Contents lists available at ScienceDirect International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh Design of interfaces with lithographically patterned adhesive pads for gluing at the microscale Pieter Samyn a, ⁎ , Jürgen Rühe b , Oswald Prucker b , Markus Biesalski c a Hasselt University, Institute for Materials Research (IMO-IMOMEC), Applied and Analytical Chemistry, Agoralaan Gebouw D, B-3590 Diepenbeek, Belgium b University of Freiburg, Institute for Microsystems Technology (IMTEK), Laboratory for Chemistry and Physics of Interfaces, Germany c Technical University Darmstadt, Ernst-Berl-Institute for Technical and Macromolecular Chemistry, Macromolecular Chemistry and Paper Chemistry, Germany ARTICLE INFO Keywords: Adhesion Patterning Microscale Lithography ABSTRACT The creation of small adhesive pads by traditional dispensing methods is technically limited. However, the miniaturisation of micromechanical components requires the parallel development of adhesive pads with sizes in the sub-50 μm range combining good geometrical confinement and mechanical strength. Therefore, a new de- sign of interfaces with adhesive pads of 32–8 μm are presented through local deposition of a liquid adhesive by means of “top-down” or “bottom-up” patterning. Using lithography and photochemical process, the shape of the adhesive pads is first stabilized by partial cross-linking and effective adhesive bonding with a counterface subsequently takes place during full cross-linking. The parameters for photochemical cross-linking of the ad- hesive pads are optimised and the mechanical performance of the patterned adhesive interfaces is evaluated. For “top-down” patterned adhesive interfaces, the geometrical stabilisation of the adhesive pads requires relatively long cross-linking times consequently resulting in low mechanical strength. For “bottom-up” patterned adhesive interfaces, the formation of adhesive pads is controlled by self-organisation of the adhesive over chemically structured substrates and requires short cross-linking times for geometrical stabilization, leading to higher mechanical strength during adhesive bonding. The fabrication of adhesive pads by a “bottom-up” approach is further discussed in relation to the influences of processing parameters on dewetting of the adhesive. 1. Introduction Gluing is a straightforward and flexible tool for assembling complex systems, integrating various micro-electronic, optical and/or fluidic components onto different substrates. However, the ongoing miniatur- ization of microsystem components requires a parallel development of appropriate assembly techniques. The latter presents new challenges to adhesive technologies with respect to dimensions of local adhesive pads: a tiny amount of adhesive should be applied on a targeted posi- tion with good precision and high strength. At present, high-precision bonding at the microscale is limited because of poor control over the deposition process, geometrical stability and/or viscosity of the cross- linked adhesives. Techniques for deposition of liquid adhesive are based on dropwise application onto the surface by means of dispensing methods. Conventional dispensers allow to handle volumes of 0.01 ml, while high-end microdispensers operate with volumes of 1 nl and some recent developments employ techniques that can deliver single drops with volumes as low as 5 pl. The ink-jet printing is a traditional technology to control the precise deposition of liquid droplets. For unfilled or low- viscosity fluids such as inks, the fluid partially dries during ink-jetting and forms relatively confined drops on the substrate. For printing filled and/or high-viscosity materials such as adhesives, however, the dro- plets are applied under wet conditions and yield proportionally larger dimensions after deposition due to capillary forces and spreading on the substrate: e.g., adhesive drops in the range of 150 μm easily spread up to 1 mm [1], or the deposition of 80 μm adhesive drops by ink-jet printing of thermoset epoxies results in drop diameters of 150 to 200 μm. The reliability of dispensing tiny amounts of adhesive has been improved under industrial conditions by integrating sensors and design of specific dispending heads [2]. Other units comprise a positive-dis- placement dispensing system with integrated adhesive tempering, process gas supply and process optics [3]. Other techniques such as screen printing enable to form drops with a diameter of about 100 μm and line-widths of 80–150 μm. Micro-stencils with 100 μm pitch and 50 μm apertures were recently developed for printing solder pastes [4]. Whereas most printing technologies are in general optimised for one specific substrate such as paper, adhesive technologies should be more flexible. The confinement of adhesives into small geometries requires sophisticated designs with glue guiding channels [5], or stamping and https://doi.org/10.1016/j.ijadhadh.2018.05.021 Accepted 22 October 2017 ⁎ Corresponding author. E-mail address: pieter.samyn@uhasselt.be (P. Samyn). International Journal of Adhesion and Adhesives 85 (2018) 88–99 Available online 25 May 2018 0143-7496/ © 2018 Elsevier Ltd. All rights reserved. T