JP2010225725A - Thin film varistor and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film varistor that improves the humidity resistance without deterioration of initial characteristics of an element associated with provision of a protection film, and that has a high reliability. <P>SOLUTION: The thin film varistor has: an insulating base body 1; a ZnO varistor layer 2 formed on the base body; surface planar type electrodes 3a and 3b arranged on the surface of the ZnO varistor layer 2 so as to oppose to each other via a predetermined gap 13 between electrodes; a buffer layer 4 arranged so as to cover at least an area exposed to the gap 13 between the electrodes in the surface of the ZnO varistor layer, and which consists of a high-resistance ZnO thin film; and a protection film 5 arranged so as to cover the buffer layer. At least one selected from a group consisting of a silicon oxide thin film, a silicon nitride thin film, and a silicon nitride oxide thin film is used as the protection film. An average diameter of crystal grain of the surface of the ZnO thin film having varistor characteristics is set to 50 nm or greater. The high-resistance ZnO thin film that serves as the buffer layer is formed by carrying out sputtering film formation in an atmosphere including oxygen. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a thin film varistor and a method of manufacturing the same, and more specifically, a surface planar electrode is disposed on a surface of a ZnO thin film having a varistor characteristic disposed on an insulator substrate so as to face a predetermined gap. The present invention relates to a thin film varistor and a manufacturing method thereof.

  As a varistor, in addition to a single plate type varistor that has been known for a long time, a laminated type varistor (see Patent Document 1), or a surface of a thin film having a varistor characteristic disposed on an insulator substrate, is used. Various shapes and structures such as a structure in which electrodes are arranged so as to face each other through a gap (hereinafter referred to as a planar type) (see Examples 1 and 2 (especially FIGS. 3 and 5) of Patent Document 2) It has been known.

  And, in the case of the structure in which both of the pair of comb-like electrodes are provided on the same plane as in Example 2 of Patent Document 2, “Because the ZnO varistor thin film 14 is sandwiched between the electrodes in the plane direction, The interelectrode capacitance can be reduced, and an element excellent in high-speed response can be configured "(see paragraph 0058 of Patent Document 2).

  In any of the above conventional varistors, it is desirable to form a protective film on the element surface. However, particularly in the case of a varistor having a planar electrode structure, a portion that exhibits varistor characteristics between the electrodes (active region) ) Is exposed on the surface, so if a protective film is not provided for protection, the characteristics may change over time due to the influence of moisture in the air. It is essential to provide

By the way, it is known that, for example, a SiO ₂ film or the like is used as such a protective film (see Patent Document 1). In the case of a laminated structure such as Patent Document 1, the interaction between the protective film and the active region is as follows. Almost no problem.

  On the other hand, in the case of a varistor element having a planar type electrode structure, the active region and the protective film are in direct contact, and it is necessary to thoroughly examine the interaction that occurs between them. This is because, in the active region of the varistor element, a fine structure in which a barrier structure is intentionally formed at the grain boundary is given in order to obtain strong nonlinearity, and it is easily affected by the external environment including the protective film. by.

 In this regard, Patent Document 2 only discusses characteristics without a protective film, and considers what measures should be taken in order not to adversely affect the characteristics of the active region. In reality, the problem of what kind of protective film should be provided in order to stably and satisfactorily maintain the characteristics of the active region has not been solved.

JP-A-5-335115 JP 2008-218592 A

  The present invention solves the above-described problems, and can improve the moisture resistance and suppress air discharge and the like without causing deterioration of the initial characteristics of the varistor element by providing a protective film. An object of the present invention is to provide a thin film varistor having high performance and a method for manufacturing the same.

In order to solve the above problems, the thin film varistor of the present invention is
An insulating substrate;
A ZnO varistor layer comprising a ZnO thin film having varistor characteristics formed on the substrate;
A pair of surface planar electrodes disposed on the surface of the ZnO varistor layer so as to face each other with a predetermined inter-electrode gap;
A buffer layer made of a high-resistance ZnO thin film having a resistivity of 10 ⁷ Ωcm or more, disposed so as to cover the surface of the ZnO varistor layer exposed in the interelectrode gap;
And a protective film disposed so as to cover the ZnO varistor layer with the buffer layer interposed therebetween.

