Thermische und elektrische Leitpfade in Cordierit durch lasergestütztes Dispergieren von metallischen Hartstoffen und Wolfram

Abstract

Ceramic substrates are commonly used in electronic techniques. These substrates have to meet specific requirements for applications in microelectronics. Beside a moderate mechanical strength, a high thermal conductivity and a good insulating these materials should show a low dielectric permittivity and small dielectric loss. Glass-ceramics based on cordierite offer a low thermal expansion and a good thermal shock resistance. Due to their low dielectric permittivity they have a high potential for use as substrate-materials for highly integrated microwave circuits. The low thermal conductivity can be increased by generating thermal conducting paths using a flexible process. Thus an enhancement of applications can be obtained. In the present study a laserinduced surface-modification process was examined to fabricate thermal and electrical conducting paths. The ceramic substrates were produced by sintering of powders with cordierite composition. Using a CO 2 -laser the cordierite-ceramic was remelted and different additives (TiC, WC, W) with higher thermal and electrical conductivity were embedded. In order to reduce thermoshock the substrate which was precoated with the additive material must be preheated. To achieve this a vacuum-furnace was developed which allowed an in-situ laser treatment. The composite structure which developed after solidification of the ceramic melt was characterised by microstructural examinations and determination of thermal and electrical conductivity. The result of the characterisation has been used to find optimised process-parameters. The result of the laserinduced modification process depended strongly on the selected additive and the process parameters. Paths fabricated with TiC introduced into the melt showed a significant increase of thermal and electrical conductivity over small dimensions. Because of the existing inhomogeneities of the microstructure (cracks, pores) this effect was not reproducible over larger distances. The areas modified with WC showed also an increase of thermal and electrical conductivity. However their adhesion strength to the substrate was limited due to the fact that the WC-particles were not embedded into the substrate but stayed top of it as a surface layer. The additive W showed a very good compatibility with the cordierite-substrate. Using optimised process-parameters e.g. an adjusted level of preheating temperature and power density conducting paths were fabricated which offered a very good adhesion to the substrate and showed a microstructure with homogeneously distributed W-particles inside the ceramic matrix. Follow this procedure any planar structures could be generated on the cordierit-ceramic, which had a ten times higher thermal conductivity than the substrate and an electrical resistivity of 10 - 5 -10 - 6 Ωm.