Reverse Pulse Plating of Thick and Crack-Free Magnetic Layers for MEMS Manufacturing

Abstract

The integration of hard magnetic alloys in the form of thick films 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 is compatible with MEMS fabrication and is characterized by many significant advantages: high deposition rates, no necessity of a vacuum system, low cost, possibility to deposit alloys and composites. Despite of these advantages, electrodeposition methods capable to yield the thick hard magnetic films suitable for MEMS sensors/actuators have not been successfully developed yet. The reason resides in the problems typically encountered in the deposition of hard magnetic alloys: high residual stress, presence of cracks and inadequate surface finishing.In this context, Reverse Pulse Plating (RPP) may constitute a possible solution to address the challenges of hard magnetic alloys electrodeposition [2]. This technique can significantly reduce internal stresses and hydrogen embrittlement, inhibiting thus cracks formation. In addition, it can simplify electrolyte formulation, eliminating the need of excessive amounts of additives. Finally, it can enhance magnetic properties and refine the grain structure of the deposits, resulting in better mechanical properties.In the present work, RPP of thick and crack-free layers of CoNiP and CoPtP is investigated. These two materials are between the most studied for potential MEMS applications [3, 4]. Indeed, whereas CoPtP offers high coercivities (> 3000 Oe) and excellent remanences, CoNiP represents a low cost alternative characterized by good magnetic properties (with HC up to 2000 Oe). Co-rich CoNiP and CoPtP are deposited from a chloride based acidic electrolyte [3] and from a tartrate based bath, respectively, using ultra-fast RPP. The resulting coatings, characterized by thickness up to 20 µm, are characterized to assess their morphology, phase composition and magnetic behavior.[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)[4] D. Mallick et al., J. Appl. Phys. 125, 023902 (2019)