Investigation of the ageing performance of AlN-ceramic used in high-power-semiconductors

Abstract

The maximum voltage of a high power semiconductor is not only limited by the processes in the silicon or thermal problems, but also by the maximum electric field and breakdown strength distribution in the packaging. The selection of aluminium nitride ceramics (AlN) as insulation material for this application is based on its high thermal conductivity (/spl lambda//sub AlN//spl ap/170 W/(m.K)). Taking into account the aim of increasing the operating voltage of for example IGBTs (Insulated Gate Bipolar Transistor) from 6.5 kV up to voltage levels above 10 kV the electric characteristics of AlN become much more important. Therefore the dielectric breakdown and the ageing performance of unmetallized industrially produced AlN substrate material, DCB (Direct Copper Bonding) copper metallized and AMB (Active Metal Brazing) copper metallized AlN substrates are examined in this study. The investigation shows a huge difference between the ac and dc breakdown strength which can attributed to the microstructure of the ceramic. A degradation of the material during the electric ageing cannot be determined. The DCB metallized AlN substrates show a similar behaviour but the breakdown voltage is lower because of the inhomogeneous field distribution caused by the geometry of the metallization. In contrast to this the breakdown behaviour of the AMB metallized AlN substrate is quite different. In most cases external flashovers occur although the sample geometry is equal to the DCB samples. The reason for this behaviour can also be attributed to the microstructure of AlN. The investigation of AlN-ceramic elucidates a development potential towards higher operating voltages.