JPS6286910A - Manufacture of piezoelectric thin film resonator - Google Patents

Abstract

PURPOSE:To obtain a piezoelectric thin film resonator having high sharpness by making the lower electrode of an alkali resistant piezoelectric thin film resonator function as a stopping layer of etching when etching a supporting substrate made of a sintered body of zinc oxide by an alkaline etching solution. CONSTITUTION:To prevent deterioration of characteristic of a piezoelectric thin film made of zinc oxide 14 due to moisture absorption, a protective film 16 made of acid resistant and alkali resistant material is formed on the piezoelectric thin film and upper electrode 15. A mask 17 is formed by photoresist on a metallic layer 13 formed on the whole face of the main surface of a supporting substrate 11, and an area 13a of the metallic layer 3 slightly larger than an area facing the upper electrode 15 is removed by etching. Then, the supporting substrate 11 is etched by 30% caustic soda from removed area 13a of the metallic layer 13. This etching is stopped when arrived at a lower electrode 12 made of alkali resistant metal formed on a main face 11a of the supporting substrate 11. A protective film 20 is formed from the face of inner wall of a recessed part 18 to the metallic layer 13.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method of manufacturing a piezoelectric thin film resonator that can be used in the VHF and UHF frequency bands.

(Prior art) In general, the so-called bulk wave resonator, which utilizes the basic thickness vibration of a piezoelectric substrate such as ceramic or quartz, is limited to the thickness of the piezoelectric substrate due to limitations in processing technology and mechanical strength of the piezoelectric substrate. The limit is several tens of μm, and therefore the usable resonance frequency is also limited to several tens of MHz.

Therefore, in recent years, so-called diaphragm-type pressure sensors, which can utilize semiconductor manufacturing technology as is and operate in the VHF and UHF ultra-high frequency bands, have a configuration as shown in FIGS. 5 and 6. 1! Thin film resonators have been studied (for example, Japanese Patent Application Laid-open Nos. 60-68706, 68710-1983, and 60-68-
(See Publication No. 68711).

The piezoelectric thin film resonator has a silicon oxide thin film 2 formed on one main surface of a silicon single crystal support weir temporary I, a first electrode 3, a piezoelectric thin film 4 of zinc oxide, and a first electrode 3 on the silicon oxide thin film 2. 2 electrodes 5 are sequentially formed, while the silicon single crystal support substrate 1 is anisotropically etched into a diaphragm shape from the other main surface of the silicon single crystal support substrate 1 toward the piezoelectric thin film 4. A recess 6 is formed to enable thickness vibration.

By the way, the formation of the recess 6 by the above-mentioned anisotropic etching of the silicon single crystal support substrate 1 utilizes the difference in etching speed between the (100) plane and the (111) plane of the silicon single crystal support substrate 1, and the etching When the etching reaches the silicon oxide thin film 2 from the silicon single crystal support substrate 1, this silicon oxide thin film 2 is also etched. For this reason, the silicon oxide thin film 2 in the recess 6 of the silicon single crystal support substrate l
There was a problem in that the thickness of the piezoelectric thin film resonator became non-uniform and the sharpness Q of the piezoelectric thin film resonator during resonance decreased.

(Objective of the Invention) An object of the present invention is to provide a method for manufacturing a piezoelectric thin film resonator having high sharpness.

(Structure of the Invention) Therefore, in the present invention, the lower electrode of the piezoelectric thin film resonator formed on one main surface of the support substrate made of a sintered body of zinc oxide is formed of an alkali-resistant metal, and the support substrate is The supporting substrate is etched from the other main surface of the substrate toward the lower electrode using an alkaline etching solution until etching is stopped at the lower electrode. That is, in the present invention, when etching a supporting substrate made of a sintered body of zinc oxide with an alkaline etching solution, the lower electrode of the piezoelectric thin film resonator having alkali resistance functions as an etching stop layer. be.

(Effects of the Invention) According to the present invention, since the etching of the support substrate is stopped at the lower electrode of the piezoelectric thin film resonator, the thickness of the lower electrode does not become uneven due to etching, and the etching has high sharpness. Piezoelectric thin film resonators can be manufactured. Also,
According to the invention, since a support substrate of zinc oxide is used,
It is possible to obtain a piezoelectric thin film resonator that is cheaper than one using a single crystal support substrate of silicon, and since the piezoelectric thin film and the support substrate are made of the same material, the thermal expansion coefficients of both are equal. A piezoelectric thin film resonator with stable temperature characteristics can be obtained.

