Effect of CaF2 Addition the Thermal conductivity and Electrical Resistivity of AlN Ceramics

Abstract

The high thermal conductivity and high electrical resistivity of aluminum nitride (AlN), coupled with its nontoxic nature, makes it promising material for the insulating substrate application. It is important issue to control the electrical resistivity or conductivity of AlN ceramics in an appropriate levels as well as thermal conductivity when they are used for semiconductor manufacturing equipment. Generally, sintering additives are used for the densification of highly thermal conductive AlN ceramics. It is well known that additives leave the residual grainboundary phases and affect on the thermal conductivity of the polycrystalline AlN. However, the effects of sintering additives and processing parameters on the electrical resistivity of AlN ceramics are less understood. In this study, the effect of calcium fluoride on microstructural development and related electrical conductivity of AlN ceramics were investigated. Calcium fluoride is known as one of sintering aids which is known to leave minimum residual grain-boundary phases. According to microstructural investigations, the grain boundaries were free of any second phase, providing direct grain-to-grain contacts. Electrical resistivity of CaF2-doped AlN ceramics was characterized by AC impedance spectroscopy at high temperature and revealed a variation of several orders of magnitude at different amount of additives. The specific grain-boundary resistivities were 1 – 3 orders of magnitude higher than the grain resistivities in all specimens. The charge carrier transport across the grain boundaries occurred through the grain-to-grain contacts, whose properties were determined by the space-charge layer. In order to analyze electrical resistivity of grain boundary, more detailed grain boundary analysis was conducted by transmission electron microscopy and photoluminescence. The grain boundary blocking and segregation effects in electrical conductivity were investigated in detail.