The main purpose of the presented work is to manufacture and experimentally validate the specifically designed surface topography. The latter cannot be readily achieved in a single production step due to the presence of pronounced undercuts. Therefore, the functional microstructure is at first prepared by injection molding replication of truncated micro-cones from a microstructured master. The subsequent bonding to a planar back plate enables the preparation of undercut structures on LGPs made of PMMA without the use of any primer, which would lower the optical quality of the LGP surface.
Injection molding of truncated micro-cones
To produce optical microstructures by injection molding, microstructured mold inserts were manufactured in brass by precision milling at LT Ultra-Precision Technology (Herdwangen-Schönach, Germany). These mold inserts were then fitted into a dedicated injection molding tool designed for versatile process validation in context with polymer replication on the micro- and nanoscale (Rytka et al. 2015). Detailed information on the molding tool can be found in the latter reference. The mold insert of primary interest in this work is presented in Fig. 4.
Variothermal injection molding was employed to assure accurate replication of the master structures, thereby avoiding incomplete filling of the micro-cavities by the PMMA (PMMA 7 N, Evonik) melt. All injection molding trials were carried out on an Arburg 320 A (Lossburg, Germany) employing a melt temperature of 240 °C, an injection velocity 20 cm3 s−1, and a holding pressure of 750 bar for 12 s prior to demolding. Variothermal cycling of the mold temperature (90 °C at the moment of injection, 50 °C at part ejection) was accomplished with the help of a 2-chamber water system composed of two control units (HB-160Z2) with a pump capacity of 60 l min−1 and a cooling power of 30 kW combined with a dedicated switching unit (HB-VS180-20) from HB-Therm (St. Gallen, Switzerland).
The injection molded micro-cone arrays displayed in Fig. 5 show very good replication fidelity. The corrugations on the side walls are replicated from chatter marks of the brass insert, which originate from the mold insert manufacturing process.
Bonding of undercut microstructures made of PMMA
As already mentioned in the previous section, the conical microstructures produced by replication actually represent the mirrored version of the designed structure. Thus, the bonding of micro-cone arrays to a flat plate is essential for the creation of a functional prototype with undercut micro-features and validation of their illumination performance characteristics (i.e. LID). Using classical thermal bonding the substrates are exposed to high stresses, which ultimately leads to deformations of the optical microstructures and concomitant deterioration of the resulting illumination characteristics. Exposure to a solvent (like isopropanol or ethanol) for the purpose of solvent bonding causes a significant reduction of optical surface quality. Besides the increase in surface roughness, a disturbing clouding effect appears, accompanied by non-satisfactory LID performance. The use of a UV glue layer causes menisci at the edges of undercut structures and thereby distorts their optical behavior.
By applying a UV-treatment, adapted from earlier work on microfluidics (Truckenmüller et al. 2004; Tran et al. 2013), these problems could be overcome and the microstructures were successfully bonded to the LGP back plate without any noticeable deterioration of the surface quality.
Exposure of PMMA to short wavelength UV irradiation causes chain scission within an approximately 400 nm thick surface layer (corresponding to the depth of UV light penetration). Thereby the molecular weight is substantially lowered, which causes a significant reduction of the glass transition temperature within the surface layer (Chidambaram et al. 2017). If this surface layer is heated above the glass transition temperature the mobility of PMMA chains rises and secondary valance bonds are broken. The polymer chains of the compressed surfaces diffuse into the other surface and hook into each other. During cooldown, they begin forming secondary valence bonds once again and are firmly bonded after cooling. Consequently, the surface can be thermally bonded at lower temperature without significant deterioration of the shape of the microstructures. A positive side effect of the UV-treatment is a concomitant selective surface smoothening (reduction in surface roughness) of the boundary layer, which occurs during UV-treatment and subsequent thermal treatment during the bonding process (Chidambaram et al. 2017).
The UV-assisted bonding process is schematically outlined in Fig. 6. To bond the PMMA microstructures to the planar PMMA sheet the joining surfaces were irradiated by UV-light with a wavelength of 172 nm (465 mJ cm−2) for 60 s (EX-mini, Hamamatsu Photonics). Following this initial step, the two parts were bonded at 103 °C (which is below the glass transition temperature of the used PMMA, i.e. 110 °C) employing a load of 11.2 kg (corresponding to a contact pressure of 336 kPa) for 20 min.
The above described process resulted in a dual-component microstructured device characterized by high bonding strength (not quantitatively measured) and good preservation of the micro-cone geometries. Figure 7 shows a well-bonded functional prototype of the vertical illumination device and the cross section of a PDMS replica used for control of the final micro-cone geometry.