Fabrication and Sensitivity Analysis of Quartz Sensor Chips with Modified Electrodes

Abstract

The quartz crystal microbalance (QCM) is an ultra-sensitive mass-detecting device, consists of an AT-cut quartz disk sandwiched between two metal electrodes. The application of a driving voltage through the electrodes initiates a shear mode vibration of the quartz disk at a resonant frequency determined by the thickness of the quartz disk and mass loading on the surface. Under certain conditions, the shift in the frequency is linearly related to the mass loading. In this work, QCM sensor disks with modified electrode geometries are successfully designed and fabricated. Unlike the conventional QCM disks, which utilize identical circular, concentric electrodes (‘plate-plate’ type), the fabricated QCMs have either a plat electrode on one side, a ring electrode on the other side (‘ring-plate’ type) or both sides ring electrode (‘ring-ring’ type). The differential sensitivity distribution of the two types of QCM is analyzed by a numerical analysis program. For both the ‘ring-plate’ and ’ring-ring’ type QCM, the mass sensitivity is concentrated on the fully electroded regions. With the increase of the internal diameter of ring electrode, the sensitivity profile becomes sharper in the fully electroded regions, and the values of the peak sensitivity become higher. By measuring the frequency changes caused by polymer coatings and protein adsorption, we prove that the modified QCMs can be used properly as mass-detecting sensors in both solid phase and liquid phase measurements, with retained overall mass sensitivity and sufficient stability. Ultimately, this study is motivated by the possibilities of establish dual-probed sensing systems, involving QCM measurements and optical measurements, as the ring electrodes allow a light transmission from one side to the other side of the quartz disks.