Some chemical and electrochemical aspects of the chemical mechanical polishing of copper

Since the early 1990s, the growing interest in copper as an interconnect metal has led to a large increase in the number of studies covering various aspects of copper chip processing. One of the important topics studied is the chemical mechanical polishing (CMP) of copper, the main method used for material removal. Several copper CMP observations have gone without adequate explanation despite their importance to the CMP process. This study investigates three of these observations.;The first observation concerns the discrepancy between the observed polishing rate and the electrochemically determined corrosion rate. In general, the electrochemically determined corrosion rates are found to be much lower than the polishing rate. In this study, copper CMP is studied as a corrosive wear process, where the enhanced dissolution is due to passive film rupture. The most common CMP slurry ingredients were selected: benzotriazole (BTAH) as a corrosion inhibitor and ferric ion and hydrogen peroxide as oxidizers.;The first step was to study the kinetics of the Cu(I)BTA film growth. The rate of growth of this film was monitored as a function of potential, BTAH concentration, and pH. The growth of the film followed a 3D growth model, with progressive nucleation at low pH and instantaneous nucleation at higher pH. By modeling the surface coverage of Cu(I)BTA as a function of time, it was shown, for the first time, that if the Cu(I)BTA film is ruptured during a CMP process, there is not enough time for it to grow back.;The second topic studied deals with the dissolution of copper in hydrogen peroxide solutions. The literature reports that the polishing rate of copper increases with hydrogen peroxide concentration then drops at higher concentrations. Copper oxide has been detected on the metal surface although the bulk solution pH and metal ion concentration are too low for precipitation to occur. In this study, the near surface pH of copper dissolving in hydrogen peroxide was found to be ∼2.3 pH units higher than the bulk value. At high metal dissolution rates, the surface concentration of the metal ions increases rapidly, exceeding the solubility limit and precipitating an oxide.;Another effect of pH was investigated, this time for the dissolution of copper in hydroxylamine solutions; the third topic of this study. The literature shows that the polishing rate of copper in hydroxylamine solutions exhibits a peak at pH ∼6. Aqueous stability diagrams and electrochemical measurements showed that the increase in the dissolution rate of copper coincided with the regions of stability of the copper-hydroxylamine complexes. (Abstract shortened by UMI.)...