Abstract:
A method for fabricating a crystal unit, including a preparing step, a bonding step, and a separating step, is provided. The preparing step prepares a quartz plate and a supporting substrate with a recess that is larger than the vibrating region on a surface of the supporting substrate. The recess corresponds to a vibrating region in the crystal unit. The bonding step bonds the quartz plate to the surface of the supporting substrate such that the quartz plate is separated from the supporting substrate in the recess. The separating step separates the quartz plate into the vibrating region and the framing portion by performing dry etching on the quartz plate such that the quartz plate has a shape that connects the vibrating region to the framing portion via a supporting beam. The framing portion surrounds the vibrating region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of Japan application serial no. 2011-219204, filed on Oct. 3, 2011. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     TECHNICAL FIELD 
     This disclosure relates to a crystal unit and a method for fabricating the crystal unit. More particularly, this disclosure relates to a crystal unit appropriate for downsizing and a method for fabricating this crystal unit. 
     DESCRIPTION OF THE RELATED ART 
     A crystal unit is widely used in various electronic devices as a frequency reference source or a time reference source. The crystal unit generally includes a plurality of excitation electrodes on a surface of a quartz plate (in other words, a crystal sheet), which functions as a vibrating plate. The crystal unit further hermetically encloses the quartz plate inside a container, and includes an excitation electrode that is electrically extracted to the outside of the container. The crystal unit has a resonant frequency, which is defined by a shape, a size, and a crystal orientation of the quartz plate. Obtaining a desired resonant frequency of the crystal unit needs a precise contour routing of the quartz plate. 
     There has been a growing need for a downsized crystal unit as an electronic device has been downsized while power consumption of an electronic device has been reduced. Etching process on a quartz plate is becoming mainstream using photolithography so as to fabricate a downsized crystal unit. The quartz plate itself is formed of silicon dioxide, thus being processed with an etching technique that is generally used in a fabrication process of semiconductor devices. However, a quartz crystal, which is made of single-crystal silicon dioxide, has crystal anisotropy. For example, in the case where wet etching with hydrogen fluoride is performed to process the quartz crystal, the etching does not transfer a mask shape as it is and the etching progresses depending on a crystal orientation. This makes control of the shape extremely difficult. 
     Therefore, contour routing of a quartz plate by the method of dry etching is attracting attention, and the method of dry etching is hard to be affected by the crystal anisotropy. For example, Japanese Unexamined Patent Application Publication No. 8-242134 discloses a fabrication of a crystal unit by reactive ion etching using an etching gas that flows on a quartz substrate. However, an oxide single crystal such as the quartz crystal does not have good workability in a dry etching process. Thus, cracking, chipping and the like during etching easily occur. These cracking, chipping, and the like probably occur due to, for example, a temperature of the quartz plate during the dry etching reaching equal to or more than 200° C. A method is proposed to prevent cracking and chipping during dry etching. The method laminates or bonds a quartz-crystal wafer on a supporting substrate so as to support the quartz-crystal wafer and then performs dry etching on the quartz-crystal wafer in that state. In this configuration, the supporting substrate supports the whole quartz plate, thus preventing cracking and chipping of the quartz plate even in the case where temperature increases during etching. For example, Japanese Unexamined Patent Application Publication No. 2010-177540 discloses dry etching on a quartz-crystal wafer that is laminated on a supporting substrate, which is formed of silicon, glass or the like, via an adhesive film. 
     A crystal unit has been downsized, and MEMS (Micro-electromechanical systems) technology has been developed by applying a semiconductor device fabrication technique. Accordingly, a supporting substrate itself, which is used to support the quartz-crystal wafer during dry etching, is used as a structural member (for example, a container) of the crystal unit and is being studied. However, laminating and bonding the crystal unit on the supporting substrate may result in a vibrating region bonded to the supporting substrate or a vibrating region in contact with the supporting substrate, thus impeding the quartz plate from vibrating at a resonant frequency. This results in marked deterioration of characteristics of the crystal unit. 
