Wafer direct bonding: tailoring adhesion between brittle materials

It is a well-known phenomenon that two solids with sufficiently flat surfaces can stick to each other when brought into intimate contact in ambient air at room temperature. The attraction between the two bodies is primarily mediated through van der Waals forces or hydrogen bonding. Without a subsequent heating step, that type of bonding is reversible. Annealing may increase the energy of adhesion up to the cohesive strength of the materials concerned. The wafer bonding phenomena in brittle materials systems, especially in silicon, is reviewed in the experiment. The focus is on low temperature bonding techniques. The pivotal influence chemical species on the surfaces have on the subsequent type of bonding (van der Waals, hydrogen, covalent bonding, mechanical interlocking) is discussed. Methods of modifying the surface chemistry for tailoring bonding properties are addressed. The paper is aimed at providing an overview of the current understanding of the factors determining the bondability and strength of the bonding obtainable. The authors assess the present state of the experimental methods for determining basic parameters governing the adhesion. A number of examples illustrate the applicability of fusion bonding for as diverse fields as opto-electronics, microsystems technology, and fabrication of advanced substrates like silicon-on-insulator wafers.