Patent Publication Number: US-11641185-B2

Title: Direct write sensors

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 15/246,907, filed Aug. 25, 2016, which claims priority to U.S. Patent Application No. 62/214,233, filed Sep. 4, 2015, entitled “Direct Write Sensors”, the entireties of which are incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under contract No. DE-AC05-00OR22725 awarded by the US Department of Energy. The government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to acoustic wave sensors, and more particularly to methods and designs for making acoustic wave sensors. 
     BACKGROUND OF THE INVENTION 
     Bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices have been fabricated for decades using traditional semiconductor integrated circuit methods. These methods are mature and optimized to produce &gt;1 billion high fidelity, matched filters for RF and microwave communications such as cell phones, walkie-talkies, and other devices. However, relatively sophisticated and expensive equipment is required to produce these devices. 
     High performance acoustic wave structures have recently been exploited for sensor applications. Acoustic wave structures have been demonstrated to perform as temperature, strain and hydrogen sensing devices, employing traditional photo-lithography for device fabrication. Photo-lithography is time consuming and comparatively expensive, and accordingly new methodologies for manufacturing such sensors are necessary. 
     SUMMARY OF THE INVENTION 
     A method of making an acoustic wave sensor includes the steps of providing a piezoelectric substrate layer and printing on the substrate layer a sensor layer comprising a first interdigitated acoustic wave transducer, and at least one other feature selected from the group consisting of a sensing film, an interdigitated acoustic wave transducer, and a Bragg reflector. A second interdigitated acoustic wave transducer or one or more Bragg reflectors can be positioned on an opposing side of the sensing film, if present, from the first interdigitated acoustic wave transducer. An insulation layer can be printed. An antenna can be printed in an antenna layer, and the insulation layer can be interposed between the antenna layer and the sensor layer. An electrical connection can be printed between the antenna and the first interdigitated acoustic wave transducer. The printing method can be performed by aerosol jet direct digital printing. 
     The acoustic wave sensor can be a bulk acoustic wave sensor. The Q factor of the bulk acoustic wave sensor can be greater than 1000. 
     The piezoelectric substrate can be provided as a film and moved roll to roll during the printing process. 
     The method can further include the step of controlling the movement of the aerosol jet thorough a control system and at least one processor. 
     A plurality of acoustic wave sensors can be printed on the piezoelectric substrate, and the method can further include the step of separating the substrate and the acoustic wave sensors into individual acoustic wave sensors. 
     The sensing film can include a hydrophilic material. The sensing film can include palladium. The sensing film can include graphene. The sensing film can include a carbon nanotube array. The sensor can have a maximum dimension of less than 2 mm 2 . 
     The sensor layer and antenna layer can be printed on opposing sides of the piezoelectric substrate layer. At least a portion of the sensor layer and the antenna layer can be printed simultaneously. 
     An acoustic wave sensor can include a piezoelectric substrate layer and a sensor layer joined to the substrate layer and comprising a first interdigitated acoustic wave transducer, and at least one other feature selected from the group consisting of a sensing film, an interdigitated acoustic wave transducer, and a Bragg reflector. A second interdigitated acoustic wave transducer or one or more Bragg reflectors can be positioned on an opposing side of the sensing film, if present, from the first interdigitated acoustic wave transducer. All of the interdigitated acoustic wave transducers and Bragg reflector can be aerosol jet printed. 
     An antenna layer can be provided and an insulation layer can be interposed between the antenna layer and the sensor layer, and the insulation layer and antenna layer can be aerosol jet printed. At least one of the sensor layer and antenna layer can include a dielectric matrix material. The sensor layer and the antenna layer can be printed on opposing sides of the piezoelectric substrate layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There are shown in the drawings embodiments that are presently preferred it being understood that the invention is not limited to the arrangements and instrumentalities shown, wherein: 
         FIG.  1    is a schematic diagram of a surface acoustic wave device according to the invention. 
         FIG.  2    is a schematic diagram of a surface acoustic wave device functionalized by a chemical specific film. 
         FIG.  3    is a schematic diagram of aerosol jet printing of surface acoustic wave devices on a piezoelectric wafer. 
