Acoustic wave-based sensors using mode conversion in periodic gratings

Using periodic gratings etched into the surface of a piezoelectric plate, s urface acoustic waves (SAW) call be converted into bulk waves and vice vers a with high efficiency. If parallel grating structures are fabricated on op posite surfaces of a piezoelectric plate, a SAW also can be directed from o ne surface to the other. Using such structures, acoustic wave-based sensors can be designed that utilize SAW for the detection of chemical analytes on an electrode-free surface, i.e., the back surface. As a result, spurious s ensor response and electrode aging that may occur when a chemical analyte c omes in contact with the transducers are minimized. The design principles o f these grating-based SAW sensors are explained, and the mass sensitivity i s investigated using chemical vapor deposited thin polymer films, a type of material used in many practical chemical sensor applications. Experimental results are presented for the detection of nitrogen dioxide (NO2) in sub-p pm concentrations.