JPH01259504A - Manufacture of laminated thin film with resistance nonlinear in voltage - Google Patents

Abstract

PURPOSE:To provide the title thin film of a high nonlinear voltage exponent without requiring any heat treatment because of its having high adhesive force to a substrate by forming a zinc oxide film in the atmosphere of oxygen gas. CONSTITUTION:A nonlinear voltage-resistance thin film is formed by alternately superimposing a ZnO film layer 2 and a Bi2O3 film layer 3 by a rf(radiofrequency) sputtering method. The ZnO film is formed in the atmosphere of O2. Formation speeds of the films are dependent upon various conditions such as target temperature and the like. Accordingly, desired film thicknesses can be obtained by adjusting the various conditions and time periods required for the film formations. Thus, a varistor element thin film layer which has a strong adhesive force to the substrate can be yielded without any heat treatment, characteristics of which can be controlled with ease.

Description

[Detailed Description of the Invention] [Background of the Invention] (Field of Industrial Application) The present invention relates to a method for manufacturing a voltage nonlinear resistance laminated thin film used for overvoltage protection devices in electric circuits, switching elements of drive circuits, etc. , more specifically, zinc oxide (Zn
Thin film layer mainly composed of O) and bismuth oxide (Bi203)
This invention relates to a method of manufacturing a voltage nonlinear resistance laminated thin film formed by laminating thin film layers mainly composed of.

(Prior art and problems to be solved by the invention) Voltage nonlinear resistance elements are generally called varistors, and their excellent nonlinear voltage-current characteristics are widely used for the purpose of voltage stabilization or surge absorption in circuits. ing. A typical example is zinc oxide varistor.

Zinc oxide varistors, which are currently commercially available and used in large quantities, contain ZnO as the main component, Bi, Co, ~1n, Cr.
It was created using the so-called powder metallurgy method, in which oxides such as oxides were mixed, granulated, baked at high temperature in air, and then electrodes were attached.

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. the result,
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 Bi2O3 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).

JP-A No. 58-86702 discloses that argon (
(Ar) A method for manufacturing a varistor laminated film is disclosed, which is characterized in that one or more ZnO films and Bi2O3 films are alternately laminated by a high-frequency sputtering method in a gas atmosphere, and then heat-treated at a temperature equal to or higher than the crystallization temperature of Bi2O3. ing. However, since this method requires heat treatment, there are practical limitations. That is, when it is necessary to form a film on a substrate with low heat resistance, such as a resin film substrate, heat treatment is difficult, and therefore this method cannot be applied.

Also, the Institute of Electronics, Information and Communication Engineers technical study group material CPM85-12
No. 6 discusses the case where a ZnO film is grown in Ar gas and a Bi 2 O film is grown in oxygen (02) gas. However, in this document, ZnO film and Bi2O3
Only the study results for the case of a two-layer film are described, and the study of film formation conditions for practical use is incomplete. Furthermore, the two-layer film has asymmetrical characteristics, which has practical limitations as a varistor element.

[Summary of the invention]

(Means for Solving the Problems) The present invention provides a solution to the above points, and provides a method for manufacturing a voltage non-linear resistance thin film in which ZnO film layers and Bi2O3 film layers are alternately laminated by high frequency sputtering method. , when a ZnO film is formed in an atmosphere containing 02,
This invention was completed based on the discovery that a varistor element thin film layer with strong adhesion to various substrates can be obtained without heat treatment, and that its characteristics can be easily controlled.

Summary: The present invention uses high-frequency sputtering to
A method for producing a voltage non-linear resistance laminated thin film comprising alternately forming and laminating a zinc oxide film and a bismuth oxide film on a substrate, the zinc oxide film being formed in an atmosphere in the presence of oxygen gas; It is characterized by:

Effects of the Invention According to the present invention, the adhesion to the substrate is strong, and therefore it can be processed into microelements, the voltage non-linearity index is high without heat treatment as in the conventional method, and the rising voltage is clear and can be formed as desired. For example, it is possible to provide a zinc oxide-bismuth oxide-based voltage nonlinear resistance laminated thin film having a relatively low value.

[Detailed description of the invention]

(1) Formation of laminated film by sputtering method The high-frequency sputtering method of the present invention is different from the high-frequency sputtering method that should be produced by this method, except that the film is formed under the conditions of the present invention described below. Other than modifications, there is no difference from the conventional high-frequency sputtering method.

