Abstract:
A surface acoustic wave sensor or identification device has a piezoelectric substrate, an interdigitated transducer (IDT) input/output mounted on a substrate for receiving a radio frequency (RF) signal and propagating a corresponding surface acoustic wave along a surface of the substrate. An IDT reflector array is mounted on the substrate and operable to receive a surface acoustic wave and reflect the surface acoustic wave in modified form back to the IDT input/output for transmission of a corresponding modified RF signal from the device. The IDT reflector array has at least one reflector segment whose reflectivity characteristics are controlled to control the nature of the modified RF signal.

Description:
FIELD OF INVENTION 
   The present invention relates to SAW sensors and identification devices. 
   BACKGROUND OF THE INVENTION 
   Surface acoustic wave (SAW) sensors and identification devices are passive radio frequency (RF) devices capable of exchanging information over both wired and wireless media depending upon the specific application. 
   SUMMARY OF THE INVENTION 
   According to the invention, SAW sensors and identification devices are configured with selectable reflector arrays which provide the capability of offering reflective segments of the reflector array, which consecutively contains multiple data bits of information within. As each SAW sensor or identification device is interrogated by an RF signal, the newly elongated reflected signal contains a data stream similar to the data selected within each reflective segment of the reflector array and is returned back to the interrogator. 
   The data embedded within the reflector array resembles a pulse position type of modulation (PPM) wherein a reflector segment within the array which is “on” reflects the interrogation signal and a reflector segment within the array which is “off” does not reflect the interrogation signal. This on/off state is achieved by controlling the load attached to the interdigital transducer (IDT) of the reflector segment. If the split finger electrode IDT load is open circuited, the IDT will reflect an incident SAW. Conversely, if the split finger electrode (IDT) load is shorted, the reflection capability of the IDT is greatly reduced. The reflector segment can also translate, by means of an altered magnitude and phase response, values of its load between the limits of an open circuit and a short circuit. 
   There are three ways of selecting the data of each reflective segment of the reflector array. The first is during the fabrication of the SAW device and is well suited for producing a random number of data bit configurations from a single fabrication process. All SAW devices are identically fabricated with all reflective segments set to “off”. A further processing step would then involve the laser trimming and subsequent opening of a split finger pair of electrodes with any reflector segment to produce an “on” segment. Such laser trimming can be computer controlled to produce a selective batch of coded devices. 
   The second way also involves fabricating identical SAW devices, but with fluidic channels positioned over an “on” split finger pair of electrodes within each reflector segment. A conductive fluid would then be selectively positioned within certain fluidic channels which, in the limit, effectively short the split finger electrodes of the IDT to produce an “off” state. Result is a selectively coded reflective array. Such positioning of the conductive fluid within the fluidic channels may result from sensor attributes by an intelligent process or by a selective acoustic wave. 
   The third way is comparable to the classification of electrochemical microsensors which measure resistance or the ability to measure current through an analyte. This way is similar to the second except that the fluidic channel is continuous so that a fluid analyte can flow over the split finger electrodes. The fluid analyte can be controlled by a micropump or by electric fields such as electro osmotic flow or by surface acoustic waves. This allows the metallized split finger electrodes to behave as ion-selective electrodes (ISEs). The conductivity of the analyte effectively controls the load of the reflector segment, thereby producing a magnitude and phase response characteristic of the properties of the analyte. The polymeric ion-selective membrane can also be photo patterned within the split finger electrode region to provide conductor sensitivity for certain vapor or liquid analyte being sampled via the fluidic channel. 
   A major aspect of this invention is thus the use of selectable reflector segments. The reflectors are selectable by microfluidic or intelligent trimming techniques to select and control the reflection magnitude and phase characteristics of a split finger IDT. Several of these IDT&#39;s may be configured as part of a total reflective array which contains a modifiable coded sequence. 