  In the thin film varistor of the present invention, the exposed region of the surface of the ZnO varistor layer other than the gap between the electrodes is also covered with the buffer layer, and is covered with the protective film via the buffer layer. It is desirable that

  In the thin film varistor of the present invention, it is desirable that the protective film is at least one selected from the group consisting of a silicon oxide thin film, a silicon nitride thin film, and a silicon nitride oxide thin film.

  In the thin film varistor of the present invention, it is desirable that the average diameter of crystal grains on the surface of the ZnO thin film having varistor characteristics is 50 nm or more.

Moreover, the manufacturing method of the thin film varistor of this invention is a manufacturing method of the thin film varistor in any one of Claims 1-4,
A high-resistance ZnO thin film serving as the buffer layer is formed by sputtering film formation in an atmosphere containing oxygen.

The thin film varistor of the present invention is a high resistance ZnO thin film having a resistivity of 10 ⁷ Ωcm or more so that a surface planar type electrode is disposed on the surface of the ZnO varistor layer and the ZnO varistor layer exposed in the interelectrode gap is covered. And a protective film is formed so as to cover the ZnO varistor layer through the buffer layer, so that the initial characteristics of the element due to the provision of the protective film are not deteriorated and the moisture resistance is reduced. It is possible to obtain a highly reliable thin film varistor capable of improving the performance and suppressing air discharge and the like.

That is, if a protective film (eg, SiN _x film or SiO ₂ film) is directly formed on the surface of a ZnO thin film having a very steep nonlinear current-voltage (IV) characteristic, the surface resistance of the ZnO thin film decreases. Element characteristics deteriorate. Therefore, in the present invention, in order to prevent this, first, a ZnO thin film having high resistance is formed as a buffer layer on the ZnO thin film, and a SiN _x film or a SiO ₂ film as a moisture-resistant protective film is formed through this buffer layer. I did it. Accordingly, it is possible to form a protective film with high moisture resistance without causing deterioration of the initial characteristics of the element due to provision of the protective film, and to provide a highly reliable thin film varistor with excellent moisture resistance. Is possible.

  Note that a highly resistive ZnO thin film can be efficiently formed by supplying sufficient oxygen (approximately 1.8% or more) at the time of film formation by sputtering, for example.

Further, deterioration of the initial characteristics by providing the protective film, i.e., the decrease in resistance occurs in the course of the protective film on the surface of the ZnO varistor layer made of SiN _x or SiO ₂ is formed, the ZnO side oxygen atom By bonding with Si atoms, oxygen atoms are removed from the vicinity of the surface of the ZnO varistor layer, and the defect portion generates two electrons. And this tendency is considered to appear conspicuously in a thin film having a non-linearity having a barrier structure at the grain boundary, but between a ZnO thin film (ZnO varistor layer) having varistor characteristics and a protective film such as SiO ₂ , This can be avoided by forming a ZnO thin film having a high resistance as a buffer layer.

  In the thin film varistor of the present invention, a region (active region) exposed in the interelectrode gap on the surface of the ZnO varistor layer is covered with a high resistance ZnO thin film that is a buffer layer, so that the region other than the interelectrode gap is formed. Even if the exposed ZnO varistor layer is not covered with the buffer layer and the protective film, the object of the present invention is achieved. However, the exposed region other than the gap between the electrodes on the surface of the ZnO thin film having varistor characteristics is also buffered. By adopting a configuration in which the protective film is covered via the layer, it is possible to improve the moisture resistance more reliably and improve the reliability.

  In the thin film varistor of the present invention, the protective film is at least one selected from the group consisting of a silicon oxide thin film, a silicon nitride thin film and a silicon nitride oxide thin film, thereby providing an element by providing the protective film. Thus, it is possible to provide a highly reliable thin film varistor having excellent moisture resistance without deteriorating the initial characteristics.

  In addition, when the average diameter of crystal grains on the surface of the ZnO thin film having varistor characteristics is set to 50 nm or more, it becomes possible to form a good grain boundary barrier structure and to form grain boundaries in the gap between electrodes. Can be reduced, and the varistor voltage (breakdown voltage) can be reduced.

