Lasers and Monolayers Pattern Surfaces

Hank Hogan

Semiconductors have long enjoyed the benefits of being small, but the same hasn't been true for other devices. Many photonic gadgets being developed require the formation of tiny, complex structures. One possible way to achieve this is through the use of self-assembled monolayers, which are chemical coatings that construct themselves, as templates or molds for construction. A research team at Lucent Technologies Inc.'s Bell Labs has combined 193-nm lithography with chemical monolayers to fabricate patterned surfaces that could produce unusual photonic materials.

"Colloidal self-assembly has been proposed as a means to assemble three-dimensional dielectric structures with interesting photonic properties, including optical bandgaps," explained Pierre Wiltzius, the director of semiconductor research at Bell Labs and a member of the research group.

How It Works

The researchers, who reported their work in the Oct. 9, 2000, issue of Applied Physics Letters, began by coating a substrate with a self-assembled monolayer. They exposed the substrate to 193-nm pulses from an excimer laser, using a mask to produce a 532-nm repeating pattern of lines and spaces. A rinse in a chemical bath washed away the exposed material from the substrate, which the researchers then immersed in a second self-assembled monolayer with different chemical properties. The second monolayer attached itself to the substrate in the repeating gaps that had been created in the first.

The finished substrate thus consisted of alternating, chemically different stripes. In subsequent processing, different minerals, polymers and other substances would attach themselves to the alternating stripes.

Key in the demonstration was the UV exposure time of only a few minutes. This is in contrast to other techniques, which may require hours. The 532-nm feature size was also much smaller than previously achieved.

Wiltzius said that this feature size is by no means the minimum size possible. The basic limiting factor is about half the wavelength of the light, so the technique could be used to produce features smaller than 100 nm.