Effect of metal impurities on shading and thermal conductivity of aluminum nitride

Because aluminum nitride (AlN) has a high thermal conductivity and a high electrical resistivity, it is used for substrates and packaging for high power or high density circuits. Silicon chips need to be protected from faults caused by exposure to ultra-violet rays. AlN packaging and substrates have to be shaded for UV protection. The characteristics of AlN are influenced by a number of factors. Specifically, metallic impurities affect the shading and thermal conductivity, and are particularly easily mixed with AlN as minerals or solutions during processing. It is very important to understand how metallic impurities influence the characteristics of AlN, and either refine processing to eliminate such effects, or add metal to achieve the desired industrial characteristics. Iron (Fe), silicon (Si), magnesium (Mg), and titanium (Ti) are metals that are easily mixed, so it is important to understand their influence. The metallic impurities Fe, Si, Mg, and Ti were added to raw powder AlN and uniformly distributed at a molecular level. The powder was sintered and tested for any effects on shading or reduction in thermal conductivity. Some Ti compounds were found at the grain boundaries, but these did not reduce the AlN thermal conductivity and provided efficient shading. In contrast, Si and Mg compounds were not found at the grain boundaries but formed solid solutions in the AlN grains, so they significantly reduced the AlN thermal conductivity and provided no shading. The effect of Fe was intermediate between that of Ti and Si and was characterized by its tendency to form solid metal ion solutions in the AlN grains.