1. Field of the Invention
The invention relates to a technology for mass production of polymeric micro fuel cells. Specifically, the invention relates to the production and replication of polymeric parts used as substrates and electrodes for micro fuel cells, and it relates to the production of a metal mold of the polymeric parts with smoothly curved channel walls, which otherwise would need to be formed with time-consuming laser ablation or etching processes. The invention further encompasses fuel cells and parts thereof made by the methods of the invention.
2. Description of Background Art
Fuel cells have gained renewed interest for applications in high power consumption electronic devices such as data acquisition and communication devices. Since the energy is stored as a ‘reservoir’ of fuel rather than as an integral part of the power source, fuel cells have advantages over batteries. A miniaturized fuel cell power source can be realized through an approach that combines thin film materials with MEMS (micro-electro-mechanical system) technology. Conventional techniques for producing micro-scale polymeric parts, including Replication Technologies and Micromachining Methods, are described below.
Replication Technologies.
The production of micro-scale polymeric parts via LIGA (German acronym for lithography, electroplating and molding) is a multi-step process requiring mask production, synchrotron exposure of a polymethylmethacrylate (PMMA) substrate (typically PMMA bonded to a metallized silicon wafer or a solid metal plate), development of the PMMA, electroplating to fill the cavities left within the PMMA mold, lapping and final dissolution of the remaining PMMA. Such technology is described in U.S. Pat. No. 5,378,583. A constraining step in this process is the requirement for access to one of the very limited number of synchrotron facilities. Furthermore, the synchrotron exposure only allows the fabrication of channels with straight channel walls and with sharp top corners, which makes subsequent coating of the channel surface with an electrically conducting layer having low resistance impossible.
Thus, a key challenge in LIGA is the replication of metallic molds with smoothly curved relief structures. Many applications require a polymeric substrate with a thin conducting surface, which is coated by line-of-sight methods, such as sputtering or evaporation. The straight deposition pathway precludes coating of a channel wall made by LIGA, or made by other micromachining techniques such as DRIE (deep reactive ion etching).
U.S. Pat. No. 5,073,237, titled “Method Of Making Molds For Electrodeposition Forming Of Microstructured Bodies”, discloses a two-layer substrate that consists of a sputtered or vapor deposited film of metal or carbon on an insulating polymer base such as PMMA. The substrate is used in the standard embossing process, during which the metal film along the walls of the embossed features is stretched and disrupted to form a discontinuous and therefore non-conductive array of isolated spangles of the deposited film. The film in the bottom of the embossed features is not disrupted in this manner, and provides a conductive contact for subsequent electroplating of the features.
Electroplating of micro-features with high aspect ratios with conductive walls and bases tends to close off the channel (feature cavity) before it has been completely plated up from the bottom. The special curved structure in the present invention can prevent such a problem.
U.S. Pat. No. 5,676,983, titled “Tool For Making A Microstructured Plastic Mold From Which Structures Can Be Formed Galvanically”, and U.S. Pat. No. 5,795,519, titled “Process Of Making A Microstructured Plastic Mold”, again utilize a two-layer substrate. These devices provide an embossing master tool in which the features have smooth walls, but the top surfaces of the features possess rough surfaces having points and ridges adapted to penetrate into the electrically insulating layer. This enhanced penetration allows the embossing tool to more efficiently expose the electrically insulating layer at the bottom of the embossed cavities.
None of the above processes provide a simple and versatile method of replicating plastic parts with smoothly walled features. Many require the pre-fabrication of specific plastic substrates, which contain a conducting layer adhered to a non-conducting layer with precise height requirements. Thus, there remains a need in the micro-fabrication art for a general method capable of replicating polymeric structures and parts needed for micro fuel cell applications.
Micromachined Micro Fuel Cells.
U.S. Pat. No. 5,753,385, U.S. Pat. Appl. No. 20030138685, titled “MEMS-based thin film fuel cell”, U.S. Pat. Appl. No. 20040043273 “Solid oxide MEMS-based fuel cells”, U.S. Pat. Appl. No. 20040048128 “Solid polymer MEMS-based fuel cells”, and U.S. Pat. Appl. No. 20040072039 “MEMS-based fuel cells with integrated catalytic fuel processor and method thereof” claim MEMS-based fuel cells of a solid oxide type (SOFC), solid polymer type (SPFC), or proton exchange membrane type (PEMFC).
In one example in U.S. Pat. No. 5,753,385, the thin film solid oxide fuel cell (TFSOFC) stack was formed using physical vapor deposition (PVD) techniques. The host substrate used was a silicon wafer covered by a thin layer of silicon nitride. A layer of nickel was first deposited, followed by a layer yttria-stabilized zirconia (YSZ). The conditions during the deposition were adjusted in order to achieve smooth, dense, continuous films, thus avoiding pinhole formation which could result in electrical shorting through the electrolyte layer.
The present invention was conceived and developed to address the problems with these conventional techniques.