     Although Japanese Unexamined Patent Application Publication No. 2010-56833 is not related to dry etching of a quartz plate, this publication discloses a configuration to downsize a surface acoustic wave (SAW) device that includes a piezoelectric substrate formed of quartz crystal or the like. The configuration forms a resin layer on a surface of the piezoelectric substrate, and also includes a void in the resin layer which is in a forming portion of an interdigital transducer (IDT) on the piezoelectric substrate. Thus, the forming portion of the interdigital transducer is not in contact with the resin layer. 
     Laminating and bonding the quartz plate on the supporting substrate is an effective method for preventing cracking and chipping during processing the quartz plate by dry etching. However, using the supporting substrate bonded to the quartz plate directly as a structural portion of the crystal unit causes the vibrator in contact with the supporting substrate or the vibrator bonded to the supporting substrate. Thus, a problem arises in that marked deterioration of characteristics of the crystal unit occurs. 
     A need thus exists for a crystal unit and a method for fabricating the crystal unit which is susceptible to the drawback mentioned above. 
     SUMMARY 
     According to an aspect of this disclosure, a method for fabricating a crystal unit includes preparing, bonding, and separating. The preparing prepares a quartz plate and a supporting substrate with a recess that is larger than the vibrating region on a surface of the supporting substrate. The recess corresponds to a vibrating region in the crystal unit. The bonding bonds the quartz plate to the surface of the supporting substrate such that the quartz plate is separated from the supporting substrate in the recess. The separating separates the quartz plate into the vibrating region and the framing portion by performing dry etching on the quartz plate such that the quartz plate has a shape that connects the vibrating region to the framing portion via a supporting beam. The framing portion surrounds the vibrating region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings, wherein: 
         FIG. 1A  is a cross-sectional view of a quartz wafer according to an embodiment of this disclosure; 
         FIG. 1B  is a plan view of the quartz wafer according to the embodiment of this disclosure; 
         FIG. 2A  is a cross-sectional view of a supporting substrate of this disclosure; 
         FIG. 2B  is a plan view of the supporting substrate of this disclosure; 
         FIG. 3  is a cross-sectional view illustrating a manufacturing process of a crystal unit of this disclosure; 
         FIG. 4A  is a cross-sectional view illustrating the manufacturing process of the crystal unit of this disclosure; 
         FIG. 4B  is a plan view illustrating the manufacturing process of the crystal unit of this disclosure; 
         FIG. 5A  is a cross-sectional view of the crystal unit of this disclosure; 
         FIG. 5B  is a plan view of the crystal unit of this disclosure; 
         FIG. 6  is an enlarged view of a supporting portion in a vibrating region of this disclosure; 
         FIG. 7A  is a cross-sectional view of a sealing member of this disclosure; 
         FIG. 7B  is a bottom view of the sealing member of this disclosure; 
         FIG. 8A  is a cross-sectional view illustrating the manufacturing process of the crystal unit of this disclosure; 
         FIG. 8B  is a plan view illustrating the manufacturing process of the crystal unit of this disclosure; 
         FIG. 9A  is a cross-sectional view illustrating the manufacturing process of the crystal unit of this disclosure; 
         FIG. 9B  is a plan view illustrating the manufacturing process of the crystal unit of this disclosure; 
         FIG. 10A  is a cross-sectional view of the crystal unit of this disclosure; 
         FIG. 10B  is a plan view of the crystal unit of this disclosure; 
         FIG. 11  is an exemplary shape of a quartz plate of this disclosure; and 
         FIG. 12  is an exemplary shape of another quartz plate of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Next, preferred embodiments of this disclosure will be described by referring to the attached drawings. 
     A crystal unit according to this disclosure is fabricated by bonding a quartz plate and a supporting substrate together, and then processing the quartz plate by dry etching. The supporting substrate has a recess, which is larger than a vibrating region, on a surface of the supporting substrate corresponding to a vibrating region of a crystal unit. 
