For a Real Close-up, a Little Light

Hank Hogan

Researchers from the University of Texas at Austin have shed new light on near-field scanning optical microscopy. Incorporating a nanometer-scale LED in the probe tip of an atomic force microscope, they acquired optical and topographic images of a test pattern at a resolution of 400 and 50 nm, respectively. It is, they say, the first time that apertureless images have been acquired with a light source embedded in a scanning probe tip.

Researchers at the University of Texas at Austin have integrated a light source into the tip of a near-field scanning microscope. A schematic shows the setup, with the probe attached to a tuning fork that helps provide topographic information. The images on the right show optical and topographic data collected from a chromium test pattern. Courtesy of Xiaojing Zhang.

Eventually, the technique could be used for near-field imaging and perhaps for subwavelength photolithographic patterning, and it has attracted the attention of semiconductor companies and of advanced materials research consortia, said biomedical engineering assistant professor Xiaojing Zhang.

The optical resolution for sensing and patterning depends on the size of the light source, he explained. “Theoretically, that can be down to the size of a single nanoparticle — a few nanometers.”

The group started with a silicon probe tip, the type found in an atomic force microscope. The group members then used a focused ion beam instrument made by FEI Co. of Hillsboro, Ore., to mill a gap in the tip and reshape it to create electrodes at its very end. They used the ion beam to create a nanometer-size p-doped region for the LED.

With the probe tip modified to include a light source, they mounted it in a near-field scanning microscope from Veeco Instruments Inc. of Plainview, N.Y. They used a quartz tuning fork to vibrate the tip while it approached a sample, thereby providing topographic measurements. They collected the transmitted light from the embedded LED and measured it with a single-photon-counting detector from Micro Photon Devices srl of Bolzano, Italy.

After using the setup to look at a test pattern of 30-nm-thick chromium on a glass substrate, the investigators measured the topographic profile of the chromium triangles while optically imaging the test structure, with the former at a better resolution than the latter.

According to postdoctoral associate Kazunori Hoshino, making the light source smaller will improve the resolution, and such a reduction will be possible through an improved definition of the light source. “We know we can make it better,” he said.

Other possible improvements include the use of organic LEDs, which might allow the right emission wavelength across a wide spectrum from the smallest possible device. An ultraviolet light source incorporated into the tip also would be of great interest to the semiconductor industry because of its potential for nanolithography applications.

Applied Physics Letters, March 31, 2008, Vol. 92, 131106.