JPH01297803A - Voltage nonlinear resistance laminated thin film and manufacture thereof - Google Patents

Abstract

PURPOSE:To increase a voltage nonlinear index and decrease its variation with time, by laminating a zinc oxide thin film and a strontium titanate thin film alternately. CONSTITUTION:A CrAu vaporization film 2 is formed on the substrate 1 of glass and the like and then, a ZnO film 3 is laminated in a mixed gas atmosphere of Ar and O2 gases and further, a SrTiO3 film 4 is laminated in an atmosphere of an O2 gas alternately on the above film 2 with a high frequency sputtering process. Voltage nonlinear indices and rise voltage values regarding the above films are determined according to the film thicknesses of ZnO and SrTiO3 films 3 and 4. Consequently, the ZnO film 3 is 0.1-5mum thick or it is preferable to have its thickness of 0.3-2mum and then, the SrTiO3 film 4 is 0.01-1mum thick or it is suitable to have its thickness of 0.05-0.7mum. Finally, electrodes 5 and 6 are mounted.

Description

[Detailed Description of the Invention] [Background of the Invention] (Industrial Application Field) The present invention relates to a voltage nonlinear resistance laminated thin film used for overvoltage protection devices in electric circuits, switching elements of drive circuits, etc., and a method for manufacturing the same. More specifically, a voltage nonlinear resistance laminated thin film formed by laminating a thin film layer mainly composed of zinc oxide (Z n O) and a thin film layer mainly composed of strontium titanate (S r T IOa ) and a method for producing the same. Regarding.

(Prior art and problems to be solved by the invention) A voltage nonlinear resistance element is generally called a varistor, and its excellent nonlinear voltage-current characteristics are widely used for the purpose of making circuit voltage stable or absorbing surges. has been done. A typical example is zinc oxide varistor.

Zinc oxide varistors, which are currently commercially available and in use in large quantities, are made by mixing oxides such as Bi, Co, Mn, and Cr with ZnO as the main component, granulating the mixture, and then firing it at high temperature in air. It was then constructed by attaching electrodes using the so-called powder metallurgy method.

Furthermore, recently, a strontium titanate ceramic composition has been developed as a varistor having capacitance and is commercially available. This was also created by the so-called powder metallurgy method, in other words, a molded body made of raw materials containing 5rTiO as the main component and small amounts of Nb2O6, NaF1SiO2, Bi2O3, etc. was heated for 1300 hours in a nitrogen gas atmosphere containing hydrogen gas. It is made by firing at ~1450°C and then firing at 1000°C in air.

On the other hand, in recent years, electronic components have become smaller and more densely packaged, and semiconductor ICs and LSIs are increasingly being used.
Similarly, varistor elements that protect these elements from surges are also required to be ultra-miniaturized. However, there is a limit to miniaturization of the varistor element using the powder metallurgy method described above. For example, with this method 0.1
It is difficult to fabricate a thin film element with a diameter smaller than 100 mm.

Based on these circumstances, a method for manufacturing a varistor laminated thin film by sputtering of ZnO and B12O3 has been proposed (Japanese Patent Laid-Open No. 58-867).
Publication No. 02, Institute of Electronics, Information and Communication Engineers Technical Study Group Material CPM8
5-126).

However, zinc oxide ZnO1 bismuth oxide Bi2O
Regarding the voltage-current characteristics of the laminated film consisting of 3, an extremely high voltage non-linearity index α can be obtained by strictly controlling the fabrication conditions in the region where the rise voltage is around 10 volts or more, whereas when the rise voltage is several volts or more, In the following regions, the value of α tends to decrease, and it is difficult to obtain a laminated film having characteristic values equivalent to those in the vicinity of 10 volts or more.

This system also has the disadvantage that voltage-current characteristics tend to change over time, which is thought to be due to the complex crystal structure of B12O3.

