A micromachined wet cell for a love-wave liquid sensor

In this paper we present a silicon micromachined wet cell for use with a Love-wave liquid sensor. The Love-wave sensor is composed of an electronic amplifier and an acoustic Love-wave delay-line on a piezoelectric substrate. Together they form an oscillator. Liquid is placed in intimate contact with the Love-wave sensor; corresponding to its viscosity the acoustic wave velocity changes, which is observed through a change in the oscillation frequency. An issue that arises in a sensor of this type is that the input impedance of the interdigital transducers (IDTs) of the delay-line changes dramatically due to the dielectric properties of the liquid above them. This adds electrical load to the amplifier and affects the oscillator's performance by reducing its resolution and sensitivity. The electric loading of the IDTs by the liquid also leads to unwanted sensitivity with respect to the electrical properties of the liquid. The wet cell was designed to overcome this disadvantage. By virtue of this cell the liquid is directed only over the wave propagation path, and so the transducers are protected from the liquid's influence. In designing the cell, bubble formation in the liquid, chemical inertness, bonding aspects and temperature effects were all considered. The design utilizes a silicon micromachined channel that guides the liquid between the transducers. Furthermore a heater for controlling the temperature of the liquid has been incorporated. Experiments have shown that placing thin side walls of a silicon micromachined channel in the propagation path of the wave adds little to the insertion loss. Losses of only 6dB or less were recorded, which confirms the suitability of this configuration. In addition to viscosity sensors this design can be applied to a broad range of Love-wave liquid sensors, including those in the biochemical area.