Embossing of microscale features into Pb and Zn was carried out using LIGA (Lithographie, Galvanoformung, Abformung) fabricated Ni mold inserts with features 100 microns in diameter and 500 microns in height. Molding was carried out at 300 °C with both uncoated Ni inserts and Ni inserts coated with Ti-containing hydrocarbon (Ti-C:H). The coatings were applied using a high-density inductively coupled plasma (ICP) assisted hybrid chemical vapor deposition (CVD)/physical vapor deposition (PVD) technique. This technique is shown to produce coatings conformally onto LIGA fabricated high aspect ratio microstructures (HARMs). The performance of the molding process was characterized using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy both in terms of the features generated and the insert condition after molding. The present results indicate that in molding metals that are not reactive with Ni no coating is necessary to produce the microfeatures. This study also demonstrates that in molding Zn, where significant metal/insert chemical interactions exist, surface engineering of the mold insert is necessary to obtain satisfactory performance. Conformal deposition of engineered ceramic coatings onto Ni microscale mold inserts is an effective means for achieving micromolding of reactive metals.
This paper describes a process to fabricate three-dimensional multilevel high-aspect-ratio microstructures (HARMs) for magnetoelectronic devices using aligned x-ray lithography in conjunction with electrodeposition. In this process, x-ray masks were constructed on a seed layer coated polyimide membrane with ultraviolet (UV) patterned and electrodeposited gold absorbers. The optically transparent polyimide allows one to align and print large areas (>4 inch in diameter) with high alignment accuracies. Patterns that contain 5-10 μm diameter posts and 7-10 μm wide lines were printed to 100-120 μm polymethyl methacrylate (PMMA) resist prepared on silicon wafers using x-ray lithography. Nickel-iron was electroplated to form ferromagnetic HARMs, while electroplated gold formed circuits. The composition profile measured with an electron probe x-ray microanalyzer (EPMA) suggested that iron content increases as NiFe plating proceeds inside the recess. The electrodeposition resulted in well-defined NiFe structures with aspect-ratios up to 20:1, smooth sidewalls and top surfaces. To isolate the magnetic structures and circuits, both wet chemical etching and sputter etching were explored to remove seed layer, and the latter yielded complete removal without noticeable damage to the features. A complete aligned x-ray exposure and electrodeposition protocol applicable to universal multilevel microstructures was established.
High-aspect-ratio microstructures (HARMs) have a variety of potential applications in heat transfer, fluid mechanics, catalysts and other microelectromechanical systems (MEMS). The aim of this work is to demonstrate the feasibility to fabricate high performance particulate metal-matrix composite and intermetallic micromechanical structures using the LIGA process. Well-defined functionally graded Ni-Al2O3 and Ni-Al high-aspect-ratio microposts were electroformed into lithographically patterned PMMA holes from a nickel sulfamate bath containing submicron alumina and a diluted Watts bath containing microsized aluminum particles, respectively. SEM image analysis showed that the volume fraction of the alumina reached up to around 30% in the Ni-Al2O3 deposit. The Vickers microhardness of these composites is in the range of 418 through 545, which is higher than those of nickel microstructures from a similar particle-free bath and other Ni-based electrodeposits. In the work on Ni-Al electroplating, a newly developed diluted Watts bath was used to codeposit micron-sized aluminum particles. The intermetallic compound Ni3Al was formed by the reaction of nickel matrices and aluminum particles through subsequent annealing at 630 degrees Celsius. WDS and XRD analyses confirmed that the annealed coating is a two-phase (Ni-Ni3Al) composite. The maximum aluminum volume fraction reached 19% at a cathode current density of 12 mA cm-2, and the Vickers microhardness of the as-deposited coatings is in the range 392 - 515 depending on the amount of aluminum incorporated.