JPS595722A - Electrode structure of thin film of zinc oxide - Google Patents

Abstract

PURPOSE:To obtain an electrode structure of a thin zinc oxide film with stable electric characteristics by interposing a Ta layer between the thin zinc oxide film and an Al layer and preventing the diffusion of Al into the thin zinc oxide film. CONSTITUTION:The thin zinc oxide film is used as a piezoelectric body for a surface acoustic wave device, various oscillators, etc. A tuning fork type oscillator which uses the invented electrode structure of the thin zinc oxide film as shown in a figure is constituted by providing the electrode structure of the thin zinc oxide film consisting of the zinc oxide film 12, Ta layer 13, and Al electrode 14 to a metallic tuning fork 11 made of ''Elinvar'', etc. This electrode structure is obtained by forming the Ta layer 13 on the thin zinc oxide film 12 formed by sputtering on the tuning fork 11 by an electron beam method to a 150Angstrom thickness and forming the Al electrode 14 thereupon to a 1mum thickness by the electron beam method. This electrode structure is free of diffusion of Al into the thin zinc oxide film 12, which functions as the piezoelectric body electrically stably.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrode structure of a zinc oxide thin film exhibiting stable characteristics.

Zinc oxide thin films are used as piezoelectric materials in surface acoustic wave devices, tuning fork vibrators, vibrator elements, etc.

An example of the use of this zinc oxide thin film will be explained based on a tuning fork vibrator.

FIG. 1 is a side view showing an example of a tuning fork vibrator. In FIG. 8, 1 is a main body of a tuning fork vibrator, 2.3 is a leg portion of this main body 1, and a side wall 2a of the leg portion 2.3.

Zinc oxide source II 4.5 is formed in 3a.

This zinc oxide thin film 4.5 is formed by vacuum evaporation, sputtering, ion blasting, or the like. 6
．． 7 shows a ρ electrode formed on the zinc oxide thin film 4.5.

This Aj2 electrode 6.7 was chosen because it is cheap and can be bonded, and it is made by electron beam evaporation method etc.
It is formed with a film thickness in the range of 000 to 10000A.

However, the electrode structure of the zinc oxide thin film as described above has the following drawbacks. That is, since the ρ electrode itself exhibits a high affinity, there is a drawback that Aβ diffuses into the zinc oxide thin film and the electrical characteristics deteriorate. That is, a phenomenon was observed in which the electrical properties of the zinc oxide film, such as the vibration frequency, were significantly changed due to the diffusion of trivalent Aβ into zinc oxide, which is a divalent semiconductor. Furthermore, when a high-temperature load life test was conducted, the above-mentioned phenomenon was further accelerated, and the deterioration of the electrical characteristics became even more significant.

Therefore, when forming a zinc oxide thin film, it is necessary to consider the entire structure including the electrode, and it is necessary to further improve the conventional electrode structure.

The present invention was made against this background, and an object thereof is to provide an electrode structure of a zinc oxide thin film exhibiting stable characteristics.

The present invention will be described in detail below based on examples.

FIG. 2 is a side view showing an example in which the zinc oxide thin film electrode structure according to the present invention is applied to a tuning fork vibrator.

11 is a metal tuning fork made of Erimba or the like, 12 is a zinc oxide thin film, 13 is a single layer of Ta, and 14 is an electrode. Of these, the Ta layer 13 is formed by an electron beam method, a sputtering method, an ion beam method, a resistance heating vapor deposition method, or the like.

FIG. 3 is a perspective view showing an example in which the electrode structure of the zinc oxide thin film according to the present invention is applied to a sound element vibrator in a bending vibration mode.

In the figure, reference numeral 21 indicates a vibrator body, which is composed of a vibrator 22 and a frame 23 that supports the vibrator 22 with a support portion 24. 25 is a zinc oxide thin film formed on the surface of the vibrator 22. 26 is a T formed on the zinc oxide thin film 25.
The a-layer 27 is a ρ electrode formed on the Ta layer 26.

FIG. 4 is a side view of an example in which the present invention is applied to another bending vibration mode vibrator.

In the figure, reference numeral 31 is a substrate made of ceramics, plastic, rubber, etc., and on the surface of this substrate 31 is an Aρ electrode. FIG.

In the figure, 41 is a zinc oxide thin film, 42 is a Ta layer formed on both sides of the zinc oxide thin film 41, and 43 is a ρ electrode formed on the 18 layer 42.

FIG. 6 is a side view showing an example in which the present invention is applied to a thickness vibration mode vibrator.

