Methods for wafer to wafer bonding using microstructures

Abstract

Wafer-to-wafer bonding using, e.g., solder metal bonding, glass bonding or polymer (adhesive) bonding is improved by profiling one or both of the wafer surfaces being bonded to define microstructures therein. Profiling means providing other than the conventional planar bonding surface to define cavities therein. The bonding material fills the cavities in the microstructures. For instance, a system of ridges and trenches (e.g. in cross-section vertical, slanted, key-holed shaped, or diamond-shaped) are microstructures that increase the surface area of the wafers to which the bonding material can adhere. Use of the key-hole shaped or diamond-shaped profile having a negative slope at the trench interior substantially increases the bonding force. Where electrical leads feed through a bond, the seal to the bond at that point is improved by either using lengthened feedthroughs which extend the length of a substantial portion of the bond or by use of doped (diffused) conductive wafer regions rather than metallized feedthroughs. In some cases the bonded surfaces include spacers which define a precisely defined distance between the two wafers, thus providing an exactly specified thickness of the bonding material. In some cases the edges of the bonded assembly are sealed by a sealant which extends through narrow capillary spaces between the two bonded wafers, thus hermetically sealing together the two wafers.