Microlens array fabrication by enhanced thermal reflow process: Towards efficient collection of fluorescence light from microarrays

This paper describes two enhanced resist reflow methods for the fabrication of microlens arrays and demonstrates their use for integrated biomolecular fluorescence detection on printed microarrays. A PDMS (polydimethylsiloxane) microlens array was fabricated by a double soft lithography approach using a photoresist microlens array as master mold. Additionally, by using both a careful control of the surface wettability and thermal treatments, we demonstrate the possibility to extend the resist reflow process in order to tune the diameters of microlens array over a large range by using a unique photomask pattern.We introduce an enhanced reflow on hydrophobic surfaces obtained by fluorosilane treatment and identify a threshold shrinkage temperature (Tshrinkage) of 140 °C, above which the diameter of microlenses can be then reduced down to 40% compared with the initial pattern on the photomask. Furthermore, on hydrophilic substrates, achieved by an accurate incomplete development of the photoresist, we demonstrate a nearly perfect linear dependency (1.4 μm/°C) of microlens diameter spreading up to 70% the initial diameter inside a temperature reflow window of 110–140 °C. For both approaches, above a freezing temperature (Tfreezing) of 170 °C, the microlens profile characteristics are temperature independent.By using high numerical aperture microlens array, we provide a proof of concept for the integration and enhanced light collection of the fluorescent signals collected form a microarray of fluorescent spots thus showing the potential of the concept for biophotonic integration.