Patent Document

TECHNICAL FIELD  
       [0001]     Embodiments relate to the field of surface acoustic wave sensors. Embodiments also relate to processing quartz wafers and sensor packaging.  
       BACKGROUND  
       [0002]     Surface acoustic wave (SAW) devices are commonly used to filter signals in electronic devices and are also used as sensors due to acoustic wave sensitivity to the physico-chemical measurands such as pressure and temperature. It is well known to those skilled in the art of surface acoustic wave devices that these SAW sensors can be either SAW delay lines or SAW resonators fabricated to be responsive to different non-electric measurands. The main component of a SAW device is a comb metal structure called the interdigital transducer (IDT), which is used to generate surface acoustic waves from an applied electric signal and vice-versa by the piezoelectric effect developed in the piezoelectric crystals/polycrystals on which the IDT is deposited. Actually, a simple example of a SAW delay line device could be obtained from two IDT structures separated by a certain distance on the same substrate.  FIG. 18 , labeled as “prior art” illustrates a SAW delay line device  1810 , where two comb structures are separated by a distance “d” and patterned onto a piezoelectric substrate  1801 . An electrical input signal is passed to the SAW delay line through the input IDT  1811  having a first input pad  1802  and a second input pad  1805 . The first input pad is electrically connected to the first comb electrode  1803 . The second input pad is connected to the second comb electrode  1804 . The input IDT  1811  converts the electrical input signal into an acoustic signal that propagates along the surface of the piezoelectric substrate  1801  to the output IDT  1812  that is also patterned onto the same piezoelectric substrate  1801 . The output IDT  1812  converts the acoustic signal into an electrical output signal that can be obtained from the first output pad  1806  and the second output pad  1807 . The SAW sensor is obtained when the propagation velocity of the acoustic wave is changed in the presence of a physical measurand from outside. If the output IDT  1812  is replaced by a single or a series of reflectors, a reflective SAW delay is obtained, which can be used as a wireless SAW sensor when the first input pad  1802  and second input pad  1805  are connected to an antenna. For simplicity reasons, the SAW sensor shall herein be represented by an IDT pattern.  
         [0003]     Stress and strain on the piezoelectric substrate cause the acoustic signal to change. The changes can be detected in the electrical output signal. Stress and strain produce a smaller measurable effect on an acoustic signal propagating along a thick substrate than along a thin substrate. Thinning an area of the substrate under the SAW sensor enhances the measurable effect. The thinned area is called a diaphragm. The act of thinning the substrate under the SAW sensor is called releasing the diaphragm.  
         [0004]     A cover is often attached to one side of a SAW sensor. The cover can protect sensor. The cover can also produce a reference chamber if it isolates a sealed volume when it is attached to the SAW sensor. Reference chambers are often used in SAW pressure sensors. Attaching a cover to a SAW sensor, however, can introduce stress and strain in the sensor. The cover induced strain can cause poor sensor measurements. The process of bonding the sensor and cover together can introduce strain. Furthermore, if the cover material is different from the sensor substrate material then environmental effects such as temperature changes can cause unintended and inconsistent strain on the SAW sensor.  
         [0005]     Ideally, a quartz cover can be bonded to a quartz sensor substrate to minimize unintentional environmental effects. Direct quartz bonding techniques can produce a chemical bond that attaches one quartz surface directly to another quartz surface. In one technique, silanol groups (Si—OH) are produced on both quartz surfaces, the surfaces are pressed together and the assembly is heated to around 450° C. During the heating treatment, silanol groups on the SAW quartz wafer will react with silanol groups on the quartz cover wafer forming covalent bonds Si—O—Si between the two wafers with oxygen atoms covalently bonded to both wafers acting as a bridge and making a strong wafer bonding. A water molecule will be released for each formation of Si—O—Si covalent bond. In another technique, a plasma treatment creates reactive dangling bonds on each surface and then the surfaces are pressed together. Those skilled in the art of quartz processing know of these and other techniques, particularly direct quartz bonding techniques, for bonding quartz surface.  
         [0006]     Other circuit elements in addition to a SAW sensor are required for producing measurements. Typically, those other circuit elements include a printed circuit board (PCB) and one or more antennas. The antennas are often patterned directly onto the PCB as traces. The SAW sensor, and any other necessary circuit elements, is attached to the PCB using any of a variety of techniques known to those skilled in the art of electronics manufacture. A SAW sensor module is a populated PCB having a SAW sensor. SAW sensor modules must often be sealed, such as with gel, epoxy, or another material in order to keep unwanted material from the SAW sensor. Those skilled in the art of SAW sensor modules know of many sealing materials and techniques applicable to SAW sensors.  
