JPH02148803A - Thin film type voltage-dependent nonlinear resistor - Google Patents

Abstract

PURPOSE:To provide a thin film type voltage-dependent nonlinear resistor having a large nonlinear coefficient, excellent characteristic stability, small size and high practical utility by incorporating a predetermined content of cobalt in first and second zinc oxide thin film layers and a predetermined content of manganese in a lead oxide thin film layer. CONSTITUTION:In a thin film type voltage nonlinear resistor, a lower electrode layer 2 made of a thin metal film is formed on a board 1, at least one set of a first zinc oxide (ZnO) thin film layer 3a and a zinc oxide (PbO) thin film layer 4 are sequentially laminated on the electrode layer, a second zinc oxide thin film layer 3b is formed on the laminate, and an upper electrode layer 5 is formed on the thin film layer. Cobalt of 0.1-0.5mol-% in terms of CoO is contained in the first and second zinc oxide layer, and manganese of 0.05-0.1mol-% in terms of MoO is further contained in the zinc oxide layer.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a symmetrical structure obtained by laminating a dielectric film containing zinc oxide as a main component and an insulating film containing lead oxide as a main component on a substrate. The present invention relates to a thin film type voltage non-linear resistor exhibiting voltage-current characteristics, and in particular to a small 'a model voltage non-linear resistor with excellent stability of characteristics.

[Prior Art] A voltage nonlinear resistor (hereinafter referred to as a varistor) usually exhibits a non-ohmic symmetrical voltage-current characteristic expressed by the following formula [I].

α (/i − (V/V ・ )
・” [Eng.] Here, I is the current value flowing through the element, ■
is the voltage applied to the element, and the voltage V when the current is i ampere
1 is called the rising voltage. Usually, this value takes the voltage value at 1 mA. α is called a nonlinear coefficient, and it can be said that the larger the value, the better the characteristics.

Generally, a varistor is used by being inserted into an electric circuit as a one-noge absorption element, an abnormal voltage suppressor, or a voltage stabilizing element for the purpose of protecting the electric circuit from abnormal voltage.

Varistors known so far have zinc oxide as a main component, and various additives such as B i 203,
coo, MnO, Sb203.

Cr2 03 , N + O, B2 03 , PbO
.

By adding and mixing a small amount of 5i02 etc. and sintering,
To obtain a varistor that exhibits excellent voltage nonlinearity, B! A method using 2C)3 is well known. The varistor obtained by this method is the first to be obtained as a ceramic sintered body, and utilizes the properties of zinc oxide grain boundaries. That is, during sintering, along with the growth and sintering of ZnO crystal grains that are necessary for n-type semiconductors, a liquid phase of, for example, Bi2O3 is generated at the grain boundaries of the crystal grains, and various additives are segregated there. , which utilizes the phenomenon of forming potential barriers against electrons at zinc oxide grain boundaries. Therefore, until now, excellent voltage nonlinear resistors have only been available in bulk type ceramic sintered bodies.

Considering application to electronic equipment, it is desirable to form a voltage nonlinear resistor on a substrate using a thin film or a thick film.

Various attempts have been made to fabricate a voltage nonlinear resistor (varistor) using a thin film or a thick film on a substrate.

However, since varistors using thick films also utilize zinc oxide crystal grains and grain boundaries, the thickness is limited to 5
~ Several times more than 10 microns is required. Even with the method of forming parallel electrodes on the surface of a thick film, it is necessary to provide a spacing between the electrodes that is several times the size of the ZnO crystal grains (50 to 100 microns), and there is a limit to miniaturization. ,
There are also problems in that it is difficult to increase surge resistance and is susceptible to surface contamination.

In addition, the liquid phase of Bi2O3 is extremely reactive and reacts with most oxides and metals, so even if it can be formed into a thick film, the interface with the substrate will be severely altered, reducing the portion that can effectively function as a device. There is also the problem. Problems arise such as reactions between hismuth and wiring metal materials during the rough fabrication of wiring patterns and devices, and wiring breakage at steps with a thickness of 10 microns or more. There were problems in that it was difficult to control the rising voltage, it was difficult to reduce the shape and size to 100 microns or less because it was dependent on printing technology, and the nonlinear coefficient could only be about 10 at most.

Attempts to produce varistors in the form of thin films were made in 1979, for example.
Journal of Applied Physics, Volume 50, pp. 555-558 (Journaof
Applied Physics, vol, 50. p,
555-558 (1979)), a zinc metal film with a thickness of about 100% was deposited on a fused silica substrate by sputtering.
By depositing a Zr1O1 film and a Bi2O3 thin film of approximately 6,000 layers each on top of that, and attaching Act 2,000 as an electrode on top of that, an asymmetrical voltage non-linearity with a rising voltage of 2 to 3V was created. It has been revealed that an element can be obtained. This device consists of ZnO thin film and B
It utilizes the interface of the i2031 film, and Z
This utilizes the so-called semiconductor-insulator interface phenomenon, which is the interface between nO and Bi2O3, which is an insulator.

