JPS61218214A - Piezoelectric thin film resonator - Google Patents

Abstract

PURPOSE:To obtain excellent piezoelectricity without giving damage to a thin film constituting air gap by using a metallic film provided to a substrate in a bridge shape as a lower electrode, providing a piezoelectric thin film onto the lower electrode in a bridge shape and providing the upper electrode onto the piezoelectric thin film so as to be opposed to the lower electrode. CONSTITUTION:The metallic film 2 constituting the lower electrode is formed onto the substrate 1 so as to form the air gap layer 5 and the piezoelectric thin film 3 and the upper electrode 4 are formed sequentially on the film 2. The bridge form of the metallic film 2 on the substrate 1 is used as a support of the air gap layer 5 so as to prevent damages to the edge of the air gap 5a (substrate 1 and a bridge shape coundary) of the air gap 5a produced to a dielectric film made of SiO2 in a conventional resonator due to micro crack. Since the piezoelectric thin film 3 is formed onto a comparatively uniform metallic film 2, that is, onto the lower electrode, deterioration in the orientation of the axis C is not given and the piezoelectric thin film with a large electromechanical coupling coefficient is obtained and a resonator characteristic with a large response is obtained. Further, the change in the opposed area due to the shift of position of the upper/lower electrodes is minimized, then the variance in the characteristic is made less to improve the productivity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a piezoelectric thin film resonator using a piezoelectric thin film suitable for VHF band and F band.

[Technical background of the invention and its problems]

In recent years, with advances in material technology and processing technology, higher density integration of semiconductor elements has been promoted.

However, the reality is that the development of miniaturization of high frequency band passive components, particularly resonant circuit components such as resonators and filters, is lagging behind that of semiconductor devices.

There is a strong desire to develop small passive components that can be integrated with semiconductor devices.

Conventionally, in relatively low frequency bands from several MHz to several MHz, piezoelectric substrates such as crystal or lead titanate ceramics have been used as resonators and filters, and vibrators that utilize their thickness vibration have been put into practical use and widely used. There is. The resonant frequency of this vibrator is determined by its geometric shape such as length, width, or thickness. However, since such piezoelectric substrates are subject to mechanical strength and processing constraints, the thickness of the substrate is limited to a few tens of micrometers using a simple mechanical polishing method, and therefore the distribution resonance frequency of the substrate is at most The limit was about several tens of MHz.

Therefore, if a higher frequency is required, higher-order thickness 1d root motion will be used, but in this case, the electromechanical coupling coefficient in case 1 is inversely proportional to the square of the order, so it becomes extremely small. Therefore, the capacitance ratio increases and the spurious resonance comes to a position close to the first resonance point, making it difficult and impractical to realize a broadband resonator filter or a broadband oscillator for a voltage-controlled oscillator.

In contrast, recently ultra-compact VHF, U
Resonators using piezoelectric thin films are being researched with the aim of creating HFHF dual resonators.

An example of such a piezoelectric thin film resonator is @Progr.
Ess in the Development of
Miniature Thin Film SAW Re
5onator and Filter Techno
logicProceedings of the 36
th Annunl Symposium on Free
Quency Control June 1982 Issue No. 5
Some are disclosed on pages 37 to 548, etc.

This uses anisotropic etching technology on a semiconductor crystal substrate such as silicon to form a hole on the back surface of the substrate, leaving a predetermined thickness of the semiconductor thin film as part of the vibrating part, and then forming a lower vibration electrode on top of the semiconductor thin film. A resonator is formed by forming a piezoelectric thin film and an upper electrode for vibration, and has the following first-order characteristics.

1) Since the vibrating part can be formed extremely thin, 1
It can be operated in a fundamental mode or a low-order mode in a frequency band of 00 MHz to several GHz.

2) Since the electromechanical coupling coefficient can be increased,
A resonator with a low capacitance ratio can now be realized and can be used as a broadband resonant circuit.

3) Because the vibrating part is composed of a composite vibrating membrane.

A piezoelectric film and a dielectric film having frequency temperature coefficients of opposite sign can be combined. As a result, a resonator with better temperature characteristics than the piezoelectric material itself can be created, and depending on the configuration conditions, the temperature coefficient can be reduced to zero.