The ZnO source and B 1203 ri are usually ZnO and Bi2O3 according to conventional methods.

In the present invention, ZnO by high frequency sputtering method
The film is formed in an atmosphere containing oxygen (0□) gas.

Formation of a ZnO film by sputtering can be performed in an atmosphere of only 02, but it can be performed in a mixed gas atmosphere of 02 and a sputtering atmosphere gas commonly used in high-frequency sputtering, such as argon (Ar) gas. is more preferable. The mixing ratio of this sputtering atmosphere gas, for example Ar, and the mixed gas of 02 is 1 to 20:1, preferably 5 to 10:1, in terms of volume ratio. An inert gas, particularly a mixed gas of Ar and 02, is preferable because it allows a transparent laminated film having ZnO as the sublayer or base layer to be formed on the resin film with good adhesion.

The formation of the Bi2O3 film can be performed in an Ar and/or 0.degree. atmosphere, but is preferably performed in a 0.degree. atmosphere.

The pressure of this sputtering atmosphere gas is 5×10~I
X 10-2 torr, preferably 5 to = 37 X 10 'torr, more preferably 6 X 1
0-3 torr.

Since the present invention does not require sintering of the film layer after sputtering, a wide variety of substrates can be used to form a laminated film by applying the present invention. Substrates that can be used in the present invention include glass substrates, resin films such as polyethersulfone (PES) films, polyethylene terephthalate (PET) films, and substrates with metal coatings formed on the surfaces as electrodes, such as gold A glass substrate with a chromium-deposited film, sapphire, silicon, etc. are preferable. 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 can be obtained, but the power is about 0.1 to 0.2 KW and the frequency is about 13. It is preferable to perform this at about 56 MHz.

In the laminated structure according to the present invention, the type, arrangement, and number of layers of each layer are arbitrary. However, as described above, the two-layer structure is not preferable because the characteristics as a varistor become asymmetrical. Further, it is preferable that the outer layer or surface layer is a ZnO layer.

Therefore, the formation of the laminated film of the varistor element according to the present invention is as follows:
For example, in the case of a varistor element with a three-layer laminated structure as shown in FIG.
It is desirable to form three films and then a ZnO film in this order.

Since the film formation rate generally depends on conditions such as applied voltage and target temperature, a desired film thickness can be easily obtained by adjusting these conditions and film formation time. As will be described later, the characteristics of the produced varistor element largely depend on the thickness of the ZnO film and the Bi2O3 film, so it is important to control the film thickness during film formation.

The thicknesses of the ZnO and Bi2O3 thin films are not particularly limited as long as they can achieve the desired voltage non-linearity index α and desired rising voltage values, which will be described later. 1-5 μm.

The thickness of the Bi2O3 film is preferably in the range of 0.01 to 1 μm. As a general tendency, the thickness of the ZnO film is 0.1 μm.
Hereinafter, it is preferable to avoid a laminated film in which the Bi2O3 film has a thickness of 0.01 μm or less, since there is a risk of destruction as the applied voltage increases. In addition, even if both thin films exceed the above range, the change in the rise voltage value will be slow,
Not economical.

In the present invention, after the laminated film is formed, it can be used as a varistor element immediately without performing any 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, it is possible to extract minute elements of 100 μm×100 μm or less from a laminated film of about 50 mm×50 mm formed on a substrate.

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.

Furthermore, in order to improve the adhesion to the substrate, it is also effective to form a ZnO film with a thickness of about 1000 Å or less on the substrate in advance in an Ar atmosphere, and then form an @ layer film thereon.

Furthermore, a thin film formed by forming only a ZnO film on a substrate can of course be used as a substrate for forming a varistor element.
There is also the possibility of using the O film as a material for other electronic components. Examples of such uses include gas sensors, surface acoustic wave filters, transparent conductive films, and the like. In particular, the ZnO film obtained by the present invention is advantageous in that it has strong adhesion to the substrate and can be made transparent depending on the formation conditions.

(2) Characteristics of varistor element The voltage-current characteristics of the laminated film 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.

This voltage nonlinearity index α largely depends on the sputtering atmospheric gas conditions. Particularly, it is preferable when 02 is present in the sputtering atmospheric gas or when this atmospheric gas is 02, since this voltage nonlinearity index α can be made extremely large.

