비선형성을 고려한 압전 박막 트랜스듀서의 특성 분석과 작동 조건 최적화

Abstract

We have characterized a piezoelectric microcantilever sensor (PEMS), a typical thin-film piezoelectric transducer, considering its nonlinear characteristics, and we have optimized the operating conditions of the PEMS for mass sensors used in bio-application based on the results of that characterization. We developed a linear quasi-static model for analyzing linear characteristics of the PEMS such as mass sensitivity, quality factors, coupling factors, and so on. We also developed a nonlinear model for analyzing the high-power operating PEMS that leads to nonlinear damping and a nonlinear spring effect. During the fabrication of the PEMS, we improved the electrical properties of silicon-based PEMSs by reducing parasitic effects. Parasitic effects were reduced by increasing the silicon resistivity, which is proportional to parasitic resistance

reducing the electrode pad area, which is inversely proportional to parasitic capacitance

forming a deep trench near the electrodes. We have investigated the hysteretic characteristics of the PEMS. These hysteretic characteristics are a typical nonlinear characteristics such as polarization (P) – applied voltage (V), capacitance (C) – V, and displacement (D) – V hysteresis loops, and we have compared the characteristics of the PEMS with Pb(Zr52Ti48)O3 and with Pb(Zr30Ti70)O3. Hot poling process at 150°C for 30 minutes was applied on the PEMS to provide a comparison to the characteristics of the PEMS after the poling process at room temperature. The creep and ageing effects of the PEMS were investigated over the course of 12 hours, and the effects were well fitted with logarithmic function. The maximum conductance, the resonant frequency, the quality factor, the electromechanical coupling coefficient, and the piezoelectric constant measured as varying applied direct current (DC) bias voltage

each measured parameter as a function of the DC bias was plotted as a type of hysteresis loop for understanding the DC bias effect on the nonlinear electromechanical characteristics of the PEMS related to the domain switching of the PZT. The resonant frequency and quality factor of the PEMS decreases as the alternating current (AC) voltage increases in amplitude as a result of the nonlinear spring effect and nonlinear damping effect. For maximizing the performance of the PEMS in terms of maximum conductance, the coupling coefficient, the quality factor, and the resonant frequency stability, we have evaluated the environmental and electrical effects on the performance of the PEMS and optimized the operating conditions. Variations of the resonant frequency resulting from the effects of environmental conditions such as variations of temperature, pressure, medium, and relative humidity (RH) were measured. To achieve less than 0.5 pg as a minimum detectable mass with the PEMS, the environmental and electrical conditions were deuced from the experimental results. For the environmental conditions, variations of temperature, pressure and RH should be less than 3.4 °C, 0.17 bar and 3.1 % respectively. For the electrical conditions, the DC bias should be high enough, approximately ±10V, and the amplitude of AC voltage should be low enough, approximately in the range of 5-50 mV. Hot poling also should be applied to maintain high maximum conductance and coupling factor. Moreover, it takes more than six hours to stabilize the resonant frequency due to the time dependent variation of the PEMS.After adopting some optimized operating conditions, Hepatitis B surface antigen (HBsAg) concentrations were measured in the range of 0.1–100 ng/ml with the PEMS, and the PEMS distinguished between the positive and the negative human serum of HBsAg using a “dip and dry” technique.