Benzophenone doped polydimethylsiloxane: Resist platform for MeV electron beam lithography assisted microlens array fabrication

Abstract

Poly-dimethylsiloxane (PDMS) is known to provide ubiquitous platform for micro Total Analysis System (μ-TAS) and Lab-On-Chip (LOC) development. Although soft lithography is extensively accepted as a fabrication tool for PDMS, direct write lithography techniques also have drawn considerable attention due to some of its own advantages. Even though e-beam lithography in PDMS is reported previously, the present manuscript reveals establishment of doped PDMS as a positive tone, self-standing and self-developable resist platform for 6 MeV e-beam lithography in normal ambience. For microstructures fabrication, undoped PDMS and photoinitiator doped PDMS as resist materials are processed using contact printing mode lithography with use of the mask containing array of circles on it. Effects of lithography processing parameters on various resist compositions are investigated to achieve improved geometry and surface profile of the fabricated microstructures. Lithographic investigations on the undoped PDMS shows microstructures with sloppy sidewalls depicting inverted bell shape geometry and cone like nanostructures inside it deteriorating surface profile. Whereas in case of doped PDMS cone like nanostructures are completely vanished and inverted bell shape geometry transformed into clean and uniform concave shape profile, which is then identified as finely produced micro lens array (MLA). The lensing quality of MLA is tested and the images produced by MLA are clear and uniform in size and shape, which confirms formation of microlenses accountable as an optical component in many applications. The observed concave geometry of fabricated MLA is resulted out of energy gradient generated during the e-beam exposure of resist stamp through mask. Similarly, an electrostatic interaction is recognized as governing mechanism for e-beam interaction with PDMS. Photoinitiator plays crucial role in energy absorption and distribution which resulted in seamless control over formation of lateral and vertical dimensions thereby giving optical smoothness to surface of MLA. The optimized lithographic parameters and composition of the resist platform, can be used to fabricated MLA with fine imaging quality and desired location of focal plane according to demand of application.