[Summary of the invention]

The present invention provides a solution to the above points, and exhibits excellent characteristics with an extremely high voltage nonlinearity index α even in the range of rising voltages of several volts or less, and has stable voltage nonlinearity with little aging. The purpose of the present invention is to provide a resistive laminated film and a method for manufacturing the same, and an SrTiO3 thin film produced in an oxygen (0) gas atmosphere by a high-frequency sputtering method exhibits nonlinear characteristics in which the current increases rapidly at an applied voltage of around 1 volt. It was completed based on the findings.

Summary: The present invention is characterized in that a zinc oxide thin film and a strontium titanate thin film are alternately laminated.
It is a voltage non-linear resistance laminated thin film.

The present invention also provides a method for producing a strontium titanate film in which a thin strontium titanate film is formed on a substrate by a high-frequency sputtering method, the strontium titanate film being formed in an atmosphere in the presence of oxygen gas;
It is characterized by:

Furthermore, the present invention provides a method for producing a voltage non-linear resistance laminated thin film, which comprises alternately forming and laminating a zinc oxide film and a strontium titanate film on a substrate by high-frequency sputtering. The method is characterized in that the formation is performed in an inert gas atmosphere or an atmosphere in which oxygen gas is present, and the strontium titanate film is formed in an atmosphere in which oxygen gas is present.

Effects According to the present invention, there is provided a stable voltage nonlinear resistance laminated film that exhibits excellent characteristics with an extremely high voltage nonlinearity index α even in the range of a rise voltage of several volts or less, and exhibits little deterioration over time, and a method for manufacturing the same. be able to.

[Detailed description of the invention]

(1) Varistor element of the present invention and its characteristics Laminated structure according to the present invention, that is, a varistor element C, ZnO film and 5rT
i03 films are alternately laminated, but the arrangement and number of layers are arbitrary as long as the effects of the present invention can be obtained. However, it is preferable that the outer layer or surface layer is a ZnO film. Particularly preferred is a varistor element having a three-layer structure as shown in FIG.

The voltage-current characteristics of the laminated film of the present invention can be expressed as in the following equation, as in the case of a normal varistor element.

Here, ■ is a current, ■ is a voltage, C is a constant, and α is a voltage nonlinear index. The voltage nonlinearity index α indicates the degree of voltage nonlinearity, and it can be said that the larger this value is, the better the nonlinearity of the varistor element is. Further, the voltage when the current increases rapidly in response to a change in voltage is called a rising voltage, and it is also important as a characteristic of the varistor element that a varistor element having a desired value of rising voltage can be easily obtained.

Here, the values of the voltage nonlinearity index α and the so-called rise voltage of the varistor element of the present invention are mainly determined by ZnO and 5
It depends on the rTiOa film thickness. Therefore, in the present invention, by controlling this film thickness, it is possible to easily obtain a varistor element having a desired voltage nonlinearity index α and a desired rise voltage.

The film thicknesses of the ZnO and S r T I Oa thin films are not particularly limited as long as they can achieve the desired voltage nonlinearity index α and the desired rising voltage values.
The thickness of the nO film is preferably 0. The thickness of the SrTiO3 film is preferably in the range of 0.01 to 1 μm, more preferably 0.05 μm.
~0.7 μm. -The tendency for boat fishing is that the thickness of the ZnO film is 0.1 μm or less, and the thickness of the 5rTi03 film is 0.0
It is preferable to avoid a laminated film having a thickness of 1 μm or less since it may be destroyed as the applied voltage increases. If both thin films exceed the above range, the rise voltage value will be relatively too high and its change will be slow, making it uneconomical.

The varistor element of the present invention can be manufactured by any known purposeful thin film forming method, and a specific example of a suitable manufacturing method is as follows.

(2) Formation of laminated film by sputtering method The high-frequency sputtering method of the present invention does not require any modification to the high-frequency sputtering method, except that the film is formed under the conditions of the present invention described below. Other than that, there is no difference from the conventional high frequency sputtering method.

