Abstract:
A surface acoustic wave device sensor configured so as to have at least two different modes of operation. An acoustic response is obtained from each of the different modes of operation. The different modes of operation are a combination of a temperature effect and a measurand effect. The measurand effect is caused by the absorption and/or adsorption of a substance into a selective coating on the piezoelectric substrate. The two different modes of operation are effected differently by the temperature effect and therefore can be used to effectively eliminate the temperature effect by simultaneously solving equations representative of the different modes of operation. The present invention eliminates the need to provide other relatively more complicated temperature compensating structure or to maintain the device at a predetermined constant temperature. The present invention can be used to detect different chemicals or substances.

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, sold, imported, and licensed by or for the Government of the United States of America without the payment to me of any royalty thereon. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to a detector or sensor for detecting a substance, and more particularly to a surface acoustic wave sensor that compensates for temperature variations. 
     BACKGROUND OF THE INVENTION 
     Surface acoustic wave sensors are often used to detect the presence of substances, such as chemicals. A surface acoustic wave or SAW device acting as a sensor provides a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Surface acoustic wave devices are fabricated using photolithographic techniques with comb-like interdigital transducers placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration. The selectivity of a surface acoustic wave device sensor is generally determined by a selective coating placed on the piezoelectric material. The absorption and/or adsorption of the species to be measured into the selective coating causes mass loading, elastic, and viscoelastic effects on the device. The change of the acoustic property due to the absorption and/or adsorption of the species can be interpreted as a delay time shift for the delay line surface acoustic wave device or a frequency shift for the resonator surface acoustic wave device. However, the response of the surface acoustic wave sensor is also effected by environmental changes, such as temperature, pressure, stress, among others. These environmental changes degrade the response of the surface acoustic wave sensor. Temperature generally has the severest effect on the response, which may cause a misinterpretation. In the past, low temperature coefficient material has been selected to reduce the temperature effect. However, this has not always been successful because the selective coating used on the piezoelectric material may change the temperature characteristics of the material. Precision control of the temperature of the sensor has also been utilized, with temperature being controlled in the range of milidegrees. However, such precise temperature control is difficult, and temperature gradients between the sensor and a temperature sensor for the temperature control generally cannot be avoided. To separate the temperature effect from the measurand effect in surface acoustic wave sensors, the upper harmonic mode operation utilizing dispersion in the layered structures and the two device configuration with perpendicular direction as a convolver has been suggested. In bulk acoustic wave resonator sensors, the dual mode operation of a SC-cut quartz resonator was suggested for a temperature compensation. Temperature compensation in other devices is known. For example, in U.S. Pat. No. 4,535,638 entitled “Resonator Transducer System With Temperature Compensation” issuing to EerNisse et al on Aug. 20, 1985. Therein disclosed is an apparatus including an oscillator such as a quartz crystal, which is caused to resonate by the oscillator at two frequencies. The vibratory element is selected so that the two frequencies both vary with variations in force applied to the element and with variations in temperature of the element. Another device is disclosed in U.S. Pat. No. 5,869,763 entitled “Method For Measuring Mass Change Using A Quartz Crystal Microbalance” issuing to Vig et al on Feb. 9, 1999, which is herein incorporated by reference. Therein disclosed is a quartz crystal resonator excited in two different modes at the same time such that the mass change and the temperature change can be measured independently. The change in mass can be calculated accurately, independent of temperature effects. 
     Accordingly, there is a need to provide for temperature compensation in a surface acoustic wave sensor for detecting the presence of a substance or chemical. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a surface acoustic wave sensor that is operated with a crystal cut and propagation direction that can be operated simultaneously with a combination of different modes such as a surface acoustic wave (SAW)(or Rayleigh wave) mode, leaky surface acoustic wave (LSAW) mode or pseudo surface acoustic wave (PSAW) mode, and harmonics modes, such as the upper odd harmonics, from a single device layout. Each of these different modes have different temperature coefficients. The intrinsic dual mode operation of a single surface acoustic wave device sensor is utilized to separate the temperature effect from the measurand effect. The multi-mode or two-mode response is represented by multiple equations which may be solved to separate the response due to the temperature changes from the response due to the measurand. 
