Acoustic sensors for physical, chemical and biochemical applications

Since the late 1950s, it has been recognized that one can make sensitive high-resolution sensors by exploiting the effects of various measurands on propagating ultrasonic waves. The earliest and best-known acoustic-wave sensor is the quartz-crystal microbalance (QCM) employed as a mass sensor for monitoring film thickness in thin-film deposition systems and by researchers studying chemical interactions with thin films deposited on the crystals. H. Wohltjen first demonstrated that a rugged and sensitive gravimetric acoustic sensor could be made in the form of a delay-line oscillator employing an electronic amplifier and a surface acoustic wave (SAW) delay line operating at hundreds of MHz. Additional ultrasonic sensor modes and structures have been demonstrated: flexural modes in thin micromachined plates and narrow rods, thickness resonators made by thin-film deposition rather than mechanical thinning of single-crystal piezoelectrics, and transverse mode devices such as SAW devices having surface modifications or a single-crystal plate-mode device. Demonstrated applications of these devices include: measuring the density and viscosity of liquids, determining the rheological properties of polymer films, quantitative detection of the concentrations of volatile organic vapors with sensor arrays, detecting the outputs of a gas chromatographic column, and both selective and sensitive detection of biochemically active compounds by employing antibody-antigen, enzyme-substrate, and other receptor-protein pairs.