Fab Method Aids Biodetectors

A more flexible way of making diagnostic chips may hold the key to producing tiny medical devices capable of testing for viruses or cancer.

The new fabrication method, bottom-up manufacturing, was developed by an interdisciplinary team at Pennsylvania State University. It involves manufacturing nanowires off chip and then anchoring them to create a resonator array in the form of a tiny diving board.

In the traditional top-down fabrication process, nanoresonators are carved from silicon, a process that works well for producing many devices that are nearly identical, but which means any chemical probes or other changes have to be made after the devices are fabricated on the chips.

"Diagnostic chips can be made more useful by assembling, at predetermined locations on the chip, large numbers of nanowires pretreated off chip," said Rustom B. Bhiladvala, research assistant professor, electrical engineering, Penn State. "Using this new bottom-up method, our group has demonstrated that thousands of single wires can be successfully aligned and anchored to form tiny diving-board resonator arrays."

The bottom-up method, although not producing identical devices, is more flexible because researchers manufacture nanowires off chip using any inorganic or organic material that will produce nanowires. They can attach probe molecules to the wires off chip using a variety of chemicals and they can attach each group of nanowires and their probes to the chips in the numbers and at the locations desired.

"We can achieve high device-integration yields, but the devices are not as uniform as top-down manufactured devices," said Theresa S. Mayer, professor of electrical engineering. "However, we can access materials that are not easy to integrate into the devices with top-down methods. We can also integrate wires treated off-chip with entirely different probe molecules that are attached to the wires using condition optimized for that molecule."

The researchers described their bottom-up method in a recent issue of Nature Nanotechnology. They fabricated resonator arrays with nanowires made of single crystal silicon or polycrystalline rhodium attached at one end and suspended over a depression. This type of device can detect target molecules when they bind to the probe molecules on the nanowires and change the wire's vibration.

To create the diving-board-like resonators, they used a layer of photoresist -- a light-sensitive material which, when exposed to light, can then be easily removed chemically -- to create an array of tiny rectangular wells on the chip. These wells were aligned above an insulated electrode on the chip surface. A solution of nanowires, with probes already attached, flows over the chip surface while the electrodes produce an electric field. The electric field grabs the nanowires and pulls them to the surface where they align perpendicular to the electrode. The aligned nanowires skate along the electrodes and when they reach a well, drop down into it.

Once a wire is in a well, that wire repels other wires allowing, for the most part, only one wire per well. The number of wires in the solution is controlled depending on the number of wells so only a few wires remain on the chip outside the wells.

"One of the biggest challenges of self-assembly is whether we can control where the wires go and control the defects," said Mayer, associate director of Penn State's Material Research Institute and director of the university's National Science Foundation (NSF) National Nanotechnology Infrastructure Network site. "This new method allows integration of the nanowires with high yield."

In the case of the resonators, once the wires are in the depressions, the researchers switch to a top-down approach, placing a layer of a different photoresist on top of the chip and removing a small cube of photoresist around the tip where the wire anchor will be built. They then electrodeposit metal into the tiny square holes, anchoring the nanowire in place. Dissolving the photoresist leaves the suspended nanowire and at the same time removes the nanowires that did not make it into wells.

By choosing the well depth and the thickness of the original photoresist layer, the height of the resonator above the chip surface can be adjusted. An added benefit of bottom-up fabrication is that the nanowires with their probe molecules retain their functions after integration. The researchers also showed that, after the resonator chip arrays were fabricated, target molecules did selectively bind to only those nanowires treated with the correct probe molecules.

The researchers tested many silicon and rhodium nanoresonators by measuring their vibration at high vacuum and found that the electroplated anchors were uniform, not too far from rigid and did not show high energy losses. They also found that both types of wires show negligible effects of air damping at pressures as high as about a thousandth of an atmosphere, which can be reached using small and inexpensive vacuum pumps. They showed that both nanowire dimensions and material properties affect the loss due to air damping at one atmosphere. The quality of the response at this modest vacuum is such that these resonators are strong candidates for sensitive resonance-based detection schemes.

"Bottom-up fabrication is an entirely new nanomanufacturing approach and we need to create devices that have properties that match what we can now make using top-down fabrication," said Mayer. "Our vision is to make large arrays of nano-size devices with multiple probes for multiple targets by placing different groups of functionalized nanowires sequentially on chips."

Assisting Bhiladvala and Mayer with the research were electrical engineering graduate student Mingwei Li, chemistry graduate student Thomas J. Morrow, recent chemistry PhD recipient James A. Sioss, recent materials science and engineering PhD recipient Kok-Keong Lew, professor of materials science and engineering and electrical engineering Joan M. Redwing, and associate professor of chemistry Christine D. Keating.

The National Institutes of Health and the NSF provided support for the research, and members of the Penn State team have filed a provisional patent on the assembly process.

For more information, visit: www.live.psu.edu