Also, the voltage at which the current increases rapidly in response to a change in voltage is called the rising voltage, but this value depends on the presence of 02 in the sputtering atmosphere gas and the thickness of the thin film layer, especially the ZnO film thickness. . Therefore, in the present invention, by controlling the ZnO film thickness, it is possible to form a varistor element having a desired rise voltage value. Further, according to the present invention, it is possible to obtain a varistor element having a lower rise voltage value than the conventional one.

〔Example〕

The present invention will be explained in further detail by giving Examples and Comparative Examples below.

In these Examples and Comparative Examples, high-purity zinc oxide disks (100 mm diameter x 5 mm thickness) and high-purity bismuth oxide disks (100 mm diameter x 5+n+s thickness) were used as targets in the high-frequency sputtering method.

The atmospheric gas pressure during sputtering is approximately 6 x 10-3
torr, 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 deposited film of gold or the like refers to a glass substrate on which chromium and gold are formed to a thickness of approximately 2000 mm.

In the following examples and comparative examples, as shown in FIG. 1B,
A glass substrate 1' with a chromium/gold vapor deposited film 4 attached to its surface.
A first Zn0 layer 2, a second Bi2O3 layer 3, and a third Zn0 layer 2 are formed in a sputtering atmosphere as shown in Table 1. Further, in order to apply a voltage and measure changes in current, 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.

In Examples 1 to 2, Z
This is an example in which a varistor element was obtained by forming an nO film in an O2 gas atmosphere and a Bi2O film in an O2 gas atmosphere to the thickness shown in Table 1.

The voltage-current characteristics of these varistor elements are as shown in FIG.

FIG. 2 shows that the varistor element obtained in this example is
This shows that there is a clear rising voltage value, that is, the voltage nonlinearity index α has an extremely large value.

Figure 2 also shows that the value of the rise voltage mainly depends on the thickness of the ZnO film, and that by controlling this film thickness, it is possible to obtain a varistor element with a desired rise voltage value. It shows.

Note that, contrary to this example, the film was formed in Ar gas with a thickness of 0.4
The voltage-current characteristics of a laminated film consisting of a 0.5 μm thick ZnO film formed in 02 gas and a 0.5 μm thick ZnO film formed in 02 gas are as follows.
Although it exhibited characteristics similar to those of Example 6, which is a laminated film sample composed of an O film, it was unstable and impractical.

Examples 7-11 These examples show that the ZnO film was coated with Ar at a volume ratio of 5:1.
This is an example in which varistor elements were obtained in the same manner as Examples 1 to 6 except that the Bi 2 O film was formed in a mixed gas atmosphere of gas and O2 gas.

The voltage-current characteristics of these varistor elements are as shown in FIG.

Figure 3 shows that if 0□ gas is present as the sputtering atmosphere gas, a varistor element with a large voltage nonlinearity index α can be obtained, and when the thickness of the thin film layer is changed, the voltage of the varistor element - This shows that the current characteristics change significantly.

In Examples 12 to 1, the ZnO film was coated with Ar at a volume ratio of 10:1.
This is an example in which varistor elements were obtained in the same manner as Examples 7 to 11, except that they were formed in a mixed gas atmosphere of gas and 02.

The voltage-current characteristics of these varistor elements are as shown in FIG.

FIG. 4 shows the ratio of 02 gas to Ar gas in Example 1.
Even if it is smaller than 1 to 15, if 02 gas exists in the sputtering atmosphere gas, a varistor element with a large voltage nonlinearity index α can be obtained, and if the thickness of the thin film layer is changed, , which indicates that the voltage-current characteristics of the varistor element change significantly accordingly.

Comparative Examples 1 to 4 In these comparative examples, a ZnO film was prepared in an Ar gas atmosphere.
This is an example in which a varistor element was obtained by forming a Bi 2 O film in an O2 gas atmosphere to the thickness shown in Table 3.

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

FIG. 5 shows that even if the ZnO film is formed in an Ar gas atmosphere,
It is shown that the voltage at which the current rapidly increases (rise voltage) mainly changes depending on the film thickness of zinc oxide ZnO. However, it can be seen that the voltage nonlinearity index α of the varistor element formed with the ZnO film in the A gas atmosphere is not so large, and the slope of the change in voltage-current characteristics is gentle.

