Three-dimensional silicon fabrication using microloading effects with a rectangular aperture mask

Three-dimensional silicon fabrication is demanded in optical lenses for collecting far infrared radiation. We propose a method for the fabrication of shapes, which have depths varying in the x and y directions, vertical walls and smooth surfaces. A mask design with rectangular apertures was used in a two-stage etching process consisting of anisotropic etching to rough out the three-dimensional shape followed by isotropic etching to smooth the surface. The relationship between the etching depth and area and the aspect ratio (length-to-height ratio) of the rectangular apertures was determined experimentally. Based on this relationship, we describe a procedure for designing rectangular apertures. We fabricated a convex microlens with a diameter of 150 μm and a height of 4.3 μm surrounded by a vertical wall. The arithmetic mean surface roughness of the microlens was 100 nm.