Research Of Film Bulk Acoustic Resonators On Flexible Subsrates

With the development of wireless communication technology,film bulk acoustic devices have their distinctive effects on RF front-end applications in recent decade.Film bulk acoustic resonator(FBAR)is a component based on piezoelectric technology.Compared to the devices based on electromagnetic wave,FBAR devices are about 4~5 orders of magnitude smaller.Meanwhile FBAR has lower insertion loss,higher quality value(more than 1000),higher working frequency,and compatibility with CMOS technology.Bulk acoustic devices behave much better than the surface acoustic wave device in power capacity.Because of these advantages,film bulk acoustic resonator devices quickly occupied not only the RF communications market but also biochemical testing sensors market.According to the way of limiting the sound waves,film bulk acoustic resonator can be classified three types: FBAR,SMR,and back-trench FBAR.The produce processes for all of them are complex and costy.In order to improve the technology of the bulk acoustic resonator,we studied a new FBAR structure based on flexible substrate.The acoustic impedance of flexible materials is very small,which makes the boundary of substrate can efficiently reflect the acoustic waves.This FBAR can ignor the air gap or Bragg refrector,so it greatly reduce the complexity of bulk acoustic technology and broaden its application.Firstly,we analyzed the theory of the FBAR based on flexible substrate.Three new kinds of models are proposed and the simulations are carried from two aspects: electrical performance and thermodynamic performance.We used PI as the flexible substrate.It was found that the PI substrate can obtain the device with good performance as well.The Q value of the device is 1300 when the thickness of PI substrate is 125?m.When the PI layer is too thin,a large number of reflected waves from the bottom boundary of the PI layer will be found,leading to the parasitic resonance mode.When the PI layer is thicker than 10 ?m,parasitic resonance mode will be effectively inhibited.In the analysis of thermodynamics,we found the maximum steady-state temperature rises by 33 K with every 2?m increase in thickness of PI layer.When we put the PI layer into a groove on silicon substrate,the maximum steady-state temperature decreases by 142.44 K with the PI thickness of 10?m.Then,we fabricated the FBAR based on flexible substrate.The optimum conditions for the growth of Mo electrode and AlN piezoelectric layer on flexible substrate were investigated.The effects of sputtering power,gas flow,and substrate temperature on the properties of the films were investigated.It was found that the water-cooled sputting on PI substrate is a better choice which is different from the situation on silicon.The optimal produce processes were researched during the fabrication of FBAR based on flexible substrate by MEMS technology.Finally,the FBAR device was successfully fabricated and tested.