4) Since it can be formed using the same technology as a general integrated circuit, an ultra-small resonator can be easily formed and can be incorporated as a part of an integrated circuit.

However, this resonator has the following serious drawbacks.

(1') Pm etching solution (pyrocatechol C6H4 (O
H) 2, ethylenediamine NH2(CH2) 2N
Etching speed of up to 50% (mixture of H2 and water H20)
Since the thickness is as small as 1 tm/Hr, the thickness of a commonly used 3-inch diameter silicon substrate is 400 #l, and it takes about 8 hours to etch this, making productivity extremely low and mass production difficult.

(Since holes are formed in the peeled substrate itself, its mechanical strength is weak and it becomes difficult to handle during the manufacturing process.

(Since the piezoelectric thin film is formed in a vacuum after forming the four-hole portions, the temperature distribution on the substrate surface becomes uneven.

Therefore, the crystal orientation of the piezoelectric thin film itself is disturbed and the film quality and piezoelectricity are deteriorated, so that the electromechanical coupling coefficient becomes smaller, the vibration loss increases, the capacitance ratio of the resonator increases, and the Q decreases.

(4) When assembling a resonator into a part of an integrated circuit, even if a protective film is used, other integrated circuits are often damaged during the hole formation process, resulting in poor yield.

In order to eliminate these drawbacks, the applicant of the present application has previously proposed a cavity type resonator as shown in FIGS. 3 and 4.

As shown in the figure, this air gap type resonator has five
A gap layer 23 is formed between the dielectric film 22 such as i02 and the substrate 21.
It is characterized by the fact that it is partially protruded so as to form a . In FIGS. 3 and 4, 24 is a quadrilateral piezoelectric thin film formed on the dielectric material @22, 25.26
are a lower electrode and an upper electrode formed sandwiching this piezoelectric film 24, and the dielectric film 22 forms part of the vibrating film and the support body.

This resonator has many advantages, such as good mass production, improved mechanical strength, uniform temperature distribution during film formation, and less damage during integration.

However, when the present inventor conducted detailed experiments on the illustrated resonator, it was found that the following new problem occurred. When a 5iOz film is used as the dielectric film 22 forming a part of the support part, a part of the bridge-shaped part of the gap opening 27 may be damaged.The reason for this is that the internal stress of the 5i02 film It is thought that microcracks are concentrated in the bridge-shaped portion of the 5iOz film, which is a hard material.

To prevent damage, make the gap opening 27 smaller.
There is a method of forming the 5i02 film 22 sufficiently thick, but the size of the void opening 27 is determined by the dimensions of the lower electrode 25 and the lower electrode 26, and if the area of the lower electrode 25 is reduced, the C axis of the piezoelectric thin film 24 There is a drawback that the orientation deteriorates and good piezoelectricity cannot be obtained. Furthermore, increasing the thickness of the 5i02 film has the disadvantage that the usable range in the high frequency band becomes narrower.

Furthermore, the illustrated resonator has a lower mold 't25 and an upper electrode 26.
Since the electrodes 25 and 26 are arranged linearly, the area of the opposing portions of the electrodes 25 and 26 is greatly affected by the positional error of the electrode pattern, which tends to cause variations in the resonator characteristics.
There is a drawback that the yield is poor and the productivity is significantly reduced.

In addition, it is difficult to divide the lower electrode or the upper electrode into a plurality of comb-like V-sections and make them face each other to form a film that elastically couples a plurality of resonators.

[Purpose of the invention]

This invention was made in order to eliminate the above-mentioned drawbacks, and it is possible to obtain a piezoelectric thin film having good piezoelectricity without damage to the thin parts constituting the voids, and which is not affected by positional deviation during electrode pattern formation. Easily reduce VC with an elastic coupling filter! The object of the present invention is to provide a piezoelectric thin film resonator that can be formed into a piezoelectric thin film resonator.

