Portland, Ore. - Nanoscale sculpting of three-dimensional microelectromechanical systems has been enabled by careful characterization of a femtosecond laser's critical intensity. By varying a femtosecond pulsed laser's intensity, University of Michigan researchers report, materials can be selectively vaporized in 3-D, enabling MEMS ablation tolerances as small as 10 nanometers.
"We have MEMS designers beating down our door," said Alan Hunt, an assistant professor in the department of biomedical engineering (Ann Arbor). "They bring us these structures they want to make but can't figure out how and we say, sure we can help you make that. We believe our technique will be a real enabler for MEMS."
Hunt originally sought to find out whether the femtosecond pulsed laser at the University of Michigan's Center for Ultrafast Optical Science could selectively vaporize structures within living cells.
But his characterization of the femtosecond laser's critical intensity-the threshold where a material vaporizes-enabled such a sharp and predictable tool that Hunt enlisted a team of electronics materials experts to characterize its use to sculpt 3-D nanoscale MEMS.
Next for the group are microfluidic devices such as labs-on-a-chip. A longstanding problem in microfluidic devices is that since the channels down which fluids flow are formed lithographically on a single plane they cannot cross. If one channel has to cross another, it must be on a different level. But there are no vias to enable microfluidic channels to jump up and down levels. Hunt's tool easily cuts vias to connect microfluidic channels in the third dimension, enabling them to hop over each other like wires in a circuit diagram.
"Of course, you have to provide some place for the vaporized material to go, but in microfluidic devices that is easy," said Hunt. "You just put fluid inside the device and the vaporized material floats away."
In contrast with current theory, which holds that multiple photon ionization causes optical breakdown, Hunt insists that self-terminated Zener-impact ionization and Zener-seeded avalanche ionization are the dominant mechanisms in optical breakdown. Zener ionization sets the smallest minimum feature size, which ultimately depends on valence electron density.
In general, femtosecond pulse sizes enable a laser to vaporize material so quickly that the surrounding material outside the focused spot is not damaged. Usually, the temperatures necessary to melt a material, no matter how fast you heat it, will diffuse into the surrounding areas, partially melting them, too. But the femtosecond laser is so fast that the normal diffusion effect is completely absent, Hunt said.
"The effect becomes nonlinear when you get your laser pulse down into the femtosecond range-diffusion is no longer the limiting factor. We are using so little energy-less than 10 nanojoules," said Hunt, "which would ordinarily be considered too little for machining, but the effect is so localized that in glass we can cut 20-nm holes with just 4 nJ."
Unlike electron-beam lithography, femtosecond lasers can be focused beneath the surface of a substrate, leaving nothing but a vapor trail while they ablate tunnels under the surface of once-solid materials.
Like electron-beam lithography, designers will probably reserve this relatively tedious step for the difficult tasks rather than use it to fabricate the planar elements of a MEMS chip. For instance, one problem in MEMS is getting 3-D parts to break away from the supports that held them during fabrication. Likewise, MEMS parts often have problems severing their last remaining support, Hunt said.
"One common problem in MEMS that our technique solves, is breaking structures away from their last remaining support," he said. "This is very simple using a femtosecond laser near its critical intensity."
Using photolithography for all the features desired on each plane of a MEMS chip, and then using the femtosecond laser to selectively form vias and to break away parts should be the natural order of MEMS fabrication, Hunt said.
"We are not MEMS designers, but we think we have created a tool that can really help with fabricating MEMS chips," he said. "All the equipment you need is readily available and we have described the technique well enough that other labs should be able to get started right away-providing they have access to a femtosecond laser."