Research On Magnetoelectric Surface Wave Resonator Based On Flexible Substrate

Flexible electronics is an emerging electronic technology,which has a wide application prospect due to its good bending flexibility and low processing cost.The flexible magnetoelectric acoustic surface wave?MSAW?resonator can relief the constraint on the Young's modulus effect of magnetostrictive layer by the rigid substrate,and has great application potential in highly sensitive magnetic sensors.However,it is difficult to integrate magnetostrictive layer and piezoelectric layer on flexible substrate,which limits the development of this field.This thesis proposes to integrate amourphous FeCoSiB magnetostrictive film and piezoelectric ScAlN film on polyimide?PI?substrate successively,and then fabricate interdigital transducers?IDT?to form the IDT/ScAlN/FeCoSiB/PI flexible substrate structure.On the basis of design and simulation,the flexible magneto-acoustic surface resonator is sucessfully obtained through the detailed exploration of material selection and integration process.First,COMSOL software was used to establish a model for the electro-acoustic analysis of the MSAW device.The wavelength,the thickness of FeCoSiB and ScAlN layers were studied and optimized,which proved the feasibility of the proposed soft MSAW resonator.The devices show both Rayleigh-wave and A1 Lamb-wave propagation characteristics.Next,the preparation processing of magnetostrictive and piezoelectric on Si substrate was explored.An in-situ bias magnetic field of 270Oe was applied during sputtering to induce uniaxial in-plane anisotropy.The effects of sputtering pressure and power on the magnetic properties of FeCoSiB films were investigated.In order to suppress the vertical magnetic anisotropy caused by large film thickness,SiO₂ was inserted between each layer of magnetic films as the isolation layer,forming the FeCoSiB/SiO₂ multilayer structure.The effect of annealing on the properties of thick multilayers was investigated.Then,the influences of nitrogen-argon ratio,bias voltage and annealing temperature on the orientation of ScAlN film were investigated.Finally,the optimized ScAlN film has a low FWHM of?002?rocking curve down to 3.2^o and a surface roughness of 1.32 nm,which is suitable for the acoustic surface wave resonator.Finally,the flexible MSAW device was fabricated.PI layer and imine treatment were applied to the flexible PI substrate to improve substrate flatness.Amorphous FeCoSiB magnetostrictive films were sputtered on the substrate.The surface fluctuation of flexible substrate caused by stress was relieved by annealing the film at150?for one hour.Multilayered FeCoSiB film with thickness of 600nm and good soft magnetic properties was obtained.Next,ScAlN films were deposited on the FeCoSiB/PI substrates.In consideration of thermal accumulation caused by high sputtering power and the mismatched thermal expansion coefficient of the ScAlN film and the PI substrate,we adopted multiple sputtering and hard mask techniques to deposit thick ScAlN films.Finally,the IDT transducer with finger width of 6?m and wavelength of 24?m was fabricated by lift-off on the ScAlN/FeCoSiB/PI substrate.The S₁₁ parameters of flexible MSAW resonator are measured suing vector network analyzer and microwave probe station.The test results show that the devices have two resonance peaks in the range of 10-120 MHz,which locate at 28.32 and 93.69MHz,respectively.By comparing the measured results with those by COMSOL simulation,the mode at 28.32MHz is assigned to Rayleigh wave mode and the mode93.69MHz is assigned to A1 Lamb wave mode.The wave velocity,electromechanical coupling coefficient and quality factor of the device at different resonance frequencies are calculated and analyzed.At 28.32 MHz,the wave velocity is 680 m/s,Q value is8.79,and K² is 9.70%¹2.2%.At 94.25 MHz,it is 2248m/s,Q value is 12.66,and K²is 5.1%.