Materials for Interdigital Transducers

The excitation and detection of surface acoustic waves, flexural plate waves, and other plate waves on piezoelectric substrates is most readily accomplished by use of an interdigital transducer (IDT). The comb-like structure of the IDT is typically made from a lithographically patterned thin metal film that has been deposited onto the surface of a piezoelectric substrate. The metal film used to make the IDT must be thick enough to offer low electrical resistance and thin enough so that it does not present an excessive mechanical load to the AW. Typical IDTs have thickness of -100-200 nm. 

SAW platform Coating material Gas, vapor Operating frequency (MHz) Max. sensitivity (Hz/ppm) 

DMMP Dimethyl methylphosphonate, DNT 2,4-Dinitrotoluene Source Data from Afzal and Dickert (2011) 

Aluminum has the advantage that it adheres well to common oxide substrates, is easy to deposit, is only 17 % less conductive (for an equivalent thickness) than Au, and is far less dense. The lower density is significant because reflections of AWs from Au IDT fingers in delay-line applications can cause appreciable pass band ripple in the IDT frequency response. Al s main disadvantage is the relative ease with which it corrodes this problem is sometimes addressed, particularly for (non-sensor) commercial applications of SAW devices, by passivating the Al using a relatively impermeable layer of a material such as SijN or AIN. 

Although it is not yet common for AW devices, other areas of microelectronics have demonstrated the utility of more exotic metallization, such as Pt-Ti, for demanding, high-temperature applications this combination would also be very corrosion resistant, though the relatively high density and poor conductivity of Pt are less than optimal for SAW devices. 

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