<title>Refractive microlenses for ultraflat photolithographic projection systems</title>

Abstract

We report on the fabrication of high-quality microlens arrays on 4' ' , 6' ' and 8' ' -fused silica wafers. Refractive, pianoconvex microlenses are fabricated by using photolithography; a reflow or melting resist technique and reactive ion etching.A diffraction-limited optical performance (p-v wave aberrations < X/8, Strehl ratio > 0.97) is achieved. Aspherical lens profiles are obtained by varying the etch parameters during the reactive ion etching transfer.The microlens arrays are used for Microlens Projection Lithography (MPL) and within UV-light illumination systems. Microlens Projection Lithography is an innovative technique using KARL SUSS Mask Aligners equipped with an ultra-flat microlens-based projection system. The projection system consists of 500.000 identical micro-objectives side-by-side. Each micro-objective consists of 3 to 4 microlenses. A fully symmetrical optical design eliminates coma, distortion and lateral color. The lens system is frontal-and backside telecentric to provide a unit magnification (+1) over the whole depth of focus. Each micro-objective images a small part of the photomask pattern onto the wafer. The partial images from different channels overlap consistently and form a complete aerial image of the photomask. Microlens Projection Lithography provides an increased depth of focus (> 50 microns) at a larger working distance (> 1 mm) than standard proximity printing. Microlens Projection Lithography allows photolithography on curved or non-planar substrates, in V-grooves, holes, etc. using a KARL SUSS Mask Aligner.

Investigation of aspherical microlens based on reflow technique

“…To ensure that the surface of the lens is consistent with the design [14] result and achieves the desired coupling efficiency, the surface of the profile, surface mean square root roughness and reflectivity of the surface, and other key parameters are characterized. The results are as follows.…”