A Study On Thin Film Bulk Acoustic Resonator (FBAR) For Mass Sensor Application

As a novel type RF MEMS device, thin film bulk acoustic resonator (FBAR) has in the last two decades made tremendous advances and has already been successfully commercialized, primarily driven by filter applications for telecom industry. At the same time, the development of FBAR sensors has paved the way for a second major application of FBAR technology, and therefore is becoming the next hotspot. Compared wih the traditional mass sensor, Quartz Crystal Microbalance (QCM), FBAR sensors have the advantages of high resonant frequency, high mass sensitivity, small size, CMOS-compatibility.We mainly studied the four aspects of FBAR sensor: the theory analysis and modeling of dual-mode FBAR, preparation of AlN piezoelectric thin film, FBAR prototype device fabrication and testing, and modeling of FBAR mass sensor. The main results of this dissertation are listed as follows:1. Resonant mechanism and vibration mode of thickness excited dual-mode FBAR based on AlN with tilted c-axis orientation was researched. The electrical impedance expression and the improved Mason equivalent circuit model of dual-mode complex FBAR were presented. It was found that piezoelectric parameters, electromechanical coupling coefficient, bulk acoustic wave velocity and vibration mode of AlN films depend on the tilt angle 0. It showed that pure longitudinal modes occur at 0°nd 66.5°, pure shear modes occur at 47.6°nd 90°, and both modes occur at the other angles. The maximum electromechanical coupling coefficient of longitudinal and shear modes appear atθ=0°nd 34.4°, respectively.2. Based on the reactive sputtering modeling, fabrication process of AlN films with preferred oriented and high deposition rate was studied. Theoretical analysis and experimental results showed that increasing the system pumping speed, using magnetron sputtering technique etc. may weaken or even eliminate the hysteresis effect of reactive sputtering process; neither too high nor too small N2 concentration benefits orientation of AlN and at the N2 concentration of 25%, AlN thin film has the most favorable orientation. Finally, in the optimized process, FWHM of AlN (002) diffraction peak is 0.28°, and the deposition rate is 2.1μm/h. A reactive sputtering process for growing AlN thin films with a c-axis inclination of 22°has also been developed and the deposition rate is 1.85μm/h.3. A new Al/W all-metal Bragg reflector structure was proposed and fabricated. AlN based solidly mounted resonator (SMR) utilizing 5-layer Al/W Bragg reflector was fabricated and measured, using the probe station and vector network analyzer. The device has a resonance frequency of around 2.25GHz and a Q value of around 170. This structure has a good adhesion between metal film, small thermal stress, large heat capacity, fast deposition rate, etc and has been applied for patents. Besides, AlN based SMR using SiO2/W Bragg reflector was also fabricated.4. Back-etched FBAR prototype devices were prepared, including thickness field excited longitudinal mode FBAR and lateral field excited shear mode FBAR. The longitudinal mode FBAR has a resonance frequency of 1.03GHz and a Q value of around 1350, while the shear mode FBAR has a resonance frequency of 1.4GHz and a O value of 370.5. A dual-mode FBAR mass sensor model was established and the results showed that the mass sensitivity of longitudinal mode FBAR SL(θ) and shear mode FBAR Ss(θ) with different inclination are -1403cm2/g～-1366cm2/g and -1344 cm2/g～-1309 cm2/g, respectively. The mass sensitivity of FBAR sensor with a certain tiled angle depends on the electromechanical coupling coefficient. The finite element model of FBAR mass sensor was established. Both longitudinal and shear mode FBAR sensors’ resonance characteristics were analyzed using ANSYS software. The results showed that the sensors of both modes have the maximum mass responsibility when the mass load located at the center of FBAR; the mass response of longitudinal and shear mode FBAR are -8150kHz/ng,-7500kHz/ng respectively. The farther the mass load deposits from the center, the lower of the FBAR sensor’s response and the response changes faster in the shear mode FBAR sensor than in the longitude mode.