Figure 1. Schematic representation of the sintering process. (a) Nanoparticles dispersed in solvent, (b) Solvent evaporation due to heating, (c) Evaporation of other ink additives and (d) Sintering of nanoparticles/increase in grain size (from Ref. 3).
Figure 2a. Silver nanoparticles before (left) and after (right) sintering. (from Ref. 6).
Figure 2b. Inkjet printed gold structures on PET (left) and paper (right). The edge sharpness is insufficient (from Ref. 6).
The inkjet printing of conductive materials has attracted much scientific and commercial interest in recent decades, with users finding the technical benefits of inkjet printing particularly persuasive in addition to its wide range of potential applications. Printed electronics are, in fact, predicted to enjoy a significant market growth (up to $48.2 billion USD by 2017),1 a growth that will be spread over potential applications such as displays, photovoltaic, RFIDs, sensors, memories, and printed circuit boards.1,2 In technical terms, inkjet printing—like alternative printing techniques—competes with conventional semiconductor technology (photolithography, vacuum deposition etc.) but it offers distinct advantages, especially in the manufacture of customised products or with flexible or sensitive substrates (contactless material application).1 The much lower initial machine costs, less maintenance and the sparing use of material and ink in drop-on-demand printing processes also greatly enhance the profitability of the manufacturing processes.3 Even so, inkjet printing has so far failed to become established in the electronics industry despite the increasing commercial availability of electrically conductive ink systems.
The aim of the present article therefore is to throw some critical light on the technical challenges facing the field of inkjet-printed electronics, and to a initiate a fundamental debate about electrically conductive inks, beginning with a description of the chemistry of some selected conductive inks and their sintering processes, and then considering technical problems involved in their practical use.
The Chemistry of Conductive Inks
The main constituent of inks used for printed electronics is undoubtedly the conductive material itself. In addition to conductive polymers, use is made of organometallic compounds, metal and metal-oxide nanoparticles, other metallic precursors as well as a variety of carbon materials. The carrier fluid is also of decisive importance for the printability of conductive inks, with frequent use being made of solvent systems that are based on acetates, glycols, cyclohexanon, NMP, MEK etc. Aqueous ink systems or UV-curable formulations are far less common for this type of ink.1,2
The formulation of these inks must take into account not just their chemical composition but also their rheological parameters (e.g. viscosity, surface tension, wetting properties, etc.) so as to guarantee printability and storage stability (sedimentation, etc.); they should also contain adhesion promoting additives such as resins in order to ensure the adequate mechanical stability of the printed films.1,3
Although many different conductive inks might be conceivable based on the above statements, a trend towards metal and metal-oxide nanoparticle inks is clearly discernible among commercial ink systems. Silver nanoparticle inks in particular are in widespread use, especially since the resulting sintered films display good conductivities and oxidation stability. However the high price of the inks is critical for these systems (they contain between 20 and 80 percent by weight of silver), one reason why much international research is focusing on copper and nickel inks. Although these inks have good conductivities, their practical deployment is still being hampered by oxidation reactions and diffusion processes.4,5
Drying and Sintering Process
The conductivity of printed films is primarily determined by the conductive material present in the ink. But because the ink carrier fluid, viscosity regulator, etc. are also deposited in the printing process, they must be removed in a subsequent drying operation. In the case of nanoparticle inks, a more intensive thermal treatment is needed to achieve the heat-sealing (sintering) of the nanoparticles as well as drying the ink film. This sintering step, which can be performed thermally (often at between 200 and 300°C) by laser, pressure, microwave irradiation, or plasma treatment is essential for an appropriately high final conductivity. It must be remembered, however, that sintering very largely depends on the process conditions, i.e.: it can be affected by time, the size and shape of the particles, the thickness of the print films, etc. as well as by the temperature profile. The sintering process is presented schematically in Figure 1, with electron microscopy images of the process shown in Figure 2a.
Although the printed electronics industry already has a very good grasp of ink chemistry, realising morphologically and geometrically identical structures still presents a major technical challenge in inkjet printing. For practical use, the printed structures should have as few voids (e.g.: pin holes) as possible, present smooth surfaces, and have well defined, straight print edges (see Figure 2b) so as to ensure uniform and repeatable electrical conductivities while avoiding the adverse effects of overheating due to inadequate local conductivity. These aspects—which are to a certain extent exacerbated by the drop-like structure in inkjet printing—are the subject of worldwide research projects and have already yielded some elegant solutions such as, for example, the use of substrates that are structured with hydrophilic and hydrophobic areas.1 It is, however, equally important to be able to optimize inkjet printers for electronic applications, which must facilitate a satisfactory positional accuracy of the droplets as well as a uniform droplet mass distribution over the printhead.2
Here it must be clearly stated that much research and development work is still needed before comprehensive solutions to these technical challenges are found. It is only when this R&D effort has been successfully concluded that we will see inkjet technology accepted for widespread use in electronic production.
Inkjet printing is an interesting alternative in the production of electronic components and circuits, and a wide variety of different applications are conceivable. Despite the significant progress achieved in ink chemistry in recent decades, numerous technical challenges still bar the way to the wider practical use of inkjet printing in the electronics industry. Specifically, the realisation of morphologically and geometrically identical structures with few voids, smooth surfaces, and well defined straight print edges appears to be a major obstacle given the current state of the art. A considerable amount of research and development work is still needed before inkjet printing will be in a position to compete with conventional semiconductor technology.
Dr. Ing. Stefan Kappaun is Head of Chemical Research at Durst Research Center. For more information please visit www.myprintresource.com/10005046
1. Nir, M. M.; Zamir, D.; Haymov, I.; Ben-Asher, L.; Cohen, O.; Faulkner, B.; de la Vega, F. "Electrically Conductive Inks for Inkjet Printing" in: Magdassi, S. (Ed.) "The Chemistry of Inkjet Inks", World Scientific, 2010.
2. Subramanian, V. "Printed Electronics" in: Magdassi, S. (Ed.) "The Chemistry of Inkjet Inks", World Scientific, 2010.
3. Shridar, A.; Blaudeck, T.; Baumann, R. R. "Inkjet Printing as a Key Enabling Technology for Printed Electronics", Sigma Aldrich Material Matters, 6 (1), 12, 2010.
4. Magdassi, S.; Grouchko, M.; Kamyshny, A. "Copper Nanoparticles for Printed Electronics: Routes Towards Achieving Oxidation Stability", Materials, 3, 4626, 2010.
5. Wolf, S.; Feldmann, C. "Cu2X(OH)3 (X = Cl-, NO3-): synthesis of nanoparticles and its application for room temperature deposition/printing of conductive copper thin-films", Journal of Materials Chemistry, 20, 7694, 2010.
6. Tobjörk, D.; Österbacka, R. "Paper Electronics", Advanced Materials, asap, 2011.