Fabrication and performance characteristics of high-frequency micromachined bulk acoustic wave quartz resonator arrays

This paper reviews the fabrication and performance of micromachined quartz resonator arrays. Using inductively coupled plasma etching techniques, we have successfully fabricated micromachined quartz resonator arrays with fundamental frequencies in the range of 25-85 MHz in an array format. These resonators have been experimentally evaluated for their performance in viscous (liquid) and viscoelastic (a biomolecular film in liquid) loading conditions. The paper discusses the ultimate sensitivity to mass and other properties of the adsorbates/contacting materials onto high-frequency quartz resonator surfaces. Measuring the frequency and Q-factor changes at the fundamental and third overtone of a 66 MHz resonator upon adsorption of immunoglobulin G (IgG) protein film on a hexadecanethiol functionalized surface, we were able to deduce: (i) the film thickness = 18 nm, (ii) density = 1040 kg m^-3, (iii) elastic modulus = 6.7 MPa and (iv) viscosity = 5.5 mPa s. Furthermore, from the adsorption isotherm for the IgG film, two different Langmuir equilibrium constants (K) were deduced. In the low-concentration region K = 2.13 × 10⁸ M^-1 and in the high-concentration region K = 6.53 × 10⁶ M^-1 were obtained. The thickness and density values obtained for IgG are consistent with the bilayer model predicted from interfacial packing of spherical protein molecules as a function of the molecular weight, and K values are consistent with earlier reported values for adsorption of IgG films. This is the first reporting of the elastic modulus and viscosity of IgG films in phosphate buffer solution.