JPH0722209A - Multilayer varistor - Google Patents

Abstract

PURPOSE:To provide a multilayer varistor in which the voltage nonlinear characteristics can be ensured stably by forming an energy barrier positively while enhancing the breakdown resistance against electrostatic discharge. CONSTITUTION:When a multilayer varistor 1 is produced by embedding at least a pair of inner electrodes 3 in a sintered semiconductor ceramic 1 while sandwiching a ceramic layer 2 exhibiting voltage nonlinear characteristics and presenting at least one crystal particle 7 touching both inner electrodes 3 in the ceramic layer 2 between the inner electrodes 3, the content of Bi in the sintered semiconductor ceramic 4 is set at 0.5wt.% or above when expressed in terms of Bi2O3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laminated varistor functioning as a voltage non-linear resistor, and more specifically, it is capable of reliably forming an energy barrier and improving a stable voltage resistance while improving the breakdown resistance by electrostatic discharge. The present invention relates to a structure capable of obtaining linear characteristics.

[0002]

2. Description of the Related Art In recent years, microcomputers have been installed in various devices such as industrial equipment, home electric appliances, and communication equipment. In a device equipped with this microcomputer, since the semiconductor element is subjected to digital control processing, there is a possibility that the semiconductor element may be destroyed or malfunction due to electromagnetic interference such as electrostatic discharge.

In order to solve such a problem of EMI noise, it is necessary to prevent the noise from being emitted from the device and not to enter the device. Since this EMI noise often goes in and out via electric wires in the power supply section and the signal wiring section, a noise filter is connected near the input / output section of the power supply section and the signal wiring section to absorb the noise. As such a noise filter, conventionally, a capacitor element, a composite element of a capacitor and an inductor, an inductor element,
Alternatively, a varistor element is used.

The above-mentioned varistor element was mainly of a disk type. With this disk type, however, the miniaturization of electronic devices in recent years and the miniaturization for the purpose of improving the mounting density accompanying the integration of electronic components, Also, it cannot meet the demand for lower voltage. As a device that can meet such requirements, a laminated varistor has been proposed in place of the disk type (see, for example, Japanese Patent Publication No. 58-23921). According to this laminated varistor, since it is possible to reduce the number of grain boundaries between internal electrodes without growing semiconductor crystal grains enormously, it is possible to reduce the operating voltage and reduce the size. Can also handle.

By the way, the voltage non-linear characteristic of the laminated varistor is exhibited by the energy barrier formed at the joint interface between the internal electrode and the semiconductor crystal grain and at the joint interface (crystal grain boundary) between the crystal grains. In this case, the immunity to a high-speed, high-voltage pulse such as electrostatic discharge is overwhelmingly larger at the interface between the electrode and the crystal particles, and experiments have shown that the interface between particles easily breaks. Has become. However, the laminated varistor currently available on the market has a drawback that the breakdown voltage against electrostatic discharge is low because it includes many bonding interfaces between crystal grains.

In order to solve the above-mentioned drawbacks, the present inventors have proposed a laminated varistor in which crystal grains in contact with both internal electrodes are present in the ceramic layer between the internal electrodes. That is, the presence of the crystal particles in contact with both internal electrodes in the ceramic layer between the internal electrodes can increase the number of bonding interfaces between the internal electrodes and the crystal particles that exhibit the voltage non-linear characteristic, and thus the electrostatic discharge can be increased. The breakdown voltage can be improved.

[0007]

However, in the above laminated varistor, although the breakdown voltage can be improved, the energy barrier during firing is not sufficiently formed, and as a result, stable voltage non-linear characteristics cannot be obtained. There is.

An object of the present invention is to provide a laminated varistor capable of reliably forming an energy barrier and obtaining stable voltage non-linear characteristics while improving breakdown breakdown voltage against electrostatic discharge.

[0009]

When the cause of the insufficient formation of the energy barrier during the above-mentioned firing was examined, the following points were found. That is, the height of the energy barrier formed at the bonding interface between the internal electrode and the crystal particles is not only determined by the difference in the work function between the electrode and the crystal particles, but exists at the bonding interface between the crystal particles. The impurity level has a great influence because it also exists at the bonding interface between the electrode and the crystal grain.

The present inventors have made further studies from the viewpoint that the formation of an energy barrier can be improved by allowing such impurity levels to exist at the electrode interface. It is formed by adsorption of ions, and this phenomenon is caused by Bi in the sintered body.
The existence of is involved. However, conventionally, a large amount of Bi is scattered during this firing, and thus the amount of Bi remaining at the electrode interface is reduced. As a result, it was found that the insulation resistance of the sintered body sharply decreased, which hindered the formation of the energy barrier. Therefore, in order to increase the insulation resistance of the sintered body, a certain amount of Bi is added to the electrode interface.
It is necessary to make the amount remain, and it is thought that the energy barrier can be reliably formed by finding the lower limit amount of this Bi amount, and the present invention has been achieved.

