Better control of the processes for making and handling carbon nanotubes means they’re now actually starting to show up in some early electronics applications. SEMI checked in with some examples of companies actually seeing some revenue from sales of CNT related products.

Suppliers of CNT gas sensors and AFM probe tips now on the market are growing the tubes in situ with CVD processes. Progress towards consistent and well characterized CNT solutions for wet deposition of random networks is propelling development of transparent conductive films and other biomedical sensors. And those developing CVD processes say they’re making strides towards better control of properties for eventual use in semiconductor interconnect as well.

Nanomix, Inc. (Emeryville, CA) has a commercial hydrogen sensor to warn of the danger of explosion on the market, and another industrial gas sensor and a CO₂ sensor for respiratory monitoring in the final stages of product development. Marketing Vice President Bill Perry says the company is now, however, concentrating its efforts on the much larger market for low-cost nitric oxide sensors for asthma monitoring, targeted for 2009. The gas is a biomarker for the airway inflammation that marks the progression of asthma, so people could easily monitor their condition to adjust their medication to avoid attacks.

Nanotubes potentially make good sensors because they’re already on the same nanoscale as many of the molecules and proteins of interest. They create an exposed conducting channel, unlike CMOS, that can be in direct contact with the environment, and all the current flows on the surface of the tubes. Put particles that react with a target substance on this sensitive conductive framework, and the electrical characteristics change in target-specific ways.

Nanomix’s initial products are for very small niches, but the company has churned out well over a million of its basic chips with networks of CNTs sensitized for different materials. It grows the random networks directly on to 6-inch wafers with a commercial CVD tool, then sends the wafers out to a commercial foundry to add the electrode pattern on top. The chips are mounted on small circuit boards with the necessary supporting circuitry to complete the sensor. Perry says the company can control the density, thickness and length of the tubes, and, to some degree, the concentration of semi-conductive versus metallic CNTs as well, to adjust the sensitivity of the network. But what turns out to matter most is just having a consistent process, so the average electrical characteristics are known.

Going forward, however, Nanomix is working on moving to using flexible substrates with printed electrodes for a range of biomolecule detection, including DNA, bacteria, viruses and glucose. The aim is to bring down costs—but also to speed up development. “You can certainly iterate faster,” noted Perry, “If you’re not using silicon.”

Also now casting its lot with wet coating of random CNT networks for unique new medical sensors is the startup Alpha Szenszor (Woburn, MA). Though co-founder Charles Lieber has been known to argue that nanowires allow much better control of properties than CNTs, and his lab at Harvard has done much work on nanowire sensors, the company will now license Nantero, Inc.’s (Woburn, MA) solution coating technology for random network CNTs. The company has said it expects to have its first product for convenient low-cost detection of disease on the market in a very short time frame, and aims eventually at detecting biomarkers for everything from HIV to cancer to heart disease.

In August Brewer Science, Inc. (Rolla, MO) started selling a consistent semiconductor grade CNT solution developed with Nantero, so researchers can actually take the material into a fab and start trying out production of some of those intriguing one-off CNT devices they’ve been developing in the lab. Nantero has shown the solution can be applied with a standard track and processed with close to conventional CMOS technologies, but it’s also now working with Hewlett Packard Co. (Palo Alto, CA) on thermal inkjet printing of nonvolatile CNT memory layers on flexible substrates for applications such as low-cost RFID tags.

Brewer Science will buy commercial CNTs from a number of qualified suppliers, purify them to <50 ppb trace metals, and disperse the individual unbundled CNTs in a benign, surfactant free solution with a shelf life of > six months. Characterization shows consistent films across the substrate and across time, at different densities. “We’re showing data that it’s fab compatible, and that the purification is a repeatable process,” says Nantero President Greg Schmergel. “Before this announcement, typically if you bought some of the material out there on January 1, and more on January 2, it would have different characteristics.” The solution, however, like the available CNT supply, remains a mix of metallic and semiconducting nanotubes. Schmergel says the availability of a material that can be used in the fab is generating interest in new CNT development projects. “Before, they knew that even if they spent the effort to develop a new device, they could never manufacture it,” he notes.

Nantero has also developed a metrology method to assure the random CNT network does in fact make a continuous connection. It checks for conductive paths that light up where the metal-CNT contact is charging under the SEM beam. Areas where the CNT film does not make connection to the metal do not show up in the SEM image.

Eikos, Inc. (Franklin, MA), meanwhile, is demonstrating scalable production technologies for its transparent conductive films of CNT networks infused with binder, that it says meet industry standards for electrical and optical performance and environmental stability. The company says the printed CNT films may be better suited to flexible applications than the more brittle ITO, and look likely to be a viable lower cost alternative within the next year or two, particularly for applications like touch screens and electronic paper that need patterning.

Eikos purifies CNTs, disperses them in a variety of solvents, prints them with a variety of conventional printing technologies on a range of substrates, and then infuses them with a polymer binder. The material has sheet resistance of 100 ohm/sq with around 90 percent light transmission for films 550 nm thick, roughly similar to ITO for less demanding applications, says Chris Weeks, business development manager for the product until recently.

“Carbon nanotubes films are beginning to be equal or lower cost for some transparent conductive applications, especially where value added patterning is needed,” says Weeks. Though the single walled tubes themselves are still expensive, only very small amounts are needed—the film can be as thin as 30 nm for 500 ohms/square—and the films can be applied and patterned with low-cost printing processes. And, if the fragmented market for single walled nanotubes consolidates as expected and some big suppliers start producing in larger volumes, prices are likely to come down sharply over the next several years. “When single wall tubes get down to the current cost of multiwall tubes, CNT conductive films will be the lowest cost alternative, even for antistatic coatings,” he says.

“CNTs seem to be able to solve some real world problems,” concurs Lawrence Gasman, principal analyst at Nano Markets (Glen Allen, VA), noting the better performance on flexible substrates than ITO, and better durability outdoors than OLEDs, for displays, signage and lighting.