Abstract:
Improved SAW pressure sensors and manufacturing methods thereof. A SAW wafer including a number of SAW transducers disposed thereon may be provided. A cover wafer may also be provided, with a glass wall situated between the cover wafer and the SAW wafer. The cover wafer may be secured to the SAW wafer such that the glass wall surrounds the SAW transducers. In some instances, the glass wall may define, at least in part, a separation between the cover wafer and the SAW wafer. One or more contours may also be provided between the cover wafer and the SAW wafer such that at least one of the contours surrounds at least one of the SAW transducers when the cover wafer is disposed over and secured relative to the SAW wafer.

Description:
TECHNICAL FIELD 
     The present invention relates generally to SAW (surface acoustic wave) sensors, and more specifically to wafer level packaging of SAW sensors. In particular, the present invention relates to methods of forming a number of SAW sensors simultaneously. 
     BACKGROUND 
     SAW devices can be used to measure a variety of strain-related properties such as temperature, stress, acceleration, and other mechanical parameters, via suitably arranged SAW transducers typically employing a deformable piezoelectric material. Examples of known piezoelectric material include quartz, lithium niobate, lithium tantalate, and treated composites bearing a thin film of a material such as zinc oxide. 
     In some instances, SAW sensors may be used in measuring pressure. A need remains for improved SAW sensors adapted for measuring pressure, including air pressure within a tire, as well as other applications. A need remains for effective, low-cost manufacturing methods of such SAW sensors. 
     SUMMARY 
     The present invention relates to improved SAW sensors adapted for measuring pressure, as well as to effective, low-cost manufacturing methods of such SAW sensors. A SAW sensor includes any SAW device, such as a SAW delay line, SAW resonator, and the like, which can be used for sensing a physical or chemical condition. 
     An example embodiment of the present invention may be found in a method of simultaneously producing a number of SAW sensors. In the example method, a SAW wafer having a number of SAW transducers disposed thereon is provided. A cover wafer is also provided, and a protective wall is provided between the SAW wafer and the cover wafer such that an interior portion of the SAW wafer and an interior portion of the cover wafer are protected and sealed against the outside. The cover wafer may be disposed over the SAW wafer such that the protective wall surrounds the SAW transducers. In some cases, the protective wall may be a protective glass frit wall and may provide and define, at least in part, a separation between the cover wafer and the SAW wafer. In some instances, the glass wall may bond the SAW wafer to the cover wafer. 
     In some instances, a number of raised contours may also be provided between the cover wafer and the SAW wafer such that at least one of the raised contours surrounds at least one of the SAW transducers when the cover wafer is disposed over and secured relative to the SAW wafer. In some cases, the SAW wafer may be a quartz SAW wafer, and the cover wafer may be a quartz cover wafer, but this is not required in all embodiments. 
     After the cover wafer and SAW wafer are secured together, a back side of the SAW wafer may be patterned, such as by etching, to form a number of pressure-sensing diaphragms, each corresponding to an individual pressure sensor on the SAW wafer. In some instances, patterning the SAW wafer includes forming a patterned metal layer on the SAW wafer and a protective metal layer on the cover wafer. The protective metal layer and the glass wall may protect the SAW wafer and the cover wafer during etching. 
     A subsequent dicing process may include making a first series of cuts through the cover wafer without cutting the SAW wafer. A second series of cuts may be made through the cover wafer without cutting the SAW wafer, the second series of cuts being parallel to the first series of cuts. A third series of cuts may be made through the cover wafer and the SAW wafer, the third series of cuts being orthogonal to the first series of cuts and the second series of cuts. A fourth series of cuts may be made through the cover wafer and the SAW wafer, the fourth series of cuts being orthogonal to the third series of cuts. 
     In some cases, a wall trench may be provided in the cover wafer, and the protective wall may be provided at least partially within the wall trench. In some cases, a number of contour trenches may also be provided in the cover wafer, and the raised contours may be provided at least partially within the contour trenches. 
     Another example embodiment of the present invention may be found in a quartz stack including a quartz SAW wafer and a quartz cover wafer. A number of SAW transducers may be provided on a surface of the quartz SAW wafer. A glass frit wall may be provided between the quartz SAW wafer and the quartz cover wafer such that the glass frit wall surrounds the SAW transducers. The glass frit wall may secure the quartz SAW wafer to the quartz cover wafer. A number of glass contours may also be provided between the quartz SAW wafer and the quartz cover wafer such that at least one of the glass contours surrounds at least one of the SAW transducers. In some instances, at least one of the glass contours is a rectangular glass frit contour, but this is not required. 
     The glass frit wall may have a height that defines a spacing between the quartz SAW wafer and the quartz cover wafer. Alternatively, the glass frit wall may, in some instances, have a height that is greater than the spacing between the quartz SAW wafer and the quartz cover wafer. In such cases, the quartz cover wafer may include a wall trench, and the glass frit wall may be at least partially disposed within the wall trench. The quartz cover wafer may also include a number of contour trenches, in which case, the glass contours can be at least partially disposed within the contour trenches. 
     A SAW pressure sensor may be separated from the glass frit bonded wafers by dicing the quartz stack described above. The SAW pressure sensor may include a pressure reference chamber that is formed by a portion of the SAW wafer bearing a SAW transducer, a portion of the cover wafer and one of the glass contours. The pressure reference chamber may bear a desired reference pressure as a result of the pressure reference chamber being sealed at the desired reference pressure. In some instances, the SAW pressure sensor may include an overpressure stop, but this is not required. 
     In some instances, the cover wafer portion may be smaller than the SAW wafer portion as a result of the dicing process used to separate the SAW pressure sensor from the quartz stack. The SAW pressure sensor may further include conductive leads that are disposed on the SAW wafer portion and that extend outwardly from the SAW transducer beyond the cover wafer portion. 
     The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures, Detailed Description and Examples which follow more particularly exemplify these embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a top view of a cover wafer in accordance with an illustrative embodiment of the present invention; 
         FIG. 2  is a top view of a stack incorporating the cover wafer of  FIG. 1 ; 
         FIG. 3  is a partial cross-sectional side view taken through  FIG. 2  at line  3 - 3 ; 
         FIG. 4  is a view of the partial cross-section of  FIG. 3 , showing a subsequent processing step in accordance with an illustrative embodiment of the present invention; 
         FIG. 5  is a view of the partial cross-section of  FIG. 4 , showing a subsequent processing step in accordance with an illustrative embodiment of the present invention; 
         FIG. 6  is a partial top view of the stack of  FIG. 2 , showing a dicing pattern; 
         FIG. 7  is a partial cross-section of the stack of  FIG. 6 ; 
         FIG. 8  is a cross-sectional side view of a SAW sensor formed by the dicing pattern shown in  FIGS. 6 and 7 ; 
         FIG. 9  is a partial cross-sectional side view of a cover wafer in accordance with an illustrative embodiment of the present invention; 
         FIG. 10  is a view of the partial cross-section of  FIG. 9 , showing a subsequent processing step in accordance with an illustrative embodiment of the invention; and 
         FIG. 11  is a cross-sectional view of a SAW sensor formed in accordance with an illustrative embodiment of the present invention. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
     DETAILED DESCRIPTION 
     The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Although examples of construction, dimensions, and materials are illustrated for the various elements, those skilled in the art will recognize that many of the examples provided have suitable alternatives that may be utilized. 
       FIG. 1  is a top view of an illustrative cover wafer  10 . In some instances, cover wafer  10  may include or be formed from quartz, but this is not required. The illustrative cover wafer  10  has a top surface  12  bearing several structures of interest. A protective wall, such as a glass wall  14 , may be positioned on the top surface  12 , proximate a periphery  16  of the cover wafer  10 . In some instances, glass wall  14  may be disposed about 1 or 2 millimeters inward from periphery  16  of the cover wafer  10 . However, it is contemplated that the glass wall  14  may be spaced inward any suitable distance from the periphery  16  of the cover wafer  10 . 
     Glass wall  14  may be formed having any suitable dimensions. For example, and in some cases, glass wall  14  may be about 2 or 3 millimeters in width (parallel to top surface  12 ) and about 15 to 75 micrometers in height (orthogonal to top surface  12 ). In some instances, glass wall  14  may be a glass frit wall, formed using known glass frit techniques such as screen printing. 
     In the illustrative embodiment, a number of raised contours  18  may also be provided on surface  12 . In some instances, the raised contours  18  may be glass frit contours formed using known glass frit techniques such as screen printing, but this is not required. The illustrated embodiment of  FIG. 1  shows a total of twenty four raised contours  18  disposed on surface  12 . It should be noted, however, that surface  12  may include fewer or more raised contours  18 , as desired. In some instances, surface  12  may include many more raised contours  18 , sometimes evenly disposed about most of surface  12 . At least some of the raised contours  18  may be dimensioned to surround a SAW transducer when the cover wafer  10  is disposed over and secured to a SAW wafer. In some instances, raised contours  18  may have a largely rectangular shape, and may be dimensioned to accommodate the particular SAW transducer or transducers used, but this is not required. 
       FIG. 2  shows an illustrative stack  20  in which cover wafer  10  has been inverted and disposed over a SAW wafer  22 , as better seen in  FIG. 3 . Cover wafer  10  may be secured to SAW wafer  22  using any suitable technique. For example, cover wafer  10  may be secured to SAW wafer  22  using a thermal annealing process, which may be performed at a temperature of about 450 degrees C. In some instances, SAW wafer  22  may be a quartz wafer. 
     As best illustrated in  FIG. 6 , a number of SAW transducers  24  may be disposed on SAW wafer  22 . The SAW transducers  24  do not appear in detail in  FIGS. 2 and 3 , as these are cross-sections. SAW transducers  24  may be formed upon SAW wafer  22  using any suitable technique such as a metallization process. SAW transducers  24  may be inter-digitized SAW transducers. It should be noted that the wafer-level packaging described herein may be applicable to sealing other quartz devices as well. 
     It can be seen that raised contours  18  may be dimensioned to surround SAW transducers  24 , while permitting related circuitry and/or conductive leads  25  (shown schematically in  FIGS. 3-5 ) to extend beyond raised contours  18 . Circuitry and/or conductive leads  25  is best described with respect to  FIG. 6 . In  FIG. 6 , circuitry and/or conductive leads  25  can be seen as including several conductive leads ending with conductive pads  27 . While the illustrative embodiment shows three conductive leads ending with three conductive pads  27  extending from either side of each SAW transducer  24 , it will be appreciated that in some cases, only two leads ending with conductive pads  27  or less, or perhaps four or more leads ending with conductive pads  27 , may extend from each SAW transducer  24 , as desired. 
     As a result, electrical communication with SAW transducers  24  may be achieved without impacting sealing around SAW transducers  24 .  FIG. 3  also shows that glass wall  14 , in the illustrated embodiment, defines a separation between cover wafer  10  and SAW wafer  22 . This separation, as will be discussed subsequently, may in some cases provide advantages during dicing steps. Moreover, as will be discussed, this separation may help define a relatively large pressure reference chamber  26 , when desired. 
     In order to form a SAW pressure sensor, it may be useful to provide a pressure sensing diaphragm by deep wet etching of the quartz on the back side of the quartz wafer  22 . One way to accomplish this, and as shown in  FIG. 4 , includes forming a front side protective masking layer  28  and a back side masking layer  30 . Layer  28  and layer  30  may be formed of any suitable material using any suitable techniques. In some instances, layer  28  and layer  30  may both be metallic layers such as a Cr/Au layer deposited using techniques such as sputtering, vapor deposition, and the like. 
     In  FIG. 5 , it can be seen that back side masking layer  30  has been patterned. This patterning may be accomplished using any suitable technique such as a photolithographic process. In some instances, it may be useful to employ a double side aligner so that the future pressure diaphragm is correctly aligned with respect to the corresponding SAW transducer  24 . 
     Once patterning has been complete, deep wet etching of a portion of the back side of SAW wafer  22  may be completed. This may results in voids  32  that will eventually form and release the pressure diaphragms. It should be noted that during deep etching, glass wall  14  may help protect SAW transducers  24  from damage that may otherwise result from the deep etching process. While a portion of glass wall  14  may be etched away by the etchant, glass wall  14  may have sufficient width to largely withstand the etching process. If there is a high etch rate of the glass wall with respect to the quartz etching, a local laser densification treatment may be performed on glass wall  14  to decrease the etching rate. Once the etching step has occurred, front side protective masking layer  28  and back side masking layer  30  may be removed using any suitable technique. 
       FIGS. 6 and 7  show, in combination, an illustrative dicing or cutting pattern for cutting stack  20  to form a number of individual SAW pressure sensors. In an initial cutting step, cover wafer  10  may be cut along the direction TR 1 . As can be seen in  FIG. 7 , this cutting may penetrate completely through cover wafer  10  but not contact SAW wafer  22 . In a second cutting step, cover wafer  10  may be cut along the direction TR 2 , which in the illustrative embodiment, is parallel to direction TR 1 . Once again, this cutting step may penetrate completely through cover wafer  10  but not contact SAW wafer  22 . 
     Next, a series of cuts may be made along direction TR 3 , which in the illustrative embodiment, may be perpendicular to directions TR 1  and TR 2 . This cutting step may penetrate completely through both cover wafer  10  and SAW wafer  22 . In a further cutting step, a series of cuts may be made along direction TR 4 , which in the illustrative embodiment, are perpendicular to direction TR 3  and thus parallel to directions TR 1  and TR 2 . As a result of these cutting processes, stack  20  may be reduced to a number of individual SAW pressure sensors  34 , as shown in  FIG. 8 . In some instances, as illustrated, it can be noted that as a result of the dicing process, there is no cover above at least a portion of the electrodes  25  and conductive pads  27 , which permits electrical communication between the SAW pressure sensor  34  and external circuitry such as a signal conditioning circuit or an antennae. 
       FIG. 8  shows an illustrative SAW pressure sensor  34  having a cover  36 , corresponding to a portion of cover wafer  10  ( FIG. 7 ), and a base  38  that corresponds to a portion of SAW wafer  22  ( FIG. 7 ). It can be seen that cover  36 , base  38  and raised contours  18  may define a pressure reference chamber  26 . Base  38  may include a pressure sensing diaphragm  40 , formed by the deep etching step discussed previously. SAW pressure sensor  34  may subsequently be provided within a housing or package using, for example, standard plastic encapsulation technology, if desired. 
       FIGS. 9 and 10  show an illustrative cover wafer  42  in accordance with another illustrative embodiment of the present invention. As discussed previously, the spacing determined by the height of glass wall  14  ( FIG. 1 ) can provide SAW pressure sensor  34  with a relatively large pressure reference chamber  26 . As a result, variations in chamber volume caused by movement of diaphragm  40  may be relatively small, thereby providing a relatively stable and constant reference pressure. However, this large spacing may make it more difficult to provide an overpressure stop for the diaphragm. 
       FIG. 9  shows a cover wafer  42 , which in some instances may be quartz, with a wall trench  44  and a number of contour trenches  46  formed within a surface  48 . Wall trench  44  and contour trenches  46  may be formed using any suitable technique, such as depositing and patterning a mask layer, followed by deep wet etching. In  FIG. 10 , a glass material  50  is disposed at least partially within wall trench  44  while a number of raised contours  52  are disposed at least partially within contour trenches  46 . Glass material  50  may, in some instances, be a glass frit wall, formed using known glass frit techniques such as screen printing. Similarly, raised contours  52  may be formed using known glass frit techniques such as screen printing, but this is not required in all embodiments. 
     It should be appreciated that cover wafer  42  may be used in place of cover wafer  10  ( FIG. 1 ). The resulting stack may be processed as discussed with respect to  FIGS. 4 through 7 , and may result in a SAW pressure sensor  54  ( FIG. 11 ) having a smaller pressure reference chamber  62 . As seen in  FIG. 11 , SAW pressure  54  has a top  56  corresponding to an appropriate portion of cover wafer  42 . A base  58  includes a pressure sensing diaphragm  60  and a SAW transducer  24  (as shown in  FIG. 6 ). 
     SAW pressure sensor  54  may be considered as including an overpressure stop, as pressure sensor diaphragm  60  can, if subjected to a sufficiently large pressure differential, actually move far enough to physically contact top  56 . In this, top  56  functions as an overpressure stop as it may limit physical movement of pressure sensor diaphragm  60 . 
     The invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the invention can be applicable will be readily apparent to those of skill in the art upon review of the instant specification.