Temporary adhesives for wafer bonding: Deep reactive ion etching application

Deep reactive ion etching is a critical step for Micro-ElectroMechanical Systems devices fabrication (MEMS). Some applications are bulk silicon etching with stop on oxinitride membranes, using so both side of a wafer. Usually, a first deep silicon etch is performed on the backside to form a thin silicon membrane with a thickness of a few micrometers. This membrane is then etched from the front side to delimitate active areas and to release the membrane structure. In other cases, vias are performed through the membranes. Nevertheless it´s necessary to use a carrier wafer where the device wafer is bonded on. Our approach is to bond the two wafers with an adhesive which can be considered as an intermediate layer. Temporary wafer bonding requires the adhesive to be easily removed without damaging the features on the active side of the device wafer over a short debonding time to increase throughput. It will also allow to continue the MEMS fabrication. The conventional adhesives (tapes, waxes, Fomblin...) used for bonding were studied. Only the Fomblin showed good results compared to the other adhesives. Fomblin presents the inconvenient of a removing process which takes 40 minutes to 1 hour; and a very bad surface state which makes it not re-usable for other applications. With temporary adhesives, we achieved a better processing window. We will detail deposition process and bonding process conditions of the adhesive, demonstrating the benefits of using the wafer bonding technique which offers a good thermal transfer. We´ll demonstrate that after the bonding, the wafers can move to lithography operations and be processed as single wafers to pattern the structures to be etched. After etching, we proceed to debonding in a few minutes. One other advantage is the good surface state of the wafer which can continue the flow of the process.