ABSTRACT
When mirror-polished, flflat, 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.
Wafer bonding refers to the phenomenon wherein mirror-polished, flflat, 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 single-crystal 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 specifific application, may be in the range of many microns down to a couple of nanometers. Modififications of this generic process flflow are quite common for specifific 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 (SiO2), and a silicon handle substrate that acts as mechanical support. In the fabrication of SOI substrates by wafer bonding, the silicon wafer forming the top layer has to be oxidized before bonding and thinned down to between 0.1 and 10 µm after bonding. SOI devices that are radiation hard are able to operate at high temperatures and have potentially higher packing density and a lower power consumption than devices on conventional silicon substrates. These features make such devices especially attractive for handheld and battery-operated electronic equipment.
In spite of the dominance of silicon-related applications, wafer-bonding technology is by no means restricted to silicon wafers. Proper polishing and control of the chemistry of the surfaces make it possible to bond a variety of solids independently of their structure (amorphous, polycrystalline, single-crystal), their crystallographic orientation and lattice parameter, or the thickness of the wafers. Wafer bonding, therefore, allows the fabrication of material combinations that previously were ruled out by most materials scientists, solid state physicists,and electrical engineers, because these material combinations were not possible by the conventional approach of epitaxial growth. Wafer bonding can also be used as a specifific joining technique for many applications, especially in the area of microsystems technologies, but also in the areas of nonlinear optics and light-emitting diodes.
We begin with a short history of wafer bonding and then discuss silicon/silicon wafer bonding in some detail because it is the best-investigated and economically most important example of such bonding. We discuss bonding of dissimilar materials and nonsilicon materials, a promising thinning approach involving hydrogen implantation, and various methods used to perform wafer bonding with high bonding strength at temperatures as low as room temperature. Finally, we discuss the use of wafer bonding to fabricate compliant substrates.Because of the large number of papers published on wafer bonding over the last decade, we do not give an exhaustive list of references. We rather refer to the proceedings of a series of symposia devoted to wafer bonding (1–4), recent review articles (5–14), and a special 1995 issue of the Philips Journal of Research (15). The subject of SOI devices has been treated extensively elsewhere (16, 17) and is not specififically discussed in this review.
