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Semiconductor wafer bonding has increasingly become a technology of choice for materials integration in microelectronics, optoelectronics, and microelectromechanical systems. The present overview concentrates on some basic issues associated with wafer bonding such as the reactions at the bonding interface during hydrophobic and hydrophilic wafer bonding, as well as during ultrahigh vacuum bonding. Mechanisms of hydrogen-implantation induced layer splitting ͑''smart-cut'' and ''smarter-cut'' approaches͒ are also considered. Finally, recent developments in the area of so-called ''compliant universal substrates'' based on twist wafer bonding are discussed.

In microsystems technologies, frequently complex structures consisting of structured or plain silicon or other wafers have to be joined to one mechanically stable configuration. In many cases, wafer bonding, also termed fusion bonding, allows to achieve this objective. The present overview will introduce the different requirements surfaces have to fulfill for successful bonding especially in the case of silicon wafers. Special emphasis is put on understanding the atomistic reactions at the bonding interface. This understanding has allowed the development of a simple low temperature bonding approach which allows to reach high bonding energies at temperatures as low as 1508C. Implications for pressure sensors will be discussed as well as various thinning approaches and bonding of dissimilar materials. q 1999 Elsevier Science S.A. All rights reserved.

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