         FIG.  4    is a schematic diagram of multi-head printing of surface acoustic wave devices. 
         FIG.  5    is a schematic diagram of multi-head printing on both top and bottom surfaces of moving media. 
         FIG.  6    is a schematic diagram illustrating pulsed thermal processing of printed surface acoustic wave structures. 
         FIG.  7 A  is a schematic diagram of standard aerosol jet printing and  FIG.  7 B  is a schematic diagram of aerosol jet printing augmented by electrostatic/electromagnetic focusing. 
         FIG.  8    is a perspective schematic view of a surface acoustic wave device. 
         FIG.  9    is a cross-section of the surface acoustic wave device. 
         FIG.  10    is a perspective schematic view of a surface acoustic wave device with functionalizing film. 
         FIG.  11    is a cross-section. 
         FIG.  12    is a schematic cross-section of a surface acoustic wave device with printed fractal antenna. 
         FIG.  13    is a bottom view. 
         FIG.  14    is a schematic cross-section of the surface acoustic wave device with a printed spiral antenna. 
         FIG.  15    is a bottom view. 
         FIG.  16    is a schematic cross-section of a bulk acoustic wave device showing an acoustic wave propagating through the piezoelectric substrate, and with a functionalizing film on top of device. 
         FIG.  17    is a schematic cross-section of a multi-layer bulk acoustic wave device and a string of reflected signals. 
         FIG.  18    is a schematic cross-section of a variable layer thickness bulk acoustic wave device providing both frequency and time diversity in reflected signals. 
         FIG.  19    is a schematic diagram of an interdigitated transducer launching an acoustic wave. 
         FIG.  20    is a schematic diagram of a Bragg reflector returning an acoustic signal to the interdigitated transducer. 
         FIG.  21    is a schematic diagram of an interdigitated transducer and Bragg reflector and launched acoustic wave and reflected acoustic wave signal. 
         FIG.  22    is a schematic diagram and photograph of corresponding printed elements in a Bragg reflector. 
         FIG.  23    is a photograph of printed interdigitated transducer electrodes. 
         FIG.  24    is a schematic diagram of a dual channel surface acoustic wave device. 
         FIG.  25    is a schematic cross-section of a dual channel surface acoustic wave device showing antenna connections. 
         FIG.  26    is a schematic diagram of a dual channel surface acoustic wave device functionalized by hydrogel for moisture sensing. 
         FIG.  27    is a schematic diagram of a dual channel surface acoustic wave device functionalized by a Palladium film for hydrogen sensing. 
         FIG.  28    is a schematic diagram of a dual channel surface acoustic wave device functionalized by graphene nanostructures for methane detection. 
         FIG.  29    is a schematic diagram of the dual channel surface acoustic wave device functionalized by carbon nanotubes for CO 2  detection. 
         FIG.  30    is a photograph of printed dual channel surface acoustic wave devices. 
         FIG.  31    is a schematic diagram illustrating an array of surface acoustic wave devices printed simultaneously and sequentially. 
         FIG.  32    is a plot of sheet resistance (ohms/Sq) versus annealing temperature (° C.) for printed structures by an ultrasonic atomizer. 
         FIG.  33    is a plot of sheet resistance (ohms/Sq) versus annealing temperature (° C.) for printed structures by a pneumatic atomizer. 
         FIG.  34    is a photograph of a 10 μm line printed on a piezoelectric polymer (PV DF) substrate with 50 nm silver particles in a liquid suspension. 
         FIG.  35    is a schematic diagram of an ultrasonic atomizer print head. 
         FIG.  36    is a schematic diagram of a pneumatic atomizer print head. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method of making an acoustic wave sensor includes the steps of providing a piezoelectric substrate layer and printing on the substrate layer a sensor layer comprising a first interdigitated acoustic wave transducer, and at least one other feature selected from the group consisting of a sensing film, an interdigitated acoustic wave transducer, and a Bragg reflector. A second interdigitated acoustic wave transducer or one or more Bragg reflectors can be positioned on an opposing side of the sensing film, if present, from the first interdigitated acoustic wave transducer. An insulation layer can be printed. An antenna can be printed in an antenna layer, and the insulation layer can be interposed between the antenna layer and the sensor layer. The antenna can alternatively be provided separately, and printed or produced by other means. An electrical connection can be printed between the antenna and the first interdigitated acoustic wave transducer. The printing method can be performed by aerosol jet direct digital printing. 
