Figure 1. Stainless steel components fabricated using
the electrochemical machining capabilities at PNNL.
Dean W. Matson and Peter M. Martin
The objective of this project has been to develop high precision microfabrication facilities and expertise to support and enhance PNNL investments in microchemical and energy management processes. Capabilities covered under this project included laser machining, deep UV and standard UV lithography, electrochemical machining, and electrochemical plating. A major emphasis during the past year was placed on expanding microfabrication capabilities and processes that were established during the first year of the project. During the past year a second UV laser micromachining station having 12" stages was procured and brought on-line. Chemical electrodeposition processes were developed and expanded to include nickel, gold, copper, and permalloy. Electrochemical machining capabilities were developed and were demonstrated for the fabrication of microscale components.
A second objective of this project was to develop a lamination process suitable for the production of all-metal microchannel components. During the past year, conditions were established to allow production of solid stainless steel chemical reactors containing complex series of internal microchannels.
Development of microfabrication technologies at PNNL is critical to the overall success of Micro Chemical and Thermal Systems. The purpose of this project has been to expand on existing microfabrication capabilities and to develop new capabilities and microfabrication processes. Fabrication needs for microtechnology applications are unique, not only in the size scales involved, but often in terms of materials and product characterization requirements. Under this project we have developed capabilities and expertise to provide PNNL staff with high quality precision microfabrication methods needed to compete in this area.
During FY 98, laser micromachining capabilities at PNNL were expanded by the acquisition and installation of a Potomac model LMT-5000 excimer laser micromachining workstation with 12î machining stages. This system complemented an existing Potomac LMT-4000 laser machining workstation having 4î stages. The LMT-4000 machining station had become an invaluable tool for fabricating a broad range of small devices containing microchannels and other flow features. It has also proven highly useful as a tool to etch patterns in photoresist or polymer substrates to be used as templates in the production of electrodeposited metal molds. Procurement of the second, larger laser machining station significantly reduced the heavy backlog of work on the smaller system. It also allowed machining of microchannels and other features on larger substrates, as well as the repetition of a machined design over a larger surface. Development of laser micromachining processes has allowed the fabrication of a various devices suitable for a broad range of microfluidic applications. Although excimer lasers are best suited for machining of plastics and other materials that absorb strongly at the laser wavelength (248 nm for KrF lasers), we have demonstrated the capability to produce small-scale laser-machined features in metals for specialized applications.
Electrochemical deposition capabilities were expanded during FY 98 to allow formation of nickel, copper, gold, and permalloy deposits for microfabrication efforts. The existing electrodeposition station was upgraded to allow continuous filtering and stirring of all baths, heating and cooling capabilities, and the addition of a pulsed power supply for improved coating characteristics. We were able to demonstrate the fabrication of a microscale metal mesh by electrodepositing nickel into a pattern generated on photoresist-covered copper using a standard lithographic process.
Electrochemical machining offers an alternative approach to the formation of high aspect ratio millimeter-scale metal components without the costs and long lead-times associated with x-ray based processes such as LIGA. An electrochemical machining station procured during FY 97 was retrofitted with a pulsed power supply to improve its efficiency and machining rate. Preliminary stainless steel parts have been produced using this capability (Fig. 1). Quality of the machined parts produced using this process suggests its application for the fabrication of actuator components for simple microscale valves and pumps.
Figure 1. Stainless steel components fabricated using
the electrochemical machining capabilities at PNNL.
Additional microfabrication capabilities developed at PNNL during FY 98 include the acquisition of a deep UV lithographic exposure source and a wet bench for use with the lithography processes. The deep UV source, when used with thick photoresist and the electrodeposition process, should allow fabrication of high aspect ratio metal micromolds for injection molding plastic components. One new staff member having previous microfabrication expertise was also hired during FY 98 and was partially supported by this project.
A second major focus of this project was the continued development of the lamination/diffusion bonding process for fabrication of metallic microchannel components. We had previously shown that by stacking machined pieces of metallic shimstock and diffusion bonding the stack under elevated temperature and pressure, solid metal components could be produced containing microchannel arrays suitable for heat exchange processes. Furthermore, the microchannels could be produced either internal to the component or on an exterior surface, depending on the requirements of the application. During FY 98 we refined the bonding process to allow production of stainless steel components. Figure 2a shows a cross section of an internal series of microchannels produced in a demonstration stainless steel component using this process. Dimensions of the individual channels were 125 µm wide by 10 mm high by 90 mm long. Widths of the microchannels produced by this fabrication method are determined by the thickness of the shim material containing the machined channel. Figure 2b shows the stainless steel shims from which the demonstration device shown in Figure 2a was produced. The lamination/bonding process developed under this program has been used to produce several different small-scale microchannel devices for chemical processing applications. During the past year one invention report and one patent application have resulted directly from this technology. Additional development of the lamination/diffusion bonding process for producing microchannel chemical processing devices will take place under a separate project within the MS&E Initiative during future years.
Figure 2a
Figure 2b
a) Cross section of microchannels produced
in a stainless steel demonstration device using
the lamination/diffusion bonding process.
b) Machined stainless steel shims used to produce the device.
P. M. Martin, D. W. Matson and W. D. Bennett (1998) "Microfabrication Methods for Microchannel Reactors and Separations Systems" Proceedings of AIChE 2nd International Microreaction Technology Topical Conference, March 8 - 12, New Orleans, LA
D. W. Matson, P. M. Martin, A.L.Y Tonkovich, and Gary Roberts (1998) "Fabrication of a stainless steel microchannel microcombustor using a lamination process" SPIE Proceedings 3514.
P. M. Martin, D. W. Matson and W. D. Bennett (1998) "Microfabrication Methods for Microchannel Reactors and Separations Systems", submitted for publication to Chemical Engineering Communications.
P. M. Martin, D. W. Matson and W. D. Bennett, "Microfabrication Methods for Microchannel Reactors and Separations Systems" presented at the AIChE 2nd International Microreaction Technology Topical Conference, March 8 - 12, New Orleans, LA
D. W. Matson, P. M. Martin, A.L.Y Tonkovich, and Gary Roberts "Fabrication of a stainless steel microchannel microcombustor using a lamination process" presented at the SPIE conference on Micromachining and Microfabrication, Sept. 20-22, Santa Clara, CA.
|