330 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 2, APRIL 2006 Magnetic Composite Electroplating for Depositing Micromagnets Shan Guan and Bradley J. Nelson, Member, IEEE, Member, ASME Abstract—This paper reports a novel magnetic composite materials deposition technique called magnetic composite elec- troplating (MCE). Thin films and micromagnets arrays of a composite matrix consisting of magnetic particles and a ferro- magnetic alloy have been fabricated based on this technique. In a typical MCE process, magnetic particles are electrochemically and mechanically embedded into electroplated ferromagnetic thin films to form a magnetic particle-alloy composite. The magnetic particle selected is a barium ferrite magnet and the ferromagnetic matrix is a pulse-reverse electroplated CoNiP alloy. The particle embedded fraction (w.t. %) directly affects magnetic properties and is experimentally determined by its energy disper- sive spectrum (EDS). Various factors including electrolyte particle concentration, applied current, electrolyte pH, and the presence of cationic surfactants affecting the particle embedded fraction are experimentally investigated. Arrays of -CoNiP magnets with a variety of dimensions and features as small as 8 have been realized by MCE. Experimental analysis shows that the composite exhibits magnetic properties, such as a high coercivity of up to 1.75 , particularly well suited for MEMS actuators. [1196] Index Terms—Electroplating, composite electoplating, mag- netics, microactuators, micromagnetics. I. INTRODUCTION I N recent years, magnetic MEMS actuators have attracted attention because of the advantages they provide, such as long distance movement, low power consumption and large actuation force [1]–[3]. Furthermore, MEMS actuators using permanent magnets have proven to be superior in achieving bi-directional microactuation with low power consumption [4]. To achieve these advantages of hard magnetic materials, the magnetic anisotropy energy ( Volume, where is the anisotropy of magnetic thin films) in the magnetized films should be maximized. However, the deposition of magnetic ma- terials, especially hard magnetic materials, is quite challenging for MEMS since most deposition processes are incompatible with MEMS fabrication [5]. To date, various micromachining techniques such as screen printing, microassembly, sputtering, and electroplating have been used for depositing and integrating magnets into MEMS devices. Screen printing can produce strong polymer-based Manuscript received November 10, 2003; revised September 7, 2005. This work was supported in part by Seagate Technologies. Subject Editor O. Tabata. S. Guan is with the Department of Mechanical Engineering, University of Minnesota-Twin Cities, Minneapolis, MN 55455 USA. He is also with the Re- search and Development, Kodak Versamark, Inc. (An Eastman Kodak Com- pany), Dayton, OH 45420 USA (e-mail: Shan.Guan@kodakversamark.com). B. J. Nelson is with the Institute of Robotics and Intelligent Systems (IRIS), ETH Zurich, 8092 Zurich, Switzerland (e-mail: bnelson@ethz.ch). Digital Object Identifier 10.1109/JMEMS.2005.863707 magnets onto MEMS actuators and is a relatively inexpensive process compared to other micromachining processes. Mag- netic particles made of ferrite [6], [7] and rare-earth materials (FeNdPrTiZr) [8] have been embedded into commercially available polymers. Among them, the strongest magnetic properties (demonstrated by Vollmers et al.) have a coercivity of and a residual induction of 0.34 tesla. Screen printing is favorable when depositing film with a thickness greater than 100 . However, the limited dimensional control of the screen printing process typically limits the minimum printed feature size to 100 or larger [8]. In addition, the mechanical properties of binding agents (typically a polymer) narrow its application. An alternative technique for depositing hard magnets for MEMS is sputtering, which is processed under a high vacuum. While it is a common technique used to deposit magnetic films with a thickness of 1 to 2 , the process becomes time consuming when a much thicker film must be deposited. Another technique for integrating permanent magnets into microdevices is microassembly, which achieves functional assemblies by positioning, orienting, bonding, and assembling micronscale components. Microassembly can integrate magnets with the strongest magnetic properties, and a microassembly process for hybrid magnetic MEMS has been demonstrated by Vikramaditya et al. [9]. The assembled micromagnets have a residual induction of 0.6 T and an intrinsic coercivity of 4.1 A/m. The obvious disadvantages of mi- croassembly are the relatively high manufacturing cost as well as the complexity of equipment and process sequences. State-of-the-art microassembly techniques integrate magnets with sizes larger than 100 100 100 with MEMS devices. These limitations narrow the applications of microassembly. Electroplating is a mature micromachining technique for MEMS device fabrication. Most electroplating techniques are compatible with MEMS fabrication and features as small as 2 have been deposited by electroplating [5]. It is a powerful technique in achieving diverse MEMS materials with relatively low cost. The main challenges of electroplating are the often high residual stresses in the deposited materials and the control of a variety of parameters that affect the properties of the deposited films. In addition to depositing materials such as Au, Cu, and Ni, electroplating has been used for depositing hard magnetic materials. As reported, CoNiMnP, CoNiP, CoPt, and CoPtP hard magnetic alloys have been successfully deposited from aqueous electrolytes [10]–[13]. Furthermore, electro- plated CoNiMnP arrays have been integrated into a MEMS- 1057-7157/$20.00 © 2006 IEEE