JP2004304020A - Simulation method of dielectric breakdown protective circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a simulation method for quickly and accurately analyzing the dielectric breakdown protective resistance of a dielectric breakdown protective circuit. <P>SOLUTION: A dielectric breakdown protective element constituted of an insulation gate type field effect transistor is replaced with an equivalent circuit using a bipolar transistor, and a current flowing from the drain of the dielectric breakdown protective element to a substrate is expressed by a first current source by an impact ionization current and a second current source by the current based on an electron / hole pair thermally generated in a depletion layer. The first current source is expressed by the product of a drain current in the case without impact ionization, a coefficient accompanying the impact ionization and a function for adjustment. The function for the adjustment is fixed when a drain voltage is smaller than a prescribed value and decreases when the drain voltage is larger than the prescribed value. With the first and second current sources as input, circuit simulation is repeatedly executed until desired dielectric breakdown protective resistance is obtained while changing the function for the adjustment. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for simulating an electrostatic breakdown protection circuit, and in particular, using a circuit simulator to determine the ESD resistance of a protection circuit for protecting a semiconductor memory or a semiconductor logic circuit element from electrostatic discharge (ESD). The present invention relates to a simulation method of an electrostatic discharge protection circuit to be simulated.
[0002]
[Prior art]
2. Description of the Related Art In a semiconductor device, there is a problem that the semiconductor device discharges due to external charges due to static electricity, causing deterioration or destruction of characteristics of an internal circuit. In order to protect a semiconductor device from such an electrostatic breakdown, an ESD protection circuit is used.
In the conventional countermeasures against the decrease in the ESD resistance of the ESD protection circuit, a method of searching for a semiconductor device having a high ESD resistance has been implemented by repeatedly manufacturing a semiconductor device and repeatedly performing a withstand voltage test of the manufactured semiconductor device while changing process conditions. I have. Recently, with the miniaturization of the element structure of a semiconductor, the element area of an ESD protection circuit has been reduced, and it has become difficult to secure ESD resistance. Therefore, in the above-described method, it takes a long time to search for a semiconductor device having high ESD resistance. As a result, the development period of the semiconductor device increases. Therefore, it is important to evaluate the ESD resistance of the ESD protection circuit in advance by simulation.
[0003]
In the current-voltage characteristics when the current generated by the electrostatic breakdown flows from the drain of the ESD protection element to the substrate, when the drain voltage increases, the current-voltage characteristics shift to a linear region, a saturation region, and an avalanche region, and the drain current is reduced. To increase. Further, there is a snapback region (snapback characteristic) in which the drain voltage decreases as the drain current increases. The quality of the ESD resistance largely depends on the snapback characteristics. It can be determined that the ESD resistance is better as the inclination angle of the snapback characteristic curve is larger and the snapback voltage (drain voltage at which snapback starts) is lower.
[0004]
Methods for simulating ESD resistance include a method using a device simulator and a method using a circuit simulator. The former has the drawback that the analysis range is as narrow as several transistors and the calculation time is long. The latter has an advantage that the layout data can be reflected and the calculation time is short, but the ESD resistance cannot be evaluated accurately. In order to overcome the drawbacks of the method using a circuit simulator, a method of simulating ESD resistance by circuit simulation using an equivalent circuit of an ESD protection circuit in consideration of snapback characteristics has been proposed (for example, see Patent Document 1). .
[0005]
[Patent Document 1]
JP 2001-339052 A
[Problems to be solved by the invention]
In this type of ESD protection circuit simulation method, the circuit simulation is repeatedly performed while changing the resistance value in the equivalent circuit based on experience until a desired current-voltage characteristic (snap-back characteristic) is obtained. When the resistance value in the equivalent circuit is changed, the relationship between the coefficient for expressing the current source due to the impact ionization current generated in the high electric field region of the drain and the drain voltage changes. For this reason, it is necessary to change the relationship between the coefficient representing the current source by the impact ionization current input to the circuit simulator and the drain voltage. In an actual circuit simulation, a table representing a relationship between a coefficient for representing a current source by an impact ionization current and a drain voltage is used. Therefore, the table must be re-created every time the relationship between the coefficient representing the current source due to the impact ionization current and the drain voltage changes. That is, each numerical value in the table must be recalculated using a predetermined calculation formula. As a result, the time required for obtaining the desired snapback characteristics increases.
