JPS5829211A - Thin film piezoelectric oscillator - Google Patents

Abstract

PURPOSE:To obtain characteristics with a wide specific band by providing thin film members on both surfaces of a piezoelectric thin film with electrodes between, and thus forming a piezoelectric oscillator. CONSTITUTION:On a substrate 31 having an opening made by etching, a thin film 33 of SiO2 is provided and on this thin film 33, an electrode 36 is formed. On the electrode 36, a piezoelectric thin film 35 is formed and on the thin film 35, an electrode 37 and a thin film 34 of SiO2 are formed. In this case, when the total thickness of the thin films 33 and 34 is a half as great as the (t) thickness of the piezoelectric thin film 35, the electromechanical coupling coefficient of multiple-frequency oscillation of degree of an even number decreases abruptly in accordance with an increase in the thickness of the thin film 34. When the thin films 33 and 34 have the same characteristics, the electromechanical coupling coefficient of basis oscillation is maximized with the thin films 33 and 34 equal in thickness.

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 thin film member and a piezoelectric thin film. be. Thin film piezoelectric vibrators basically have a structure with electrodes on both sides of a thin piezoelectric plate, and by changing the shape of the electrodes, they can be used for filters, oscillators, etc.
It is an element with a wide range for L.

Generally, piezoelectric vibrators used at high frequencies, such as several tens of MHz or more, use thickness vibration of a piezoelectric thin plate whose plate surface is sufficiently wide compared to its thickness as a vibration mode.

Since the resonant frequency of thickness vibration is inversely proportional to the thickness of the piezoelectric thin plate, the thickness must be reduced in order to use it at a high frequency. However, when the thickness is less than about 40 microns, parallel plane polishing of piezoelectric materials such as bulk crystals and ceramics becomes extremely difficult. It is difficult to produce a cover. On the other hand, if we use odd-order harmonics of thickness vibrations of relatively thick materials such as bulk crystals or ceramics, the resonance of the shade will be 3 times or 5 times the fundamental wave at the same thickness. The frequency is obtained,
This is used as an overtone oscillator in oscillators, etc.
ing. However, when using the 0th harmonic, the electromechanical coupling coefficient kn is equal to the electromechanical coupling coefficient of the fundamental wave by /n2 of □.
If the frequency is doubled and resonance occurs at this time, the electromechanical coupling coefficient decreases, resulting in the filter's fractional bandwidth and the oscillator's control range becoming too narrow, which is often not suitable for practical use.

Another method using harmonics is to create a piezoelectric thin film on a substrate and cause the substrate to vibrate at a higher order in a resonant mode where the thickness of the piezoelectric thin film is half a wavelength. The electromechanical coupling coefficient has a maximum value when the wavelength is equal to an integer multiple of the half wavelength of the sound wave propagating in the substrate, but this value also decreases as the resonant mode becomes higher order. In this method, even if the substrate is thin, it is extremely difficult to make the substrate thin, and the electromechanical coupling coefficient is small because the electromechanical coupling coefficient is small. Even if it were, the fractional bandwidth of the filter and the control range of the oscillator would be narrow, making it unsuitable for practical use.

As a method of obtaining an insulator that improves the drawbacks of the above methods, electrodes (public,
(Aluminum, etc.) Piezoelectric thin film (CdS.

Or CdTe) electrodes (gold, aluminum, etc.) are formed by etching the substrate corresponding to the vibrating part #IJ, and the vibrating part becomes a piezoelectric W1. 2. Description of the Related Art A thin film piezoelectric vibrator is known which is made of a film and whose outer edge is supported by a substrate.

(Japanese Patent Publication No. 45-25579) Since the vibrating part of this vibrator is thin, it is possible to use fundamental vibration or low-order harmonic vibration even at high frequencies of 50 MHz or higher, and this made it possible to use IIM filters with a wide fractional band. be able to. However, this thin film piezoelectric exciter uses Cd as a piezoelectric thin film.
Since the S ring uses CdTe, it has the disadvantages of a small acoustic quality factor (Q), a large thermal protection coefficient, and a thin vibrating part, so it has low mechanical strength. In addition, ZnO, which can be made into a thin film as a piezoelectric material,
However, the above-mentioned drawbacks cannot be improved much. In the above-mentioned Japanese Patent Publication No. 45-25579, 5i on a silicon substrate
A Qz thin M is formed, and an electrode, a piezoelectric thin film, and an electrode are formed on it in this order, and the part of the substrate corresponding to the vibrating part is removed by etching. A thin-film piezoelectric vibrator is also disclosed, which is made of a thin film and has an outer edge supported by a substrate.The effect of forming the 08iOz thin film is to stop etching at the 8i02 thin film part, as well as to improve the mechanical strength of the piezoelectric vibrator. This has the effect of increasing the optical strength and reducing the temperature coefficient of the resonator characteristics. This vibrator will be explained in more detail using FIGS. 1 and 2.

