Research On Key Technologies Of Shear Wave Mode Thin-film Bulk Acoustic Wave Resonators

Thin film bulk acoustic resonator(FBAR)is widely used in the research of piezoelectric sensing technology,biosensing technology and communication technology because of its small structure size,easy integration and higher quality sensitivity.The longitudinal wave excited by FBAR has too much loss in liquid environment,which makes it unable to work effectively in liquid environment,hindering the development of FBAR in the field of liquid sensors or biosensors.The shear wave mode FBAR has a higher quality factor in the liquid.Because the loss of the shear wave is lower than that of the longitudinal wave.Therefore,it is possible to maintain high sensitivity of the shear wave mode FBAR sensor in liquid environment.There are two methods to excite the shear wave of the FBAR device.One is to control the c-axis tilt growth of the piezoelectric film so that the c-axis has a certain tilted angle with the vertical electric field direction,i.e.c-axis tilted FBAR.The other is lateral field excitation(LFE)FBAR.The piezoelectric thin film adopts vertical c-axis orientation growth.And the excitation electrode and the ground electrode are placed on the same side of the piezoelectric film to form an excitation electric field.The LFE-FBAR device has a mature manufacturing process and a simple structure.But the parasitic effect between the parallel electrodes will cause the resonance performance of the sensor to be polluted.At the same time,some vertical electric fields exist under the electrodes and at the edge of the electrodes.It makes piezoelectric thin film to excite some longitudinal wave not pure shear wave mode.The theoretical resonance performance of c-axis tilted FBAR is better than LFE-FBAR.However,how to effectively control the c-axis tilted angles and to prepare a piezoelectric thin film with a single orientation and excellent quality are currently the biggest problems.The main work and achievements of this manuscript are shown as follows:1.We had exhaustively introduced the research background and significance of the subject,the research progress of shear wave mode FBAR and FBAR as liquid sensors at home and abroad.2.Introduced the working principle of FBAR.And calculated the piezoelectric properties of piezoelectric thin films with different c-axis tilted angles through Maxwell equation,piezoelectric effect and acoustic wave transmission equation,and adding the conversion matrix.A Mason electrical equivalent circuit model was built.And the boundary conditions of the gas and liquid were added to derive the working state of the FBAR device in liquid environment.3.Studied the preparation technology of key point of FBAR-the piezoelectric thin film.1)The thin films were prepared by radio frequency(RF)magnetron sputtering.This method makes the thin films have good uniformity,high density,low roughness and high film formation quality.2)Explored the relationship between the sputtering time and thickness of thin films.It is found linear relationship between these two parameters within a certain range(120-180 min).In the early and later stages of sputtering,phenomena such as insufficient sputtering and sputtering fatigue will occur,resulting in a slower thin film formation rate.3)Explored the effect of the relative motion between the target and the substrate,the sputtering pressure and the annealing treatment on the piezoelectric thin film deposition.We had sputtered Zn O piezoelectric thin film with c-axis orientation and c-axis tilted orientation.The angle between the target and the substrate determines the caxis tilted angle.And the high arrival energy of the atoms is beneficial to the tilted growth of the crystal grains.The atoms can be given a high arrival energy by increasing the sputtering rate.However,the crystal grains will reunite under the high sputtering rate.Therefore,it is very important to choose appropriate sputtering conditions to deposit thin films.What's more,annealing can promote the growth of piezoelectric crystals,making piezoelectric crystal denser.4.A two-dimensional finite element model(FEM)of the back-etched LFE-FBAR device was built by COMSOL Multiphysics simulation software.The influence of electrode spacing,electrode width,piezoelectric layer thickness,support layer thickness and the choice of materials were analyzed on the resonance performance of FBAR.We optimized the FBAR structure and selected appropriate size parameters to lay the foundation for the future production of FBAR devices.The three-dimensional structure of the FBAR device is drawn by using Auto CAD software,which is convenient for later processing and preparation.We used Auto CAD software to draw the mask layout of the FBAR devices,which is convenient for later processing and preparation.At the same time,use the ADS software to analyze the test results of the FBAR device.5.We prepared sensitive membrane for heavy metal ion detection.Because FBAR is a quality-sensitive sensor,the disadvantage is poor selectivity.Therefore,in order to use FBAR as a sensor,we increased the selectivity of the device by coating a sensitive film on the sensor.We chose thioglycolic acid(TGA)as a sensitive film coated on bulk acoustic wave resonator to adsorb heavy metal ions.Besides,we used TGA to modify Cd Te nanospheres for signal amplification and increased the sensitivity of the device.The sensitivity of the enhanced sensor is 6 times that of the unenhanced.The detection limits of the device for lead ion,cadmium ion and copper ion are 0.096,0.089 and 0.189?g / L,respectively.However,this device cannot detect the type of heavy metal ions in unknown liquids.So we proposed a scheme combining the sensor with the electrochemical stripping method to improve the selectivity of the sensor.