A Study On The Mass Loading Effect And Its Applications Of Bulk Acoustic Wave Resonators

The mass loading effect of bulk acoustic wave(BAW)resonators is one of the important theoretical basis for its application in filters and sensors.The conventional inertial mass loading effect occurs when the connection between mass loading and resonator is rigid,which induces a negative frequency shift.It is often used to form the passband of filter or detect the mass change on the surface of BAW resonator.On the contrary,the elastic mass loading effect relys on the elastic linker between mass loading and resonator,which induces a positive frequency shift.It can be used to detect the stiffness of linker.Thus this work is focus on the theoretical study of these two kinds of mass loading effect and its application in wideband filter and biomolecular stiffness sensor.The main achievements of this work are listed as follows:1.The influence of inertial mass load effect on the bandwidth,flatness and return loss of the passband in the BAW filter is analyzed.A method of compensating the bandwidth generated by the inertial mass load effect through increasing the effective electromechanical coupling coefficient of the resonator is proposed to increase the bandwidth of the filter effectively,and to keep good insertion loss and return loss in the passband simultaneously.2.A modified lattice configuration with only two auxiliary inductors is proposed to achieve a wideband filter response using AlN-based BAW resonators.These two inductors could increase the effective electromechanical coupling coefficient of every resonator in the filter.When this topology is adopted in muti-stage configuration,the inductors paralleled in two facing ports can be combined into one equivalent inductor,which enables more compact integration of filter chip.For proof-of-concept validation purposes,a prototype film bulk acoustic wave filter in this configuration is designed and fabricated.The measurement results present a 12% relative bandwidth with a good insertion loss and deep out-of-band rejection,which verify the feasibility and superiority in the bandwidth of the proposed topology.3.The working principle of elastic mass loading effect based on the particle-resonator coupled system is investigated by the mathematical analysis of general block-spring model.A concept of detection window is defined to reinterpret the critical stiffness as a threshold of positive and negative frequency shifts.Furthermore,the Mason model and finite element method(FEM)are introduced to verify the calculation result of above mechanical model.The mechanism of inducing positive frequency shift by the elastic mass loading effect is first explained in the perspective of acoustic resonant mode of BAW resonators.To further understand this phenomenon occurred in fluid environment,FEM model is built up with liquid layer added above the SMR surface.4.A 2.5GHz solidly mounted resonators(SMRs)is designed and fabricated as the key component in the biosensor.Biotin-streptavidin,antibody and antigen binding system are used as model molecular linkers to establish the particle-resonator system.The positive frequency shift of the system is used to estimate the stiffness of biomolecules connecting between the SMR and attached particles.Three sets of experiment are conducted to study the relationship between the positive frequency shifts and different particle diameters,particle densities and linker stiffness.The preliminary results suggest that this system has great potential as a promising biosensor for biomolecular stiffness detection in real time.