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 step. The silicon shadow mask is micromachined such that it fits in a self-aligned fashion into the removed area of the device wafer and thus approaches the apertures close to the surface. Furthermore, owing to self-alignment between shadow and target, the lateral overlay precision is improved. The use of this technique is demonstrated for direct evaporation of metal patterns 10 x 50 mu m(2) in size on the backside of a membrane of a bimorph-actuated device that is recessed by 500 mu m. The same pattern dimensions were achieved in positive and negative resist by exposing it through the shadow mask aperture. We also show an application that uses a different type of shadow 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.