We developed a fabrication process for microoptical elements with continuous profiles. In contrast to gray tone lithography with the cost intensive HEBS-glass or direct writing by laser or electron beam and the closely connected expensive equipment, the presented technique allows a low budget fabrication of continuous profiles with smooth surfaces. We use conventional binary photolithography with standard DNQ-Novolak based photoresist, simple smoothing techniques and proportional transfer by dry etching. All variations of the procedure are based on the local depth control with the local filling factor of a periodic pattern in a binary photomask. The filling factor of the mask defines the resist volume, which corresponds to an effective layer thickness. With the aid of smoothing techniques after development, the effective resist layer thickness is transformed to the real local profile thickness. Thus, continuous change of the filling factor in the periodic mask pattern results in a smooth height profile. Furthermore, it is possible to fabricate continuous height profiles with just one lithographic step if the mask pattern can not be resolved by the exposure-system. This can be achieved by the use of smaller periods or by increasing the gap between mask and substrate. The need of further surface smoothing depends on the smoothness demands. With the help of continuous resist profiles, fabricated by smoothing of binary resist patterns and also by using non-resolvable masks, combined with further binary structuring after the proportional transfer, three dimensional waveguide taper for low loss fiber-waveguide coupling via mode matching were successfully manufactured.

Date Published: 19 October 2005
PDF: 11 pages
Proc. SPIE 5965, Optical Fabrication, Testing, and Metrology II, 596504 (19 October 2005);

Published in SPIE Proceedings Vol. 5965:
Optical Fabrication, Testing, and Metrology II
Angela Duparré; Roland Geyl; Lingli Wang, Editor(s)