Resonant surface acoustic wave chemical detector

1. Apparatus for chemical detection comprising:

a pair of interdigitated transducers (IDTs) formed on a piezoelectric substrate; and

a layer of adsorptive material deposited on a surface of the piezoelectric substrate between the IDTs, wherein:

each IDT is conformed, and is dimensioned in relation to an operating frequency and an acoustic velocity of the piezoelectric substrate, so as to function as a single-phase uni-directional transducer (SPUDT) for surface-confined Rayleigh waves at the operating frequency, wherein the operating frequency is greater than 900 MHz, and is further conformed and dimensioned to have an input impedance at the operating frequency of about 50 ohms;

the piezoelectric substrate is a wafer of Y-propagation, Z-cut lithium niobate whereby acoustic waves launched into the piezoelectric substrate by at least one of the IDTs comprise surface-confined Rayleigh waves;

the pair of IDTs is spaced apart along a propagation axis and mutually aligned relative to said propagation axis so as to define an acoustic cavity that is resonant to surface-confined Rayleigh waves at the operating frequency, wherein a distance between each IDT of the pair of IDTs lies in the range from 100 wavelengths of the operating frequency to 400 wavelengths of the operating frequency;

each IDT comprises a plurality of metal strips formed on the surface of the piezoelectric substrate and extending substantially perpendicularly to the propagation axis, said metal strips herein denominated fingers, that form a repeating pattern of at least 50 but not more than 100 repeat units along said axis;

the apparatus comprises a further plurality of fingers, herein denominated velocity-correcting fingers, that are formed in a repeating pattern along said propagation axis on at least a portion of said piezoelectric substrate that lies between said IDTs;

the IDT fingers and the velocity-correcting fingers collectively form a single periodic pattern in which continuity of mass loading due to the IDT fingers is continued all the way across the piezoelectric substrate;

each IDT is conformed as a floating electrode uni-directional transducer (FEUDT); and

the IDTs have mutually facing edges, and the adsorptive layer covers the velocity-correcting fingers and extends from one said mutually facing edge to the other mutually facing edge without substantially encroaching on either of the IDTs.

2. The apparatus of claim 1 , wherein

each repeat unit comprises:

two driver fingers that are electrically connected to opposite signal-carrying conductors;

two floating fingers that are individually electrically floating; and

two shorted fingers that are electrically shorted to each other.

3. The apparatus of claim 1 , wherein:

the apparatus comprises at least one measurement sensor, at least one reference sensor, and an electronic circuit connected to the measurement and reference sensors;

the measurement sensor comprises the said IDT pair formed on the piezoelectric substrate and the said layer of adsorptive material deposited on the surface of the substrate between the IDTs;

the reference sensor comprises a further IDT pair formed on the piezoelectric substrate; and

the electronic circuit is conformed to detect phase differences between respective signals transmitted across the measurement and reference sensors.

4. The apparatus of claim 3 , wherein:

each said IDT pair includes a transmitting IDT and a receiving IDT;

the electronic circuit includes a precision oscillator connected to the transmitting IDT of at least one said measurement sensor and at least one said reference sensor so as to drive acoustic oscillations of said transmitting IDTs;

the electronic circuit includes a phase detector connected to the receiving IDTs of said driven measurement and reference sensors;

the phase detector is conformed to provide an output signal that is indicative of said detected phase differences; and

the electronic circuit further comprises a matched filter connected to the output of the phase detector, wherein the matched filter is conformed to reject low-frequency drift components and high-frequency noise from the phase-detector output and to correlate an output response signal from the phase detector with a template representing an expected response signal.

5. The apparatus of claim 4 , wherein the precision oscillator is an oven-controlled crystal oscillator.

6. The apparatus of claim 1 , wherein the operating frequency is at least 1.8 GHz.

7. The apparatus of claim 1 , wherein the pair of IDTs is separated by a distance selected to achieve a desired tradeoff between sensitivity and insertion loss.

8. Apparatus for chemical detection comprising at least one measurement sensor, at least one reference sensor, and an electronic circuit connected to the measurement and reference sensors, wherein:

the measurement sensor comprises a pair of interdigitated transducers (IDTs), consisting of a transmitting and a receiving IDT, formed on a piezoelectric substrate and spaced apart along a propagation axis and mutually aligned relative to said axis so as to define an acoustic cavity that is resonant to surface-confined Rayleigh waves at an operating frequency, wherein the operating frequency is greater than 900 MHz;

the piezoelectric substrate is a wafer of Y-propagation, Z-cut lithium niobate whereby surface-confined Rayleigh waves are launched into the piezoelectric substrate by at least one of the IDTs;

the measurement sensor further comprises a layer of adsorptive material deposited on a surface of the substrate between the IDTs;

the reference sensor comprises a further pair of IDTs, consisting of a transmitting and a receiving IDT, formed on a piezoelectric substrate and spaced apart along a propagation axis and mutually aligned relative to said axis so as to define an acoustic cavity that is resonant to surface acoustic waves (SAWs) at the operating frequency;

each IDT of said measurement and reference sensors is conformed, and is dimensioned in relation to the operating frequency and an acoustic velocity of the piezoelectric substrate, so as to function as a floating electrode uni-directional transducer (FEUDT) for surface-confined Rayleigh waves;

the FEUDT conformation of each IDT includes a repeating pattern of at least 50 but not more than 100 repeat units, each repeat unit comprises a plurality of metal strips, herein denominated fingers, that project substantially perpendicularly to the propagation axis; and each repeat unit comprises two driver fingers that are electrically connected to opposite signal-carrying conductors, two floating fingers that are individually electrically floating, and two shorted fingers that are electrically shorted to each other;

each of said measurement and reference sensors comprises a further plurality of fingers, herein denominated velocity-correcting fingers, that are formed in a repeating pattern along said propagation axis on a portion of the respective substrate surface that lies between the IDTs of the respective IDT pair so as to form, continuously with the IDT fingers, a single periodic pattern in which continuity of mass loading due to the IDT fingers is continued all the way across the piezoelectric substrate;

the adsorptive layer of the measurement sensor covers the velocity-correcting fingers and extends from an edge of one IDT of the pertinent IDT pair to an edge of the other IDT of said pair without substantially encroaching on either of said IDTs;

the electronic circuit includes a temperature controlled precision oscillator connected to the transmitting IDT of the measurement sensor and to the transmitting IDT of the reference sensor so as to drive acoustic oscillations of said transmitting IDTs;

the electronic circuit includes a phase detector connected to the receiving IDTs of the measurement and reference sensors, wherein the phase detector is conformed to provide an output signal that is indicative of phase differences between respective acoustic signals transmitted across the IDT pairs of the measurement and reference sensors;

the electronic circuit further comprises a matched filter connected to the output of the phase detector, wherein the matched filter is conformed to reject low-frequency drift components and high-frequency noise from the phase-detector output and to correlate an output response signal from the phase detector with a template representing an expected response signal;

each of the IDTs has an input impedance of about 50 ohms; and

the pair of IDTs is separated by a distance of at least 100 wavelengths but is not more than 400 wavelengths of the surface-confined Rayleigh waves at the operating frequency.