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Wetting behaviour of LTCC and glasses on nanostructured silicon surfaces during sintering
Keywords: lyophilic nanostructured silicon surfaces, pressure-assisted sintering process, Wetting behaviour
Wetting behaviour of LTCC and glasses on nanostructured silicon surfaces during sintering S. Gropp1, M. Fischer2, L. Dittrich1, B. Pawlowski3, J. Müller2, and M. Hoffmann1, 1Chair for Micromechanical Systems, IMN MacroNano®, lmenau University of Technology, Am Helmholtzring 1, 98693 Ilmenau, Germany 2Electronics Technology, IMN MacroNano®, Ilmenau University of Technology, Gustav-Kirchhoff-Straße 1, 98693 Ilmenau, Germany 3Fraunhofer Institute for Ceramic Technologies and Systems, Michael-Faraday-Str. 1, 07629 Hermsdorf, Germany To combine the excellent mechanical characteristics of silicon for active MEMS with the electrical properties of LTCC a compound substrate was originally investigated at the IMN MacroNano® at Ilmenau University of Technology in 2008. The silicon on ceramics (SiCer) compound is ideal for RF-MEMS devices because passive RF components such as coils could be included into the LTCC while the silicon could processed with classic thin-film technology. The SiCer compound is manufactured from a silicon substrate with homogeneous nanostructuring, a lamination step of LTCC layers and silicon, followed by a pressure-assisted sintering process. This procedure is time-consuming (process time approx. 24 h) and requires cost-intensive technology. Furthermore, there is the risk of plastic deformation in the silicon due to the high temperatures in combination with high pressure. In this work we present modified nanostructured silicon surfaces for an optimized bond interface to improve the wetting process. For that purpose we tested the wetting properties of BGK, a sodium containing base material developed for anodic bonding, and BCT (Bondable Ceramic Tape), for direct silicon bonding, on different nanostructured silicon surfaces at 870 °C and 900 °C. Further, we investigate the behaviour of Borofloat 33, AF32, and AF45, applied as nanoscale glass powder on nanostructured surfaces. The results were analyzed by microscope and SEM. We show that the high surface tensions of the molten glasses at high temperatures in combination with the lyophobic surface of the structured silicon prohibit forming of an interlocking bond between the two substrates. To overcome this behaviour and to reduce the pressure during the sintering process we start to modifier the silicon surface. With mechanical processing of the surface of the silicon there is no possibility to change the lyophobic characteristic into lyophilic. Therefore, we treat the nanostructured silicon surface chemically by adding additional thin layers of metals or thermal SiO2. First investigations with additional sputtered metals on the nanostructured silicon show a lyophilic characteristic of the surface and a better bond behaviour of the SiCer compound. This work is founded by the German Research Foundation within the projekt MUSIK (DFG HO 2284/3-1)
Sebastian Gropp,
Chair for Micromechanical Systems, IMN MacroNano®, lmenau University of Technology, Am Helmholtzring 1, 98693 Ilmenau, Germany
Ilmenau, Thüringen
Germany


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