JPS61218215A - Piezoelectric thin film resonator - Google Patents

Abstract

PURPOSE:To increase the strength of structure, to eliminate damages of the air gap layer and to improve the reliability of products by using a bridge shape formed with the 1st dielectric film, the piezoelectric thin film and the 2nd dielectric film so as to form the air gap layer. CONSTITUTION:The 1st dielectric film 2 is formed to form the air gap layer 7 on the substrate 1 and the lower electrode 3, the piezoelectric thin film 4, the upper electrode 5 and the dielectric film 6 are formed on it sequentially. The thickness of the air gap layer 7 is enough to be several times or over of vibration displacement in the operating frequency of the resonator but it is desired to provide the thickness of several hundreds of Angstrom - several mum because of the ease of manufacture. Through the constitution above, since the bridge shape air gap is formed by the lamination of the 1st dielectric film 2, the piezoelectric film 4 and the 2nd dielectric film 6, the strength of structure is increased, damages such as cracks due to deflection are prevented, the reliability of products is improved and the air gap layer region is formed, then a filter provided with plural resonators and utilizing acoustic coupling is formed easily.

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 UHF 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.

Therefore, in application fields such as wireless communication equipment and Nicomedia-related equipment, there is a strong desire to develop small passive components that can be integrated with semiconductor elements in the VHF band and the square band.

Conventionally, in relatively low frequency bands from several MHz to several MHz, piezoelectric substrates such as crystal or lead titanate-based ceramics have been used as resonators and filters, and vibrators that utilize the thickness vibration of these substrates have been put into practical use and widely used. There is. The resonant frequency of the resonator is determined by the geometrical shape such as length, width, or thickness. However, since such piezoelectric substrates are subject to mechanical strong electric current and processing limitations, the thickness of the substrate is limited to a few tens of micrometers using a simple mechanical polishing method, and therefore the fundamental resonant frequency of the substrate is only a few tens of MHz at most. was the limit.

Therefore, if higher frequencies are required, higher-order thickness vibrations are used, but the electromechanical coupling coefficient in this case is inversely proportional to the square of the order, so it becomes extremely small, so the capacitance ratio Furthermore, since the spurious resonance is located close to the desired resonance point, it has been difficult and impractical to realize a broadband resonator Fugolta or a broadband oscillator for a voltage-controlled oscillator.

On the other hand, recently the basic mode for thickness photography is
is an ultra-compact VH that operates with relatively low-order overtones.
Resonators using piezoelectric thin films are being researched with the aim of realizing F2, UHF band resonators.

As such a piezoelectric thin film resonator, for example, Progre
ss in the Development of
Miniature Thin Film SAW Re5
Onator and Filter Technology
-ogy” Proceedings of the
36th Annual Symposium on
Freguenc7 Control 1982 6
Some are disclosed on pages 537 to 548 of the Japanese issue. This method 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 section, and then forming an excitation layer on top of it. ' is a resonator formed by forming poles, a piezoelectric thin film, and an upper electrode for excitation, and has the following characteristics.

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

2 + [Because the air-mechanical 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) Since the vibrating section is composed of a composite vibrating membrane, it is possible to combine a piezoelectric film and a dielectric film having a frequency temperature coefficient of opposite sign. 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, it is possible to easily form an ultra-small resonator and also to incorporate it as a part of an integrated circuit.

However, this resonator has the following serious drawbacks.

1') PED etching solution (pyrocatechol C6H4 (O
H) 2, ethylenediamine NH2(CH2) 2-N
Etching speed of up to 50% (mixture of H2 and water H20)
Due to the small Rn/Hr, the thickness of the commonly used 3-inch diameter silicon substrate is 400 mm, so it takes about 8 hours to etch it, making it extremely low in productivity and difficult to mass-produce.

2) Since holes are formed in the substrate itself, its mechanical strength is weak and handling during the production process becomes difficult.

3) Since the piezoelectric thin film is formed in a vacuum after forming the voids, the temperature distribution on the substrate surface becomes non-uniform.

Therefore, the crystal orientation of the piezoelectric thin film itself is disturbed, and the film quality and piezoelectricity deteriorate, resulting in a decrease in the electromechanical coupling coefficient, an increase in vibration loss, and an increase in the capacitance ratio of the resonator.
Q decreases.

