Thin Film Electroacoustic Devices for Biosensor Applications

Abstract: Biosensors are today important devices within various application areas.In this thesis a new type of label-free biosensor device is studied, which is fabricated using the same processes used for the fabrication of integrated circuits. This enables tighter integration and further sensors/biosensor miniaturization. The device is a so-called Thin Film Bulk Acoustic Resonator (FBAR). Within this thesis a low temperature reactive sputtering process for growing AlN thin films with a c-axis inclination of 20-30o has been developed. This enables shear mode FBAR fabrication suitable for in-liquid operation, essential for biosensor applications. Shear mode FBARs were fabricated operating at frequencies above 1GHz exhibiting Q values of 100-200 in water and electromechanical coupling factors kt2 of about 1.8%. This made it possible to move the thickness excited shear mode sensing of biological layers into a new sensing regime using substantially higher operation frequencies than the conventionally used quartz crystal microbalance (QCM) operating at 5-20MHz. Measured noise levels of shear mode FBARs in contact with water showed the resolution to be in the range 0.3ng/cm2 to 7.5ng/cm2. This demonstrated the FBAR resolution without any averaging or additional stabilization measures already to be in the same range as the conventional QCM (5ng/cm2), suggesting that FBARs may be a competitive and low cost alternative to QCM. The linear thickness limit for sensing of biomolecular layers was concluded to be larger than the thickness of the majority of the molecular systems envisaged for FBAR biosensor applications. A temperature compensated shear mode FBAR composite structure was demonstrated with retained coupling factor and Q-value by utilizing the second mode of operation. Understanding has been gained on the sensor operation as well as on how the design parameters influence its performance. Specifically, sensitivity amplification utilizing low acoustic impedance layers in the FBAR structure has been demonstrated and explained. Further, temperature compensated Lamb mode (FPAR) devices were also studied and demonstrated with optimized electromechanical couplings.