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Platinum Metals Rev., 2001, 45, (1), 14

Chemical Fluid Deposition of Conformal Palladium Films


Thin films of palladium (Pd) and Pd alloys have many industrial applications, such as in catalysis, gas sensors and microelectronics, where Pd film is used as contacts in integrated circuits and as seed layers for electroless plating. Pd film is presently prepared by vacuum sputtering and electroplating, but for complex topographies consistent coverage is problematic.

High-purity, contour-following conformal Pd film may be prepared by chemical vapour deposition (CVD), but at temperatures > 200°C. A drawback to this is that low deposition temperatures are needed by microelectronics to suppress mechanical stress during device fabrication and to minimise interdiffusion and reactions between adjacent layers. The thickness of CVD-produced film also varies, giving inconsistent coverage for high-aspect ratio features.

Researchers at The University of Massachusetts, U.S.A., have now succeeded in depositing highly pure, conformal Pd films onto silicon wafers and polyimide at low temperature using chemical fluid deposition (J. M. Blackburn, D. P. Long and J. J. Watkins, Chem. Mater ., 2000, 12, (9), 2625-2631). This involves the chemical reduction of soluble organopalladium compounds in supercritical carbon dioxide (CO2) at 40-80°C and 100-140 bar pressure.

Deposition by hydrogenolysis of π-2-methylallyl-(cyclopentadienyl)palladium(II) in CO2was completed in less than 2 minutes at 60°C, yielding pure, continuous, 100-200 nm thick, reflective Pd films free from ligand-derived contamination. Conformal coverage was observed on a patterned silicon wafer with features as small as 0.1 μm wide × 1 μm deep.