The ZnO source and S r T t Oa source are usually ZnO and S r T IOs according to conventional methods, but in order to adjust characteristics such as resistance, they may be added according to the principle of valence control. Modifications can be made depending on the purpose, such as using a target with added agents.

In the present invention, ZnO by high frequency sputtering method
The film is formed in an atmosphere in which 02 gas is present, that is, 02 gas, and a sputtering atmosphere gas commonly used in high-frequency sputtering, such as argon (Ar).
It is carried out in an atmosphere of an inert gas such as 02 gas or a mixed gas of 02 gas and an inert gas. Especially in an atmosphere with only 02 gas,
That is, it is preferable to carry out in an atmosphere of 02 gas. Ar
Z in a gas or mixed gas atmosphere of Ar gas and 02 gas
When an nO film is formed, the voltage non-linearity index α of the obtained varistor element tends to be smaller and the characteristics tend to be slightly inferior compared to when the film is formed in an 02 gas atmosphere. However, depending on the type of substrate, ZnO may have a better adhesion state than a film formed in an 02 gas atmosphere.
A membrane layer is obtained. That is, by appropriately selecting the atmosphere for forming the ZnO film, it is possible to form a ZnO film, and even a ZnO-8rTiO3 laminated film, on various substrates, which is advantageous.

The mixing ratio of the mixed gas of O2 gas and inert gas, such as Ar gas, is 1:1 to 20. Preferably 1:5
~10.

The formation of the 5rTiO film is performed in an atmosphere in which 02 gas is present, preferably in an atmosphere containing only 02 gas, that is, in an 02 gas atmosphere. The formation of this S r T IOa film was performed in 02
Although it is most preferable to carry out the process in a gas atmosphere, a mixed gas of 02 gas and a sputtering atmosphere gas commonly used in the above-mentioned high frequency sputtering method may be used.

The pressure of this sputtering atmosphere gas is 5 x 10
-I x 10-2 torr, preferably 5-7 x
10 torr, more preferably 6 X 10-”t
orr.

In the present invention, since the ZnO film and the 5rT103 film can be formed on various substrates, the present invention can be applied to a wide variety of substrates. In particular, according to the present invention, a ZnO film with strong adhesion can be formed on various substrates, and as a result, it is possible to stably form a ZnO-8rTiO3 laminated film with a ZnO film as a base layer on various substrates. Substrates that can be used in the present invention, particularly on which a ZnO film can be formed, include glass substrates, resin films such as polyethersulfone (PES) films, polyethylene terephthalate (PET) films, and metals as electrodes. A substrate with a coating formed on its surface, such as gold.
Glass substrates with chromium-deposited films, sapphire, silicon, etc.
is preferred. In particular, when a laminated film is formed on a PES film or a PET film, it is possible to easily obtain a minute laminated film element by cutting the laminated film together with the film into a desired size and then melting the film on the substrate. It is advantageous because it can be done.

The frequency of the high-frequency power and high-frequency voltage during sputtering of the present invention is not particularly limited as long as the desired effect is obtained, but the power is about 0.1 to 0.3 KW and the frequency is about 13.56 MHz. preferable.

As described above, in the varistor element according to the present invention, the arrangement of each layer and the number of layers are arbitrary. However, as described above, it is preferable that the outer layer or surface layer is a ZnO layer.

Therefore, in the case of a varistor element having a three-layered laminated structure as shown in FIG.
It is preferable to first form a ZnO film on a substrate, then a 5rTiOa film, and then a ZnO film.

As mentioned above, the characteristics of the created varistor element are Zn
It is important to control the film thickness during film formation because it largely depends on the film thickness of the O film and the S r T t Oa film. Film formation speed generally depends on various conditions such as applied voltage, type of atmospheric gas, atmospheric gas pressure, target-substrate distance, and target temperature, so by adjusting these conditions and film formation time, Desired film thickness can be easily obtained.

In the present invention, after the laminated film is formed, it is possible to use it as a varistor element immediately without performing post-processing such as sintering.

Since the laminated film formed by the method of the present invention has strong adhesion to the substrate, the formed laminated film can be cut together with the substrate into a desired size by means such as a dicer. Thereby, a minute laminated film element can be easily obtained. Furthermore, as mentioned above, if the substrate is a resin film, by dissolving the resin film after cutting,
It is also possible to isolate only the laminated film. According to this method, for example, 5 0++ unX 50
It is also possible to collect minute elements of 100 μm×100 μm or less from a laminated film with a thickness of mm.

In order to use the laminated film formed as described above as a varistor element, it is necessary to form electrodes on this laminated film, which can be done by a conventional method. It is also possible to form a laminated film on a glass substrate or the like on which a vapor-deposited film of gold or the like has been formed in advance, and use this vapor-deposited film of gold or the like as a part of the electrode.

In addition, in order to improve the adhesion to the substrate (4), a ZnO film with a thickness of about 1000 Å or less was coated on the substrate in advance in an Ar atmosphere.
It is also effective to form a film and then form a laminated film thereon.

Furthermore, the characteristics of the ZnO-8rTiO3-based multilayer film of the present invention are stable, with less change over time compared to the characteristics of the ZnO-Bl20a-based multilayer film. As far as the present inventors know, for example, the voltage-current characteristics of a ZnO-B t 20a-based laminated film vary in the diameter of the laminated film, and it may take more than 10 [] to stabilize. , ZnO-8
It is normal for rTiO3-based laminated films to require no more than a few days.

〔Example〕

The present invention will be explained in further detail by giving examples and reference examples below.

In these examples and reference examples, high-purity zinc oxide sintered disks (1,00 mm diameter x 5 mm thickness) and high-purity strontium titanate sintered disks (1,00+++
n diameter x 5 + nm thickness) was used.

The atmospheric gas pressure during sputtering is approximately 6×10”t.
orr, a high-frequency power source with a power of 0.15 KW was used, and the frequency of the high-frequency voltage was 13.56 MHz.

Further, hereinafter, a glass substrate with a gold vapor-deposited film refers to a glass substrate on which chromium and gold are formed to a thickness of 2,000 mm.

Example 1 In this example, S
r TiOa film by high frequency sputtering method,
This is an example formed on various substrates. The film formation status is as shown in Table 1.

According to this example, it can be seen that the 5rTi03 thin film largely depends on the type of substrate and the high frequency sputtering atmosphere. In particular, when the substrate is a glass substrate, a glass substrate with a gold vapor-deposited film, or a glass substrate with a Zno film, and when the sputtering atmosphere is an oxygen gas atmosphere,
It can be seen that a good SrTiO3 thin film can be formed.

Table 1 (Example 1) Example 2 In this example, a single layer film of SrTiO3 was formed on a glass substrate with a gold vapor deposited film by high frequency sputtering in an 02 gas atmosphere to the film thickness shown in Table 2. This is an example of the formation. Electrodes were attached on the surface of the gold vapor deposited film and on the formed 5rTiOa film, and the current-voltage characteristics of this 5rTiO3 film were measured. The results are shown in FIG.

As shown in FIG. 1, the S r TiOa single layer film of this example exhibits voltage nonlinear resistance characteristics in which the current increases rapidly near the applied voltage of 1.

Furthermore, FIG. 1 shows that this so-called rising voltage is 5rTi
This shows that it is possible to control the Oa film thickness to a desired value by controlling the Oa film thickness.

