Frequency interference between two quartz crystal microbalances

Multichannel quartz crystal microbalances (MQCM) are very attractive for biosensor applications because these devices can be miniaturized using microelectromechanical systems micromachining technology, leading to substantial reductions in cost, diagnostic time, and sample volumes. However, the miniaturization also brings about frequency interference between adjacent microbalances and the measuring accuracy will thus be affected by the extent of frequency coupling. In this paper, a theoretical analysis of the interference between two QCM on a monolithic quartz crystal is performed. It is found that both the separation between the QCM and the ratio of the electrode widths can significantly affect the frequency interference and mass sensitivity. The frequency interference is particularly strong when the two QCM have the same width or the ratio of the electrode widths is near specific values related to excited overtones. The sensitivity to mass loading decreases significantly when the geometric parameters fall within such strong interference regions. Therefore, the size and separation of the electrode structures must be carefully chosen in the design process for small quartz devices.