JPS5886702A - Method of producing varistor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a varistor that is ultra-compact and suitable for reducing the size and weight of equipment.

An element whose resistance rapidly decreases as the applied voltage increases is generally called a varistor. Conventionally, as a varistor, a ZnO varistor made of ZnO and a trace amount of a metal oxide such as B12O3 is known. Zn
O varistor is made of Zn□□□ with 0.1 and several mol of Bi.
It is obtained by molding a powder to which 2O3, C020C02O31°5b2o3, etc. are added, and sintering it in air at 1250°C.

The ZnO varistor thus obtained can have a rising voltage to the side by changing the thickness. When the voltage-current (I) characteristic is approximated as vac: constant l=(), α (voltage nonlinear index) is 30 to 6.
A value of about 0 was obtained, and the non-ohmic property was also extremely excellent. However, this method has the disadvantage that it is difficult to obtain a small size and good characteristics.

, the non-ohmic nature of the ZnO varistor is due to the barrier of the grain boundaries of the ZnO particles, and therefore the rising stain pressure (is the number of grain boundaries connected in series between the electrodes, in other words, the number of grain boundaries of the device It is proportional to the thickness. Therefore, if the desired rise voltage is 1
If you try to obtain a varistor with a varistor, the thickness of the element will automatically be determined. The rise voltage of the normally obtained device is 100 to 200 V per 11x, and therefore, for many consumer applications, a sintered body with a thickness of about 11aIL is required.

On the other hand, in recent years, electronic components have become smaller and more dense, and various light and compact consumer electronic devices are being developed using this technology. Many semiconductor ICs and LSIs are used in these devices, and protecting them from abnormal overvoltages (surges) has become an important issue.

For these applications, any Zt+O varistor is unsuitable due to its rise voltage and feature size.

Therefore, there is a need for an ultra-small surge protection element suitable for these ultra-small circuits.

The present invention was developed based on this situation, and provides a method for manufacturing a micro-varistor suitable for application to these micro-circuits, and details thereof will be described below along with examples.

1 to 3 show cross-sectional views of a varistor obtained using the method of the present invention. Hereinafter, one embodiment of the present invention will first be described based on FIG. 1.

Gold (Au) was deposited to a thickness of 400 mm on an alumina substrate 1 using a so-called vacuum evaporation method in which a raw material was heated and evaporated in a vacuum chamber to form an electric layer of 2a. Next, this f 1
x 10-' Tor r O It is mounted on a metal electrode in an airtight tank with an exhaust part that can create a high vacuum, and facing it, a sintered body made of zinc oxide (ZnO) and a bismuth oxide (Bi A metal oxide sintered body 3 consisting of O) was attached to each electrode (target). This airtight tank is filled with argon gas as a trace amount of atmospheric gas,
After setting the pressure in the airtight tank to i 2 x 10 −2 Torr, a high frequency sputtering power of 10 oW was applied between both electrodes to cause discharge. In this method, the source material of the target is sputtered and evaporated by the collision of gas molecules separated by electric discharge, forming a film on the substrate.
This is a so-called high frequency sputtering method. Using this method, ZnO film 3'f! : A thickness of 6,000 layers was applied, and a B Z 203 film 4 of a thickness of SOO layers was deposited on top of the layer. Thereafter, the electrodes 2a and 2b were taken out from the airtight tank and heat-treated in the air at a temperature of SOO°C, and then Au was applied to a thickness of 2,000 by vacuum evaporation on the Bi2O3 film 4 to give a conductivity of 2b. Voltage (T) - Current (I) between
When examining the characteristics, rectification was observed in the V-I characteristic, the reverse characteristic had a rising voltage of 6 V, (Z was about 50), and the forward characteristic had a rising voltage of 1 V or less, α is 3
About 0 was obtained. That is, the varistor shown in FIG. 1 manufactured by the above method is a so-called asymmetric varistor that has directionality in the V-■ characteristic.

FIG. 2 shows a cross-sectional view of the varistor obtained in the second embodiment of the present invention, in which the B12O3 film 4iZno film 3
A so-called symmetrical varistor, which has a sandwiched structure and has no directionality in V-1 characteristics, was obtained. The rising voltage in this case is about iv.

