JPS58101403A - Method of producing thin film 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 extremely 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. As a conventional varistor, a ZnO varistor made of ZnO and a trace amount of metal oxide such as Bi2O3 is known. ZnO varistor contains 0.1 to several moles of ZnO powder'17) Bt20
3. It is obtained by molding powder to which Co2O3, MnO2, and 5b203fx are added and sintering it in air at around 1260°C. Z obtained in this way
The rise voltage of the nO varistor can be controlled by changing the thickness. In addition, when the voltage-current (I) characteristic is approximated as ■ I=()a C: constant, 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 ZnO varistors is due to the grain boundary barriers of ZnO particles.
Therefore, the rising voltage is proportional to the number of grain boundaries connected in series between the electrodes, in other words, to the thickness of the device. Therefore, in order to obtain a varistor having a desired rise voltage, the thickness of the element is automatically determined. The rise voltage of normally obtained devices is 100 to 200 V per 1 mI, and therefore, in many consumer applications, a sintered body with a thickness of about 1 m11 is required. In recent years, electronic components have become smaller and more dense, and various lightweight and compact consumer electronic devices have been developed using this technology.
protection from these abnormal overvoltages (surges) has become an important issue. For these applications, the aforementioned ZnO varistors are unsuitable due to their rise voltage and feature size. Therefore, there is a need for a micro-sized surge protection element suitable for these micro-circuits. The present invention was made based on these circumstances, and provides a manufacturing method for obtaining an ultra-small thin film varistor suitable for application to these ultra-small circuits. Explain in detail.

<Example 1> In FIG. 1, the quartz glass substrate 1 is heated to 1×1σ7 Tor.
A sintered body of a composition (ZnO) consisting of zinc oxide (ZnO) and an additive effective as a varistor is mounted on a metal electrode in an airtight tank having an exhaust part that can be made into a high vacuum of n.
Oxide powder consisting of O varistor sintered body) and barium oxide (Bad) was attached to each electrode (target). This airtight tank was filled with argon gas as a trace amount of atmospheric gas, and the pressure inside the airtight tank was set to 2×1σ2 Torr, and then a high frequency sputtering power of 200 W was applied between both electrodes to cause discharge. This method is a so-called high-frequency sputtering method in which the source material of the target is sputtered and evaporated by the collision of gas molecules separated by electric discharge to form a film on the substrate. Using this method, the BaO film 2 was formed to a thickness of 2000 Å.

Furthermore, ZnO barista composition is added on top of this! Is was deposited to a thickness of 6 μm and laminated. Thereafter, it is taken out from the airtight tank and immersed in SO'C water to dissolve BaO and separate the quartz glass substrate 1 and the ZnO varistor composition film 3.
Only the nO varistor film 3 was taken out and fired in air at 1200°C for 1 hour to obtain a thin film varistor sintered body. Further, as shown in FIG. 2, an AP with a thickness of 2000 mm was formed on both sides of the thin film varistor sintered body 4 by vacuum evaporation to form an electrode 6. When we examine the voltage (g) - current (I) characteristics between these electrodes, we find that the rising voltage is approximately 6V and a is approximately B0, indicating that it is a so-called symmetrical varistor with no directionality in the V-I characteristics. there were.

In the above example, the substrate and the ZnO varistor composition film were separated and the ZnO varistor composition film was fired to obtain a single thin film varistor. In addition to this method, a method for obtaining a thin film varistor is to provide an electrode (for example, Au, Pt, etc.) that can withstand the high temperature of the firing process on a substrate, form a ZnO varistor composition film on this, and then fire it together with the substrate. Alternatively, there is a method of forming a ZnO varistor composition film directly on a substrate and firing it as it is. However, in any of these methods, the ZnO varistor composition film peeled off, foamed, and cracked at the firing temperature due to the difference in thermal expansion coefficient between the substrate or electrode and the ZnO varistor composition film.

