Materials self-assemble into circuitry in the millisecond it takes them to solidify, research shows. When the undoped material is sprayed into preformed vias, waveguides solidify. Doping with molecules that recognize specific conditions builds instant sensor arrays.

Called "surfactant-templated silica mesophases," the materials can self-assemble into photonic pathways. "By adding ligands that exhibit molecular recognition, we can literally spray on sensor arrays," said project leader Jeff Brinker, a senior scientist at Sandia and a professor at the University of New Mexico .

Other team members are Hongyou Fan, Aaron Stump, Victor Perez-Luna and Gabriel Lopez from UNM's Center for Micro-Engineered Materials and Department of Chemical and Nuclear Engineering. Participating Sandia researchers include Yunfeng Lu (former UNM student and Sandia post-doc, now at Applied Materials Inc. in Santa Clara, Calif.), Scott Reed, Tom Baer and Randy Schunk.

Brinker's team has harnessed the tendency of detergent-type molecules to exhibit two phases with opposite reactions to water, one hydrophobic (water-repellent) and one hydrophilic (water-soluble). During evaporation, those molecules form spherical cavities called pores. For instance, a drop of the material dries to form a hollow "dome" and a line of the material dries to form a hollow "tube" flattened on its bottom side.

By spraying the material in a line on a silicon substrate, the pores spontaneously line up to form a tiny waveguide of any length and just 25 angstroms in diameter. Mild heating causes the pores to meld into a solid, permanent structure. "We're experimenting now with using modified ink-jet printers to literally print waveguides and sensor arrays directly on silicon sheets," said Brinker.

So far, Brinker's team has focused on fluids and gases that lead to existing sensor chips, such as for handheld chemical analyzers. But with the addition of ligands that exhibit molecular recognition, such as molecules that change color in response to the environment, spray-on sensor arrays can be produced.

"For instance, we've proven the concept with a fluidic guide containing ligands that react to pH by changing color," said Brinker. Ligands exist that exhibit molecular recognition for a variety of spray-on sensor types, such as for light, heat, magnetic fields, electric fields, specific gases and specific liquids.

Microelectronic future

For the future, however, Brinker's team will concentrate on microelectronic devices that can spray directly onto flexible silicon sheets. Eventually, the team envisions spraying whole computer displays onto a flexible silicon sheet from a standard ink-jet printer using color composition software to mix the materials as appropriate for different parts of the circuitry.

"We're adapting our materials so that we can load them directly into ink-jet printer cartridges, then with ordinary color composition software, we can mix them together in 64,000 different combinations," said Brinker.

No matter how alluring "spray-on computer displays" might sound, it will likely be many years before even simple active devices make it out of the lab. To get the ball rolling, Brinker said the team's next accomplishment will be to create passive circuits with characteristics that are difficult to obtain using conventional means. For instance, Brinker believes that the ink-jet mixing strategy can be used to create devices with dielectric coefficients much lower than can be achieved with conventional lithography.

"There are many types of microelectronics that we can create more easily than by conventional means, such as materials that are strong, hard and hydrophobic, but with a very low dielectric constant," said Brinker.