Janet I. Hur

Fabrication of Very-High-Aspect-Ratio Micro Metal Posts and Gratings by Photoelectrochemical Etching and Electroplating

A high-yield fabrication process for dense arrays of very-high-aspect-ratio (VHAR) freestanding metal posts and gratings is developed. Silicon molds of regularly arranged through-holes or trenches are first fabricated by photoelectrochemical etching. By studying the etching parameters, including geometry constraint, current density and potential, electrolyte concentration, and etching time, we succeed to produce dense arrays of VHAR holes ( depth = 610 μm; diameter = 5 μm; pitch = 14 μm) and trenches (depth = 320 μm ; width = 4 μm; pitch = 8 μm) with yields higher than 99% on 2-cm2 processing areas. The VHAR molds are then filled with metals using a new bottom-up electroplating technique, which features an intermittent vacuum degassing to remove the air and hydrogen bubbles from such deep and narrow voids during the plating. Zinc and nickel are successfully electroplated in high quality, and the freestanding metal structures are obtained by removing the silicon molds by XeF2 etching. Obtained are maximum aspect ratios of 120 : 1 for posts (height = 600 μm; diameter = 5 μm; pitch = 14 μm) and over 60 : 1 for gratings (height = 250 μm; width = 4 μm; pitch = 8 μm) with yields higher than 99% on ~ 0.5-1-cm2 samples.

Synthesis and Properties of a Photopatternable Lithium‐Ion Conducting Solid Electrolyte

One of the important considerations for the development of on-chip batteries is the need to photopattern the solid electrolyte directly on electrodes. Herein, the photopatterning of a lithium-ion conducting solid electrolyte is demonstrated by modifying a well-known negative photoresist, SU-8, with LiClO4 . The resulting material exhibits a room temperature ionic conductivity of 52 µS cm-1 with a wide electrochemical window (>5 V). Half-cell galvanostatic testing of 3 µm thin films spin-coated on amorphous silicon validates its use for on-chip energy-storage applications. The modified SU-8 possesses excellent mechanical integrity, is thermally stable up to 250 °C, and can be photopatterned with micrometer-scale resolution. These results present a promising direction for the integration of electrochemical energy storage in microelectronics.

Membranelss micro fuel cell chip enabled by self-pumping of fuel-oxidant mixture

Despite the significant research in micro fuel cells, miniaturizing a whole fuel cell system to below a few millimeters did not appear feasible until very recently, because ancillary parts (e.g. external pump, gas separator) are needed to complete a self-standing system. Building on the recently discovered self-pumping fuel cell free of the ancillary parts, here we design and test new self-pumping fuel cell architecture free of the membrane electrode assembly (MEA). Benefiting from the selectivity of catalysts on each electrode under slow flow rates, we find that the device can be run in a mixed stream instead of the two streams of unmixed laminar flows of fuel and oxidant, greatly simplifying the design. Proof-of-concept device verifies the mechanism of embedded fuel pumping and CO2 degassing, while producing 8 mW/cm2 of peak power density.

High yield dense array of very-high-aspect-ratio micro metal posts by photo-electrochemical etching of silicon and electroplating with vacuum degassing

A high yield fabrication of a dense array of very-high-aspect-ratio (VHAR) freestanding metal posts over a large area is presented. The fabrication is based on photo-electrochemical etching of silicon and bottom-up electroplating of the metal. By using various new techniques, most notably degassing-assisted electroplating, freestanding nickel posts with aspect ratio and height up to 85∶1 and 425 µm, respectively, are realized. While the increase in the aspect ratio of the posts may appear more revealing, the dramatic improvement in yield and uniformity is often more important for applications. The report is over our entire processing area (3 cm2), unlike most others in the literature (1 mm2 – 1 cm2).

A microstructured cathode for fuel cell with self-regulated O 2 bubble creation and consumption

We introduce a monolithic cathode that self-regulates oxygen supply, ideally complementing the recently developed monolithic anode that self-pumps the fuel. Developed is a microstructured cathode that generates oxygen, consumes generated oxygen bubbles as needed, and stops the generation when not consumed, all in a self-regulating fashion. Cyclic voltammetry confirms that the cathode is capable of reducing oxygen even in the presence of simulating a mixed-reactant condition. Half-cell tests show reasonable current outputs, suggesting a complete (full) fuel cell of no moving parts is feasible.