In the method of manufacturing a thin film varistor of the present invention, a high resistance ZnO thin film (buffer layer) is formed by sputtering film formation in an atmosphere containing oxygen, so that the resistivity is 10 ⁷ Ωcm or more. It is possible to efficiently form a preferable buffer layer made of a high-resistance ZnO thin film with little characteristic fluctuation even when contacted with a protective film made of a silicon compound or the like formed thereon, and has excellent moisture resistance. It is possible to provide a highly reliable thin film varistor.

It is a figure which shows the thin film varistor concerning Example 1 of this invention, Comprising: (a) is a top view, (b) is front sectional drawing. It is a figure which shows the state before forming the buffer layer of the manufacturing process of the thin film varistor concerning Example 1 of this invention. It is a figure which shows the state after forming the buffer layer and protective film of the manufacturing process of the thin film varistor concerning Example 1 of this invention. It is a figure which shows the current-voltage characteristic of the thin film varistor (element before forming a buffer layer and a protective film) concerning Example 1 of this invention. It is a figure which shows the magnitude | size of the insulation resistance investigated about each sample (thin film varistor) produced in Example 1. FIG. It is a figure which shows the current-voltage characteristic of the thin film varistor (element before forming a buffer layer and a protective film) concerning Example 2 of this invention. It is the figure which calculated | required the number of the grain boundaries between electrode gaps from the SEM photograph about the thin film varistor of Example 2 of this invention, and plotted the relationship with a breakdown voltage. It is a figure which shows the relationship between the number of grain boundaries between electrodes (between electrode gaps), and a breakdown voltage of the ZnO varistor layer which comprises the thin film varistor produced in Example 2 of this invention.

  Examples of the present invention will be described below to describe the features of the present invention in more detail.

  FIG. 1 is a diagram schematically showing the configuration of a thin film varistor according to one embodiment of the present invention, in which (a) is a plan view and (b) is a front sectional view.

As shown in FIGS. 1A and 1B, this thin film varistor is formed on a silicon substrate (substrate) 1 having an SiO ₂ film 1a formed on the surface, which is an insulating substrate, and on the Si substrate 1. A ZnO varistor layer 2 made of a ZnO thin film having varistor characteristics, and a pair of surface planar electrodes 3a disposed on the surface of the ZnO varistor layer 2 so as to face each other with a predetermined inter-electrode gap 13. 3b and the resistance of the ZnO varistor layer 2 disposed so as to cover the region exposed in the interelectrode gap 13 and the other exposed region, that is, the region where the surface planar electrodes 3a and 3b are not disposed. A buffer layer 4 made of a high-resistance ZnO thin film with a rate of 10 ⁷ Ωcm or more and a protective film 5 disposed so as to cover the exposed region of the ZnO varistor layer 2 with the buffer layer 4 interposed therebetween. Note that the buffer layer 4 and the protective film 5 are removed at the center of the surface planar type electrodes 3a and 3b for connection to the outside, and the surface planar type electrodes 3a and 3b are exposed.

In this thin film varistor, as the Si substrate 1, a Si substrate having a thickness of 500 μm and a SiO ₂ film having a thickness of 650 nm formed on the surface is used.

  The ZnO varistor layer 2 is a 0.6 μm thick ZnO thin film formed by high frequency magnetron sputtering.

  Further, the surface planar type electrodes 3 and 3b are made of an Al thin film formed by electron beam evaporation after forming an electrode lift-off pattern with a photoresist, and the film thickness thereof is about 500 nm.

The buffer layer 4 is disposed so as to cover the region of the surface of the ZnO varistor layer 2 exposed in the interelectrode gap 13 and the peripheral portion of the surface planar type electrode and the ZnO varistor layer. 4, a high-resistance ZnO thin film having a resistivity of 10 ⁷ Ωcm or more is used. In Example 1, the buffer layer 4 has a thickness of about 80 nm.

