Patent Publication Number: US-2022239277-A1

Title: Crystal element and crystal device

Description:
FIELD 
     The present disclosure relates to a crystal element of a thickness-shear vibration mode and to a crystal device that includes the crystal element. Examples of the crystal device may be a crystal unit, a crystal oscillator, and the like. 
     BACKGROUND 
     The crystal element of the thickness-shear vibration mode is acquired by forming excitation electrodes made of a metal film pattern on both main surfaces of an AT-cut crystal plate (for example, see Japanese Patent Application Laid-open No. 2016-34061). The oscillation frequency of the crystal element is inversely proportional to the thickness of the crystal plate. That is, the higher the oscillation frequency, the thinner the crystal plate. 
     The crystal device generates a specific oscillation frequency by using the piezoelectric effect and the inverse piezoelectric effect of the crystal element. A typical crystal device has a structure in which the crystal element is housed in a package and airtightly sealed by a lid. 
     SUMMARY 
     A crystal element according to the present disclosure includes, defining that a front side of two faces in a front-and-back relation is a first face, a back face is a second face, and a dimension of a direction vertically going through the first face and the second face is a thickness: a vibration part that includes a first face and a second face; a flat plate part that includes a first face and a second face, the flat plate part having a thickness thicker than a thickness of the vibration part and being disposed in an outer edge of the vibration part on a plan view; a fixing part that includes a first face and a second face, the fixing part having a thickness thicker than the thickness of the flat plate part and being disposed in an outer edge of the flat plate part on a plan view; an excitation electrode disposed on the first face and the second face of the vibration part; a mounting electrode disposed at least on one of the first face and the second face of the fixing part; and a wiring electrode that electrically connects the excitation electrode and the mounting electrode. 
     A crystal device according to the present disclosure includes: the crystal element according to the present disclosure; a base body where the crystal element is disposed; and a lid that, together with the base body, airtightly seals the crystal element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view illustrating a crystal element according to a first embodiment; 
         FIG. 2  is a plan view viewed through from the back side of the crystal element; 
         FIG. 3  is a sectional view taken along the line Ic-Ic of  FIG. 1 ; 
         FIG. 4  is a plan view illustrating a crystal plate according to the first embodiment; 
         FIG. 5  is a sectional view taken along the line IIb-IIb of  FIG. 4 ; 
         FIG. 6  is a plan view for describing examples of the dimensions of the crystal plate illustrated in  FIG. 4 ; 
         FIG. 7  is a sectional view illustrating a crystal plate according to a first example; 
         FIG. 8  is a sectional view illustrating a crystal plate according to a second example; 
         FIG. 9  is a sectional view illustrating a crystal plate according to a third example; 
         FIG. 10  is a sectional view illustrating a crystal plate according to a fourth example; 
         FIG. 11  is a sectional view illustrating a crystal plate according to a fifth example; 
         FIG. 12  is a perspective view illustrating a crystal device according to a second embodiment; and 
         FIG. 13  is a sectional view taken along the line IVb-IVb of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
     Recently, the mechanical strength of the crystal plates tends to be deteriorated since the crystal plates become thinner as the oscillation frequency of the crystal elements becomes higher. For example, when the oscillation frequency is 150 MHz, the thickness of the crystal plate is about 11 In this case, since the crystal plate is too thin, distortion is likely to occur in the crystal plate due to the stress. When distortion occurs in the crystal plate, the vibration balance of the vibration part is decreased. Thereby, the electrical characteristic of the crystal element is deteriorated. Examples of the deterioration in the electrical characteristic may be an increase in the equivalent series resistance value and deterioration in the frequency temperature characteristic (occurrence of a dip and the like). This issue is prominent particularly in the crystal element with the oscillation frequency of 150 MHz, since the crystal plate thereof is considerably thin. 
     The crystal element according to the present disclosure includes a vibration part, a flat plate part that is thicker than the vibration part and disposed in an outer edge of the vibration part, and a fixing part that is thicker than the flat plate part and disposed in an outer edge of the flat plate part, so that it is possible to achieve a configuration in which the outer edge of the thin vibration part is supported by the thick plate part, and the outer edge of the flat plate part is supported by the still thicker fixing part. As a result, even if the vibration part becomes thinner as the oscillation frequency becomes higher, it is possible to maintain the mechanical strength of the crystal element and to secure the stable electrical characteristic thereby. 
     Hereinafter, modes for embodying the present disclosure (hereinafter, referred to as “embodiments”) will be described with reference to the accompanying drawings. Note that same reference signs are used for substantially the same structural elements in the specification and the drawings to avoid duplicated explanations as appropriate. Shapes in the drawings are illustrated in such a manner to be easily comprehended by those skilled in the art, so that dimensions and ratios thereof are not necessarily consistent with the actual ones. 
     First Embodiment 
       FIG. 1  is a plan view illustrating a crystal element  10 ,  FIG. 2  is a plan view viewed through from the back side of the crystal element  10 , and  FIG. 3  is a sectional view taken along the line Ic-Ic of  FIG. 1 .  FIG. 4  is a plan view illustrating a crystal plate  21 ,  FIG. 5  is a sectional view taken along the line IIb-IIb of  FIG. 4 , and  FIG. 6  is a plan view for describing examples of the dimensions of the crystal plate  21 . Hereinafter, explanations will be provided with reference to those drawings. 
     The front side out of the two faces in a front-and-back relation is defined as “first face”, the back side thereof is defined as “second face”, and the dimension in a direction vertically going through the first face and the second face is defined as “thickness”. The crystal element  10  of the first embodiment includes: a vibration part  11  having a first face  111  and a second face  112 ; a flat plate part  12  having a first face  121  and a second face  122 , which is thicker than the vibration part  11  and disposed in an outer edge of the vibration part  11  on a plan view; a fixing part  13  having a first face  131  and a second face  132 , which is thicker than the flat plate part  12  and disposed in an outer edge of the flat plate part  12  on a plan view; excitation electrodes  141 ,  142  disposed on the first face  111  and the second face  112  of the vibration part  11 ; mounting electrodes  151 ,  152  disposed at least on one of the first face  131  and the second face  132  of the fixing part  13 ; and wiring electrodes  161 ,  162  that electrically connect the excitation electrodes  141 ,  142  and the mounting electrodes  151 ,  152 . 
     The first faces  111 ,  121 ,  131  and the second faces  112 ,  122 ,  132  may be configured as follows. The first face  121  of the flat plate part  12  and the first face  131  of the fixing part  13  are on different planes. The second face  122  of the flat plate part  12  and the second face  132  of the fixing part  13  are on different planes. The first face  111  of the vibration part  11  and the first face  121  of the flat plate part  12  are on different planes. The second face  112  of the vibration part  11  and the second face  122  of the flat plate part  12  are on different planes. That is, the first faces  111 ,  121 ,  131  of each of the vibration part  11 , the flat plate part  12 , and the fixing part  13  are disposed in a step-like manner, and the second faces  112 ,  122 ,  133  thereof are also disposed in a step-like manner. 
     On a plan view, it may also be defined that the vibration part  11 , the flat plate part  12 , and the fixing part  13  are in a substantially rectangular shape, the flat plate part  12  is disposed to surround all sides of the rectangular shape of the vibration part  11 , and the fixing part  13  exists only on one side  120  of the rectangular shape of the flat plate part  12 . When the flat plate part  12  is disposed to surround all sides of the vibration part  11 , the vibration part  11  comes to form a recessed part that is provided in the center of the flat plate part  12 . Note here that “substantially rectangular shape” includes a square, a rectangle with four rounded corners, and the like. Furthermore, the flat plate part  12  may surround not all sides of the vibration part  11  but three sides or two sides thereof. In that case, the flat plate part  12  including the vibration part  11  forms a substantially rectangular shape. The fixing part  13  may not be disposed only on the one side  120  of the flat plate part  12  but may by disposed to surround two sides, three sides, or all sides thereof. In that case, the fixing part  13  including the flat plate part  12  forms a substantially rectangular shape. 
     The first face  121  and the second face  122  of the flat plate part  12  may respectively include inclined faces  121   a  and  122   a  that become thinner as leaving away from the fixing part  13 . There may be either one of the inclined faces  121   a  and  122   a . Furthermore, the inclined faces  121   a  and  122   a  are formed by wet-etching when the crystal axis of the crystal plate is set in the manner illustrated in the drawings. 
     The crystal element  10  may further include a through-hole  17  opened through in the thickness direction between the mounting electrodes  151 ,  152  and the vibration part  11 . In the first embodiment, the through-hole  17  is formed in the inclined faces  121   a  and  122   a.    
     In  FIG. 4  and  FIG. 6 , a distance  114  from a center  113  of the vibration part  11  to the fixing part  13  may be defined to be longer than a distance  124  from a center  123  of the flat plate part  12  to the fixing part  13  on a plan view. 
     In  FIG. 4  and  FIG. 6 , assuming that the direction perpendicular to the one side  120  of the flat plate part  12  where the fixing part  13  is disposed is defined as the length direction on a plan view, a distance  125  from the fixing part  13  to the vibration part  11  in the flat plate part  12  may be defined to be one half or more and twice or less of the dimension (length  115 ) of the vibration part  11  in the length direction. 
     In  FIG. 4  and  FIG. 6 , assuming that the direction in parallel to the one side  120  of the flat plate part  12  where the fixing part  13  is disposed is defined as the width direction on a plan view, a distance  126  from the outer edge of the flat plate part  12  to the vibration part  11  in the width direction of the flat plate part  12  may be defined to be larger than the dimension (length  116 ) of the vibration part  11  in the width direction. 
     Next, the crystal element will be described in more detail. 
     The crystal element  10  operates in the thickness-shear vibration mode, and the oscillation frequency (fundamental wave) is 150 MHz or more, for example. The vibration part  11 , the flat plate part  12 , and the fixing part  13  are formed with a single crystal plate  21 . The excitation electrodes  141 ,  142 , the mounting electrodes  151 ,  152 , and the wiring electrodes  161 ,  162  are formed with a metal pattern of a same material. 
     The crystal plate  21  is an AT-cut crystal plate. That is, assuming that a rectangular coordinate system XYZ with the X-axis (electrical axis), the Y-axis (mechanical axis), and the Z-axis (optical axis) in the crystal is rotated by 30° or more and 50° or less (for example, 35°15′) to define a rectangular coordinate system XY′Z′, a wafer cut in parallel to the XZ′ plane is the raw material of the crystal plate  21 . Furthermore, the longitudinal direction of the crystal plate  21  is in parallel to the X-axis, the lateral direction is in parallel to the Z′-axis, and the thickness direction is in parallel to the Y′-axis. 
     Referring to  FIG. 6 , examples of the dimensions are as follows. As for the crystal plate  21 , a length  211  is 700 to 100 μm, and a width  212  is 400 μm. As for the vibration part  11 , the length  115  and the width  116  are both 100 μm. The distance  126  from the outer edge of the flat plate part  12  to the vibration part  11  is 150 μm. The length  133  of the fixing part  13  is 50 to 200 μm. 
     A pair of excitation electrodes  141  and  142  is in a substantially elliptical shape on a plan view, and provided at roughly the center of the first face  111  and the second face  112  of the vibration part  11 , respectively. From the excitation electrodes  141 ,  142 , the wiring electrodes  161 ,  162  used for connection but not contributing to excitation are extended to the mounting electrodes  151 ,  152 . That is, the excitation electrode  141  is electrically connected to the mounting electrode  151  via the wiring electrode  161 , and the excitation electrode  142  is electrically connected to the mounting electrode  152  via the wiring electrode  162 . Note that the excitation electrodes  141  and  142  are not limited to be in a substantially elliptical shape but may be in a substantially circular shape, a substantially rectangular shape, or the like, for example. 
     While both of the mounting electrodes  151  and  152  are disposed in the second face  132  of the fixing part  13 , at least one of those may be disposed in the first face  131  of the fixing part  13 . In that case, the mounting electrodes  151  and  152  may be electrically connected to a package or the like via a wire. 
     The metal pattern configuring the excitation electrodes  141 ,  142  and the like forms a laminate including a base layer made of chromium (Cr) and a conductor layer made of gold (Au), for example. That is, the base layer is disposed on the crystal plate  21 , and the conductor layer is disposed on the base layer. The base layer mainly plays a role for providing an adhesion force with respect to the crystal plate  21 . The conductor layer mainly plays a role for providing electrical conduction. 
     As the metal pattern manufacturing steps, there are following methods in which “to provide a metal film” is referred to as “to deposit a film (deposition)”. The methods may be: a method which deposits a film on the crystal plate  21 , forms a photoresist pattern, and performs etching; a method which forms a photoresist pattern on the crystal plate, deposits a film, and performs lift-off, a method which deposits a film while covering the crystal plate with a metal mask, and the like. For the deposition, sputtering, vapor deposition, or the like is used. 
     The crystal element  10  can be manufactured in a following manner by using photolithography and an etching technique, for example. 
     First, a corrosion resistant film is provided on the entire surface of the AT-cut crystal wafer, and a photoresist is provided thereon. Subsequently, a mask where a pattern of the external shape (including the through-hole  17 ) of the crystal plate  21  and the flat plate part  12  is drawn is superimposed on the photoresist, which is then exposed and developed to expose a part of the corrosion resistant film. In this state, wet-etching is performed on the corrosion resistant film. Thereafter, wet-etching is performed on the crystal wafer by using the remaining corrosion resistant film as the mask to form the external shape (uncompleted) of the crystal plate  21  and the flat plate part  12  (a flat plate part process step). Then, the external shape (uncompleted) of the crystal plate  21  and the vibration part  11  are formed in the same manner (a vibration part process step). Subsequently, the external shape (until being completely etched) of the crystal plate  21  is formed (an external shape process step). Note that “to simultaneously etch the first face and the second face of the crystal plate  21 ” is referred to as “double-sided etching”, and “to etch either one of the first face and the second face of the crystal plate  21 ” is referred to as “one-sided etching”. In each of the steps, double-sided etching is used. 
     Thereafter, the remaining corrosion resistant film is removed from the crystal wafer, and a metal film to be the excitation electrodes  141 ,  142 , and the like is provided on the entire surface of the crystal wafer. Subsequently, a photoresist mask with a pattern of the excitation electrodes  141 ,  142 , and the like is formed on the metal film, and an unnecessary part of the metal film is removed by etching to form the excitation electrodes  141 ,  142 , and the like. Thereafter, an unnecessary part of the photoresist is removed to form a plurality of crystal elements  10  on the crystal wafer. At last, each of the crystal elements  10  is cut out from the crystal wafer into a piece to acquire individual crystal elements  10 . 
     The operation of the crystal element  10  is as follows. An alternating voltage is applied to the crystal plate  21  via the excitation electrodes  141  and  142 . This causes thickness-shear vibration of the crystal plate  21  such that the first face  111  and the second face  112  shift from each other, thereby generating a specific oscillation frequency. As described, the crystal element  10  operates to output signals of a specific oscillation frequency by using the piezoelectric effect and the inverse piezoelectric effect of the crystal plate  21 . At this time, the thinner the thickness of the crystal plate  21  between the excitation electrodes  141  and  142  (that is, the vibration part  11 ), the higher the oscillation frequency. 
     Next, actions and effects of the crystal element  10  will be described. 
     (1) As described above, the crystal element  10  of the first embodiment includes: the vibration part  11  having the first face  111  and the second face  112 ; the flat plate part  12  having the first face  121  and the second face  122 , which has a thickness thicker than the thickness of the vibration part  11  and is disposed in the outer edge of the vibration part  11  on a plan view; the fixing part  13  having the first face  131  and the second face  132 , which has a thickness thicker than the thickness of the flat plate part  12  and is disposed in the outer edge of the flat plate part  12  on a plan view; the excitation electrodes  141 ,  142  disposed on the first face  111  and the second face  112 ; the mounting electrodes  151 ,  152  disposed at least on one of the first face  131  and the second face  132 ; and the wiring electrodes  161 ,  162  that electrically connect the excitation electrodes  141 ,  142  and the mounting electrodes  151 ,  152 . 
     The crystal element  10  of the first embodiment includes the vibration part  11 , the flat plate part  12  that is thicker than the vibration part  11  and disposed in the outer edge of the vibration part  11 , and the fixing part  13  that is thicker than the flat plate part  12  and disposed in the outer edge of the flat plate part  12 , so that it is possible to achieve the structure in which the outer edge of the thin vibration part  11  is supported by the flat plate part  12  and the outer edge of the thick flat plate part  12  is supported by the still thicker fixing part  13 . As a result, even when the vibration part  11  becomes thinner as the oscillation frequency becomes higher, the mechanical strength of the crystal element  10  can be maintained, thereby making it possible to secure a stable electrical characteristic. 
     Now, an example of the effects of the crystal element  10  will be described in a specific manner. A crystal element formed with a vibration part and a fixing part without a flat plate part will be discussed as a comparison example. In this comparative example, the stress is concentrated on the boundary between the thin vibration part and the thick fixing part, and distortion is likely to occur in the thin vibration part. Therefore, when the vibration part becomes still thinner as the frequency becomes higher, the vibration part becomes more likely to be distorted. On the contrary, with the crystal element  10 , the stress generated between the vibration part  11  and the fixing part  13  is dispersed or absorbed in the flat plate part  12 . Therefore, the vibration part  11  is not likely to be distorted even when the vibration part  11  becomes thinner. As the stress source, there may be the gravity, the tension of the metal pattern, or the like. 
     Furthermore, since the vibration part  11  in the crystal element  10  with the oscillation frequency of 150 MHz or more in particular is considerably thin, it is necessary to pay close attention for the damages thereof and the like when mounting. With the crystal element  10 , handleability can be improved by being mounted to a package via the fixing part  13  that is thicker than the flat plate part  12 . 
     (2) The first faces  111 ,  121 ,  131  are disposed in a step-like manner, and the second faces  112 ,  122 ,  132  are also disposed in a step-like manner. In this case, the front side and the back side of the crystal plate  21  are in a similar configuration. Thereby, the tension of the metal pattern on the front side and the tension of the metal pattern on the back side cancel each other, so that distortion of the vibration part  11  can be suppressed further. Moreover, since the flat plate part  12  and the vibration part  11  can be formed by double-sided etching, the etching time per manufacturing step can be shortened to about one half of the case of one-sided etching. 
     Furthermore, such effects are increased by forming a vertically symmetric structure with the front side and the back side of the crystal plate  21 . In addition, the vibration state in the upper half part and the lower half part of the crystal plate  21  becomes the same since the upper side and the lower side are symmetric with respect to the center of gravity of the crystal plate  21 . Therefore, it is possible to improve the vibration balance, and to reduce the CI (crystal impedance) value. 
     (3) The vibration part  11 , the flat plate part  12 , and the fixing part  13  are in a substantially rectangular shape, the flat plate part  12  is disposed to surround all sides of the rectangular shape of the vibration part  11 , and the fixing part  13  is disposed only on the one side  120  of the rectangular shape of the flat plate part  12 . In a case where the vibration part  11  is in a substantially rectangular shape, the etching residue when the vibration part  11  is formed by wet-etching can be easily managed compared to a case where the vibration part  11  is in a substantially circular shape or elliptical shape. It is because the shape of the etching residue becomes simple since the crystal face exposed to the outer edge of the vibration part  11  becomes simple when the vibration part  11  is in a substantially rectangular shape. As a result, it is possible to suppress disconnection of the wiring electrodes  161  and  162  in the outer edge of the vibration part  11 . Furthermore, when the flat plate part  12  is disposed to surround all sides of the vibration part  11 , it is possible to support all sides of the thin vibration part  11  with the thick flat plate part  12  so that distortion generated in the vibration part  11  can be reduced further. 
     (4) The first face  121  of the flat plate part  12  has the inclined face  121   a  that becomes thinner as leaving away from the fixing part  13 , or the second face  122  of the flat plate part  12  has the inclined face  122   a  that becomes thicker as leaving away from the fixing part  13 . In that case, following effects can be achieved. The thickness of the flat plate part  12  (the inclined faces  121   a  and  122   a ) in the vicinity of the fixing part  13  becomes thicker toward the fixing part  13 . Therefore, the stress transferred to the vibration part  11  side from the fixing part  13  side is absorbed or dispersed by the inclined faces  121   a  and  122   a  (gradual step), so that distortion of the vibration part  11  can be suppressed further. Furthermore, since the vibration generated in the vibration part  11  is gradually damped as going toward the fixing part  13 , thereby reducing the influence of the vibration reflected by the fixing part  13  upon the vibration part  11 . Therefore, it is possible to decrease the CI value since the influence of the fixing part  13  for the vibration of the vibration part  11  is suppressed by the cross sectional shape of the flat plate part  12  in the vicinity of the fixing part  13 . Even though only one of the inclined faces  121   a  and  122   a  provided therein, the effect is increased further by providing the both. 
     (5) The through-hole  17  opened through along the thickness direction is further provided between the mounting electrodes  151 ,  152  and the vibration part  11 . In that case, the stress transferred to the vibration part  11  side from the fixing part  13  side is absorbed or dispersed by the through-hole  17 , so that distortion of the vibration part  11  can be suppressed further. In other words, when fixing the fixing part  13  to a package, it is possible to reduce distortion generated in the flat plate part  12  and also reduce distortion generated in the vibration part  11  as a result. 
     Furthermore, by confining the vibration energy of the vibration part  11  in the through-hole  17 , the CI value can be decreased. Moreover, by forming the through-hole  17  in the inclined faces  121   a ,  122   a , such an effect is increased in cooperation with the function of the inclined faces  121   a ,  122   a.    
     (6) The distance  114  from the center  113  of the vibration part  11  to the fixing part  13  is longer than the distance  124  from the center  123  of the flat plate part  12  to the fixing part  13 . In that case, the vibration part  11  is distant from the fixing part  13 , so that influence of the stress when fixing the fixing part  13  to the package can be reduced. 
     (7) The distance  125  from the fixing part  13  to the vibration part  11  in the flat plate part  12  is one half or more and twice or less than the length  115  of the vibration part  11 . When the distance  125  is less than one half of the length  115  of the vibration part  11 , it is likely to have the influence of the stress from the fixing part  13 . When the distance  125  exceeds twice of the length  115  of the vibration part  11 , the flat plate part  12  is likely to be distorted so that the vibration part  11  is likely to be distorted as well. 
     (8) The distance  126  from the outer edge of the flat plate part  12  to the vibration part  11  in the width direction of the flat plate part  12  is longer than the width  116  of the vibration part  11 . In that case, the width direction of the vibration part  11  can be supported by the flat plate part  12  in a sufficient dimension so that distortion generated in the vibration part  11  can be reduced further. 
     Other Examples 
       FIG. 7  to  FIG. 11  are sectional views illustrating crystal plates  21  to  25  of each of first to fifth examples. Hereinafter, explanations will be provided with reference to those drawings. 
     Crystal plates with various sectional shapes can be acquired by forming the flat plate part and the vibration part of the first embodiment by performing either double-sided etching or one-sided etching. Those crystal plates will be described as the first to fifth examples of the first embodiment. 
     The crystal plate  21  of the first example illustrated in  FIG. 7  is the same as those illustrated in  FIG. 3  and  FIG. 5 . Hereinafter, same reference numerals are used for the structural elements of the crystal plates  22  to  25  corresponding to those of the crystal plate  21 . 
     When the flat plate part  12  is formed by double-sided etching, the first face  121  of the flat plate part  12  and the first face  131  of the fixing part  13  are on different planes, and the second face  122  of the flat plate part  12  and the second face  132  of the fixing part  13  are on different planes. 
     When the flat plate part  12  is formed by one-sided etching, the first face  121  of the flat plate part  12  and the first face  131  of the fixing part  13  are on a same plane, and the second face  122  of the flat plate part  12  and the second face  132  of the fixing part  13  are on a same plane. Alternatively, the second face  122  of the flat plate part  12  and the second face  132  of the fixing part  13  are on a same plane while the first face  121  of the flat plate part  12  and the first face  131  of the fixing part  13  are on different planes. 
     When the vibration part  11  is formed by double-sided etching, the first face  111  of the vibration part  11  and the first face  121  of the flat plate part  12  are on different planes, and the second face  112  of the vibration part  11  and the second face  122  of the flat plate part  12  are on different planes. 
     When the vibration part  11  is formed by one-sided etching, the first face  111  of the vibration part  11  and the first face  121  of the flat plate part  12  are on a same plane while the second face  112  of the vibration part  11  and the second face  122  of the flat plate part  12  are on different planes. Alternatively, the second face  112  of the vibration part  11  and the second face  122  of flat plate part  12  are on a same plane while the first face  111  of the vibration part  11  and the first face  121  of the flat plate part  12  are on different planes. 
     As for the crystal plate  21  of the first example illustrated in  FIG. 7 , the flat plate part  12  and the vibration part  11  are both formed by double-sided etching. As described in the first embodiment, it is required to perform etching three times, which are “double-sided etching of the flat plate part  12 +double-sided etching of the external shape”→“double-sided etching of the vibration part  11 +double-sided etching of the external shape”→“double-sided etching of the external shape”. 
     As for the crystal plate  22  of the second example illustrated in  FIG. 8 , the flat plate part  12  is formed by double-sided etching, and the vibration part  11  is formed by one-sided etching. In this case, it is required to perform etching only twice, which are “double-sided etching of the flat plate part  12 +double-sided etching of the external shape”→“one-sided etching of the vibration part  11 +double-sided etching of the external shape”. While the vibration part  11  is formed on the first face  121  of the flat plate part  12  in the second example, the vibration part  11  may be formed on the second face  122  of the flat plate part  12 . In that case, it is possible to have the same sectional shape as that of the second example by inverting the crystal plate. 
     As for the crystal plate  23  of the third example illustrated in  FIG. 9 , the flat plate part  12  is formed by one-sided etching, and the vibration part  11  is formed by double-sided etching. In this case, it is required to perform etching only twice, which are “one-sided etching of the flat plate part  12 +double-sided etching of the external shape”→“double-sided etching of the vibration part  11 +double-sided etching of the external shape”. While the flat plate part  12  is formed on the second face  132  side of the fixing part  13  in the third example, the flat plate part  12  may be formed on the first face  131  side of the fixing part  13 . In that case, it is possible to have the same sectional shape as that of the third example by inverting the crystal plate (same for the fourth and fifth examples). 
     As for the crystal plate  24  of the fourth example illustrated in  FIG. 10 , the flat plate part  12  and the vibration part  11  are both formed by one-sided etching. In this case, it is required to perform etching only twice, which are “one-sided etching of the flat plate part  12 +one-sided etching of the external shape”→“one-sided etching of the vibration part  11 +double-sided etching of the external shape”. 
     As for the crystal plate  25  of the fifth example illustrated in  FIG. 11 , the flat plate part  12  and the vibration part  11  are both formed by one-sided etching like the crystal plate  24  of the fourth example. Note, however, that the vibration part  11  is formed on the second face  122  of the flat plate part  12  in the fifth example, while the vibration part  11  is formed on the first face  121  of the flat plate part  12  in the fourth example. In this case, it is required to perform etching only twice, which are “one-sided etching of the flat plate part  12 +one-sided etching of the external shape”→“one-sided etching of the vibration part  11 +double-sided etching of the external shape”. 
     The configuration, operations, and effects of the crystal element that includes the crystal plates  21  to  25  according to each of the examples are the same as those of the crystal element of the first embodiment. 
     Second Embodiment 
       FIG. 12  is a perspective view illustrating a crystal device  60  according to a second embodiment, and  FIG. 13  is a sectional view taken along the line IVb-IVb of  FIG. 12 . Hereinafter, a crystal device including the crystal element of the first embodiment will be described as the crystal device  60  of the second embodiment by referring to those drawings. 
     As illustrated in  FIG. 12  and  FIG. 13 , the crystal device  60  according to the second embodiment includes: the crystal element  10  of the first embodiment; a base body  61  where the crystal element  10  is disposed; and a lid  62  that, together with the base body  61 , airtightly seals the crystal element  10 . The base body  61  is also referred to as a package, which is configured with a substrate  61   a  and a frame  61   b . The space surrounded by the top face of the substrate  61   a , the inner face of the frame  61   b , and the bottom face of the lid  62  forms a housing part  63  of the crystal element  10 . The crystal element  10  outputs reference signals used in an electronic device and the like, for example. 
     In other words, the crystal device  60  includes: the substrate  61   a  that includes a pair of electrode pads  61   d  on its top face and four external terminals  61   c  on its bottom face; the frame  61   b  disposed along the outer peripheral edge of the top face of the substrate  61   a ; the crystal element  10  mounted on the pair of electrode pads  61   d  via a conductive adhesive  61   e ; and the lid  62  that, together with the frame  61   b , airtightly seals the crystal element  10 . 
     The substrate  61   a  and the frame  61   b  are made of a ceramic material such as alumina ceramics or glass ceramics, for example, and integrally formed into the base body  61 . The base body  61  and the lid  62  are roughly in a substantially rectangular shape on a plan view. The external terminals  61   c , the electrode pads  61   d , and the lid  62  are electrically connected via a conductor formed on the inner side or the side face of the base body  61 . More specifically, each of the external terminals  61   c  is disposed at the four corners of the bottom face of the substrate  61   a . Two external terminals  61   c  among those are electrically connected to the crystal element  10 , and the remaining two external terminals  61   c  are electrically connected to the lid  62 . The external terminals  61   c  are used for being mounted to a printed wiring board or the like of an electronic device and the like. 
     As described above, the crystal element  10  includes: the crystal plate  21 ; the excitation electrode  141  formed on the top face of the crystal plate  21 ; and the excitation electrode  142  formed on the bottom face of the crystal plate  21 . Then, the crystal element  10  is bonded on the electrode pads  61   d  via the conductive adhesive  61   e , and plays a role to oscillate the reference signals of the electronic device and the like by the stable mechanical vibration and piezoelectric effect. 
     The electrode pad  61   d  is for mounting the crystal element  10  in the base body  61 , and a pair of which is disposed adjacent to each other along one side of the substrate  61   a . The pair of electrode pads  61   d  connects the mounting electrodes  151  and  152 , respectively, and fixes the crystal element  10  on the substrate  61   a  with a cantilever support structure with one end of the crystal element  10  being a fixed end and the other end of the crystal element  10  being the free end isolated from the top face of the substrate  61   a.    
     As for the conductive adhesive  61   e , conductive powder as a conductive filler is contained in a binder such as a silicone resin. The lid  62  is formed with an alloy containing at least iron, nickel, or cobalt, for example, and airtightly seals the housing part  63  in a vacuum state or filled with a nitrogen gas or the like by being joined to the frame  61   b  by seam welding or the like. 
     With the crystal device  60  that includes the crystal element  10 , it is possible to provide a stable electrical characteristic. The crystal device  60  is mounted on the surface of the printed board configuring an electronic device by fixing the bottom faces of the external terminals  61   c  to the printed board by soldering, gold (Au) bumps, a conductive adhesive, or the like. Furthermore, the crystal device  60  is used as an oscillation source in various kinds of electronic devices such as a smartphone, a personal computer, a clock, a game console, a communication device, or an in-vehicle device such as a car navigation system, for example. 
     Others 
     While the present disclosure has been described above by referring to the embodiments, the present disclosure is not limited thereto. As for the configuration and details of the present disclosure, various modifications occurred to those skilled in the art can be applied. For example, while the shape of the vibration part is described to be in a substantially rectangular shape, it is also possible to be in other patterns (a circular shape, an elliptical shape, a polygonal shape, and the like). 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2019-101757, filed on May 30, 2019, the entire contents of which are incorporated herein by reference.