Researches On Novel Structure SAW Mass Sensor And Isolation Technologies Of SAW Integrated Acoustic Path

The designing technology of a surface acoustic wave (SAW) mass sensor with high equivalent Q_e value and narrow passband is very important to improve the tiny mass loading sensitivity, reliability and accuracy of the SAW mass sensor. In this work, the designing methods of SAW filter such as slanting metal array reflector, multistrip coupler (MSC) and apodized interdigital transducer (IDT) are all employed in SAW sensor design. A Y type dual path surface acoustic wave mass sensor with slanting reflector and apodized IDT is studied. The equivalent Q_e value of the sensor in oscillation mode can reach up to more than 104, which greatly improves the measurement accuracy and the reliability. The P-Matrix calculation model is built up, taking into consideration the edge reflection of the sensor. The properties of the sensor are analyzed in both experimentally and theoretically using P-matrix calculation model. The reason why a high equivalent Q_e value can be reached is analyzed as well. The frequency domain response is simulated by traditional P-Matrix model combined with Fabery-Perot model. Several new chip edge absorbing technologies compatible with standard SAW manufacture process are researched as well to prevent the SAW signal interference among different acoustic paths in an integrated SAW device. The relationship between the absorbing efficiency of viscous-elastic thin film and its geometric size of the film is deduced by perturbation theory.The main research content of this work: 1 Designing and testing of a novel structure sensora) A Y type dual path delay-line surface acoustic wave mass sensor with slanting reflector and apodized IDT is studied by using various designing technologies of SAW filter. With the help of oscillator circuit, a high equivalent Q_e value can be achieved, which greatly improves the measurement accuracy and the reliability. Input transducer with apodized aperture is designed by numerical Fourier transfer method. toreject the side lobe and improve the transition bandwidth so as to achieve a high equivalent Q_evalue in oscillation mode. MSC technology, which controls SAW energy distribution, is adapted to realize acoustic pathparallel connection. These not only shunt the disturbance of bulk wave, but also keepmeasurement path and reference path in good symmetry. Slanting reflector is used to realize the attenuation of spurious signal resulted from the edgereflection. The geometric parameters of the metal array can be obtained according to thesimulation result of the SAW distribution in the reflector simulated.b) The new structure sensor is fabricated on the 128°Y-X LiNbO₃ substrate. The temperature properties and the mass loading sensitivity of the dual path mass sensor are measured both in impedance match mode and oscillation mode. The experiment results show: The temperature compensation can be realized by dual path structure of Y type mass sensor.After compensation by dual path, the temperature coefficient can be drastically reduce to 6.2Hz/℃, which is 3 orders decrease in magnitude lower than that of single path. Therefore, themeasurement discrepency resulted from temperature can be neglected. A mass loading sensitivity of 5.13 GHz . cm²/g(the relative sensitivity is 4.2m²/kg) is obtainedin impedance match mode. While the value comes to 5.24GHz .cm²/g (the relative sensitivity is4.3m2/kg) in oscillation mode. The experiment results have been published by Sensor and Actuator :B.2 The P matrix model is built up to study the frequency domain properties of the SAW mass sensor theoretically and verified by experiments.a) P matrix model is built up taking the chip edge reflection into consideration, The insertion loss of transition signals and spurious reflective signals of the sensors with different structures are calculated and compared. The frequency response of devices with or without slanting reflector is simulated using a combination of P matrix model and Fabery-Perot model.b) The frequency and time domain response of devices with or without slanting reflector are tested. Comparison is made between the experimental results and the reported results.c) Both theoretical and experimental results show that the advantages of using apodized IDT compared with uniformed IDT are the TTI and multiple reflections can be decreased by about 2dB; the side lobe rejection can be increased by lSdB; and the short term frequency stability is increased by -1.81 dBc/Hz. The equivalent Qe value is improved from 4800 to 8689 in oscillation mode; The advantages of using the slanting reflector are bandpass ripple due to edge reflection can be reduced by 2dB; the effective Qe value is improved from 8689 to 12477 in oscillation mode; the short term frequency stability is increased by -1.2 dBc/Hz.3 New isolation and absorbing technologies are researcheda) The photoimageable polymide as a new absorberThe attenuation model of SAW using photoimageable polymide thin film is deduced from perturbation theory. The theoretical results shown that, when the acoustic path is covered by the photoimageable polymide thin film, the propagation loss of SAW is proportional to the center frequency of the device, to the width and thickness of the polyimide thin film. The testing results indicated that, when the photoimageable polymide thin film with a thickness of 5um was coated on the surface of a SAW device (center frequency of 165MHz), the propagation loss of the SAW became 9.5dB/mm. So good isolation results can be achieved.b) SAW attenuation by using surface stress due to Ti diffusion and proton exchanging process. The crystal lattice of the LiNbC>3 is deformed by Ti diffusion or proton exchanging process in local area. Thus the stress due to lattice deformation will result in the SAW propagation loss. The testing devices are fabricated in lift-off process. The experimental results indicated that, the SAW propagation loss is 7.07dB/mm in the stress area, when the Ti is deposited on the local diffusion area with a thickness of 2000A and diffused at high temperature. The SAW propagation loss is 5.82dB/mm in the stress area, after proton exchanging process.Now the application of SAW sensors become more and more widely. Various new structures of SAW sensors are researching to satisfy the requirement of sensitivity, accuracy, and reliability. In the future, more and more research works will be carried on the development of new structure designing.