Low-Temperature Bonding of Ceramics by Sol-Gel Processing
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Low-Temperature Bonding of Ceramics by Sol-Gel Processing C.J Barbé, D.J. Cassidy, G. Triani, B.A. Latella, D.R.M. Mitchell, A. Day, K. Short, J.R. Bartlett, J.L. Woolfrey and G.A. Collins. ANSTO Materials Division, PMB 1 Menai NSW 2234, Australia. ABSTRACT Sol-gel bonding was produced between smooth, clean substrates of silicon and polycrystalline alumina by spin-coating solutions containing partially hydrolysed silicon alkoxides. The two coated-substrates were assembled and the resulting sandwich was fired at temperatures ranging from 300 to 600°C. The coatings and bonded substrates were investigated using SEM, TEM and micro-indentation. For silicon wafers, an optimum water-to-alkoxide molar ratio of 10 and hydrolysis water pH of 2 was found. Such conditions led to relatively dense films (>90%), resulting in bonds with a fracture energy of 3.5 J/m2, which is significantly higher than those obtained using hydrophilic wafer bonding (typically 1.5 J/m2). Poly-crystalline alumina substrates were similarly bonded at 600°C; the optimised silica sol-gel chemistry yielded interfaces with fracture energy of 4 J/m2. INTRODUCTION Traditional methods for bonding ceramics involve either diffusion bonding at temperatures exceeding 1000°C or the use of organic polymers. However, the former approach is incompatible with the packaging requirements of modern electronic components (which cannot sustain temperatures exceeding 300°C), while the latter approach yields bonds with significant permeability towards chemical vapours and gases, precluding application in caustic, corrosive or in vivo environments. Sol-gel processing, which essentially involves low-temperature, inorganic polymerisation reactions, should overcome these limitations and form strong, low-permeability ceramic-ceramic bonds at low temperature. Although sol-gel bonding of silicon wafers has been reported previously 1, our inability to reproduce the results, and the consistent production of particulate coating solutions which resulted in very weakly bonded specimens, has led us to develop our own chemistry and bonding process. Furthermore, to bond polycrystalline alumina substrates, thicker films are required, re-enforcing the need for a more versatile sol-gel process. The influence of the sol-gel chemistry on the coating morphology, and the bond interfacial energy ultimately achieved, has been studied in detail and reported elsewhere 2. In this paper, we will focus on two aspects of the sol-gel bonding: the nature of the sol-gel bond in the model silicon system and extending this work to the bonding of a more challenging polycrystalline substrates, such as alumina.
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EXPERIMENTAL Procedure Standard sol-gel bonding solutions were prepared by adding a 0.01 M solution of HNO3 to a solution of tetraethylorthosilicate (TEOS) in ethanol. A water-to-alkoxide mole ratio (W) of 10 was used, and the solutions were aged for one day prior to use. The bonding procedure was conducted in a Class 1000 clean room environment. Silicon wafers (Virginia Semiconductors) were pre
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