Abstract:
A surface acoustic wave sensor to measure physical, biological or chemical parameters is claimed. Using different piezoelectric substrate materials, piezoelectric substrates with different thicknesses or metallizations with different thicknesses or patterns are used to distinguish between the effects of different physical, biological or chemical parameters.

Description:
[0001]    This application claims benefit of priority from U.S. Provisional Application No. 61/197,911, filed Nov. 3, 2008, which is incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a surface acoustic wave (SAW) device and more particularly to a surface acoustic wave (SAW) sensor to measure physical parameters such as temperature and strain. 
       BACKGROUND OF THE INVENTION 
       [0003]    Surface acoustic wave (SAW) devices are widely used as band-pass filters or resonators. Surface acoustic wave (SAW) devices are electronic components that generate guided acoustic waves along the surface of the device. Any changes to the characteristics of the propagation path affect the velocity, phase or amplitude of the acoustic waves propagating along the surface of the device. These changes can easily be measured. The changes in frequency, phase or amplitude can be correlated to a physical quantity such as temperature, pressure or strain, or the detection of the presence of a specific gas. Thus, the device can be used as a sensor. 
         [0004]    SAW sensors are very sensitive because the propagating acoustic wave has its energy concentrated close to the device surface. SAW devices are typically fabricated on single crystal anisotropic substrates that are also piezoelectric, such as quartz. A piezoelectric material produces electrical charges when it is subjected to mechanical stress. This phenomenon is reversible. A SAW sensor used to measure temperature, pressure, strain or the presence of a gas, typically includes a pair of spaced apart interdigital electrodes formed by a metal and disposed on the surface of the substrate. The interdigital electrode pair creates mechanical stress in the substrate when an electric field is applied. The (oscillatory) electric field creates a mechanical wave that propagates along the surface of the substrate. A second pair of interdigital electrodes converts the received mechanical wave back into an electric signal that is then compared to the original signal. 
         [0005]    One of the difficulties of achieving acceptable performance parameters with SAW sensors is that quartz undergoes an α to β transition at about 570° C. and loses its piezoelectric properties. Also, aluminum (Al), the most widely used metallization for SAW electrodes becomes soft when the temperature exceeds a few hundred degrees and actually melts at 660° C. For extended temperature ranges materials other than quartz have to be used. Materials such as LiNbO 3 , materials from the LGX family of crystals or gallium phosphate can be used to extend the temperature range. 
         [0006]    Another difficulty with SAW sensors is the fact that they cannot easily differentiate between different physical parameters. For example, a typical SAW sensor cannot easily distinguish between temperature and strain or temperature and pressure. Various physical parameters influence the propagation properties of a mechanical wave and the sensor cannot distinguish among them. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The preferred embodiments of the present invention overcome the problems discussed, particularly the problem of distinguishing between the influences of various physical parameters on the properties of the SAW device. This is accomplished by using different substrates on parts of the device or by using different thicknesses of the substrate or the metalization layers. Also, different metallization ratios (i.e. different width to periodicity ratios) can be used to generate different SAW parameters that can be used to distinguish between different parameters. As will be shown in the detailed description of the different embodiments of the invention, this will affect the frequency behavior of the device differently and will allow the determination of temperature, strain, pressure or the presence of a specific gas separate from each other, thus distinguishing between these parameters. 
         [0008]    Various objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic representation of a prior art surface acoustic wave (SAW) sensor with interdigital fingers. 
           [0010]      FIG. 2  is a schematic representation of a surface acoustic wave (SAW) sensor with reflectors. 
           [0011]      FIG. 3  is a schematic representation of a surface acoustic wave (SAW) sensor with different metallization ratios to separate the effect of temperature and strain. 
           [0012]      FIG. 4  is a schematic representation of one embodiment of a surface acoustic wave (SAW) sensor to separate the effect of temperature and strain based on sensor devices with different substrates. 
           [0013]      FIG. 5  is a schematic representation of an embodiment of a surface acoustic wave (SAW) sensor to separate the effect of temperature and strain based on different thicknesses of the piezoelectric substrate. 
           [0014]      FIG. 6  is a schematic representation of an embodiment of a surface acoustic wave (SAW) sensor to separate the effect of temperature and strain based on different thicknesses of the metallization. 
           [0015]      FIG. 7  is a schematic representation of an embodiment of a surface acoustic wave (SAW) sensor with a layered structure. 
           [0016]      FIG. 8  is a schematic representation of an embodiment of a surface acoustic wave (SAW) sensor with a split electrode arrangement. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring now to the drawings,  FIG. 1  shows a prior art surface acoustic wave device that will be used to explain the functionality of such a device. On top of one of the surfaces of a piezoelectric substrate  11  an interdigital structure (IDT)  12  is formed that acts as a generator for a surface acoustic wave. Interdigital electrode structures in the form of interleaved fingers are typically used. The material of the piezoelectric substrate  11  is typically quartz, lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 3 ). The piezoelectric substrate  11  can also be formed from a material of the LGX group (e.g. langasite, La 3 Ga 5 SiO 14 ), gallium phosphate (GaPO 4 ) or the like. These materials are particularly useful for high temperature operation since they do not exhibit a phase transition until very high temperatures (e.g. +1200° C. for langasite). The material of the interdigital structure  12  is typically aluminum (Al); however, other metals such as platinum (Pt), gold (Au), tungsten (W) or the like can also be used. The material for the interdigital structure is typically deposited by sputtering, vacuum evaporation, or chemical vapor deposition with a thickness of typically a fraction of a micron to several microns. 
         [0018]    A (oscillatory) voltage  13  is applied to the interdigital structure  12 . A series resistance  14  is usually present acting as a source resistance. The periodic voltage  13  applied to structure  12  generates periodic strain in the piezoelectric substrate that travels along the surface of the SAW device as a surface acoustic wave  15 . The surface acoustic wave  15  interacts with a second interdigital structure  16  and is converted back into an electric signal that produces a current  17  that flows through a load impedance  18 . 
         [0019]    The distance between two fingers of the same polarity is termed the electrical period q of the IDT. The maximum electroacoustic interaction is obtained at the frequency f 0 , the mid-frequency of the transducer. At this frequency the wavelength λ 0  of the surface acoustic wave precisely corresponds with the electrical period q of the IDT, so that all wave trains are superimposed in-phase and transmission is maximized 
         [0000]        q=λ   0   =v/f   0   [1]
 