       FIG. 1A  is a cross-sectional view of a quartz plate  10  before bonding.  FIG. 1B  is a plan view of the quartz plate  10  before bonding.  FIG. 1A  illustrates the cross-sectional surface taken along the line I-I of  FIG. 1B . The quartz plate  10  is, for example, cut out from a single quartz crystal as a GT-cut quartz plate, and has approximately a rectangular shape. A plurality of crystal units may be faired at the same time using a quartz plate (that is, a quartz-crystal wafer) with a size corresponding to sizes of the plurality of crystal units, and then the quartz plate is divided into individual crystal units by dicing. In this case, a portion corresponding to one crystal unit has approximately a rectangular shape. While a case of the GT-cut quartz plate will be described as an example here, the crystal orientation is not limited to GT cut. For example, the crystal orientation may employ AT-cut or another cut. 
     The quartz plate  10  includes a vibrating region  11  as a center portion. The quartz plate  10  also includes excitation electrodes  12  and  13  on respective main surfaces of the quartz plate  10  corresponding to the vibrating region  11 . Here, the crystal unit is assumed to be a GT-cut crystal unit using a contour vibration mode. The vibrating region  11  has an oval shape with high circularity. The excitation electrodes  12  and  13  have the same shape as the vibrating region. The quartz plate  10  includes a metal layer  14  that is fouled along an outer periphery of its main surface (a first main surface) at the upper side of the drawing. The metal layer  14  is used for bonding with a sealing member, which is described below. On the first main surface, the quartz plate  10  further includes coupling electrodes  15  and  16  in a region between the excitation electrode  12  and the metal layer  14 . The coupling electrode  15  is electrically connected to the excitation electrode  12  via an extraction electrode  17  in a long thin shape. An electrically conducting path  18  is formed in a position where the coupling electrode  16  is formed and passes through the quartz plate  10  to the other main surface (a second main surface) side of the quartz plate  10 . The electrically conducting path  18  is formed as a so-called through hole electrode. An extraction electrode  19  is formed on the second main surface of the quartz plate  10  and electrically connects an end portion of the electrically conducting path  18  and the excitation electrode  13 . The extraction electrodes  17  and  19  are disposed in a region where supporting beams  21  and  22  are formed, as described below. The supporting beams  21  and  22  hold the vibrating region  11  at the framing portion  20 , which is the outer periphery portion of the quartz plate  10 . Here, the crystal unit is assumed to be a GT-cut crystal unit. The supporting beams  21  and  22  are formed to extend in mutually opposite directions from the vibrating region  11  by dry etching described below. 
     The excitation electrodes  12  and  13  and the extraction electrodes  17  and  19  are formed by a method such as vapor deposition or sputtering. The metal layer  14  and the coupling electrodes  15  and  16  are formed as follows. For example, a chromium (Cr) film and a gold (Au) film are formed on the quartz plate  10  in this order by sputtering, vapor deposition, or plating. Then, the metal layer  14  and the coupling electrodes  15  and  16  are patterned so as to be made of the same material with the same thickness. The excitation electrode  12 , the metal layer  14 , the coupling electrodes  15  and  16 , and the extraction electrode  17  may be formed at the same time. 
       FIG. 2A  is a cross-sectional view of a supporting substrate  30  before bonding.  FIG. 2B  is a plan view of the supporting substrate  30  before bonding.  FIG. 2A  illustrates a cross-sectional surface taken along the line II-II of  FIG. 2B . The supporting substrate  30  is, for example, formed of silicon (Si) and the like. The supporting substrate  30  includes a recess  31  on its top surface corresponding to positions of the vibrating region  11 , the supporting beams  21  and  22 , and the coupling electrode  16  in the quartz plate  10 . The recess  31  has a shape slightly larger than a shape including the vibrating region  11 , the supporting beams  21  and  22 , and the coupling electrode  16 . The recess  31  is formed such that the supporting substrate  30  is processed by wet etching or dry etching, and has a depth of, for example, about 1 μm. 
