Generation of submicrometer structures by photolithography using arrays of spherical microlenses

This paper describes the fabrication of arrays of spherical microlenses by self-assembly of microspheres and the use of these arrays for nearfield photolithography to generate repetitive microstructures in photoresist. We used these arrays of microspheres to fabricate two types of elastomeric membranes: (i) membranes that have microspheres embedded in their surface and (ii) membranes that have hemispherical wells in their surface. Both types of membranes act as amplitude masks that pattern the intensity of illumination in the near field incident on the photoresist. Microspheres in the first type of membrane act as convergent lenses that generate recessed microstructures in positive photoresist. Hemispherical wells in the second type of membrane act as divergent lenses that produce protrusive microstructures in positive photoresist. This method can generate dense, regular arrays of microstructures with a variety of profiles--circular or hexagonal holes, circular posts, hollow posts, and cones--depending on the sizes and refractive indices of the spherical lenses and the distance between the lenses and the photoresist. This technique provides a simple route to large areas (>4 cm2) of repetitive, simple microstructures.