Ultraviolet-Assisted Release of Microelectromechanical Systems From Polyimide Sacrificial Layer

Process heating of microelectromechanical systems (MEMSs) devices hardens polyimide sacrificial layers, complicating the final release and lowering yield for delicate structures. This paper reports ultraviolet (UV)-assisted release, which is demonstrated on an MEMS cantilever fabricated by an eight-mask photolithographic process. A commercial co-developable polyimide ProLift 100 (Brewer Science) sacrificial layer was used. The process subjects the device to multiple heat treatment steps. Both wet chemical etching and dry reactive ion etching were explored. During the former, large sheets of hardened polyimide floated free of the substrate to damage delicate MEMS structures. The latter is typically slow, so that grass appears during long exposures to plasma ions. The solution reported here is UV exposure prior to release. Optical constants of the sacrificial layer material, which were baked to simulate thermal histories during various fabrication steps, were measured to understand the effectiveness of UV exposure. Wet and dry etch rates were measured as a function of UV dose. Finally, the advantages of UV pretreatment were demonstrated during the release of actual MEMS cantilevers.