[0007]
An object of the present invention is to provide a simulation method for an ESD protection circuit that can quickly and accurately analyze the ESD resistance of the ESD protection circuit.
[0008]
[Means for Solving the Problems]
FIG. 1 shows the basic principle of the present invention.
In the method for simulating an electrostatic discharge protection circuit according to the first aspect, first, an electrostatic discharge protection element constituted by an insulated gate field effect transistor is replaced with an equivalent circuit using a bipolar transistor. Next, a current flowing from the drain of the electrostatic discharge protection element to the substrate is divided into a first current source by an impact ionization current and a second current source by a current based on an electron-hole pair thermally generated in a depletion layer. Represent. Further, the first current source is represented by a product of a drain current when there is no impact ionization, a coefficient associated with the impact ionization, and an adjustment function. The adjusting function is constant when the drain voltage is lower than a predetermined value, and decreases when the drain voltage is higher than the predetermined value. Then, the circuit simulation is repeatedly performed using the first and second current sources as inputs and changing the adjustment function until a desired resistance to electrostatic discharge protection is obtained.
[0009]
In the present invention, the electrostatic breakdown protection resistance obtained by circuit simulation can be adjusted only by multiplying the coefficient accompanying impact ionization by the adjustment function. For this reason, when adjusting the electrostatic breakdown protection resistance, the relationship between the coefficient associated with impact ionization and the drain voltage can be easily changed. Therefore, a desired resistance to electrostatic discharge protection can be obtained in a shorter time than when the resistance value in the equivalent circuit is changed. Also, by expressing the current flowing from the drain of the electrostatic discharge protection element to the substrate by the first and second current sources, the resistance to electrostatic discharge protection can be accurately evaluated.
[0010]
In the method for simulating the electrostatic discharge protection circuit according to the second aspect, the adjustment function is represented by A / (1 / A + 1 / B) using a parameter A given by a constant and a parameter B given by a function of a drain voltage. Represent.
By expressing the adjustment function with a simple formula, the adjustment function can be easily changed. That is, the resistance to electrostatic breakdown protection obtained by circuit simulation can be easily adjusted.
[0011]
In the method for simulating the electrostatic discharge protection circuit according to the third aspect, the current-voltage characteristics of the drain obtained by the circuit simulation are compared with expected values to determine whether or not a desired resistance to electrostatic discharge protection has been obtained. The expected value is the drain current-voltage characteristic obtained by device simulation of the electrostatic discharge protection device.
The quality of the electrostatic breakdown protection resistance depends on the current-voltage characteristics of the drain. For this reason, by comparing the current-voltage characteristics of the drain obtained by the circuit simulation and the device simulation, it is possible to easily determine whether or not the desired resistance to electrostatic discharge protection has been obtained.
[0012]
In the method for simulating an electrostatic discharge protection circuit according to claim 4, the slope of the linear region after snapback in the current-voltage curve of the drain obtained by the circuit simulation and the snapback in the current-voltage curve of the drain obtained by the device simulation. When the difference from the inclination of the subsequent linear region is equal to or smaller than a predetermined value, it is determined that the desired electrostatic breakdown protection resistance has been obtained.
[0013]
The quality of the electrostatic breakdown protection resistance generally depends on the snapback characteristic in the current-voltage characteristic of the drain. For this reason, by using the comparison result between the slopes of the linear regions after snapback obtained by the circuit simulation and the device simulation for the determination of the result of the circuit simulation, it is determined whether or not a desired electrostatic breakdown protection resistance is obtained. Can be accurately determined.
[0014]
In the method for simulating an electrostatic discharge protection circuit according to claim 5, a snapback start point in a drain current-voltage curve obtained by circuit simulation and a snapback start point in a drain current-voltage curve obtained by device simulation. Is smaller than or equal to a predetermined value, it is determined that the desired electrostatic breakdown protection resistance has been obtained.
[0015]
The quality of the electrostatic breakdown protection resistance generally depends on the snapback characteristic in the current-voltage characteristic of the drain. For this reason, by using the comparison results between the snapback start points obtained by the circuit simulation and the device simulation, respectively, for determining the result of the circuit simulation, it is possible to accurately determine whether or not a desired electrostatic discharge protection resistance is obtained. Can be determined.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments will be described with reference to the drawings. 2 to 6 show one embodiment of the simulation method of the electrostatic discharge protection circuit according to the present invention.
