Application of 3D glycerol-compensated inclined-exposure technology to an integrated optical pick-up head*

Abstract

This paper proposes a novel method for integrating inclined mirrors with precise angles and smooth surfaces in on-chip Fresnel lens optical pick-up-head applications. A glycerol-compensated inclined-exposure technology was adopted for 45° mirror pair fabrication. This technology can fabricate 19–90° inclined structures onto SU-8 negative tone resist with thickness from 100 to 1000 µm. Glycerol is employed as an index matching material during exposure to compensate for the refractive index difference between the air and SU8, thus extending the possible inclined angles from 54° (in air) down to 19° (in glycerol). To eliminate reflected induced patterns, CK-6020L resist is employed as an antireflection layer. The surface roughness, measured by AFM, is below 7 nm for various inclined angles. Device fabrication has been successfully completed and test results demonstrated a functional VCSEL (vertical cavity surface emitting laser) beam focusing on a 5 µm spot. Further beam size reduction into the sub-micrometer range requires an objective lens with high numerical aperture, which is not within the scope of this paper. The fabricated optical components are robust and accurately aligned. This new integrated approach provides an easy and low-cost fabrication method for optical pick-up heads eliminating the need for manual assembly.

Improved process for high yield 3D inclined SU-8 structures on soda lime substrate towards applications in optogenetic studies

“…However, inclined exposure has inherent limitations on the achievable angled structure (>54º) due to the index of refraction mismatch between air, the glass substrate, and SU-8. Here, we use glycerol (n = 1.6) immersion to compensate for the index mismatch and allows the achievable angle to be extended down to 19º [4].…”

“…Backside inclined exposure has already been used to achieve high aspect ratio microneedles [3]. The achievable angle was limited to > 54º due to the refractive index mismatch between air (n = 1), the glass photomask (n = 1.53), and SU-8 (n = 1.67) [4]. To create SU-8 inclined structures down to 19º, index matching with a glycerol (n = 1.6) bath was employed (also requires anti-reflection coating on the silicon wafer substrate) [4].…”

“…The achievable angle was limited to > 54º due to the refractive index mismatch between air (n = 1), the glass photomask (n = 1.53), and SU-8 (n = 1.67) [4]. To create SU-8 inclined structures down to 19º, index matching with a glycerol (n = 1.6) bath was employed (also requires anti-reflection coating on the silicon wafer substrate) [4]. Tight contact between the mask and wafer is required; a gap shifts the inclined structure due to a longer UV light propagation path [4].…”

“…To create SU-8 inclined structures down to 19º, index matching with a glycerol (n = 1.6) bath was employed (also requires anti-reflection coating on the silicon wafer substrate) [4]. Tight contact between the mask and wafer is required; a gap shifts the inclined structure due to a longer UV light propagation path [4]. In practice, tight contact can be difficult to establish.…”

Collective Micro-Optics Technologies for VCSEL Photonic Integration

“…For instance, SU-8 was used to fabricate integrated guiding holes for coupling plastic optical fibers to VCSELs [39]. Three-dimensional SU-8 microprisms were also directly integrated on VCSEL surface by inclined exposure photolithography for in-plane optical coupling [40]. Moreover, SU-8 microprisms were fabricated by electron beam grey-scale lithography for static VCSEL beam steering [41].…”