Piezoelectric vibrator and manufacturing method thereof

Abstract: In the piezoelectric vibrator, a plate includes a thin quartz plate (1) portion for generating vibrations, and non-contact electrodes (3,3') for applying an alternating electric field are arranged above the thin quartz plate portion at both sides thereof so as not to contact with the thin quartz plate portion. The electrodes may be formed on a supporter (2,2') which can be bonded directly with the thin quartz plate. The electrodes may also be formed above the thin quartz plate and supported by the plate outside the thin quartz plate portion. The piezoelectric vibrator has a superior Q of resonance at high frequencies.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a piezoelectric vibrator comprising a piezoelectric plate made of quartz, said piezoelectric plate comprising a vibration portion, and a pair of supporters bonded to said piezoelectric plate at the opposite sides thereof, each for supporting an electrode; whereby said supporters, said piezoelectric plate and said electrodes being arranged in such a way that a gap is provided between each electrode and the vibration portion of said piezoelectric plate, said gap being sufficiently big that the vibration portion of said piezoelectric plate does not contact said electrodes if an alternating electric voltage is applied to the electrodes. The present invention also relates to a method for manufacturing such a piezoelectric vibrator. Description of the Prior Art Recently, communication devices are made more compact and operate at higher frequencies according to the development of mobile communication technology. These devices need vibrators and high frequency filters, and piezoelectric vibrators can be used for such vibrators and high frequency filters. A prior art piezoelectric vibrator such as a quartz vibrator is prepared by forming a quartz plate of a prescribed shape and by forming electrodes on the top and bottom planes thereof. Vibrations are generated in the piezoelectric vibrator by applying an alternating electric voltage between the electrodes. A quartz vibrator having good characteristics at high frequencies is needed for mobile communication devices. Among high frequency characteristics, vibration frequency and Q of resonance (which is in correspondence to an inverse of loss) are important. The vibration frequency of quartz vibrator is inversely proportional to the thickness of quartz plate if thickness-shear vibration of quartz is used. Usually, a quartz vibrator which can be produced easily usually has vibration frequency of 50 MHz or less in the fundamental mode and has 30 µm or more of thickness of quartz plate. In order to increase the vibration frequency, it is needed to make the thickness of quartz plate thinner. For example, the vibration frequency of 167 MHz can be available for a thickness of 10 µm, and the vibration frequency of 330 MHz can be available for a thickness of 5 µm. For example, E. A. Gerber et al. produces quartz vibrators for frequencies of hundreds MHz to more than 1 GHz by using polishing, etching, ion trimming or the like ("Advances in Microwave Acoustic Frequency Sources", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 10, October 1986, 1004). However, the Q of resonance decreases with increasing frequency. This is ascribed partly to the increase in electrode loss due to the skin effect at the electrodes. This is also ascribed to the weight of the electrode itself which contributes the loss more with decreasing thickness. Then, there is an upper limit on the thickness of the electrode though the increase in thickness decreases the resistance of the electrode. Therefore, if quartz is formed to be thin with polishing or the like, the vibration frequency increases whereas the Q of resonance decreases, or a piezoelectric vibrator of good high frequency characteristics cannot be produced. On the other hand, a quartz vibrator with a non-contact electrode is also known. For example, a quartz vibrator disclosed in Japanese Patent laid open Publication 2-121512/1990 intends to decrease temperature hysteresis. A quartz plate having a concave section is interposed and fixed between top and bottom plates on which an electrode is formed. Gaps are provided between the quartz plate and the electrodes. A specified metal (SUS 630) is used for metallic plates for holding the quartz plate, and the quartz plate is fixed with the metallic plates or holders with screws at a fixing angle of the quartz plate determined so that the