In the method for simulating an electrostatic discharge protection circuit according to the present invention, first, an electrostatic discharge protection element (ESD protection element) constituting the electrostatic discharge protection circuit is replaced with an equivalent circuit.
FIG. 2 shows an equivalent circuit of the ESD protection element.
The equivalent circuit 100 is an equivalent circuit of an ESD protection element composed of an nMOSFET (MOSFET: Metal-Oxide-Semiconductor Field-Effect Transistor). Note that the equivalent circuit 100 is configured for a region (a source, a drain, a well, and the like) formed in the semiconductor substrate. Therefore, the equivalent circuit 100 does not include the gate of the nMOSFET. A well 103 is formed in a p-type substrate 104 by introducing a p-type impurity. The source 101 and the drain 102 of the nMOS transistor structure are formed by introducing an n-type impurity into the well 103.
[0017]
The equivalent circuit 100 has a bipolar transistor TR that is a parasitic element in order to reproduce the snapback characteristic. The collector of the bipolar transistor TR and one end of each of the current sources Ileakc, ξIc, and the junction capacitance Cd are connected in parallel to a drain terminal D via a drain resistance Rd. The other ends of the current sources Ileakc and ξIc and the junction capacitance Cd are connected to the base of the bipolar transistor TR. The base of the bipolar transistor TR is connected to a base terminal B via base resistors Rb and Rbs connected in parallel. The emitter of the bipolar transistor TR is connected to the source terminal S via the source resistor Rs. In the bipolar transistor TR, the emitter current Ie flows through the emitter, the collector current Ic flows through the collector, and the base current Ib flows through the base. The substrate current Isub flows through the substrate 104 via the base resistors Rb and Rbs. Note that the substrate current Isub changes depending on whether it is due to holes in the substrate 104 or to holes generated by impact ionization in the depletion layer near the drain 102. The base resistance Rbs is provided to represent a change in the substrate current Isub.
[0018]
The current sources ξIc (first current source) and Ileakc (second current source) are provided for evaluating ESD resistance, and represent a current flowing from the drain 102 of the ESD protection element to the substrate 104. The current source Ileakc represents a current based on an electron-hole pair thermally generated in a depletion layer between the drain 102 and the well 103. The current source ξIc represents an impact ionization current generated when electrons flowing between the source 101 and the drain 102 are accelerated by an electric field of a depletion layer near the drain 102. The current source ξIc is given as a table representing the relationship between the coefficient 伴 う due to impact ionization and the drain voltage.
[0019]
Parameters such as the resistances Rd and Rb are calculated using, for example, a device simulator Medici. The current sources Ileakc, ξIc, junction capacitance Cd, and the like are tabulated as a function of the drain voltage. Since the above-described method of calculating the bipolar parameter is described in Japanese Patent Application Laid-Open No. 2001-339052, a detailed description is omitted here.
[0020]
Next, the parameters described above are input to a circuit simulator, and a circuit simulation is performed. At this time, a drain voltage is supplied to the drain terminal D from a separately modeled electrostatic source such as a person or a machine.
Next, the current-voltage characteristics of the drain obtained by the circuit simulation are compared with expected values to determine whether or not a desired resistance to electrostatic discharge protection has been obtained. The expected value is a current-voltage characteristic of the drain obtained by performing a device simulation of the ESD protection element in advance.
[0021]
FIG. 3 shows an example of the current-voltage characteristics of the drain obtained by the circuit simulation.
When the snapback characteristic is reproduced by the circuit simulation, the slope of the linear region X after the snapback and the start point SP of the snapback (the drain voltage Vds and the drain current Ids) are accurately reproduced in the drain current-voltage curve. is important.
[0022]
FIG. 4 shows a part of the linear region X of FIG.
In the present embodiment, the slope of the linear region after snapback in the drain current-voltage curve Lc obtained by the circuit simulation, and the slope of the linear region after snapback in the drain current-voltage curve Ld obtained by the device simulation, It is determined whether or not the difference is equal to or smaller than a predetermined value. The slopes of the current-voltage curves Lc and Ld are, for example, (Idc2-Idc1) / (Vd2-Vd1) and (Idd2-Idd1) / (Vd2-Vd1) using preset drain voltages Vd1 and Vd2, respectively. Is calculated.
