Influence of the crystal orientation on the electrical properties of AlN thin films on LTCC substrates

In this study, the influence of the crystal orientation on the electrical properties of sputter deposited aluminium nitride (AlN) thin films on low temperature co-fired ceramics (LTCC) substrates is investigated. The degree of c-axis orientation can be tailored by the deposition conditions such as plasma power, gas pressure and gas composition in the deposition chamber. Due to the large surface roughness of LTCC substrates (R_a = ∼0.4 μm) the quality of thin films is lower compared to silicon. Between areas of columnar grains arranged perpendicular to the LTCC surface, defects like voids are generated due to the wavy surface characteristics. The impact of crystal orientation and temperature up to 400 °C on the electrical performance is evaluated, as these layers are targeted as potential candidates for dielectric heat spreaders on multilayered ceramic substrates for high frequency applications. These AlN thin films having a good c-axis orientation exhibit lower leakage current levels over the complete temperature range compared to those with a poor alignment with respect to this crystallographic plane. The leakage current behaviour, however, is dominated according to the Pool-Frenkel electron emission independent of the degree of c-axis orientation.