Improved quartz crystal microbalance technique

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Abstract
The importance of using the most pertinent mathematical description of the mass load versus frequency relation for quartz crystal thin-film thickness monitors is reviewed. The different usable crystal load ranges of the so-called frequency- and period-measurement techniques in comparison with the Z-Match technique are calculated for most of the commonly used deposition materials. A new thin-film thickness monitoring procedure is described, which takes the influence of the acoustic film properties on the mass load versus frequency slope into consideration without need for the explicit knowledge of the acoustic impedance ratio z of the deposited film and the quartz crystal. It is shown how the effective z value in the composite resonator built by the quartz crystal and the deposited foreign layer can be derived from a measurement of a quasiharmonic overtone resonance frequency in addition to the commonly practiced exclusive measurement of the fundamental resonance frequency. The presently established relation between the mass load or the deposited film thickness and the corresponding resonance frequency of the loaded crystal is enhanced by a more rigorous one-dimensional composite resonator description. The introduced so-called series resonance relation is of significance for the accurate determination of the specific acoustic impedance ratio z of the deposited film and the crystal. Furthermore, the deviations of the frequency spectra of plano-convex shaped crystals from that of the one-dimensional (infinite plate) geometry are investigated and taken into account. The achieved accuracy in the z determination from the first and third quasiharmonic frequencies justifies the applicability of the two-frequency or auto slope accommodation technique introduced in this paper.