Investigation of diffractive-refractive microlens array fabricated by focused ion beam technology

A microfabrication process for plano-convex microlens arrays with continuous relief for refractive and diffractive usage by focused ion beam (FIB) technology is introduced in detail. Two FIB techniques—milling and deposition of $SiO2$—are used for the fabrication. A 9×9 array of micro diffractive optical elements (DOEs) with a singlet f number of 2 and a numerical aperture (NA) of 0.25, and a 9×9 refractive microlens array with a singlet diameter of 60 μm, NA of 0.1, and f number of 5 were designed and fabricated by the milling method. In addition, the method of FIB-induced $SiO2$ deposition for microlenses is utilized. The lens profile and focusing characteristics were measured with a WYKO NT2000 interferometer and a BeamScope-5PTM beam scanner, respectively. The focused spot size is about 6.7 μm (full width at half maximum) and 5.1 μm (at $1/e2)$ for the diffractive lens and the refractive lens array, respectively. The rms thickness errors within a diameter of 27 μm for the DOEs and within 60 μm for the refractive lenses (produced by milling) are 11 nm, and 15 nm, respectively. The wave aberrations are 1.7 and 1.5 waves, respectively. © 2001 Society of Photo-Optical Instrumentation Engineers.