Relations among composition, microstructure, and mechanical and dielectric properties from 1 MHz to 100 GHz of aluminum nitride substrates

This study evaluated the bonding, microstructure, and thermal, electrical, and mechanical properties of six commercially available aluminum nitride (AlN) substrates, to establish the relations among their compositions, structures, and properties, which could provide information relevant to their potential applications. Y ₂O ₃ was used as a sintering aid for all six AlN substrates, and produced a high sintering density (≥ 3.32 g/cm ³). AlN substrates can be classified into two categories—those containing Y ₄Al ₂O ₉ and YAlO ₃ grains and those containing Y ₃Al ₅O ₁₂ and YAlO ₃ grains. Scanning electron microscopy revealed that all AlN substrates contained both transgranular and intergranular fractured grains, with average sizes ranging between 4.2 and 6.8 μm, and granular secondary-phase yttrium aluminates. Transmission electron microscopy showed very few grain boundaries with glassy-phase Al ₂O ₃. The phases obtained using Raman spectroscopy and X-ray photoelectron spectroscopy were in accordance with those identified from the X-ray diffraction patterns. The thermal conductivities of the AlN substrates ranged from 172 to 188 W/mK. The dielectric constant and dielectric loss of the different AlN substrates at 100 GHz ranged from 7.9 to 8.3 and 0.00010–0.00053, respectively. The electrical resistivity of the AlN ceramics ranged from 1.15 × 10 ¹² to 5.61 × 10 ¹⁴ Ω·cm, and the DC dielectric strength varied from 43.63 to 53.52 kV/mm. The flexural strength of the AlN substrates varied over a wide range, from 403 to 551 MPa. Substrates containing yttrium aluminates with irregular shapes and larger sizes were associated with lower flexural strength. The microhardness of the AlN substrates ranged from 1015 to 1298 Hv, and substrates with a large grain size and wide grain-size distribution exhibited lower microhardness. When considering the overall physical properties, AlN 4 exhibited the best performance among the samples studied.