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KEY WORDS: silicon-on-insulator, surface energy, UHV bonding, compliant substrate
ABSTRACT
When mirror-polished, flat, and clean wafers of almost any material are brought into contact at room temperature, they are locally attracted to each other by van der Waals forces and adhere or bond. This phenomenon is referred to as wafer bonding. The most prominent applications of wafer bonding are silicon-on-insulator (SOI) devices, silicon-based sensors and actuators, as well as optical devices. The basics of wafer-bonding technology are described, including microcleanroom approaches, prevention of interface bubbles, bonding of III-V compounds, low-temperature bonding, ultra-high vacuum bonding, thinning methods such as smart-cut procedures, and twist wafer bonding for compliant substrates. Wafer bonding allows a new degree of freedom in design and fabrication of material combinations that previously would have been excluded because these material combinations cannot be realized by the conventional approach of epitaxial growth.
INTRODUCTION
Wafer bonding refers to the phenomenon wherein mirror-polished, flat, and clean wafers of almost any material, when brought into contact at room temperature, are locally attracted to each other by van der Waals forces and adhere or bond. Wafer bonding is alternatively also known as direct bonding, fusion bonding, or more colloquially as gluing without glue. In most cases, the wafers involved in actual applications are semiconductor wafers consisting of singlecrystal materials such as silicon or gallium arsenide used in microelectronics or optoelectronics. The bonding at room temperature is usually relatively weak compared with that of covalently or ionically bonded solids. Therefore, for many applications, the room-temperature-bonded wafers have to undergo a heat treatment to strengthen the bonds across the interface. Frequently, one of the two wafers is then thinned down to a thickness that, depending on the specific application, may be in the range of many microns down to a couple of nanometers. Modifications of this generic process flow are quite common for specific applications; e.g. no heating step or no thinning step may be involved, or the heating and bonding steps may be combined.
At present, the most prominent applications of wafer bonding are in the areas of silicon-on-insulator (SOI) devices and silicon-based sensors and actuators. SOI structures consist of a thin, top layer of single-crystal silicon, a layer of silicon dioxide (Si02), and a silicon handle...