thermal expansion coefficient of the metallic plates agrees with that of the quartz plate at the fixing angle. The thermal hysteresis decreases because the thermal expansion coefficient of the metallic plates is chosen to agree with that of the quartz plate. In the structure of the quartz vibrator, the effect of the mass of the electrodes on the vibration can be vanished so that Q at high frequencies can be improved. However, because screws are used, it is difficult to provide a constant gap of micrometer order reproducibly. If the gap cannot be adjusted at a high precision for a quartz plate of thickness of 10 µm or less, the stability of Q on production and on the characteristics under operation are not sufficient. Further, because screws are used, it is difficult to seal the vibration element in a compact size. The thermal expansion coefficient cannot be agreed only at a certain direction, and it is difficult to realize stability in a wide temperature range. The characteristics are liable to change due to drop or mechanical vibrations. It is also known to use an adhesive instead of screws to provide gaps and to fix the holders. However, an adhesive is an organic material and has low heat-resisting property, and the characteristics may change for soldering at 220 °C or more and the temperature characteristics on use or on transport are not stable. Further, if the sealing is performed, gas may be generated from the adhesive and this may change the characteristics. A similar quartz vibrator is also disclosed in Japanese Patent laid open Publication 5-22071/1993, wherein a quartz plate and a pair of plates each having a non-contact electrode are fixed mechanically with screws. Further, an elastic surface wave element disclosed in Japanese Patent laid open Publication 3-6912/1991 has a non-contact type electrode with a gap between the piezoelectric plate and the electrode. This element also uses the non-contact type electrodes like the present invention. However, the structure of the electrode is different from that of the present invention because an elastic surface wave is generated. Further, a bridge or supporter of the electrode is formed by using deposition on the piezoelectric plate, while the supporter of the electrode is bonded directly with the piezoelectric plate in the present invention. The Japanese patent application JP-A-61-41215 discloses a piezoelectric vibrator according to the precharacterising clause of claim 1. This known resonator contains a pair of electrode plates for supporting a chrystal plate. The bonding between the electrode plates and the chrystal plate is realized by ordinary bonding agents. For manufacturing a piezoelectric vibrator having a good Q at high frequencies it is necessary to provide an extremely small gap of 10 µm or less between the piezoelectric plate and the electrodes. Such an accurate positionning of the chrystal plate between the electrode plates, however, is not possible if a bonding agent is provided between chrystal plate and electrode plate. Goetz, G. G., Fathimulla, A. M. in: "Generalized Bonding", 1989 IEEE SOS/SOI Technology Conference, October 3-5, 1989, Nevada, US; p. 125-126 report on research results concerning a bonding and etchback approach to SOI (BESOI). The authors of this article report that the concept of BESOI has been successfully applied by bonding Ge to Si, and Si to fused quartz. SUMMARY OF THE INVENTION An object of the present invention is to provide a quartz vibrator having a good Q at high frequencies. A second object of the present invention is to provide a quartz vibrator which can be sealed in a compact size. A third object of the present invention is to provide a quartz vibrator having good temperature characteristics. A fourth object of the present invention is to provide a quartz vibrator stable against mechanical vibrations. A fifth object of the present invention is to provide a quartz vibrator which can be produced in a large amount. According to the present invention, a piezoelectric vibrator is provided which comprises a piezoelectric plate made of quartz, said piezoelectric plate comprising a vibration portion, and a pair of supporters bonded to said piezoelectric plate at the opposite sides thereof, each for supporting an electrode; whereby said supporters, said piezoelectric plate and said electrodes being arranged in such a way that a gap is provided between each electrode and the vibration portion of said piezoelectric plate, said gap being sufficiently big that the vibration portion of said