ALIGNED WAFER BONDING - A KEY TO 3-DIMENSIONAL MICROSTRUCTURES

Successful fabrication of critically aligned three dimensional structures has been achieved by combining precision alignment procedures and techniques for direct silicon bonding. This produces three dimensional bonded layers that might include combinations of mechanical, electronic and/or optical elements formed in separate prefabricated layers. We call this technique aligned wafer bonding. The precise aligned bonding of the features was done with an Optical Associates Hyperline 400 Infrared Aligner. This machine can hold two imprinted wafers face to face while projecting an infrared image of the surfaces to a viewing screen. An array of alignment marks were etched into the surface of silicon wafers with hot potassium hydroxide. These V-grooves were then precisely aligned and the wafers were brought into contact for initial bonding. Subsequent high temperature annealing was used to strengthen and complete the chemical bonding. The instrumentation used in this work required alignment features with a vertical dimension of 30 micrometers to produce a suitable infrared image. We found that the apparent size of the images produced by the optical system limited the accuracy in precision alignment. However, with reduced wafer separation, we achieved wafer alignment with an accuracy of better than 5 micrometers. This technique would generally be used for the precision alignment and bonding of complementary micromechanical, electrical, or optical structures during the formation of three dimensional devices. The details of the aligned wafer bonding and its applications are presented.