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Nanobumps Created with Polystyrene Spheres and 248nm or 308nm Laser Pulses

Reema Piparia, Department of Chemical Engineering and Material Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, MI 48202, Erhard W. Rothe, Chemical Engineering, Wayne State University, 5050 Anthony Wayne Drive, Detroit, MI 48202, and Ronald J. Baird, Department of Electrical and Computer Science Engineering, Wayne State University, 5050 Anthony Wayne Drive, Detroit, MI 48202.

Introduction: Laser-processing of material surfaces has been extensively investigated. The smallest size of the resulting features was, until recently, limited by optical diffraction to ≈ λ. That limit has been circumvented with the use of near-field optical-microscopes in which light is directed into a fiber tip. By locating surfaces within the tip's near-field radiation, researchers have caused ablation, etching, nanolithography, oxide reduction, and other chemistry. Unfortunately these microscope-based processes are slow.

As an alternative approach, thousands of nanoscale features can be rapidly formed on a surface. Such arrays are formed by self assembly of two-dimensional arrays of spheres from a colloidal suspension of monodisperse spheres. When a laser shot strikes such an array, the light concentrates on the exit side of each sphere. At the surface, which is within the sphere's near-field, the fluence is typically 5-500x larger than the original laser beam.

Methods and Materials: In this study, arrays with 1 μm and 3 μm diameter polystyrene spheres were created on a silicon wafer by spin coating. The surface was then radiated with 248-nm or 308-nm excimer-laser light. The 248-nm laser was followed by a spatial homogenizer that produced an area of uniform fluence. In contrast, the 308-nm laser beam was treated only by allowing it to pass through a spherical lens. The resulting surface structures were characterized by optical microscopy and Atomic Force Microscopy. In order to demonstrate the difference between using 248-nm and 308-nm radiation, we performed calculations for a free (i.e., no surface interaction) sphere. Mieplot was used to compute light absorption within the spheres.

Results and Discussion: With an array of 1-μm spheres, we form an array of nanobumps, each at the location where a sphere had been. These nanobumps are on the order of 50-nm high and 200-nm wide. Our experiments show a large difference between the results from 248-nm and 308-nm excimer-laser wavelengths. The process at 308nm is substantially different than that at 248-nm because a polystyrene sphere with a 1μm diameter absorbs about half of 248-nm radiation incident upon it while transmitting almost all 308-nm light. The absorbed energy at 248nm can lead to a sequence of phenomena, including a violent sphere destruction that will occur within duration of the laser pulse. Possible applications of these nanobump arrays will be discussed.