Film bulk acoustic resonators (FBAR) are developed for sensor applications. FBARs built as surface mounted resonators (SMR) consist of a piezoelectric layer sandwiched by two electrodes. An acoustic Bragg mirror prevents energy losses caused by acoustic waves propagating into the SI wafer.
These Si-integrated thin-film resonators are arranged as arrays on a chip. The resonance frequency of the individual elements is typically in the GHz range. Its resonance is detuned by binding of molecules on the resonator surface according to the mass attachment, which can be electronically detected. The selectivity for individual elements, i.e. the binding of certain molecules in a key-in-keyhole system, is achieved by a biochemical functionalization. The advantage of the thin-film arrays compared to classical quartz crystal resonators is the possibility to integrate the resonator array into a CMOS read-out circuit and the implementation of a multiplicity of elements on a very small area.
FBARs consist of several thin films which thicknesses being comparable to the wave length of the acoustic waves. This brings difficulties because the sensor response does not strictly follow the Sauerbrey equation but more complex models have to be used. However this gives rise to many variations for the stack design, e.g. influencing the temperature stability or sensitivity.
Currently, influences on the temperature coefficient of frequency are studied theoretically and verified in experiments. Mass sensitivity has been determined and simulations showed increased sensitivity and the possibility of temperature compensation for improved stack designs.
Simulated stacks will now be tested in experiments.
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