating gate 12 as data without disappearing even after the power is turned off.

The floating gate device 1 having the above structure is simulated as follows using a device simulator. First, control gate 1
A mesh point is set at each position where the numerical calculation area below 3 is subdivided into a mesh. Next, a boundary condition between the floating gate 12 and the control gate 13 is set, and a voltage is applied to the control gate 13 corresponding to each mesh point. Then, the voltage is measured at each electrode.

However, in the floating gate device 1 having the above structure, the voltage of the floating gate 12 cannot be directly measured. Therefore, in the simulation of the floating gate device 1, various characteristics are obtained by estimating the charge based on the electric capacitance of each part of the device.

[0007]

However, the simulation method of the floating gate device 1 described above has the following problems. That is, since the above-described simulation method includes approximate elements, various characteristics of the device obtained by simulation and characteristics of the device obtained by measurement are significantly different.

In the floating gate device 1, the floating gate 12 and the control gate 1
It is difficult to perform a simulation with a device simulator, because the prediction of the boundary conditions between the 3 and 3 is complicated and the required value cannot be calculated unless the correct boundary conditions are set. Moreover, in the above-mentioned simulation, the number of mesh points set in the numerical calculation area is set more than that of a normal single gate device. For this reason, the calculation in the device simulator takes time as much as the number of mesh points increases.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to solve the above-mentioned problems and to provide a method for obtaining accurate device characteristics in a short time in a floating gate device simulation by using a device simulator.

[0010]

In order to solve the above-mentioned problems, the following three steps are performed in the floating gate device simulation method of the present invention. First, in the first step, the data of the diffusion concentration of impurities of the structure and components of the floating gate device to be simulated is obtained. In the second step, a voltage is applied to the control gate of the floating gate device to measure the drain voltage. And in the third step,
The floating gate voltage is calculated according to the following calculation method using the data obtained in the first step and the value measured in the second step. First, the channel coefficient related to the capacitance between the control gate and the floating gate based on the data obtained in the first step is multiplied by the voltage applied to the control gate in the second step to obtain a first value. Term. In addition, the drain coefficient related to the electric capacitance between the floating gate and the drain based on the data obtained in the first step is multiplied by the drain voltage measured in the second step to obtain a second term. Further, the charge of the floating gate is divided by the total capacitance of the floating gate device to obtain the third term. Then, the floating gate voltage is calculated by adding the first term, the second term, and the third term.

[0011]

In the above simulation method, the floating gate voltage is calculated from the data regarding the floating gate device to be simulated, the voltage applied to the control gate, and the measured value of the drain voltage. Therefore, the simulation is performed by regarding the floating gate and the control gate as a single gate.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a schematic cross-sectional view of a floating gate device for performing simulation. As shown in the figure, the floating gate device 1 has two gates, a floating gate 12 and a control gate 13, on the upper surface side of the substrate 11. The periphery of each of the floating gate 12 and the control gate 13 is covered with an oxide film not shown here. Further, a source 14 and a drain 15 in which impurities are diffused are formed on the substrate 11 at positions corresponding to both sides of the floating gate 12. Then, terminals 13a, 14a, 15a are connected to the source 14, drain 15 and control gate 13, respectively.

A method of simulating the floating gate device 1 will be described with reference to the flowchart of FIG. 1 and FIG. First, in the first step, data on the structure and impurity diffusion of the floating gate device 1 to be simulated is obtained. (S1) Here, data relating to the electric capacitance of each part of the floating gate device 1, the charge, the concentration of impurities diffused in the source 14 and the drain 15, and the like are obtained from design values or experimental values.

In the second step, in parallel with the first step, the source 14 is grounded and a voltage is applied to the control gate 13, and the voltage applied to the drain 15 at that time is measured. (S2) Here, for example, mesh points are set in each part where the numerical calculation area of the floating gate device 1 is subdivided into a mesh shape. Then, a predetermined voltage is applied to the control gate 13 corresponding to each mesh point,
The voltage applied to the drain 15 is measured. In this case, the floating gate 12 and the control gate 13 are regarded as one gate electrode. Therefore, the floating gate device 1 is replaced with an equivalent single gate device 2 as shown in FIG. Then, only the lower part from the floating gate 12 in FIG. 2 is set as a numerical calculation area, and mesh points are set.

