OPE Journal

OPV, SMART CITIES & SUSTAINABILITY 18 No 37 | November 2021 | OPE journal with the semi-dry ink and a negative image of the final pattern is removed from the PDMS. In the semi-dry state, the ink exhibits high viscosity and cohesion, which enables the high-resolution patterning process with 100% ink transfer. Finally, in the third step, the ink remaining on the PDMS is transferred to the receiving substrate. Unlike any other printing method, the process enables features that are close to those obtained using conventional photo lithography such as i) uniform layer thickness, ii) cross-sectional profile with low taper angle, iii) low line-edge roughness and iv) feature sizes close to nominal. These features are enabled by the patterning being done on the semi-dry ink, which avoids imper- fections arising from substrate-ink interactions such as wetting and solute flow occurring in conventional printing methods. VTT’s sheet-to-sheet flat-bed reverse-offset printer is shown in Figure 2. The ROP-printer is compatible with 8” substrates (190 x 190mm maximum print area) and is inside a semi-clean and HVAC-controlled laboratory environment. The system is equipped with high-precision machinery components and servomotors and the machine structure is constructed to diminish any internal or external vibrations. The ROP-printer has a coating unit coupled with a PDMS cylinder and cliché and substrate tables in line. Both tables have a high flatness and are made with thermally stable materials. A coating step is done by manually adding the ink into a capillary coater. The off and set steps are then carried out automatically. The PDMS cylinder rotation is synchronised with the lateral movement of the cliché and substrate tables and a suitable nip pressure is adjusted by the indentation of the PDMS Figure 2: VTT’s sheet-to-sheet flat-bed reverse-offset laboratory printer, compatible with an 8” wafer (19 cm x 19 cm printing area) Figure 3: Alignment pattern (Ulvac L-Ag RPO ink) (a) and alignment-accuracy results in machine direction (MD) (b) and cross direction (CD) (c). The average error and its standard deviation are also given