piezoelectric plate does not contact said electrodes if an alternating electric voltage is applied to the electrodes. Said pair of supporters being directly bonded to said piezoelectric plate without any adhesives; said bonding is due to adsorption of hydroxide groups adhered to the opposing surfaces and/or due to constituent atoms of said plate and said supporter. A method for manufacturing a piezoelectric vibrator according to the present invention comprises the steps of providing a piezoelectric plate made of quartz with a vibrating portion; providing supporters each supporting an electrode; bonding said supporters to said piezoelectric plate at the opposite sides thereof such that a gap is provided between each electrode and the vibration portion of said piezoelectric plate, said gap being sufficiently big that the vibration portion of said piezoelectric plate does not contact said electrodes if an alternating electric voltage is applied to the electrodes, whereby said bonding step comprises the steps of making the bonding surfaces of said pair of supporters and said piezoelectric plate hydrophile; and bringing the bonding surfaces of said pair of supporters and said piezoelectric plate into contact. An advantage of the present invention is that a piezoelectric vibrator has good Q of resonance at high frequencies. Another advantage of the present invention is that a piezoelectric vibrator has good temperature characteristics. A third advantage of the present invention is that a compact piezoelectric vibrator can be provided. A fourth advantage of the present invention is that the mechanical stability of a piezoelectric vibrator can be improved. A fifth advantage of the present invention is that the productivity of a piezoelectric vibrator can be improved. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, and in which: Fig. 1(a) is a sectional view of a first example of a piezoelectric vibrator of the present invention, and Fig. 1(b) is a plan view of the piezoelectric vibrator; Fig. 2(a) is a schematic sectional view of a second example of a piezoelectric vibrator of the present invention, and Fig. 2(b) is a plan view of the piezoelectric vibrator; Fig. 3(a) is a schematic sectional view of a third example of a piezoelectric vibrator of the present invention, and Fig. 3(b) is a plan view of the piezoelectric vibrator; Fig. 4(a) is a schematic sectional view of a fourth example of a piezoelectric vibrator of the present invention, and Fig. 4(b) is a plan view of the piezoelectric vibrator; Fig. 5(a) is a schematic sectional view of a fifth example of a piezoelectric vibrator of the present invention, and Fig. 5(b) is a plan view of the piezoelectric vibrator; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the views, examples of structures and manufacturing methods of piezoelectric vibrators of the present invention will be explained below. Embodiment 1 Figs. 1(a) and (b) show a first example of a piezoelectric vibrator. Fig. 1(a) shows a section of the structure of the piezoelectric vibrator and Fig. 1(b) is a top view of the structure shown in Fig. 1(a), wherein reference numeral 1 denotes a piezoelectric plate or a quartz plate for generating vibrations, and reference numerals 2, 2' denote a supporter of a non-contact electrode 3, 3', made of quartz or the like. The quartz plate 1 is bonded directly with the supporters 2, 2' without an adhesive. The supporter 2, 2' has a recess 2" at the center for forming the electrode 3, 3' and another part 6, 6' having the same level as the recess 2" for connecting the electrode 3, 3' to the external. Thus, the part 6, 6' does not hinder the connection of the connection portion 3" to external electrodes 5, 5' which are provided for the connection to an external circuits at the same level as the recess 2". The electrodes 3, 3' are opposed to each other via the quartz plate 1 and they are connected via connecting parts 7, 7' to the external electrodes. The connection parts 7, 7' of the electrodes, the external electrodes 5, 5' and the connecting part 6, 6' of the supporter 2, 2' are provided at two different directions from the center from each other, as shown in Fig. 1(b). Reference numerals 4, 4' denote gaps between the quartz plate 1 and the electrode 3, 3'. The supporters 2 and 2' and the plate 1 have cut-out portions 5" in order to expose the surface for forming the external electrodes 5, 5'. The electrodes 3, 3' do not contact with the vibrating portion at the center of the quartz vibration plate 1 directly due to the air