Evaluation of aluminum step coverage quality in contact structures using a modulated reflectance imaging technique

Correlation was obtained between contact step coverage quality and modulated reflectance signal using a two-laser beam technique. Among its advantages over the common cross-section SEM evaluation, this technique would enable statistical in-line wafer quality and reliability control. The results of modulated reflectance, or thermal wave (TW), experiments are in agreement with the step coverage trends that are intuitively expected, suggesting a `higher TW signal for smaller step coverage´ hypothesis. The following general trends have been observed (a) smaller contacts have a higher TW signal than larger diameter contacts, (b) vertical wall contacts have a larger TW signal than sloped wall contacts, and (c) contacts at the edge of the wafer have a larger TW signal than those at the center. However, a one-to-one (contact specific) verification of the hypothesis using SEM analysis has eluded the authors. Wafers were prepared with contacts of various sizes and profiles in either BPSG or LPCVD oxide. They were then put through a standard barrier metallization process. Al-1% Si-0.5% Cu (1 μm thick) depositions were then carried out under various conditions so as to provide a variety of aluminum film morphologies around and within the contacts