     The movement of the aerosol jet thorough a control system and at least one processor. Any suitable control mechanism and processor or controller is possible. 
     A plurality of acoustic wave sensors can be printed on the piezoelectric substrate, and the method can further include the step of separating the substrate and the acoustic wave sensors into individual acoustic wave sensors. The individual sensors can be cut by any suitable device and packaged as individual sensors or groups of sensors. 
     The sensing film can be any suitable material. The sensing film can for example include a hydrophilic material. The sensing film can include palladium. The sensing film can include graphene. The sensing film can include a carbon nanotube array. The sensor can have a maximum dimension of less than 2 mm 2 . The acoustic wave sensor can be printed without a sensor film to measure temperature, because the piezoelectric material alone can be temperature sensitive in the desired temperature range. 
     The sensor layer and antenna layer can be printed on opposing sides of the piezoelectric substrate layer. The antenna can connect to more than one sensor. Sensors can be printed on both sides of the piezoelectric substrate layer. Vias can be created through the substrate and suitable electrical connections can be provided through the vias by any suitable method, including aerosol jet printing. At least a portion of the sensor layer and the antenna layer can be printed simultaneously. 
     An acoustic wave sensor can include a piezoelectric substrate layer and a sensor layer joined to the substrate layer and comprising a first interdigitated acoustic wave transducer, and at least one other feature selected from the group consisting of a sensing film, an interdigitated acoustic wave transducer, and a Bragg reflector. A second interdigitated acoustic wave transducer or one or more Bragg reflectors can be positioned on an opposing side of the sensing film, if present, from the first interdigitated acoustic wave transducer. All of the interdigitated acoustic wave transducers and Bragg reflectors can be aerosol jet printed. 
     An antenna layer can be provided and an insulation layer can be interposed between the antenna layer and the sensor layer, and the insulation layer and antenna layer can be aerosol jet printed. At least one of the sensor layer and antenna layer can include a dielectric matrix material. The sensor layer and the antenna layer can be printed on opposing sides of the piezoelectric substrate layer. Many antenna designs can be utilized including, without limitation, dipole antennas, gap loaded Archimedean spiral antennas, fractal antennas, and patch antenna arrays. 
       FIG.  1    is a schematic diagram of a surface acoustic wave device  10  according to the invention. The device  10  includes a piezoelectric substrate  14 . An interdigitated transducer  18  has interdigitated transducer lines  26  and contacts  34  which connect to antenna  42 . An interdigitated transducer  22  has interdigitated transducer lines  30  and contacts  38  which connect to antenna  46 . 
       FIG.  2    is a schematic diagram of the surface acoustic wave device  10  functionalized by a chemical specific film  50 . The film  50  can be printed by aerosol jet printing. The film  50  can also be applied by other methods. 
       FIG.  3    is a schematic diagram of aerosol jet printing of surface acoustic wave devices on a piezoelectric wafer. An aerosol jet print head  54  includes an outer housing  58  and neck  62 . An inner sheath  66  defines an annular space  74  for the flow of sheath gas that serves to focus and direct the aerosol  74  into a fine jet  78 . Movement of the print head  54  can be controlled by a processor and control mechanisms to direct the jet  78  so as to create sensors  82  on the substrate  86 . The substrate  86  can be in the form of a wafer and can be moved in the direction of arrows  90  and  94  to permit the printing of a multitude of sensors on the same wafer. 
       FIG.  4    is a schematic diagram of multi-head printing of surface acoustic wave devices. This is shown schematically by print head  54 A which moves according to direction arrow  104 , and print head  54 B which can move in the same or a different direction as shown by arrow  108 . Accordingly multiple sensors  100  can be printed rapidly and in some cases simultaneously onto substrate  98 . 