α was approximately 50. Figure 3 shows a cross-sectional view of the varistor obtained in the third embodiment of the present invention, which is a symmetrical varistor, but has a structure with an increased number of laminated layers, and has a high rising voltage. Got a barista. In this case, the rising voltage was about 18V, and α was about 60. The non-ohmic property of the device obtained by the present invention is due to the heterojunction between the ZnO film and the B12O3 film, and therefore the rising voltage is
This can be controlled by alternately laminating ZnO films and B12O3 films and arbitrarily selecting the number of laminated layers.

Note that in any of the above embodiments, the ZnO film 3. B
The manufacturing method of the 12o3 film 4 is the same. The same applies to the heat treatment conditions after film formation; regardless of the number of laminated layers, the amorphous B12O formed by sputtering is heat treated in air at a temperature of 80°C.
The metal oxide film whose main component is Z! The beterojunction formed at the boundary with the No. 10 film was stabilized, and non-ohmic properties with excellent stability could be obtained. Films formed by sputtering and whose main component is Bi2O3 in an amorphous state are heat-treated at a temperature below the crystallization temperature (in the case of each of the above examples, the crystallization temperature was 600°C), the v-I characteristic is It was unstable and could not be put to practical use as a ballista.

In other words, when the film was formed, the metal oxide film itself and its petrojunction were incomplete and unstable due to the amorphous state, but by heat treatment at a temperature higher than the crystallization temperature, a stable film and a stable heterojunction can be obtained. Since this is a bond, non-ohmic properties with excellent stability can be obtained.

In the embodiments described above, a combination of alternately laminating at least one layer of a ZnO film and a film of B12O3 was described. bismuth (B
i) Elements other than i), such as praseodymium (Pr)
, rare earths such as lanthanum (La), and barium (Ba
), alkaline earth metals such as strontium (Sr), and metal oxides with higher resistivity than ZnO, such as lead (PB), can also be heat-treated in air at temperatures above the temperature at which crystallization occurs. By doing so, Z
Good non-ohmic properties can be obtained between the nO film and an excellent varistor can be manufactured. Note that non-ohmic properties cannot be obtained in the case of a film made of a metal oxide having a resistivity lower than that of ZnO. In addition, in the above example, B was used as the metal oxide.
Although 12O3 alone was used, 5b203°Sio, which is known as an effective additive for sintered ZnO varistors, was used.
2. Cr2O2, Ga2o32MnO2, B2O3, N
It is possible to further improve the varistor characteristics by adding, for example, 1°. Similarly, AQ2o is added to the ZnO film.
3. By adding additives such as Ga2o3 it is possible to improve the varistor properties.

In addition to the alumina substrate, ceramic substrates such as zirconia, single crystal substrates such as sapphire, glass substrates such as quartz, and metal substrates such as platinum, gold, and stainless steel are used as substrate materials that are stable against temperature. Excellent varistor characteristics can be obtained even if the varistor is manufactured by the same method as in the example.

Although the high-frequency sputtering method was used as the method for forming the film in each of the above examples, Z n O
It is possible to form a film mainly composed of f and a film mainly composed of Bi2O3, and therefore it is also possible to manufacture a varistor having non-ohmic properties.

As is clear from the above description, according to the present invention, it is possible to manufacture an ultra-compact device suitable for downsizing and reducing the weight of equipment.

[Brief explanation of drawings]

1...Alumina substrate, 2a, "2b..."
・Electrode, 3...ZnO film, 4...B1
2o3 membrane.

Claims (2)

[Claims] (1) An electrode is provided on a substrate made of a temperature-stable material, and a film mainly composed of zinc oxide (ZnO) and a metal oxide with a higher resistivity than ZnO are deposited on this electrode by high-frequency sputtering. 1. A method for manufacturing a varistor, comprising alternately stacking at least one layer of films containing metal oxide as a main component, and then heat-treating the film at a temperature higher than that at which crystallization of the film containing metal oxide as a main component occurs. (2) A method for manufacturing a varistor according to claim (1), characterized in that a film containing bismuth oxide (B1203) as a main component is used as the film containing a metal oxide as a main component.