Studies on cracking issues of pulsed electrodeposited Nickel-Tungsten thin films

Nickel-tungsten (Ni-W) film has wide applications either as surface protective coatings for tooling, storage media, and plastic mold or as a thin film structure member in microelectronic and MEMS devices. However, so far little has been truly understood about the stresses developed in electro-deposited Ni-W films. One of the major problems with Ni-W deposits is the high film stress generated during the deposition. Without proper control, the stress can lead to film failure by cracking. Hydrogen embitterment which was due to hydrogen evolution reaction during the deposition and rather large atomic radius difference between nickel and tungsten, built up tensile stresses which may be released by crack formation and propagation. The current work investigates the effect of various parameters on the development of Ni-W film plating stress. The investigation will be conducted by three approaches: bath chemistry, pulsed parameters and chemical additives. It was found that complexing agent and pre-plating surface treatment had certain influence on the morphology of the Ni-W alloys. However, cracks could not be effectively eliminated. By application of pulsed plating technique, it is found with the decreasing of current density, the residual stress decreased. The longer off timing also helped to relieve the residual stress. By addition of the reverse pulse plating, the surface became much smoother. Several chemical additives were also investigated. Saccharine helped to reduce the quantity of hydrogen evolution and thus this additive could be used to alleviate the residual stress. NaH2PO2 was not useful as the tungsten content dramatically reduced to nearly zero by addition of this chemical. By adding the stabilizer Pb(Ac)2 and saccharine into the plating bath, the residual stress has been improved a lot compared with those deposited before.Bachelor of Engineering (Materials Engineering