Therefore, according to the present invention, at least a pair of internal electrodes are embedded in a ceramic sintered body so as to overlap with each other with a ceramic layer exhibiting a voltage non-linear characteristic interposed therebetween, and the ceramic layer between the internal electrodes is embedded in both internal electrodes. In a laminated varistor in which at least one crystal particle contacting the electrode is present, the content of Bi contained in the sintered body is B
It is characterized in that it is 0.5 wt% or more in terms of i ₂ O ₃ .

Here, Bi ₂ O ₃ remaining in the sintered body is
To increase the amount to 0.5 wt% or more, the thickness of the ceramic layer,
This can be achieved by appropriately setting the firing temperature and the like. Specifically, the residual Bi ₂ O ₃ amount can be increased as the thickness of the ceramic layer is reduced and the firing temperature is set lower. For example, thickness 2
By setting the temperature to 1000 to 1150 ° C. with the ceramic layer of 0 μm, the Bi ₂ O ₃ content is set to 3.5 to 0.5 wt%.
You can

[0013]

According to the laminated varistor of the present invention, the amount of Bi ₂ O ₃ contained in the sintered body is set to 0.5 wt% or more.
As described in the mechanism described above, the presence of Bi can form an impurity level at the bonding interface between the internal electrode and the crystal particles, and can increase the insulation resistance of the sintered body. As a result, an energy barrier during firing can be sufficiently formed, and stable voltage nonlinear characteristics can be obtained.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. 1 and 2 are views for explaining a laminated varistor according to an embodiment of the present invention. In the figure, 1 is a laminated varistor of this embodiment. The varistor 1 has a rectangular parallelepiped shape, and a semiconductor ceramic layer 2 containing ZnO as a main component and an internal electrode 3 made of Pt are alternately laminated, and a ceramic layer 6 as a dummy is formed above and below the semiconductor electrode.
Are laminated and laminated, and the laminated body is integrally sintered to form a sintered body 4.

One end face 3a of each internal electrode 3 is a sintered body 4
Are alternately exposed to the left and right end faces 4a and 4b, and the remaining end faces are located inside the ceramic layer 2 and embedded in the sintered body 4. The left and right end faces 4a of the sintered body 4 are
An external electrode 5 made of Ag is formed on 4b,
The external electrode 5 is electrically connected to the one end surface 3a of each internal electrode 3.

Further, the ceramic layer 2 sandwiched between the internal electrodes 3 of the above-mentioned sintered body 4 is a portion that exhibits a voltage non-linear characteristic. The ceramic layer 2 has a thickness of 20 μm or less and is composed of crystal grains having an average grain size of about 10 μm. Further, in the ceramic layer 2, a large number of crystal grains 7 contacting both of the internal electrodes 3 facing each other are provided.
Exists.

The Bi content in the sintered body 4 is 0.5 wt% or more in terms of Bi ₂ O ₃ . As a result, a predetermined amount of impurity level is formed at the bonding interface between the internal electrode 3 and the crystal particles 7.

According to the laminated varistor 1 of the present embodiment, since the amount of Bi ₂ O ₃ contained in the sintered body 4 is 0.5 wt% or more, the bonding interface between the internal electrode 3 and the crystal grain 7 is burned. Union 4
It is possible to form the impurity level necessary for increasing the insulation resistance of. As a result, an energy barrier during firing can be sufficiently formed, and stable voltage nonlinear characteristics can be obtained.

Further, in this embodiment, the internal electrodes 3 facing each other are used.
Since the crystal particles 7 contacting both of them are present in the meantime,
The joint interface between the crystal grains 7 can be reduced to increase the joint interface between the internal electrodes 3 and the crystal grains 7, and the breakdown voltage against high-speed, high-voltage pulses such as electrostatic discharge can be improved, and the structure can be used as a noise countermeasure component. The reliability can be improved.

Next, a characteristic test conducted to manufacture the laminated varistor 1 of this embodiment and confirm its effect will be described. First, a method of manufacturing the laminated varistor 1 will be described.

ZnO (96.5 mol%), Bi ₂ O
₃ (1.0 mol%), Co ₂ O ₃ (1.0 mol%), M
nO (1.0 mol%) and Sb ₂ O ₃ (0.5 mol%) are mixed in the respective ceramic materials at the above molar ratios,
A raw material powder is prepared by adding 1.0% by weight of glass powder composed of B ₂ O ₃ , SiO ₂ , and ZnO.

Next, a butyral organic binder is mixed with the above raw material powder to form a slurry, and the slurry is used to form green sheets having a thickness of 20 μm and 50 μm, respectively, and each green sheet has a predetermined size. A large number of ceramic layers 2 and 6 are formed by cutting into a rectangular shape.

