Author: Erik Gretler LindbergPublisher:
ISBN:
Category : Chemical engineering
Languages : en
Pages : 169
Book Description
Commercial plating systems typically utilize an electrolyte containing the plated ion in combination with additive mixtures consisting of multiple organic surface active species. These additives adsorb on the plated electrode, modifying the deposit properties, texture, and distribution. While some of the mechanistic details of the additives adsorption and interactions have been characterized, the effects of convective flow and particularly of complex current waveforms remain uncharted. A specific motivation for the research reported herein, is the preferential fill, by electroplated copper, of blind and open vias in printed circuit boards that is achieved utilizing special additives in combination with the application of periodic reverse current waveform in the presence of ferric ions and complex flow. The process, which is widely utilized, has been developed empirically. Its optimization requires understanding the effects of each process parameter, its quantification, and the development of a comprehensive quantitative model. The additives utilized in this study are polyethylene glycol (PEG) which is a copper deposition inhibitor, and bis-sodiumsulfopropyl-disulfide (SPS), which is a weak accelerator. Those very same additives enable the bottom-up metallization of semiconductor interconnects; however, due to the much larger metallized features (hundreds of microns vs. few nanometers) and much longer deposition time (order of hour instead of a few seconds), the challenges facing the plating process herein, are different and, in many cases, more complex. A quantitative model describing competitive adsorption of additives and polarization effects was developed to address deficiencies in current theories. This model, invoking heterogeneous adsorption energy sites, accounts for the steady-state additives (SPS and polyethylene glycol, `PEG') coverages, subject to competitive adsorption, as a function of additives concentration in solution, and accurately predicts polarization as a function of time. An extension of this model has been applied to periodic reverse plating, explaining and quantitatively modeling the preservation of increased polarization associated with pulsing as compared to DC. This maintained polarization, which is generated by the periodic reverse current, is the key to enabling the continued bottom-up plating over time periods exceeding a few minutes. The model qualitatively explains the polarization dependence on the magnitude of the anodic pulse potential or current, and lack of dependence on the anodic pulse length. Another effect of the periodic reverse pulsed current is in enhancing the deposit thickness uniformity. This is achieved due to the preferential dissolution at the short high current anodic pulses, of any deposit asperities produced during the regular longer term deposition at the lower cathodic current densities of the pulse waveform. This effect has been quantified and modeled. The significance of the addition of ferrous/ferric ions to the process has been analyzed. The ferrous ions depolarize the dimensionally stable anodes, preventing the rapid oxidation of the sulfur-containing additive bis-sodiumsulfopropyl-disulfide (SPS). Perhaps more importantly, the ferric ions etch the plated copper through a transport limited reaction. Determining quantitatively the transport dependence of this reaction, and applying this to analyze current efficiency data obtained in a commercial plating machine which incorporates complex flow, it was possible to determine quantitatively the average transport rates prevailing in the industrial scale machine. This determination allows the use of lab-scale rotating disk electrodes (RDE) to simulate industrially relevant plating conditions. Furthermore, this transport information enabled the detailed simulation of the flow and transport rates in and around the plated vias, an essential component for quantitative modeling of the process. A computer implemented model has been used to simulate feature fill incorporating the effects of periodic pulse reverse plating, the additives induced polarization effects, and the transport dependent etching of the plated copper. The simulation results agree well with via fill plating experiments conducted in the lab, using a PCB coupon mounted on a RDE.