     Accordingly, it is an object of the present invention to provide a surface acoustic wave device sensor that provides accurate analysis of the exposed substance independent of temperature. 
     It is an advantage of the present invention that the surface acoustic wave device compensates for temperature variations. 
     It is a feature of the present invention that the surface acoustic wave device sensor operates with a multi-mode. 
     These and other objects, advantages, and features will be readily apparent in view of the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a surface acoustic wave device sensor in a delay line configuration. 
     FIG. 2A is a plan view of a surface acoustic wave device sensor in a resonator configuration with a one-gate resonator. 
     FIG. 2B is a surface acoustic wave device sensor in a resonator configuration having a two-gate resonator. 
     FIG. 3 is a perspective view of a surface acoustic wave device sensor. 
     FIG. 4 is a cross section of the device illustrated in FIG. 3 taken along line  4 — 4 . 
     FIG. 5 is a block diagram illustrating the method steps of the present invention. 
     FIG. 6 is a schematic diagram illustrating the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a plan view of a surface acoustic wave (SAW) device sensor  10  having a delay line configuration. The SAW device sensor  10  comprises a piezoelectric material  12  on which are placed input interdigital electrodes  14  and output interdigital electrodes  16 . The interdigital electrodes  14  and  16  form transducers converting electrical energy into a surface acoustic wave. The surface acoustic wave is represented by arrow  18 . Depending upon the crystal cut of the piezoelectric material and the propagation direction, a surface acoustic wave device can be operated simultaneously with the combination of a surface acoustical wave (SAW) (or Rayleigh wave), leaky surface acoustic wave (LSAW) or pseudo surface acoustic wave (PSAW), and harmonic modes from a single device layout or configuration. As an example, both surface acoustic waves and leaky surface acoustic waves exist in a thirty-six degree rotated Y-cut of lithium tantalate (LTO) with the free-surface acoustic velocities of 3125 meters per second and 4227 meters per second, respectively. Additionally, both surface acoustic wave (SAW) and leaky surface acoustic wave (LSAW) modes exist in a one hundred twenty-eight degree rotated Y-cut of lithium niobate (LNO) with velocities of 3580 meters per second and 4693 meters per second, respectively. There are many other possible selections of piezoelectric material, cut, and propagation directions that may be utilized in practicing the present invention. All of these may be determined without any undue experimentation. It is only necessary that the different modes have different temperature coefficients. 
     FIG. 2A is a plan view of another sensor. A surface acoustic wave device sensor  110  has a resonator configuration with one gate. Formed on the piezoelectric material  112  are interdigital electrodes  117 , acting as a transducer, and reflectors  122 . 
     FIG. 2B illustrates another embodiment of a surface acoustic wave device sensor  210  having a resonator configuration with two gates. A piezoelectric material  212  has formed thereon two interdigital electrodes  117  and  117 ′, acting as transducers. Bounding the two electrodes  117  and  117 ′ are two reflectors  222 . 
     FIG. 3 is a perspective view illustrating another embodiment of the present invention. A surface acoustic wave device sensor  310  has a delay line configuration. An input interdigital electrode  314  and an output interdigital electrode  316  are formed on and coupled to a piezoelectric substrate  312 . The piezoelectric substrate  312  has a selective coating  324  thereon. The selective coating  324  is selected such that a particular species to be measured is absorbed by the selective coating  324  changing the acoustic properties of the surface acoustic wave device. Different selective coatings are well known. This change in acoustic properties is detected and is used to identify or detect the substance or species absorbed and/or adsorbed by the selective coating  324 . 
     FIG. 4 is a cross section taken along line  4 — 4  in FIG.  3 . FIG. 4 more clearly illustrates the structure of the present invention. The selective coating  324  need not cover the entire planar surface of the piezoelectric substrate  312 , but need only cover a portion thereof. The selective coating  324  illustrated covers the electrodes  314  and  316  and the entire planar surface of the piezoelectric substrate  312 . 
     In operation, because the crystal cut and propagation direction of the surface acoustic wave device is selected to operate simultaneously in different modes, the intrinsic dual mode operation of a single surface acoustic wave device sensor may be utilized to separate the temperature effect from the measurand effect. The temperature effect is the change in acoustic property due to a change in temperature, and the measurand effect is the change in acoustic property due to the absorption and/or adsorption of the substance or material exposed to the selective coating. Accordingly, the multi-mode or two mode responses can be denoted by multiple or two simultaneous equations. 
     
       
           r   1   =f   1 ( m )+g 1 ( T ) 
       