[Summary of the invention]

In the piezoelectric thin film resonator of the present invention, a metal film is provided in a bridge shape on a substrate so that a void layer is formed, this metal film is used as a lower electrode, a piezoelectric thin film is provided in a bridge shape on this lower electrode, and further It is characterized in that an upper electrode is provided on the piezoelectric thin film so as to be at least partially opposed to the lower electrode.

〔Effect of the invention〕

According to this invention, by forming the metal film in a bridge shape as part of the supporting portion of the void layer, damage due to microcracks is eliminated, and product reliability and yield can be improved. . Further, since the piezoelectric thin film is uniformly formed on the lower electrode, that is, the metal film, there is no deterioration of C-axis orientation, and resonator characteristics with good piezoelectricity and large response can be obtained. Moreover, since the change in the facing area due to the positional deviation of the upper and lower electrodes can be minimized, variations in the characteristics of the resonator and filter can be reduced and productivity can be increased. Furthermore, since a plurality of resonators can be provided in a region corresponding to the gap layer, a filter using acoustic coupling can be formed in the same vibrating section.

[Embodiments of the invention]

The present invention will now be described in detail with reference to the drawings.A metal film 2 constituting a lower electrode is formed, and a piezoelectric thin film 3 and an upper electrode 4 are sequentially formed thereon. Such a piezoelectric thin film resonator is manufactured as follows. First, Z is chemically dissolved onto the substrate l using a method such as a sputtering method or a photoetching method in advance.
Forming a void-forming substance film such as nO in a rectangular shape,
A lower electrode of a metal film 2 made of Au--Ti or the like is formed in a band shape using means such as vacuum evaporation or hot etching so that at least a part of the gap-forming material film protrudes thereon. At this time, the metal film 2 is formed into a bridge-like structure with a part protruding. A piezoelectric thin film such as ZnO is formed on this metal 1 [2 by using means such as RF magnetron sputtering method,
) The butt part 2 of the metal film 2 using the lithography technique
A resist pattern is formed at a desired position except for a, and using this as a mask, an etching solution is used to form a piezoelectric thin film 3 of a predetermined size. Furthermore, an upper electrode 4 is formed on this piezoelectric thin film 3 so as to be opposed to a part of the metal film 2 using means such as a vacuum evaporation method. Finally, piezoelectric thin film 3 and polarization 4
After covering it with a protective film such as photoresist, it is immersed in an etching solution that dissolves the void-forming material (in the case of ZnO, a dilute acid solution such as HCl) to dissolve the void-forming material film. At this time, the metal film 2 and the above-mentioned protective film do not dissolve at all, and only the gap-forming substance film dissolves from the vicinity of the gap openings 5 provided at two locations, and finally, all the gap-forming substance dissolves. A gap layer 5 is formed between the substrate 1 and the metal film 2, and a piezoelectric thin film resonator is completed. here.

Sky 1! It is sufficient that the thickness of the iS layer 5 is several times or more the imaging displacement at the operating frequency of the resonator, but from the viewpoint of ease of fabrication, it is preferably from several hundred amps to several μm.

This piezoelectric thin film resonator causes the piezoelectric thin film 4 formed in the region corresponding to the void layer 5 to vibrate around the electrode facing part by applying an electric signal between the metal film 2 and the lower electrode 4. Operates as a vibrator.

FIG. 1(b) shows the structure of a fill 1 formed by the same method as in FIG. 1(a) above, in which another upper electrode 4a is provided adjacent to the upper electrode 4. When an energy trap state occurs near the resonance frequency due to the electrode load effect, a part of the vibrational energy is acoustically transmitted to the other upper electrode 4a side, and the induced vibrational energy is taken out from the upper electrode 4a. This allows it to function as a filter.

