Chemical fluid deposition: Reactive deposition of thin metal films from supercritical carbon dioxide solutions
The fabrication of microelectronics with increasing complexity and decreasing dimensions is placing stringent requirements on the deposition of metal films. These requirements include the conformal coverage of complex topographies, complete filling of narrow (<100 nm), high aspect ratio features, reduction in process temperatures, and reduction of environmentally objectionable wastes from current processing technologies. Physical deposition techniques, such as sputtering, are essentially line of sight methods and thus, uniform coating of the walls and the bottom of high aspect ratio features is difficult. Current chemical deposition strategies include both liquid and vapor phase techniques. Liquid phase techniques include electrolytic plating, which has several drawbacks including the production of heavy metal containing aqueous waste streams. Additionally, for deposition to take place, electrolytic plating requires the presence of a conducting metal seed layer, which must be deposited using another metallization method. Chemical vapor deposition (CVD) often encounters precursor volatility constraints, which leads to mass transport limited conditions and poor step coverage.
Chemical fluid deposition (CFD) is a fundamentally new approach that meets all of the aforementioned demands and eliminates many of the drawbacks associated with other deposition techniques. CFD involves chemical reduction of organometallic compounds dissolved in supercritical fluids to yield highly pure metal films onto a substrate. Precursor concentrations are several orders of magnitude above those employed in CVD, which prevents mass transport limitations and promotes excellent step coverage of very narrow features. Moreover, since the effluent of the CFD often contains only CO2, hydrocarbons, and H2, this technique offers considerable environmental advantages over aqueous plating baths.
In this work, the versatility and effectiveness of the CFD process is demonstrated. High purity, conformal films of platinum, palladium, and rhodium were deposited in simple hot-wall batch reactors at low temperature (40–80°C). The deposition of copper in a cold-wall batch reactor demonstrates the ability of the process to completely fill 100nm wide trenches with aspect ratios (height:width) of 10:1 without the requirement of a metallic seed layer. Furthermore, a study of the deposition rates of nickel at different conditions indicates that reaction rates are comparable to CVD even at temperatures almost 200°C lower.