Inkjet-Assisted Lithography of Templates for the Electroforming of Micromagnets for MEMS Applications
Authors: Bernasconi Roberto , Cesaro Riccardo , Magagnin Luca 
Published: August 2024

The integration of hard magnetic alloys in the form of patterned layers is one of the most interesting challenges for the manufacturing of energy efficient magnetic microelectromechanical systems (MEMS) [1]. With respect to current state of the art devices, whose working principle mostly relies on the exploitation of Lorentzian forces, MEMS based on permanent magnets can potentially present lower power consumption, larger displacements and stronger actuation forces. However, despite these attractive advantages, it is generally difficult to integrate thick hard magnetic layers with currently employed MEMS fabrication techniques.
Electrodeposition is a technique that can be exploited to accomplish this task and that is characterized by many significant advantages: high deposition rates, no necessity of a vacuum system, low cost, possibility to deposit both alloys and composites. In addition, Reverse Pulse Plating (RPP) [2] can be used to mitigate the challenges connected to the deposition of hard magnetic alloys: high residual stress, presence of cracks and inadequate surface finishing. It can also simplify electrolyte formulation, eliminating the need of excessive amounts of additives.
Regardless of the material selected, electrodeposition needs a template in order to confer a specific shape to the magnetic layer. Such template is normally manufactured using state-of-the-art technologies like standard lithography. This technique, however, is characterized by the necessity to use photomasks, which are costly and poorly adaptable to geometrical variations. Furthermore, the initial investment for the lithography equipment is typically high. A possible alternative is represented by the use of inkjet printing (IJP). This technology is characterized by low costs, high flexibility and optimal productivity. It can be applied to the micrometric patterning of electroformed layers by following a novel approach described in the present work for the first time: inkjet-assisted lithography (IAL). Briefly, droplets of a dispersion of silver nanoparticles are printed on the surface of a spin coated layer of negative photoresist. Silver is opaque for the UV radiation and it constitutes a mask for the resist. The resist is exposed and developed, forming thus templates usable for electroforming.
The present work investigates the application of this approach to the production of thick and crack-free CoNiP micromagnets, which are RPP deposited inside IAL templates. CoNiP was selected due to its low-cost and good magnetic properties (with HC up to 2000 Oe). The alloy is electrodeposited from a chloride based acidic electrolyte [3], using ultra-fast RPP, in micrometric circular pores created via IAL. The diameter of the pores, and consequently the diameter of the finished magnets, can be efficiently tuned by changing the speed of the jetted droplets of silver ink. Conversely, the thickness of the magnets can be tuned by varying the deposition time. The resulting micromagnets, characterized by diameters in the 20-50 µm range and thickness up to 10 µm, are characterized to assess their morphology, phase composition and magnetic properties. The micrometric magnets here described may potentially find application for the realization of powerless MEMS, energy harvesting systems or microelectronic components.
[1] N. M. Dempsey, “Hard Magnetic Materials for MEMS Applications” in: Nanoscale Magnetic Materials and Applications, Springer, Boston, MA (2009)
[2] S. Pané et al., Electrochim. Acta 56, 8979-8988 (2011)
[3] D. Y. Park et al., Electrochim. Acta 47, 2893-2900 (2002)