Field-assisted diffusion bonding and bond characterization of glass to aluminum

The bonding of glass wafers to aluminum foils in multi-layer assemblies was investigated by the fieldassisted diffusion bonding process. Bonding was effected at temperatures in the range 350–450 C and with an applied voltage in the range 400–700 V under a pressure of 0.05 MPa. The experimental parameters of voltage and temperature were the main factors in influencing the ionic current leading to the formation of the depleted layer. The peak current in three-layer samples (glass/aluminum/glass) during bonding is twice that for the case of the two-layer samples (aluminum/glass). SEM and EDS analyses showed the presence of transition layers near the glass/aluminum interface, and XRD data demonstrated the phase structure of the glass/aluminum interface. The tensile strength of the bonded material increased markedly with increasing temperature and applied voltage. Fracture occurred in the glass phase near the interface with the aluminum. Finite element analysis showed the residual deformation in three-layer samples to be significantly lower than in two-layer samples. The symmetry in three-layer samples resulted in the absence of strain, an important advantage in MEMS fabrication. Introduction Field-assisted diffusion bonding or anodic bonding is a key technique in the joining of alkali-rich glass (or ceramics) to semiconductors or metals. The technique, first described by Wallis and Pomerantz in 1969 [1], has become an important process in the formation and assembly of microelectromechanical systems (MEMS), pressure sensors, accelerometers, and microfluidic devices [2–4]. Field-assisted diffusion bonding has been extensively investigated to join glass to silicon [5]. Investigations have also been carried out for bonding of glass to other materials, including SiO2 [6], Si3N4 [7], and various metals [8–10]. Bonding is effected by imposing a DC field across the glasssilicon layers at typical temperatures in the range 350– 400 C, with the glass being biased as the cathode. The field causes the dissociation of the alkali oxides in the glass and results in the migration of the metal ions (typically Na) away from the interface creating a depletion layer. The subsequent diffusion of oxygen ions to the silicon and the ensuing reaction to form Si–O bonds contribute to the bonding process [11, 12]. The microstructure and the composition related to the formation of the bond have been investigated [13–15]. Requirements in more complicated circuitry and microelectrical devices have led to a focus on multi-layers of glassmetal entities for such applications as silicon ink-jet printers, biomedical devices, and micro-chemical analyzing system [16–18]. In the previous investigations on field-assisted diffusion bonding of glass-metal systems [19–22], layers of silicon, aluminum, or silicon nitride were prepared by sputtering or physical vapor deposition techniques. In the present article, the bonding mechanism and the factors influencing bonding of glass to aluminum are investigated in multi-layer systems, utilizing aluminum foils and glass wafers. C. R. Liu X. Y. Lu Q. S. Meng (&) Department of Materials Science & Engineering, Taiyuan University of Technology, Taiyuan 030024, China e-mail: [email protected] J. F. Zhao Z. A. Munir Department of Chemical Engineering & Materials Science, University of California, Davis, CA 95616, USA Y. P. Zhao Mechanical Research Institute of the Chinese Academy of Science, Beijing 100086, China 123 J Mater Sci (2008) 43:5076–5082 DOI 10.1007/s10853-008-2583-4