Development of analytical sensors for measurements in harsh chemical environment

Electroless plating is a wet chemical process used to deposit metals onto electronic substrates. However, the mechanism of the plating process is not yet clearly understood. This thesis focuses on the mechanism and analytical chemistry of electroless plating. The experimental aims were two-fold: first, to understand how the chemical composition of electroless plating baths (specifically Ni, Co and Au) influenced the plating rate and the morphology of plating; and second, to develop chemical sensors for monitoring the efficiency of electroless plating baths.; We have studied the effect of bath ligands, oxygen and stabilizers on Ni baths using UV/Vis spectrophotometry, cyclic voltammetry (CV), chronopotentiometry, quartz crystal microbalance (QCM), scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS) and X-ray fluorescence (XRF). We found that the interaction between the half reactions (i.e., the oxidation of reducing agent and the reduction of metal ion) in plating increased the plating rate. We also studied the effect of different stabilizers on a nickel bath and found that thiourea exhibited a bimodal effect on the nickel bath, while lead acetate completely inhibited the plating, whereas potassium iodate significantly increased the plating rate causing plating-out. Based on these observations, we proposed a mechanism for the existence of bimodal effect of thiourea as a stabilizer. This mechanistic study was extended to cobalt bath chemistry following a similar procedure and methodology. Another aim of the project was to develop analytical sensors for monitoring the bath chemistry. Therefore continuous monitoring is required to replenish the multiple components affecting the plating rate. Consequently, we reported the design of a novel continuous fluorescence oxygen sensor that could be used for highly alkaline gold baths. This oxygen sensor was tested at an industrial site and it was found to provide remarkable sensitivities, rapid response time and adequate stability compared to conventional detection methods. A novel sensing chemistry method was developed for monitoring the high hydroxide content of plating baths based on 2-pyridylazo compounds.; The project has contributed to the understanding of the mechanism of electroless deposition in many ways: first was the investigation of the mixed potential theory (MPT) for three different Ni bath chemistries, and the discovery that two baths obeyed MPT whereas, one bath did not. Consequently, this departure from MPT was explained according to ligand atmospheric size (LAS) concept. Secondly, this work has shown the existence of bimodal distribution in the effect of stabilizers on Ni baths. (Abstract shortened by UMI.)...