In addition, since the ZnO varistor composition film obtained by sputtering has good adhesion to the electrode and substrate, it tends to react at the firing temperature, resulting in unstable V-I characteristics and poor yield. there were. In order to solve these problems, the present invention takes out only the ZnO varistor composition film obtained by sputtering and fires it at 1200''C, so that ZnO crystallizes and becomes particles, and additives such as B12O3 are precipitated at the grain boundaries. By the way, the amorphous ZnO varistor composition film formed by sputtering crystallizes and forms barriers at the grain boundaries. The temperature at which the varistor was formed was 800°C. Therefore, it was not possible to obtain excellent non-ohmic properties by firing at a temperature below this temperature. The material does not cause foaming or peeling, and its V-I characteristics are stable.Furthermore, it can be made into a chip and easily incorporated into a high-density packaging circuit, greatly contributing to the miniaturization and light weight of devices.

<Example 2> In FIG. 1, the quartz glass substrate 1 is heated to IX1σ7 Tor.
A sintered body of a composition consisting of zinc oxide (ZnO) and an additive effective as a varistor (ZnO
Oxide powder consisting of varistor sintered body) and calcium oxide (Cab) was attached to each electrode (bouget). After the preparation step of this high frequency sputtering method, the same process as in Example 1 was performed. However, CaO in Figure 1
Membrane 2 has a different dissolution rate in water than the BaO membrane of Example 1, but the only difference is the time for immersion in water.
This is basically the same idea as the first embodiment. Therefore C
After dissolving the aO film in water, only the ZnO varistor composition film was taken out and fired in air at 1000°C for 1 hour.

As a result, a thin film varistor equivalent to that of Example 1 was obtained.

<Example 3> A thin film varistor was manufactured in the same manner as in Examples 1 and 2. The only difference is that the target of high-frequency sputtering equipment is water-soluble strontium oxide (SrO).
All other methods were the same as in Example 1 and Example 2.

As a result, the characteristics and yield of the thin film varistor were the same as in Example 1. Although quartz glass was used as the substrate material in the embodiments of the present invention, ceramic substrates such as alumina and zirconia, single crystal substrates such as sapphire, and metal substrates such as gold and stainless steel can be used as substrates as long as their surfaces are smooth. be. Furthermore, in this example, barium oxide (Bad), calcium oxide (Cab), or strontium oxide (8rO) may be used as the water-soluble alkaline earth oxide film between the substrate and the ZnO varistor composition film. Potassium nitrate (KNO3) was used as a nitrate film in addition to the alkaline earth oxide film.

Parium nitrate A (Ba(NOa)2) - Lead nitrate (Pb(N
03) Even if membranes such as 2) are used, they are similarly water-soluble, and the same effects as alkaline earth oxide membranes such as BaO membrane, CaO membrane, and SrO membrane can be obtained, and it is considered possible to use them. . Furthermore, although the high frequency sputtering method was described as a method for forming the film in the embodiment, methods other than the sputtering method, such as electron beam evaporation method and ion blating method, may also be used.

[Brief explanation of the drawing]

FIGS. 1 and 2 are diagrams for explaining the method for manufacturing a thin film star (1) of the present invention. FIG. 1 is a cross-sectional view showing the structure in the thin film forming process, and FIG. FIG. 1...Substrate, 2...BaO film, CaO
film, 8rO film, 3... ZnO varistor composition film,
4... Thin film Lister sintered body, 6... Electrode. Name of agent: Patent attorney Toshio Nakao and one other person 11 Figure 11

Claims (2)

[Claims] (1) An alkaline earth oxide film is provided on the substrate by high-frequency sputtering, and then a composition film (ZnO varistor composition film) consisting of zinc oxide (ZnO) and an additive effective as a varistor is laminated, and then A thin film varistor, characterized in that the substrate and the varistor composition film are separated by immersion in water to dissolve the alkaline earth oxide film, and the varistor composition film is fired at a temperature higher than the temperature at which crystallization occurs. manufacturing method. (2) Barium oxide (Ba
d) The method for manufacturing a thin film varistor according to claim 1, characterized in that the method uses step d).