There's a big gap in small-scale machining. Miniature machining techniques have long produced precision metal components with critical dimensions as small as a millimeter. And emerging micromachining technologies can now turn out silicon MEMS components with sub-micron resolution. But what of those jobs that fall between these two extremes of the small-machining spectrum? Gilbert Benavides, a researcher at Sandia National Laboratories, calls this middle ground "meso-scale" machining.
Meso-scale components, which typically have minimum feature sizes hovering around 25 microns, stand to become more important in a variety of applications. According to Benavides, Sandia first became interested in meso-technologies as a way to work within the "fixed volume" of existing nuclear weapons during upgrades. But civilian uses stand to take off too, including sub-miniature actuators, motors, gears, cutting tools, and particle screens.
Yet, it's the growth of MEMS, or "microelectromechanical systems," that will likely create the biggest opportunity for meso-scale components. Benavides explains that MEMS chips often have to mate mechanically and electronically with larger systems. "To package the MEMS stuff, you need meso-scale stuff," he says. To cite an example, Sandia has a patented interface device that makes both the fluidic and electric connections to a microfluidic IC, a type of MEMS device. One side of this meso-scale interface component acts as a fluid-handling manifold and has holes measuring 200 microns across. The other side provides a plug-in connection to a larger circuit board.
Meso-scale machining differs from micromachining in ways that transcend size. For one, meso-machining methods can tackle a much wider variety of materials than silicon-based surface micromachining techniques. Benavides reports that Sandia has produced meso-scale components in a variety of metals, ceramics, plastics, and rare-earth magnets. The metals line-up has included stainless steel, aluminum, brass, copper, tungsten carbide, and even more exotic alloys like Kovar and Hiperco. For another, meso-scale machining techniques offer more flexibility in the geometry department. MEMS micromachining usually involves the etching away of silicon and tends to produce low aspect ratio features. "There's not much topography on micromachined parts," says Benavides, who refers to these features as "2 ½-D." The various meso-machining techniques, meanwhile, excel at making high aspect ratio features, including trenches, holes, and contoured surfaces.
And just what are the techniques for producing these meso-scale features? Speaking during the International Manufacturing Technology Show (IMTS) in Chicago, Benavides outlined some of the machining methods that can do the trick:
Focused ion beam (FIB) machining. This method bombards the workpiece with a tightly controlled beam of accelerated gallium ions. "The process essentially knocks atoms off the workpiece," Benavides says. Not surprisingly, FIB is slow, removing just 0.5 cubic microns/second. But it has its advantages too. "It works with any material," Benavides notes. And it can produce high-aspect-ratio features as small as 200 nanometers, he adds. Sandia has used FIB to machine meso-scale parts directly. As an example, Benavides has machined a sine wave with a 7-micron wavelength into a silicon wafer.
Micro-milling and turning. FIB can also be used indirectly, as a way to produce micro-cutting tools. Sandia engineers have fashioned six-sided tungsten carbide and tool steel end mills that measure 25 microns across. "That's about one-third the diameter of a human hair," Benavides notes. These tools have seen use for jobs such as 25 micron wide by 25 micron deep trenches in brass and aluminum. Sandia engineers have likewise produced miniscule turning tools such as a single-crystal-diamond threading tool with an 18-micron cutting edge. This tool has seen use cutting tiny helical grooves in aluminum. The challenges with these micro-tools, of course, only start with their fabrication: Benavides reports significant hardships in mounting and indicating micro-tools. "The set-up can take some time," he says.
Laser machining. Laser ablation can remove material layer by layer, and Sandia has worked with two systems capable of meso-scale resolution. The excimer laser is one. But because it has a relatively long pulse it tends to impart too much thermal energy to the workpiece, which can produce debris as the metal melts and splashes, Benavides reports. "We're not doing much with the excimer machine right now," he says. "It does an iffy job on metals needing small features." One good application for the system involves the use of masks to make multiple, high-aspect-ratio holes in one pass, he adds. More promising for meso-scale machining is Sandia's femto-second laser. With its short, 100 femto-second pulse "width," this laser does not suffer from the thermal problems associated with the excimer, Benavides explains. "It's not a thermal process at that pulse width," he says. "So there's no debris problem." Sandia initially used its femto-second machine only for holes, but recently has been able to tackle numerically controlled contours. For example, Benavides has cut snowflake patterns into 316 stainless steel.
Micro-EDM. Electro-discharge machining is hardly new, but Sandia has been working on ways to apply both EDM to meso-scale features. "EDM is the most mature process at least in the sense that you can buy a machine," Benavides says. Using wire and sinker machines from Agie Ltd. (Davidson, NC), Sandia engineers can fabricate features as small as 25 microns with a contour tolerance of 3 microns, Benavides reports. The wire EDM, for example, has produced a stainless steel ratchet gear that measures 175 microns from root to tooth tip and has a surface finish (Ra) of 0.36 microns. In the case of the sinker machine, Sandia uses EDM along with LIGA, an electroforming method capable of sub-micron features. LIGA is limited to electroplateable materials. So Sandia uses the method to make copper EDM electrodes. "EDM gives us LIGA-sized parts, but in engineering materials like stainless steel," he says. Sandia has used its sinker machine for parts such as a 0.15-mm-thick Kovar screen with 0.05-mm wide webbing sections. And it's worth noting that micro-EDM, while already capable of meso-scale parts, is getting even better. At the IMTS show, Agie showed its latest wire EDM machine, called Vertex. It improves tolerance to ±0.5 microns from ±1.5 microns in past models, according to Ken Farr, an Agie applications engineer. "This improvement makes EDM even more suitable for the microfluidics applications," Farr says.
For more information on EDM machines from Agie, enter 544 at www.designnews.com/info.