Electrical Performance of Co-Fired Alumina Substrates at High Temperatures

Abstract

A 96% polycrystalline alumina (Al2O3) based prototype packaging system with Au thick-film metallization successfully facilitated long term testing of high temperature SiC electronic devices for over 10,000 hours at 500°C previously. However, the 96% Al2O3 chip-level packages of this prototype system were not fabricated via a commercial co-fire process which is more suitable for large scale commercial production. The co-fired alumina materials adopted by the packaging industry today usually contain several percent of glass constituents to provide better adhesion and sealing at interfaces formed during a co-firing process at temperatures usually lower than the regular sintering temperature for alumina. In order to answer the question if co-fired alumina substrates can provide reasonable high temperature electrical performance comparable to those of regular 96% alumina sintered at 1700°C, this paper reports on the dielectric performance of a selected high temperature co-fired ceramic (HTCC) alumina substrate and a low temperature co-fired ceramic (LTCC) alumina (polycrystalline aluminum oxides with glass constituents) substrate from room temperature to 550°C at frequencies of 120 Hz, 1 KHz, 10 KHz, 100 KHz, and 1 MHz. Parallel-plate capacitive devices with dielectrics of these co-fired alumina and precious metal electrodes were used for measurement of the dielectric properties of the co-fired alumina materials in the temperature and frequency ranges. The capacitance and AC parallel conductance of these capacitive devices were directly measured by an AC impedance meter, and the dielectric constant and parallel AC conductivity of the dielectric were calculated from the capacitance and conductance measurement results. The temperature and frequency dependent dielectric constant, AC conductivity, and dissipation factor of selected LTCC and HTCC co-fired alumina substrates are presented and compared to those of 96% alumina. Metallization schemes for co-fired alumina for high temperature applications are discussed to address packaging needs for low power 500°C SiC electronics.