Plasma-activated silicon–glass high-strength multistep bonding for low-temperature vacuum packaging

Abstract

Si-glass bonding is one of the most crucial vacuum packaging methods in semiconductor devices; however, high temperatures (>300 °C) are required to achieve a high bonding strength, which limits its applications. Current low-temperature Si-glass bonding requires a humid environment and is not compatible with vacuum environments. In this study, a low-temperature vacuum multistep bonding method based on plasma activation was proposed. Traditional plasma-activated low-temperature bonding relies on water-molecule bridging in the atmosphere to lower the bonding temperature. In contrast, the proposed multistep bonding process uses plasma-activated vacuum anodic bonding for initial bonding, and then utilizes direct bonding to balance the bonding surface energy and sufficiently eliminate the bonding surface micro gap, thus achieving a high bonding strength of 18 MPa at 150 °C under vacuum. The effects of plasma activation on the hydrophilicity, surface morphology, and chemical conditions of the wafer surfaces were investigated. The morphologies of the bonding interfaces under different bonding conditions were characterized, and cross-sectional elemental scanning was used to analyze the microscopic factors. Based on the aforementioned investigations and the gap closure theory, the mechanism of multistep bonding was analyzed. Finally, the proposed bonding method was validated by fabricating micro alkali metal vapor cells. The results demonstrate that the proposed method meets the requirements for vacuum device packing at low temperatures.