Abstract:
A surface mount quartz crystal resonator includes a quartz crystal-based plate including a central portion adapted to resonate at a desired frequency, and a border substantially surrounding a peripheral region of the central portion. The border includes a first region physically separated from the central portion, and a second region joined to the central portion. A base plate is provided which is secured to the plate so that the central portion of the plate is free to resonate relative to the base plate. A cover plate is provided and is secured to the plate so that the plate is located between the base plate and the cover plate. At least one, and preferably both, of the base plate and the cover plate are made of quartz.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to quartz crystal resonators and methods for making same. More particularly, the invention relates to surface mount quartz crystal resonators, and methods of making such resonators, which are straightforward in construction, inexpensive to manufacture, and effective and durable in use. 
     Quartz crystal resonators, because of their frequency accuracy and stability, are indispensable in modern electronics, for example, in telecommunications, computers, entertainment equipment and the like, as well as in other applications, many of which are well known. As used herein, a quartz crystal resonator, is a device comprising a piezoelectric quartz crystal element in the form of a thin plate, for example, a circular or rectangular plate, and an enclosure which can be sealed by some means to form a hermetic seal. Electrical terminals are provided which pass into the enclosure to provide the means to apply an alternating voltage across the quartz crystal element causing the element to vibrate. The piezoelectric quartz element has a set of thin conductive metallic electrodes deposited onto its major surfaces. The over lapping area of the electrode on one side of the plate with that of the electrode on the other side of the plate defines the resonating portion of the quartz element. The piezoelectric quartz crystal element resonates when the alternating voltage has the frequency of the resonant frequency of the quartz element, is applied. The resonant frequency of the quartz crystal element is determined by the piezoelectric and elastic constants of quartz, the dimensions of the quartz element, the metallic electrodes and other secondary factors. 
     Conventionally, surface mount quartz crystal resonators were made up of an electroded piezoelectric quartz crystal plate and a ceramic enclosure or base. The quartz crystal resonator plate is fixed in or on the ceramic enclosure by electrically conductive epoxy applied at two points on one end of the quartz crystal plate. A metal cover is welded to a metal flange on the ceramic enclosure. Alternately, a ceramic cover, is joined to the ceramic enclosure or base by means of adhesive or by reflow of low melting point glass. 
     The ceramic enclosures or bases are of laminated ceramic construction which employs a combination of cofired metallic depositions, metal vias and ceramic-to-metal seals. These ceramic components require a high level of technology to manufacture, are comparatively expensive, and have historically been in short supply. 
     It would be advantageous to provide surface mount quartz crystal resonators which are straightforward in construction, inexpensive to manufacture and effective and durable in use. 
     SUMMARY OF THE PRESENT INVENTION 
     New surface mount quartz crystal resonators and methods for making same have been developed. The present resonators are straightforward in construction, relatively inexpensive to manufacture, effective in use, for example, in electronic equipment such as computers, handheld cell phones, wireless control and data transmission systems and the like, and do not rely on materials or components which have historically been in short supply. For example, the present surface mount quartz crystal resonators do not require, and preferably do not include, the ceramic enclosures or bases referred to above. Thus, the present invention avoids dealing with such ceramic enclosures or bases and the problems attendant thereto. Preferably, the present quartz crystal resonators are encapsulated in a base plate of quartz and a cover plate of quartz. Importantly, the present surface mount quartz crystal resonators when installed in the application circuit have substantial, preferably enhanced, resistence to shock and vibration. Also, the present resonators can be produced with a reduced height and/or profile relative to the resonators of the prior art. The present methods of producing surface mount quartz crystal resonators are straightforward to practice and provide a cost effective approach to producing surface mount quartz crystal resonators. 
     One important aspect of the present invention relates to resonator plates, which are a major component of the present quartz crystal resonators. In general, the present resonator plates comprise a quartz crystal-based plate or plate member including a central portion or region having a peripheral region, for example, around the width and length of the central portion. The central portion is adapted to resonate at a desired frequency, preferably in response to an alternating voltage being applied across the central region. A border or border portion is provided which substantially surrounds the peripheral region of the central portion. The border includes a first region physically separated or spaced apart from the central region, and a second region joined to the central portion. 
     In use, the central portion of the quartz crystal-based plate resonates at a desired frequency preferably in response to the application of an alternating voltage, while the border of the plate remains substantially stationary, as will be described hereinafter. Thus, only a portion of the quartz crystal-based plate resonates. The other portion, that is the border, of the plate is used to support the resonating central portion and to provide part of the housing or enclosure of the surface mount resonator. 
     The first region of the border which is physically separated from the central portion of the plate member preferably is formed by removal of quartz from a solid quartz crystal plate. In one embodiment, a solid quartz crystal plate is provided and a quantity of quartz is removed, for example, forming a slot, so that the first region of the border is spaced apart, for example, by the formed slot, from the resonating central portion. 
     The outer periphery of the quartz crystal-based plate may be of any suitable geometric shape, for example, suitable for use in a surface mount quartz crystal resonator. Particularly useful geometric shapes include a substantially circular shape, a substantially rectangular shape and the like. In one particularly useful embodiment, the quartz crystal-based plate has a rectangular outer periphery and includes a slot located between the central portion and the first region. The slot is located inwardly of the outer periphery along at least three sides of the rectangular outer periphery. 
     The central region of the quartz crystal-based plate preferably is provided with electrodes to facilitate the application of an alternating voltage. In one particularly useful embodiment, a first electrode is provided on the top surface of the central portion and a second electrode is provided on an opposing bottom surface of the central portion. 
     The thickness of the central portion may be substantially uniform or may be variable. In one very useful embodiment, the thickness of the central portion preferably is reduced in the region or regions of the central portion which are outside of the resonant region defined by the overlapping electrodes. For example, the thickness of the central region may be reduced in one or more regions of the central portion on which neither first nor second electrode is provided. This feature will be discussed in detail hereinafter. 
     In another very useful embodiment the thickness of the central portion is essentially uniform but is substantially or significantly reduced relative to the thickness of the border. This feature will be discussed in detail hereinafter. 
     In another broad aspect of the present invention, resonator assemblies are provided which comprise a quartz crystal-based plate or plate member, as described elsewhere herein, and a base plate secured to the plate so that the central portion of the plate is free to resonate relative to the base plate, for example, in response to an appropriate alternating voltage being applied thereto, across the plate. The base plate is secured to the border of the quartz crystal-based plate, preferably along substantially the entire outer portion of the border. This securement of the base plate to the border of the quartz crystal-based plate provides a substantially strong mechanical bond between the plate member and the base plate. This enhances the durability of the present resonators, for example, relative to the prior art resonators, which enhances the effective life of the present resonators. 
     Although the base plate and quartz crystal-based plate can be secured using various techniques, it is preferred that adhesives be employed. Thus, the assembly preferably includes an adhesive located between the base plate and the border of the quartz crystal-based plate. This adhesive is effective in securing the base plate to the border. A suitable adhesive may be employed. One particularly useful class of adhesives are epoxy-based adhesives. 
     Although the base plate may be comprised of any suitable material of construction, for example, metals, glasses, ceramics and the like, the preferred material of construction is quartz. The use of quartz is very effective in reducing costs while substantially matching the physical characteristics of the quartz crystal-based plate. 
     The base plate preferably includes a plurality of base electrodes positioned so that one base electrode is in electrical connection with the first electrode of the central portion of the plate member and another base electrode is in electrical connection with the second electrode of the central portion. Such base electrodes are very effective in providing the alternating voltage signal from a remote source to the resonating central portion of the plate member. 
     The electrodes described herein may be constructed of any suitable electrically conductive material. However, it is preferred that such electrodes comprise metals. The electrodes can be provided in any suitable manner. Preferably, the electrodes are provided by vacuum deposition onto the surface, as desired. 
     Quartz crystal resonators, in accordance with the present invention, include the quartz crystal-based plate and base plate, as described elsewhere herein, and, in addition, a cover plate secured to the quartz crystal-based plate so that the plate is located between the base plate and the cover plate. Preferably, the base plate and the cover plate are both secured to the border of the quartz crystal-based plate. More preferably, both the base plate and the cover plate are secured to substantially the entire outer portion of the border so that the resonator is firmly mechanically bonded together and the resonating central portion of the quartz crystal-based plate is hermetically sealed or enclosed. 
     In one embodiment, a first adhesive is provided which is located between the base plate and the border and is effective in securing the base plate to the border, and a second adhesive is provided and located between the cover plate and the border and is effective in securing the cover plate to the border. The compositions of the first and second adhesives may be the same or different, preferably the same. 
     Although any suitable material may be employed as the cover plate, the cover plate preferably comprises quartz. Thus, in one particularly useful embodiment, the quartz crystal-based plate, base plate and cover plate all comprise quartz. In one useful embodiment, at least one of the base plate and the cover plate includes an outwardly extending recess. This feature will be described in more detail hereinafter. 
     In another broad aspect of the present invention, methods for producing quartz crystal resonators are provided. Such methods include providing a solid quartz crystal plate. Quartz is removed from the solid quartz crystal plate to form a quartz crystal plate member including a central portion, a border and a space, preferably a slot, between the central portion and the border including a first region separated from the central portion and a second region joined to the central portion. First and second electrodes are placed on the top surface and the opposing bottom surface of the plate member, respectively. The plate member is secured to an electroded base plate so that the central region is free to resonate relative to the base plate, preferably in response to an alternating voltage being applied to the central region. The plate member is secured to a cover plate so that the plate member is located between the base and the cover plate. 
     In one embodiment, the base plate and the cover plate both comprise quartz and the securing steps include the use of adhesives to secure the plate member to the base plate and the plate member to the cover plate, respectively. The securing steps are effective to both mechanically bond the base plate, the plate member and the cover plate together, and form a hermetically sealed periphery. Electrically conductive adhesive, preferably electrically conductive epoxy adhesive, is employed to make contacts between the electrodes which are deposited on the central portion and the electrodes which are deposited onto the base plate which complete the electrical circuit of the resonator. 
     Each and every feature described herein, and each and every combination of two or more of such features, is included within the scope of the present invention provided that the features included in such a combination are not mutually inconsistent. 
     These and other aspects and advantages of the present invention are set forth in the following detailed description and claims, particularly when considered in conjunction with the accompanying drawings in which like parts bear like reference numerals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top front view, in perspective, of a piezoelectric quartz crystal resonator plate in accordance with the present invention; 
     FIG. 2 is a perspective illustration of the plate shown in FIG. 1 with conductive metallic electrodes coating the major surfaces; 
     FIG. 3A is a perspective illustration of the upper internal surface electrode pattern of a quartz base plate in accordance with the present invention; 
     FIG. 3B is a perspective illustration of the lower external surface electrode pattern of the quartz base plate in accordance with the present invention; 
     FIG. 4 is a perspective illustration of the piezoelectric quartz resonator plate shown in FIG. 2 bonded to the quartz base plate; 
     FIG. 5 is a perspective illustration of a surface mount quartz crystal resonator in accordance with the present invention; 
     FIG. 6 is a cross-sectional view of an alternate embodiment of a surface mount quartz crystal resonator in accordance with the present invention; and 
     FIGS. 7A and 7B are a partial top view and a cross-sectional view, respectively, of another piezoelectric quartz resonator plate in accordance with the present invention. 
     FIGS. 8A and 8B are a partial top view and a cross-sectional view, respectively, of a further piezoelectric quartz resonator plate in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a rectangular piezoelectric quartz crystal plate  10  is shown. A decoupling slot  11  has been formed in the plate  10  by removing a narrow section of quartz along three sides of the plate  10  and partially along fourth side  14 . The forming of the decoupling slot  11  creates a central resonant part  13  of quartz and a border  12  of quartz around the peripheral region  13 A of central resonant part. The border  12  includes a first region  12 A which is spaced apart, by slot  11 , from resonant part  13 , and a second region  12 B which is joined to the resonant part. The amount of removal along the fourth side  14  of the plate  10  depends on how strong the second region  12 B joining the border  12  and the resonant part  13  is desired to be. Second region  12 B is strongest if no quartz is removed parallel to the fourth side  14  of the plate  10 . In the embodiment of the invention being described, the length of the quartz plate  10  is shown to be parallel with the X Axis, referred to as the crystallographic axes of the quartz crystal and the AT-cut of quartz crystal which is commonly employed for high frequency quartz crystal resonators. The width of plate  10  is parallel to the Z′ Axis and the thickness of the plate is parallel to the Y′ Axis. 
     Without wishing to limit the invention, typical dimensions of quartz plate  10  include a length in the range of about 3.2 mm to about 12 mm, for example, about 7.5 mm; a width in the range of about 2.5 mm to about 5.5 mm, for example, about 5 mm; and a thickness dependent on the resonating frequency according to the following relationship              t   =     1.65   F             EQN   .              1                                
     where t is thickness in mm and F is the resonating frequency in MHZ which can often range from about 8 MHz to about 50 MHz. 
     With the plate  10  formed as shown in FIG. 1, stresses which are applied to the sides of the plate  10  to the first region  12 A of border  12  outside of the decoupling slot  11  have no influence on the resonant characteristics of the resonant part  13 . Stresses applied to the fourth side  14  result in some strain in the resonant part  13 . However, because the resonant part  13  is free to move without interference from the other three sides of the border  12  of plate  10 , the effect of the strain from the fourth side  14  on the resonant characteristics are very small, if noticeable at all. 
     The forming of the quartz plate  10  in the way shown in FIG. 1 provides a resonant part  13  of the plate  10  which is substantially mechanically isolated from the border  12  of the plate so that the border of the plate may be incorporated into the enclosure structure of the quartz crystal resonator to be made from plate  10  without detriment to the resonant characteristics of the final product. 
     In order to apply an electrical signal across the resonant part  13  of the quartz plate  10 , conductive metallic electrodes  14  and  15  are vacuum deposited opposite each other, on the top or first major surface  13 B and the bottom or second major surface  13 C, respectively, of resonant part  13  of the quartz plate  10 , as is shown in FIG.  2 . 
     The dimensions of the electrodes  14 ,  15  depend on the values of the parameters of the equivalent electrical circuit which the final quartz crystal resonator is being designed to meet and the size limitations that may apply because of the application involved. However, the thickness of the electrodes  14 ,  15  and the density of the metal that is employed for the electrodes  14 ,  15  are primary factors that determine the reduction in frequency of the resonant part  13  of the quartz plate  10  from the frequency that is apparent when no electrodes have been applied to the resonant part  13 , which is referred to as the unelectroded frequency. The ratio of the amount of the reduction of the resonant frequency to the unelectroded frequency is commonly called the mass loading of the electrode, which is expressed by the equation,                Δ   =           f   u     -     f   e         f   u       =     mass                 loading         ,           EQN   .              2                                
     where f u  is the unelectroded frequency of the resonant part, and f e  is the frequency of the electroded resonant part. A result of acoustic wave considerations shows that the thickness shear wave that is driven between the electrodes  14 ,  15  at frequency f e  cannot propagate into the unelectroded areas of the resonant part  13  and the amplitude of the acoustic displacement exponentially decreases as the wave radiates towards the edges  18 ,  19  of the resonant part  13 . 
     Since the energy of the wave is proportional to the square of the acoustic displacement, the energy of the wave also exponentially decreases as it radiates from the electrode edges  16 ,  17  toward the edges  18 ,  19  of the resonant part  13  of the quartz plate  10 . This phenomenon is termed energy trapping and is well known in the quartz device industry. Energy that reaches the edges  18 ,  19  of the resonant part  13  is lost from the resonator either by dispersion or absorption of the acoustic wave. The larger the value of Δ, the greater the rate of exponential decreasing of the amplitude of the acoustic displacement and the greater the amount of energy trapping. When the amount of acoustic energy which is lost by inadequate energy trapping is large, then the equivalent series resistance is large. 
     It is normally desired that a quartz crystal resonator have a relatively small equivalent series resistance. Therefore, the value of Δ and the length of quartz plate between the edges of the electrodes  16 ,  17  and the edges  18  and  19  of the resonant part  13  are design considerations in determining the dimensions of the quartz plate  10  and the resonant part  13  so that the quartz crystal resonator meets the requirements of the intended application, such as they may be. 
     Conductive metallic appendages extend from the top electrode  14  and the bottom electrode  15  to terminal electrode areas  20 ,  21  of the quartz crystal resonator plate  10 . The significance of the terminal electrode areas  20 ,  21  is that they line up with base terminal electrode areas  26 ,  27  on the upper internal surface  23  of the quartz base plate  22  which is shown in FIG.  3 . Conductive epoxy is applied so that it connects the terminal electrode area  20  to base terminal electrode area  26  and terminal electrode area  21  to base terminal electrode area  27 . 
     The quartz plate  10 , including electrodes  14  and  15 , is bonded to quartz base plate  22 , which is shown in FIGS. 3A and 3B, using a conventional epoxy adhesive. The quartz base plate  22  has about the same crystallographic orientation as the quartz crystal plate  10  so that the thermal expansion characteristics of the base plate and the plate  10  are substantially or essentially the same. However, for applications having somewhat less stringent requirements or specifications, base plates of quartz having crystallographic orientations dissimilar from the plate  10  or of materials other than quartz can be employed. FIG. 3A shows the upper internal surface  23  of base plate  22 , while FIG. 3B shows the lower external surface  24  of the quartz base plate  22 . 
     The lateral dimensions of the quartz crystal plate  10  are essentially the same as the quartz base plate  22 . The metallic electrode pattern  25 ,  28  on the upper internal surface  23  is such that metallic electrode leads extend from each of the terminal electrode areas  26  and  27  to metallic electrodes  25  and  28 , respectively, which in turn wrap around the edges of the base plate  22  to connect to terminal electrode areas  29  and  30 , respectively, on the lower external surface  24  of the quartz base plate  22 . A conventional conductive epoxy adhesive is applied so that it connects the terminal electrode areas  20 ,  21  on the quartz plate  10  with base terminal electrode areas  26 ,  27  on the upper internal surface  23 , which are in turn connected via the metallic electrode pattern with the terminal electrode areas  29 ,  30  on the lower external surface  24 . The metallic electrodes  14 ,  15  which drive the resonant part  13  of the quartz plate  10  are thus connected to terminals  29 ,  30  on the lower external surface  24  of the base plate  22 . 
     In FIG. 3B, four terminal electrode areas  29 ,  30 ,  31 ,  32  are shown on the lower external surface  24  of the quartz base plate  22 . However, only two terminal areas  29 ,  30  are part of the electrical circuit. The two other terminal areas  31 ,  32  are functional only in that they are soldered or fixed to the application printed circuit board and aid in locating and holding the final surface mount quartz resonator in place. The number of electroded terminal areas could be reduced to two and their location on the lower external surface  24  would be that which best fits the requirements of the application. The conductive metallic electrode patterns on the surfaces  23  and  24  of the quartz base plate  22  are vacuum deposited thin metallic films. However, they can be placed on the surfaces by other means as well. 
     The assembly of the quartz crystal plate  10  to the quartz base plate  22  utilizes conventional epoxy adhesive  34  as is shown in FIG.  4 . Epoxy adhesive  34  is applied to the perimeter of either the plate  10  or the base plate  22 . The application of the epoxy adhesive around the plate perimeter forms epoxy adhesive  34  into a frame having a width is less than the width of the border  12  of the quartz crystal plate  10 . The plate  10  and base plate  22  are then positioned one on top of the other and seated so that the epoxy adhesive  34  completely contacts the facing surfaces of both plates. Care is taken to insure that no epoxy adhesive bridges the slot  11  in the quartz plate  10  between the border  12  and the resonant part  13 . The epoxy adhesive  34  has the dual purpose of mechanically bonding the two plates  10  and  22  together and forming a hermetic seal around the joining perimeter. Note that the outer perimeter  33  of adhesive layer  34  substantially coincides with the outer perimeters of plates  10  and  22 . The thickness of the epoxy adhesive layer  34  is sufficient to keep the resonant part  13  from touching or making contact with the quartz base plate  22  when the part  13  is resonating. 
     Conventional conductive epoxy  36  is applied so that it connects the terminal electrode areas  20  and  21  of the quartz plate  10  and the terminal electrode areas  26  and  27 , respectively, on the base plate  22 . After the application of the epoxy adhesive  34  and  36 , the adhesives are allowed to cure in accordance with the specification of the manufacturers of the adhesive. Adhesives  34  and  36  can be cured at the same time. 
     After the adhesives  34  and  36  have been properly cured, the assembly  39  of the plate  10  and base plate  22  may be tested before further processing. This can be accomplished by contacting the terminal electrode areas  29 ,  30  and using the appropriate instruments for performing the tests that are required. As is normal in the case of quartz crystal resonators, the frequency preferably is adjusted to the required frequency before completing the assembly and sealing on the cover. 
     As shown in FIG. 5, the quartz cover plate  35  has essentially the same dimensions as the quartz base plate  22  but the quartz cover plate has no functional electrodes. The quartz cover plate  35  is transparent and the electrodes of the quartz crystal plate  10  and the quartz base plate  22  can be seen through the cover plate. However, the surface of the quartz cover plate  35  is used for marking the device for identification. 
     The quartz cover plate  35  is bonded to the assembly of the quartz plate  10  and the quartz base plate  22  in substantially the same way the quartz base plate  22  was joined to the quartz resonator plate. Conventional epoxy adhesive  38  is applied to the border  12  of the quartz plate  10 . The width of the application of epoxy adhesive  38  around the perimeter of the quartz plate  10  is narrower than the border  12  to insure that excess epoxy adhesive does not bridge the slot  11  between the border  12  and the resonant part  13  of the quartz plate  10 . If excess epoxy adhesive should be inadvertently placed onto the resonant part  13  the resonant characteristics would be detrimentally affected depending on how much epoxy adhesive was so placed. The outer perimeter  37  of epoxy adhesive  38  substantially coincides with the outer perimeter of plate  10 , base plate  22 , and cover plage  35 . 
     After the epoxy adhesive  38  is applied, the cover plate  35  is then placed on top of the assembly and seated so that the perimeter of the cover plate  35  is completely in contact with the epoxy adhesive  38  and no voids are present. This latter step is performed in a glove box which contains dry nitrogen gas so that the quartz crystal resonator part  13  is hermetically sealed and filled with the inert gas dry nitrogen. The epoxy adhesive  38  is cured in the same inert gas in accordance with the specifications of the adhesive manufacturer. 
     The epoxy adhesive has a certain viscosity and surface tension which supports the three quartz plates  22 ,  10  and  35  during assembly and keeps them from touching after cure. It is important the central resonant part  13  not be in contact with either the quartz base plate  22  or the quartz cover plate  35 . Such contact may prevent resonance from occurring or result in high equivalent series resistance of the resonator. If for any reason the design may require that the central resonant part be thicker than the perimeter border then the central part of the quartz cover and base plate can be recessed sufficient, as described hereinafter, so that contact is avoided. 
     A feature of the invention lies in the fact that the temperature coefficients of expansion of the quartz crystal plate  10 , the quartz base plate  22  and the quartz cover plate  35  are the same so the stresses which arise from the use of dissimilar materials for the base and cover according to old art are avoided. 
     A feature of this invention, resulting from the structure of the resonator assembly  40 , is that the resonant part  13  of the quartz crystal resonator plate  10  is supported within the cavity which is formed by the border  12  of the quartz plate  10 , the quartz base plate  22 , the cover plate  35  and the thicknesses of the epoxy adhesives  33  and  38  around the perimeter bonding the assembly together. The quartz plate  10  is not separated from the base plate  22  and the cover plate  35 , as is often the case with the prior art. The strength of the quartz supporting the resonant part  13  is much greater than two small dots of conductive epoxy which supports the resonator element of surface mount quartz crystal resonators in the prior art. 
     FIG. 6 illustrates an alternate embodiment of a surface mount quartz crystal resonator assembly in accordance with the present invention. Except as expressly described herein, this alternate resonator assembly, shown generally at  140 , is structured and functions in a manner similar to that described previously with regard to resonator assembly  40 . Components of alternate resonator  140  which correspond to components of resonator assembly  40  are identified by the same reference numeral increased by 100. 
     With reference to FIG. 6, the primary difference between alternate resonator assembly  140  and resonator assembly  40  is that the cover plate  135  and the base plate  122  include recesses. In particular, base plate  122  includes an outwardly extending recessed area  50 , and cover plate  135  includes an outwardly extending recess area  52 . These recessed areas or regions  50  and  52  can be produced by conventional methods. Such recessed areas are designed to provide additional space within which resonate part  113  can resonate without coming in contact with either the base plate  122  or the cover plate  135 . This embodiment is particularly useful when it is desired to reduce the size, for example, the profile, of the resonator. 
     FIGS. 7A and 7B illustrate another embodiment of a quartz crystal plate in accordance with the present invention. Except as expressly described herein, this other quartz crystal plate, shown generally at  210 , is structured and functions in a manner similar to that previously described with regard to plate  10 . Components of other plate  210  which correspond to components of plate  10  are identified by the same reference numeral increased by 200. 
     The other plate  210  deals with the energy trapping phenomenon, which has been previously discussed with reference to FIG.  2 . In some applications, which require small overall size, the design of the resonant part  213  of the quartz resonator plate  210  require a very large value of Δ to achieve the necessary energy trapping for the design to result in an acceptably low value of the equivalent resistance. Normally the desired value of Δ, mass loading is achieved by the thickness of the electrode  14  which is deposited onto the resonant part  13  of the quartz plate  10 . However, because the metallic electrode material has a much lower internal mechanical Q than quartz, using electrodes which are relatively thick increases the equivalent resistance. 
     An alternative to increasing the electrode thickness is shown in FIGS. 7A and 7B. The frequency of the thickness shear mode which is employed in this embodiment of the invention is inversely proportional to the thickness of the quartz resonator plate  210  and can be expressed by the EQN. 1, noted above. If the thickness of the resonant part  213  is reduced, as shown in FIG. 7, between the edges of the electrode  216 ,  217  and the edges of the resonant part  218 ,  219  then the frequency in those regions  60 ,  62  will be much higher than the frequency in the electroded region  64 . For example, if the thickness is decreased by 10% then the frequency, ignoring other factors, will increase by about 10%. Since the mass loading, Δ, as given by EQN. 2, is proportional to the difference between the frequency in the unelectroded region  60 ,  62  and the frequency in the electroded region  64 , then the mass loading Δ can be increased without having excessively thick electrodes but by decreasing the thickness of the quartz plate outside of the electroded area  64 . This method enables the designs to achieve high levels of energy trapping and good values of equivalent resistance, and still be of small, compact size. 
     Quartz resonators of higher frequency are becoming of increased importance. However, as given in EQN. 1 as the frequency of the resonator increases its thickness must correspondingly decrease. The thinner the quartz plate the more fragile it becomes and the more difficult it is to process through the manufacturing process. One alternative is to use overtones of the fundamental frequency which allows for a thicker plate to be used for higher frequencies. However, the equivalent circuit of an overtone mode resonance has higher equivalent resistance and much lower motional capacitance than the fundamental mode resonance. For this reason in many applications the fundamental mode is required. 
     FIGS. 8A and 8B illustrate a further embodiment of a quartz crystal plate in accordance with the present invention. Except as expressly described herein, this further quartz crystal plate, shown generally at  310 , is structured and functions in a manner similar to previously described with regard to plate  10 . Components of further plate  310  which correspond to components of plate  10  are identified by the same reference numeral increased by 300. 
     As shown in FIGS. 8A and 8B, the central portion  313  of the quartz crystal plate  310  is reduced in thickness so that its resonant frequency meets that required by the application. This may be accomplished using any suitable technique, for example, by selectively etching the central portion  313 , while leaving the border  312  to be much thicker and stronger. The thickness transition  70  lies between the border region  312 B and the central portion  313 . Using this approach a very thin central portion  313  plate  310  having high resonant frequencies can be achieved without sacrificing the thickness and strength of the border  312 . 
     While this invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto and that it can be variously practiced within the scope of the following claims.