JPH0568128B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high frequency piezoelectric vibrator for VHF and UHF using a piezoelectric thin film, and more particularly to a thin film piezoelectric vibrator having a vibrating part of a composite structure consisting of a combination of a silicon thin film and a piezoelectric thin film. be.

Generally, a piezoelectric vibrator used at a high frequency of several tens of MHz or more uses thickness vibration of a piezoelectric thin film whose plate surface is sufficiently wide compared to its thickness as a vibration mode.

The resonant frequency of thickness vibration is inversely proportional to the thickness of the piezoelectric thin film, so in order to use it at high frequencies, the thickness must be made thinner, but when the thickness is less than about 40 microns, processing such as parallel plane polishing is required. It becomes very difficult. Therefore, it is difficult to mass produce vibrating piezoelectric vibrators with a thickness of 50 MHz or more using bulk piezoelectric crystals or piezoelectric ceramics.

As a method of obtaining a vibrating piezoelectric vibrator with a thickness of 50 MHz or more by reducing the thickness of the vibrating portion, a thin film piezoelectric vibrator having the structure shown in FIGS. 1 and 2 is known. This thin film piezoelectric vibrator is manufactured by forming a semiconductor or insulator thin film member 13 on a substrate 11, forming holes 12 in the substrate 11 by etching, and then sequentially forming a base electrode 14 and a base electrode 14 on the thin film member 13. It is manufactured by forming a piezoelectric thin film 15 and an upper electrode 16, and a composite diaphragm consisting of a generally non-piezoelectric thin film member 13 and a piezoelectric thin film 15 is supported at its peripheral portion by a substrate 11. It has a structure.

In the thin film piezoelectric vibrator having the structure shown in FIGS. 1 and 2, silicon having a (100) surface is generally used as the substrate 11, and an etching solution (hereinafter referred to as EDP solution) consisting of ethylenediamine, pyrocatechol, and water is used as the substrate 11. Alternatively, the holes 12 can be precisely created using anisotropic etching using an aqueous potassium hydroxide (KOH) solution.

As the thin film member 13, the above EDP liquid or
A material with an etching rate as low as possible for a KOH aqueous solution is required, and various oxides and nitrides have been proposed as materials suitable for this purpose, but the most ideal material is a silicon thin film. This is because silicon thin film is a single crystal, so it has high mechanical strength and acoustic quality.
This is because the factor Q is large. In particular, it is known that silicon doped with boron at a high concentration has an extremely low etching rate with respect to EDP liquid. Highly doped silicon thin films are used as thin film members.

However, since a silicon thin film doped with boron at a high concentration has a very high conductivity, the conventional thin film piezoelectric vibrator shown in FIGS. 1 and 2 had the following serious drawbacks. In other words, in such a vibrator, an extraction electrode 17 of the upper electrode is necessary for wiring by wire bonding, etc., but as a result of the capacitance between this extraction electrode and the silicon thin film being added in parallel to the vibrator, The disadvantage is that the capacitance ratio of the vibrator becomes apparently large. The dimensions of the upper electrode of a thin film piezoelectric vibrator are usually
The width is about 100 to 200 μm, and even if the extraction electrode is made as small as possible, it needs to be about 100 μm square, so the apparent capacitance ratio of the vibrator becomes 2 to 3 times larger. For this reason, when a conventional thin film piezoelectric vibrator is used, it is not possible to sufficiently widen the control range of the oscillator or the fractional bandwidth of the filter.

An object of the present invention is to provide a thin film piezoelectric vibrator that eliminates the above-mentioned drawbacks, and the most important feature of the present invention is that the silicon thin film has a two-layer structure consisting of a low-resistance layer and a high-resistance layer.

A base electrode, a piezoelectric thin film, and a piezoelectric thin film are placed on the silicon thin film in order.
In a thin film piezoelectric vibrator having a structure in which a vibrating part is formed with an upper electrode and an outer edge of the vibrating part is supported by a substrate from which a portion corresponding to the vibrating part is removed, the silicon thin film is This is a thin film piezoelectric vibrator characterized by having a two-layer structure of a resistance layer and a high resistance layer.