The structure of this thin film thickness single oscillator is shown in FIG. In FIG. 1, 11 is a substrate, and 12 is a hole formed in the substrate by etching. 13 is thin g, 14 is piezoelectric thin film, 15
．． Reference numeral 16 denotes a pair of electrodes provided on both sides of the piezoelectric thin film.

The electromechanical coupling coefficient of the thin film piezoelectric vibrator with the structure shown in Figure 1 is 2.
varies depending on the ratio of the thickness t' of the thin film 13 and the thickness t of the piezoelectric thin film 14.

Then, k is the thickness t of the thin 111i%13 and the piezoelectric thin [
According to the comparison of the thicknesses of 14t', t' changes as shown in FIG. In FIG. 2, the horizontal axis is the thickness ratio, the vertical axis is the electromechanical coupling coefficient, and the numbers written on the curve indicate the order of vibration. As expected from FIG. 2, when fundamental vibration is used in the thin film rolling vibrator having the structure shown in FIG.

A large mechanical viscosity coefficient can be obtained when the thickness is sufficiently thin, for example, /l<0.2, and therefore it can be used in a filter with a wide fractional bandwidth or an oscillator with a wide control range. However, the 3rd or 4th order Shocho Hankyoru also has a relatively large ``furrow mechanical coupling constant''.
As can be expected from the figure, there is a drawback in that an unnecessary response occurs in the pulse IjL due to the 3rd or 4th order ^-wave co-moat. On the other hand, as can be seen from FIG. 2, the thickness t of the thin film 13 is advantageous in increasing the mechanical strength of the mover portion.
The electromechanical coupling coefficient of fundamental vibration decreases as
Furthermore, the electromechanical coupling coefficient of the second harmonic excitation increases. Therefore, when the thickness of the thin film 13 is thick, it has the disadvantage that the filter's specific band width becomes narrower or the control range of the oscillator becomes narrower.
This has the disadvantage that an unnecessary response occurs due to the next harmonic resonance.

The purpose of the present invention is to eliminate the drawbacks of the conventional thin film piezoelectric vibrators as described above, and to realize a thin film piezoelectric vibrator that has a large electromechanical coupling coefficient for basic #i motion and an extremely small electromechanical coupling coefficient for higher order vibrations. Therefore, by using the thin-film pressure transducer of the present invention, a filter with a wide fractional bandwidth or an oscillator with a wide control range can be realized without an unstable response on the recording frequency side. The most distinctive feature of the thin film pressure vibrator of the present invention is that the vibrating portion has a multilayer structure consisting of a thin FIJ, member, electrode, piezoelectric thin film, electrode, and thin film member.

The present invention will be explained in detail below. Figure 8g3 is a diagram showing the basic structure of the thin film piezoelectric vibrator of the present invention. In FIG. 3, 31 is a substrate made of silicon, crystal, etc.
32 is a hole formed in the substrate by etching.

33 and 34 are thin film members.

35 is a pressure '4## film, and 36.37 is an electrode provided at the interface between the piezoelectric thin film and the upper and lower thin film members. Any material may be used for the substrate 31 as long as it can form holes by etching. One of the desirable materials is silicon whose surface is a (100) plane. Such silicone can be prepared by using an etching solution such as KOH% ethylenediamine (10
By eliminating the anisotropy of etching, in which the etching speed of the (111) surface is extremely low compared to the etching speed of the (111) surface, the spread of etching in the surface direction is extremely small, and therefore holes can be formed with high precision. It has the advantage of being able to control the dimensions of The piezoelectric thin film 35 is ZnU.