         [0020]    The relationship between the electrical and mechanical power density of a surface wave is described by the material-dependent piezoelectric coupling coefficient k 2 . Around k −2  overlaps of the transducer are required to convert the entire electrical power applied to the IDT into the acoustic power of a surface wave. 
         [0021]    The velocity v of a surface wave on the substrate, and thus the propagation time τ and the mid-frequency f 0  of a surface wave component, can be influenced by a range of physical variables. In addition to temperature mechanical forces such as static elongation, compression, shear, bending and acceleration have a particular influence upon the surface wave velocity. This facilitates the remote interrogation of mechanical forces by surface wave sensors. 
         [0022]    In general, the sensitivity S of the quantity x to a variation of the influence quantity y can be defined as: 
         [0000]    
       
         
           
             
               
                 
                   
                     S 
                     y 
                     x 
                   
                   = 
                   
                     
                       1 
                       x 
                     
                      
                     
                       
                         ∂ 
                         x 
                       
                       
                         ∂ 
                         y 
                       
                     
                   
                 
               
               
                 
                   [ 
                   2 
                   ] 
                 
               
             
           
         
       
     
         [0023]    To first order, the influence of the quantity y (which can be temperature, strain et al.) on the mid-frequency f 0  and propagation time τ can be calculated as follows: 
         [0000]        v ( y )= v ( y   0 )·[1 −S   y   v ·( y−y   0 )]  [3]
 