(Example) Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

First, M n CO! to zinc oxide (ZnO)! l and V 2
05, manganese (M n) and vanadium (
V) is prepared so that it is contained in an amount of 5 atomic % and 0.1 atomic %, respectively, and wet-mixed. This is calcined and pulverized in a wet mill, then pressure molded into a flat plate shape and sintered at about 1200°C for 2 hours. The resulting thickness was 10 mm, 50
A sintered body of 50+n+n (not shown) was cut into slices with a thickness of 0°5 mm, and mirror-polished with #8000 SiC to a thickness of 0.3+n+ as shown in Fig. 1(a).
n flat support substrates II are created.

As described above, the reason why manganese is added to zinc oxide is to increase the insulation resistance of the support substrate 11, and the support substrate 11 has an insulation resistance of IQIO ohms or more. Further, the reason why vanadium is added to zinc oxide is to improve the sinterability of zinc oxide and obtain a sintered body with high density.

Next, on one main surface 11a of the support substrate ti, as shown in FIG.
As shown in b), the lower electrode 1 has a constant modulus steel composition consisting of iron (Fe), nickel (Ni) and chromium (Cr).
2 is formed by a sputtering method. In this sputtering, since the support substrate 11 is an oxide and the formed lower electrode 12 is easily oxidized, Ar:Ht=80
It is preferable to carry out the reaction in a mixed gas of 20:20. Note that this lower electrode 12 is, as will be described later, a support substrate! It functions as a stop layer for the etching described in (2) and also functions as one electrode of the piezoelectric thin film resonator.

On the other hand, a metal layer 13 is formed over the entirety of the other main surface 11b of the support substrate II. This metal layer I3 is formed on the support substrate 11
It is formed to suppress side surface etching when etching from the other main surface 11b side, and is formed from an iron-nickel alloy, which is likely nickel having alkali resistance.

Next, as shown in FIG. 1(C), the lower electrode I2
Zinc containing 2 at % of nickel is oxidized by reactive sputtering to form a piezoelectric thin film 14 made of C-axis oriented zinc oxide crystals on one main surface lla of the support substrate 11 covering the substrate. The thickness t of the piezoelectric thin film 14 is determined by t=N/f, where f (Hz) is the intended resonance frequency and N (Hz, mm) is the frequency constant specific to the composite vibrating membrane.

On the piezoelectric thin film 14, as shown in FIG. 1(d), an upper electrode I having the same constant elastic steel composition as the lower electrode 12 is provided.
5 is formed by a sputtering method.

After that, in order to prevent the piezoelectric thin film 14 made of zinc oxide from absorbing moisture and deteriorating its characteristics, acid-resistant and acid-resistant A protective film 16 made of an alkaline material is formed. This protective film 16 is preferably a paraxylylene vapor-deposited film, which has a small mass and a small thickness, so that the thickness vibration of the piezoelectric thin film I4 is not inhibited.

The thickness of the protective film 16 is set to 05 μm to 10 μm.

On the other hand, as shown in FIG. 1(f), on the metal layer 13 formed on the entire surface of the other main surface 11b of the support substrate 11,
A mask I7 is formed using photoresist, and a region 13a of the metal layer 13 that is slightly larger than the region facing the upper electrode 15 is covered with ferric oxide (Baume Be'45°), hydrochloric acid
Remove by etching with 0% solution at 50°C.

After that, from the region 13a where the metal layer (3) was removed,
As shown in FIG. 1(g), the support substrate 2 is etched with 30% caustic soda (NaOH). This etching stops when it reaches the lower electrode 12 formed on one main surface 11a of the support substrate 11 and made of an alkali-resistant earth metal. □Thereby, the lower electrode 12 is attached to the support substrate 11.
A recess 18 is formed to allow thickness vibration of the piezoelectric thin film 14 sandwiched between the upper electrode 15 and the upper electrode 15 .

After forming the recesses 18 in the support substrate II by etching as described above, in order to prevent the support substrate II made of zinc oxide from absorbing moisture, the four parts are etched as shown in FIG. 1(h). From the inner wall surface of 18 to the metal layer 13,
A protective film 20 is formed.