In this embodiment, as the buffer layer 4, a sample having a high resistance buffer layer made of additive-free ZnO and a sample having a buffer layer made of a high resistance ZnO thin film containing Bi ₂ O ₃ and Er ₂ O ₃ are used. Was made.

Further, the protective film 5 formed on the upper surface of the buffer layer 4 is a SiO ₂ film formed by high-frequency magnetron sputtering, like the ZnO varistor layer, and has a film thickness of 250 nm.
Next, a manufacturing method of this thin film varistor will be described.

[Preparation of Example Sample]
(1) Preparation of Substrate As a substrate, as shown in FIG. 2, a Si substrate (thickness 500 μm) 1 having a SiO ₂ film 1a with a thickness of about 650 nm formed on the surface was prepared.

(2) Formation of ZnO Varistor Layer A ZnO thin film (ZnO varistor layer) 2 having varistor characteristics was then formed on the Si substrate 1. In forming the ZnO varistor layer 2, a high frequency magnetron sputtering method was used, and the ZnO varistor layer 2 (FIG. 2) was formed on the SiO ₂ film 1a on the surface of the Si substrate 1 under the following conditions.

As a target for forming the a) target ZnO varistor layer, the ZnO sintered body, the Bi ₂ O ₃ 6 wt%, the Er ₂ O ₃ 4% by weight inclusive thickness 5 mm, a diameter of 100 mm, What was affixed on the copper backing plate was used.
b) Conditions for high-frequency magnetron sputtering A substrate placed facing the target is set, exhausted to a back pressure of 10 ⁻⁴ Pa, argon gas is introduced, pressure is 2.1 × 10 ⁻¹ Pa, RF power is 250 W. The ZnO varistor layer was formed under the conditions of no substrate heating. The substrate holder was revolved.
The film formation time was 75 minutes, and the ZnO varistor layer was grown to a film thickness of about 0.6 μm.
After the film formation, heat treatment was performed at 500 ° C. for 1 hour in a nitrogen atmosphere containing 2% oxygen.

(3) Formation of surface planar type electrode Then, after forming a lift-off pattern for the electrode with a photoresist, surface planar type electrodes 3a and 3b having a predetermined pattern are formed by an electron beam evaporation method using an Al thin film having a thickness of about 500 nm. Formed. Thereafter, lift-off was performed in an organic solvent to form a pair of surface planar electrodes 3a and 3b having a pattern as shown in FIGS. 2 and 1A and 1B.

In this example, a sample (varistor element) in which the value of the width W (see FIG. 1B) of the interelectrode gap 13 between the pair of surface planar electrodes 3a and 3b was 1.5 μm was produced.
And the current-voltage characteristic was investigated about the obtained varistor element, ie, the element before forming the buffer layer 4 and the protective film 5. FIG.

  FIG. 4 shows current-voltage characteristics of a varistor element (an element before the buffer layer 4 and the protective film 5 are formed) having a width W of the interelectrode gap 13 of 1.5 μm. As shown in FIG. 4, a varistor voltage (breakdown voltage) with a current value of 1 mA was about 90V. The nonlinear coefficient α was about 50.

(4) Formation of Buffer Layer Next, the varistor element having the above initial characteristics is made of a highly pure ZnO thin film made of additive-free ZnO and having a resistivity of 10 ⁷ Ωcm or more under the following conditions. Was formed as a buffer layer 4 (FIG. 3).

a) As a target, a target containing Bi ₂ O ₃ and Er ₂ O ₃ and a target made of high-purity ZnO without additives (non-added ZnO target) were prepared.
b) The gas flow rates were argon: 50 SCCM and oxygen: 5 SCCM.
c) The film formation time was 10 minutes.
Other conditions were the same as those in the case of forming the ZnO varistor layer.

Then, in this embodiment, as described above, by using a target containing _{Bi 2 O 3, Er 2 O} 3, and a non-doped ZnO target containing no _{Bi 2 O 3, Er 2 O} 3, the buffer layer A sample having a buffer layer formed of a ZnO thin film containing Bi ₂ O ₃ and Er ₂ O ₃ (a sample of sample number 4 in Table 1 after completion), Bi ₂ O ₃ , Er _{2 is formed.} A sample (a sample of sample number 8 in Table 1 after completion) having a buffer layer made of a ZnO thin film having a high purity and high resistance and containing no O ₃ was prepared.

