Influence of gold thin-film interlayers on anodic bonding of copper microstructures produced by LIGA

Neither pure copper nor solid gold can be anodically bonded to glass. It is only the gold coating on the copper which allows a joint to be built up as a result of the copper ions diffusing into the gold layer, but not many of them being able to migrate into the glass. To encapsulate microstructures produced by the LIGA technique, anodic bonding of gold-coated copper to Coming 0211-type glass was studied. For demonstration purposes, a glass platelet made of Coming 0211 was anodically bonded to a LIGA linear actuator consisting of electroplated copper coated with 1 mu m of gold. A better understanding about the decisive parameters in anodic bonding was obtained by varying the bonding temperature and the thickness of the gold layer. Glass can be bonded on to the entire surface of gold layers 0.5-1 mu m thick at temperatures as low as 300 degrees C; however, when the systems cool to room temperature, stress-induced cracks arise in the glass. On the other hand, thicker gold layers of 2.5 to 10 mu m thickness require higher bonding temperatures for the same period of heating, but prevent the occurrence of such cracks because of their higher ductility.