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2001
Journal Article
Titel
Precise polymer micro-optical systems
Abstract
UV moulding enables for the generation of thin polymeric films carrying micro-optic elements on top of arbitrary substrates like glass, Silicon, III/V-semiconductors by a process which is performed in a modified contact mask aligner. The elements are characterized by high precision and stability, temperature stable and precise pitch, index homogeneity, uniformity across the wafer, and they fulfill additional requirements for a practical application (AR-coating, separation in a dicing saw). The basic UV-reaction moulding technology was described already, e. g. in /1/. This technology has now been improved and completed. This concerns first of all the investigation of the influence of the technology on the precision of replicated refractive optical surfaces. The basic technology is a combination of lithography, reflow and UV-moulding. In contrast to variable dose writing/exposure or etching, where the absolute precision depends on some dissolution/ etching rate (and is therefore propo rtional to the total height of the element), the reflow process used here results in a "minimal" surface which is primarily independent of the total sag. Additional factors influencing the accuracy of the structures (e. g. reaching the equilibrium state in the reflow process, control of the shrinkage) have been investigated. In the UV-moulding process, the effective shrinking has been reduced down to 1 to 2%. UV-moulded lenses, e. g., show a high precision of the optical surface as well as low wavefront aberrations. Best fit differences to an ideal spherical or cylindrical surface are 10nm rms/ 70nm p.v. for Ø200µm, sag 10µm, N.A.= 0.1 and about 50nm rms/ 300nm p.v. for Ø700, sag 75µm, N.A. = 0.2. The improved accuracy of the technology allows for a number of new applications, first of all collimation lens arrays for fiber ribbons. They must have sags of 40 to 60 µm and a pitch accuracy of < 0.5 µm. That was realized by UV-moulding on borofloat glass. Other applications will be de scribed. The combination of UV-Lithography (mask process) with UV-moulding (using a replication tool) can be applied to integrate waveguides with path converting prisms or other micro-optic elements. Additionally, the binary process can be added to improve the characteristics of UV-moulded structures., e. g. one can define areas which are free of polymer (for electrical bonding etc.). The technology was extended to a double-sided replication in order to generate micro-optical systems on a wafer scale. In contrast to mounted wafer stacks, these systems can be easily separated into chips by a dicing saw. Furthermore, the substrate may show some additional functionality, e. g. it could be a cylindrical gradient index lens fabricated by ion exchange in glass, or can carry pinhole structures. One promising field of application of those micro-optical subsystems is laser diode collimation and beam forming.