[Problems to be Solved by the Invention] However, these devices have asymmetric voltage-current characteristics, have a small nonlinear coefficient, cannot control the rise voltage, cannot be made thin, and cannot be made small to about 100 microns or less. There were problems such as difficulty in forming and lack of stability of properties, and the range of application was limited.

If a symmetrical thin-film varistor could be realized, it would be possible to provide an ultra-small varistor array at low cost, which would have an extremely wide range of applications and be highly practical. As mentioned above, it can be easily inferred that in order to manufacture a thin film type varistor, a stacked structure in which a Bi2O3 thin film is sandwiched between upper and lower ZNOI films is sufficient. However, even with this structure, the above-mentioned problems could not be solved. Considering the cause of this, Bi2O3 has a low melting point of 880°C and is prone to crystal transformation, which changes due to the heat generated during varistor operation, resulting in unstable characteristics. it is conceivable that.

The characteristic change of the varistor is the rising voltage (VllIIA)
v11I after applying a DC voltage of about 80% of
The rate of change in IA, ie, Δv11IIA/v11IIA, is evaluated. In practice, this value is desired to be within ±10%, but in the thin film varistor with the structure described above, this value is within ±10%.
There were only cases exceeding 0%.

The present invention has been made in order to solve the conventional problems as described above, and provides a thin film voltage nonlinear resistor that has a large nonlinear coefficient, excellent stability of characteristics, and is small and highly practical. The purpose is to

[Means for solving the problems] In order to solve the various problems of thin film varistors,
Sufficient consideration must be given to the properties of the ZrO thin film as a semiconductor, the control of the interface between ZnO and the insulator, the stability of the insulator layer, and the stability of the electrode material used. As a result of examining various materials to replace the highly unstable B12O3, the present inventor found that a thin film varistor that solved the various problems mentioned above could be produced by having a predetermined configuration as shown below. This led to the invention.

That is, in the present invention, a lower electrode layer made of a metal thin film is formed on a substrate, and a first zinc oxide (ZnO) layer is formed on the electrode layer.
At least one thin film layer and a lead oxide (PbO) thin film layer are sequentially formed.
A thin film type voltage nonlinear resistor comprising a first zinc oxide thin film layer, a second zinc oxide thin film layer formed on the laminate, and an upper electrode layer made of a metal thin film formed on the thin film layer. The second zinc oxide thin film layer contains 0.1 to 0.5 mol% of cobalt in terms of CoO, and the lead oxide thin film layer contains 0.05 mol% of manganese in terms of MnO. ~0
．． This is a thin film type voltage non-linear resistor characterized by containing 1 mol%.

The structure of the thin film type varistor of the present invention is as shown in FIG. 1, which also serves as an example and shows its basic configuration. FIG. 1(a) is a cross-sectional view, and FIG. 1(b) is a plan view. be. Note that the sectional view of FIG. 1(a) is taken along the line AA- in FIG. 1(b).

In FIG. 1(a), the underlying substrate 1 is made of a material that has good surface flatness and can withstand the subsequent process conditions of film formation in an oxidizing atmosphere at a maximum temperature of 400°C, such as glass, alumina, or It is not particularly limited as long as it is made of sapphire or the like. The lower electrode layer 2 applied first needs to be resistant to similar atmospheric conditions, and is suitably made of, for example, gold, platinum, palladium or ruthenium metal. The reason why the above-mentioned materials are suitable as electrode film materials is partly due to the difference in work function between the zinc oxide thin film layer 3a (first zinc oxide thin film layer) and the metal material, or because the zinc oxide thin film This may also be due to the presence or absence of an oxide film layer on the metal surface just before layer 3a is formed. In short, it seems that any material that is stable and exhibits low resistance in an oxidizing atmosphere is sufficient.

Next, cobalt is converted to CoO and is 0.1 to 0.5 mol%.
A target is prepared in advance so that the added zinc oxide thin film @3a has the same composition, and the film is formed by sputtering or the like. A normal powder metallurgy method can be used to prepare the target, and the target may be prepared by mixing and firing powder raw materials.

Next, convert manganese into MnO and convert it to 0.05 to 0.1.
The lead oxide thin film layer 4 containing mol% and the zinc oxide thin film @3b (second zinc oxide thin film layer) meeting the same conditions as above may be formed by sputtering.

At this time, conditions were suitable such that the additives cobalt and manganese were slightly diffused and mixed into the zinc oxide thin film layers 3a, 3b and the lead oxide thin film layer 4. That is, it seems that good electrical characteristics can be obtained by setting the concentration change near the interface between the zinc oxide thin film layers 3a and 3b and the lead oxide thin film layer 4 to an optimum value. At this time, the concentration of cobalt is high in the zinc oxide thin film, and conversely, it is necessary to increase the concentration of manganese in the lead-lead thin film.

