Inkjet Deposition Assisted Electroforming of Functional Microstructures

Abstract

Inkjet (IJ) printing is a materials deposition technology based on the controlled emission of droplets from a nozzle. Such droplets, which contain the material of interest in the form of a solution or a dispersion, are deposited on a substrate, allowing thus controlled patterning. In the last few decades, IJ evolved from a mere graphical and visual tool to a versatile methodology able to find application in a great number of industrial fields: electronics, displays production, ETC. IJ printing proved to be one of the most flexible and efficient technologies to pattern materials on a wide variety of rigid and flexible substrates.More recently, IJ found application also in the field of microfabrication. In particular, it was employed to manufacture microelectromechanical systems (MEMS) [1], microelectronic components and sensors [2]. Many of these applications require the presence of IJ printed conductive or magnetic layers to allow actuation or sensing. By employing IJ, however, it is difficult to obtain highly conductive and mechanically stable layers. This issue is connected to the nature of the IJ technology, which rely on the use of fluids, either solutions or dispersions. As a consequence, the variety of printable conductive and magnetic materials is limited. Metals IJ printing, in particular, is normally based on the use of nanoparticle suspensions or on reactive IJ deposition [3]. Both methods, however, cannot outperform bulk metallic layers. Conductive polymers IJ printing is possible as well, but final layers are characterized by limited mechanical properties and poor conductivity.To expand IJ applicability to micromanufacturing, we developed a novel approach to indirectly pattern bulk metallic microstructures by combining IJ printing and electroforming. The technique requires the presence of a conductive substrate. Initially, a layer of SU-8 photoresist is printed [4] on the substrate to form a negative of the final pattern. Subsequently, the positive pattern is growth by mean of electrodeposition inside the negative SU-8 pattern. SU-8 is then removed, leaving an indirectly printed metal pattern. It is also possible to dissolve the substrate in a suitable aggressive solution, leaving thus freestanding electroformed planar structures. Moreover, this latter approach can be employed to transfer metal patterns on nonconductive substrates. The methodology hereby presented can be potentially used for the production of MEMS, microelectronic components and microrobots.To provide an applicative example, we present the microfabrication of free-standing functional microdevices. In detail, a negative pattern was printed on an aluminum substrate. Then, different metallic layers (Cu, NiFe and Cu) were sequentially electrodeposited inside. Copper was deposited to provide mechanical stability, while NiFe was deposited to allow magnetic actuation of the final devices. Subsequently, SU-8 was stripped from the surface and the aluminum substrate was dissolved in a NaOH solution. As a result, freestanding electroformed microdevices were obtained. They were morphologically characterized by mean of SEM and AFM. Finally, they were actuated by applying a controlled magnetic field. The high level of precision achieved during the actuation tests performed suggests possible applications in the field of micromanipulation.[1] G. K. Lau et al., Micromachines 8, 194 (2017)[2] N. K. Mani et al., “Bioelectrochemical Interface Engineering”, chapter 19, John Wiley & Sons (2020)[3] P. Calvert, “Reactive Inkjet Printing: A Chemical Synthesis Tool”, chapter 10, RSC publishing (2017)[4] R. Bernasconi et al., Polymer 185, 121933 (2019)