Comparison of high-frequency quartz crystal microbalances' (QCMs') mass sensitivities to theory

There is a current need for a sensor which can measure minute outgassing or erosion over very long time spans in the space environment. One way of addressing this need is a QCM with very stable output and high mass sensitivity. In order to increase the mass sensitivity. In order to increase the mass sensitivity of the QCM, the crystal has to oscillate at a higher frequency. In the past, assurance that the mass sensitivity at 10MHz as predicted by theory has been provided by nine different experimenters using the same or different techniques. When 15MHz QCMs with an increased theoretical sensitivity became available, they were experimentally exposed to the same molecular source flow as the 10MHz QCMs, to measure their response. It proved to be identical to theory. Historically, QCM sensor discussions have dealt exclusively with plano-plano crystals, i.e., both sides flat and parallel. Now, however, increases in frequency beyond 15MHz call into question whether we still have plano-plano crystals or whether plano-convex now best describes the crystals. Since the diameter of the high frequency crystal has to be less in order for it to oscillate, it becomes harder and harder to assure true plano-plano crystal performance as the fundamental frequency is raised. In this paper, we will discuss experiments which have been performed comparing the mass sensitivity of 25MHz to 15MHz crystals, or the mass range that is available with these high sensitivity crystals. We will also address the plano-plano versus plano-crystals' sensitivities.