gaps 4, 4' when the vibrating portion vibrates. Further, because the electrodes 3, 3' are provided near the vibration portion of the quartz plate 1, a strong electric field is applied to the quartz vibration portion when an electric field is applied between the top and bottom electrodes 3, 3'. By using this structure, a quartz vibrator of very good Q of resonance at high frequencies can be produced. For example, if the quartz plate 1 has 4 µm of thickness, the electrode 3, 3' has 2 µm of thickness and the gap 4, 4' has 1 µm of thickness, about 420 MHz of resonance frequency and Q of about 10,000 are realized. On the contrary, if the gaps 4, 4' are not provided, Q decreases to about 5,000. Thus, the loss at high frequencies can be improved to a large extent. An example of a manufacturing method of the present invention will be explained below. First, a quartz plate of 40 µm of thickness is provided, and a portion to be remained to be thick for handling is masked with a metallic film made of for example chromium-gold on both top and bottom planes. By using the mask, a portion of very thin thickness is formed at a part of the quartz plate with an etchant of hydrofluoric acid. By controlling the temperature and the time accurately on etching, the thin portion can be formed to have for example 4 µm of thickness. This thin portion is provided for the quartz plate 1 shown in Fig. 1(a). Next, quartz plates are provided, and by using a similar mask and a similar etching technique, quartz plates 2, 2' for supporting electrodes are produced which are thick at the periphery and thin at the center to form a recess 2". Similarly, a connecting part 6, 6' and a cut-out portion 5" are formed. Then, the masks are removed with an etchant for chromium-gold. The sizes of the quartz plates or supporters 2, 2' are as follows: The thickness at the periphery is 100 µm, while the thickness at the recess is 97 µm or it is concave by 3 µm in the recess 2". The recess 2" is circular and its diameter is for example 0.5 mm, and the total diameter is 1 mm by including the thick portion. Next, electrodes 3, 3', 7, 7', 5, 5' for activating vibrations and for connecting to the external are formed on the supporter 2, 2' with an ordinary vacuum deposition, photolithography, etching and lift-off techniques. The diameter of the electrodes 3, 3' for activation is for example 300 µm. The shape of the recess 2", 6, 6' is formed so as to be able to form an electrode 3" for connection with the external. The electrodes 3, 3', 7, 7', 5, 5' are made of for example chromium-gold, and the thickness thereof is 2 µm. A pair of such electrode supporters 2, 2' are formed. Next, the surface of the quartz plate 1 and the quartz plates 2, 2' for supporting the electrodes are made hydrophile. In concrete, the surface is rinsed lightly with hydrofluoric acid and it is immersed in pure water for several minutes. After the hydrophile treatment, the quartz plate 1 is interposed by the two quartz plate 2, 2' so that the electrodes 3, 3' oppose with each other. The planes in contact are adsorbed strongly each other by hydroxide groups adhered to the surface. Then, they are subjected to heat treatment. The water content is extracted gradually from the contact planes and oxygen and silicon atoms of constituent atoms of the quartz are bonded or the adhesion strength increases more. The heat treatment temperature is needed to be 100 °C or higher in order to improve the adhesion strength, and the adhesion strength increases with increasing heat treatment temperature. Actually, the heat treatment temperature is 300 to 500 °C. Finally, the thick portion of the quartz plate remained for handling is removed, and a piezoelectric vibrator of a structure shown in Figs. 1(a) and (b) can be obtained. That is, the thickness of the vibration portion is 4 µm, the thickness of the electrodes is 2 µm and the thickness of the gap is 1 µm. Therefore, a piezoelectric vibrator having a superior Q at high frequencies can be fabricated as shown in Figs. 1(a) and (b). Embodiment 2 Figs. 2(a) and (b) show a second example of a piezoelectric vibrator. Fig. 2(a) shows a section of the structure of the embodiment while Fig. 2(b) is a top view of the structure shown in Fig. 2(a), wherein reference numeral 11 denotes a quartz plate as a piezoelectric plate for generating vibrations, and reference numeral 12, 12' denotes a supporter of a non-contact electrode 13, 13', made of quartz or the like. The electrodes 13, 13' are formed at the center of the supporters 12, 12'. The piezoelectric plate 11 is bonded directly with the supporters 12, 12' without an adhesive. The piezoelectric plate 11 includes a thin portion 11' for generating vibrations at the center and a thick portion 11" around the thin portion 11'. The piezoelectric plate 11 has a concave part 16, 16' further for connecting the electrode 13, 13' via connection portion 17, 17' to external electrodes 15, 15' provided to be connected to an external circuit. The electrodes 13, 13' are opposed to each other with respect to the vibration portion 11' of the quartz plate 11 and they are connected via the connecting parts 17, 17' to the external electrodes. The connection parts 17, 17' of the electrodes, the external electrodes 15, 15' and the connecting part 16, 16' of the supporter 12, 12' are provided at different directions from each other from the center, as shown in Fig. 2(b). The top and bottom supporters 12 and 12' and the piezoelectric plate 11 have cut-out portions 15" in order to expose the surface for forming the external electrodes 15, 15'. Reference numerals 14, 14' denote gaps between the quartz plate 11 and the electrode 13, 13'. The electrodes 13, 13' do not contact with the vibration portion 11' of the quartz plate 11 directly via the gaps 14, 14' when the vibrating portion 11' vibrates. Further, because the electrodes 13, 13' are provided near the vibration portion 11', a strong electric field is applied to the vibration portion 11' when an electric field is applied between the top and bottom electrodes 13, 13'. By using this structure, a quartz vibrator of very good Q of resonance at a high frequency can be produced. As an example, if the quartz plate 11 has 10 µm of total sum of thickness by adding 4 µm for the upper and lower planes, the electrode 13, 13' has 2 µm of thickness and the gap 14, 14' has 1 µm of thickness, about 420 MHz of resonance frequency and Q of about 10,000 are realized. On the contrary, if the gaps are not provided, Q decreases to about 5,000. Thus, the loss at high frequencies can be improved to a large extent. Next, an example of a manufacturing method is explained. First, a quartz plate of 40 µm of thickness is provided, and a portion 11 of 10 µm of thickness is fabricated as in the first method of Embodiment 1. Further, by repeating the same procedures, a thin portion 11' of 4 µm of thickness is formed in the thick portion 11 of 10 µm of thickness. The process explained in the first example of manufacturing method in Embodiment 1 is carried out by using silicon wafer or glass instead of quartz for plates for supporting electrodes. Next, the connection part 16, 16' and the cut-out portion 15" are formed similarly. Then, the electrodes 13, 13', 17, 17' and 15, 15' are formed on the supporter 12, 12'. Next, the surfaces of the quartz plate 11 and of the supporter 12, 12' are made hydrophile, as to the first method. Then, the supporter 12, 12' are put on the quartz plate 11 so as to oppose the electrodes 13, 13' via the thin portion 11'. Then, the heat treatment is performed. Therefore, a structure shown in Figs. 2(a) and (b) can be produced, and good high frequency characteristics can be realized. Embodiment 3 Figs. 3(a) and (b) show a third example of a piezoelectric vibrator wherein reference numerals 1 - 7 denote parts having names and functions similar to the piezoelectric vibrator shown in Fig. 1(a) and (b). Fig. 3(a) shows a section of the structure of the embodiment while Fig. 3(b) is a top view of the structure shown in Fig. 3(a). In the piezoelectric vibrator shown in Fig. 3(a), a sealant 8, 8' of glass of low melting point embeds the openings of the connection portion 6, 6' for connecting electrodes 7, 7' and this seals the vibration portion of the piezoelectric vibrator hermetically. This electrode structure is similar to that of Embodiment 1, and the advantages of the electrodes 13, 13' are quite the same. Further, as explained above, the opening to the external is sealed with the glass sealant 8, 8' while the piezoelectric plate 1 and the supporter 2, 2' are sealed at the contact portions with direct bonding. Then, it can be said that the quartz vibrator is kept sealed by the direct bonding of the quartz plate 1 and the supporters 2, 2' and the sealant 8, 8'. Thus, the vibration portion is sealed hermetically. The melting point of the glass sealant 8, 8' may be selected to be 300 - 500 °C, and the direct bonding is performed at 300 - 500 °C as mentioned above. Therefore, the characteristics of the vibrator is stable below this temperature, or the temperature characteristic is very stable. Because an organic substance is not included inside the sealed structure, there is no gas generation in the inside due to secular change. By adopting this structure, the piezoelectric vibrator is stable against the environment for a long time. Previously, it is needed to case a piezoelectric vibrator to seal hermetically. However, in the above-mention structure, it is not needed further to be cased in a case so that the quartz vibrator can be made compact. Next, an example of a manufacturing method is explained. First, the processes as explained in Embodiment 1 are carried out to form a structure of piezoelectric vibrator shown in Figs. 1(a) and (b). Then, the piezoelectric vibrator is put in an nitrogen or inert gas environment, and the space 6, 6' open to the external is filled with a glass of low melting point. Because the direct bonding and the sealing with a sealant has good air tightness, the vibration portion can be sealed hermetically. Further, the structure as illustrated in Figs. 2(a) and (b) can also be sealed at the openings of portions 16, 16' for connecting the electrodes to the external with a glass of low melting point, and similar advantages can be realized. Embodiment 4 Figs. 4(a) and (b) show a fourth example of a structure of a piezoelectric vibrator wherein reference numerals 11 - 17 denote parts having names and functions similar to the piezoelectric vibrator shown in Figs. 2(a) and (b). Fig. 4(a) shows a section of the structure of this example while Fig. 4(b) is a top view of the structure shown in Fig. 4 (a). The piezoelectric vibrator shown in Figs. 4(a) and (b) differ from that shown in Figs. 2(a) and (b) as to the structure of the piezoelectric plate 11. The piezoelectric plate 11 comprises a thin portion 11' for generating vibrations at the center and a thick portion 11" around the thin portion 11'. The thin portion 11' further comprises a vibration portion 11' and a thin portion 9' thinner than the vibration portion 11' and provided around the vibration portion 11'. Thus, vibration energy can be confined more in the vibration portion 11', while advantages similar to those of Embodiment 2 are available. The Q of resonance increases further and the undesirable, spurious mode can be reduced. The reduction of the spurious mode can be realized if the thickness of the thin portion 9 is thinner than the vibration portion 11'. Embodiment 5 Figs. 5(a) and (b) show a fifth example of a structure of a piezoelectric vibrator of Embodiment 5 wherein reference numerals 1 - 7 denote parts having names and functions similar to the piezoelectric vibrator shown in Figs. 1(a) and (b). Fig. 5(a) shows a section of the structure of Embodiment 5 while Fig. 5(b) is a top view of the structure shown in Fig. 5(a). This structure is the same as that of Embodiment 1 shown in Figs. 1(a) and (b) except the material of the supporters 21, 21'. The supporters 21, 21' for supporting the electrodes 3, 3' have the same shape as the supporters 2, 2' in Figs. 1(a) and (b), but they are made of silicon or glass instead of quartz. Because the silicon comprises silicon atoms as a constituent element and the glass comprises silicon and oxygen atoms as main constituent elements, the direct bonding can be performed as in Embodiment 1. Therefore, a structure similar to that shown in Figs. 1(a) and (b) can be obtained and good high frequency characteristics can be realized. Next, an example of a manufacturing method is explained. The process explained in Embodiment 1 is carried out by using silicon wafer or glass instead of quartz for plates for supporting electrodes. Because the silicon wafer includes silicon as a constituent element and the glass includes silicon and oxygen as main constituent elements, the direct bonding is possible by using the processing for hydrophile treatment with hydrofluoric acid etchant and pure water and heat treatment as in Embodiment 1. Therefore, a structure similar to that in Embodiment 1 can be produced and good high frequency characteristics can be realized. Further, a structure and characteristics similar to those in Embodiment 2 can also be obtained similarly. That is, the process explained in Embodiments 2 is carried out to form a structure of piezoelectric vibrator by using silicon wafer or glass instead of quartz for the plates for supporting electrodes. The above-mentioned examples are explained further below. The improvement of Q at high frequencies is remarkable if the thickness of the quartz plate is 10 