Process Modeling of the Laser Induced Surface Modification of Ceramic Substrates for Thermal and Electrical Lines in Mic
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PROCESS MODELING OF THE LASER INDUCED SURFACE MODIFICATION OF CERAMIC SUBSTRATES FOR THERMAL AND ELECTRICAL LINES IN MICROSYSTEMS Herbert Gruhn, Roland Heidinger, Magnus Rohde, Sabine Riidiger, Johannes Schneider* and Karl-Heinz Zum Gahr* Forschungszentrum Karlsruhe, Institute for Materials Research I, *Universitdt Karlsruhe (TH), Institute of Materials Science and Engineering II, P.O.B. 3640, 76021 Karlsruhe, Germany ABSTRACT Laser induced surface modification has been used to fabricate conducting paths in ceramic substrates. For the purpose of process simulation and prediction of process parameters a finite element model has been developed to simulate the thermal behaviour of the substrate during laser surface interaction. The results of the model calculation have been verified experimentally for alumina and Cordierite substrates. Using this model the width and the depth of the fabricated lines could be predicted as a function of the laser power and velocity. The stresses due to thermal mismatch are estimated and identified as the likely reason for crack formation which reduces the functionality of the conducting paths. Further developments will consider different ceramic substrates such as PZT. INTRODUCTION For many applications in microsystems technology the question of thermal management is of high interest. Very important are also robust conducting paths with a high electric conductivity. Today such paths between active elements on ceramic substrates are formed by lithographic methods in thin and thick film technology. A possible alternative is based on the laser induced surface modification [I] (see figure 1). Three different processes are considered. The remelting
process is regarded as the fundamental process in which the substrate surface is melted locally by the laser beam and solidifies. In the injection process particles are directly sprayed into the laser melted surface. This is in contrast to the precoating process. In the first step the substrate is precoated with the particles to be dispersed. In a second step the laser beam remelts the substrate under the precoating and intermixes both. The cross-sections of solidified lines produced by the three processes in substrate material Cordierite (2MgO - 2SIO 2 - 5A120 3) are presented in figure 1. To avoid cracking by thermal shock during laser treatment the ceramic substrates are preheated slightly below sintering temperature. The problem of the oxidation of metallic precoatings during substrate preheating was solved by the development of a special vacuum furnace which allows laser material treatment in vacuum. The advantages of the processes are free design and a good bonding to the substrate particularly for higher temperature applications. The aim of this work is to fabricate small lines with a width down to 200 gim which show a significant increased thermal and electric conductivity compared to the ceramic. The ceramic substrate material considered in this study is Cordierite because of its outstanding properties such as low thermal expansion, good
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