Abstract:
The present disclosure provides a mesa-type AT-cut quartz-crystal vibrating piece, in which amount of the vibrating unit is adjusted to appropriate amount, in order to inhibit unnecessary vibration and to prevent degradation. The mesa-type AT-cut quartz-crystal vibrating piece ( 30 ) for vibrating piece vibrates at 38.400 MHz comprises a rectangular excitation unit ( 31 ), a peripheral region ( 32 ) formed in periphery of the excitation unit and thinner than the excitation unit. The thickness difference h (μm) between one principal surface of the excitation unit and the adjacent peripheral region is obtained by the following equation: h=(0.2×Mx)−143 The length of the x-axis direction of the crystallographic X-axis is Mx (μm).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Japan Patent Application No. 2010-178229, filed on Aug. 7, 2010, in the Japan Patent Office, the disclosures of which are incorporated herein by reference in their respective entireties. 
       FIELD 
       [0002]    The present disclosure pertains to the mesa-type AT-cut quartz-crystal vibrating piece and the quartz-crystal device comprising the same. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    One of the representative thickness-shear mode vibrating pieces is an AT-cut quartz-crystal vibrating piece. The quartz-crystal devices accommodating such AT-cut quartz-crystal device within their respective package are commonly used in different types of electric devices as a standard frequency sources. These quartz-crystal devices are being reduced in size, and recently developed AT-cut quartz-crystal vibrating pieces may include a curved, beveled, or convex, region at the outer periphery of the principal surface of the AT-cut quartz-crystal vibrating piece, in order to efficiently acquire energy trapping behavior. Curve processing, such as bevel or convex processing as disclosed in Japan Unexamined Patent Application No. 2002-018698, forms the sloped region on the outer periphery of the quartz-crystal vibrating piece by a barrel polishing method. 
         [0004]    However, the AT-cut manufacturing method by wafer method has advanced in recent years, which made contouring more difficult. Therefore, mesa-type, processing is applied to the AT-cut quartz-crystal vibrating piece, in place of the contouring method, to reduce the thickness of the periphery of the quartz-crystal vibrating piece. 
         [0005]    However, in the prior art mesa-type AT-cut quartz-crystal vibrating piece, the vibrating energy generated on the vibrating unit includes unnecessary vibrating energy generated on outer periphery of the vibrating unit which degrades the AT-cut quartz-crystal vibrating piece. 
         [0006]    The present disclosure provides a mesa-type AT-cut quartz-crystal vibrating piece, in which the thickness of the peripheral region of the vibrating unit is adjusted to reduce unnecessary vibration and to prevent degradation. 
       SUMMARY 
       [0007]    In its first aspect, the present disclosure provides a mesa-type AT-cut quartz-crystal vibrating piece vibrating at 38.400 MHz. The mesa-type AT-cut quartz-crystal vibrating piece comprises: a rectangular excitation unit surrounded by a peripheral region having a thickness less than the excitation unit. The term “thickness difference” as used in this disclosure refers to the thickness difference between the central excitation unit and the peripheral region surrounding the excitation unit. The thickness difference is evenly distributed with respect to the first (top) and second (bottom) principal surfaces of the quartz-crystal vibrating piece. A height h (μm) thickness difference between one principal surface of the excitation unit and the adjacent peripheral region is obtained by the following equation (1): 
         [0000]        h =(0.2× Mx )−143  (1)
 
         [0008]    wherein the length of the rectangular excitation unit in x-axis direction of the crystallographic X-axis is denoted as Mx (μm). 
         [0009]    In its second aspect, the present disclosure provides a mesa-type AT-cut quartz-crystal vibrating piece vibrating at 32.736 MHz. The mesa-type AT-cut quartz-crystal vibrating piece comprises: a rectangular excitation unit surrounded by a peripheral region having a thickness less than the excitation unit. A height h (μm) for the thickness difference between one principal surface of the excitation unit and the adjacent peripheral region is obtained by the following equation (2): 
         [0000]        h =(0.1× Mx )−87  (2)
 
         [0010]    wherein the length of the rectangular excitation unit in x-axis direction of the crystallographic X-axis is denoted as Mx (μm). 
         [0011]    In its third aspect, the present disclosure provides a mesa-type AT-cut quartz-crystal vibrating piece. The mesa-type AT-cut quartz-crystal vibrating piece comprises: a rectangular excitation unit surrounded by a peripheral region having a thickness less than the excitation unit. A height h (μm) for the thickness difference between one principal surface of the excitation unit and the adjacent peripheral region is obtained by the following equation (3): 
         [0000]        h =(1.8× Mx/t   0.7 )−92  (3)
 
         [0012]    wherein the length of the x-axis direction of the crystallographic X-axis is Mx (μm). 
         [0013]    In its fourth aspect, the present disclosure provides a mesa-type AT-cut quartz-crystal vibrating piece. The mesa-type AT-cut quartz-crystal vibrating piece comprises an outer frame surrounding the peripheral region of the excitation unit and supporting the excitation unit at the peripheral region thereof. 
         [0014]    In its fifth aspect, the present disclosure provides a quartz-crystal device. The quartz-crystal device comprises a mesa-type AT-cut quartz-crystal vibrating piece in any one of the first to third aspects; a base having a recess for containing the mesa-type AT-cut quartz-crystal vibrating piece; and a lid for sealing the recess. 
         [0015]    In its sixth aspect, the present disclosure provides a quartz-crystal device. The quartz-crystal device, comprises the mesa-type AT-cut quartz-crystal vibrating piece of the fourth aspect having a first (top) principal surface and a second (bottom) principal surface; the lid having a first surface to be bonded onto the first (top) principal surface of the outer frame; and the base having a second surface to be bonded onto the second (bottom) principal surface of the outer frame. 
         [0016]    The present disclosure provides a mesa-type AT-cut quartz-crystal vibrating piece, in which the thickness of the peripheral region of the vibrating unit is reduced in order to inhibit unnecessary vibration and to prevent degradation of the AT-cut quartz-crystal vibrating piece. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1A  is a perspective view of the quartz-crystal device  100 . 
           [0018]      FIG. 1B  is a cross-sectional view of the quartz-crystal device  100 . 
           [0019]      FIG. 1C  is a cross-sectional view of the quartz-crystal device  100  taken along B-B line. 
           [0020]      FIG. 2A  is a plan view of the AT-cut quartz-crystal vibrating piece  30 . 
           [0021]      FIG. 2B  is a cross-sectional view of  FIG. 2A  taken along D-D line. 
           [0022]      FIG. 3A  is a graph showing the correlation between the thickness difference h at specific vibrating frequency and the length Mx of the excitation unit  31  in X-axis direction. 
           [0023]      FIG. 3B  is a graph showing the correlation between the thickness difference h, length Mx of the excitation unit  31  in X-axis direction and thickness t of the excitation unit  31 . 
           [0024]      FIG. 4A  is a perspective view of the quartz-crystal device  200 . 
           [0025]      FIG. 4B  is a cross-sectional view of the  FIG. 4A  taken along E-E line. 
           [0026]      FIG. 5A  is a plan view of the lid  210 . 
           [0027]      FIG. 5B  is a plan view of the AT-cut quartz-crystal vibrating piece  230 . 
           [0028]      FIG. 5C  is a plan view of the base  220 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Various representative embodiments are described below with reference to the respective drawings. It will be understood that the scope of the disclosure is not limited to the described embodiments, unless otherwise stated. 
       First Embodiment 
       [0030]    &lt;Overall Configuration of Quartz-Crystal Device  100 &gt; 
         [0031]      FIG. 1A  is a perspective view of the quartz-crystal device  100 . The quartz-crystal device  100  is constituted of the lid  10 , base  20  and AT-cut quartz-crystal vibrating piece  30  (refer to  FIG. 1B ) mounted in the base  20 . The AT-cut quartz-crystal vibrating piece has a principal plane (YZ plane) that is inclined, with respect to the Y-axis of the crystal axes (XYZ), from the Z-axis to the Y-axis direction by 35°15′. Therefore, the quartz-crystal device  100  is described below by designating a longitudinal direction thereof as the X-axis direction, designating the thickness direction thereof as the Y′-axis direction, and designating the width direction thereof that is perpendicular to the X-axis and Y′-axis directions as the Z′-axis direction. In this explanation below, inclination to the Y′-axis is denoted as +Y′-axis and declination to the Y′-axis is denoted as −Y′-axis. 
         [0032]    A cavity  24  is formed inner side of the base  20  (refer to  FIG. 1B ), and an AT-cut quartz-crystal vibrating piece  30  is situated onto the cavity  24 . The external electrodes  21  are formed on lower main surface of the base  20 . The lid  10  is disposed on +Y′-axis direction to the base  20 , so as to seal the cavity  24 . The lid  10  is fabricated by materials such as ceramic, glass, quartz-crystal and metal. The base  20  is fabricated by materials such as ceramic, glass and quartz-crystal. 
         [0033]      FIG. 1B  is a cross-section of the quartz-crystal device  100 .  FIG. 1B  is a cross-section of the  FIG. 1A  taken along A-A line or the  FIG. 1C  along C-C line, which will be explained hereafter. The recess on the base  20  forms the cavity  24 . The connection electrodes  22  are formed on lower side of the cavity  24 , and the connection electrodes  22  are electrically connected to the external electrodes  21  via conductor (not shown). An excitation unit  31  is formed on the AT-cut quartz-crystal vibrating piece  30 , and a peripheral region  32  is disposed on outer periphery of the excitation unit  31 . The thickness of the peripheral region  32  is less than the thickness of the excitation unit  31 . The excitation electrodes  33  are formed on both the first (upper) principal surface and the second (lower) principal surface of the excitation unit  31 . The extraction electrodes  34  are formed on the peripheral region  32 , and the extraction electrodes  34  are electrically connected to the excitation electrodes  33 . The excitation electrodes  33  and the extraction electrodes  34  comprise a chrome (Cr) layer on surfaces of the quartz-crystal and a gold (Au) layer overlays the surface of the chrome (Cr) layer. The extraction electrodes are electrically connected to respective connection electrodes  22  via electrically conductive adhesive  41 . Thereby, the excitation electrodes  33  are electrically connected to respective external electrodes  21 . 
         [0034]      FIG. 1C  is a cross-section of the  FIG. 1B  along B-B line. On the cavity  24  disposed on the base  20 , two connection electrodes  22  are formed. One connection electrode  22  is electrically connected to the excitation electrode  33  situated on upper surface of the excitation unit  31  of the AT-cut quartz-crystal vibrating piece  30 , and the other connection electrode  22  is electrically connected to the excitation electrode  33  situated on lower surface of the excitation unit  31 . 
         [0035]      FIG. 2A  is a plan view of the AT-cut quartz-crystal vibrating piece  30 . The AT-cut quartz-crystal vibrating piece  30  has same respective X-axis direction as the crystallographic X-axis of the quartz-crystal. Also, Z′-axis direction of the AT-cut quartz-crystal vibrating piece  30  has same respective axial direction as direction of the quartz-crystal from the Z-axis to the Y-axis direction tilted by 35°15′. Length Gx of the AT-cut quartz-crystal vibrating piece  30  in X-axis direction is, for example, 990 μm, and width Gz of the AT-cut quartz-crystal vibrating piece  30  in Z′-axis direction is, for example, 700 μm. Also, length of the excitation unit  31  in the X-axis direction is denoted as Mx and the width in the Z′-axis direction is denoted as Mz. 
         [0036]      FIG. 2B  is a cross-section of  FIG. 2A , taken along D-D line. The thickness of the peripheral region  32  of the AT-cut quartz-crystal vibrating piece  30  is denoted as Gy, thickness of the excitation unit  31  is denoted as t, and the thickness difference between the excitation unit  31  and the peripheral region  32  is denoted as h. In the disclosed embodiments, the thickness difference between the thickness t of the excitation unit  31  and the thickness Gy of the peripheral region  32  is evenly distributed with respect to the first (top) and second (bottom) principal surfaces of the excitation unit  31 . Reference letter h is used to designate the height of an elevational step between one principal surface (top or bottom) of the excitation unit  31  and the peripheral region  32 . As is shown in  FIG. 2B , the total thickness difference between the thickness t of the excitation unit  31  and the thickness Gy of the peripheral region  32  is actually 2×h. A thickness difference between the excitation unit  31  and peripheral region  32  are formed on both +Y′-axis and −Y′-axis sides of the AT-cut quartz-crystal vibrating piece  30 , and both thickness differences are leveled at same height. The thickness difference h is positive, and the thickness t of the excitation unit  31  is greater than the thickness Gy of the peripheral region  32 . 
         [0037]    &lt;Thickness Difference h&gt; 
         [0038]    By performing an experiment for determining the optimum length of the AT-cut quartz-crystal vibrating piece  30 , it was found that the thickness difference h correlates to the length Mx of the excitation unit  31  in X-axis direction and the vibrating frequency. Also, it was found that the thickness difference h correlates to the length Mx of the excitation unit  31  in x-axis direction and thickness t of the excitation unit  31 . Hereinafter, the above-identified correlations are explained. 
         [0039]      FIG. 3A  is a graph showing a correlation between the thickness difference h and length Mx of the excitation unit  31  in X-axis direction at predetermined vibrating frequency. The abscissa of the graph indicates the length Mx (μm) of the excitation unit  31  in X-axis direction, and the ordinate indicates the height (μm) of the elevational section. The white circle in  FIG. 3A  indicates the value at vibrating frequency of 38.400 MHz, and the black circle indicates the value at vibrating frequency of 32.736 MHz. The inventor of the present disclosure manufactured forty AT-cut quartz-crystal vibrating piece  30  at thickness difference of h=1 μm and vibrating frequency of 38.400, each quartz-crystal vibrating piece having different length Mx. Similarly with other thickness differences, forty AT-cut quartz-crystal vibrating pieces  30  were manufactured having different lengths Mx. Then, the inventor of present disclosure measured CI (crystal impedance) value of each AT-cut quartz-crystal vibrating piece  30 . Each of the forty AT-cut quartz-crystal vibrating pieces  30  had different CI value, and the CI value had quadric curve (parabola) relation to the length Mx. Each white circle and black circle indicated in  FIG. 3  is inputted with the length Mx which produces the lowest CI value at respective vibrating frequency and thickness difference h. 
         [0040]    In  FIG. 3A , for both 32.736 MHz and 38.400 MHz, the thickness difference h increased as the length Mx of the excitation unit  31  in the X-axis direction extends longer. Also, the amount of each vibrating frequency of  FIG. 3A  is aligned in a straight line. In  FIG. 3A , the solid straight line  50  denotes the 38.400 MHz and dotted-dash line denotes the 32.736 MHz. The solid straight line  50  can be calculated by the following equation: 
         [0000]        h =(0.2× Mx )−143  (1)
 
         [0041]    Also, the dotted-dash line  51  can be calculated by the following equation: 
         [0000]        h =(0.1× Mx )−87  (2)
 
         [0042]      FIG. 3B  is a graph showing the correlation between the thickness difference h, length Mx of the excitation unit  31  in X-axis direction and thickness t of the excitation unit  31 . 
         [0043]    The abscissa of the  FIG. 3B  indicates Mx/t 0.7 , and the ordinate indicates the thickness difference h (μm). In  FIG. 3B , the relationship between the thickness difference h at the vibrating frequency of 32.736 MHz and 38.400 MHz and at Mx/t 0.7  wherein the CI value becomes the lowest, is indicated in black dot. The measurement was taken with the thickness t of the vibrating frequency  31  being 51.0 μm at the vibrating frequency of 32.736 MHz, and 43.5 μm at the vibrating frequency of 38.400 MHz. The black dots indicated in  FIG. 3B  were mostly on the solid line  52  or on neighborhood of the solid line  52 , and the position of the black dots can be approximated by the solid line  52 . The solid line  52  can be calculated using the following equation: 
         [0000]        h =(1.8× Mx/t   0.7 )−92  (3)
 
         [0044]    The equation (1) and equation (2) can determine the thickness difference h for excitation unit  31  for lowering the CI value, when manufacturing the AT-cut quartz-crystal vibrating piece  30  having the vibrating frequency of either 38.400 MHz or 32.736 MHz, and the value of the excitation unit  31  on the mesa portion is predetermined. Also, the equation (3) allows calculating the thickness difference h of the excitation unit  31  for lowering the CI value by using the thickness t of the excitation unit  31  and the length Mx of the excitation unit  31 , regardless of the vibrating frequency. 
       Second Embodiment 
       [0045]    The AT-cut quartz-crystal frame may be surrounded by the outer frame. An explanation regarding the AT-cut quartz-crystal vibrating piece having surrounded by the outer frame is explained hereafter. 
         [0046]    &lt;Configuration of the Quartz-Crystal Device  200 &gt; 
         [0047]      FIG. 4A  is an exploded view of the quartz-crystal device  200 . The quartz-crystal device  200  includes the lid  210 , AT-cut quartz-crystal vibrating piece  230  and the base  220 . The lid  210  is disposed on upper side of the quartz-crystal device  200 , the base  220  is disposed on lower side, and the quartz-crystal vibrating piece  230  is sandwiched between the lid  210  and base  220 . The external electrodes  221  are disposed on the lower main surface of the base  220 . Lid  210  and base  220  are fabricated of glass or quartz-crystal material. 
         [0048]      FIG. 4B  is a cross-section of  FIG. 4A  along E-E line. On the AT-cut quartz-crystal vibrating piece  230 , an excitation unit  231  and a peripheral region  232  are formed, and the outer frame  235  is formed surrounding the peripheral region  232 . The outer frame  235  is situated to support the peripheral region  232 . The excitation electrodes  233  are formed on both the first (upper) principal surface and the second (lower) principal surface of the excitation unit  231 , and the extraction electrodes  234  are formed on the outer frame  235  and extend to the peripheral region  232 . The AT-cut vibrating piece  230  vibrates at predetermined vibration per minute, when voltage is applied from a pair of the excitation electrodes  233 . The lid  210  is bonded by contacting the principal surface of the outer frame  235  and the first surface  211  on the −Y′-axis side of the lid  210 , and the base  220  is bonded by contacting the other principal surface of the base  220  and the second surface  223  on the +Y′-axis side. The connection electrodes  222  are formed on the base  220  on the +Y′-axis side, and when bonding the connection electrodes  222  and AT-cut quartz-crystal vibrating piece  230 , the connection electrodes  222  are bonded to the extraction electrodes  234 . Also, the connection electrodes  222  are electrically connected to the external electrodes  221  via conductor (not shown). 
         [0049]    The thickness of the excitation unit  231  of the AT-cut quartz-crystal vibrating piece  230  is designated by t, and the thickness of the peripheral region  232  is designated by Gy. The elevated section of the excitation unit  231  and the peripheral region  232  indicates the leveled surface formed on the +Y′-axis side and −Y′-axis side, which is designated by h. Further, the AT-cut quartz-crystal vibrating piece  230  is fabricated of synthetic quartz-crystal, has same respective X-axis direction as the crystallographic X-axis of the quartz-crystal, and has same respective axial direction as the direction of the quartz-crystal from the Z-axis to the Y-axis direction tilted by 35°15′, which is similar feature as the AT-cut quartz-crystal vibrating piece  30 . 
         [0050]      FIG. 5A  is a plan view of the lid  210 . The lid  210  has a rectangular principal surface having a longitudinal axis in the X-axis direction and a short axis in the Z′-axis direction. On peripheral unit on the surface on −Y′-axis side, a first surface  211  is formed, which is connected to the outer frame  235  of the AT-cut quartz-crystal vibrating piece  230 , and a recess  212  is formed on center, surrounded by the first surface  211 . 
         [0051]      FIG. 5B  is a plan view of the AT-cut quartz-crystal vibrating piece  230 . The AT-cut quartz-crystal vibrating piece  230  and the outer frame  235  are connected by the connecting arms  236 . The extraction electrodes  234  extracted from the excitation electrodes  233  on the excitation unit  231  is formed through the peripheral region  232  and connecting arms  236  to corners of the outer frame  235 . The extraction electrodes  234  are connected to the connection electrodes  222 , formed on the base  220 , at connection points  237  on corners of the outer frame  235 . Connection points  237  are formed on corners of the outer frame  235 , indicated with dotted oval circle in  FIG. 5B . Length Gx of the peripheral region  232  on the AT-cut quartz-crystal vibrating piece  230  in X-axis direction is, for example, 990 μm, and width Gz of the peripheral region  232  on the AT-cut quartz-crystal vibrating piece  230  in Z′-axis direction is, for example, 700 μm. Also, length of the excitation unit  31  in X-axis direction is denoted as Mx and the width in the Z′-axis direction is denoted as Mz. 
         [0052]      FIG. 5C  is a plan view of the base  220 . The second surface  223  is formed on outer periphery of the base  220  in +Y′-axis side surface, which is bonded to the outer frame  235  of the AT-cut quartz-crystal vibrating piece  230 , and a recess  225  is formed inside of the second surface  223 . On part of the second surface  223  in +Y′-axis side of the base  220 , the connection electrodes  222  are formed, which is electrically connected to the connection point  237  of the extraction electrodes  234  on the AT-cut quartz-crystal vibrating piece  230 . 
         [0053]    Although the outer frame is situated on the AT-cut quartz-crystal vibrating piece, since the peripheral region and the outer frame are connected by the connecting arms, excitation energy for unnecessary vibration, generated on the peripheral region, do not change largely. Therefore, the equation (1), equation (2) and equation (3) can be applied to the AT-cut quartz-crystal vibrating piece. 
         [0054]    As mentioned above, although optimal embodiments of the present disclosure were explained in detail, it will be understood by a person skilled in the art that the disclosure encompasses various alterations and modifications to the embodiments, within the technical scope of the invention.