Nucleation and Adhesion of Electrodeposited Copper on Anodized Thin-Film Aluminum for LIGA Microfabrication

Abstract

The nucleation and electrodeposition of copper from a pyrophosphate electrolyte onto anodized thin-film aluminum was used to provide well-adhering electroforms in high aspect-ratio resist molds made by X-ray lithography. Cyclic voltammetry and chronoamperometry methods were employed to characterize the impact of process variables on initiating copper deposition onto an anodized aluminum film. Adhesion of the electroplated films was evaluated using a 90°peel test. Several aluminum films were studied to elucidate the influence that specific alloying elements have on the electrodeposition after anodization. The alloy content of the aluminum, specifically, small amounts of copper ͑1-4%͒, was found to significantly enhance nucleation. Under certain anodization conditions, small amounts of copper in the original aluminum film provided facile nucleation while aluminum alloys without copper could not be electroplated. The addition of copper to the aluminum alloys anodized at room temperature reduced the nucleation overpotential by more than 300 mV and increased the nuclei densities by two orders of magnitude. With sufficient adhesion of the electrodeposited metal to withstand both plated-metal stresses and planarization, through-mold electrodeposits were fabricated within resist molds ranging in height from 250 to 2800 m, possessing features having lateral dimensions down to 10 m.Through-mold electrodeposition, such as that employed by LIGA ͑lithographie, galvonoformung, abformtechnik͒ microfabrication, often requires a metallized substrate to act as the plating base after resist exposure and development. 1,2 This substrate must be conductive, provide facile initiation of electrodeposits, and provide good adhesion for both the plated metal and the resist. Various substrates and approaches have been attempted to improve LIGA processing, underlying the need for a better substrate material. 3,4 Graphitic carbon has been attempted in the past, and while good poly͑methyl methacrylate͒ ͑PMMA͒ adhesion and metal deposition was achieved, poor adhesion of the metal deposit was claimed. 5 Alternative integration schemes can be utilized to circumvent the poor metal adhesion by avoiding the planarization, but such approaches are not always suitable. 6 Improved adhesion was demonstrated through the use of a buffer layer of polyimide between the substrate and the PMMA. 7 The approach requires a dry etch of the polyimide after PMMA development that may be undesirable if minimal sidewall roughness and strict feature tolerance are preferred. Another option for an X-ray resist substrate is beryllium, which has been used in X-ray mask fabrication. Environmental and safety concerns make the routine use of beryllium increasingly unattractive.Aluminum is a highly desirable alternative to these previously investigated substrate materials because its low X-ray absorption reduces secondary radiation emitted during exposure. With higher-Z substrates, this interaction of the radiation with the substrate-resist, interface can compromise adhesion of...

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