Flexible Single Crystalline SAW Devices And Their Temperature/Strain Sensing Applications

With the rapid development of Internet of Things(IoT)and the accelerated layout of SG communication,the fields such as smart industry,smart logistics,smart healthcare,city brain are facing important development opportunities.In order to build a connection between the physical world and the internet server,obviously the wireless sensor technologies play an important role.Inspired by this,the market demand for wireless sensors is growing rapidly.In particular,wireless passive sensors are more popular in the fields such as wearable electronics and industry monitoring.Surface acoustic wave(SAW)sensors is one of the representative wireless passive sensing technologies,and they have many merits such as compact size,low cost,simple fabrication process,good robustness,etc.In recent years,surface acoustic wave sensors are moving towards flexibility,and show great prospects for applications in flexible wearable electronics,implantable electronics,medical electronics,etc.This work focuses on the research of flexible single crystalline SAW devices and their sensing applications.The application environments cover room temperature,(extremely)low temperature and high temperature harsh environments.Two kinds of devices are mainly studied,which are respectively single crystalline thin film lithium niobate(TF-LN)based flexible multi-mode SAW temperature/strain sensor,and flexible multi-mode LGS SAW temperature/strain sensor.The latter is mainly for(extremely)low temperature and high temperature harsh environments applications.The main contents and achievements of this research are as following:(1)The appropriate methods are determined for the simulation and acoustic parameter calculation of flexible LiNbO3 or LGS SAW devices:Both Iteration method and three-dimensional(3D)finite element method(FEM)show considerably high simulation precision for LiNbO3 and LGS SAW devices with different cuts.Two-dimensional(2D)FEM shows acceptable precision for the simulation of LiNbO3 SAW device,but significant errors for LGS SAW device.This is due to the non-negligible shear horizontal displacement in LGS SAW device,which leads to the leaky wave in the body of LGS.(2)Successful process schemes for the fabrication of high-frequency high-performance flexible single crystalline SAW devices are provided:High performance?433 MHz Pt/LGS SAW devices were fabricated by using projection/stepper lithography and AZ 5214 E photoresist based reversal-bake process.LGS SAW devices with Al2O3/Pt/Al2O3/Pt/Al2O3 composite electrodes were fabricated by using AZ 5124 E positive photoresist process and ion beam etching(IBE).The thinning process of 50 ?m flexible TF-LN SAW devices is designed.A thermal sliding temporary bonding adhesive based grinding-thinning process of LGS SAW devices is proposed.(3)Aiming at the difficulties in the package and testing system design of high temperature SAW sensors,a systematic research is carried out and feasible schemes are offered:The key parameters in the selection of strain adhesive are discussed.A method to improve the bonding strength between LGS and adhesive by using Al2O3 transition layer is proposed.The test package of high temperature SAW sensor is designed based on silver/platinum paste wire bonding and an all ceramic platform.A high temperature SAW strain measurement system based on high temperature reference strain gauge and micrometer is built.(4)Aiming at the shortcomings of flexible polycrystalline ZnO or AlN film SAW devices,such as poor piezoelectric performance,etc.,single crystalline LiNbO3 thin film based flexible SAW devices are proposed,and their temperature/strain sensing applications are investigated:TF-LN SAW devices have dual acoustic modes which are respectively Rayleigh mode and Thickness shear mode,and both modes exhibit linear frequency response to strain in the range of±3500??.This wide strain measurement range is at least 5 times that of traditional SAW strain sensors based on rigid substrates,and is slightly larger than that of current flexible ZnO film SAW sensors,which indicates the excellent flexibility of TF-LN SAW strain sensors.Also,TF-LN SAW strain sensors show a hysteresis of less than 1.5%,a strain detection limit of about 2.8?? and good fatigue performance.Compared with thickness shear mode,Rayleigh mode has larger strain sensitivity(up to?193 Hz/??)and slightly higher frequency temperature coefficient.By a method of difference,the two acoustic modes can be used for strain and temperature decoupling or temperature self-calibration.(5)Aiming at the shortcomings of current high temperature SAW sensors,such as the lack of flexibility and large strain sensing capability,etc.,flexible multi-mode LGS SAW temperature/strain sensors facing harsh environment(-196?1000 ?)are proposed.The acoustic modes of flexible Pt/LGS SAW devices with different cuts are analyzed in detail,and the results show that there are mainly Rayleigh wave mode and shear horizontal leaky wave mode.The low and high temperature sensing characteristics of different acoustic modes are systematically investigated.The Peak 3 of(0°,138.5°,27°)and(0°,138.5°,117°)devices have higher temperature sensitivities and monotonic frequency-temperature characteristics in positive temperature range,particularly the latter shows monotonic frequency response in the temperature range from extremely low(e.g.liquid nitrogen temperature-196?)to very high temperature(e.g.1000?).Based on(0°,138.5°,72°)cut,a wide-range(-60?700?)linear LGS SAW temperature sensor with high sensitivity(-167 ppm/?)is proposed for the first time.The effects of Al2O3 passivation layer on the frequency,Q factor,temperature coefficients and temperature resistance of LGS SAW devices are studied.The LGS SAW devices with composite electrodes can maintain piezoelectric properties at least under 1000 ?.The Rayleigh mode of(0°,138.5°,117°)cut device shows the highest strain sensitivity.The strain sensitivities of LGS SAW devices are measured at different temperature,and a detection range of at least 2100 ?? as well as a sensitivity of-1065 Hz/?? are achieved at 650? for the first time.Finally,a proposal for Q factor improvement of LGS SAW device is offered.