The influence of moisture on surface properties and insulation characteristics of AlN substrates

The influence of moisture on the surface properties and insulation characteristics of sintered aluminum nitride (AlN) substrates was investigated. Two AlN substrates, one nontreated and the other with an oxidizing heat treatment, were exposed to moisture under several sets of conditions. For the nontreated substrate, changes in weight and surface properties were not observed, and neither was degradation of insulation characteristics under high temperature and humidity test conditions (80°C, 90% RH). Under more severe conditions of a pressure cooker test (121°C, 2 atm, water vapor), the surface morphology changed and the insulation characteristics were degraded. These changes are attributed to formation of a new surface layer on the substrate. Verification of this hypothesis was undertaken by carrying out an immersion test in warm water (80°C). Clear changes in surface morphology were seen. Meanwhile, the oxidized substrates were found to experience no changes in weight or surface morphology under the pressure cooker test. It is concluded that the thermal oxidation process is an effective way to protect the AlN substrates against moisture and the subsequent degradation of the insulation characteristics