Design And Fabrication Of SAW Resonator Based On Relaxor Ferroelectric Single Crystals

The surface acoustic wave(SAW)devices,which takes piezoelectric substrate as one of the core components,are devices that realize various functions via electroacoustic signal conversion.Because of its advantages of simple structure,excellent performance,miniaturization,low insertion loss,good consistency and etc.,it has attracted much attention from researchers and usually used as electronic warfare and radar in military fields,and as communications and broadcast television systems in civil fields.SAW resonator is an important branch among them,and has been widely used in sensor systems in the fields of physics,chemistry and biology.In addition,resonators also have an important impact in the field of wireless communications.Among them,the piezoelectric material,the core component of the SAW resonator,plays a decisive role in the maximum bandwidth that the device can achieve.With the development of science and technology,preparation of miniaturized and broadband SAW devices has aroused increasing interest in whole world,and it is urgent to prepared piezoelectric materials for broadband devices.Therefore,this thesis focuses on manganese-doped lead niobium indium acid lead-niobium magnesium magnesium-titanium Lead acid 0.5mol% Mn-0.3Pb(In1/2Nb1/2)O3-0.4Pb(Mg1/3Nb2/3)O3-0.3Pb Ti O3(referred to as Mn-PIMNT)and sodium bismuth titanate-barium titanate Na0.5Bi0.5Ti O3-0.05 Ba Ti O3(NBBT)single crystal,because both of the two new types relaxed ferroelectric single crystals have high-voltage electrical constants,high dielectric constants,and high electromechanical coupling coefficients.The main study are the following three aspects:(1)The [001] oriented Mn-PIMNT single crystal was selected to study the influence of temperature and electric field on the macro-electrical properties(including dielectric,ferroelectric and piezoelectric properties),micro-domain structure and phase structure of the single crystal.Three dielectric anomalies were found near 115°C(TR-M),122°C(TM-T),and 155°C(TT-C)in the dielectric performance analysis,and the dielectric constant has a strong correlation to frequency in those three situations.The strain curve shows that the high-temperature phase transition induced by electric field is discontinuous and has the characteristics of first-order phase transition.In addition,the parameters of coercive field(EC),residual polarization(Pr),maximum strain(Smax),and longitudinal electrostriction coefficient(Q)show different trends with temperature changes.The value of Pr and EC gradually decrease with the increasing temperature,when the temperature approaches TR-M and TM-T,Pr decreases sharply,while Smax and Q increase significantly.The fatigue properties in a bidirectional electric field were also studied,and it was found that the single crystal still exhibited excellent electrical stability even after cyclic loading 105 times under a 20 k V/cm electric field.The piezoelectric microscope was used to study the domain structure and its changing rules under different temperatures and external electric fields.As a result,it was found that the Mn-PIMNT single crystal has a stripe domain structure with a length of several micrometers and a width of several hundred nanometers,and there is still a uniform and stable domain structure above the Curie point,which is related to the characteristic of the dispersed phase transition of the relaxed ferroelectric.And the polar nano-domains(PNRs)above TC also help to form a stable domain structure in the local area.(2)A Finite Element Method(FEM)was used to establish a SAW resonator simulation model of Mn-PIMNT relaxed ferroelectric single crystals polarized in different directions.the intrinsic relationships between the properties(propagation speed v,electromechanical coupling coefficient K2,wastage,etc.)of the single crystal and the polarization direction,cut shape and post-processing were studied.The results show that the wave velocity of Rayleigh SAW obtained by Z-cut Mn-PIMNT single crystal decreases with the increase of the thickness of the gold electrode,with a range of 1200 m/s to 2400 m/s and v[111]>v[011]>v[001].Among them,the Mn-PIMNT single crystal polarized in the [001] direction has a weak resonance peak due to the energy leakage to the substrate,and the obtained K2 is valueless.For the single crystal polarized in the [011] and [111] directions,when the electrode thickness is large enough,the value of K2 tends to be stable,and K2[011]> K2[111],the variation range is 9% ~ 27%.When h Au=10%?,K2[011] achieved a larger value of 24% and K2[111] was 23.8%,which was about 2.3 times higher than that of the thickness of the zero electrode.When ?=25°,K2[111] = 24%,which is higher than the maximum value that can be achieved by Z-cut and Y-cut substrates,so the optimized cut shape is 25°Z-X.In addition,by researching photolithography and reactive ion etching microfabrication technology,a microfabrication preparation method for an interdigital transducer was established,and a SAW resonator prototype device based on this substrate was developed.The IDT line width was uniform,and reached 0.8 ?m.(3)The surface acoustic wave excitation and propagation characteristics of NBBT lead-free piezoelectric single crystals under different model parameters is systematic theoretical studied.The parameters such as IDT thickness,cut shape,and resonator configuration of SAW were studied to pursue the optimal SAW with the characteristics of high velocity,high electromechanical coupling coefficient and low loss.The calculation results of NBBT with optimized electrode type and thickness show that the SAW resonator based on Z-X NBBT can achieve a larger K2(~ 18%)and a smaller propagation loss at h Au=13%?,the K2 value is more than four times that of traditional piezoelectric materials.In addition,when the cut type is Y-50°X,the second-order Sezawa SAW is excited,and its resonance frequency(fr ~ 750 MHz)is twice the first-order Rayleigh SAW mode,and K2 is about 6.8%.Parasitic response can be suppressed by changing the thickness of the electrode or the shape of the substrate to obtain the ideal resonance mode.This study fully illustrates the surface acoustic wave propagation characteristics of lead-free NBBT and explores the application prospects of NBBT in wideband surface acoustic wave devices.