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The demand for ever faster, cheaper electronics is pushing the lithography-based manufacturing techniques standard in the semiconductor industry to their limits. Now researchers report a cheap, fast lithography technique that uses arrays of flexible polymer nano pens to precisely pattern millions of complex structures in parallel. The technique, which the researchers have used to create an integrated circuit (and lilliputian versions of the Olympics logo), can be employed to make lines whose sizes range from a few nanometers to millimeters thick.

The technique, developed by Chad Mirkin, a chemist at Northwestern University and director of the International Institute for Nanotechnology, uses arrays of pyramid-shaped polymer pens whose tips are dipped in solutions of chemicals that may feature almost any molecule, including proteins and acids; the pens are then traced over a surface by a mechanical arm to create millions of structures in parallel. The width of the lines drawn by each pen can be carefully controlled by varying the force exerted on the flexible pen tips. Because Mirkin’s pens trace out designs programmed by computer software, they can quickly switch between complicated designs, making possible the creation of complex patterns whose features are very close together.

Mirkin has used the pens to pattern acid on a silicon wafer coated with gold; he then etched, based on the pattern, a gold integrated circuit. Polymer-pen lithography also shows promise for patterning biological molecules. Indeed, says Mirkin, the technique could work with almost any molecular “ink,” including proteins for capturing and studying cells. The arrays of polymer pens cost less than a dollar each to make.

Polymer-pen lithography is an improvement over dip-pen lithography, a technique that Mirkin has been developing since 1999. Dip-pen lithography uses arrays of sharp, stiff cantilevered probes–the same ones used for atomic force microscopy. Mirkin created a company, NanoInk, to commercialize the technology. But, he acknowledges, “its ultimate utility has been limited by problems with throughput, cost, and complexity.” The size of its molecular strokes has been restricted to a relatively narrow range, the cantilevers are prone to breaking, and the number of structures that can be made in parallel is limited.

“If this works,” says Grant Willson, an engineer at the University of Texas at Austin, “it will speed the process” of patterning structures with nano pens. The new version of dip-pen lithography could make the technology much more commercially practical. But Mirkin’s technique will be competing in a crowded field, notes Willson. Researchers aiming to pack circuits with ever smaller features for ever faster chips are taking many different nanofabrication approaches. Some, for example, are creating optical antennas to focus light into very small beams to extend the capabilities of photolithography. Others have turned to beams of electrons or ions, or use heat deformation to form patterns.

Harald Fuchs, director of the Interface Physics Group at the University of Münster, in Germany, says that the major advantage of Mirkin’s technique over other nanofabrication methods is precision and flexibility. The pens could be used to write a pattern in one molecular ink, get dipped in another, and then write another layer. To make even more complex patterns, says Fuchs, each pen tip could be dipped in a different ink.

Mirkin says that Northwestern is talking to companies, including his own NanoInk, about commercializing polymer-pen lithography. The technique, he says, will make the dip-pen technology “accessible to a large number of people.”

Source:www.technologyreview.com

In an important step toward the development of practical invisibility cloaks, researchers have engineered two new materials that bend light in entirely new ways. These materials are the first that work in the optical band of the spectrum, which encompasses visible and infrared light; existing cloaking materials only work with microwaves. Such cloaks, long depicted in science fiction, would allow objects, from warplanes to people, to hide in plain sight.

Both materials, described separately in the journals Science and Nature this week, exhibit a property called negative refraction that no natural material possesses. As light passes through the materials, it bends backward. One material works with visible light; the other has been demonstrated with near-infrared light.

The materials, created in the lab of University of California, Berkeley, engineer Xiang Zhang, could show the way toward invisibility cloaks that shield objects from visible light. But Steven Cummer, a Duke University engineer involved in the development of the microwave cloak, cautions that there is a long way to go before the new materials can be used for cloaking. Cloaking materials must guide light in a very precisely controlled way so that it flows around an object, re-forming on the other side with no distortion. The Berkeley materials can bend light in the fundamental way necessary for cloaking, but they will require further engineering to manipulate light so that it is carefully directed.

One of the new Berkeley materials is made up of alternating layers of metal and an insulating material, both of which are punched with a grid of square holes. The total thickness of the device is about 800 nanometers; the holes are even smaller. “These stacked layers form electrical-current loops that respond to the magnetic field of light,” enabling its unique bending properties, says Jason Valentine, a graduate student in Zhang’s lab. Naturally occurring materials, by contrast, don’t interact with the magnetic component of electromagnetic waves. By changing the size of the holes, the researchers can tune the material to different frequencies of light. So far, they’ve demonstrated negative refraction of near-infrared light using a prism made from the material.

Researchers have been trying to create such materials for nearly 10 years, ever since it occurred to them that negative refraction might actually be possible. Other researchers have only been able to make single layers that are too thin–and much too inefficient–for device applications. The Berkeley material is about 10 times thicker than previous designs, which helps increase how much light it transmits while also making it robust enough to be the basis for real devices. “This is getting close to actual nanoscale devices,” Cummer says of the Berkeley prism.

