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Optical Patterning Structures Films

Photonics Spectra
Jan 2001
Kevin Robinson

Researchers at Sandia National Laboratories have developed an inexpensive method for controlling the structure of thin films. Using self-assembly methods and optical patterning, they create ordered porous composite nanostructures that may find applications as low-dielectric-constant materials, sensor and adsorbent materials, waveguides and filters.

Group leader C. Jeffrey Brinker, a senior scientist at Sandia and a professor at the University of New Mexico, explained that the method is based on simple evaporative techniques that dispense liquid inorganic and organic precursors and structure-directing agents onto a surface. The researchers then assemble the molecules into precise structures, "some like the packing of soda straws, others like jungle gyms," he said.

Ultraviolet Patterning

The key is an optical patterning process that uses UV light to fine-tune the structures with angstrom precision. The evaporative self-assembly process creates a mesoporous silica film with photosensitive molecules dispersed evenly throughout, Brinker said. Irradiating the film with UV light through a photomask creates highly acidic local features.

The researchers then may selectively etch the unexposed regions with a basic solution and heat the film to create a mesoporous silica architecture in the photodefined areas, or they may skip the etching and heat the film to produce the self-assembled mesoporous silica everywhere. The relative pore size, refractive index, volume fraction porosity and surface area of the exposed and unexposed regions depend on the UV dose.

In a paper published in the Oct. 6, 2000, issue of Science, the researchers and their colleagues at the university and at Vienna University of Technology in Austria and Applied Materials of Santa Clara, Calif., cite potential uses for the technique. Because the UV-exposed regions are hydrophobic, introduced water will collect as small droplets on the unexposed areas. The method thus could precisely direct the deposition of water-soluble ingredients. "Conceivably, we could make patterns of DNA," Brinker said.

Moreover, because the technique enables the user to tailor the refractive index of the film, giving either the unexposed or the exposed region a lower refractive index, it may be used to produce waveguides.

Brinker said his group is investigating three-dimensional patterning with the technique and the ability to tune pore size, which could lead to the development of size-selective membranes and sensor coatings. The researchers also would look into the use of photoresists instead of a photoacid to define the open and closed pore channels.

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