Wafer-Bonding and Thinning Technologies
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down to between 0.1 /j-m and 10 yam after bonding. In spite of a certain dominance of silicon-related applications, waferbonding technology is not restricted to silicon wafers. Proper polishing and control of the chemistry of the surfaces allows one to bond a. whole variety of solids independent of their structure (amorphous, polycrystalline, single-crystalline), their crystallographic orientation, and their lattice parameter or the thickness of the wafers. Wafer bonding therefore allows the fabrication of material combinations that—by most materials scientists, solidstate physicists, or electrical engineers— would previously have been excluded because these combinations cannot be realized by the conventional approach of epitaxial growth. Wafer bonding can also simply be used as a specific joining technique for all kinds of applications especially in the growing area of microsys-
terns technology but also in the areas of nonlinear optics and improvement of conventional light-emitting diodes. Because of the large number of papers published on wafer bonding over the last decade, we cannot give an exhaustive list of references. We rather refer to the proceedings of a series of symposia devoted to wafer bonding,1"4 to recent review articles,5"8 to a special 1995 issue of the Philips Journal of Research, and to a book
on this subject.910 After wafer bonding, one of the wafers is subsequently polished or etched down to a thickness suitable for SOI applications. The other wafer serves as a mechanical substrate and is called handle wafer (Figure 1). Wafer Bonding In 1985 Lasky et al. proposed a mechanism to describe the bonding process between two oxidized wafers at both room temperature and high temperature. 11 This initial work was used to produce SOI by the bonding of two oxidized wafers together at room temperature, by annealing of the bond at high temperature in an oxygen ambient, and by thinning of one of the wafers to a desired SOI thickness. Since then the process has been perfected. Silicon-wafer bonding usually involves the following steps: (1) The surfaces of two mirror-polished silicon wafers, which may or may not contain structures such as cavities, are conditioned and prepared for the bonding process. The wafers may be thermally oxidized or may just contain a native oxide that is made hydrophilic by a proper surface treatment. The wafer surfaces are then typically covered by one or two monolayers of water.
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Figure 1. Bonding of two oxidized silicon wafers (left) and polishing/etching back of one of the wafers.
MRS BULLETIN/DECEMBER 1998
Wafer-Bonding and Thinning Technologies
(2) The two mirror-polished wafer surfaces are brought into contact for bonding at room temperature in air in a sufficiently clean environment (either in a conventional clean room or in a "microclean room"; see, e.g., Reference 12) in order to avoid particles between the wafers. (3) Directly after room-tempera
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