Nanoscale Bow Ties Act as Optical Antennas
Researchers at Stanford University in California have used bow-tie-shaped optically resonant metallic structures to create resist features smaller than 30 nm using an 800-nm laser beam in a two-photon absorption process. The technique could have applications in semiconductor manufacturing as a means of creating very small features using relatively long wavelength sources.
An atomic force microscopy image reveals the resist covering a bow-tie antenna that was exposed to the 27-μW output of an 800-nm laser (upper left). The gap in the bow tie is 36 nm and is clearly visible in the cross section (upper right). A simulation depicts the structure in the resist (bottom). The enhanced electric field in the gap is responsible for this subdiffraction-limit imaging.
A bow-tie antenna is shaped like two truncated triangles that point toward each other with a small gap between them. The researchers constructed an array of the 75- to 85- nm-long features from a 20-nm gold film and a 4-nm titanium sticking layer atop a fused silica/ITO substrate using electron-beam lithography. The gap widths within the bow ties ranged from 16 to 40 nm, and a 3-μm spacing separated the structures on the array.
Over this array, they spread SU-8 resist and exposed it using a Ti:sapphire laser from Coherent Inc. of Santa Clara, Calif., operating at 800 nm, focusing the beam to a 300-nmwide spot that they scanned across the sample. Using scanning electron and atomic force microscopy (AFM), they determined that the resist was exposed with power levels as low as 27 μW. At those power levels, the bow ties therefore must have enhanced the electric field near the gap in the triangles. These experimental results agreed with simulations. The researchers have some ideas as to how the effect might be used for nanolithography.
“We have fabricated a bow-tie antenna on an AFM cantilever,” said Arvind Sundaramurthy, a graduate student at the university. “In this configuration, it acts as a scanning light source that can be used to pattern small features of less than 30 nm on resist, and the resist patterns can be transferred onto a mask by etching.”
Besides being used in lithography, the nanoantennas have application in apertureless near-field scanning optical microscopy. The lack of an aperture would boost the throughput compared with an aperture-based system. Sundaramurthy further noted that the field enhancement near the antenna could serve as a molecule trap, enabling the spectroscopic interrogation of small numbers of molecules — or potentially single molecules.
Nano Letters, online Feb. 9, 2006, doi: 10.1021/nl052322c.
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