Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee Research paper Vapour-assisted roll-to-roll nanoimprinting of micropillars on nanocellulose films Tapio Mäkelä ⁎ , Ari Hokkanen, Asko Sneck, Teemu Ruotsalainen, Alexey Khakalo, Tekla Tammelin VTT Technical Research Centre of Finland Ltd., Tietotie 3, FI-02150 Espoo, Finland ARTICLE INFO Keywords: Nanoimprinting Hot embossing Roll-to-roll R2RNIL R2RVANIL Surface modification Cellulose nanofibrils TEMPO CNF Optical effect Diffraction grating ABSTRACT Here we demonstrate a straightforward vapour-assisted roll-to-roll nanoimprinting lithography (R2RVANIL) patterning method to biobased nanocellulosic films with the aim to fabricate microstructured films with dif- fraction effect. In the R2RVANIL technique a roll with pillar structures is softly pressed against nanocellulose film in the presence of hot water vapour replicating the pattern onto the film. The printed pillar structures are compared with the structures produced using traditional thermal R2RNIL. The manufacturing method is de- monstrated using optically transparent TEMPO-oxidised cellulose nanofibrils (TEMPO CNF). TEMPO CNF films possess high tensile strength and modulus whereas the tensile strain is low, they are very hygroscopic - a property which improves material processing at elevated humidities, thus overcoming a lack of a clear softening point. In this context, the water molecules are acting as plasticising molecules thereby softening the relative brittle nanocellulose film structures. Patterned pillars were achieved due to the swelling and softening of the film which was contacted with moist air. R2RVANIL method appears to be more gentle and surface properties such as roughness are different from the ones achieved via conventional thermal NIL method. The diffraction grating is demonstrated using laser beam compared to calculated diffraction patterns. Water vapour assisted patterning methods are beneficial, for example, in the future optics and electronics where films cannot be processed under high pressure and, on the other hand, high production volumes are needed. 1. Introduction TEMPO-oxidised cellulose nanofibrils (TEMPO CNF) are renewable nanoscaled cellulosic material, with intriguing properties such as the good film forming, good thermal stability and excellent optical trans- parency. Therefore, TEMPO CNF films are potential substrates for op- tical applications and active support film for printed electronics in the future [1–4] i.e. they can be considered as a substitutes for petroleum- based materials with unique and tuneable physical properties [5,6]. CNF films offer also a moderate smoothness, high density and high water uptake capability which make them an attractive materials for solutions where recycling and biodegradation is important [4,6]. Tar- geting optics and electronic application, a novel surface modification and patterning techniques become highly essential. Conventional pho- tolithography techniques are not suitable or too slow for high volume manufacturing due to the brittleness of nanocellulosic films, and therefore new techniques are needed. Holograms, diffractive optical elements and, for example backlights applications have been demonstrated for plastic substrates using thermal NIL or hot embossing techniques [7–10]. Thermal roll-to-roll nanoimprint lithography (R2RNIL) is used for large area nano- and micropatterns [11–15]. R2RNIL has been used in nano- and [16] and micropatterning [14–16]. The feature size replicated in the film can be much below 1 μm, but typically 1–10 μm[17–19]. Due to the fragile nature of TEMPO CNF film thermal R2RNIL method for surface pat- terning is demanding [3]. More gentle method where water vapour is used to improve printing properties with lower printing force is re- quired. The aim of this study was to demonstrate microstructured patterns on TEMPO CNF films using water vapour-assisted roll-to-roll nanoim- printing (R2RVANIL) method [21]. Presented method relies on the in- troduction of hot water vapour on the surface of the TEMPO CNF film. As previously shown by Hakalahti et al. [25], due to its highly hygro- scopic nature, TEMPO CNF film can significantly uptake water mole- cules only within seconds simultaneously softening the film structure. Therefore, the film swelling and softening are contributing positively on the mechanical imprint process where the nanocellulose film structure is periodically patterned in a controlled manner. Moreover, the https://doi.org/10.1016/j.mee.2020.111258 Received 10 January 2020; Received in revised form 31 January 2020; Accepted 17 February 2020 ⁎ Corresponding author. E-mail address: tapio.makela@vtt.fi (T. Mäkelä). Microelectronic Engineering 225 (2020) 111258 Available online 19 February 2020 0167-9317/ © 2020 Elsevier B.V. All rights reserved. T