IR-sintering efﬁciency on inkjet-printed conductive structures on paper substrates Cristina Gaspar a, ⁎, Soile Passoja b , Juuso Olkkonen b , Maria Smolander b a Aalto University, Finland b VTT Technical Research Centre of Finland, Finland abstract article info Article history: Received 24 August 2015 Received in revised form 5 October 2015 Accepted 10 October 2015 Available online xxxx Keywords: Paper substrate IR-sintering Thermal sintering Inkjet-printing Conductive patterns Nanoparticles The use of ﬁbre-based materials as substrates in printed electronics has been increasing, mainly due to its attrac- tive characteristics, such as low-cost or wide availability. Additionally, paper enables recycling and it shows at- tractive features, such as high thermal stability, when compared to traditional polymer-based substrates (Tobjörk and Österbacka, 2011 [1]). Nevertheless, one of the drawbacks of using paper substrates is that the sur- face usually is very rough, typically, showing roughness values above 10 μm 2 . In most cases, printing structures that need to be highly uniform, without disruptions, require additional coating. Inkjet printing provides sharp detail reproduction and strict lines on printed structures. Sintering is required for drying the ink. Thermal sintering is the traditionally used method, but requires long periods of time and promotes the ageing of the paper due to a long exposure at high temperature. When printing conductive structures on paper alternative photonic sintering methods such as IR-sintering show some attractive characteristics. IR-sintering is compatible with roll-to-roll fabrication, providing low-cost, fast and localized sintering, which makes it suitable for ﬁbre- based substrates (Tobjörk et al., 2012 [3]). This work has been carried out to study and compare the efﬁciency of thermal and IR sintering of conductive structures on different paper and polymer substrates. All substrates were printed using silver based ink, which was applied on the substrate surface by inkjet. Resistivity values of the printed structures were used to compare the performance on the substrates. IR-sintering showed the best re- sults in terms of achieved conductivity of the printed lines when using short sintering time of no more than 10 min. The conductivity values of the inkjet-printed silver lines on Lumi silk substrate reached about 40% of the bulk silver value after IR-sintering, whereas with thermal-sintering this value only reached about 20% of the bulk silver value. IR-sintering improves the sintering process, increasing the conductivity of the printed struc- tures and at the same time reducing signiﬁcantly the sintering time. In the case of Lumi silk substrate, high con- ductivity was observed after only two minutes of sintering time when IR-oven was used. Both techniques can be used in a roll-to-roll mass manufacturing process, enabling the fabrication in large scale of ﬂexible electronic devices, on paper substrates, without the need for extra steps, such as coatings. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Printed electronics has applications in several different areas, such as power sources and printed batteries [4] or sensors [5]. Printing enables cost ef ﬁcient mass manufacturing of electrodes and other functional- ities, such as deposition of functional materials on large and ﬂexible variation of substrates like plastic [6] , paper [7] and fabrics [8] in a controlled way. From the ﬂexible substrates, paper shows a wide range of ad- vantages over the other substrate materials, such as low-cost, environ- mental friendliness and recyclability. Fibre-based substrates have characteristics that will inﬂuence the output of printing functional inks. Properties such as roughness or porosity of the substrate surface are crucial, especially when aiming for printing conductive structures. Nevertheless, the surface of the paper substrates allows tailoring e.g. coating can be added to improve these properties. Bollström et al. (2013) [9] studied the use of a multilayer coated paper and its inﬂuence on printability of functional inks. They tailored different properties of paper structure, such as thickness, porosity, surface energy and absorp- tivity of the paper, in order to improve the printing of the different deposited materials. Despite the good results of this study, coating of ﬁbre-based substrates requires additional stages using different mate- rials which increases the costs and is highly time consuming. Our work shows the use of pre-coated, commercial paper substrates, used and studied without any surface treatment, used-as-is. Inkjet-printing provides a low-cost deposition technique, due to its characteristics i.e. relatively high production speed, excellent detail re- production, low cost and the possibility for contactless printing. Dimen- sions and thickness of the inkjet-printed layers can be controlled by rheological properties of the ink, such as optimal viscosity and surface Microelectronic Engineering 149 (2016) 135–140 ⁎ Corresponding author. E-mail address: Cristina.Henriques.Gaspar@aalto.ﬁ (C. Gaspar). http://dx.doi.org/10.1016/j.mee.2015.10.006 0167-9317/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee