Development of high temperature electrodeposited LIGA MEMS materials

Abstract

In the current work, new electrodeposited materials including Ni-W alloys, Ni-Al 2 O 3 nanocomposites and Ni based superalloys were developed for use in high temperature MEMS applications. Ni-W alloys with different compositions (Ni 5 at% W and Ni 15 at% W) were electrodeposited from an alkaline ammonia citrate electrolyte in the form of compact thick (> 100 μm) layers and LIGA microspecimens. The mechanical investigations demonstrated in the as-deposited state higher strength and a brittle behaviour compared to pure nickel; while in the annealed state (300 - 700 °C), the strength remained high and additional plasticity was noticed. Higher W content resulted in higher strength up to a 700 °C annealing temperature, whereas lower W content led to more softening after annealing. These properties were confirmed by the nanocrystalline microstructure showing grains smaller than 100 nm in the as-deposited state that slightly increased in size after annealing but still remaining in the submicron range. This microstructural stability can be explained by the effect of W atoms impeding the boundaries' movements. In order to incorporate Al 2 O 3 into compact and thick layers and microspecimens, an electrolyte was developed and optimized based on an alkaline ammonia citrate sulfate solution containing hydrophilic nanoparticles electrostatically dispersed. The incorporation of the particles was evidenced by the detected nanosized agglomerates presenting an Al 2 O 3 content of about 3 wt% and 1.3 wt% for the layers and the microspecimens, respectively, and the increase in strength and brittleness in the as-deposited state compared to pure nickel. After annealing, Ni-Al 2 O 3 remained strong, which was in agreement with the slight increase in grain size showing microstructural stability till an annealing temperature of 600 °C. This may be due to the pinning effect of the particles on the grain boundaries. For use in the formation of superalloys, Ni-Al composites were electrodeposited from a similar solution as Ni-Al 2 O 3 at a neutral pH and containing metallic aluminum particles. It has been shown that the size of the particles affects the structure of the electrodeposited layer that was non-compact when incorporating microparticles, in contrast to nanoparticles resulting in defect free material. Moreover, the electrolyte was unstable even after a short aging time (24h). A first model, based on the determination of the diffusion distance of aluminum in nickel, was presented predicting the annealing parameters necessary for the formation of the high temperature resistant y' phase.