Next, in the third step, the first and second steps are performed.
The voltage of the floating gate 12 is calculated by using Equation 1 using the values obtained in the step. (S3)

## EQU1 ## Vfg = .alpha.ch.times.Vcg + .alpha.d.times.Vd + Qfg / Ctot Each value used in the above equation 1 will be described below. Vfg is the voltage of the floating gate 12 calculated here.
αch is a channel coefficient related to the electric capacity between the floating gate 12 and the control gate 13.
Vcg is the voltage applied to the control gate 13 in the second step. αd is the floating gate 12
Is the drain coefficient related to the electric capacity of the portion where the drain 15 and the drain 15 overlap. Vd is the voltage applied to the drain 15 measured in the second step. Qfg is the charge of the floating gate 12. Ctot is the total potential capacitance of the floating gate device 1. Above αch, αd
, Qfg, Ctot are calculated from the data relating to the floating gate device 2 obtained in the first step.

In the above mathematical expression 1, when the source 14 is not grounded, the third term has a negligibly small value, so the third term may be deleted.

Then, in the fourth step, it is judged whether or not the calculation of the voltage of the floating gate 12 has been performed for all the necessary points of the mesh points set in the second step. (S4) Here, when it is determined that the process has not been performed, the process returns to the second process and the subsequent processes are sequentially repeated. On the other hand, if it is determined that the operation has been performed, the calculation of the voltage of the floating gate 12 ends.

Then, using the voltage of the floating gate 12 calculated as described above, various characteristics of the floating gate device 1 are obtained.

In the above method, the voltage of the floating gate 12 is calculated from the data on the structure and impurity diffusion of the floating gate device 1 to be simulated and the measured voltage value. Therefore, it is not necessary to set the boundary condition between the floating gate 12 and the control gate 13 to estimate the voltage of the floating gate 12. Therefore, the floating gate 1
2 and the control gate 13 are regarded as one gate electrode, and a mesh point is set only below the floating gate 12 to perform a simulation.

By incorporating the above first to fourth steps into the device simulator of the single gate device 2 shown in FIG. 3, a floating gate device is simulated in the same manner as the single gate device 2. You can

[0021]

As described in the above embodiments, according to the present invention, the floating gate voltage is calculated from the device structure and the diffusion of impurities and the measured voltage value. Therefore, various characteristics of the device can be obtained by an accurate simulation. You can get it. In addition, by calculating the voltage of the floating gate as described above, the floating gate device can be replaced with a single gate device for simulation. Therefore, the mesh points may be arranged only below the floating gate. Therefore, it is possible to perform the simulation using the simulator of a normal single gate device,
Since the calculation time is shortened, the floating gate device can be simulated in a short time.

[Brief description of drawings]

FIG. 1 is a flowchart illustrating an example.

FIG. 2 is a schematic sectional view of a floating gate device.

FIG. 3 is a schematic cross-sectional view of a single gate device.

[Explanation of symbols]

 1 Floating gate device 11 Substrate 12 Floating gate 13 Control gate 14 Source 15 Drain Vfg Floating gate voltage αch Channel coefficient Vcg Control gate voltage αd Drain coefficient Vd Drain voltage Qfg Floating gate charge Ctot Total capacitance

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location H01L 29/788 29/792

Claims (1)

[Claims] 1. A method of simulating a floating gate device, comprising: arranging a floating gate above a substrate on which a source and a drain are formed, and further arranging a control gate above the floating gate. The structure and impurity of the floating gate device. A second step of applying a voltage to the control gate to measure a drain voltage, and the control gate and the control gate based on the data obtained in the first step. The channel coefficient related to the capacitance between the floating gate and the floating gate is multiplied by the voltage applied to the control gate in the second step to obtain the first term, and the floating gate based on the data obtained in the first step. The drain coefficient related to the capacitance between the drain and Then, the drain voltage measured in the second step is multiplied to obtain the second term, and the charge of the floating gate is divided by the total capacitance of the floating gate device to obtain the third term. And a third step of calculating a floating gate voltage by adding the third term and the third term, and a floating gate device simulation method.