Electrodeposited Co-Ni alloys for MEMS

Abstract

The electrodeposition process has been studied and optimized in order to obtain homogeneous Co-Ni deposits on Si/SiO2/Ti/Ni substrates. The electrochemical and magnetic characterization of the deposited layers shows the features of a soft magnetic material, i.e. high saturation magnetization (1.2 T) and low coercivity, which may be varied as a function of the electrodeposition parameters. Good adherence, brilliant aspect, smooth surfaces and a constant deposition rate have been obtained in all the processed samples. The compatibility of the Co-Ni plating process with the main microelectromechanical systems (MEMS) fabrication techniques, surface and bulk, has been evaluated and these results widely demonstrate the versatility of this magnetic layer in MEMS production. Due to the high selectivity and homogeneity of the deposition process, it has been possible to pattern the deposits with a definition down to 3 μm and an aspect ratio of 4.6 has been achieved. A novel method to liberate the patterned structures by sacrificial etching of the seed layer has been developed. Uniform and homogeneous deposits of Co-Ni alloy have also been obtained in devices fabricated by bulk technologies. For these devices an anisotropic wet etching procedure in tetramethyl ammonium hydroxide (TMAH) has been optimized to release the magnetic layer. Low stressed free-standing structures have been obtained by both surface and bulk methods and are presented in this paper.