Anodic Bonding at Room Temperature
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Anodic Bonding at Room Temperature Volker Baier1, Andreas Gebhardt2 and Stefan Barth3 1 Institute for Physical High Technology, Micro Systems Division, Winzerlaer Straße 10, D-07745 Jena, Germany, e-mail: [email protected] 2 VITRON Spezialwerkstoffe GmbH, Otto-Schott-Straße 13, D-07745 Jena, Germany, e-mail: [email protected] 3 Hermsdorfer Institut für Technische Keramik e. V., Magnetwerkstoffe, Marie-Curie-Straße 17, D-07629 Hermsdorf, Germany ABSTRACT Special phosphate glasses (niobium-phosphate glasses) are used for anodic bonding experiments. They are optimized particularly with regard to elevated alkali ion conductivity at room temperature due to high contents of Li or Na ions. The compounds anodically bonded at room temperature show the strength of the breakage of the glass. Only a few seconds up to a few minutes at voltages of maximum 500 V are necessary for bonding. The glasses have a thermal expansion coefficient, which is not fitted to that of silicon. Therefore, stability of the bonded compounds is achieved only at temperature jumps up to 100 K in the case of bulk samples. For higher temperature jumps the probability of breakage increases continuously. It is possible to get thin films of these glasses with ac and dc magnetron sputter techniques for example on silicon. These thin layers are anodically bondable and the compounds with silicon show a higher temperature stability (at least 233 to 473 K). INTRODUCTION Commonly used glasses for anodic bonding like Pyrex bond glass (# 7740) are bonded typically at 623 - 673 K. With Li-aluminosilicate-β-quartz glass-ceramic (HOYA # PS-100)1 and Li-alumino-boro-silicate glass2 bond temperatures of 473 K are reached. But anodic bonding at lower temperatures gets more and more important as a method for hermetic encapsulation of many components, because this process is one of the last of the many manufacturing steps and, therefore, high temperatures are often not allowed. For the anodic bonding process it is necessary to have a high ion conductivity in the bond glass and the thermal expansion coefficients of the bond partners should be well fitted to each other (for example silicon to the bond glass) because of the temperature range of operation. On the other hand, the condition of thermal fitting of the expansion coefficients is of lower importance, if it is possible to bond at ambient temperature. Therefore, the search for glasses with high ion conductivity at room temperature is of main interest. EXPERIMENT Preparation of glasses Two glasses are prepared in the system R2O-Nb2O5-P2O5 with R = Li or Na. The composition is optimized for high alkaline-ion conductivity at room temperature3.
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The reagents Na2CO3, NH4H2PO4 and Nb2O5 are mixed and then the homogeneous powder is slowly moistened in a beaker with some water. Now the formation of CO2 begins. After the end of the degassing the slurry is heated at 353 K for about 18 h. The glasses are obtained by melting of the raw materials at 1273 K in a Pt crucible for about 2 h. Finally, the melts are cast into a
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