Bonding properties of metals anodically bonded to glass

Anodically bonded Pyrex-/Metal double stack were investigated for applications in the sensor encapsulation field. The use of metals can increase the robustness of the packaging and eliminate the use of glue. Titanium anodically bonded to glass can lead to biocompatible systems The anodic bonding of metallic sheets of Invar, Kovar, Alloy 42 and titanium to ion-containing glasses, Pyrex and Foturan, was evaluated in terms of samples preparation, bonding parameters, and bonding characteristics. At a bonding temperature below 300 °C, the stress induced to Pyrex was smaller when the Invar was used, however, a weak bonding was obtained at the lowest bonding temperatures investigated. In comparison with Invar and Alloy 42 bonded to Pyrex, Kovar induced a smaller stress for bonding temperatures higher than 350 °C. For bonding temperatures in between 300 and 350 °C, a similar value of stress was obtained for Kovar and Alloy 42 bonded to Pyrex as well as a high bonding strength. A post-annealing step at a temperature of and higher than the bonding temperature was shown to decrease the bonding stress and can be used to improve the bonding strength of samples bonded at low temperature. Kovar and Alloy 42 bonded to Pyrex at temperatures of and higher than 250 °C were tight to liquid at a pressure of 1.5 bars. In the case of titanium, Pyrex and Foturan were successfully bonded to titanium thin films and sheets, respectively. A proper selection of metals and bonding parameters led to levels of residual stress, strength and tightness that make anodic bonding of metals to glass a suitable technique for the assembling and packaging of microsystems, for instance piezoresistive silicon sensors and microfluidic devices.