     
     
       
           r   2   =f   2 ( m )+ g   2 ( T ) 
       
     
     where, 
     r represents the acoustic response with the subscripts referring to the mode, either surface acoustic wave or leaky surface acoustic wave or surface acoustic wave and their harmonic modes; 
     f(m) represents the acoustic response function due to the measurand, material or substance being measured or detected, with the subscripts referring to the mode; and 
     g(T) represents the acoustic response function due to the change in temperature, with the subscripts referring to the mode. 
     Of course, in the above, the acoustic response function f(m) is calibrated by obtaining the response of the selective coating to a measurand under a stabilized temperature condition. Additionally, the acoustic response function g(T) is calibrated for the temperature change effects without exposing it to a measurand; for example, in a vacuum or dry air. By simultaneously solving these two equations, the response due to the temperature changes can be separated from those due to the measurand effect. Therefore, the need to control temperature is unnecessary or made less strict. It is possible to utilize the teachings of the present invention to utilize the multi-mode operation in more than the two fundamental modes only. By utilizing more modes from higher harmonic modes, simultaneous equations may be solved to compensate for more effects, including other environmental effects, other than the temperature and measurand effects. For example higher order odd harmonic modes of the fundamental SAW mode may be used. Multiple harmonic modes may be used provided the different harmonic modes have a different response to an environmentally effect, such as temperature or pressure. Additionally, the present invention may be utilized in an array of sensors that have multiple coatings which may be utilized to increase the selectivity by a pattern recognition technique for chemical sensor applications. For example, an array of surface acoustic wave device sensors may be fabricated on a planar surface with selective coatings associated with each separate surface acoustic wave device in the array. 
     FIG. 5 is a block diagram illustrating the method steps of the present invention. Block  28  represents the initial calibration of the surface acoustic wave device with a selective coating for an environmental effect, such as temperature, and the measurand effect. Block  30  represents the step of exposing the surface acoustic wave device having a multi-mode of operation to an analyte or substance to be determined, with each mode having a different response to an environmental effect, such as a temperature effect, and a measurand effect or effect due to absorption and/or adsorption of a substance to be detected. Block  32  represents the step of exciting the surface acoustic wave device such that a surface acoustic wave is transmitted through a selective coating resulting in a modified acoustic property due to the absorption and/or adsorption of the analyte or substance. Block  34  represents the step of detecting the surface acoustic wave device response. Block  36  represents the step of separating the measurand effect from the temperature effect. This step effectively represents the simultaneous solution of the two equations to solve for the measurand effect, effectively eliminating the temperature effect. Block  38  represents the step of determining the analyte. This step is achieved by detecting the changed acoustic properties of the selective coating with a substance absorbed therein. Based upon the change in acoustic properties, the analyte can be determined by known techniques. 
     FIG. 6 is a schematic illustration of the present invention. A power source  40  provides power to drive an input signal transducer  42 . The input signal transducer  42  is coupled to a multi-mode surface acoustic wave device  44 . The multi-mode surface acoustic wave device  44  has a selective coating thereon. An output signal transducer  46  is coupled to the multi-mode surface acoustic wave device  44  and provides a response due to a first mode. An output signal transducer  48  is coupled to the multi-mode surface acoustic wave device  44  and provides a response due to a second mode. The input signal transducer  42  and output signal transducers  46  and  48  are coupled to a signal processor  50 . The signal processor  50  determines any delay time shift, in a delay line surface acoustic wave device, or a frequency shift, in a resonator surface acoustic wave device. This change in acoustic properties results in detection and characterization of the analyte, material, or substance absorbed in the selective coating. By obtaining an output signal from multiple modes of operation, different environmental effects, such as temperature, can be effectively eliminated. 
     Accordingly, it should be appreciated that the present invention, in providing a surface acoustic wave device sensor that can effectively determine or detect an analyte or substance substantially independent of temperature, makes possible the use of the present invention in many different applications. The need to control temperature is greatly reduced. This eliminates the need for other more complicated structures that have been utilized to compensate for temperature coefficients or their effect on a sensor. The concept of the present invention in utilizing different modes in a surface acoustic wave device sensor may be applied to effectively eliminate other effects that may interfere with the measurand effect, such as different environmental effects other than temperature. 
     Although the preferred embodiment has been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit and scope of this invention.