   Such election and control of the modifiable coded sequence may be achieved by varying the conductivity of select pairs of split finger electrodes within the IDT&#39;s of the reflective array, which in effect alters the load resistance of the IDT&#39;S, and which then alters the IDT&#39;s reflection properties to modify the coded sequence. 
   Invention enables manufacturing costs of SAW sensor and identification devices to be lowered by permitting the fabrication of identical devices and then selectively trimming certain reflector segments to produce a controlled batch of coded devices. 
   With the use of fluidic channels, the invention enables field selectable programming of the reflective segments which allows variable information from a single sensor, a network of sensors, financial smart card, or any other variable data apparatus including ZigBee applications to be entered into such reflective segments and then embedded into the reflected interrogation signal. The movement of the conductive fluid within the fluidic channels can be controlled by the attributes of the sensor or by an intelligent processor. 
   The invention is also applicable to the analyses of chemical materials in both laboratory and/or wireless applications. Since a SAW device is very small in profile and completely passive, a wireless electrochemical application will also work well as in-situ implants to monitor various chemical ionic responses. 
   The invention has various advantages. A first advantage is that a method is provided to lower the manufacturing costs by fabricating identical SAW devices and then implementing a computer controlled laser trimming process on certain split finger pairs of electrodes within selected reflective segments to produce a controlled batch of coded devices. 
   A second advantage is that its provides the ability of using sensor attributes such as pressure, temperature, centrifugal force and other physical characteristics of sensor transducers, including acoustic wave movement motion, to control the conductive fluid within the fluidic channels of the reflective array to provide a means of transcribing data to the device. 
   A third advantage is the ability of an analogue sensor to be interrogated by an RF signal and have the reflected RF signal turned back to the interrogator with the digital representation of the sensor embedded into it. The combination of the extended reflective array and the ability for the sensor attributes to turn “on” and “off” certain segments of the reflective array allows for a digitization of the sensor&#39;s analog quantity. 
   A fourth advantage relates to the ability of a reflective array to reflect an interrogation signal which is characteristic of the resisted properties of a vapor or liquid analyte. This allows the combination of SAW and microfluidic technologies to form an electrochemical sensor. The split finger electrodes of the SAW IDT and therefore the IDT&#39;s reflective signature react to chemical changes within the fluidic channel to produce an ion-selective electrode (ISE). Signal processing techniques performed at the interrogation unit would separate out the differences of the reflective signal to distinguish certain properties of the vapor or liquid analyte. This reaction may also implement a polymeric material within the fluidic and electrode regions to support ionic measurements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: 
       FIG. 1  is a block diagram of a simple system in accordance with one embodiment of the invention comprising a reader transceiver interrogating sole identification or sensor devices with an RF signal via a wired or wireless media, 
       FIG. 2  is a schematic perspective view of basic elements of a wireless sole identification or sensor device, 
       FIG. 3  is a diagrammatic view of elements of a split finger IDT reflector segment, 
       FIG. 4  is a similar view of elements of a selectable reflector array suitable for laser trimming, 
       FIG. 5  is a similar view of elements of a selectable reflector array suitable for fluidic control, 
       FIG. 6  shows magnitude and time diagrams of reflected SAW waveforms from a reflector array, 
       FIG. 7  is a diagrammatic view of elements of a selectable reflector array suitable for a chemiresistor, 
       FIG. 8  shows magnitude and time diagrams of reflected sole waveforms applied to a chemiresistor type sensor, and 
       FIG. 9  is a diagrammatic view of elements of a selectable reflector with modified metallized regions within fluidic channels. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to the drawings,  FIG. 1  shows a selectable reflective array which can be used for sensor and radio frequency identification devices (RFID). A base interrogator  110  initiates a sequence of events to query a remote SAW device  120  or, as part of a certain protocol, several remote SAW devices  123 ,  125 . An intelligent process initiates a sequence of events where the base interrogator  110  transmits, via a base antenna  114  or wired interface  135 , and interrogation signal  115 ,  116  which propagates towards the antennas  130 ,  133 , of the remote SAW devices  120 ,  123 , or is transmitted via the wired interface  135 . Once received within the selectable reflector SAW device  120 ,  123  and  125 , the interrogation acoustic signal is selectively reflected with encoded data and retransmits from the antennas  130 ,  133 , or via the wired interface  135 . The encoded data wave form  140 ,  143  and  145  returns to the base interrogation unit  110  via its antenna  114  or wired interface  135  to be processed. The data processed at the base unit represents the data embedded into the selectively reflective acoustic signal. 
