Temperature Dependent Dielectric Properties of Polycrystalline 96% A1 2 O 3
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Temperature Dependent Dielectric Properties of Polycrystalline 96%Al2O3 Liang-Yu Chen1 and Gary W. Hunter2 Ohio Aerospace Institute/NASA Glenn Research Center at Lewis Field, Cleveland, OH 44135 2 NASA Glenn Research Center at Lewis Field, Cleveland, OH 44135 1
Abstract Polycrystalline Al2O3 substrates have been proposed and tested for high temperature micro devices packaging intended for operation at temperatures up to 500oC. The dielectric properties of this material, including dielectric constant and effective volume conductivity, at elevated temperatures are of interest, especially for RF packaging applications. This article reports temperature dependent dielectric properties of polycrystalline 96% Al2O3 substrates from room temperature to 550oC measured by the AC impedance method at 120 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz. We observed negative temperature coefficients of volume electrical conductivity of 96% Al2O3 at 1 k, 10 k, and 100 kHz between room temperature and 50oC. The dielectric constant of the material increases significantly with temperature at frequencies below 10 kHz. The physical mechanisms of these dielectric behaviors of 96% Al2O3 at elevated temperatures are discussed. Introduction Polycrystalline aluminum oxide (Al2O3) is an important packaging substrate material which has low cost, high thermal stability, and high electrical resistivity. It has been extensively used in high reliability electronics packaging. Various thin-film and thickfilm metallizations have been developed for Al2O3 substrates. In addition to conventional electronics packaging, Al2O3 has also been proposed for high temperature electronic devices and high temperature MEMS packaging [Salmon, 1998 and Chen 2002]. The packaging applications in elevated temperature environments, especially, those for high temperature radio frequency (RF) devices [Hunter, 2003, Spry and Neudeck, 2004] generated interests in the temperature dependence of dielectric constant and volume electrical conductivity of Al2O3 substrates. Limited data of temperature dependent dielectric properties of Al2O3 have been reported previously [Antula 1967]. In this article we report temperature and frequency dependent dielectric properties of polycrystalline 96% Al2O3 substrates in a temperature range from room temperature to 550oC measured by the AC impedance method at 120 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz. Experimental Details The room-temperature dielectric constant of polycrystalline 96% Al2O3 has been reported to be 9.5 between 1 k and 100 MHz, and room temperature dissipation factor has been reported to be 0.001 at 1 kHz, 0.0014 at 1 MHz, and 0.0004 at 100 MHz [MetWeb]. A ‘capacitor’ for measuring temperature dependent dielectric properties of 96%Al2O3 substrate was fabricated on a 3.5 in. x 3.5 in. x 0.15 mil 96%Al2O3 substrate (used as the
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dielectric) with gold (Au) thick-film metallization (used as the two electrodes) on both sides of the substrate. The surface roughness of the substrate was ~ 20 microinch (rms). The electrode/met
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