Piezoelectric materials with hexagonal crystal systems such as CdS and Roux, and P
Various piezoelectric materials can be used, such as bTiOs, PZT, BaTiOs, etc. Among them, ZnO can be used to produce thin films with good reproducibility in which the C-axis is oriented perpendicular to the substrate surface and has high resistivity by sputtering, CV diode, ion blating, etc. It can be said to be an optimal material because it is capable of trapping energy. The object of the present invention can be achieved no matter what material is used as the thin film member, such as an insulator, a semiconductor such as silicon, or a metal thin film. Among them, it is known that when 5i02 thin films are used as the thin film members in FIGS. 33 and 34, the elastic constant of the piezoelectric thin film generally has a negative temperature coefficient, whereas the elastic constant of 8iQz has a positive temperature coefficient. Therefore, a thin film piezoelectric vibrator with low temperature dependence can be realized. In addition, silicon has high mechanical strength and a large acoustic quality factor (Q), so silicon thin films can be used even at high frequencies such as 300 MHz and above, which require a thin vibration area. If it is used as a thin film member, the mechanical strength of the vibrating part can be improved. Furthermore, even when a filter with particularly low loss or a resonator with high resonance sharpness is required, the requirements can be realized by using a silicon thin film as the thin film member due to its large acoustic quality factor (Q). In this case, a silicon thin film is used as the thin film member shown in FIG. 33, and a SiOg thin film is used as the thin film member 34, which is desirable from the viewpoint of temperature characteristics. However, the material of this thin film member is SiO2 or Sk.
However, other insulating materials, semiconductor materials, and even metal materials can be used.

Next, the relationship between the electromechanical coupling coefficient and the thickness of the thin film in the thin film piezoelectric vibrator of the present invention having thin films formed above and below the piezoelectric thin film will be explained using Figures 4 to 6. In the structure shown in FIG. 3, the piezoelectric thin film 35 is a ZnO thin film with a thickness t, and the thin film members 33 and 34 are both SiO! In the case of a thin film and the total thickness of these two thin films is equal to 6, the thickness t33 of the thin film member 33 and the thin film member 34
The change in the electromechanical coupling coefficient when changing the ratio t3)'t33 to the thickness t34 is obtained from a theoretical formula. The numbers attached to the curves in FIG. 4 indicate the order of vibration. 1λ33=0 corresponds to the case of the structure having a thin film member on only one side in the conventional example described above. As is clear from FIG. 4, as the thickness of the thin film member 34 increases, the electromechanical coupling coefficient of fundamental vibrations increases rapidly, and conversely, the mechanical coupling coefficient of even-order higher-order vibrations rapidly decreases. I will do it.

That is, compared to the conventional structure, by forming thin films above and below the piezoelectric thin film and gradually changing the thickness of these thin films so that they are equal, the characteristics of the vibrator are greatly improved.

In particular, when the thickness of the thin film members on both sides increases, the electromechanical coupling coefficient of fundamental vibrations becomes maximum, and even-order higher-order vibrations are completely suppressed, resulting in optimal vibrator characteristics.

The same variation as shown in FIG. 4 is also shown when the thin film member 33 and the thin film member 34 are made of different materials, but in this case, the thickness of each thin film member 34 has an equal ratio to the sound velocity in each thin film. At such a thickness ratio, the electromechanical coupling coefficient of fundamental vibration becomes maximum, and even higher-order vibrations are completely suppressed. For example, when the thin film member 33 has a sonic velocity of 843
0/, and the thin film member 34 has a sound velocity of 596
For a Si□z thin film of 0/2, the thickness ratio 1n is 0.71
The electromechanical coupling coefficient of the fundamental vibration is maximized, and even higher-order vibrations are completely suppressed.

Next, a case will be described in which the thicknesses of the thin films are the same, or when the thin film materials are different, the ratio of the thicknesses is such that each thickness has an equal ratio to the speed of sound.

FIG. 5 shows whether the thin film members 33 and 34 in FIG. This is a diagram showing the stress distribution of vertical vibration in a vibrating part formed to have a ratio equal to the ratio of sound speed in The stress distributions for higher-order vibrations of the second, third, and fourth orders are A, C, C, and II. In FIG. 5, 41 is a piezoelectric thin film, 42.43 is a thin film member, and 44.45 is an electrode. Thin film member 42°4
3, the stress is zero outside each point, and the direction of the stress is opposite on the left and right of the point where the stress is zero. @5 As can be seen from Figures B and D, in such a multilayer structure, the integral of stress in the piezoelectric thin film, that is, between the electrodes, becomes zero in even-numbered high-order vibrations such as the second and fourth orders. , therefore even-order higher-order vibrations are not excited. Furthermore, as can be seen from Figure 5A, in such a multilayer structure, the piezoelectric thin film is located near the maximum stress during fundamental vibration, so it is difficult to place a thin film member on only one side of the piezoelectric thin film as in the past. A larger electromechanical coupling coefficient can be obtained compared to the structure of the present invention, and even compared to the case where the Wi films on both sides have different thicknesses. In FIG. 5, the thin film member 42.4
3 is made of the same material with equal thickness, or is made of different materials and the ratio of the thickness of each thin film member is equal to the ratio of sound speed in each thin film. As shown in Fig. 3, even if the thickness of the thin film member is not limited as described above, as long as the thickness of the thin m member is relatively close, a multilayer structure as shown in Fig. 3 can be used.
A comparison with the conventional structure shown in FIG. 1 provides a thin film pressure source vibrator that suppresses even-numbered high-order vibrations and has a large electromechanical coupling coefficient for fundamental vibrations.