     Next, the second main surface of the quartz plate  10  is bonded to the top surface of the supporting substrate  30 . The bonding is assumed to be direct bonding, which does not use any adhesive and the like. For example, the supporting substrate  30  and the quartz plate  10  are bonded together by surface activated bonding. The surface activated bonding radiates Ar+ ion in a vacuum so as to activate surfaces. Then, the activated surfaces are bonded together by pressure bonding. This eliminate the need for heating or the like, which is an advantageous effect.  FIG. 3  illustrates a cross-sectional shape after the bonding corresponding to the cross-sectional surface taken along the line I-I of  FIG. 1B  and the cross-sectional surface taken along the line II-II of  FIG. 2B . The excitation electrode  13  and the extraction electrode  19 , which are formed on the second main surface of the quartz plate  10 , are arranged in the recess  31  of the supporting substrate  30 . Thus, the excitation electrode  13  and the extraction electrode  19  are not directly in contact with the supporting substrate  30 . Portions to be the vibrating region  11  of the quartz plate  10  and the supporting beams  21  and  22  are also not in contact with the supporting substrate  30 . 
     Next, a passivation film  35  such as resist, which is to be a mask in dry etching, is formed on the first main surface of the quartz plate  10 .  FIG. 4A  and  FIG. 4B  illustrate the quartz plate  10  and the supporting substrate  30  with the passivation film  35 .  FIG. 4A  is a cross-sectional view while  FIG. 4B  is a plan view.  FIG. 4A  illustrates a cross-sectional surface taken along the line IV-IV of  FIG. 4B . The passivation film  35  is patterned by a technique such as photolithography. While the passivation film  35  is &amp;limed on the vibrating region  11  of the quartz plate  10 , the framing portion  20 , and the supporting beams  21  and  22 , the passivation film  35  is not formed in a region that separates the vibrating region  11  from the framing portion  20 . Here, a portion of the passivation film  35  corresponding to the vibrating region  11  is formed only in a region inside the recess  31  of the supporting substrate  30 , which is important. 
     Next, etching is performed using the passivation film  35  as a mask until the etching makes a hole through the quartz plate  10  and then the passivation film  35  is removed.  FIG. 5A  is a cross-sectional view after etching.  FIG. 5B  is a plan view after etching.  FIG. 5A  illustrates a cross-sectional surface taken along the line V-V of  FIG. 5B . This embodiment has a feature that performs dry etching with the vibrating region  11  of the quartz plate  10  separating from the supporting substrate  30 . An etching gas of the dry etching may employ, for example, a fluorocarbon (CF)-based gas, which is generally used for dry etching of silicon dioxide. 
     With the above processes, the quartz plate  10  is separated into the vibrating region  11  and the framing portion  20 , which surrounds the vibrating region  11 . The quartz plate  10  is processed such that the vibrating region  11  and the framing portion  20  have shapes that are mutually connected via the supporting beams  21  and  22 . The vibrating region  11  and the supporting beams  21  and  22  are arranged in the region of the recess  31  on the supporting substrate  30 . The vibrating region  11  and the supporting beams  21  and  22  are not in contact with the supporting substrate  30 . Vibration of the vibrating region  11  is not impeded by the supporting substrate  30 .  FIG. 6  is an enlarged view of a portion that supports the vibrating region  11  in the quartz plate  10 . For ease of explanation, respective electrodes are omitted in the drawing.  FIG. 6  illustrates that the vibrating region  11  is arranged in the recess  31  so as not to be in contact with the supporting substrate  30 . 
     The coupling electrodes  15  and  16  are electrically connected with the respective excitation electrodes  12  and  13 . The quartz plate  10 , which is being supported by the supporting substrate  30 , is housed, for example, inside a ceramic package. The coupling electrodes  15  and  16  are each extracted to outside in an electrically conductive state. This consequently completes fabrication of the crystal unit. 
     In the case where the crystal unit is formed such that the quartz plate  10 , which is being supported by the supporting substrate  30  as illustrated in  FIG. 5A  and  FIG. 5B , is housed inside the container, a problem arises in that it is difficult to reduce a dimension in the height direction of the crystal unit. Therefore, use of the supporting substrate  30  itself may be employed as the container of the crystal unit. In this case, the following configuration is preferred. The sealing member is bonded to the first main surface of the quartz plate  10 . The vibrating region  11  is hermetically enclosed in a space that is formed of the supporting substrate  30 , the framing portion  20  of the quartz plate  10 , and the sealing member. 