(5) Formation of Protective Film Next, as shown in FIG. 3, an SiO ₂ film was formed as the protective film 5 on the buffer layer 4 formed in the above (4) under the following conditions.
a) As a target, high-purity SiO ₂ was used.
b) The gas flow rates were argon: 30 SCCM and oxygen: 13 SCCM.
c) The RF power was 500W.
d) The film formation time was 60 minutes. Other conditions were the same as those in the case of forming the ZnO varistor layer.

(6) Etching of buffer layer and protective film (exposure of surface planar electrode)
After forming the protective film 5 as described above, an etching pattern is formed with a photoresist, and the protective film (SiO ₂ film) 5 and the buffer layer (high resistance ZnO thin film) 4 are selectively removed by dry etching. The surface planar type electrodes 3a and 3b were partially exposed (center portion). As a result, samples Nos. 4 and 8 (thin film varistors) having the structure shown in FIG. 1 were obtained.

[Preparation of sample for comparison]
For comparison, a sample (sample No. 2 in Table 1) in which a buffer layer containing Bi ₂ O ₃ and Er ₂ O ₃ is formed in an oxygen-free atmosphere and a protective film is provided via the buffer layer is provided. Produced. Note that the sample No. 2 in which the buffer layer is formed in an oxygen-free atmosphere is a sample that does not have the requirement that the resistivity is 10 ⁷ Ωcm or more, which is a requirement of the present invention.

Furthermore, for comparison, a thin film varistor (sample No. 6 in Table 1) in which a SiO ₂ protective film was directly formed on a ZnO varistor layer (a sample not satisfying the requirements of the present invention) was produced.

  Further, in the step of producing each sample of sample numbers 2, 4, 6, and 8 described above, a ZnO varistor layer produced in the same batch as the ZnO varistor layer for each sample of sample numbers 2, 4, 6, and 8 was prepared. Samples Nos. 1, 3, 5, and 7 in Table 1 (thin film varistors not provided with a buffer layer and a protective film) were prepared for comparison of basic characteristics not including differences between batches.

  Table 1 and FIG. 5 show the magnitudes of the resistance values examined for the samples prepared as described above. In FIG. 5, the horizontal axis indicates the distribution in the plane (distance from the substrate center).

In order to facilitate understanding, each sample in Table 1 is organized and described here.
(a) Sample No. 2 is a sample that does not satisfy the requirements of the present invention, in which a buffer layer containing Bi ₂ O ₃ and Er ₂ O ₃ is formed in an oxygen-free atmosphere and a protective film is formed thereon. A thin film varistor.
(b) The sample No. 4 is a thin film varistor having the requirements of the present invention, in which a buffer layer containing Bi ₂ O ₃ and Er ₂ O ₃ is formed in an aerobic atmosphere and a protective film is further formed thereon. It is.
(c) Sample No. 6 is a thin film varistor that does not have the requirements of the present invention, in which a SiO ₂ protective film is directly formed on a ZnO varistor layer without forming a buffer layer.
(d) Sample No. 8 is a thin film having the requirements of the present invention, in which a buffer layer not containing Bi ₂ O ₃ and Er ₂ O ₃ is formed in an aerobic atmosphere, and a protective film is further formed thereon. It is a varistor.
(e) Samples Nos. 1, 3, 5, and 7 are composed only of ZnO varistor layers made of the same batches as the respective ZnO varistor layers constituting the samples Nos. 2, 4, 6, and 8 described above. It is a thin film varistor (thin film varistor not provided with a buffer layer and a protective film) that does not have the requirements of the present invention.

Hereinafter, the insulation resistance of each sample (varistor element) will be described with reference to Table 1 and FIG.
In FIG. 5, the data of each sample excluding the samples of sample numbers 2 and 6 are partially overlapped to make it difficult to read the resistance value, but FIG. The difference between the resistance value of the sample and other samples can be confirmed, and it is shown from the idea that it is meaningful to recognize that the resistance value is sufficiently large. Is shown in Table 1.

