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

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Vol. 471; p. 144719
• Main Authors: Yu, Mingzhi, Zhao, Libo, Wang, Yongliang, Xia, Yong, Ma, Yintao, Wang, Yanbin, Han, Xiangguang, Chen, Yao, Lu, Shun, Luo, Guoxi, Zhu, Nan, Yang, Ping, Wang, Kaifei, Lin, Qijing, Jiang, Zhuangde
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-09-2023
• Subjects: High bonding strength; Low-temperature vacuum packaging; Plasma-activated; Silicon–glass multi-step bonding; Plasma-activated; High bonding strength; Silicon–glass multi-step bonding; Low-temperature vacuum packaging
• ISSN: 1385-8947; 1873-3212
• DOI: 10.1016/j.cej.2023.144719

•Multistep bonding involves plasma activation, anodic bonding, and direct bonding.•High bond strength of 18 MPa was achieved under vacuum at 150 °C.•Plasma activation forms dangling Si–O bonds on the surface, rendering the surface porous for easy bonding.•Long-term direct bonding maximizes the bond strength by balancing the free surface energy and strain energy at the bonding interface.•The bonding mechanism of the multistep bonding process was established based on gap closure theory.
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.