Any method that can be expected to produce such a phenomenon in a thin film is not limited to sputtering, and may also be a vapor deposition method using an ion beam or plasma. Alternatively, heat treatment may be performed after film formation is completed.

Any material may be used for the upper electrode layer 5 as long as it does not particularly deteriorate the characteristics.

[Example] Next, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic cross-sectional view and a plan view of an embodiment of the present invention, and the manufacturing method thereof is as follows.

First, as the substrate 1, fused silica and sapphire substrates were used. When using palladium or platinum as the lower electrode layer 2, in consideration of adhesion to the substrate, Ti was applied in advance by magnetron sputtering on only 500 people, and then palladium or platinum was deposited on 3000 people using the same sputtering method. . On the other hand, when ruthenium (Ru) was used as the lower electrode layer 2, it was deposited directly on the substrate-F to a thickness of 3000 nm.

Using oxide powder with a purity of 99.9% or more as Coo,
This powder and ZnO powder, which also has a purity of 99.9% or higher, were mixed using pure water using a ball mill method, and then molded into a diameter of 15 cm and a thickness of 1 cm at a pressure of 2 tons/cm2, and heated at 1200°C. The sintered body sintered for 1 hour was used as a sputter target for a zinc oxide thin film layer. The zinc oxide thin film layer 3a containing cobalt was formed to a thickness of 5,000 wafers by high frequency magnetron sputtering at a substrate temperature of 300°C.

The lead oxide layer 4 was formed by mixing MnO with PbO powder and compacting it at 2 tons/cm 2 to a thickness of 2,000 mm using the same method as described above using sputter targets 1 to 1.

Further, the zinc oxide thin film layer 3b containing cobalt is again deposited at 50%
00 people, and laminated and formed films in sequence under the same conditions as above. Finally, a gold electrode was deposited using an electron beam heating method.
A film was formed to a thickness of A to form the upper electrode layer 5.

(The electrical characteristics of the q element were evaluated as 1fiu by measuring the voltage-current characteristics with a curve tracer and DC.

The nonlinear coefficient is calculated from the measured voltage values v11nA and VlomA at current values of 1mA and 10rTIA [
Calculated according to the following formula:

Figures 2 and 3 show the results of measuring the nonlinear coefficient (α) when varying the amount of cobalt added in the zinc oxide thin film layer and the amount of manganese added in the lead oxide thin film layer, respectively.
As shown in the figure. Note that the dependence on each additive was measured under conditions in which the amounts of other additives were set to the optimum amounts, and all values exceed the values calculated in terms of oxides.

As is clear from FIGS. 2 and 3, the thin film varistor according to the present invention has excellent nonlinearity.

Table 1 shows the change rate ΔV1mA/V1mA of ■1111A after applying a DC voltage of 80% of the rising voltage (vllIIA) to mark 7]0 for 100 hours. Table 1 shows the characteristics of a conventional example in which bismuth oxide was used instead of lead oxide according to the present invention. As can be seen from the table, the nonlinear coefficient and the rate of change of VllIIA clearly show better characteristics in the case of the present invention.

d3 in Table 1. In this example, only the thin film varistor having the structure shown in FIG. Needless to say, a thin film varistor with excellent stability can be obtained.

[Effects of the Invention] As mentioned above, the thin film varistor according to the present invention is a highly practical element that has not only nonlinear characteristics but also stable characteristics, and can be easily mounted on various substrates. Since it can be formed, it can be easily fabricated not only into ultra-small varistor arrays, but also into 10-micron sizes, such as those required for liquid crystal display elements, by using photolithographic microfabrication technology, making it possible to have a wide range of applications.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view and a plan view of an embodiment of the present invention,
FIGS. 2 and 3 are characteristic diagrams showing the dependence of the nonlinear coefficient on the amount of additives. 1...Substrate 2...Lower electrode 11 layer 3
a, 3b...Zinc oxide thin film layer

Claims (1)

[Claims] (1) A lower electrode layer made of a metal thin film is formed on the substrate, and at least one set of a first zinc oxide (ZnO) thin film layer and a lead oxide (PbO) thin film layer are sequentially laminated on the electrode layer,
A thin film type voltage nonlinear resistor comprising: a second zinc oxide thin film layer formed on the laminate; and an upper electrode layer made of a metal thin film formed on the thin film layer; In the zinc oxide thin film layer, cobalt is converted to CoO and contains 0.1
~0.5 mol% is contained in the lead oxide thin film layer, and further, in the lead oxide thin film layer,
A thin film type voltage nonlinear resistor characterized in that it contains manganese in an amount of 0.05 to 0.1 mol% in terms of MnO.