  From Table 1 and FIG. 5, it was confirmed that the samples Nos. 1, 3, 5, and 7 having a buffer layer and a protective film and having only a ZnO varistor layer have high insulation resistance. . However, since these samples do not have a protective layer, they are easily affected by the external environment and have practical problems.

In the case of the sample of sample number 2 in which the buffer layer containing Bi ₂ O ₃ and Er ₂ O ₃ was formed in an oxygen-free atmosphere, high insulation resistance could not be obtained. This is because when a buffer layer containing Bi ₂ O ₃ and Er ₂ O ₃ was formed in an oxygen-free atmosphere, a buffer layer having a sufficiently high resistance value could not be formed.

In addition, in the case of the thin film varistor of sample number 6 in which the SiO ₂ protective film was directly formed on the ZnO varistor layer without forming the buffer layer, a high insulation resistance could not be obtained. This is because the protective film is formed directly on the ZnO varistor layer, and the insulation resistance is reduced due to the influence.

On the other hand, in the sample (sample Nos. 4 and 8) provided with a protective film through a buffer layer formed in an atmosphere containing oxygen, a good insulation resistance is maintained even after the protective film (SiO ₂ film) is formed. It was confirmed that it would sag.

Incidentally, the sintered sample 4 was formed using a target containing Bi ₂ O ₃ and Er ₂ O _3, a sample with a buffer layer containing Bi ₂ O ₃ and Er ₂ O _3, Sample No. 8 the samples were formed using a non-doped ZnO target containing no Bi ₂ O ₃ and Er ₂ O _3, is a sample with a buffer layer containing no Bi ₂ O ₃ and Er ₂ O _3, both good Maintains a good insulation resistance. Thus, as the buffer layer, those containing Bi ₂ O ₃ and Er ₂ O _3, or containing no Bi ₂ O ₃ and Er ₂ O _3, it is understood that may be provided either of the buffer layer. However, in consideration of various characteristics other than the insulation resistance, it is more desirable to form a high-purity buffer layer that does not contain impurities.

Note that the high-resistance buffer layer formed here is thin and integrated with the ZnO varistor layer, and thus its characteristics cannot be obtained independently. Therefore, a single-layer oxygen-added ZnO film was formed under similar conditions, and the resistivity was examined. As a result, the resistivity was estimated to be approximately 1.2 × 10 ⁷ Ωcm. Therefore, if the buffer layer has a resistivity of about 1.2 × 10 ⁷ Ωcm, the resistance does not deteriorate even if a protective film such as SiO ₂ or SiN _x is formed thereon.

  From Example 1 described above, when the protective film is provided via the buffer layer having the requirements of the present invention, the moisture resistance is improved without deteriorating the initial characteristics of the element due to the provision of the protective film. It was confirmed that a highly reliable thin film varistor capable of suppressing air discharge and the like can be obtained.

A ZnO varistor layer was formed on a Si substrate having the same SiO ₂ film on the surface as that used in Example 1. The film formation time was 225 minutes, and the ZnO varistor layer was grown to a film thickness of about 1.7 μm. Other conditions were the same as those in Example 1 above.
Then, after the formation of the ZnO varistor layer, a surface planar type electrode was formed by the same method and the same conditions as in Example 1 above.
In Example 2, a sample (with a width W of the inter-electrode gap 13 between the pair of surface planar electrodes 3a and 3b shown in FIG. 1B having a value of 1 μm, 1.5 μm, and 2.5 μm ( A varistor element was manufactured.

  The current-voltage characteristics of the obtained varistor element, that is, the element before forming the buffer layer and the protective film were examined. FIG. 6 shows current-voltage characteristics of a varistor element (an element before forming the buffer layer and the protective film) having a gap width between electrodes of 1.5 μm. From FIG. 6, a varistor voltage (breakdown voltage) with a current value of 1 mA was about 58V. The nonlinear coefficient α was about 50.