         [0000]        f   0 ( y )= f   0 ( y   0 )·[1 −S   y   f ·( y−y   0 )]  [4]
 
         [0000]      τ( y )=τ( y   0 )·[1 +S   y   τ ·( y−y   0 )]  [5]
 
         [0024]      FIG. 2  shows another type of surface acoustic wave (SAW) device. Here one set of an interdigital structure  22  is placed on one of the surfaces of a piezoelectric substrate  21 . Reflectors  23  and  24  are placed a certain distance away from the interdigital structure  22  but on the same surface of the piezoelectric substrate  21 . A (periodic) voltage applied to the interdigital structure  22  converts the electrical signal into a surface acoustic wave  25  that propagates along the substrate  21  and is reflected by the reflectors  23  and  24 . Here the interdigital structure  22  converts the mechanical energy back into an electrical signal. 
         [0025]    If only the differential propagation times or the differential phase between the individual reflected pulses are evaluated, the sensor signal is independent of the distance between the reader and the transponder. The differential propagation time τ 2-1 , and the differential phase φ 2-1  between the two received pulses is obtained from the distance L 2-1  between the two reflectors, the velocity v of the surface wave, and the frequency f of the interrogation pulse. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           τ 
                           
                             2 
                             - 
                             1 
                           
                         
                         = 
                         
                           
                             2 
                             · 
                             
                               L 
                               
                                 2 
                                 - 
                                 1 
                               
                             
                           
                           v 
                         
                       
                       ; 
                     
                      
                     
                       
 
                     
                      
                     
                       
                         ϕ 
                         
                           2 
                           - 
                           1 
                         
                       
                       = 
                       
                         
                           2 
                            
                           π 
                            
                           
                               
                           
                            
                           
                             f 
                             · 
                             
                               τ 
                               
                                 2 
                                 - 
                                 1 
                               
                             
                           
                         
                         = 
                         
                           
                             4 
                              
                             π 
                              
                             
                                 
                             
                              
                             
                               f 
                               · 
                               
                                 L 
                                 
                                   2 
                                   - 
                                   1 
                                 
                               
                             
                           
                           v 
                         
                       
                     
                   
                    
                   
                       
                   
                 
               
               
                 
                   [ 
                   6 
                   ] 
                 
               
             
           
         
       
     
         [0026]    The measurable change Δτ 2-1  or Δφ 2-1  when a physical quantity y is changed by the amount Δy is thus: 
         [0000]      Δτ 2-1 =τ 2-1   ·S   y   τ   ·Δy; Δφ   2-1 =2 πf·τ   2-1   ·S   y   τ   ·Δy   [7]
 
         [0027]    The influence of the physical quantity y on the surface wave transponder can thus be determined only by the evaluation of the phase difference between the different pulses of the response signal. 
         [0028]    In a reflective delay line the available path is used twice. However, if the IDT is positioned between two fully reflective structures, then the acoustic path can be used many more times due to multiple reflections. Such an arrangement is called a surface wave one-port resonator. The distance between the two resonators must be an integer multiple of the half wavelength λ 0  at the resonant frequency f 1 . The displacement of the mid-frequency Δf 1  and the displacement of the associated phase Δφ 1  of a resonator due to a change of the physical quantity y with loaded Q factor are: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           f 
                           1 
                         
                       
                       = 
                       
                         
                           
                             - 
                             
                               
                                 f 
                                 1 
                               
                                
                               
                                 ( 
                                 
                                   y 
                                   0 
                                 
                                 ) 
                               
                             
                           
                           · 
                           
                             S 
                             y 
                             f 
                           
                           · 
                           Δ 
                         
                          
                         
                             
                         
                          
                         y 
                       
                     
                     ; 
                   
                    
                   
                     
 
                   
                    
                   
                     Δϕ 
                     = 
                     
                       2 
                        
                       
                         Q 
                         · 
                         
                           
                             Δ 
                              
                             
                                 
                             
                              
                             
                               f 
                               1 
                             
                           
                           
