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9/27/04

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Oxide Dispersion Strengthening of Nickel Electrodeposits for Microsystem Applications S.H. GOODS, T.E. BUCHHEIT, R.P. JANEK, J.R. MICHAEL, and P.G. KOTULA Oxide dispersion strengthened nickel (ODS-Ni) electrodeposits were fabricated to net shape using the lithography, electroforming, and molding (LIGA) process in a nickel sulfamate bath containing a suspension of 10 nm diameter Al2O3 particulates. Mechanical properties were compared to baseline specimens fabricated using an identical sulfamate bath chemistry without the particulates. Results revealed that the as-deposited ODS-Ni exhibited significantly higher yield strength and ultimate tensile strength (50 pct) than the baseline material. This increase in as-deposited strength was attributed to Orowan strengthening. The ODS-Ni also showed improved retention of room-temperature strength after annealing over a range of temperatures up to 600 °C. Nanoindentation measurements revealed that the properties of the dispersion-strengthened deposit were uniform through its thickness, even in narrow, high aspect ratio structures. Microscopy revealed that this resistance to anneal softening was due to an inhibition of grain growth in the presence of the oxide dispersion. At elevated temperatures, the strength of the ODS-Ni was approximately 3 times greater than that of the baseline material although with a significant reduction in hot ductility.

I. INTRODUCTION

ELECTRODEPOSITION of metals in thick polymethylmethacrylate (PMMA) or SU-8 photoresist molds has enabled the fabrication of high aspect ratio microstructures, i.e., large height-to-width ratio, for various microsystems applications.[1–5] Electrodeposited microsystem components afford the possibility of the design, fabrication, and assembly of mechanical and electromechanical devices at a size scale that is smaller than that achievable through conventional small scale machining. Also, these components have higher toughness than those realized through silicon surface micromachining. In many cases, individual components of a microsystem device will function as springs or flexures, and therefore, the structural materials used will be required to have specific predefined mechanical properties. As the cross-sectional areas of these load-bearing components decrease (to save space and weight, for example), the stresses they experience remain quite high or may even increase relative to their conventionally machined counterparts,[6] requiring the use of exceptionally high-strength materials. The current study is directed toward realizing enhanced mechanical properties in net-shape electrodeposited structures for microsystem applications fabricated via the LIGA process. LIGA is an acronym derived from the German words: lithographie, galvanoformung, and abformung (lithography, electroforming, and molding). The direct version of this process is capable of producing discrete, free-standing metallic parts with lateral dimensions ranging from 10 to S.H. GOODS, Technical Staff Member, is with the Micros