  Although the width of the electrode gap is 1.5 μm, which is the same as that of Example 1, the varistor voltage of the sample of Example 1 is about 90V, and the varistor voltage of the sample of Example 2 is about 58V. The reason why the varistor voltage is smaller in the sample is that the ZnO varistor layer is grown to a thickness of about 1.7 μm (in Example 1, the thickness of the ZnO varistor layer: 0.6 μm). This is because the number of grain boundaries in the gap is reduced. This is clear from the following analysis.

FIG. 7 shows the state of crystal grains by SEM observation of the surface of the ZnO varistor layer.
Table 2 shows the relationship between the width W of the interelectrode gap 13, the number of grain boundaries existing in the electrode gap 13, and the varistor voltage of the sample produced in this example.

  In Example 2, although the film formation time of the ZnO varistor layer is different and the diameter of the crystal grain is also different, the number of grain boundaries between the electrodes (between electrode gaps) is obtained from the SEM photograph (see Table 2), and the breakdown voltage and When the relationship is plotted, as shown in FIG. 8, it is almost on a straight line, and its inclination is approximately 2.7V.

  This indicates that the breakdown voltage per grain boundary is constant regardless of the grain size if the crystal grains grow to a certain extent (approximately 100 nm). Note that the breakdown voltage of 2.7 V corresponds to 2.0 to 3.5 V, which is a breakdown voltage per grain boundary in a ZnO bulk element (ceramic varistor).

In a ceramic varistor, it is considered that a barrier structure is formed at the grain boundary, and that it develops non-linear electrical characteristics. In the thin film varistor of this example, the number of grain boundaries between electrode gaps, breakdown voltage, If the relationship shown in FIG. 8 is established between the two, the same mechanism is considered to be reproduced in the thin film. In the process of forming such a fine structure in which a barrier structure is intentionally formed at the grain boundary, oxygen is strongly involved, and therefore, with an oxide such as SiO ₂ formed on the surface, When the interaction is large and the protective film is formed directly on the surface of the ZnO varistor layer, the resistance is deteriorated. However, even in such a varistor element, the SiO ₂ protective film is formed through the buffer layer (high-resistance ZnO thin film) in the same manner as in the above-described embodiment, so that the resistance is not deteriorated. A protective film excellent in moisture resistance could be formed, and a highly reliable thin film varistor could be obtained.

  In each of the above embodiments, a thin film varistor having a square planar electrode is shown as the surface planar electrode. However, the surface planar electrode has a square shape as in the above embodiment. The shape is not limited, and various shapes such as a comb-like electrode can be used.

  The present invention is not limited to the above-described embodiments in other respects, but relates to the constituent material of the substrate, the method of forming the buffer layer, the kind of the constituent material of the protective film, and the like within the scope of the invention. Can be added.

1 Silicon substrate (base)
1a SiO ₂ film 2 ZnO varistor layer 13 electrode gap 3a, 3b the surface planar electrode 4 a buffer layer 5 protective film

Claims (5)

An insulating substrate;
A ZnO varistor layer comprising a ZnO thin film having varistor characteristics formed on the substrate;
A pair of surface planar electrodes disposed on the surface of the ZnO varistor layer so as to face each other with a predetermined inter-electrode gap;
A buffer layer made of a high-resistance ZnO thin film having a resistivity of 10 ⁷ Ωcm or more, disposed so as to cover the surface of the ZnO varistor layer exposed in the interelectrode gap;
A thin film varistor comprising: a protective film disposed so as to cover the ZnO varistor layer with the buffer layer interposed therebetween.   The exposed region of the surface of the ZnO thin film other than the gap between the electrodes is also covered with the buffer layer, and is covered with the protective film via the buffer layer. The thin film varistor described.   3. The thin film varistor according to claim 1, wherein the protective film is at least one selected from the group consisting of a silicon oxide thin film, a silicon nitride thin film, and a silicon nitride oxide thin film.   4. The thin film varistor according to claim 1, wherein an average diameter of crystal grains on the surface of the ZnO thin film having varistor characteristics is 50 nm or more. It is a manufacturing method of the thin film varistor in any one of Claims 1-4, Comprising:
A method of manufacturing a thin film varistor, comprising forming a high-resistance ZnO thin film, which is the buffer layer, by sputtering film formation in an atmosphere containing oxygen.