In the figure, 51 is a substrate made of 3i, 5i02, etc.
On the substrate 51, /l electrodes 52, 18 layers 53 are sequentially formed. Further, on the 18 layers 53 is a zinc oxide thin film 5.
4 is formed. A cavity 51a is formed in the substrate 51 corresponding to the position where the zinc oxide thin film 54 is formed. On the -L side of the zinc oxide thin film 54, a Ta layer 55 and an A electrode 56 are sequentially laminated.

Next, as a specific example, an example in which the electrode structure of the zinc oxide thin film according to the present invention is applied to the tuning fork vibrator shown in FIG. 2 will be described.

To explain with reference to FIG. 2, a thin zinc oxide rfA layer 12 is formed on the vibrator 11 by sputtering, a Ta layer 13 is formed on it to a thickness of 150A by an electron beam, and then a Aρ electrode 1 consisting of 1μ
4 was formed by an electron beam method. In this way, a vibrator with a vibration frequency of 32 Kl-1z was created.

A DC voltage of 20 V was applied to this vibrator, and it was left at a temperature of 120° C. for 10,000 hours. When the -C measurement of the temporal change characteristics of the vibration frequency at this time was performed on 20 samples, the results shown in FIG. 7 were obtained. The solid line in the figure is based on this embodiment. Furthermore, the broken line shows the results of measurements made in the same manner for a conventional example consisting of only a ρ electrode. This vibration frequency change characteristic over time (ΔF
/Fo) was determined from the following formula.

Also, the series resonant resistance (Ro) and the parallel resonant resistance (R
A) was also measured in the same manner, and the results are shown in FIGS. 8 and 9, respectively.

As is clear from FIGS. 1 to 9, the device according to the present invention has a smaller change in vibration frequency over time, a smaller change in Ro over time, and a smaller change in Ra over time than the conventional example. effects have been obtained.

Here, the reason why Ro and Ra were measured is as follows.

First, if we show both circuits for a zinc oxide thin film, the first
It will look like Figure 0. In the figure, Cd indicates parallel capacitance, which is a value close to the capacitance when the zinc oxide thin film is considered as a capacitor. Ro is series resonant resistance, Co is equivalent capacitance, L
o is the equivalent inductance.

Moreover, R a is approximately determined by the following equation.

From the relationship, this R corresponds to the series resonant frequency (fo).
It can be seen that if o becomes too large, a large degree of amplification is required for oscillation, resulting in a deterioration of the oscillation conditions.

Also, from the relationship between impedance and frequency shown in Figure 11, Ra corresponds to the anti-resonance frequency (fa), and if Ra becomes small, a sufficient phase change for oscillation cannot be achieved, which also reduces the oscillation conditions. It will bring about.

As is clear from FIGS. 8 and 9, according to the embodiment of the present invention, Ro,
It can be understood that the change in Ra over time is small, and from this it can be understood that the electrode structure of the zinc oxide thin film according to the present invention has stable electrical characteristics and also exhibits stable characteristics even in high temperature load life tests. This is used as a guideline to determine whether stable oscillation can be expected.

As described above, according to the present invention, a Ta layer is interposed between the normal lead oxide thin film and the ρ electrode as a diffusion prevention layer for Aβ, and compared to the conventional one, it does not exhibit sufficient characteristics for practical use. Zinc tsi can be provided. Particularly, according to the present invention, a highly reliable zinc oxide thin film with small changes in Ro and Ra and little change in frequency can be obtained in a high-temperature load life test.

[Brief explanation of the drawing]

Fig. 1 shows an example of a tuning fork vibrator (side view), Fig. 2 is a side view showing an example in which the zinc oxide thin film electrode structure according to the present invention is applied to a tuning fork vibrator, and Fig. 3 shows a tuning fork vibrator. FIGS. 4 to 6 are side views of examples in which the zinc oxide thin film electrode structure according to the present invention is applied to each vibrator; FIG. 7 is a characteristic diagram of vibration frequency change over time based on a specific embodiment of the present invention, FIG. 8 is a characteristic diagram of change over time of the same <Ro, FIG. 9 is a characteristic diagram of change over time of the same <Ra, and FIG. 11 is a diagram of the relationship between impedance and frequency. 11...Substrate, 12...Zinc oxide thin film, 13...・l"a layer, 14...Aρ electrode. Patent applicant: Murata Manufacturing Co., Ltd. the law of nature

Claims (1)

[Claims] An electrode structure of a zinc oxide thin film, characterized in that a Ta layer is interposed between the surface of the zinc oxide thin film and a β electrode.