A novel fabrication technique for three-dimensional concave nanolens arrays

A novel facile technique is proposed for fabricating three-dimensional (3D) concave nanolens arrays on a silicon substrate. The technique leverages an inherent characteristic of the polymethyl methacrylate (PMMA) resist during inductively coupled plasma (ICP) etching. The tendency for plasma ions to accumulate at the edge of the PMMA resist helps create a local electric field that causes the ions to etch the sidewall of the PMMA resist. This process progressively increases the uncovered area, resulting in a graded etched depth or a concave structure in the substrate. In addition, using a given ICP etching recipe, the time required for a PMMA resist to be removed by sidewall etching is determined by its width. The use of PMMA resist of different widths enables one to achieve structures of varying etched depths and thus a 3D lens array. Optical characteristics of the fabricated nanolens were simulated using the FDTD (Finite-difference time-domain) method, and focal lengths ranging from 150 nm to 420 nm were obtained. This type of nanolens is very useful in ultraviolet optical devices and CMOS image sensors.