A room-temperature direct-bonding method for various crystal substrates including semiconductors and oxides has been developed. In this method, the surfaces of two wafers are sputter etched by Ar ion beam and bonded in high vacuum. This method is called the surface-activated bonding (SAB). It is suitable for the bonding between different materials and fabrication of integrated substrates, because the process is free from the problems of thermal expansion mismatch. Such integrated substrates are expected to improve various microdevices and allow realization of new devices. (c) 2006 Published by Elsevier B.V.