Next, in order to explain in more detail the principle and features of the present invention as described above, Figure 6 shows a thin film pressure oscillator having the structure shown in Figure 3. As an example, the thin films 33 and 34 have the same thickness. The electromechanical coupling coefficient k for a 5iOz thin film and the piezoelectric thin film 35 made of ZnO was determined from the rational equation. @ (In lJf, the horizontal axis is thin #s33 and 34
The vertical axis represents the electromechanical coupling coefficient, and the number written on the curve represents the order of vibration. It is clear from Figure 6 that in the thin film pressure W + vibration vibration of the structure shown in Figure 3, even-numbered higher-order vibrations such as the second and fourth orders must not be excited at all71). For example, when the thin films above and below the pressure are made thicker such that /, > 0.2, the electromechanical coupling coefficient of basic transmission decreases rapidly as shown in Figure 2 in the conventional structure. On the other hand, in the structure shown in Fig. 3, increasing the thickness of the thin films above and below the 0 piezoelectric thin film, which has the feature of non-electricity and gradual decrease, to >0.2, especially the thickness of the vibrator. It is advantageous in increasing the mechanical strength of the resonator at frequencies above 300 M)iz, where a thin film is required, and in particular, the use of a silicon film further reduces the acoustic quality factor Q of the resonator. The thin film pressure 'flL of the structure of FIG. 3 according to the present invention is advantageous in terms of increasing
As a result of the pendulum having the above-mentioned features, the electromechanical coupling coefficient of the fundamental vibration is large even when the thickness of the thin films above and below the pressure film is increased, and the thin film pressure has a small inanimate response due to higher-order vibrations. 111t resonator can be used.Also, even if the thin films above and below the piezoelectric thin film are thin, the conventional structure is #! While the fourth-order higher-order vibration is strongly excited, the third-order vibration according to the present invention
In the structure shown in the figure, the fourth-order higher-order vibration is completely suppressed. In particular, in Figure 6, around /l~0.33B, the third
The electromechanical coupling coefficients of the second and fifth primary ui excitations are both small, and the electromechanical coupling coefficient of the fundamental vibration is maximum. Therefore, even if the thin films above and below the piezoelectric thin film are thin,
A thin-film piezoelectric vibrator with a large non-vibrating electromechanical coupling coefficient and a small unnecessary response due to high-order vibration can be realized.

As described above, according to the present invention, it is possible to realize a thin film piezoelectric vibrator with a large electromechanical coupling coefficient for fundamental vibrations and a small unnecessary response due to higher-order vibrations. Therefore, by using the thin film piezoelectric vibrator of the present invention, It is possible to provide a filter with a small unnecessary response, a wide fractional bandwidth, and an oscillator with a wide control range.

The present invention will be specifically described below with reference to Examples.

[Example 1]

A 0.75μmu) Si02 thin film was formed on a silicon substrate with a (100) surface using a sputtering method. Au was deposited on the 08iQ2 thin film using Cr as a base, and then partial electrodes were formed by photolithography. death,
Next, a 5 μm OJ ZnO thin layer was formed using a sputtering method.
18 were formed. Further, on the ZnO thin film, a SiQz thin film (0.75/jmO was formed using a sputtering method after forming a pole) was formed by lift-off.

Next, using the 8isNn thin film formed on the back side of the silicon substrate as a mask, the silicon substrate corresponding to the vibration site was exposed to ethylenediamine 5. Etching was performed using an etching solution consisting of rotechol and water to form holes. A thin film vibrator having the structure shown in FIG. 3 was manufactured by the above steps. The impedance characteristics of the stiff thin film piezoelectric vibrator are as shown in Figure 7, and the electromechanical coupling coefficient of the fundamental vibration determined from the impedance characteristics matches the value shown in Figure 6 and is 0.08. 5) As can be seen, the even-order higher-order vibrations were not excited, and the 3rd and 5th-order higher-order vibrations were only excited to a very small extent. [Example 2] Surface 2. A silicon substrate film in which is (ioo) plane.
It was epitaxially grown to a thickness of 12 μm.