[0023]
FIG. 5 shows the vicinity of the start point SP of the snapback in FIG.
In this embodiment, the snapback start point SPc (drain voltage Vdsc, drain current Idsc) in the drain current-voltage curve Lc obtained by the circuit simulation and the snapback in the drain current-voltage curve Ld obtained by the device simulation It is determined whether the distance d from the start point SPd (drain voltage Vdsd, drain current Idsd) is equal to or less than a predetermined value.
[0024]
If both of the above two conditions are satisfied, the simulation of the ESD protection circuit is completed. That is, a desired snapback characteristic (electrostatic breakdown protection resistance) can be obtained by circuit simulation.
If at least one of the above two conditions is not satisfied, the adjustment function f is changed. The adjustment function f is represented by A / (1 / A + 1 / B) using the parameters A and B. The parameter A is set as a constant with respect to the drain voltage. Parameter B is set as a function of the drain voltage. Note that the relationship between the parameter B and the drain voltage is tabulated. The coefficient ξ for representing the current source ξIc described above is updated to a result obtained by multiplying the coefficient ξ before adjustment of the adjustment function f by the adjustment function f. The adjustment function f is set to 1 as an initial value.
[0025]
FIG. 6 shows a process of changing the adjustment function f.
In FIG. 6, a white triangle indicates the parameter A, a white inverted triangle indicates the parameter B, a black triangle indicates the adjustment function f, a black circle indicates the coefficient ξ before the adjustment function f is changed, and a white circle indicates the adjustment function f. Shows the coefficient ξ after the change. The adjustment function f represented by the parameters A and B described above is constant when the drain voltage is smaller than a predetermined value (5V), and decreases when the drain voltage is larger than the predetermined value (5V). Therefore, when the drain voltage is smaller than the predetermined value (5 V), the coefficient ξ matches before and after the adjustment function f is changed due to the influence of the parameter A. When the drain voltage is larger than the predetermined value (5 V), the coefficient ξ becomes smaller before and after the adjustment function f is changed due to the influence of the parameter B. Thus, a new table is created in a short time only by multiplying each numerical value of the table representing the relationship between the coefficient ξ and the drain voltage by the adjusting function f. Then, the updated table is input to the circuit simulator, and the processes after the circuit simulation are performed again.
[0026]
As described above, the present embodiment has the following advantages.
Simply multiplying the coefficient ξ by the adjustment function f can adjust the electrostatic breakdown protection resistance obtained by the circuit simulation. Therefore, when adjusting the electrostatic breakdown protection resistance, the table representing the relationship between the coefficient ξ and the drain voltage can be easily updated. Therefore, a desired snap-back characteristic can be obtained in a shorter time than when changing the values of the resistors Rb, Rd and the like in the equivalent circuit 100. In addition, by expressing the current flowing from the drain 102 of the ESD protection element to the substrate 104 by the current sources ξIc and Ileakc, the resistance to electrostatic discharge protection can be accurately evaluated.
[0027]
Since the adjustment function f is represented by a simple equation, the adjustment function f can be easily changed. That is, the resistance to electrostatic breakdown protection obtained by circuit simulation can be easily adjusted.
By comparing the current-voltage characteristics of the drain obtained by the circuit simulation with the current-voltage characteristics of the drain obtained by the device simulation, it is possible to easily determine whether or not a desired electrostatic breakdown protection resistance is obtained. In particular, by using the comparison result between the slopes of the linear region after snapback obtained by the circuit simulation and the device simulation and the comparison result between the start points of the snapback for circuit simulation result determination, a desired result is obtained. It is possible to accurately determine whether or not the electrostatic breakdown protection resistance is obtained.
[0028]
In the above-described embodiment, the example in which the adjustment function f is represented by A / (1 / A + 1 / B) using the parameters A and B has been described. The present invention is not limited to such an embodiment. For example, the adjustment function f may be represented by another expression having a characteristic that is constant when the drain voltage is lower than a predetermined value and decreases when the drain voltage is higher than the predetermined value.
[0029]
In the above-described embodiment, an example is described in which both a determination process using the slope of the linear region after snapback and a determination process using the start point of snapback are performed in order to determine the result of the circuit simulation. Was. The present invention is not limited to such an embodiment. For example, in order to determine the result of the circuit simulation, only one of the determination process using the inclination of the linear region after snapback and the determination process using the snapback start point may be performed.
