Vol.5 No.3 2012

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Research paper : Development of a pressure sensor using a piezoelectric material thin film (M. Akiyama et al.)−174−Synthesiology - English edition Vol.5 No.3 (2012) as heavy metals, 3) shows three times the piezoelectric characteristic of crystal, and 4) has high elasticity coefficient (Young’s modulus: 314 GPa) and piezoelectric linearity is maintained at high pressure.[4] By selecting AlN as the detecting material, the issue of heat resistance was easily cleared.We considered using the thin film instead of monocrystal as the structure of the detecting material because, if the charge generated by the piezoelectric material is used as the output signal of the sensor, the sensor output will not be dependent on the thickness of the piezoelectric material. By using the thin film, the disadvantage of the fragility of the monocrystal can be overcome, and high durability against mechanical impact can be obtained. Moreover, it will be possible to use the semiconductor process that is readily mass-producible, and low cost can be achieved. Compared to other hopeful piezoelectric materials, there are less number of elements in AlN, and therefore it can be readily made into thin films. Hence, the authors’ research team employed the AlN thin film as the detecting material of the combustion pressure sensor for the first time in the world, and embarked on the development of technology to fabricate the AlN thin film.To investigate whether the AlN thin film showed the piezoelectric response as reported, an AlN thin film was fabricated on a silicon monocrystal substrate using the reactive sputtering method, and the piezoelectric responsiveness of AlN thin film was studied. The piezoelectric response of the fabricated AlN thin film showed good linearity in the pressure range of 0.1~1.6 MPa, and also showed good frequency property in the 0.1~100 Hz range. Therefore, it was confirmed to be an adequate candidate of the sensor material (2005).[5] The analysis of the basic property was conducted by the electric model of an AlN thin film, and good linearity similar to the combustion pressure of the engine was obtained in the 0.4~8.0 MPa range. Since the measured values and the electric model matched, it was shown that the AlN thin film could be used as the detecting material for the combustion pressure sensor (2005).[6] The electric resistance of AlN thin films decreases dramatically as the temperature increases, according to the Arrhenius equation. When the resistance decreases, the charge generated in the piezoelectric material diffuses before it can be detected by the measurement system so it cannot be measured. Therefore, we studied the volume resistivity of the AlN thin film and the temperature dependency of relative permittivity. Figure 3 shows the results. The volume resistivity of the AlN thin film was 106 cm or more at 851 °C, and the permittivity increased only slightly. Therefore, it was found that measurement was possible for the AlN thin film at 800 °C or above (2006).[7]However, since the monocrystal silicon substrate was used, the sensor element was weak against mechanical impact, and it was necessary to develop a sensor element with good Fig. 3 Temperature dependence of the relative permittivity and volume resistivity of AlN thin filmFig. 5 Temperature dependence of internal stress of AlN thin film formed on various substrate materialsFig. 4 (a) Cross sectional SEM photograph and (b) XRD pattern of AlN thin film fabricated on Inconel substrateTemperature /℃TVolume resistivity /ΩcmρRelative permittivity900800700600500400300200100005101520101210111010109108107106Volume resistivityRelative permittivity Inconel substrateAlN thin filmX-ray diffraction intensity2θ [deg.](0002) AlNInconel 30 40 50 60 70 80 (a) (b) 800700600500400300200100-2.5-2.0-0.5-1.0-0.50.00.51.0Tensile stress σT/GPaTemperature /℃TTDepoσb(AIN)InconelFerrite stainless

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