   A schematic view of the selectable reflector SAW device  120 ,  123 , or  125  is shown in  FIG. 2 . The SAW circuit is fabricated on a piezoelectric substrate  200 . The input/output IDT  205  has metallized finger electrodes placed on the surface of the piezoelectric substrate and receives the interrogation signal  116  via the attached interface  220  which may be an antenna or a wired interface. An electrical to acoustic wave transformation occurs within the IDT  205 , and an incident acoustic wave  240  propagates along the piezoelectric substrate until it reaches a reflector array  230 . The reflective array  230  has one or more reflective segments which, in turn, selectively reflect back the incident acoustic wave  240  to produce a concatenated reflected acoustic wave  250 . This concatenated reflected acoustic wave contains reflected elements of the incident acoustic wave, depending upon the selectable load conditions of each reflective segment within the reflective array  230 . The concatenated reflected acoustic wave  250  transforms within the input/output IDT  205  which converts the acoustic wave to electrical signals which are propagated back via the interface  220  to the base interrogation unit  110 . Due to the harmonic content of the reflected acoustic wave  250 , it would be desirable to implement single figure electrodes for the input/output IDT  205  to suppress the conversion of unwanted harmonic frequencies. 
   The composition of the elements of a split finger IDT reflector segment located within the reflector array  230  is illustrated in  FIG. 3 . An incident acoustic wave  300  approaches the reflector segment  310  which is fabricated such that the metallized split finger electrodes  315  are positioned in pairs which are alternately attached to the metallized lower bus bar  320  and to the upper bus bar  325 . For a split finger IDT, each finger width and adjacent space is nominally an eighth-wavelength in width. A load element  330  is electrically connected to the lower bus bar  320  and to the upper bus bar  325 . The characteristics of the reflector segment  310  can be predicted by the P-matrix notation for the reflection of a split finger IDT, which is terminated by a load admittance YL as shown in equation (1). 
           P22   ⁡     (   YL   )       =   P11     ,           ⁢     sc   +       2   ⁢     P13   2       P33     +   YL         
 
at the limits, when Y L =0 (open circuit), then the IDT achieves maximum reflection, ie. an “on” condition and, when Y L =∞ (short circuit), then a minimum of reflection occurs, ie. an “off” condition within the IDT. As Y L  is varied between the limits of an open and a short circuit, P 11  (Y L ) will vary both in magnitude and phase accordingly.
 
   The effect of the load  330  then determines the presence of the reflected acoustic wave  305 . Since there is not a total reflection of the incident acoustic wave  300 , a continuing incident acoustic wave  340  continues to propagate on to the next reflector segment of the reflector array. Depending on subsequent load terminations, a reflective wave  345  is reflected back from the subsequent reflective segments. 
   The reflector array  230  is expanded in  FIG. 4  to illustrate its various functional elements. An incident acoustic wave  400  first meets a reference IDT  410  which is continuously configured as a reflector by keeping its load as an open circuit. The open circuit load is accomplished by eliminating any electrical connection between the adjacent sets of split finger pairs of electrodes. The reference reflector  410  inserts the equivalent of a “start bit” in the reflective acoustic wave  405 . 