       FIG. 7A  is a cross-sectional view of an exemplary sealing member.  FIG. 7B  is a bottom view of the exemplary sealing member.  FIG. 7A  illustrates a cross-sectional surface taken along the line VII-VII of  FIG. 7B . A sealing member  40  is made of, for example, silicon. The sealing member  40  includes, on its bottom face, a metal layer  41  corresponding to the metal layer  14  of the quartz plate  10 . The sealing member  40  also includes coupling electrodes  42  and  43  in positions corresponding to the coupling electrodes  15  and  16  of the quartz plate  10 . The coupling electrodes  42  and  43  are made of the same material with the same thickness and by the same process as those of the metal layer  41 . The sealing member  40  includes through electrodes  44  and  45  that are implanted in positions of the respective coupling electrodes  42  and  43  for external connection. The through electrodes  44  and  45  have one ends electrically connected to the respective coupling electrodes  42  and  43 . The through electrodes  44  and  45  are formed such that copper (Cu) and gold are implanted by plating. Alternatively, the through electrodes  44  and  45  may be formed such that a tungsten (W) layer or a polysilicon layer doped with impurities are formed by a CVD (chemical vapor deposition) method. The sealing member  40  further includes, on its bottom face, a recess  48 , which is larger than the vibrating region  11 , corresponding to the vibrating region  11  of the quartz plate  10 . Thus, the vibrating region  11  is not brought in contact with the sealing member  40  when the sealing member  40  is bonded to the quartz plate  10 . 
       FIG. 8A  is a cross-sectional view of the sealing member  40  bonded to the quartz plate  10 .  FIG. 8B  is a plan view of the sealing member  40  bonded to the quartz plate  10 .  FIG. 8A  illustrates a cross-sectional surface taken along the line VIII-VIII of  FIG. 8B . The sealing member  40  is bonded to the quartz plate  10  as follows. The metal layer  14  of the quartz plate  10  and the metal layer  41  of the sealing member  40  are bonded together. Then, the coupling electrodes  15  and  16  of the quartz plate  10  are bonded to the coupling electrodes  42  and  43  of the sealing member  40 . The bonding employs, for example, a method such as surface activated bonding. The bonding results in a hermetical enclosure of the vibrating region  11  of the quartz plate  10  inside the space, which is formed of the supporting substrate  30 , the framing portion  20 , and the sealing member  40 . Especially, bonding the sealing member  40  to the quartz plate  10  in a vacuum hermetically encloses the vibrating region  11  in a vacuum. 
     Next, the through electrodes  44  and  45  are exposed by polishing (by back grinding) the sealing member  40  from the top surface side of the sealing member  40 .  FIG. 9A  is a cross-sectional view illustrating exposed through electrodes.  FIG. 9B  is a plan view illustrating the exposed through electrodes.  FIG. 9A  illustrates a cross-sectional surface taken along the line IX-IX of  FIG. 9B . After that, mounting terminals  46  and  47  are formed on the top surface of the sealing member  40  corresponding to the exposed positions of the through electrodes  44  and  45 . The mounting terminals  46  and  47  are electrically connected to the through electrodes  44  and  45 , thus being electrically connected to the excitation electrodes  12  and  13 . The mounting terminals  46  and  47  are used for mounting the completed crystal unit on a surface of a circuit board. In the case where the quartz plate  10 , the supporting substrate  30 , and the sealing member  40 , which each have the size corresponding to a plurality of crystal units, are used for forming the plurality of crystal units at one time, the quartz plate  10  is divided into individual crystal units by dicing in this phase.  FIG. 10A  is a cross-sectional view of the completed crystal unit.  FIG. 10B  is a plan view of the completed crystal unit.  FIG. 10A  illustrates a cross-sectional surface taken along the line X-X of  FIG. 10B . 
     In this configuration, the through electrodes  44  and  45  may be formed on the sealing member  40  before bonded to the quartz plate  10 . This increases degrees of freedom for materials of the through electrodes  44  and  45 . Bonding the sealing member  40  to the quartz plate  10  allows sealing and electrical wiring at the same time, which is also an advantageous effect of this method. 
