Effect of application of opposite polarity voltage on interface separation of anodically bonded kovar alloy–borosilicate glass joints

Abstract

Anodic bonding is a process for bonding metal to ion-conductive glass at a temperature lower than the glass softening point, where a DC voltage is applied to the metal and glass with an anode and a cathode, respectively. In this study, when opposite polarity voltage was applied to anodically bonded Kovar alloy–borosilicate glass joints, the bond interface separated. Because Na accumulated at the bond interface by application of opposite polarity voltage, a brittle Na and Si compound oxide layer removed from the vitrification region formed adjacent to the bond interface. It is considered that destruction of the brittle oxide layer is caused by the thermal stress owing to a slight difference in the linear expansion coefficient and the elastic stress caused by bonding between bond surfaces with imperfect flatness, resulting in separation of the bond interface. The interface separation speed decreased with higher bonding temperature and lower temperature during application of opposite polarity voltage. Because movement of alkali ions becomes easy when the bonding temperature is high, the width of the alkali ion depletion layer formed adjacent to the bond interface was large in joints with high bonding temperature. When opposite polarity voltage was applied, the potential gradient of the alkali depletion layer became large and the potential gradient of the glass bulk became gradual. As a result, movement of Na ions toward the bonding interface was suppressed and it took a long time to reach the Na concentration required to separate the interface.