Aluminum-Germanium Eutectic Bonding for MEMS: Behaviour and Solidification of Liquid Al-Ge on Different Substrates

In this paper we study the formation and behaviour of liquid Al-Ge alloy, its solidification, the resulting microstructures and the appearance of voids in bonded structure for MEMS packaging. The underneath layer influences the wettability of liquid Al-Ge as well as its solidification which can lead to undesirable Al dendrites. Each underneath layer imposes a critical value for the cooling rate above which dendrites are likely to appear. Blanket and patterned Al/Ge deposited wafers are bonded together with process parameters optimized thanks to previous results. The interfaces present a dendritic-free eutectic structure. Micro-voids are observed at the interfaces between Al and Ge phases. The Kirkendall effect and thermal stresses are two possible reasons for voiding phenomenon.