     In the crystal unit described above, the quartz plate  10  is processed by dry etching so as to separate the vibrating region  11  from the framing portion  20 . In the case where the quartz crystal, which is silicon dioxide, is etched by dry etching, using a fluorocarbon based gas as the etching gas obtains high etching rate. However, a deposited material as a by-product formed by an etching reaction adheres to a side face of the etched quartz crystal. This deposited material strongly adheres to the side face of the quartz crystal. Additionally, the deposited material includes oxide as a component. Thus, it is difficult to remove the deposited material in the case where a solvent such as acetone, which is usually used in an organic removal process in the field of semiconductor device production, is employed. Even using a dedicated remover has difficulty in removal of the deposited material, which has adhered to the side face of the quartz plate. 
     A study by inventors has revealed that while the deposited material as a by-product formed by dry etching easily adheres to a portion at an inner side of a orthogonally folded portion and the like, the deposited material adhered to a portion on an inner surface of an arc-like shape is easily removed. That is, a portion to be processed by dry etching is formed not in a rectangular shape, but in a smooth arc-like shape. This facilitates removal of the deposited material after dry etching. In this embodiment, the crystal unit has a structure that holds the vibrating region  11  using the supporting beams  21  and  22  that extend from the framing portion  20 . The vibrating region  11  has different resonant frequency and vibration characteristic depending on its shape and size. Thus, changing the shape and the size of the vibrating region  11  is not preferred in order to obtain a satisfactory vibration characteristic and a desired resonant frequency. However, a shape of an inner wall of the framing portion  20  does not affect the resonant frequency and the vibration characteristic, thus being decided freely. Accordingly, it is preferred that the framing portion  20  have an inner periphery in a shape of a circle or an ellipse in the case where the quartz plate  10  is processed by dry etching so as to separate the vibrating region  11  from the framing portion  20 . It is preferred that the framing portion  20  be processed to have a polygonal shape with rounded corners in the case where the inner periphery of the framing portion  20  has difficulty in having a shape of a circle or an ellipse due to the size of the crystal unit. The polygonal shape with rounded corners is, for example, a rectangular shape with rounded corners. The rounded corners each have a curvature radius equal to or more than 0.1 μm and equal to or less than 1 mm. 
       FIG. 11  illustrates an exemplary inner periphery, which is formed in a shape of a circle or an ellipse, of the framing portion  20 . A crystal unit in  FIG. 11  is assumed to be a GT-cut crystal unit and includes a vibrating region  11  that is formed in a shape of an ellipse with high circularity.  FIG. 12  illustrates another example of an inner periphery, which is formed in a shape of a circle or an ellipse, of the framing portion  20 . A crystal unit in  FIG. 12  is assumed to be an AT-cut crystal unit and includes a vibrating region  11  that is formed in a rectangular shape. 
     The disclosed fabrication method preliminarily forms the recess on the surface of the supporting substrate before bonding to the quartz plate. This consequently allows dry etching to proceed in the state where the vibrating region of the quartz plate is not in contact with the supporting substrate. The crystal unit thus obtained does not make the vibrating region contacted or bonded to the supporting substrate as a crystal unit. This avoids interference with vibration of the vibrating region. Forming the recess on the supporting substrate is performed by a method such as dry etching or wet etching. The recess employs any depth insofar as the depth surely prevents the vibrating region from contacting the supporting substrate. For example, the depth is set to equal to or more than 0.1 μm. The recess does not pass through the supporting substrate. Thus, the depth of the recess has a thickness smaller than that of the supporting substrate. 
     A crystal unit disclosed here includes a supporting substrate and a quartz plate bonded on a surface of the supporting substrate. The quartz plate has a shape connecting a vibrating region to a framing portion, which surrounds the vibrating region, via a supporting beam. The vibrating region is separated from the framing portion by dry etching. The surface of the supporting substrate includes a recess, which is larger than the vibrating region, corresponding to the vibrating region, so as to prevent the vibrating region from contacting the supporting substrate. 
     The recess, which is larger than the vibrating region, is preliminarily disposed on the supporting substrate, which supports the quartz plate during dry etching, corresponding to the vibrating region of the quartz plate. As a result, bonding the quartz plate to the supporting substrate prevents interference with the vibration of the quartz plate, thus allowing to obtain the crystal unit that maintains a good vibration characteristic. 
     The principles, preferred embodiment and made of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.