       FIG.  5    is a schematic diagram of multi-head printing on both top and bottom surfaces of moving media. Multiple print heads such as print heads  54 A and  54 B can be provided to print sensors  120  onto substrate  116 . The substrate  116  can be in the form of a film which moves from roll  124  to roll  128  in a roll-to-roll printing process. A print head  112  can be provided on an underside of the substrate  116  such that sensors can be printed simultaneously on both sides of the substrate  116 , or components of sensors can be printed on opposing sides of the substrate  116 , such as sensor layers on one side and antenna layers on the opposing side. The substrate  116  can be moved in the direction of arrow  132  such that a succession of sensors  120  can be rapidly printed. The process can also be configured to move vertically from roll to roll. 
       FIG.  6    is a schematic diagram illustrating pulsed thermal processing devices  136  and  144  emitting high intensity radiation  140  and  148  respectively to effect curing of the sensor layers on the substrate  116 . 
       FIG.  7    is a schematic diagram of ( FIG.  7 A ) of a standard aerosol jet print head and ( FIG.  7 B ) aerosol jet printing augmented by electrostatic/electromagnetic focusing. The focusing can be by any suitable structure such as electromagnets  152 ,  156  which focus the jet  78  to a very fine micro jet  160  enabling printing of features less than 10 μm in extent. 
       FIG.  8    is a perspective schematic view of a surface acoustic wave device  180  having interdigitated transducers  172  and  176  printed on piezoelectric substrate  168 .  FIG.  9    is a cross-section of the surface acoustic wave device  180 . 
       FIG.  10    is a perspective schematic view of the surface acoustic wave device  168  having substrate  180  with interdigitated transducers  172  and  176 , and with functionalizing film  184 . The functionalizing film  184  can also be applied by aerosol jet printing. As seen in  FIG.  11   , the features can be printed in a single pass of the substrate  180  past the print heads. 
       FIGS.  12 - 13    are a schematic cross-section and bottom view of a surface acoustic wave device  190  in which a substrate  194  has interdigitated transducers  198  and  202  printed on a 1 st  side  206 . A 2 nd  side  210  of the substrate  194  has printed thereon antennas  214  and  218 . Multiple sensors can be connected to the same antenna. The antenna  214  can have contact  222  and the antenna  218  can have contact  226  which contacts can be printed with the aerosol jet printer on a bottom side  210  of the substrate  194 . 
       FIGS.  14 - 15    are a schematic cross-section and bottom view of the surface acoustic wave device  230  with a having a substrate  234 . Interdigitated transducers  238  and  240 , and sensor film  244  can be printed on a 1 st  side  248  of the substrate  234 . On a 2 nd  side  252  of the substrate  234  can be printed spiral antennas  256  and  260 . Many different antenna designs can be printed. 
       FIG.  16    is a schematic cross-section of a bulk acoustic wave device  264  having a substrate  268 . An interdigitated transducer  272  is provided on a 1 st  side  276  of substrate  268 . An interdigitated transducer  280  is provided on 2 nd  side  284  of substrate  268 . A functionalizing film  290  is shown on top of the 1 st  interdigitated transducer  272 . An acoustic wave  294  is shown propagating through the piezoelectric substrate  268 . The interdigitated transducers  272  and  280 , as well as functionalizing film  290  can be printed on the substrate  268  by aerosol jet printing. The Q factor of the bulk acoustic wave sensor can be greater than 1000. 
       FIG.  17    is a schematic cross-section of a multi-layer bulk acoustic wave device  300  having a substrate  304  comprised of several layers  304  A-E. A 1 st  interdigitated transducer  308  can be provided on a 1 st  side  312  of the substrate  304 . A 2 nd  interdigitated transducer  316  can be provided on 2 nd  side  320  of substrate  304 . A functionalizing film  324  can be provided on top of the 1 st  interdigitated transducer  308 . The acoustic waves indicated by arrows  328  A-E can produce a string of reflected signals. All features of the device  300  can be printed on the substrate  304  by aerosol jet printing. 
       FIG.  18    is a schematic cross-section of a variable layer thickness bulk acoustic wave device  340  having a substrate  344 . The substrate  344  is comprised of layers  348 ,  352 , and  356  of variable layer thickness. A 1 st  interdigitated transducer  360  is provided on a surface  364  of the substrate  344 . A 2 nd  interdigitated transducer  368  can be provided on a side  372  of the substrate  344 . A functionalizing film  376  can be provided on top of the 1 st  interdigitated transducer  360 . Acoustic signals  380 ,  384 ,  388  will be reflected by the variable thickness layers according to the wavelengths of the signal and the properties and thickness of the layers  348 ,  352 , and  356 . The interdigitated transducers  360  and  368  and functionalizing film  376  can be printed by aerosol jet printing. The variable thickness bulk acoustic wave device  340  provides both frequency and time diversity in reflected signals. 
       FIG.  19    is a schematic diagram of an interdigitated transducer  392  having contacts  396  and  400  and interdigitated lines  404 . An acoustic wave schematically indicated as  408  is propagated in the direction shown by arrow  412 .  FIG.  20    is a schematic diagram of a Bragg reflector  416  having ends  420  and reflecting layers  424 . The Bragg reflector  416  selectively reflects signal  428  in the direction of arrow  432 . As shown in  FIG.  21   , the acoustic signal  408  is emitted and returned to the interdigitated transducer  392  as reflected signal  428 . As shown in  FIG.  22    the Bragg reflector  416  can be printed on substrate  430  by aerosol jet printing. There is shown in  FIG.  23    printed electrodes  434  and  438  of a dual channel interdigitated transducer printed on substrate  442 . A series of Bragg reflectors can also be used to create an identification code. Each Bragg reflector has a unique geometry (different line width and spacing) causing a group of returned signals each having a different frequency. 
       FIGS.  24 - 25    are a schematic diagram and cross-section of a dual channel surface acoustic wave device  450 . The device  450  includes a 1 st  electrode  454  having branches  458  and  462 . A 2 nd  electrode  466  has a terminus  470 . Interdigitated transducer lines  474  and  478  are positioned between the 1 st  branch  458  and the terminus  470  and the 2 nd  branch  462  and the terminus  470 . An antenna  490  can connect across the electrodes  454  and  466  to impart a signal. The antenna  490  can be connected by line  494  to electrode  466 , and by line  498  to electrode  454 . 
       FIG.  26    is a schematic diagram of the dual channel surface acoustic wave device  450  functionalized by hydrogel  502  for moisture sensing.  FIG.  27    is a schematic diagram of the dual channel surface acoustic wave device  450  functionalized by a Palladium film  506  for hydrogen sensing.  FIG.  28    is a schematic diagram of the dual channel surface acoustic wave device  450  functionalized by graphene nanostructures  510  for methane detection.  FIG.  29    is a schematic diagram of the dual channel surface acoustic wave device functionalized by carbon nanotubes  514  for CO 2  detection. These films can be printed by aerosol jet printing or fabricated by other means. Functionalization can also be achieved with organic compounds called amines. Amines are derived from ammonia and can be tailored for CO 2  detection, for example. The amine is also printed by aerosol jet methods. 
       FIG.  30    is a photograph of printed dual channel surface acoustic wave devices  518  printed according to the invention.  FIG.  31    is a schematic diagram illustrating how an array of surface acoustic wave devices  522  can be printed simultaneously and sequentially on a single substrate. Employing a printing process that implements a 2 mm 2  device with 2 mm pitch, results in ˜2,000 devices/wafer. A 50% decrease in sensor size more than triples the number of sensors per wafer. Multiple acoustic wave sensors as in  FIG.  30    can be provided for sensing different phenomena, and can be printed and combined to determine conditions or properties not able to be determined with a single acoustic wave sensor (such as to measure temperature and humidity in combination to determine dew point). An array of sensors such as shown in  FIG.  31    can be used to measure the same properties, and the results combined to create a statistical result with improved accuracy over the result from a single sensor. 
       FIG.  32    is a plot of sheet resistance (ohms/Sq) versus annealing temperature (° C.) for structures printed by an ultrasonic atomizer. The plot shows that sheet resistance generally decreases with increasing annealing temperature.  FIG.  33    is a plot of sheet resistance (ohms/Sq) versus annealing temperature (° C.) for structures printed by a pneumatic atomizer. The decrease in sheet resistance with annealing is not as pronounced for the pneumatic atomizer as for the ultrasonic atomizer. 
       FIG.  34    is a photograph of a 10 μm line printed with 50 nm silver particles suspended in alcohol on a piezoelectric polymer (PVDF) substrate. This figure illustrates the very fine features that can be printed for sensors according to the invention. 
       FIG.  35    is a schematic diagram of an ultrasonic atomizer print head assembly  520 . The print head  520  includes an ultrasonic transducer  524  for applying energy to the ink bath  528 . This creates droplets  522  which mix with gas from gas inlet  544  in chamber  536  and provide an aerosol in conduit  540  which exits at outlet  548  to the print head. The transducer  524  produces high-frequency pressure waves causing the ink or other liquid source material to be atomized. The atomized droplets are then entrained in the gas stream and deposited on the substrate. This device can be used for materials with viscosity in the range of 1-5 cP and particulate suspensions of 50 nm or less. 
       FIG.  36    is a schematic diagram of a pneumatic atomizer print head  560 . Gas enters the device through gas inlet  564  and mixes with in bath  568 . The aerosol so created exits at outlet  572  to the print nozzle. An exhaust  576  can also be provided. The pneumatic atomizer uses high-velocity gas stream to shear liquid ink into small droplets (1 to 5 μm). Compressed gas is expanded through the atomizer nozzle, producing a high-velocity jet. Larger droplets impact the walls, returning to the reservoir. Smaller droplets remain suspended for deposition. The pneumatic atomizer can be used for inks, pastes, or other materials with viscosity in the range of 1 to 1000 cP and particulate suspensions &lt;500 nm in size. 
     The BAW/SAW sensors of the invention can be fabricated by suitable aerosol jet equipment, such as can be obtained from Optomec, Inc. (Albuquerque, N. Mex.). Other possible fabrication methods including ink jet printing, screen printing, syringe printing, plasma pray printing, and myriad fused deposition printing methods including laser and electron beam melting, are capable of printing structures that fall into a subset of aerosol jet capabilities. Aerosol jet is a highly versatile additive manufacturing tool capable of efficiently depositing metals, semi-conductors, insulators, polymers and even biological materials in precise miniature patterns or larger structures depending upon the system configuration. 
     SAW devices rely on precise feature size printing in order to function in recognized ISM communication frequency bands compatible with many existing process control and automation systems. The invention has achieved some of the smallest aerosol jet sensor structures to date. Both a dual channel SAW device and a Bragg reflector having ˜6 um features have been produced. This feature size produces an operating frequency range of ˜125-175 MHz in the resulting SAW device. Controlling the operating frequency of acoustic wave sensors is important for two reasons: a) the operating frequency largely determines the operational communication distance between the passive sensor and the interrogator; and b) tailoring the frequency response provides each sensor with a unique identity. 
     Fabricating BAW sensors also requires precise deposition control. BAW devices rely on precise control of device layer thickness. Aerosol jet provides ˜10 nanometer layer thickness printing control. The performance of the device, commonly represented by the Q-factor, is also highly dependent on the device geometry. Aerosol jet printed BAW devices can achieve Qf&gt;1,000, enabling detection limits as small as 1 part in 10 9  (1 part/billion or 1 ppb). Similarly, SAW devices have also demonstrated sub-ppm detection limits. 
     The invention is capable of producing sensors with many different modes of sensing including: physical sensors, chemical sensors, biological sensors, location and tracking sensors, and tamper resistant seals. Possible sensor modalities include, without limitation physical sensors for temperature, strain, pressure, corrosion. Chemical sensors are used to measure humidity/moisture, CO 2 , CH 4 , HF, VOCs, H 2 , and the like. Electronic sensors measure current flow, voltage and charge. Biological sensors test for food spoilage, bacterial and fungal agents, blood sugar level, and other biometric factors. Location and tracking sensors and tamper resistant seals are also becoming more common. Sensing applications include physical sensing for energy control, condition monitoring, and presence detection. Electronic sensing is performed for grid monitoring and surveillance. Chemical sensing is used for pipeline monitoring, leak/plume detection, and accident response. Multiple sensor modalities can easily be combined to produce a unique signature or finger print. Other applications are possible. 