   Other elements of this reflective array are the individual reflective segments  430 ,  440 ,  450 , which are located linearly within the acoustic wave path. The number of reflective segments depends on the number of bits chosen for the specific sensor and RFID application. The reflective segments  430 ,  440 ,  450  within the reflector array are all fabricated as “off” segments, in that a selected pair of split finger electrodes act as a shorted load element electrically connecting the two bus bars  320 ,  325  and all of the electrode finger pairs together. These selected finger pairs are then exposed to selectable regions  435 ,  445 , and  455  of the reflective segments  430 ,  440 , and  450  respectively. During fabrication, a computer controlled trimming process selectively cuts the selective split finger electrodes to produce a controlled batch of coded reflector arrays which in effect produces a controlled batch of SAW senor and identification devices. The depiction of the selectable regions  435 ,  445 , and  455  are shown as singular regions for each of the reflective segments  430 ,  440 , and  450 . However, in practice, the selectable regions can be replicated at each side or end of the IDT. 
   An arrangement which allows for “field programming” of the reflector segments of the reflector array is illustrated in  FIG. 5 . The reference reflector  510  performs the same function of initiating an equivalent start bit as did the previous reference reflector  410  of  FIG. 4 . The remaining reflector segments  530 ,  540 , and  550  are all fabricated as “on” segments, in that the effective load between the bus bars is an open circuit. Within the first reflector segment  530 , there is located a selectable region  535  which contains an open pair of split finger electrodes and a fluidic channel  533 . When a conductive fluid fills the fluidic channel  533 , the conductive fluid effectively shorts out the pair of split finger electrodes within the selectable region  535 . This in effect electrically connects the two bus bars and all of the electrode finger pairs together to reduce the reflective characteristics of the reflecting segment  530  to produce an “off” segment. Similar sequences can occur for reflector segments  540  and  550  with selectable regions  545  and  555  and fluidic channels  543  and  553  respectively. 
   Diagrams showing amplitude versus time characteristics of the reflected acoustic waves are shown in  FIG. 6 . Both waveforms  600  and  610  can be the result of reflector arrays configured as in the computer aided trimming process shown in  FIG. 4  or as a “field programmable” reflective array with the use of fluidic channels as shown in  FIG. 5 . For the first waveform  600  of  FIG. 6 , there are a total of four signals which indicate a reflective array with one reference reflector and three reflective segments. The signals are illustrated on a vertical amplitude and horizontal time axis. The first signal  603  is the reflected response from the reference reflector  410  or  510  shown previously. The next signal  604  is the result of the incident wave continuing through the reference reflector  410  or  510  and reflecting from the first reflector segment  430  or  530 . 
   The load for the reflecting segments  430  or  530  must be an open circuit so maximum reflection occurs. It should be noted that the amplitude of signal  604  is slightly smaller than that of signal  603  due to losses within the system. Similarly, signal  605  is the result of the incident wave continuing through the first reflective segment  430  or  530  and reflecting from the n−1 reflector segment  440  or  540 . The load for the two reflecting segments  440  or  540  must also be an open circuit so maximum reflection occurs. It should again be noted that the amplitude of signal  605  is slightly lower than that of the preceding signal  604  due to losses within the system. Similarly, signal  606  is the result of the incident wave continuing through the n−1 th  reflective segment  440  or  540  and reflecting from the n th  reflector segment  450  or  550 . The load for the reflecting segments  450  or  550  must be a short circuit so a minimum of reflection occurs. It should again be noted that the amplitude of signal  606  is much lower than that of the preceding signal  604  due to the inability of the n th  reflector segment  450  or  550  to reflect an incident wave. 
   Similarly, for waveform  610 , signal  613  is also the result of a reference reflector segment, with signal  614  and signal  615  being the result of having reflector segments configured with a short circuit load, and with signal  616  resulting from having a reflector segment configured with an open circuit load. The waveform  600  and  610  can be construed as digital representations of successive load conditions, namely 1 1 1 0 and 1 0 0 1 respectively. 
