Chemical-physical characterization of colloids for chemical mechanical planarization processes of semiconductor devices.

CMP (Chemical Mechanical Planarization) is one of the most expensive processes in the semiconductor industry, and about 70% of its cost can be ascribed to the consumption of the slurry employed during the process; such aqueous suspensions typically contain concentrated oxidizing chemicals and colloidal particles mechanically active on the wafers. In order to reduce costs of ownership, the dilution of standard commercial fumed WCMP slurries with de-ionized water (in 1:1 ratio) is a common procedure used by many semiconductor manufacturers; such dilution leaves process parameters practically unaffected, requiring only minor adjustments of velocity and polishing pressures. Differently, simple dilutions at higher ratios have shown a number of problems, such as decrease in removal rate, process instability and low reliability on the devices, that make this procedure not economically sustainable. Therefore, the need for cheaper CMP processes imposes the development of slurry modifications with additives different than water, which are intrinsically cheap, non-toxic and not harmful to the whole process and to the product quality. This work begins (Chapter 1) with a brief description of the CMP process, introducing process parameters, consumables and the mechanisms of planarization, with a special focus given to metal (tungsten) CMP processes. Then, in Chapter 2, the cost of ownership of the process is analyzed, detailing the effects of dilution. Chapter 3 describes all the physico-chemical techniques used for the analysis of the four chosen WCMP slurries, to which the following chapters (from Chapter 4 to Chapter 7) are dedicated. A single chapter is dedicated to the analyses and results of each slurry. Chapter 8 summarizes the results of the analyses, reporting and comparing the determined composition of the slurries. Slurries labeled as A and C are chosen as the most interesting ones, and following analyses focus on them. The effects of dilution on ions concentration and pH are considered, and the effects on - potential are described. In Chapter 9 the interaction between slurry and wafer is considered. A first part of the chapter is dedicated to the morphological analysis of the surface of tungsten after contact with slurry. The rest of the chapter deals with CMP processed wafers, starting with the analysis of removal rate on wafers with no defined pattern or morphology varying chemical properties of the slurry. The last part of the chapter reports and compares the results obtained on patterned wafers, including recession of tungsten in silicon oxide, defect density and electrical yield. Electrochemical analysis of tungsten in slurry is introduced in Chapter 10. Values of corrosion current and potential are determined, and correlated with varying pH and with removal rate on tungsten wafers. A final chapter (Chapter 11) is dedicated to summarize the conclusions of this work. Appendix A details the X-ray powder diffraction characterization of di-silver trihydrogen para periodate. A compound not strictly related, but found during this work. Appendix B and C contain the posters and papers presented and published during this work.