Hereinafter, the present invention will be explained in detail with reference to Examples.

3 and 4 show the structure of an embodiment of the thin film piezoelectric vibrator of the present invention. 31 in Figures 3 and 4
is a silicon substrate whose surface is a (100) plane,
32 is a hole created in the substrate by etching;
33 is a silicon thin film doped with boron at a high concentration. 33' is a high-resistance silicon thin film, which is a feature of the present invention, and is epitaxially grown on the silicon thin film 33. 34 is a base electrode;
35 is a piezoelectric thin film, 36 is an upper electrode, and 37 is an extraction electrode of the upper electrode. As can be seen from FIGS. 3 and 4, in the thin film piezoelectric vibrator of the present invention, the capacitance added to the vibrator is the capacitance between the base electrode 34 and the silicon thin film 33, and the capacitance between the extraction electrode 37 and the silicon thin film 33. They are connected in series, and the capacitance is 1/3 that of the conventional structure.
It is as follows. Therefore, the capacitance ratio of the thin film piezoelectric vibrator of the present invention is approximately 1/2 that of the conventional structure shown in FIGS. 1 and 2, and by using the thin film piezoelectric vibrator of the present invention, The control range of the oscillator or the fractional bandwidth of the filter is about twice that of the conventional one.

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

A silicon thin film doped with boron at a concentration of 10 ²⁰ /cm ³ is epitaxially grown to a thickness of 1 μm on the surface of a silicon substrate whose surface is a (100) plane, and then a high-resistance silicon thin film is further grown on top of it. 3μm
It was grown epitaxially to a thickness of . Next, a Si ₃ N ₄ thin film is formed on both sides of the substrate, a window is formed in the Si ₃ N ₄ thin film on the back side, and the silicon substrate corresponding to the vibration site is etched from the back side using an etching solution consisting of ethylenediamine, pyrocatechol, and water. Then, a silicon thin film diaphragm was formed.
Next, an Au/Cr electrode was formed on the silicon thin film using a vapor deposition method, followed by a 4 μm thick zinc oxide (ZnO) thin film using a sputtering method. Finally, with the vapor deposition method
A thin film piezoelectric vibrator having the structure shown in Figs. 3 and 4 was manufactured in which an Al electrode was formed on a ZnO thin film. At the same time, a resonator with a conventional structure was manufactured in which all silicon thin films were silicon thin films doped with boron at a concentration of 10 ²⁰ /cm ³ using a completely customary process. As a result of measuring the characteristics of two types of resonators, the capacitance ratio of the resonator with the conventional structure was 80, and the capacitance ratio of the resonator with the structure of the present invention was
40, demonstrating the usefulness of the present invention.

As described above, according to the present invention, it is possible to provide a thin film piezoelectric vibrator with a smaller capacitance ratio compared to conventional structures, and by using the thin film piezoelectric vibrator of the present invention, it is possible to create an oscillator with a wide control range or a filter with a wide specific band. can be realized.

[Brief explanation of the drawing]

1 and 2 are diagrams showing the structure of a conventional thin film piezoelectric vibrator, with FIG. 1 being a plan view and FIG. 2 being a sectional view. 3 and 4 are diagrams showing the structure of the thin film piezoelectric vibrator of the present invention, with FIG. 3 being a plan view and FIG. 4 being a sectional view. 1 to 4, 11 and 31 are silicon substrates, 12 and 32 are holes, and 13, 33, and 33'
is a silicon thin film, 14, 34 is a base electrode, 15,
35 is a piezoelectric thin film, 16 and 36 are upper electrodes, 17,
37 is an extraction electrode.

Claims (1)

[Claims] 1. It has a vibrating part with a structure in which a base electrode, a piezoelectric thin film, and an upper electrode are formed in this order on a silicon thin film, and the outer edge of the vibrating part is supported by a substrate from which the part corresponding to the vibrating part has been removed. 1. A thin film piezoelectric vibrator having a structure in which the silicon thin film has a two-layer structure of a low resistance layer and a high resistance layer.