Research On Performance And On-Chip Structure Of Micro-Electro-Mechanical Bulk Acoustic Wave Resonator

With the development of miniaturization and integration of the wireless communication,a high performance radio frequency filter will be needed.As the only promising miniaturized passive frequency components Micro-Electro-Mechanical bulk acoustic wave?BAW?resonators have drawn intensive attentions of researchers.Meanwhile,their high operating frequency,high Q value and the excellent sensitivity make the BAW resonators suitable for applications in the sensor field.The Micro-Electro-Mechanical BAW resonators are fabricated by semiconductor and microfabrication process.Their qualities are strongly relied on the preparation processes,which should guaranteed by advanced manufacturing equipment.The design of the resonators involves many complex factors.In this dissertation,three aspects are carried out: the devices modeling and multiphysics analyzing,the key membrane materials preparation,and the realization of the on-chip structure resonators.Two kinds of on-chip structure BAW resonators which are flexible BAW resonators and bridge structure BAW resonators were successfully realized.The main contents and innovations of this dissertation were summarized as following:The one dimensional Mason model and MBVD model were derived based on piezoelectric theory and the theory of acoustic wave propagation in solids.The three dimensional finite element models were built to realize the multiphysics analyzing of the resonators including electric-acoustic field and thermal field.The influence of membrane materials on the performance of the resonators was studied.The proper materials which were Mo/W and AlN for metal and piezoelectric layers of the devices were obtained respectively.The amorphous AlN which was synthesized by radio frequency magnetron sputtering under water cooled condition used as structure layer instead of PECVD Si₃N₄ to reduce the residual stress.The polyimide whose acoustic impedance is close to zero was proposed to be the substrate of the BAW resonators.The finite element model was built to analyse the new flexible BAW resonators.According to the results,the thickness of polyimide has a critical value?7.5 ?m?above which the flexible devices will have good resonant performance.Compared with the structure of traditional film bulk acoustic resonator?FBAR?and solid mounted resonator?SMR?the structure of flexible BAW resonator was quite simple which greatly simplified the fabrication process.Furthermore,the new flexible BAW resonator will extend the application of the devices to the field of bio-sensing and wearable electronics.The effect of three-dimensional structure on the performance of the devices was researched.The calculation results revealed that the effective electromechanical coupling coefficient(K_eff²)is closely related to the thickness ratio of electrode/piezoelectric layers.For Mo and W electrode,the optimized thickness ratio is near 0.15 when the K_eff² obtained the maximum value.When the ratio of lateral dimension/longitudinal dimension is lower than 50/1 the influence of lateral dimension on performance of the devices can not be ignored.The lateral modes are strongly coupled to the thickness mode so the performance of the devices degraded rapidly.The electrode configuration also has a significant effect on the performance of the devices.The simulation results revealed that the resonators with square or arbitrary quadrilateral top electrode had better performance than whose with circle or triangle top electrode.The effects of a membrane over an air-gap,the Brag mirror reflector and the back-trench membrane on thermal characteristics of the devices were investigated by three-dimensional finite element thermal model.The thermal characteristics of SMR were systematically studied.The effect of Brag reflector layers,Brag reflector materials and the resonant area on the thermal performance of the devices was researched respectively.According to this study,the SMR had better thermal reliability than which of other type resonators.They are more suitable for the application under high-power situation.In order to enhance the power handling of SMR the resonant area should not be too small and the high thermal conductivity material should be used instead of SiO₂.The key membrane materials preparation of flexible BAW resonators was systematically studied.We have investigated thin tungsten films and thin molybdenum films deposited on polymer substrates by magnetron sputtering under different conditions.The influence of sputtering conditions including sputtering power,sputtering pressure and substrate temperature on microstructures,surface morphology and electrical resistivity of tungsten and molybdenum sputtered on polyimide substrates were discussed and elucidated.Compact,flat and low electrical resistivity tungsten films and molybdenum films with?110?texture were successfully obtained on polyimide substrates.On the basis of this study,the optimized deposition condition for tungsten films is: a sputtering power of 69 W,a sputtering pressure of 1 Pa and a substrate temperature of 100 °C.As for molybdenum films the optimized deposition condition is: a sputtering power of 200 W,a sputtering pressure of 1 Pa and a substrate temperature of room temperature?water cooled?.Effects of commonly used rigid substrates of Si?111?,SiO₂ and flexible substrates of polyimide on properties of tungsten films deposited by direct-current magnetron sputtering were investigated and compared.The AlN thin films deposited on polyimide substrates by radio frequency magnetron sputtering were systematically researched.The effects of deposition conditions including N₂/Ar gas ratios,sputtering power,sputtering pressure and substrate temperature on the crystal orientation,microstructure and surface morphology of AlN thin films sputtered on polyimide substrates were studied and discussed.High-oriented c-axis AlN thin films were successfully synthesized on polyimide substrates under water cooled condition.According to this study,the optimized sputtering condition for AlN films is: N₂/Ar gas ratio of 4/1,a sputtering power of 200 W,a sputtering pressure of 0.5 Pa and a substrate temperature of room temperature?water cooled?.Finally,the realization of the on-chip structure resonators was studied.In order to be compatible with microfabrication process the pre-processing method of polyimide substrates was explored and proposed.The lithography process and etching process of electrode layer and piezoelectric layer were systematically studied.The etching of piezoelectric layer is changed after which of top electrode layer to avoid the etching conflict between bottom electrode layer and top electrode layer.A low cost and compatible with common equipment condition process program was obtained to fabricate flexible BAW resonators.The prototypes were on-chip tested and the performance was good.A method to obtain gentle slope structure of sacrificial layer was proposed.This method was based on the diffraction optical principle.It was implemented by controlling the distance between the mask and the photoresist combining with the dry etching.The obtained angle of the gentle slope is about 25°.The realization of gentle slope structure is beneficial to the growth of thin films above the sacrificial layer and avoids the cracking problem at the edge of the devices because of the stress concentration.A process program of bridge structure BAW resonators was designed.The prototypes were fabricated and tested.