JPH0161253B2 - - 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 in particular has a vibration part with a composite structure consisting of a combination of a silicon thin film doped with boron at a high concentration and a piezoelectric thin film. This invention relates to thin film piezoelectric vibrators.

Generally, piezoelectric vibrators used at high frequencies, such as several tens of MHz or more, use thickness vibration of a piezoelectric thin plate whose 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 plate, 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, creating holes 12 in the substrate 11 by etching, and then sequentially forming 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 has a structure in which 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 is becoming.

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 the lowest possible etching rate for KOH aqueous solution is required, and various oxides and nitrides have been proposed as materials suitable for this purpose, but silicon thin films doped with a high concentration of boron are the most suitable. It is an ideal material. This is because a silicon thin film doped with boron at a high concentration is grown epitaxially on a silicon substrate, diffused, and
This is because since it is a single crystal made by ion implantation or the like, it has high mechanical strength and a high acoustic quality factor Q.

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 required for wiring by wire bonding, etc., but as a result of the capacitance between this extraction electrode 17 and the silicon thin film being applied 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 diameter 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 thin film piezoelectric vibrator having a conventional structure is used, the control range of the oscillator or the fractional bandwidth of the filter cannot be made sufficiently wide.

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 a structure in which a thin film of an insulating material is provided between the lead electrode of the upper electrode and the piezoelectric thin film. It is in.

That is, the present invention provides a thin film member whose outer edge is supported by a silicon substrate, a base electrode formed at the center of the thin film member, a piezoelectric thin film formed on the base electrode, and a piezoelectric thin film member formed on the base electrode. In a thin film piezoelectric vibrator having a vibrating portion consisting of an upper electrode formed on a thin film, a thin film of an insulating material is formed on the piezoelectric thin film at the outer edge, and a thin film of an insulating material is formed from the upper electrode on the piezoelectric thin film to the thin film of the insulating material. This is a thin film piezoelectric vibrator characterized in that an extraction electrode of the upper electrode is formed up to the upper electrode, and the upper electrode and the extraction electrode are a continuous and integrated electrode thin film.

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

FIGS. 3 and 4 show the structure of a thin film piezoelectric vibrator according to an embodiment 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 film doped with boron at a high concentration, 34 is a base electrode, 35 is a piezoelectric thin film, 36 is an upper electrode, 37 is an extraction electrode of the upper electrode,
38 is an insulating thin film which is a feature of the present invention. From the perspective of the present invention, it is desirable that the insulating mountain thin film 38 be made of a material with a dielectric constant as small as possible;
Various oxides and nitrides whose manufacturing methods have been established in the semiconductor industry, such as SiO ₂ and Si ₃ N ₄ , can be used. As a result of the thin film piezoelectric vibrator of the present invention as shown in FIGS. 3 and 4 having the insulating thin film 38, the capacitance between the lead electrode of the upper electrode and the silicon thin film is very small compared to the capacitance at the vibrating part. Therefore, the apparent capacitance ratio does not increase as in the conventional case, and an oscillator with a wider control range or a filter with a wider fractional band can be realized than in the conventional case.

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 was epitaxially grown on the surface of a silicon substrate having a (100) plane to a thickness of 3 μm. Next, Si ₃ N ₄ was formed on the back side of the silicon substrate.
Using the thin film as a mask, the silicon substrate corresponding to the vibration site was etched from the back side using an etching solution consisting of ethylenediamine, pyrocatechol, and water to form a silicon thin film diaphragm. Next, an Au/Cr electrode was formed on the silicon thin film using a vapor deposition method, and then a 4 μm thick electrode was formed using a sputtering method.
A zinc oxide (ZnO) thin film was formed. Furthermore, after forming a SiO ₂ insulating thin film by sputtering method,
Al electrodes were formed by vapor deposition to produce thin film piezoelectric vibrators having the structures shown in FIGS. 3 and 4. At the same time, a resonator with a conventional structure without an SiO ₂ insulating film was manufactured using exactly the same process. As a result of measuring the characteristics of two types of vibrators, the capacitance ratio of the vibrator with the conventional structure was 60, and the capacity ratio of the vibrator with the structure of the present invention was 30, 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 than the conventional structure, 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 wide specific band. A filter can be implemented.

In the above explanation, the case where a silicon thin film doped with boron to a Sakai concentration was used as the thin film member was explained, but the same effect can be obtained by applying the present invention even when other thin film members are used. .

[Brief explanation of drawings]

FIGS. 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, 13 and 33 are silicon thin films, 14 and 34 are base electrodes, 15 and 35 are piezoelectric thin films, and 16 and 36 are upper electrodes. , 17 and 37 are extraction electrodes, and 38 is an insulating thin film.

Claims (1)

[Claims] 1. A thin film member whose outer edge is supported by a silicon substrate, a base electrode formed at the center of the thin film member, a piezoelectric thin film formed on the base electrode, and a piezoelectric thin film formed on the piezoelectric thin film. In a thin film piezoelectric vibrator having a vibrating part consisting of an upper electrode, a thin film of an insulating material is formed on the piezoelectric thin film at the outer edge, and the upper electrode is extended from the upper electrode on the piezoelectric thin film to the thin film of the insulating material. 1. A thin film piezoelectric vibrator characterized in that an electrode is formed, and the upper electrode and the extraction electrode are a continuous and integral electrode thin film.