µm or less. The thickness at the vibration portion of the quartz plate is not be limited to 4 µm adopted in Embodiments 1 - 5. However, if the thickness of the quartz vibration portion exceeds 10 µm, the advantages of the non-contact electrodes become small. Therefore, it is preferable that the thickness of the quartz vibration portion is 10 µm or less. The initial thickness of the quartz plate is not limited to be 40 µm as adopted in the examples, or it may be thicker or thinner than 40 µm as far as the handling is not hindered. The gap between the thin quartz plate and the non-contact electrode have a thickness of 1 µm. However, by controlling the vacuum deposition or the etching for forming the thin quartz plate, the gap can be made larger or smaller between 0.1 - 5 µm. The gaps for the non-contact electrodes can be formed with good reproducibility with use of wet etching, pin coating, vacuum deposition and the like. The intensity of electric field increases with decreasing the thickness of the gap, while it is needed to increase the gap to overcome the bending of the electrodes when the area of the vibration portion is wide. Practically, the thickness up to 5 µm is sufficient for such a case. The Q of resonance does not decrease so low at high frequencies due to electrode loss because the gaps are formed between the piezoelectric vibration portion and the non-contact electrodes so as not to contact directly with each other, and the high frequency characteristics can be improved to a large extent. Further, the direct bonding is used for connection by using quartz both for the vibration portion and for the supporters such as explained in Embodiments 1 and 2, the thermal expansion coefficients of the vibration portion and the supporter can be chosen to be same, so that a very stable element against temperature can be obtained. Even if glass is used for the supporters as in Embodiment 5, it is relatively easy to adjust the thermal expansion coefficient for a material selected for the piezoelectric vibrator because the range for selecting a glass is wide. Therefore, the stability of the characteristics against temperature is improved by the direct bonding of the piezoelectric plate with materials having the same or a similar thermal expansion coefficient. Further, if silicon is used for the supporters, the thin quartz plate can be formed with etching easily and the productivity of the quartz vibrator is improved. Because direct bonding without an adhesive agent is used, the precision of the gap is high, and the bonding is stable for etching and heat treatment. Thus, the direct bonding is superior to increase productivity. In the above-mentioned Examples 1 - 5, the electrodes are made of chromium-gold layers (for example successive layers of a chromium layer of 0.02 µm thickness and a gold layer of 0.48 µm thickness), and the thickness thereof is for example 2 µm. On the other hand, the electrodes may also be made of an ordinary metal such as titanium, platinum, copper, silver, tungsten or nickel. Aluminum can also be used if aluminum is not used for forming the gap. The thickness of the electrodes can be controlled between 0.1 - 5 µm with vacuum deposition.

Claims: A piezoelectric vibrator comprising: a piezoelectric plate (1; 11) made of quartz, said piezoelectric plate (1; 11) comprising a vibration portion, and a pair of supporters (2, 2'; 12, 12'; 21, 21') bonded to said piezoelectric plate (1; 11) at the opposite sides thereof, each for supporting an electrode (3, 3'; 13, 13'); whereby said supporters (2, 2'; 12, 12', 21, 21'), said piezoelectric plate (1; 11) and said electrodes (3, 3'; 13, 13') being arranged in such a way that a gap (4, 4'; 14, 14') is provided between each electrode (3, 3'; 13, 13') and the vibration portion of said piezoelectric plate (1; 11), said gap (4, 4'; 14, 14') being sufficiently big that the vibration portion of said piezoelectric plate (1; 11) does not contact said electrodes (3, 3'; 13, 13') if an alternating electric voltage is applied to the electrodes (3, 3'; 13, 13'); characterized in that said pair of supporters (2, 2'; 12, 12', 21, 21') being directly bonded to said piezoelectric plate (1; 11) without any adhesives, wherein said bonding is due to adsorption of hydroxide groups adhered to the opposing surfaces and/or due to constituent atoms of said plate and said supporters (2, 2'; 12, 12', 21, 21'). The piezoelectric vibrator according to Claim 1, wherein said supporter (2, 2'; 12, 12', 21, 21') is made of quartz. The piezoelectric