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This paper describes a process to fabricate monolithic multilevel high-aspect-ratio microstructures (HARMs) for ferromagnetic devices built on silicon wafers using aligned X-ray lithography in conjunction with electrodeposition. Two X-ray masks were fabricated, each consisting of gold (Au) absorber structures on a transparent polyimide membrane. One mask was used to print a polymethyl methacrylate (PMMA) resist layer. Then, a second PMMA layer was applied to the same wafer, and the second mask was used to pattern it. Transparent alignment windows in the second mask, combined with a piezoelectrically controlled X-ray aligner, allowed for high alignment accuracy between the two print patterns over large areas (>4 inch in diameter). Au circuits were electroplated into first PMMA layer from a sulfite-based electrolyte, and nickel-iron (NiFe) ferromagnetic HARMs were formed in second PMMA resist from a sulfate-based bath. The deposition resulted in well-defined NiFe structures with aspect-ratios up to 67:1 as well as smooth sidewalls and top surfaces. Chemical composition measurements with energy X-ray dispersive spectroscopy (EDS) and wavelength X-ray dispersive spectroscopy (WDS) showed that Fe content increased during the electrodeposition process. To electrically isolate the NiFe posts and Au circuits, both wet chemical etching and sputter etching were explored to remove the exposed seed layer, and the latter approach completely removed the seed layers without damaging the electroplated features. Index Terms--LIGA, HARM, alignment X-ray lithography, X-ray mask, electrodeposition, NiFe, multilevel ferromagnetic structures, etching.

Nickel micro pin fin heat exchangers can be electroplated directly onto planar or non-planar metal surfaces using a derivative of the LIGA micromachining process. The feasibility of fabricating a relatively complex micro heat exchanger on a simple airfoil is demonstrated.

An experimental apparatus has been built to measure the elastic modulus and bending strength (modulus of rupture) of LIGA nickel posts. The apparatus uses the static cantilever beam bending approach to measure mechanical properties in a direction parallel to the growth direction. Experimental results are presented for two sets of largely identical posts constructed using an overplating method. One set was electroplated using a Watts bath, and the other set was electroplated using a sulfamate bath. For the Watts bath, the measured modulus of elasticity was slightly lower than that of bulk nickel (182 GPa), while, for the sulfamate bath, it was approximately half (93 GPa). The strength properties of the two sets of posts also differ dramatically. Microhardness measurements, Focused Ion Beam images of grain structure, and scanning electron microscopy (SEM) micrographs of failure regions are used to further characterize and explain the differences in the results. This integrated testing approach yields a consistent set of data regarding material properties, grain size/structure and failure mechanisms. Potential sources of experimental error are also identified and improvements in experiment design are suggested to reduce these errors. [603] Index Terms--Failure analysis, high-aspect ratio microstructures (HARMs), LIGA, materials testing, strength.

Conventional microheat sink design primarily focuses on the use of continuous fin arrays to optimally dissipate thermal energy from electronic components. By contrast, this paper experimentally measures the thermal and structural performance of two micro pin fin heat sinks designed for use in load bearing applications such as mechanical seals and thrust bearings. One pin fin array is of low porosity, which is more optimal for load bearing capacity, and the other is of high porosity, which is more optimal for heat dissipation. By using these two extreme cases, the thermal-structural tradeoff found in load bearing microheat sinks is demonstrated. The heat sinks are constructed of nickel, electrodeposited onto a stainless steel thrust ring using a modified LIGA technique. Under forced air cooling, the thermal performance of each is compared to a simple model based on a combination of macroscale pin fin heat sink results and classical correlations for fins in cross flow. The low porosity design is also tested under the application of a 44.5 N thrust load at 2500 rpm and found to be structurally sound. Experimental temperature profiles demonstrate a substantial benefit of the microheat sink in cooling the load bearing surface. [601] Index Terms--Forced air cooling, mechanical seals, microheat exchanger, microheat sink, thrust bearings.

This paper describes a process to fabricate monolithic multilevel high-aspect-ratio microstructures (HARMs) for ferromagnetic devices built on silicon wafers using aligned X-ray lithography in conjunction with electrodeposition. Two X-ray masks were fabricated, each consisting of gold (Au) absorber structures on a transparent polyimide membrane. One mask was used to print a polymethyl methacrylate (PMMA) resist layer. Then, a second PMMA layer was applied to the same wafer, and the second mask was used to pattern it. Transparent alignment windows in the second mask, combined with a piezoelectrically controlled X-ray aligner, allowed for high alignment accuracy between the two print patterns over large areas (>4 inch in diameter). Au circuits were electroplated into first PMMA layer from a sulfite-based electrolyte, and nickel-iron (NiFe) ferromagnetic HARMs were formed in second PMMA resist from a sulfate-based bath. The deposition resulted in well-defined NiFe structures with aspect-ratios up to 67:1 as well as smooth sidewalls and top surfaces. Chemical composition measurements with energy X-ray dispersive spectroscopy (EDS) and wavelength X-ray dispersive spectroscopy (WDS) showed that Fe content increased during the electrodeposition process. To electrically isolate the NiFe posts and Au circuits, both wet chemical etching and sputter etching were explored to remove the exposed seed layer, and the latter approach completely removed the seed layers without damaging the electroplated features. [ABSTRACT FROM AUTHOR]