Molybdenum alloy electrodeposits for magnetic actuation

This paper deals with the preparation and characterisation of alloys containing molybdenum applicable in MEMS. An electrodeposition process for achieving a homogeneous, low-stressed, soft-magnetic Co–Ni–Mo alloy is described. The electrochemical study allows setting bath composition and deposition conditions useful to perform the deposition process. Deposits with low nickel and molybdenum percentages (11–15 wt%) are virtually useful for magnetic actuation applications. Saccharine is an effective antistress agent and it also decreases grain size and surface roughness of the Co–Ni–Mo layers. The mechanical and magnetic properties of the ternary alloy have been compared with those shown by a binary Co–Mo alloy containing similar molybdenum percentages previously electrodeposited in our laboratory. Thin Co–Ni–Mo films exhibited higher microhardness values than Co–Mo films and better magnetic properties for magnetic actuation (Hc = 50 Oe, Mr = 90 emu g−1 and μr = 670). Furthermore, film appearance (such as brightness) and corrosion resistance improved due to nickel presence. Electrodeposition has been tested on silicon/seed-layer substrates and the process selectivity has been investigated on photolithographed silicon. Both alloys are compatible with fabrication techniques involved in MEMS technology. No damage was observed when silicon was removed over alloy films to get stand-alone layers. The response of a microsized valve (silicon + seed-layer + electrodeposited Co–Mo or Co–Ni–Mo film) under an external magnetic field has been explored with good results.