4) When incorporating 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 resonator has five parts on a substrate 21.
A gap layer 23 is formed between the dielectric film 22 such as iOz 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 film 22, and 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 good productivity, improved mechanical strength, and uniform temperature distribution during film formation.
It also has many advantages such as 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. In other words, when a 5i02 film is used as the dielectric film 22 serving as the support portion, microcracks may occur in a part of the bridge-shaped portion of the void opening 27 and the void layer 23 may be damaged.
The vibrating membrane of the SiO2 film and the piezoelectric thin film 24 on the top of the substrate bends and comes into contact with the substrate surface, causing vibration energy to leak into the substrate, making it difficult to obtain good resonator characteristics. If 27 is wide, 5iOz film 22
It was found that the appearance is noticeable when the surface is thin. The reason for this is that the internal stress of the 5i02 film is mainly due to the
It is thought that the cracks are concentrated in the vicinity of the bridge shape, causing microcracks and causing the vibrating membrane to warp.

[Purpose of the invention]

The present invention has been made in order to eliminate the above-mentioned drawbacks, and an object of the present invention is to provide a piezoelectric thin film resonator that prevents damage to the thin film portion constituting the void and provides good resonance characteristics.

[Summary of the invention]

In the piezoelectric thin film resonator of the present invention, the first dielectric film is provided in a bridge shape so that a gap layer is formed on the substrate, and the piezoelectric thin film is sandwiched between the first dielectric film on the bridge region and the lower electrodes are partially opposed to each other. and a bridge-shaped piezoelectric thin film region provided with an upper electrode, and a second dielectric film covering the piezoelectric thin film region and disposing a second dielectric film on the first dielectric region. .

〔Effect of the invention〕

According to this invention, since the void layer is formed by the bridge shape in which the first dielectric film, the piezoelectric thin film, and the second dielectric film are laminated, the structural strength is increased and damage to the void layer is eliminated. In addition to increasing product reliability, it is also possible to improve yield.

Further, since the gap opening can be formed wide, a plurality of resonators can be provided in a region corresponding to the gap layer, and a filter using acoustic coupling can be easily formed.

[Embodiments of the invention]

The present invention will be described in detail below with reference to one drawing.

FIGS. 1(a) and 1(b) show a perspective view and a sectional view of a piezoelectric thin film resonator according to an embodiment of the present invention.

substrate! A dielectric film 2 is formed so that a void layer 7 is formed thereon, and a lower electrode 3, a piezoelectric thin film 4, and an upper electrode 5 are formed on top of the dielectric film 2.
and dielectric film 6 are sequentially formed. Such a piezoelectric thin film resonator is manufactured as follows.

First, a rectangular film of a chemically soluble material for forming voids, such as ZnO, is formed in advance on a substrate 1 by means such as sputtering or photoetching. 5i so that at least a part of the void-forming material film protrudes above it.
A dielectric film 2 such as 02 is formed in a strip shape using a sputtering method, a photoetching method, or the like. At this time, the dielectric film 2 is formed into a partially protruding bridge-like structure.

A lower electrode 3 is formed on this dielectric film 2 using a method such as a vacuum evaporation method. Furthermore, a piezoelectric thin film of ZnO or the like is formed thereon using means such as RR magnetron sputtering, and a resist pattern is formed on the region excluding the butt portion 3a of the lower electrode 3 using photolithography technology. A piezoelectric thin film 4 of a predetermined size is formed using an etching solution as a mask. Roughly, on this piezoelectric thin film 4, an upper electrode 5 having a bonding pad 5a is formed so as to face a part of the lower electrode 3 using a means such as a vacuum evaporation method. Roughly cover the piezoelectric thin film 4.

A dielectric film 6 is formed on the dielectric film 2 except for part of the pad portions 3a and 5a by sputtering or the like.

Finally, using the dielectric film 6 as a protective film, it is etched with an etching solution (HCl in the case of ZnO) that dissolves the material for forming voids.
diluted acid solution) to dissolve the void-forming material film.

At this time, only the gap-forming material film is dissolved from the vicinity of the gap lower a provided at two locations, and finally all the gap-forming substances are dissolved to form a gap layer 7 and a piezoelectric thin film resonator is formed. be completed.

Here, it is sufficient that the thickness of the void layer 7 is at least several times the vibration displacement at the operating frequency of the resonator, but from the viewpoint of ease of manufacture, it is desirable to have a thickness of several hundred amps to several micrometers.

This piezoelectric thin film resonator is formed by applying an electric signal between a lower electrode 3 and an upper electrode 5, thereby forming a dielectric film 2 and a piezoelectric thin film in a region corresponding to a gap layer 7 centering on the electrode facing part. By vibrating the composite film composed of the dielectric film 4 and the dielectric film 6, it operates as a vibrator.

FIG. 2 shows the structure of a filter formed by the same method as in FIG. 1 above. The filter with this structure has a type in which the upper electrode of the resonator is divided into two parts, and an upper electrode 12 is provided adjacent to the upper electrode 11, and the lower electrode 13 is The upper electrodes 11 and 12 are partially opposed to each other with the piezoelectric film 14 in between. That is, two resonators are formed using the lower electrode 13 as a common electrode. At this time, the vibrations excited by one resonator propagate through the piezoelectric film, exciting another adjacent resonator, and operate as a bandpass filter through which only a specific frequency passes.

Therefore, according to such a configuration, a bridge-shaped void layer is formed by laminating the first dielectric film 2, the piezoelectric film 4, and the second dielectric film 6, which increases the structural strength and prevents breakage. Damage such as deflection can be prevented and product reliability can be improved, while yields can be improved and small quantities can be increased. Furthermore, since the void layer region can be formed widely, it becomes easier to provide a plurality of resonators and utilize acoustic coupling to form a filter. Further, the dielectric film 6 serves as a lower electrode.

Since the piezoelectric thin film and the upper electrode are coated, it is possible to prevent the effects of outside air such as humidity and increase reliability, and to locate the maximum point of the stress distribution of the vibration mode at the operating frequency in the center of the piezoelectric thin film layer. filtration, it is possible to increase the 4-air mechanical coupling coefficient. Further, the #s electric films 2 and 6 have the same wave number temperature coefficient with the opposite sign as the piezoelectric thin film 4.
By using @, a resonator with excellent temperature characteristics can be obtained.

Note that the present invention is not limited to the above-mentioned embodiments, and can be implemented with various modifications without changing the gist. In other words, the material of the dielectric film is not limited to SiO2, but may also be PSG (Phospho 5ilicat
eGlass), 5iOz doped with boron and phosphorus BPSG (Boro Phospho 5ilic)
It may be made of glass such as ate 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 not limited to ZnO, but also -AeN, Nb2O5, PbTi0
Materials such as a, TazO5, etc. can be used as piezoelectric thin films.

According to the present invention, materials such as metals, oxides, semiconductors, dielectrics, and polymeric materials can be used as the material film for forming voids, as long as they can be easily dissolved in the etching solution for creating voids.

According to this invention, when the piezoelectric thin film resonator is incorporated into an integrated circuit, a semiconductor such as SL 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.

According to this invention, a plurality of upper electrodes are arranged to face each other and perpendicular to the lower electrode, and are separated to such an extent that the elastic coupling between the electrode opposing parts can be ignored, or It is also possible to construct a multi-element resonator using grooves or a sound absorbing material between the electrodes so that each of the opposing upper and lower electrodes can be used as an independent resonator.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing one embodiment of the invention, FIG. 2 is a front view showing another embodiment of the invention, and FIGS. 3 and 4 are diagrams showing a conventional piezoelectric thin film resonator. . DESCRIPTION OF SYMBOLS 1... Substrate, 2... Dielectric film, 3... Lower electrode,
4... Piezoelectric thin film, 5... Upper electrode, 3&, 5a...
・Ponding pad part, 6... dielectric film, 7...
Gap layer, 7a... Gap opening, 11, 12... Lower electrode, 13... Lower electrode, 14... Piezoelectric thin film, 21.
...Substrate, 22...Dielectric film, 23...Void layer, 2
4...Piezoelectric thin film. 25... Lower electrode, 26... Upper electrode, 27...
void mouth. Engraving of the drawings (no change in content) (α) Cb) Figure 2 Figure 1 Figure 3 Figure 4 Procedural amendment (method) % formula % 1. Indication of the case Patent application No. 1988-58194 2. Invention Name of Piezoelectric Thin Film Resonator 3, Relationship with the Amendment Case Patent applicant (307') Toshiba Corporation 4, Agent 1-1 Shibaura-chome, Minato-ku, Tokyo 105 June 25, 1985 (Delivery date) 6. Engraving of the specification and drawings to be amended 7. Contents of the amendment Submit the engraving of the specification and drawings originally attached to the application as attached. (No change in content) 7d-l2＼

Claims (2)

[Claims] (1) A substrate, a first dielectric film provided in a bridge shape so that a gap layer is formed between the substrate, and a piezoelectric film on the first dielectric film including the bridge-shaped region. A piezoelectric thin film region having a lower electrode and an upper electrode partially facing each other with the thin film in between is provided, and a second dielectric film region covering the piezoelectric thin film and disposed on the first dielectric film region is provided.
A piezoelectric thin film resonator characterized by forming a dielectric film of. (2) The piezoelectric thin film resonator according to claim 1, wherein there is at least one pair of electrodes in a region corresponding to the void layer.