vibrator according to Claim 1, wherein said supporter (2, 2'; 12, 12', 21, 21') is made of glass. The piezoelectric vibrator according to Claim 1, wherein said supporter (2, 2'; 12, 12', 21, 21') is made of silicon. The piezoelectric vibrator according to Claim 1, wherein said supporter (2, 2'; 12, 12', 21, 21') has a recess opposite to said piezoelectric plate (1; 11) and said electrode (3, 3'; 5, 5'; 7, 7'; 13, 13') is formed on the recess. The piezoelectric vibrator according to Claim 1, wherein the thickness of said vibration portion of said piezoelectric plate (1; 11) is between 10 and 0.1 µm. The piezoelectric vibrator according to Claim 6, wherein the thickness of said vibration portion of said piezoelectric vibration plate (1; 11) is between 5 and 0.1 µm. The piezoelectric vibrator according to Claim 6, wherein the gap (4, 4'; 14, 14') between said vibration portion and said electrode (3, 3'; 5, 5'; 7, 7'; 13, 13') is between 0.1 and 5 µm. The piezoelectric vibrator according to Claim 1, wherein said piezoelectric plate (1; 11) comprises a thick plate portion around the vibration portion and wherein said supporters (2, 2'; 12, 12', 21, 21') are bonded directly to the thick plate portion of said piezoelectric plate (1; 11). The piezoelectric vibrator according to Claim 1, wherein the vibration portion of said piezoelectric plate (1; 11) is sealed hermetically by the direct bonding of the vibrating portion with said supporters (2, 2'; 12, 12', 21, 21'). The piezoelectric vibrator according to Claim 2, wherein the thermal expansion coefficient of said quartz for said supporter (2, 2'; 12, 12', 21, 21') is selected to be the same as that of said piezoelectric plate (1 ; 11). The piezoelectric vibrator according to Claim 3, wherein the thermal expansion coefficient of said glass of said supporter (2, 2'; 12, 12', 21, 21') is selected to be the same as that of said piezoelectric plate (1; 11). The piezoelectric vibrator according to Claim 1, wherein said vibration portion of said piezoelectric plate (1; 11) comprises a central portion and a peripheral portion around the central portion, the peripheral portion having a thickness smaller than the central portion, whereby vibration energy is confined in the central portion. Method for manufacturing a piezoelectric vibrator, said method comprising the steps of a) providing a piezoelectric plate (1; 11) made of quartz with a vibrating portion; b) providing supporters (2, 2'; 12, 12'; 21, 21') each supporting an electrode (3, 3'; 13, 13'); c) bonding said supporters (2, 2'; 12, 12'; 21, 21') to said piezoelectric plate (1; 11) at the opposite sides thereof, such that a gap (4, 4'; 14, 14') is provided between each electrode (3, 3'; 13, 13') and the vibration portion of said piezoelectric plate (1; 11), said gap (4, 4'; 14, 14') being sufficiently big that the vibration portion of said piezoelectric plate (1; 11) does not contact said electrodes (3, 3'; 13, 13') if an alternating electric voltage is applied to the electrodes (3, 3'; 13, 13'); characterized in that bonding step c) comprises the steps of c1) making the bonding surfaces of said pair of supporters (2, 2'; 12, 12', 21, 21') and said piezoelectric plate (1; 11) hydrophile; and c2) bringing the bonding surfaces of said pair of supporters (2, 2'; 12, 12', 21, 21') and said piezoelectric plate (1; 11) into contact. Method for manufacturing a piezoelectric vibrator according to claim 14, characterized in that the step c1) of making the bonding surfaces hydrophile consists of rinsing the surfaces with hydrofluoric acid. Method for manufacturing a piezoelectric vibrator according to claim 15, cha racterized in the step of immersing the bonding surfaces in water after rinsing them with hydrofluoric acid. Method for manufacturing a piezoelectric vibrator according to one of claims 14 to 16, characterized by the step of d) subjecting the bonding surfaces of said pair of supporters (2, 2'; 12, 12', 21, 21') and said piezoelectric plate (1; 11) a heat treatment for increasing the adhesion strength after the bonding surfaces have been brought into contact. Method for manufacturing a piezoelectric vibrator according to claim 17, characterized in that the heat treatment temperature is at least 100°C. Method for manufacturing a piezoelectric vibrator according to claim 17, characterized in that the heat treatment temperature is 300° C to 500°C.

Inventors: EDA KAZUO, NAGATA TAKASHI, EDA, KAZUO, NAGATA, TAKASHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H03H3
IPC codes: H03H9, H03H3