(164ad) Nanobumps Created with Polystyrene Spheres and 248nm or 308nm Laser Pulses

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 ≈½ 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.