Self-aligned 3D shadow mask technique for patterning deeply recessed surfaces of micro-electro-mechanical systems devices

We present a 3D shadow mask technique for patterned modification of an area that is deeply recessed from the top surface with a large topographical st ep. The silicon shadow mask is micromachined such that it fits in a self-al igned fashion into the removed area of the device wafer and thus approaches the apertures close to the surface. Furthermore, owing to self-alignment b etween shadow and target, the lateral overlay precision is improved. The us e of this technique is demonstrated for direct evaporation of metal pattern s 10 x 50 mu m(2) in size on the backside of a membrane of a bimorph-actuat ed device that is recessed by 500 mu m. The same pattern dimensions were ac hieved in positive and negative resist by exposing it through the shadow ma sk aperture. We also show an application that uses a different type of shad ow mask for the evaporation of an array of 25-mu m wide metal wires across a step of 120 mu m with no loss of dimension control. Full-wafer processing with the reusable shadow mask is demonstrated. (C) 1999 Elsevier Science S .A. All rights reserved.