Next, a vehicle is mixed with Pt to prepare an electrode paste, and the paste is screen-printed on the upper surface of each ceramic layer 2 to form the internal electrodes 3. Then, as shown in FIG. 2, the ceramic layers 2 and the internal electrodes 3 are overlapped with each other alternately, and the one end faces 3a of the internal electrodes 3 are overlapped so as to be positioned alternately, and the dummy ceramic layers are provided on the upper and lower surfaces thereof. 6 are stacked and laminated. A pressure of 2 ton / cm ² is applied in the laminating direction to perform pressure bonding to form a laminated body. In this embodiment, the ceramic layer 2 having a thickness of 20 μm and the thickness of 50 μm are used.
The ceramic layers 2 of m 2 were respectively adopted, and the ceramic layers 6 for dummy of 50 μm were adopted to prepare two kinds of laminated bodies.

Next, the ceramic layer 2 having the above-mentioned thickness of 20 μm
The laminated body using is sintered in air at 1100 ° C. for 3 hours, and the laminated body using a ceramic layer having a thickness of 50 μm is sintered in air at 1250 ° C. for 3 hours. Got After applying the Ag paste to the left and right end surfaces 4a and 4b of each sintered body 4 thus obtained, 6
The external electrode 5 is formed by baking at a temperature of 00 ° C. for 10 minutes. As a result, the laminated varistor 1 of this embodiment is manufactured.

[0025]

[Table 1]

Table 1 shows the varistor voltage V _1mA , the voltage non-linearity coefficient a, and the insulation resistance value In (resistance value when a voltage of 50% of the varistor voltage is applied) for each laminated varistor manufactured by the above method. ) Shows the result of having measured.
In addition, each of the above laminated varistor was polished and subjected to thermal etching to observe crystal particles. As a result, the average grain size of the crystal grains was 10.3 μm in the sample using the ceramic layer having a thickness of 20 μm, and 28.4 μm in the sample using the ceramic layer of 50 μm. In addition, when crystal grains contacting both the internal electrodes were examined in both of these samples, it was confirmed that both samples were present.

In Table 1, the thickness is 50 μm and the firing temperature is 1
In the case of Sample No. 2 at 250 ° C., the insulation resistance value logIR was as low as 1.9, which was almost in a short circuit state, and most of Bi was scattered during firing. On the other hand, the thickness is 20 μm
In the case of Sample No. 1 having a firing temperature of 1100 ° C, the insulation resistance value logIR was as high as 7.6 and the varistor voltage was 4.
A satisfactory value of 3 V and a voltage non-linearity coefficient of 42 is obtained.

[0028]

[Table 2]

Table 2 shows the firing temperature and B scattered during firing.
The results of examining the relationship with the amount of i ₂ O ₃ and the changes in the insulation resistance value due to this are shown. The number of crystal particles in contact with both internal electrodes was the same in each sample.

As shown in Table 2, the firing temperature is 1200 ° C.
Beyond this, most of Bi ₂ O ₃ is scattered and the residual amount is 0.36 wt% or less.
The logIR also drops sharply to 2.9. On the contrary,
When the firing temperature is in the range of 1000 ° C to 1175 ° C, the lower the temperature, the more the amount of residual Bi ₂ O ₃ becomes 0.51 to 0.51.
It is as large as 3.52 wt%, and the insulation resistance value logIR is as high as 6.7 to 7.9. From this, it is understood that the amount of Bi ₂ O ₃ remaining in the sintered body after firing can be reduced to 0.5 wt% or less by selecting the firing temperature according to the thickness of the ceramic layer.

[0031]

As described above, according to the laminated varistor according to the present invention, the amount of Bi contained in the sintered body is 0.5 wt% or more in terms of Bi ₂ O ₃ , so that it is destroyed by electrostatic discharge. The insulation resistance of the sintered body can be increased while improving the withstand voltage, and as a result, an energy barrier can be reliably formed, and stable voltage non-linear characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view illustrating a laminated varistor according to an embodiment of the present invention.

FIG. 2 is an exploded perspective view showing a method for manufacturing a laminated body of the above embodiment.

[Explanation of symbols]

 1 Multilayer Varistor 2 Ceramic Layer 3 Internal Electrode 4 Sintered Body 7 Crystal Particles

Claims (1)

[Claims] 1. At least a pair of internal electrodes are embedded in a sintered body made of a semiconductor ceramic so as to overlap each other with a ceramic layer exhibiting a voltage non-linear characteristic sandwiched therebetween, and both of the internal electrodes are embedded in the ceramic layer between the internal electrodes. In a laminated varistor in which at least one crystal particle contacting the internal electrode is present, the content of Bi contained in the above sintered body should be 0.5 wt% or more in terms of Bi ₂ O _3. Characteristic laminated varistor.