Microstructural characterization of CaO-doped AlN substrate material

Abstract

AIN ceramics are candidates for substrate materials in electronic circuits. In spite they are insulators they exhibit a very high thermal conductivity. Impurities and the unavoidable sintering additives usually degrade the thermal conductivity. With a special heat treatment it was possible to improve the thermal properties. Microstructural characterization with analytical transmission electron microscopy (TEM) on sintered and purified AIN samples was conducted to understand the reason for this behavior.AIN powder contaminated with 1 wt% oxygen and doped with 1.4 wt% CaO was pressureless sintered in a N-atmosphere to more than 98.1 % relative density. Additional annealing at 2100 K yielded a translucent product with a high thermal conductivity. With annealing for less than 24 h in a reducing or non reducing atmosphere thermal conductivity increased slowly from 41 Wm-1K-1 to 75 Wm-1K-1. This could be attributed to microstructural changes like grain growth and change in fractal dimension via SEM. A drastic increase was achieved after 44 h in a reducing atmosphere, where a thermal conductivity of 219 Wm-1K-1 was reached, approaching that of pure aluminum. Thin specimen for TEM were prepared in the usual manner from a 23.5 and 44 h annealed sample and examined in a Jeol 2000 FX. Analytical work was done with a Tracor Northern energy dispersive x-ray system (EDS) and a Gatan serial electron energy loss spectrometer (EELS).