Low-cost optical microstructures fabricated by imprinting porous silicon

We present a direct-to-device method for stamping porous silicon to produce optical microstructures. The stamping technique utilizes a reusable silicon stamp fabricated by standard lithographic methods. Large area (9mm2) stamps are applied to single layer thin films of porous silicon with a force on the order of 1kN. The process affords precise control over both lateral and vertical dimensions of patterning while maintaining large area uniformity. We demonstrate tunable imprint depths in the 10nm-120nm range as well as lateral feature sizes down to 0.25 mu m. Imprinted structures are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical diffraction experiments. By utilizing reusable stamps and a straightforward technique, the overall process can be performed at low-cost and high throughput. This enables a wide variety of optical microstructures to be readily fabricated. As an example, we present a porous diffraction grating and demonstrate proof-of-concept sensing capabilities, for exposure to water vapor as well as small molecules (3-aminopropyltriethoxysilane). Additional device structures enabled by this fabrication process are also discussed. The stamping process is expected to be applicable to other porous materials such as porous titania, porous alumina, and porous silica.