Optical EngineeringInvestigation of diffractive-refractive microlens array fabricated by focused ion beam technology
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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 9x9 array of micro diffractive optical elements (DOEs) with a singlet f number of 2 and a numerical aperture (NA) of 0.25, and a 9x9 refractive microlens array with a singlet diameter of 60 ?m, 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 ?m (full width at half maximum) and 5.1 ?m (at 1/e2) for the diffractive lens and the refractive lens array, respectively. The rms thickness errors within a diameter of 27 ?m for the DOEs and within 60 ?m 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.