         [0007]     Current technology does not, however, supply systems or methods for the batch processing of covered and sealed all quartz SAW sensors because individual quartz covers are attached to individual quartz SAW sensors. Aspects of the embodiments directly address the shortcoming of current technology by the direct quartz bonding of processed quartz substrates before dicing.  
       BRIEF SUMMARY  
       [0008]     The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole.  
         [0009]     It is therefore an aspect of the embodiments to process a quartz cover wafer to produce a sensor recess pattern and a stripe recessed pattern. The sensor recess pattern is an array of sensor recesses. The stripe recessed pattern is a series of parallel recessed zones called stripes. The two patterns are aligned to one another and with the stripes perpendicular to the rows of sensor recesses.  
         [0010]     It is another aspect of the embodiments to process a SAW quartz wafer to produce a SAW sensor pattern. The SAW sensor pattern is an array of SAW sensors. The SAW sensor pattern is arranged to align with the sensor recess pattern on the quartz cover wafer.  
         [0011]     It is yet another aspect of the embodiments to align the quartz cover wafer and the SAW quartz wafer and then to direct quartz bond them. The quartz surfaces of the two wafers are coincident, meaning touching, except in those places where the SAW sensors align with the sensor recesses and where the stripes traverse the cover. The coincident surfaces are direct quartz bonded. Any of the known direct quartz bonding techniques is sufficient, including the methods discussed above involving plasma treatment for quartz surface activation or hydrophilization treatment for silanol (Si—OH) group formation. The SAW sensor pattern is now sealed within the sensor recess pattern and stripe recess patterns. The two quartz wafers, being direct quartz bonded together, form a wafer tandem.  
         [0012]     It is a further aspect of the embodiments to release the quartz diaphragm of each SAW sensor. This operation can be done to the entire SAW sensor array at once without damaging the SAW sensors because only one side of the SAW quartz wafer can be etched. The other side, containing the SAW devices, is bonded to, sealed against, and protected by the quartz cover wafer. A metal masking layer can protect the entire surface of cover wafer during diaphragm release and also a metal masking layer can be used on the back side of the quartz SAW wafer for the selective etching of the SAW wafer in order to make quartz diaphragm. The continuous direct quartz bonding at the periphery of the wafer tandem will protect it against penetration of etching solution to the SAW surface.  
         [0013]     It is a yet further aspect of the embodiments to separate individual covered SAW sensors from the wafer tandem. The stripes are cut away, perhaps by sawing or cutting along the edges of each stripe. During processing, such as during the diaphragm release step, the outer surfaces of the wafer tandem can lose transparency. Without transparency, the stripes cannot be seen. Alignment marks can be placed on the quartz cover wafer or on the SAW quartz wafer such that the stripes can be located without being seen. The wafer tandem with stripes removed can be diced using standard wafer dicing techniques.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein.  
         [0015]      FIG. 1  illustrates a sensor recess pattern of the cover wafer in accordance with aspects of the embodiments;  
         [0016]      FIG. 2  illustrates a stripe recess pattern of the cover wafer in accordance with aspects of the embodiments;  
         [0017]      FIG. 3  illustrates a quartz cover wafer processed to produce a sensor cover pattern and a stripe recess pattern in accordance with aspects of the embodiments;  
         [0018]      FIG. 4  illustrates an expanded view of sensor recess and stripes on a quartz wafer in accordance with aspects of the embodiments;  
         [0019]      FIG. 5  illustrates a first side view along a first cut line of the expanded view of  FIG. 4  in accordance with aspects of the embodiments;  
         [0020]      FIG. 6  illustrates a second side view along a second cut line of the expanded view of  FIG. 4  in accordance with aspects of the embodiments;  
         [0021]      FIG. 7  illustrates a third side view along a third cut line of the expanded view of  FIG. 4  in accordance with aspects of the embodiments;  
         [0022]      FIG. 8  illustrates a SAW sensor array produced on a SAW quartz wafer in accordance with aspects of the embodiments;  
         [0023]      FIG. 9  illustrates an expanded view of SAW sensors aligned with sensor recesses in a wafer tandem in accordance with aspects of the embodiments;  
         [0024]      FIG. 10  illustrates a wafer tandem in accordance with aspects of the embodiments;  
         [0025]      FIG. 11  illustrates a side view of a wafer tandem in accordance with aspects of the embodiments;  
         [0026]      FIG. 12  illustrates a first side view along a first cut line  901  of the expanded view of  FIG. 9  in accordance with aspects of the embodiments;  
         [0027]      FIG. 13  illustrates a second side view along a second cut line  902  of the expanded view of  FIG. 9  in accordance with aspects of the embodiments;  
         [0028]      FIG. 14  illustrates a third side view along a third cut line  903  of the expanded view of  FIG. 9  in accordance with aspects of the embodiments;  
         [0029]      FIG. 15  illustrates an a covered SAW sensor in accordance with aspects of the embodiments;  
         [0030]      FIG. 16  illustrates a covered SAW sensor module in accordance with aspects of certain embodiments;  
         [0031]      FIG. 17  illustrates a high level flow diagram of producing a SAW sensor module having an all quartz covered SAW sensor in accordance with aspects of certain embodiments; and  
         [0032]      FIG. 18 , labeled as “prior art” illustrates a SAW sensor patterned on a substrate.  
     
    
     DETAILED DESCRIPTION  
       [0033]     The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof. In general, the figures are not to scale.  
         [0034]      FIG. 1  illustrates a sensor recess pattern  101  in accordance with aspects of the embodiments. The sensor recess pattern  101  is an array of sensor recesses that each has a sensor body recess  102  and a sensor lead recess  103 . The sensor lead recess  103  is shown as being continuous with the adjacent sensor body recess.  
         [0035]      FIG. 2  illustrates a stripe recessed pattern  203  in accordance with aspects of the embodiments. The stripe recessed pattern is a series of recessed zones  202  in the quartz cover wafer.  
         [0036]      FIG. 3  illustrates a quartz cover wafer  301  processed to produce a sensor cover pattern  302  in accordance with aspects of the embodiments. As can be seen in the figure, the sensor cover pattern  302  includes the sensor recess pattern of  FIG. 1  and the stripe recessed pattern of  FIG. 2 . Each stripe  202  is aligned perpendicular to the sensor lead recesses  103  and between the sensor body recesses  102 .  
         [0037]      FIG. 4  illustrates an expanded view of sensor recesses and stripes  202  on a quartz cover wafer  301 , in accordance with aspects of the embodiments. Three cut lines are also shown. The first cut line  401  goes across the sensor lead recesses  103 , stripe recesses  202 , and sensor body recesses  102 . The second cut line  402  goes across the stripe recesses  202 , and sensor body recesses  102 . The third cut line  403  goes across the stripe recesses  202 .  
         [0038]      FIG. 5  illustrates a first side view along the first cut line  401  of the expanded view of  FIG. 4  in accordance with aspects of the embodiments. This cut view is called the first cover wafer cut view  501 . Sensor lead recesses  103 , sensor body recesses  102 , and stripe recesses  202  can be seen in the quartz cover wafer  301 .  
         [0039]      FIG. 6  illustrates a second side view along the second cut line  402  of the expanded view of  FIG. 4  in accordance with aspects of the embodiments. This cut view is called the second cover wafer cut view  601 . Sensor body recesses  102 , and stripe recesses  202  can be seen in the quartz cover wafer  301 .  
         [0040]      FIG. 7  illustrates a third side view along the third cut line  403  of the expanded view of  FIG. 4  in accordance with aspects of the embodiments. This cut view is called the third cover wafer cut view  701 . Stripe recesses  202  can be seen in the quartz cover wafer  301 .  
         [0041]      FIG. 8  illustrates a SAW sensor array  802  produced on a SAW quartz wafer  801  in accordance with aspects of the embodiments. The SAW sensor array is made of many SAW sensors  803 .  
         [0042]      FIG. 9  illustrates an expanded view of SAW sensors  803  aligned with sensor and stripe recesses in a wafer tandem in accordance with aspects of the embodiments. The quartz cover wafer  301  lies on top of the SAW quartz wafer, which cannot be seen. The stripes recesses  202 , sensor lead recesses  103 , and sensor body recesses  102  formed into the quartz cover wafer  301  can be seen. The SAW sensors  803  formed on the SAW quartz wafer can be seen aligned with and laying within the sensor body recesses  102  and sensor lead recesses  103 .  FIG. 9  also has three cut lines. The first cut line  901  is analogous the  FIG. 4  first cut line  401 . The second cut line  902  is analogous the  FIG. 4  second cut line  402 . The third cut line  903  is analogous the  FIG. 4  third cut line  403 .  FIG. 10  illustrates a full view of the wafer tandem in accordance with aspects of the embodiments. The sensor cover pattern  302  can be seen aligned with the sensor pattern  802 . Here, the SAW quartz wafer is under the quartz cover wafer  301 .  
         [0043]      FIG. 11  illustrates a first side view of a wafer tandem  1101  in accordance with aspects of the embodiments. The quartz cover wafer  301  lies on top of the SAW quartz wafer  801 . A seam  1102  is shown between the two wafers for illustrative purposes only. In actual practice, there is no seam because direct quartz bonding is seamless. It is possible that, from the side as shown, the devices and topography between the wafers could be seen.  
         [0044]      FIG. 12  illustrates a second side view along the first cut line  901  of the expanded view of  FIG. 9  in accordance with aspects of the embodiments. The first cover wafer cut view  501  is seen overlying a portion of the SAW quartz wafer  801 . The SAW sensor  803  can be seen aligned with the sensor recesses and on top of the SAW quartz wafer  801 . The quartz diaphragm  1201  can also be seen because it has been released. At the edges of the wafer tandem there is always a continuous direct quartz bonding zone to protect the SAW wafer surface against etching liquid penetration during diaphragm release by wet etching.  
         [0045]      FIG. 13  illustrates a third side view along the second cut line  902  of the expanded view of  FIG. 9  in accordance with aspects of the embodiments. The second cover wafer cut view  601  is seen overlying a portion of the SAW quartz wafer  801 . At the edges of the wafer tandem there is always a continuous direct quartz bonding zone to protect the SAW wafer surface during diaphragm release by wet etching. The SAW sensor  803  can be seen aligned with the sensor recesses and on top of the SAW quartz wafer  801 . The quartz diaphragm  1201  can also be seen because it has been released. A portion of the SAW sensor  803  is obscured because the sensor lead recess is not present in this view.  
         [0046]      FIG. 14  illustrates a side view along the third cut line  903  of the expanded view of  FIG. 9  in accordance with aspects of the embodiments. The third cover wafer cut view  701  is seen overlying a portion of the SAW quartz wafer  801 . The SAW sensor, quartz diaphragm, sensor lead recess, and sensor body recess are not present in this view. The recessed stripes are present in this view. Examining  FIGS. 12 and 13  reveals that cutting away the stripes will reveal the SAW sensor leads.  
         [0047]      FIG. 15  illustrates a covered SAW sensor  1501  in accordance with aspects of the embodiments. A covered quartz sensor  1501  can be obtained by removing the stripes to reveal the leads of the SAW sensors  803  underneath. Next, the wafer tandem is diced in the same manner as any wafer is diced. Dicing is a common operation in wafer processing in which a processed wafer is cut into individual components. Here, dicing along the strip direction cuts through only the quartz cover wafer  301  while dicing in the perpendicular direction cuts through SAW quartz wafer  801  and the quartz cover wafer  301 . A sensor lead recessed zone  103  can be seen on the right side of covered SAW sensor  1501  after dicing. The gap between sensor lead and quartz cover can be sealed in subsequent steps aimed toward pressure reference chamber formation.  
         [0048]     It is important to note here that contaminants can get into the covered SAW sensor because the remaining part of the sensor lead recess  103 , directly above the lead portion of the SAW sensor  803  and below the quartz cover, is not sealed. Sealing the sensor lead recess protects the SAW sensor and can create a sealed reference chamber. Those practiced in the arts of sensor production or electronics manufacture know of many techniques for sealing an electronic component such as the covered SAW sensor.  
         [0049]      FIG. 16  illustrates a covered SAW sensor module in accordance with aspects of certain embodiments. The covered SAW sensor  1501  is flipped over so that the SAW quartz substrate, formally part of the SAW quartz wafer, is on top. As such, the SAW sensor leads can be bonded to the circuit substrate  1601 . One SAW sensor lead is electrically connected to a first antenna  1603  while the other is electrically connected to a second antenna  1602 . The circuit substrate can be any material used for producing printed circuit elements. A printed circuit board (PCB) is one example of a circuit substrate.  
         [0050]      FIG. 17  illustrates a high level flow diagram of producing a SAW sensor module having an all quartz covered SAW sensor in accordance with aspects of certain embodiments. After the start  1701 , the quartz cover wafer is processed  1702  to produce a sensor cover pattern. As discussed above, a sensor cover pattern has properly aligned stripes recesses and sensor recesses. Next the SAW quartz wafer is processed  1703  to produce an array of SAW sensors. Next, the wafers are direct quartz bonded  1704  to produce a wafer tandem. The diaphragm is released  1705  with the quartz cover wafer protecting the SAW sensors. Finally, the stripes are removed  1706  and the wafer tandem is diced  1707  to produce many individual covered SAW sensors.  
         [0051]     Also after the start  1701 , the circuit substrate is patterned  1708 . A printed circuit board is an example of a patterned circuit substrate. The circuit substrate can be processed in other ways as well before the covered SAW sensor is ready for attachment. Regardless, once the circuit substrate is prepared and the covered SAW sensor is ready, the covered SAW sensor is attached to the circuit substrate  1709  to produce a SAW sensor module. The SAW sensor module can be sealed  1710  before the process is done  1711 . Sealing the covered SAW sensor is discussed above. The same, or a similar, result can be obtained by sealing the SAW sensor module before attaching it to the circuit substrate  1709 .  
         [0052]     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Technology Category: 3