[0030]
In the above-described embodiment, the example in which the nMOSFET is used as the ESD protection element has been described. The present invention is not limited to such an embodiment. For example, a pMOSFET may be used as an ESD protection element.
As described above, the present invention has been described in detail. However, the above-described embodiment and its modifications are merely examples of the present invention, and the present invention is not limited thereto. Obviously, modifications can be made without departing from the scope of the present invention.
[0031]
【The invention's effect】
According to the method for simulating the electrostatic discharge protection circuit of the first aspect, it is possible to obtain a desired resistance to electrostatic discharge protection in a short time. In addition, the electrostatic breakdown protection resistance can be accurately evaluated.
In the method for simulating the electrostatic discharge protection circuit according to the second aspect, the adjustment function can be easily changed. That is, the resistance to electrostatic breakdown protection can be easily adjusted.
According to the simulation method of the electrostatic discharge protection circuit of the third aspect, it can be easily determined whether or not a desired resistance to the electrostatic discharge protection has been obtained.
[0032]
In the simulation method of the electrostatic discharge protection circuit according to the fourth and fifth aspects, it is possible to accurately determine whether or not a desired resistance to the electrostatic discharge protection has been obtained.
[Brief description of the drawings]
FIG. 1 is a flowchart showing the basic principle of the present invention.
FIG. 2 is an explanatory diagram showing an equivalent circuit of the ESD protection element.
FIG. 3 is an explanatory diagram showing an example of a current-voltage characteristic of a drain.
FIG. 4 is an explanatory diagram illustrating a determination process using the inclination of a linear region after snapback.
FIG. 5 is an explanatory diagram illustrating a determination process using a snapback start point.
FIG. 6 is an explanatory diagram showing a process of changing an adjustment function.
[Explanation of symbols]
Reference Signs List 100 Equivalent circuit 101 Source 102 Drain 103 Well 104 Substrate ξIc, Ileakc Current source Lc, Ld Current-voltage curve SP, SPc, SPd Start point of snapback

Claims (5)

Replace the electrostatic discharge protection element composed of insulated gate type field effect transistors with an equivalent circuit using bipolar transistors,
The current flowing from the drain of the electrostatic breakdown protection element to the substrate is represented by a first current source based on an impact ionization current and a second current source based on a current based on an electron-hole pair thermally generated in a depletion layer,
The first current source is configured such that the drain current in the absence of impact ionization, the coefficient associated with impact ionization, is constant when the drain voltage is lower than a predetermined value, and decreases when the drain voltage is higher than the predetermined value. Expressed as the product of the adjustment function and
A simulation of an electrostatic discharge protection circuit, wherein a circuit simulation is repeatedly performed until the desired resistance to electrostatic discharge protection is obtained while changing the adjusting function by using the first and second current sources as inputs. Method. The method for simulating an electrostatic discharge protection circuit according to claim 1,
A simulation of an electrostatic discharge protection circuit, wherein the adjustment function is represented by A / (1 / A + 1 / B) using a parameter A given by a constant and a parameter B given by a function of a drain voltage. Method. The method for simulating an electrostatic discharge protection circuit according to claim 1,
A current-voltage characteristic of the drain obtained by circuit simulation is compared with an expected value which is a current-voltage characteristic of the drain obtained by device simulation of the electrostatic discharge protection element, and a desired electrostatic discharge protection resistance is obtained. A method for simulating an electrostatic discharge protection circuit, which comprises determining whether or not the circuit has been damaged. The simulation method for an electrostatic discharge protection circuit according to claim 3,
The difference between the slope of the linear region after snapback in the current-voltage curve of the drain obtained by the circuit simulation and the slope of the linear region after snapback in the current-voltage curve of the drain obtained by the device simulation is a predetermined value. A method for simulating an electrostatic discharge protection circuit, comprising determining that a desired resistance to electrostatic discharge protection has been obtained when the value is not more than a value. The simulation method for an electrostatic discharge protection circuit according to claim 3,
When the distance between the start point of snapback in the current-voltage curve of the drain obtained by the circuit simulation and the start point of snapback in the current-voltage curve of the drain obtained by the device simulation is equal to or less than a predetermined value. A method for simulating an electrostatic discharge protection circuit, wherein it is determined that desired electrostatic discharge protection resistance is obtained.

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