Laser-induced forward transfer for printed electronics applications

Abstract

Printed electronics appeared in the 1980s as a cost-effective alternative to silicon-based electronics. Employing the techniques from the graphics industry, such as rotogravure or screen printing, it was possible to print metals, ceramics, and polymers on a wide variety of materials, including flexible and organic substrates. However, these techniques became not adequate when customization or short runs were considered since the production costs of the components and devices substantially increased. To overcome these issues, direct-write techniques, such as inkjet printing, allowed depositing materials on-demand in a digital fashion. Nonetheless, the ink was ejected in the form of droplets from a nozzle, which small diameter limited the range of printable inks; only those with low viscosity (few mPa·s) and small particle size (~100 nm) could be routinely printed without resulting in nozzle clogging. Alternatively, laser-induced forward transfer (LIFT), another digital technique, has barely any of these constraints. LIFT is a printing technique capable of depositing almost every kind of ink in a digital fashion independently of its rheology. In LIFT, a thin layer of ink is extended on a transparent donor substrate, which is placed facing the receiver substrate through a certain gap. Using a laser pulse focused on the ink donor film, a cavitation bubble is induced. The high pressure within results in its expansion, propelling the material forward towards the receiver, where it is finally deposited. Since the ink is not ejected from an output nozzle, the range of printable viscosities extends from a few mPa·s to hundreds of Pa·s, and the particles in suspension can feature sizes of up to tens of micrometers, non-achievable with other direct-write techniques. Furthermore, both the resolution of the printed features and the printing speeds are similar to those of other digital printing techniques. In this thesis, the use of LIFT is investigated with the aim of printing inks for printed electronics applications. Special attention is devoted to the transfer of conductive pads to be used as interconnects. To demonstrate the potential and possibilities of LIFT, different inks used in printed electronics applications are chosen. These inks exhibit diverse rheologies: from low to high viscosity, and with particle sizes ranging from nano- to micrometers, characteristics that make them unprintable with most of the other direct-write methods. Finally, to prove the versatility and compatibility of the technique with the desired applications, several functional components and devices are entirely printed with LIFT. The work is divided in three main sections. The first aims at the production of transparent electrodes by means of the LIFT of two silver nanowire inks on rigid and flexible substrates. The main laser parameters are varied to find the optimum compromise between the optical and electrical properties, to finally print a device consisting of conductive and transparent electrodes. The second focuses on the LIFT of high solid content silver screen printing ink. The study is divided in the fundamental study of the deposits and its correlation with the transfer dynamics, and the ability to obtain conductive interconnects on non-planarized regular paper. As a proof-of-concept, a radio-frequency inductor is printed on paper. The third consists of performing LIFT using continuous-wave laser sources for printing inks, with the aim of reducing the capital investment associated to pulsed LIFT. The laser parameters are varied to determine the optimum printing conditions and the transfer mechanism is investigated. As a final remark, a gas and temperature sensor is printed using this approach.