A Novel Bonding Technique Using Metal-Induced Crystallization of Amorphous Silicon
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0989-A06-01
A Novel Bonding Technique Using Metal-Induced Crystallization of Amorphous Silicon Markus D. Ong, and Reinhold H. Dauskardt Materials Science and Engineering, Stanford University, 416 Escondido Mall, Stanford, CA, 94305
ABSTRACT This study investigates the use of aluminum-induced crystallization of amorphous silicon as a potential bonding mechanism for a sandwich stack of films between two silicon substrates. Similar procedures using copper diffusion bonds have been in use, but these require temperatures as high as 400 ∞C. Using the crystallization of amorphous silicon as the bonding mechanism has allowed the bonding temperature to be lowered by more than 100 K. Fracture experiments for a low-k material were conducted, and the results using amorphous silicon bonding was compared to epoxy bonding control experiments. Essentially identical results were obtained for the two bonding mechanisms. Low-temperature bonding techniques are of great interest to future progress in the microelectronics industry, and these results are promising advances. INTRODUCTION Wafer to wafer bonding is a critical step in the fabrication of complex three-dimensional (3-D) structures. Complex 3-D structures have many technologically relevant applications such as increasing transistor densities in microelectronic devices while reducing interconnect RC time delay and power consumption [1], increasing efficiencies of thin film solar cells [2], and microelectromechanical systems [3]. Current state-of-the-art bonding schemes include adhesive bonding [1,3], copper diffusion bonding [4], and oxide-oxide bonding [5]. Disadvantages and common drawbacks of these techniques include insertion of thick intermediate layers, weak bonding, high temperature requirements, and incorporation of incompatible materials [1]. In this study, we demonstrate a novel bonding technique using metal-induced crystallization (MIC) of amorphous silicon (a-Si). This novel technique creates a thin, strong bond at relatively low temperatures while incorporating materials already commonly in use in microelectronics fabrication. The integration of 3-D stacks using low temperature bonding is especially desirable, particularly when incorporating organic or hybrid organic-inorganic dielectric materials that degrade at excessively high temperatures [6]. Previous studies have shown that crystallization of a-Si can be induced by some metals such as aluminum, gold, or palladium [7] at temperatures below 200 ∞C, well below the 400 ∞C required for copper diffusion bonding [8]. Therefore, this reaction is of great interest for use as a low-temperature bonding technique. This study shows that aluminum-induced crystallization of amorphous silicon is a potential bonding mechanism as it is used as the bonding technique in thin-film fracture experiments and compared to results of a control group consisting of well-established values of fracture energies measured using an epoxy bond.
EXPERIMENT Amorphous silicon was deposited on silicon substrates (40 mm x 40 mm squares) with nati
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