Patent ID: 9726646
Date: 2017-08-08
CPC Classifications: G01N

Claim:
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.