   This invention can readily be adapted to combine SAW and microfluidic technologies to form an electrochemical ion-selective sensor.  FIG. 7  illustrates how a reflective array can be arranged with three reflector segments to create a SAW based microfluidic sensor. This sensor would provide the means of sensing both vapor and liquid analyte samples. This can be accomplished by using photolithographic techniques to impose a sorptive polymeric material within the fluidic channel  730  and the selectable split finger electrodes  725 . The incident surface acoustic wave  700  is excited from an input/output IDT  205  of  FIG. 2 . The IDT  205  need not be attached to an antenna as this use could be implemented in a wireless situation or a laboratory benchtop electrochemical senor wired via a suitable interface to an intelligent processor. 
   The incident SAW  700  will reflect from the first reference reflector  710  to produce a first reflective SAW  701 . A continuing second incident SAW will propagate through the first reference reflector  710  to interact with the selectable reflector segment  720 . The conductivity of the sample fluid entering the fluidic channel  733  and exiting the fluidic channel  735  would determine the load component Y L  of equation (1). For a fluid sample low in ions, presenting a low conductivity case, the selectable reflector segment  720  will produce a maximum reflective SAW  705  from the second incident SAW  704 . For a fluid sample containing various concentrations of ions, the value of Y L  will vary, therefore producing varying magnitude and phase values of the reflected SAW  705 . At the limit of maximum concentration of ions, YL is a short circuit, therefore minimizing the second reflected SAW  705 . A continuing third incident SAW will propagate through the selectable reflector segment  720  to interact with the second reference reflector  715 . The incident SAW  708  will reflect from the second reference reflector  715  to produce a third reflective SAW  709 . All reflective SAW components  701  will propagate towards the input/output IDT  205 . 
   Three examples of the magnitude versus time responses of the electrochemical sensor of  FIG. 7  are illustrated in  FIG. 8  for various ion concentrations of the sample analyte. In the first sequence, signals  801  and  805  are from reference reflectors, and signal  803  has a magnitude and phase representative of the reference signals  801  and  805  to indicate that the analyte being measured has minimal conductivity. The second sequence sampled signal  813  shows a marked difference in both magnitude and phase with respect to the reference signals  811  and  815 . Signal processing techniques can be implemented to enhance this difference, which can then be extrapolated using equation (1) to determine the load value of Y L , which in effect determines the ionic composition of the sample analyte. The final sequence shows how the signal  823  compares with the two reference signals  821  and  825  to indicate a reflection segment with a minimum of reflection which tends, in the limit, to indicate a short circuit Y L . This condition represents a sampled analyte which has maximum ionic concentration. The time slots indicated by  831 ,  833  and  835  represent the time duration of the reflected acoustic waves and are indicative of the spatial length of the interrogation pulse  116  shown in  FIG. 1 . The time spacing between reflected pulses  842  and  844  are dependent on the spacing of the reflector segments of the reflector array. 
   The reflector segment  720  shown in  FIG. 7  has typical split finger dimensions of approximately 0.21 μm, assuming a 0.5 metallization ration, an operating frequency of 2,400 MHZ and a 128° YX-LiNb0 3  substrate. The fluidic channel  730  is dimensioned to contained one pair of split fingers. Analyte with dimensions greater than the width of the split fingers will possibly have difficulty traversing the fluidic channel and providing adequate conductivity. A modified reflector segment  920  and a larger fluidic channel  930  which overcome this problem are shown in  FIG. 9 . The width of both the upper bus bar  921  and lower bus bar  922  within the fluidic channel  930  can be varied in width during manufacturing. The width of these modified metallized regions of the upper bus bar  921  and lower bus bar  922  produce a gap width  925  which can be arranged to be suitable in dimensionality to the selected analyte. This configuration would still allow a sorptive polymeric material to be placed within the fluidic channel  930 . The flow of analyte into the fluidic channel input  933  and out the fluidic channel output  935  can be controlled by an intelligent process using pressure, electric fields such as electro osmotic flow or surface acoustic waves. 
   The advantages of the invention will now be readily apparent to a person skilled in the art from the above description of preferred embodiments. Other embodiments and advantages of the invention will also now be readily apparent to a person skilled in the art, the scope of the invention being defined in the appended claims.