As the protective film 20, a varaxylylene vapor-deposited film similar to the protective film 16 can be used.

Through the above steps, a piezoelectric thin film resonator 9 as shown in a plan view in FIG. 2 can be obtained. In this piezoelectric thin film resonator 19, the recessed portion 18 of the support substrate 2 is formed under the region 21 sandwiched between the lower electrode 12 and the upper electrode 15 of the piezoelectric thin film 14, so that the above-mentioned portion 21 of the piezoelectric thin film 14 has a thickness. It can vibrate. The lower electrode above! 2 and the upper electrode!
5 are drawn out to the peripheral edge of the support substrate 11 by lead electrodes 12a and 15a, respectively.

As described above, if the piezoelectric thin film resonator 19 is manufactured by the steps shown in FIGS. 1(a) to 1(h), the lower electrode 12 of the piezoelectric thin film resonator 19 is made of an alkali-resistant earth metal, so that This lower electrode I2 functions as a stop layer for etching of the support substrate 11 with caustic soda (alkali). As a result, the etching is stopped with the thickness of the lower electrode 12 being uniform, and the reflection characteristics of the thickness vibration generated in the piezoelectric thin film 14 on the lower electrode 12 are made uniform, and the sharpness Q of the piezoelectric thin film resonator 19 is reduced. can be increased.

In the above embodiment, the metal layer 13 formed on the other main surface 11b of the support substrate 11 in FIG. 1(e)
is not essential, and the support substrate II can be etched by forming the mask I7 directly on the other main surface 11b of the support substrate II. However, if this is done, as shown in FIG. are not formed sharply. On the other hand, the support substrate 11
If the metal layer 13 is formed on the other main surface 11b of
As shown in FIG. 4, the side walls of the recess 18 are formed sharply. The circles in FIGS. 3 and 4 indicate air bubbles generated by etching.

In the above embodiment, as shown in FIG. 1(b), the lower electrode I2 and the metal layer 13 are formed on one main surface 11a and the other main surface 11b of the supporting substrate 11, respectively. Support substrate in the same manner as described in g)! After forming the recess 18 by etching (2), the piezoelectric thin film 14, the upper electrode 15, the protective film 16, etc. may be formed on the one main surface 11a side of the support substrate 11.

[Brief explanation of drawings]

Figure 1(a), Figure 1(b), Figure 1(c), Figure 1(
d), FIG. 1(e), FIG. 1(r), FIG. 1(g), and FIG. 1(h) are explanatory diagrams of the manufacturing process of the piezoelectric thin film resonator according to the present invention, respectively, and FIG. 3 is a plan view showing the electrode structure of a piezoelectric thin film resonator manufactured by the method of the present invention, and FIGS. 3 and 4 show cases in which no metal layer is formed on the other main surface of the support substrate and cases in which a metal layer is formed on the other main surface of the support substrate. FIG. 5 is a perspective view of a piezoelectric thin film resonator manufactured by a conventional method, and FIG. 6 is a sectional view taken along line A--A in FIG. 5. DESCRIPTION OF SYMBOLS 11... Support substrate, lla, llb... Main surface, 12... Lower electrode, 13... Metal layer, 14...
... Piezoelectric thin film, I5... Upper electrode, 16... Protective film, 17... Mask, 18... Concave portion, 2
0...Protective film. Patent applicant Murata Manufacturing Co., Ltd. Representative Patent attorney Aoyama Aoyama and 2 others Figure 3
Figure 4 Figure 5 Figure 6

Claims (1)

[Claims] (1) A step of preparing a support substrate made of a sintered body of zinc oxide, forming a lower electrode made of an alkali-resistant metal on a part of the main surface of the support substrate, and covering the lower electrode with the support substrate. A step of forming a piezoelectric thin film of zinc oxide on one main surface, a step of forming an upper electrode on the piezoelectric thin film, and a step of forming a protective film having acid resistance and alkali resistance over the piezoelectric thin film and the upper electrode. a step of masking the other main surface of the supporting substrate and etching the supporting substrate from the other main surface toward the lower electrode with an alkaline etching solution until etching stops at the lower electrode. A method for manufacturing a piezoelectric thin film resonator, comprising the steps of: