Patent Publication Number: US-2022239279-A1

Title: Film bulk acoustic sensors using thin ln-lt layer

Description:
RELATED APPLICATION INFORMATION 
     This patent is a continuation of patent application Ser. No. 17/319,082, entitled FILM BULK ACOUSTIC SENSORS USING THIN LN-LT LAYER, filed May 12, 2021, which is a continuation-in-part of patent application Ser. No. 17/125,779, entitled FILM BULK ACOUSTIC RESONATORS IN THIN LN-LT LAYERS, filed Dec. 17, 2020, now U.S. Pat. No. 11,251,775, which is a continuation of patent application Ser. No. 17/090,599, entitled FILM BULK ACOUSTIC RESONATORS IN THIN LN-LT LAYERS, filed Nov. 5, 2020, now U.S. Pat. No. 10,944,380, which is a continuation of patent application Ser. No. 16/932,719, entitled FILM BULK ACOUSTIC RESONATORS IN THIN LN-LT LAYERS, filed Jul. 18, 2020, now U.S. Pat. No. 10,862,454, which claims priority from provisional patent application No. 62/875,855, entitled FILM ACOUSTIC RESONATORS IN THIN LN-LT LAYERS, filed Jul. 18, 2019, and provisional application No. 62/958,851, entitled YBAR ON ROTATED Y-CUTS OF LN, filed Jan. 9, 2020. The entire contents of each application are incorporated herein by reference. 
    
    
     NOTICE OF COPYRIGHTS AND TRADE DRESS 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever. 
     BACKGROUND 
     Field 
     This disclosure relates to sensors using film bulk acoustic resonators. 
     Description of the Related Art 
     A variety of acoustic wave resonators have been developed, primarily for use in radio frequency filters for communications equipment. One type of acoustic wave resonator is the Y-cut film bulk acoustic resonator (YBAR) described in U.S. Pat. Nos. 10,944,380 and 10,862,454. 
     In addition to applications in RF filters, acoustic bulk and surface wave resonators are widely used as sensors to detect the presence of various gases, liquids, and biological or chemical species. To function as a sensor, an acoustic wave resonator may be coated with a sensing material capable of absorbing, adsorbing, or otherwise capturing the material or species to be detected. The presence of the captured species causes a measurable shift in the resonance frequency of the acoustic wave resonator. 
     A well-known acoustic wave sensor is the quartz microbalance (QMB). A QMB is a shear bulk wave fundamental mode quartz resonator operating in 4-6 MHz frequency range. The resonance frequency is changed when a layer of molecules is deposited on its surface. The precise measurement of the frequency change allows a QMB to measure mass density changes of the order of 1 μg/cm 2 , or one molecular layer. The advantage of a QMB sensor is that it directly measures absolute surface density of attached molecular layer, independent on the other physical properties (optical, magnetic, chemical, phase state, etc.) of the layer. 
     The sensitivity of an acoustic wave sensor increases with its frequency. To allow operation at higher frequencies, film bulk acoustic resonators (FBARs) and different types of surface waves and Lamb modes in piezoelectric membranes have been proposed for use in sensors. However, most FBARs currently exploit extension mode vibrations and are not usable for liquid sensing because the liquid load introduces unacceptable high acoustic losses. Another problem is that FBAR sensors require electric contacts to electrodes on both sides of the piezoelectric membrane. Lamb wave sensors exploiting S0 mode also suffer from increased loss when submerged in liquid or having liquid deposited on one side. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  includes a schematic plan view and schematic cross-sectional views of a sensor using a Y-cut film bulk acoustic resonator (YBAR). 
         FIG. 2  includes a schematic plan view and schematic cross-sectional views of another sensor using a YBAR. 
         FIG. 3  is a chart of admittance as function of frequency of an exemplary YBAR. 
         FIG. 4  is a block diagram of a sensing system using a YBAR sensor. 
         FIG. 5  is another block diagram of a sensing system using a YBAR sensor. 
         FIG. 6  is a flow chart of a process of fabricating a YBAR sensor. 
     
    
    
     Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a simplified top view and a cross-sectional view of one period of a sensor  100  based on a Y-cut film bulk acoustic resonator (YBAR). The YBAR is made up of a piezoelectric plate  110  having essentially parallel front and back surfaces  112 ,  114 , respectively. In this context, “essentially parallel” means “parallel within reasonable manufacturing tolerances.” The piezoelectric plate  110  is a thin single-crystal layer of a piezoelectric material. The piezoelectric plate is preferably lithium niobate (LN) but may be lithium tantalate (LT), lanthanum gallium silicate, gallium nitride, or some other material. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. 
     The thickness is of the piezoelectric plate  110  may be determined from 
     
       
         
           
             
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     where F R  is a desired operation frequency, V SH  is the shear wave velocity of the piezoelectric plate, and n=1, 3, 5, . . . is the desired mode (overtone) number. n=1 is usually referred to as “fundamental mode” and n&gt;1 as “overtones”. More exact formulas relating the membrane thickness and including thickness of electrodes are known and can be used in design of the sensor. 
     The back surface  114  of the piezoelectric plate  110  is attached to a substrate  120  that provides mechanical support to the piezoelectric plate  110 . The substrate  120  may be, for example, silicon, sapphire, quartz, or some other material. The piezoelectric plate  110  may be bonded to the substrate  120  using a wafer bonding process, or grown on the substrate  120 , or attached to the substrate in some other manner. The piezoelectric plate may be attached directly to the substrate or may be attached to the substrate via one or more intermediate material layers. 
     A cavity  125  is formed in the substrate  120  such that the portion of the piezoelectric plate  110  containing the front-side and back-side conductor patterns  130 ,  132 ,  134  is suspended over the cavity  125 . “Cavity” has its conventional meaning of “an empty space within a solid body.” The portion of the piezoelectric plate (including the conductor patterns and a sensing layer  140  (described subsequently) suspended over the cavity is referred to herein as the “diaphragm” due to its resemblance to the diaphragm of a microphone. The cavity  125  may be a hole completely through the substrate  120  (as shown in Section A-A) or a recess in the substrate  120  that does not extend through the substrate  120 . The cavity  125  may be formed, for example, by selective etching of the substrate  120  before or after the piezoelectric plate  110  and the substrate  120  are attached. As shown in  FIG. 1 , the cavity  125  has a rectangular shape. A cavity of a YBAR may have a different shape, such as a regular or irregular polygon. The cavity of a YBAR may more or fewer than four sides, which may be straight or curved. 
     A first front-side conductor pattern  130  and a second front-side conductor pattern  132  are formed on the front surface  112  of the piezoelectric plate  110 . A back-side conductor pattern  134  is formed on the second surface  114  of the piezoelectric plate  110 . The back-side conductor pattern  134  is a “floating” conductor pattern, meaning that is not electrically connected to any other conductor. The back-side conductor pattern is capacitively coupled to the first and second front-side conductor patterns. The conductor patterns may be molybdenum, aluminum, copper, gold, or some other conductive metal or alloy. The back-side conductor pattern and the front side conductor patterns are not necessarily the same material. The portion of the piezoelectric plate  110  between the first front-side conductor pattern  130  and the back-side conductor pattern  134  forms a first resonator  150 . The portion of the piezoelectric plate  110  between the second front-side conductor pattern  132  and the back-side conductor pattern  134  forms a second resonator  155 . The first and second resonators  150 ,  155  are electrically in series such that an RF signal applied between the first and second front-side conductor patterns  130 ,  132  excites acoustic waves in both the first and second resonators  150 ,  155 . 
     The diaphragm forms a seal over the cavity  125  such that the first and second front-side conductor patterns  130 ,  134  are not exposed to the environment adjacent to the back-side conductor pattern  134 . 
     Ideally, when an RF signal is applied between the first and second front-side conductor patterns  130 ,  132 , the back-side conductor pattern should remain at ground potential. To this end, a capacitance of the first resonator  150  should be equal to a capacitance of the second resonator  155 . Assuming the piezoelectric diaphragm has uniform thickness, the capacitances will be equal if the area of the first resonator  150 , which is to say the area of overlap between the first front-side conductor pattern  130  and the back-side conductor pattern  134 , is equal to the area of the second resonator  155 . The back-side conductor pattern  134  will remain at ground potential when balanced signals (i.e. signals with equal amplitude and 180-degree phase difference), are applied to the first and second conductor patterns  130 ,  132 . 
     The piezoelectric plate may be Y-cut (i.e. with the Y crystalline axis of the piezoelectric material normal to the surfaces  112 ,  114 ) or rotated Y-cut (i.e. with the Y crystalline axis of the piezoelectric material rotated by a predetermined angle with respect to normal to the surfaces  112 ,  114 ). In this case, an RF signal applied between the first and second front-side conductor patterns  130 ,  132  will excite shear acoustic waves in both the first and second resonators. Rotated Y-cuts can be used to achieve shear displacements exclusively in planes parallel to the surface  112 ,  114 . Selection of the rotation angle can be used to control the electromechanical coupling of the resonators. Shear displacements parallel to the surfaces of the piezoelectric plate do not generate compressional waves in an adjacent liquid thus allowing high Q-factor operation of the resonator. The sensing (bottom) metallized surface is uniform and continuous, containing no connectors, wires, grooves or other structures, which is convenient for deposition of sensing layer  140 . The maximal amplitude of vibration on the surface guaranties high sensitivity of the sensor. 
     As shown in  FIG. 1 , the first and second front-side conductor patterns and the back-side conductor pattern  134  are rectangular in shape. The conductor patterns may be non-rectangular (e.g. trapezoidal, curved, or irregular) to suppress parasitic acoustic modes. 
     In the detailed cross-sectional view, the thickness of the piezoelectric plate  110  is dimension ts and the thickness of the conductor patterns  130 ,  132 ,  134  is dimension tm. The thickness ts of the piezoelectric plate may be, for example, 100 nm to 1000 nm. The thickness tm of the conductor patterns  130 ,  132 ,  134  may be, for example, 10 nm to 500 nm. The thickness of the conductor patterns may be the same or the first and second front-side conductor patterns and the back-side conductor pattern may have different thicknesses. 
     The piezoelectric plate  110  may be etched or otherwise removed, completely or only partially, in the area between the first and second front-side conductor patterns  130 ,  132 , forming slots  115 . The presence of the slots  115  may suppress lateral acoustic modes that might be excited by the electric field between the front-side conductor patterns  130 ,  132 . A depth tg of the slot  115  can extend partially or completely through the piezoelectric plate  110 . 
     To convert a YBAR into the sensor  100 , a sensing layer  140  is disposed on the back-side conductor pattern  134 . The sensing layer  140  may be, for example, a film, a monolayer, or a surface treatment. The sensing layer  140  may be disposed directly on the back-side conductor pattern  134  or may be coupled to the back-side conductor pattern  134  via an intermediate layer such as an adhesion promoter. 
     The sensing layer  140  is configured to selectively capture a target species from a gaseous or liquid environment. The target species may by biological or chemical. The target species may be captured by absorption into the sensing layer and/or adsorption onto the surface of the sensing layer. A variety of materials may be used for the sensing layer  140  as appropriate for the target species. For example, the sensing layer  140  may be an antigen, an antibody, an enzyme, a nucleic acid, a DNA molecule, a polymer or other organic material with a particular functional group, an inorganic layer, or some other material. 
     When used as a sensor, the sensing layer  140  is exposed to a gaseous or liquid sample medium that may, or may not, contain the target species to be detected. When the target species is present, it is captured in or on the sensing layer, which incrementally increases the mass of the diaphragm. Increasing the mass of the diaphragm reduces the resonance frequency of the first and second resonators. The change in resonance frequency can be measured by a suitable measurement system, thus detecting the presence of the target species. 
     The structure of the YBAR  100  is well suited for use as a sensor. Importantly, the back-side electrode is continuous and there is no electric field extending from the back-side electrode into the sample medium. Since there is no electric field in the sample medium, the sample medium can be conductive. Further, the dielectric permittivity of the sample medium does not influence the resonance frequency of the YBAR. The continuous back-side electrode allows uninterrupted flow of a liquid or gaseous sample medium. 
     The primary acoustic mode of the YBAR is a shear mode in which atomic displacements are parallel to the surfaces of the piezoelectric plate. Very little or no shear acoustic energy is transferred to the sample medium. 
     The back-side electrode can be made of any appropriate electrically conductive material. For example, the back-side electrode may be gold to avoid corrosion. 
     A YBAR sensor can work at 2-5 GHz frequency range, providing high sensitivity. For a YBAR using a 400 nm lithium niobate piezoelectric plate, a single molecular layer of a target species attached to the sensing layer shift the resonance frequency of the YBAR by roughly 5 MHz to 10 MHz. For another example of the sensitivity of a YBAR sensor, imagine that only one virus of 100 nm in diameter is captured by each resonator having 10×50=500 μm 2  area. In this case the relative frequency shift is of the order of 1 ppm, or absolute shift is around 5 kHz, which can be easily measured. 
     The operation of a YBAR sensor on the 3 rd  or 5 th  harmonic (overtone) is also possible. Operation at an overtone will allow the use of a 3-times or 5-times thicker piezoelectric plate for a given frequency of operation. A thicker piezoelectric plate will make a sensor more robust mechanically. 
     When a YBAR sensor is in contact with a liquid, the resonance frequency will shift due to the viscosity of the liquid. The frequency shift Δf due to viscosity can be estimated by the following formula: 
     
       
         
           
             
               
                 
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     where f R  is the resonance frequency in air, p is the liquid density, η is the viscosity of the liquid, and n=1, 3, 5 is the mode number. The viscosity effect always moves frequency down. For water, the relative shift will be of the order 0.003, or 15 MHz for a 5 GHz resonator. The attenuation due to the viscosity of water will reduce the Q-factor of the YBAR by roughly 1.5-2 times. Since the expected unloaded Q-factor of a YBAR using the fundamental mode is in the range 300-600. The resonance will remain well pronounced when the YBAR sensor is exposed to water. 
       FIG. 2  shows a simplified top view and a cross-sectional view of another YBAR  200 . The YBAR  200  is made up of a piezoelectric plate  210  attached to a substrate  220  as previously described. A cavity  225  is formed in the substrate  220  such that a portion of the piezoelectric plate  210  is suspended over the cavity  225 . 
     First and second front-side conductor patterns  230 ,  232  are formed on the front side of the piezoelectric plate (the side facing away from the cavity  225 ). The first and second front-side conductor patterns  230 ,  232  form an interleaved finger pattern (IFP) similar to an interdigital transducer or IDT used in surface acoustic wave resonators. The first front-side conductor pattern  230  includes a first plurality of parallel fingers extending from a first busbar. The second front-side conductor pattern  232  includes a second plurality of parallel fingers extending from a second busbar. The first and second pluralities of parallel fingers are interleaved and most or all of the interleaved parallel fingers are disposed on the portion of the piezoelectric plate  210  suspended over the cavity  225 . The width m of each finger will be a substantial portion of the pitch p, or center-to-center spacing, of the fingers. 
     As shown in the detail view, slots  215  may be formed in the piezoelectric plate  210  between the interleaved fingers of the first and second front-side conductor patterns  230 ,  232 . The presence of the slots  215  may suppress lateral acoustic modes that might be excited by the electric field between the front-side conductor patterns  230 ,  232 . A depth tg of the slots  215  can extend partially or completely through the piezoelectric plate  210 . The grooves also prevent spreading of vibration energy along the structure thus improving Q-factor of resonators. 
     A back-side conductor pattern  240  is formed on the back side of the piezoelectric plate  210  opposed to the first and second front-side conductor patterns  230 ,  232 . A sensing layer  240  is formed on the back-side conductor pattern  234 . A first resonator is formed between the first front-side conductor pattern  230  and the backside conductor pattern  234 . A second resonator is formed between the second front-side conductor pattern  232  and the back-side conductor pattern  234 . The first and second front-side conductor patterns may have the same number of interleaved fingers. 
       FIG. 3  is a graph  300  of the performance of an exemplary YBAR suitable for use in a sensor.  FIG. 3  is based on simulation of the YBAR using a finite element method. The exemplary YBAR includes a rotated Y-cut lithium niobate piezoelectric plate with Euler angles (0°, 81.5°, 0°). These Euler angles maximize the electromechanical coupling for the primary shear acoustic mode. The piezoelectric plate thickness in this example is 400 nm thick. The first and second front-side conductor patterns are interleaved fingers with a width w of 8μ and a pitch p of 10μ. Slots are provided in the piezoelectric plate between fingers to prevent excitation of horizontally propagating spurious waves. The back-side conductor pattern is floating. Both the front-side and back-side conductor patterns are aluminum 50 nm thick. 
     The solid curve  310  is a plot of the absolute value of the admittance of the exemplary YBAR as a function of frequency. The exemplary YBAR has a resonance  215 , where its admittance is maximum, at a resonance frequency of 3938 MHz. The exemplary YBAR has an anti-resonance  320 , where its admittance is minimum, at an anti-resonance frequency of 4929 MHz. The Q factor of the exemplary YBAR is 560 at the resonance frequency and 600 at the anti-resonance frequency. The dashed curve  330  represents an approximate frequency shift caused by absorption or adsorption of a monolayer of a subject species by the sensing layer of a YBAW sensor. 
       FIG. 4  is a schematic block diagram of a sensor system  490  incorporating a YBAR sensor  400 . The YBAR sensor  400  includes a piezoelectric plate  410 , a substrate  420  with a cavity, first and second front-side conductor patterns  430 ,  432 , a floating back-side conductor pattern  434 , and a sensing layer  440 . In addition to a YBAR sensor, a sensor system must include a means for exposing the YBAR sensor to an environment in which the target species is to be detected (an environment that may or may not contain the target species) and a measurement subsystem  460  to determine if the resonance frequency of the YBAR sensor has, or has not, been affect by the presence of the target species. 
     Ideally, the measurement subsystem  460  applies a balanced RF signal to the conductor patterns  430 ,  432 , which is to say the RF signals applied of the two conductor patterns are of opposite polarity, or 180 degrees out-of-phase, and equal in amplitude. Further, the capacitance between each of the conductor patterns  430 ,  430  and the back-side conductor pattern  434  are substantially equal. In this case, there is minimal or zero potential on the backside conductor pattern  440 . Having minimal or zero potential on the backside conductor pattern  440  minimizes the influence of the electrical characteristics (e.g. dielectric permittivity and/or conductance) of the environment in which the target species is to be detected. 
     In the sensor system  490  of  FIG. 4 , the means for exposing the YBAR sensor  400  to an environment is depicted schematically as a rectangular chamber  450  with “sample in” and “sample out” ports. Depending on the environment and the target species, the means for exposing the YBAR sensor to the environment span a wide range of possibilities. The means may be as simple as exposing a YBAR sensor to a natural environment such as the ambient air or a flowing or stagnant body of water. At a slightly higher level of complexity, a YBAR sensor may be mounted, permanently or temporarily, in a pipe, duct, or other conduit through which the liquid or gaseous environment flows during a manufacturing or distribution process. In this context, “conduit” has the broadest meaning of “a natural or artificial channel though which something, such as a fluid, is conveyed”. Similarly, a YBAR sensor may be mounted, permanently or temporarily, in a tank or other container in which the liquid or gaseous environment is stored. The means for exposing the YBAR sensor to an environment may include a dedicated tube or conduit and, optionally, a pump, to direct all or a portion of an environment to the YBAR sensor. For example, a sample of a bodily fluid (the environment) may be automatically divided into a plurality of portions routed by conduits to respective sensors including one or more YBAR sensors. 
     The measurement subsystem  460  is configured to measure the resonance frequency of the YBAR sensor  400  and thereby determine the presence or absence, and optionally the amount, of a target species in the environment presented to the YBAR sensor. As previous described, the resonance frequency of the YBAR sensor  400  will typically be 1 GHz to 5 GHz and the anticipated frequency change due to capture of the target species may be about 0.1% of the resonance frequency. 
     A variety of known techniques may be used in the measurement subsystem  460  to measure the resonance frequency of the YBAR sensor  400 . For example, the measurement subsystem  460  may be a network analyzer that measures the admittance of the YBAR sensor over a predetermined frequency range. In this case the resonance frequency of the YBAR sensor is the frequency where the admittance is greatest. For a second example, the measurement system  460  may incorporate the YBAR sensor into an oscillator circuit. The frequency of the oscillation may be determined using a frequency counter. 
     The measurement subsystem may contain additional sensors not shown in  FIG. 4 . For example, since the resonance frequency of a YBAR sensor may be dependent on temperature, the measurement subsystem  460  may include a temperature sensor and a temperature compensation circuit or processor. 
       FIG. 5  is a schematic block diagram of another exemplary measurement subsystem  560  to measure the resonance frequency of a YBAR sensor  500 . The measurement subsystem  560  includes an oscillator circuit  570  coupled to the YBAR sensor  500  and configured to output a signal having a frequency equal to the resonance frequency of the YBAR sensor. The measurement subsystem  560  also includes a reference oscillator  575  configured to output a signal having a frequency equal to the resonance frequency of a reference YBAR  510 . The reference YBAR  510  and the YBAR sensor  500  may be, to the extent possible, identical. The reference YBAR  510  and the YBAR sensor  500  may be exposed to the environment in which the target species is to be detected. For example, the YBAR sensor  500  and the reference YBAR  510  may be concurrently fabricated on the same piezoelectric plate and substrate. The YBAR sensor  500  and the reference YBAR  510  may be identical except that the reference YBAR  510  does not include a sensing layer. Both the YBAR sensor  500  and the reference YBAR  510  may be exposed to the same environment and, importantly, may be at the same temperature. 
     The signals output from the oscillator circuit  570  and the reference oscillator  575  are multiplied by a mixer  580  and input to a low pass filter  585 . The output from the lowpass filter  585  is a signal with a frequency equal to the difference between the frequencies of the signals output from the oscillator circuit  570  and the reference oscillator  575 . The frequency of the signal output from the lowpass filter  585  may then be determined, for example, by a frequency counter  590 . 
     Description of Methods 
       FIG. 6  is a simplified flow chart of a method  600  for making a YBAR sensor such as the YBAR sensor  100  of  FIG. 1 . The method  600  starts at  610  with a piezoelectric plate disposed on a sacrificial substrate  602  and a device substrate  604 . The method  600  ends at  695  with a completed YBAR or filter. The flow chart of  FIG. 6  includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in  FIG. 6 . 
     Thin plates of single-crystal piezoelectric materials bonded to a non-piezoelectric substrate are commercially available. At the time of this application, both lithium niobate and lithium tantalate plates are available bonded to various substrates including silicon, quartz, and fused silica. Thin plates of other piezoelectric materials may be available now or in the future. The thickness of the piezoelectric plate may be between 300 nm and 1200 nm. When the substrate is silicon, a layer of SiO 2  may be disposed between the piezoelectric plate and the substrate. The piezoelectric plate  602  may be, for example, y-cut or rotated y-cut lithium niobate with a thickness of 300 nm to 1000 nm bonded to a silicon wafer with an intervening SiO 2  layer. The device substrate  604  may be silicon, fused silica, quartz, or some other material. 
     At  620 , the piezoelectric plate on the sacrificial substrate  602  and the device substrate  604  are bonded. The piezoelectric plate on the sacrificial substrate  602  and the device substrate  604  may be bonded using a wafer bonding process such as direct bonding, surface-activated or plasma-activated bonding, electrostatic bonding, or some other bonding technique. 
     After the piezoelectric plate on the sacrificial substrate  602  and the device substrate  604  are bonded, the sacrificial substrate, and any intervening layers, are removed at  630  to expose the surface of the piezoelectric plate (the surface that previously faced the sacrificial substrate). The sacrificial substrate may be removed, for example, by material-dependent wet or dry etching or some other process. 
     A front-side conductor pattern, such as front-side conductor patterns  130 ,  132  of  FIG. 1 , is formed at  640  by depositing and patterning one or more conductor layers on the surface of the piezoelectric plate that was exposed when the sacrificial substrate was removed at  630 . The conductor pattern may be, for example, aluminum, an aluminum alloy, copper, molybdenum a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. A conduction enhancement layer of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the front-side conductor patterns, such as the busbars of the IFPs. 
     The front-side conductor pattern may be formed at  640  by depositing the conductor layer and, optionally, one or more other metal layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques. Further, portions of the piezoelectric plate between the conductors of the front-side conductor pattern can be removed to form grooves in the piezoelectric plate between the conductors. For example, the portions can be removed during the same or a different etching process. The portions may be removed through an entire thickness of the piezoelectric plate or only to a certain depth. 
     Alternatively, the front-side conductor pattern may be formed at  640  using a lift-off process. Photoresist may be deposited over the piezoelectric plate. and patterned to define the front-side conductor pattern. The conductor layer and, optionally, one or more other layers may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern. 
     At  650 , a cavity is formed in the back side of the piezoelectric plate, opposite the position of the front-side conductor pattern. The cavity may be formed using an anisotropic or orientation-dependent dry or wet etch to open a hole through the substrate to the back side piezoelectric plate. 
     At  660 , a back-side conductor pattern is formed on the back side of the piezoelectric plate. The back-side conductor pattern can be formed to be like back-side conductor pattern  134  of  FIG. 1 . The back-side conductor pattern can be formed by depositing and patterning one or more conductor layers on the back surface of the piezoelectric plate through the cavity formed at  650 . The back-side conductor pattern may be, for example, aluminum, an aluminum alloy, copper, molybdenum a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. 
     After the back-side conductor pattern is formed at  660 , a sensing layer, such as the sensing layer  140  of  FIG. 1 , may be formed over all or a portion of the back-side conductor pattern. 
     The sensor device may then be completed at  680 . Actions that may occur at  680  include depositing an encapsulation/passivation layer such as SiO 2  or Si 3 O 4  over all or a portion of the front side of the device; forming bonding pads or solder bumps or other means for making connection between the device and external circuitry; excising individual devices from a wafer containing multiple devices; other packaging steps; and testing. After the sensor device is completed, the process ends at  695 . 
     A variation of the process  600  starts with a single-crystal piezoelectric wafer at  602  instead of a thin piezoelectric plate on a sacrificial substrate of a different material. Ions are implanted to a controlled depth beneath a surface of the piezoelectric wafer (not shown in  FIG. 6 ). The portion of the wafer from the surface to the depth of the ion implantation is (or will become) the thin piezoelectric plate and the balance of the wafer is the sacrificial substrate. At  630 , the piezoelectric wafer may be split at the plane of the implanted ions (for example, using thermal shock), leaving a thin plate of piezoelectric material exposed and bonded to the back-side conductor pattern. The thickness of the thin plate piezoelectric material is determined by the energy (and thus depth) of the implanted ions. The process of ion implantation and subsequent separation of a thin plate is commonly referred to as “ion slicing”. 
     Other variations of the process  600  include forming the back-side conductor patterns and optionally the sensing layer on the piezoelectric plate and/or forming the cavity in the device substrate prior to assembling the piezoelectric plate and device substrate at  620 . 
     Closing Comments 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments. 
     As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items.