     BAW and SAW sensor functionality is realized by measuring the change in the acoustic wave propagation velocity within the device substrate. The following relationship describes this phenomenon by relating the interrogator operating frequency to the acoustic propagation velocity divided by the acoustic wavelength. The physical feature size of the device (BAW—layer thickness and SAW—spatial feature size and pitch) largely determines the operating parameters.
 
 v=c/λ   (1)
 
     In equation (1), v represents the electromagnetic interrogation frequency as described above. The invention is capable of printing SAW devices with 6-20 um feature size and operational frequencies ranging from 37.5-175 MHz. The device propagation velocity represented by c in equation (1) is the acoustic propagation velocity within the device substrate, not the speed of light. The acoustic velocity can range from 1,500 to nearly 4,500 m/s, depending upon substrate material selection. The λ represents the acoustic wavelength of the device determined by layer thickness or surface features for BAW and SAW devices respectively. For SAW devices being printed currently, the Bragg reflectors and IDT include ˜10 um line width and pitch. This critical feature size is generally interpreted as λ/4, the quarter-wave coupling wavelength from antenna theory. 
     
       
         
           
             
               
                 
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     In order to create an acoustic sensor, it is necessary to couple to the sensor substrate and modify the acoustic propagation velocity. There are two principle ways in which acoustic velocity is altered: a) mass loading—the substrate gains additional inertia through increased mass loading causing a mechanical impedance to the acoustic wave; and b) strain induced modification of the substrate elastic modulus—the acoustoelectric effect. Both phenomena result in a change of amplitude, phase, frequency or a time delay—the returned signal arrives later in time proportional to the changed mass or strain. 
     Surface acoustic wave devices require their substrate to be a ferroelectric material. More specifically, piezoelectric substrate materials such as lead-zirconate-titanate (PZT), lithium niobate, or quartz produce the greatest response or conversion efficiency. Polymeric substrates, such as polyvinyladene fluoride (PVDF), also exhibits ferroelectric properties, allowing acoustic devices to be printed upon flexible substrates. Other materials are possible. 
     Piezoelectric materials exhibit temperature dependent response, hence a SAW device functions as a temperature sensor without further functionalization. To sense other physical variables, the SAW device is functionalized by mechanical means. For instance, a very sensitive pressure sensor is created by functionalizing the SAW device by making the substrate a flexible element or diaphragm. This flexing is caused by changing pressure, creating a changing strain in the substrate that is detected as an acoustic velocity shift in the SAW device. 
     Sensing chemical or biological variables requires functionalization in a different manner. Humidity sensing is a relatively simple example of functionalization via mass loading. Polymethylmethacrilate (PMMA), is a reversible hydrophilic material that is readily printed by AJ methods. Hydrophilic aerogels can also be printed.  FIG.  26    illustrates a SAW humidity sensor architecture. The hydrophilic region is deposited into one of the SAW device channels. An acoustic velocity change occurs as moisture loading changes. As the film increases in mass, the acoustic velocity decreases, and vise versa. 
     Many film treatments are possible to functionalize the SAW device. Phase separated glass and metal oxide structures provide a scaffold having extremely high surface area. These high surface area structures are then functionalized by creating chemically specific absorption sites, such as, palladium nanoparticles for hydrogen sensing. A nanostructured spinodally phase separated ultra-porous silica film can be deposited on lithium niobate. 
     The sensor design and functionalization methods can be utilized with a single aerosol jet deposition head or multiple heads. The sensor designs can be scaled-up for roll-to-roll processing by employing a multi-head approach. Aerosol jet deposition can be performed with a parallel head array, but additional head arrays can also be performing additional fabrication steps down stream. A variety of materials can be printed, including conductors, insulators, semi-conductors, dielectrics, and biological materials. Feature sizes less than ˜10 μm and with no real upper limit are possible. The method is compatible with rigid and flexible substrates. The integration of sensors, antennas, and electronics is possible, with a wide range of operating communications frequencies 
     Key controls include ink selection (viscosity, conductor, insulator and dielectric), substrate selection (material, thickness, wafer or polymer film, deposition rate, nozzle size, atomization carrier gas pressure and flow rate, sheath gas pressure and flow rate, exhaust gas pressure and flow rate, scanning speed and thermal management. 
     EXAMPLE 
     Surface acoustic wave (SAW) sensors were developed using aerosol jet direct digital printing, employing an Optomec AJ200 system. Single channel and dual channel sensors were demonstrated. Sensor modalities include temperature, humidity and CO 2 . Volatile organic compound (VOC) sensors and other physical sensors such as pressure, corrosion, current and voltage are possible. The invention allows printing features as small as 5 microns, and feature size to 1 micron or less are possible. In order to fabricate these sensors, several parameters must be controlled. A 100 micron nozzle, combined with 25 ccm aerosol flow rate from an ultrasonic atomizer (UA) was utilized. The UA was set to 0.3 mA, with a nitrogen sheath gas flow rate of 22 ccm. The processing speed was 1 mm/sec, depositing Clariant silver colloid 25 ink with on a lithium niobate substrate. The total print time was 2 minutes. 
     The ease and low cost of manufacture of sensors created by the invention present the possibility of disposable sensors. Disposable sensors create a paradigm shift in measurement, control, monitoring, etc. by opening up new methods and approaches. For example, sensors embedded into products during manufacturing, sensors that are embedded into the manufacturing devices/equipment for intimate process monitoring, and sensors that flow-through the manufacturing process on test articles or the actual product itself, can be used to characterize process conditions in situ. Gaining intimate knowledge of manufacturing process conditions has never been done in this way and could lead to significant efficiency gains. Further, having process knowledge, item-by-item has also never been achieved and could be exploited for efficiency gains. For example, many thousand sheets of drywall are manufactured on a daily basis. The condition of the sheet versus the process could be measured providing pinpoint information of actual product condition, versus global process information. Disposable RF/SAW sensors could be applied to each sheet of drywall, monitoring in real-time, the temperature and moisture content of the sheet. Process conditions, such as oven temperature, residence time, etc. are then controlled in response to optimum product conditions instead of being controlled to a global fixed point. 
     There are many advantages of aerosol jet printing versus inkjet and screen printing. There is a wider possible material range (metals, alloys, resistor paste, dielectrics, polymers, adhesives). Most materials are standard off-the-shelf. There is no customization that is required. Significantly higher metal loadings are possible, as much as 50%-70 weight percent for high resolution printing. There is a 10 times smaller nano-particle droplet size to produce finer features. There is 20-30 times higher yield than inkjet per nozzle. There are higher volumetric rates and metal loading. There is superior throughput nozzle with higher material loading per droplet. There is 5 times the distance above substrate enables conformal printing on non-planar surfaces. There are much finer feature sizes (10 μm) and tighter pitch (20 μm) pitch. There is better edge definition. 
     There are many advantages of aerosol jet printing versus screen printing. Noncontact printing eliminates breakage of substrates. Noncontact process enables conformal printing on non-planar substrates. The aerosol jet process is easier to implement changes, and there is no hard tooling. Thinner layer deposits reduced material waste. Much finer feature size (10 μm) and tighter pitch (20 μm) are possible. 
     Direct printing of conductive patterns eliminates the use of toxic chemicals and saves costs. Nano silver conductive inks allow direct printing of conductive patterns for electronic devices on any surface. Hazardous chemical metallization processes are replaced by water-based ink containing metal particles to form the electronic pattern. Nano-sized silver particles can be cured at low temperatures to create an electric conductor almost as conductive as a pure silver wire. 
     The invention is capable of taking alternative forms without departing from the spirit or essential attributes thereof, and accordingly reference should be made to the following claims to determine the scope of the invention.