Polymer Microoptics in Silicon Micromachining

Licentiate thesis, 2006

MEMS (micro-electro-mechanical system) technology is an established field, which makes use of the advanced state of silicon processing to fabricate complex electro-mechanical structures and devices. Microoptics is a new, but advanced technology within the field of optics. Also within the microoptics field fabrication techniques from the microelectronics industry are frequently used and it seems natural to proceed to merge these two technologies to build so called MOEMS (micro-opto-electro-mechanical system). The efforts to develop and produce MOEMS have rapidly intensified in recent years, and some commercial applications can already be found. We have developed new fabrication processes based on the use of an amorphous fluorocarbon polymer, Cytop, for making microoptical diffractive elements and refractive lenses for integration in MEMS structures. The processing scheme we use is a reversed order protocol, meaning that the structuring of the lenses precedes the silicon microstructuring, where we introduce new processing steps and non-standard materials. The new processing steps and materials we introduce are all compatible with established silicon processing. The fabrication process for integration of diffractive optical elements makes use of hot embossing replication and was developed using two different diffractive optical structures – a small-feature, high-aspect ratio Fresnel lens and a fan-out diffractive element. The fabrication process for refractive (positive) microlenses makes use of reflow of the amorphous fluorocarbon polymer and was tested fabricating lenses with nominal diameters ranging from 25 m to 200 m. The scope of this thesis is a general approach to process development and performance characterization of the microoptical elements. We also have an application in mind: optical beam steering, which can be implemented by integrating the lenses on a translatable silicon stage. The lenses developed in this work are all suitable for this application. Our Fresnel lens is designed to use a 850 nm VCSEL as light source, and has an f-number of 2.5. The refractive lenses fabricated have f-numbers as small as ~1.9. In particular the lenses with nominal diameters of 25-50 m were found to have a perfect lens shape all over their surface, yielding high quality focusing lenses. The good optical properties of the amorphous fluorocarbon polymer used as the optical material makes our refractive lenses useful in a broad wavelength interval; in the visible and near-infrared range their transmittance is >95 %.