High-Quality NSOM Aperture Probes Made More Cheaply

David L. Shenkenberg

Near-field scanning optical microscopes (NSOM) have not been used widely or routinely because of difficulties producing high-quality probes at a reasonable cost. The technique uses probes with holes so tiny that they produce evanescent waves to increase the resolution of light microscopy beyond Abbe’s limit.

Why do we need NSOM for routine use? Because the technique is an optical one, it can image areas below the surface. Atomic force microscopy has high resolution but provides only topographical information.

Scientists used a tiny spark to create an aperture in this NSOM probe, as shown in these images taken by scanning electron microscopy using secondary electrons (top) and backscattered electrons (bottom). Images reprinted with permission of Applied Physics Letters.

NSOM has advantages over other superresolution microscopy techniques, also. It uses a simpler setup compared with stochastic optical reconstruction microscopy and gives more than fluorescence information. Likewise, stimulation emission depletion microscopy requires high-intensity lasers that can damage samples.

Most methods currently used for creating the apertures in the probes require contact with the probe. As a result, the probes frequently become damaged. Although focused ion beam technology can create high-quality probes repeatedly without contact, the process costs about $1 million, according to Marc Chaigneau at École Polytechnique in Palaiseau, France. While at Université de Nantes, he and a colleague as well as Tiberiu M. Minea at Université Paris Sud developed a patented process that does not contact the probe and that costs much less — in the tens of thousands of dollars’ range.

They developed a “plasma apparatus” to create the probes, which are made by coating optical fibers with metal. The fibers are contained in a cylindrical cathode-ray tube inside the apparatus. The probe tip forms in a wax sheath that protects the tips from the hydrofluoric acid, which moves by capillary action and shapes the wax-sheathed fiber to a point by microconvection.

These nanoscale latex balls were imaged at 100-nm resolution with the new aperture probes (left) and with atomic force microscopy (right) for comparison.

They pumped argon gas in the hollow cathode and electrically excited the gas to the plasma state, and the plasma electrons cleaned the etched fibers by Coulombian repulsion. Argon in the chamber simultaneously ionizes with the plasma electrons and sputters the silver cathode target. Ion sputtering dominates after the cleaning has occurred, and the silver atoms accumulate on the fibers.

This time, they created apertures in the probes using a tiny electrical spark in the plasma apparatus when filled with argon. To generate the spark, they aligned the probe tip perpendicular to a planar electrode fed by a power supply set to 700 V. The spark develops when the tip and planar electrode are 400 μm apart and the pressure is at 9 mbar. These conditions cause the emergence of an enhanced electric field that conveniently both generates the spark and causes the spark to align itself.

The spark can create apertures from 30 to 100 nm. The researchers performed NSOM on nanoscale latex balls using these aperture probes and achieved <100-nm resolution. They validated the result with scanning electron microscopy.

Applied Physics Letters, March 3, 2008, Vol. 92, 093106.