Dielectric breakdown in aluminium nitride

Aluminium nitride (AlN) is regarded as a useful engineering material because of its high thermal conductivity. Whilst thermal conductivity data is available for AlN, the equivalent electrical breakdown data is scare. Therefore industrially produced unmetallized aluminium nitride is subjected to AC and DC breakdown tests at different temperatures. Furthermore the breakdown strength of DCB (direct copper bonding) metallized AlN with differently shaped copper metallization edges is explored. For unmetallized AlN huge differences between ac and de dielectric strength values are found. The measured breakdown field strength for samples of different manufactures varies from 50 to 68 kV/mm (peak value) for AC and from 84 to 151 kV/mm for DC voltage. A possible reason for this behaviour can be seen in the microstructure of AlN. A statistically significant influence of the temperature on the dielectric strength cannot be discovered. For DCB-metallized AlN the difference between DC and AC dielectric strength values can also be proved. The examination demonstrates that the edge of the copper metallization is the starting point of the dielectric breakdown. A difference between both examined structures cannot be found. To explain the breakdown process a model is presented. This model is based on the theory of the breakdown process in small gas filled cracks in insulators