Table 2 (Example 2) Example 3 In this example, ZnO was
This is an example in which a varistor element as shown in FIG. 4 was obtained by forming the film and the 5rTiO film in a 02 gas atmosphere to the film thickness shown in Table 3. That is, a first layer of Zn0 layer 3 and a second layer of 5rTiO are formed on a glass substrate 1 having a gold vapor deposited film 2 on its surface.
It is constructed by forming three layers 4 and a Zn0 layer 3 as the third layer. Further, an electrode 5 was placed on the surface of the gold vapor-deposited film, and an electrode 6 was placed on the third layer of ZnO film, and the voltage-current characteristics of this varistor element were measured. Its voltage-current characteristics are shown in FIG.

FIG. 2 shows that the varistor element obtained in this example is
It is shown that it has a clear and relatively low rise voltage value. In addition, in Figure 2, the value of the rising voltage is mainly Z.
This shows that it changes depending on the nO film thickness.

1 20 1 Example 4 In this example, ZnO was produced by high frequency sputtering method.
The film was heated at 5rT in a mixed gas atmosphere of Ar gas and 02 gas.
This is an example in which a varistor element as shown in FIG. 4, similar to Example 3, was obtained by forming an iO film in an O2 gas atmosphere to a film thickness shown in Table 4.

The voltage-current characteristics of the varistor element are as shown in FIG.

FIG. 3 shows that although the varistor element obtained in this example has a slightly inferior voltage nonlinearity α compared to Example 3, it still has a relatively low rise voltage.

Reference Example When putting a voltage nonlinear resistance laminated thin film into practical use, it is necessary to be able to produce the laminated thin film on various substrates.

Table 5 shows glass substrates, glass substrates with gold evaporated films, PES
This figure shows the state of adhesion of ZnO films formed on films and PET film substrates in various sputtering atmospheric gases.

The adhesion status of the varistor element laminated film of the present invention to the substrate is as follows.
It depends largely on the adhesion status of the ZnO film (-1 layer). According to Table 3, the ZnO film adheres strongly to the glass substrate and does not peel off. On the other hand, the ZnO film adheres strongly to the glass substrate and does not peel off.
On ES or PET films, ZnO films made in Ar gas adhere strongly and become opaque if the film is thick, whereas ZnO films made in 02 gas become transparent but do not peel off. It has been found that it tends to be difficult to process such as cutting. As a result of further investigation, it was found that by including a small amount of 02 gas in the Ar gas atmosphere, the ZnO thin film becomes transparent and the adhesion strength increases.

Also, from the laminated film formed on the film, 50 μm x 50 μm
In order to collect the elements of m, a dicer was used to probe and cut the elements, and the elements with very little chipping were easily obtained.

Table 5

[Brief explanation of the drawing]

1 to 3 show voltage-current characteristics of embodiments of the present invention. The attached numbers here correspond to the sample numbers in Tables 2-4. FIG. 4 shows a cross-sectional view of a three-layer laminated film prepared on a glass substrate with a Cr--Au vapor deposited film in order to measure the voltage-current characteristics of the voltage nonlinear resistance laminated film of the present invention. 1...Glass substrate, 2...Cr*Au vapor deposited film, 3-
・-ZnO film, 4=・S r T I Oa film,
5.6... Electrode. Applicant's agent Mr. Sato

Claims (4)

[Claims] 1. A voltage nonlinear resistance laminated thin film characterized by alternately laminating a zinc oxide thin film and a strontium titanate thin film. 2. A method for producing a strontium titanate film in which a strontium titanate thin film is formed on a substrate by a high-frequency sputtering method, the method comprising forming the strontium titanate film in an atmosphere in the presence of oxygen gas. Method for manufacturing strontium film. 3. A method for producing a voltage non-linear resistance laminated thin film, which consists of alternately forming and laminating zinc oxide films and strontium titanate films on a substrate by high-frequency sputtering, in which the zinc oxide film is formed in an inert gas atmosphere. 1. A method for producing a voltage non-linear resistance laminated film, the method comprising forming a strontium titanate film in an atmosphere containing oxygen gas or in an atmosphere containing oxygen gas. 4. 4. The manufacturing method according to claim 2, wherein the strontium titanate film is formed in an oxygen gas atmosphere.