On the accuracy of the quartz-crystal microbalance (QCM) in thin-film depositions

Abstract

The quartz-crystal microbalance (QCM) is traditionally used as a thin-film deposition monitor. The most commonly used monitor crystal is AT-cut quartz vibrating in its thickness shear mode. The mass of the deposited film on the quartz crystal's surface is indicated by the change of its resonance frequency. With the introduction of the z-match EM technique in the 1970s, the accuracy of the QCM dramatically improved. This technique takes into account the mismatch of acoustic impedance between the quartz crystal and the deposited thin film. In recent years, new methodologies have been proposed to improve thhe accuracy of the QCM even further. For many years, we have carefully gathered a vast amount of data on the deposition of various metals, dielectrics and alloys on quartz crystals. In this paper, we report results of the comparison of the recently proposed methods with the Z-match technique, using the gravimetric measurement of deposited mass on the quartz crystals as the benchmark. We also report an extension formula, which broadens the validity of the z-match technique beyond a normalized frequency shift of 50%.