Investigation On High Frequency And High Power Durable SAW Filter Materials And Devices

Mobile communication is developing rapidly in the information age,and surface acoustic wave?SAW?filters face two huge challenges.One is the drop of power durability result from shrinking device sizes and increasing power level,the other is higher operating frequency caused by crowded spectrum at low frequency.Improving power durability and operating frequency of SAW filters are two key issues in the research of SAW filters now.In this paper,Al/Ni,Al/Ti/Cu/Ti and Al/Cu/Ti electrode were prepared by multilayer method.The influence of different structure on the resistance to acoustomigration and the behavior of electrode under high power were investigated.Attention was fixed on the analysis of electrode degradation mechanism and enhancing the power durablity of SAW filters.On the other hand,ZnO/6H-SiC composite piezoelectric substrate with high acoustic velocity was prepared by raido frequency?RF?sputtering.The propagation characteristics of SAW on ZnO/6H-SiC composite substrate were studied.Based on these,a high frequency SAW filter was designed.The results show that the different thickness Ni transition layers affects the texture,surface morphology and microstructures of Al films tremendously.6nm Ni transition layer improves the strength of the Al?111?texture,reduces the surface roughness,the number of grain boundaries,and improves the bonding force between the Al film and the substrate.Those effects extend 10 times the electromigration lifetime comparing with that of the non-transition layer Al film,while the power durability of the 1.5GHz SAW filter on 42^o YX-LiTaO3 substrate is increased to 2.2 times from device without transition layer to device with 6nm Ni transition layer.The interdigital transducers?IDTs?will be thinner,narrower and more vulnerable at ever high frequency,which needs to be strengthened further.Using finite element method to analyze the working state of electrode,it is found that stress and strain are mainly concentrated at the bottom edge of electrode,and the stress is mainly shear stress?yz and?xy.For this situation,the bottom of electrode is strengthened by using Al/Ti/Cu/Ti and Al/Cu/Ti multilayer to enhance the resistance to acoustomigration.In the case of 2.7 GHz SAW filter on 42^o YX-LiTaO3 substrate,the fingers width is about 60%of that in 1.5 GHz SAW filter.However,Al?102 nm?/Cu?10 nm?/Ti?5 nm?electrode increases the power durability of the 2.7 GHz high frequency SAW filter to 1.9 times that of Al-0.9wt.%Cu?135 nm?/Ti?5 nm?electrode,3 times that of Al-0.9wt.%Cu?140 nm?electrode.the high power durability of Al?102 nm?/Cu?10 nm?/Ti?5 nm?is attributed the appearance of Al₂Cu in the bottom edge of electrode.Besides,when the thickness of Cu layer is thin,Al/Cu/Ti electrode has relatively lower electrical resistance than Al-0.9wt.%Cu/Ti making it suitable for using in high frequency and high power SAW filters.By using substrates with high acoustic velocity,high frequency SAW filters can be achieved with wider finger width to ensure low process difficulty,low cost and high reliability.In this work,high quality ZnO films with strong c-axis texture on 6H-SiC substrate deposited by RF spittering have an orientation relationship of[112?0]ZnO?0002?ZnO?[11 2?0]6H-SiC?0006?6H-SiC.In ZnO/6H-SiC composite substrate with0<hZnO/?<1,phase velocity of all kinds of SAW decrease with the increase of hZnO/?.Among them,the first mode possesses the largest electromechanical coupling factor which rise first and then fall with the increase of h_ZnO/?.The first mode and second mode with high phase velocity and proper electromechanical coupling factor at large h _ZnO/?are sutible for application in high frequency SAW filters.In addition,the quality factor is found to be higher at small h_ZnO/?and frequency temperature coefficient is smaller in the fundamental mode and the second mode in one-port resonators.According to these results,a mirror-connected longitudinally coupled double mode resonant filter was designed and fabricated,whose center frequency is up to 6.2 GHz.What's more,5GHz ladder filters also have been fabricated with insertion loss of-9.7 dB.