The second material is made up of silver nanowires embedded in aluminum. “The nanowire medium works like optical-fiber bundles, so in principle, it’s quite different,” says Nicholas Fang, mechanical-science and -engineering professor at the University of Illinois at Urbana-Champagne, who was not involved in the research. The layered grid structure not only bends light in the negative direction; it also causes it to travel backward. Light transmitted through the nanowire structure also bends in the negative direction, but without traveling backward. Because the work is still in the early stages, it’s unclear which optical metamaterial will work best, and for what applications. “Maybe future solutions will blend these two approaches,” says Fang.

Making an invisibility cloak will pose great engineering challenges. For one thing, the researchers will need to scale up the material even to cloak a small object: existing microwave cloaking devices, and theoretical designs for optical cloaks, must be many layers thick in order to guide light around objects without distortion. Making materials for microwave cloaking was easier because these wavelengths can be controlled by relatively large structural features. To guide visible light around an object will require a material whose structure is controlled at the nanoscale, like the ones made at Berkeley.

Developing cloaking devices may take some time. In the short term, the Berkeley materials are likely to be useful in telecommunications and microscopy. Nanoscale waveguides and other devices made from the materials might overcome one of the major challenges of scaling down optical communications to chip level: allowing fine control of parallel streams of information-rich light on the same chip so that they do not interfere with one another. And the new materials could also eventually be developed into lenses for light microscopes. So-called superlenses for getting around fundamental resolution limitations on light microscopes have been developed by Fang and others, revealing the workings of biological molecules with nanoscale resolution using ultraviolet light, which is damaging to living cells in large doses. But it hasn’t been possible to make superlenses that work in the information-rich and cell-friendly visible and near-infrared parts of the spectrum.

Researchers at the University of California, Berkeley, have created the first integrated circuit that uses nanowires as both sensors and electronic components. With a simple printing technique, the group was able to fabricate large arrays of uniform circuits, which could serve as image sensors. “Our goal is to develop all-nanowire sensors” that could be used in a variety of applications, says Ali Javey, an electrical-engineering professor at UC Berkeley, who led the research.

Nanowires make good sensors because their small dimensions enhance their sensitivity. Nanowire-based light sensors, for example, can detect just a few photons. But to be useful in practical devices, the sensors have to be integrated with electronics that can amplify and process such small signals. This has been a problem, because the materials used for sensing and electronics cannot easily be assembled on the same surface. What’s more, a reliable way of aligning the tiny nanowires that could be practical on a large scale has been hard to come by.

A printing method developed by the Berkeley group could solve both problems. First, the researchers deposit a polymer on a silicon substrate and use lithography to etch out patterns where the optical sensing nanowires should be. They then print a single layer of cadmium selenide nanowires over the pattern; removing the polymer leaves only the nanowires in the desired location for the circuit. They repeat the process with the second type of nanowires, which have germanium cores and silicon shells and form the basis of the transistors. Finally, they deposit electrodes to complete the circuits.

The printed nanowires are first grown on separate substrates, which the researchers press onto and slide across the silicon. “This type of nanowire transfer is good for aligning the wires,” says Deli Wang, a professor of electrical and computer engineering at the University of California, Santa Barbara, who was not involved in the research. Good alignment is necessary for the device to work properly,since the optical signal depends on the polarization of light, which in turn is dependent on the orientation of the nanowires. Similarly, transistors require a high degree of alignment to switch on and off well.

Another potential advantage of the printing method is that the nanowires could be printed not only onto silicon, but also onto paper or plastics, says Javey. He foresees such applications as “sensor tapes”–long roles of printed sensors used to test air quality or detect minute concentrations of chemicals. “Our next challenge is to develop a wireless component” that would relay the signals from the circuit to a central processing unit, he says.

But for now, the researchers have demonstrated the technique as a way to create an image sensor. They patterned the nanowires onto the substrate to make a 13-by-20 array of circuits, in which each circuit acts as a single pixel. The cadmium selenide nanowires convert incoming photons into electrons, and two different layers of germanium-silicon nanowire transistors amplify the resulting electrical signal by up to five orders of magnitude. “This demonstrates an outstanding application of nanowires in integrated electronics,” says Zhong Lin Wang, director of the Center for Nanostructure Characterization at Georgia Tech.

After putting the device under a halogen light and measuring the output current from each circuit, the group found that about 80 percent of the circuits successfully registered the intensity of the light shining on them. Javey attributes the failure of the other 20 percent to such fabrication defects as shorted electrodes and misprints that resulted in poor nanowire alignment. He notes that all of these issues can be resolved with refined manufacturing methods.

The researchers also plan to work toward shrinking the circuit to improve resolution and sensitivity. Eventually, says Javey, they want everything on the circuit to be printable, including the electrodes and contacts, which could help further reduce costs.