Study On Dual-channel Quartz Crystal Microbalance

Quartz crystal Microbalance(QCM)has been used as a novel sensor in biography,chemical sensing applications in liquid for its high precision,frequency stability,low cost and simple structure.QCM is a kind of bulk acoustic wave(BAW)sensor working in thichness shear mode(TSM)vibration.According to the shift of fundamental frequency,QCM can detect the slight mass deposited on the crystal surface.When used as biosensor in liquid,the resonant frequency is not only affected by the mass change,but also by the liquid density and viscosity,which are greatly dependent on the liquid temperature.For minimizing the fluctuation induced by temperature,two QCMs have been fabricated on a single quartz wafer as dual-channel quartz crystal microbalance(DQCM).One channel works as biosensor.coated with functional membranes and the other acts as a reference sensing on the liquid property.In this paper,we discuss the main factors affecting the interference between two channels including the distance of adjacent resonators,diameter of electrode,the proportion of diameter and thickness.Different solutions to avoid the coupling between two resonators have been compared.An inverted mesa-type structure has been successfully designed and fabricated,which could not only improve mass-frequency sensitivity,but also enhance the mechanical stability.Besides,the convex structure of inverted mesa-type can reduce the coupling between two resonators.The fabrication of DQCM is completed by using well established wet etching process.The flow injection technology has been used to examine the compensation effect.A micro flow cell with three main components which were made from polymethyl methacrylate(PMMA)was designed and fabricated to assemble the fabricated resonator chip.The electrical signals of the chip are guided out using two spring-pins thereafter.The Q value of DQCM is larger than 10000 when it vibrates in air.The Q value drops to be about 1000 in pure water.An oscillator circuit is developed to drive the DQCM and measure the frequency shift.Experimental results demonstrated that the two QCMs could vibrate individually and the frequency stability can be greatly improved by using the reference sensor.