9' on this silicon thin film in the same process as in Example 1.
am, thickness 5. am's ZnO thin film, A1% and other fJIR
1.5 μm using sputtering method.
OJ 8i0z tll[e formed. Silicon, 1
The i-speed of 9iQz is 8430"/, 5960" respectively.
The ratio of the thickness of 5i02N to the thickness of the silicon thin film is equal to the ratio of the speed of sound, 0.71. Next, using the S i JIN4 thin film formed on the back surface of the silicon substrate as a mask, the silicon substrate corresponding to the vibration site was removed using an etching solution consisting of ethylenediamine, pyrocatechol, and water to form holes. A thin film piezoelectric vibrator having the structure shown in FIG. 3 was fabricated through the steps described above. This *m pressure'@fi
The custom impedance of the i-exciter is as shown in Figure 8, and the electromechanical coupling coefficient of the fundamental vibration determined from the impedance characteristics is 2, which corresponds directly to 1 in Figure 6, and ' o, o 77. As can be seen from Figure 8, the even-numbered higher-order vibrations are not excited at all, and the 5th-order higher-order vibrations are slightly excited.1. Although g3rd-order higher-order vibrations are excited, they are extremely small.

As described above, according to the present invention, it is possible to realize a piezoelectric IJJR actuator that has extremely high IL characteristics in practical terms, such as a large mechanical coupling protection ridge for fundamental vibrations and a small unnecessary response due to higher-order vibrations. Therefore, by using Akira Hongo's thin-film-thickness VIL resonator, unnecessary responses due to higher-order vibrations between one frequency are reduced, and a filter with a wide fractional bandwidth and an oscillator with a wide control range can be provided.

[Brief explanation of the drawing]

Figure tH1 is a cross-sectional view showing the structure of a conventional thin film wL resonator, in which 11 is a substrate made of silicon, crystal, etc.;
13 is a thin film member, 14 is a pressure zone thin film, and 15° and 16 are opposing electrodes. FIG. 2 shows the theoretical surface M for the instantaneous mechanical coupling coefficient of the thin film piezoelectric vibrator having the structure shown in FIG. The numbers written on the curves in the figure indicate the order of vibration. FIG. 3 is a cross-sectional view showing the structure of the thin film piezoelectric actuator of the present invention, in which 31 is a substrate made of silicon, quartz, etc., 32 is a hole formed in the substrate by etching, 33 and 34
is a thin film member, 35 is a piezoelectric thin film, and 36 and 37 are opposing electrodes. ! Figure 4 shows the ratio t between the thickness 4 of the thin film member 34 and the thickness tss of the thin film 1 member 33 for the thin film piezoelectric vibrator having the structure shown in Figure 3.
37 is a diagram showing a tss change of 2 in the electromechanical coupling coefficient for 37. The numbers written on the curves in the figure indicate the order of vibration. FIG. 5 is a diagram showing the stress distribution at the vibration site of the thin film piezoelectric vibrator according to the invention. A, B, and C in the diagram. DII, A, mouth, Ha, and 2 are the basic vibrations, @
This is the stress distribution of second-order, third-order, and fourth-order vibrations. In the figure, 41 is a piezoelectric thin film, 42.43 is a thin m member, and 4
4.45 is an electrode. FIG. 6 is a diagram showing a single-ring curve of 2 for the electromechanical coupling coefficient of the thin film piezoelectric vibrator of the present invention. The numbers written on the curves in Figure 1 indicate the order of vibration. FIG. 7 and FIG. 8 are diagrams showing the impedance characteristics of the thin film piezoelectric vibrators of Example 11 and Example 2, respectively. The horizontal axis is the frequency expressed in MH1 units, and the vertical axis is the relative value of impedance expressed in decibels. '42 times 71 3rd Figure 32 4th Figure/133 5th A Figure 6 7t Figure 7 False Frequency (A/fHz) Figure 8 01θ00 Burning Altered Shape 0 25θθ,~ eyes!
, & (niih otsu)

Claims (1)

[Claims] 1. A thin-film WIIL vibrator characterized by having a multilayer structure in which a thin film member and an electrode, a piezoelectric thin film, an electrode, and a thin film member are formed in this order on the thin film member. . 2. The thin film piezoelectric vibrator according to claim 1, wherein the thin film members formed above and below the piezoelectric thin film are made of the same material and have the same thickness. 3. Pressure - A patent characterized in that the thin film members formed above and below the thin film are made of different materials, and the ratio of the thicknesses of the upper and lower thin films is equal to the ratio of sound speeds in the respective thin films. The thin film piezoelectric vibrator according to item 1, wherein the range of d is desired.