                             f 
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   8 
                   ] 
                 
               
             
           
         
       
     
         [0029]    where f 1  is the unaffected resonant frequency of the resonator. 
         [0030]    From the equations above it is obvious that the influence quantity y can be estimated by measuring time delay (τ), phase (φ), or frequency (f) variation. Temperature, strain, and other parameters can be measured with very good accuracy (0.1° C., 0.1 μstrain, etc.) at very high rate (10 5  measurements per second). 
         [0031]      FIG. 3  shows a specific embodiment of the current invention with two SAW sensors arranged in parallel with different metalization ratios. Similar to the structure shown in  FIG. 1  each SAW sensor in  FIG. 3  is formed with an interdigital structure (IDT)  32  on top of one of the surfaces of a piezoelectric substrate  31  which serves as a generator for a surface acoustic wave. The interdigital structures of the two different SAW sensors have the same periodicity  37 / 39  that determines the frequency of the device. However, the width  30  and  38  of the fingers of the IDTs  32  are quite different. The propagation properties of a SAW sensor depend on the ratio of the width  30  and  38  to the periodicity  37  and  39  and are used to distinguish between the different physical, biological or chemical effects. 
         [0032]      FIG. 4  shows another embodiment of the current invention. Two sensors are built on different substrate materials  41  and  47 . Material  41 , for example, is quartz, whereas substrate  47 , for example, is lithium niobate. According to equations [1]-[8] the effects of strain and temperature can be separated since the basic properties of SAW propagation depend on the substrate parameters. Since the substrates are different, the basic properties of SAW propagation are different and enough variables exist to separate strain and temperature effects in equations [1]-[8]. 
         [0033]    It is known that the SAW propagation parameters depend on the thickness of the piezoelectric substrate.  FIG. 5  shows an embodiment of the current invention where a portion of the piezoelectric substrate  51  is thinner than the other portion. Interdigital structures  52  are placed on the thinner and thicker portion of the device. Reflectors  53  and  54  are placed a certain distance away from the interdigital structures  52 . Surface acoustic waves  55  and  56  are generated by the interdigital structures  52  that travel along the surface of substrate  51  and are reflected by the reflectors  53  and  54 . Since the basic SAW propagation parameters depend on the thickness of substrate  51  enough variables are available to solve equations [1]-[8] for strain and temperature independently. 
         [0034]    It is also known that the basic SAW propagation parameters depend on the thickness of the metallization, i.e. the thickness of the interdigital structures and the reflectors.  FIG. 6  shows an embodiment of the current invention where two sets of interdigital structures  61  and  66  with different thickness are placed on one of the surfaces of a piezoelectric substrate  61  with constant thickness. Two sets of reflectors  63 , 64  and  67 , 68  are placed a certain distance away from the interdigital structures  62  and  66 . The thickness of reflectors  63  and  64  is equal to or close to the thickness of interdigital structure  62  whereas the thickness of reflectors  67  and  68  is equal to or close to the thickness of interdigital structure  66 . Since the basic SAW propagation parameters depend on the thickness of the interdigital structures  62  and  66  and the reflectors  63  and  64 , or  67  and  68 , enough variables are available to solve equations [1]-[8] for strain and temperature independently. 
         [0035]      FIG. 7  shows an embodiment of the current invention where the interdigital structure  72  is placed on the surface of a backing plate  73 . The material of the backing plate is non-piezoelectric. A piezoelectric layer  71  serving as the substrate is placed on top of the interdigital structure  72 . This piezoelectric layer  71  is preferably a polycrystalline layer of a material such as zinc oxide (ZnO). An additional conductive layer  72  is placed on top of the structure. 
         [0036]    The above detailed descriptions of embodiments of the invention are not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The teachings of the invention provided herein can be applied to other systems, not necessarily to the SAW sensor systems described above. These and other changes can be made to the invention in light of the detailed description. Furthermore, the elements and acts of the various embodiments above can be combined to provide further embodiments beyond those described.