Example 17 In this example, the sputtering atmosphere was the same as that of Examples 12 to 1.
6, the thickness of the ZnO film of the first layer and the third layer was 0.5 μm1 The thickness of the Bi2O3 film of the second layer was 0.1 μm.
m, and a laminated thin film having a large value of the voltage nonlinearity α near the applied voltage lv was created.

FIG. 6 shows the voltage-current characteristics of this varistor element.

Further, FIG. 6 shows a varistor element of Example 6 in which a Bi2O3 film has a thickness of 0.1 μm and a ZnO film with a film thickness of approximately 0.5 μm is formed in an 02 gas atmosphere, and a varistor element in which a varistor element is formed in an Ar gas atmosphere. The voltage-current characteristics of the varistor element of Comparative Example 4 formed in the above are shown for comparison.

FIG. 6 clearly shows that the varistor element of this example has a larger value of the voltage non-linearity index α at an applied voltage of around 1 V than that of Example 6 and Comparative Example 4.

Comparative Example 5 Voltage of commercially available zinc oxide-bismuth oxide varistor element
The current characteristics are shown in FIG. 7 as Comparative Example 5.

Example 18 When a voltage nonlinear resistance@layer thin film is put to practical use, it is necessary to be able to produce the laminated thin film on various substrates.

Table 3 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 state of adhesion of the varistor element laminated film to the substrate largely depends on the state of adhesion of the ZnO film, which is the first layer.

According to Table 3, ZnO3 strongly adheres to the glass substrate and does not peel off. On the other hand, on glass substrates with gold vapor-deposited films and PES or PET films, ZnO films prepared in Ar gas adhere strongly and become opaque when the film thickness is thick, whereas ZnO films prepared in Ar gas It has been found that although the ZnO film becomes transparent, it tends to peel off easily and cannot be processed 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 as an atmospheric gas, 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
Processing and cutting was performed using a dicer to obtain elements of m, and elements with extremely little chipping were easily obtained.

Table 3

[Brief explanation of the drawing]

FIG. 1 shows a cross-sectional view of the voltage nonlinear resistance laminated thin film of the present invention, (A) is a three-layer laminated film prepared on various substrates, (B)
1 shows a three-layer laminated film prepared on a glass substrate with a Cr--Au vapor deposited film for measuring voltage-current characteristics. 2 to 4 and 6 show voltage-current characteristics of embodiments of the present invention. The appended numbers here correspond to the numbers of the examples in Table 2. FIG. 5 shows the voltage-current characteristics of Comparative Examples 1 to 4. FIG. 7 shows the voltage-current characteristics of a commercially available product. 1... Substrate (glass, film, etc.), 1'... Glass substrate, 2... ZnO film, 3... Bi2O3 film, 4
...Cr-Au vapor deposited film, 5,6... electrode. Applicant's agent - Engraving of the drawing by Sato (no changes to the content) Figure 1 o-4 Voltage V (V) Voltage f'iV () Lightning pressure V (V) Figure 4 Voltage V (V) Figure 5 Voltage V () Voltage V (V) Procedural amendment 1 Indication of the case 1988 Patent application No. 88651 2 Title of invention Method for manufacturing voltage nonlinear resistance laminated thin film 3 Person making the amendment Relationship with the case Patent application Hito Co., Ltd. Empaiya Airport Service 4 Agent

Claims (5)

[Claims] 1. A method for producing a voltage nonlinear resistance laminated thin film, which consists of alternately forming and laminating zinc oxide films and bismuth oxide films on a substrate by high-frequency sputtering, and in which the zinc oxide film is formed in an atmosphere containing oxygen gas. A method for manufacturing a laminated thin film, which is characterized in that the manufacturing method is carried out inside. 2. 2. The method for producing a laminated thin film according to claim 1, wherein the zinc oxide film is formed in an oxygen gas atmosphere or a mixed gas atmosphere of argon gas and oxygen gas. 3. 2. The method for producing a laminated thin film according to claim 1, wherein the bismuth oxide film is formed in an oxygen atmosphere. 4. A method for producing a zinc oxide film in which a zinc oxide film is formed on a substrate by a high-frequency sputtering method, characterized in that the formation of the zinc oxide film is performed in an atmosphere in the presence of oxygen gas. Manufacturing method. 5. 5. The method for producing a zinc oxide film according to claim 4, wherein a resin film is used as the substrate and the zinc oxide film is formed in a mixed gas atmosphere of argon and oxygen gas.