Therefore, according to such a configuration, by using the bridge-shaped metal film 2 on the substrate l as a supporting part of the void layer 5, the void opening 5, which was created in the conventional dielectric film such as Si02, is eliminated.
Damage due to microcracks at the a-end (substrate l and bridge-shaped interface) can be prevented, and the reliability of the product can be improved, as well as the yield and productivity can be increased. Furthermore, it is important to note that when the lower electrode is partially present as in the conventional case, the C-axis orientation of the piezoelectric film deteriorates during the formation process of the piezoelectric film. Since the piezoelectric thin film 3 is formed on a relatively uniform metal film 2, that is, on the lower electrode, a piezoelectric thin film with a large electromechanical coupling coefficient without deterioration of C-axis orientation can be obtained, and resonator characteristics with a large response can be obtained. can get. Further, since changes in the facing area due to positional deviation between the upper and lower electrodes can be minimized, variations in characteristics of the resonator and filter can be reduced and productivity can be increased. In addition, by removing the void-forming material film in the final step, the metal film 2 (lower electrode), piezoelectric thin film 3, and upper electrode 4 can be deposited on the void-forming material film in the intermediate step. At times, the temperature distribution becomes uniform and a film can be formed well. Thereby, a resonator with low vibration loss and a small capacitance ratio can be easily obtained.

Next, as another embodiment of the present invention, as shown in FIG. 2, an S
Some have a structure in which a chemically stable protective dielectric film 11 such as i02 or S i 3N4 is provided. Here, the same parts as in FIGS. 1(a) and 1(b) are denoted by the same reference numerals, and the description thereof will be omitted. In this way, by covering the piezoelectric film 3 and the upper electrode 4 with the dielectric film 11, it is possible to prevent the influence of outside air such as temperature and improve reliability. , Mata. By using a material having a frequency temperature coefficient of opposite sign to that of the piezoelectric thin film 3 as the dielectric film 11, a resonator with excellent temperature characteristics can be obtained. Furthermore, by using the dielectric film 11 also as a protective film during void formation, the step of removing a protective film such as photoresist can be omitted, and productivity can be increased accordingly. Note that the present invention is not limited to the above-mentioned embodiments, and can be implemented with various modifications without changing the gist. According to this invention, the material of the protective dielectric film is 5to2
(,5i02 PSG (Phospho 5ilicate Glass)
s), 8102 doped with boron and phosphorus BPSG
(Bor.

It may be made of glass such as Phospho 5 Ilicate Glass), or it may be a composite film made by laminating multiple types of dielectric films as long as the frequency temperature coefficient is opposite to that of the piezoelectric thin film.

According to this invention, the material of the piezoelectric thin film is limited to ZnO.
Materials such as 205 and PbTiO3 can be used as piezoelectric thin films.

According to this invention, the material of the material film for forming voids is not limited to ZnO, but may be any metal, oxide, semiconductor, dielectric, or polymeric material as long as it can be easily dissolved in the etching solution for forming voids. Such materials can be used as the void-forming material film.

According to this invention, when the piezoelectric thin film resonator is incorporated into an integrated circuit, a semiconductor such as St or GaAs is used as the substrate material, and when the piezoelectric thin film resonator is incorporated into a hybrid circuit or the like as an individual part, ceramic material is used as the substrate material. , glass, etc. can be used as the substrate.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing one embodiment of the present invention, FIG. 2 is a sectional view showing another embodiment of the invention, and FIGS. 3 and 4 are diagrams showing a conventional piezoelectric thin film resonator. l...Substrate, 2...Metal film (lower electrode), 2a...
- Butt part, 3... Piezoelectric thin film, 4.4a... Upper electrode. 5... Gap layer, 5a... Gap opening, 11... Dielectric film. 21...Substrate, 22...Dielectric film, 23...Void layer, 24...Piezoelectric thin film, 25...Lower electrode, 26...
...Top electrode, 27...Gap opening. Engraving of drawings (no change in content) (Q) (b) Figure 1 Figure 2 Figure 3 Figure 1 Procedures Amendment (Method) Showa eff7. 2nd day

Claims (3)

[Claims] (1) a substrate, a metal film provided so that a void layer is formed between the substrate, and a piezoelectric film formed on the upper side of the metal film including a region corresponding to the void layer; A piezoelectric thin film resonator comprising electrodes that sandwich the piezoelectric film and are provided to face at least a portion of the metal film. (2) The piezoelectric thin film resonator according to claim 1, wherein at least one electrode is provided in a region corresponding to the void layer. (3) The piezoelectric thin film resonator according to claim 1, wherein the piezoelectric film has a dielectric film formed on its upper side.