Marangoni effect in nanosphere-enhanced laser nanopatterning of siliconY. Lu, S. Theppakuttai, and S. C. Chen a)Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712~Received 3 December 2002; accepted 9 April 2003!We report a Marangoni effect in nanosphere-enhanced laser direct nanopatterning of silicon surface.A monolayer of nanosphere array was formed on the silicon substrate by self-assembly. A 248-nmexcimer laser was used to irradiate the sample surface. Due to optical field enhancement between thenanosphere and the substrate, the silicon surface was locally melted. The molten material wasredistributed due to surface tension forces, resulting in the formation of a nanodent array. Themorphology of the nanodents changed from bowl-type to ‘‘Sombrero’’ with increase of laserintensity as a result of a Marangoni effect that arises due to the competition between athermocapillary force and a chemicapillary force acting on the molten material. © 2003 AmericanInstitute of Physics. @DOI: 10.1063/1.1581387#Traditional materials processing in the nanometer rangeusing laser technology is very difficult with conventional op-tics due to the diffraction limit of the beam wavelength. Re-cently, a near-field technology has been developed to circum-vent the diffraction limit, 1,2 permitting the spot size to bereduced down to 20 nm. In most near-field techniques, thisresolution is achieved by placing a small aperture betweenthe sample and the light source. If the aperture-to-sampleseparation is maintained to much less than a wavelength, theresolution will be determined by the aperture size rather thanby the diffraction limit. This near-field technique using asingle hollow near-field tip to deliver the laser beam has beenproposed for surface modification and lithography on ananoscale due to optical field enhancement in the near-field. 3However, this approach is difficult to implement in an indus-trial setting due to sophisticated hardware to control the near-field distance ~approximately 5 nm above the surface!, pos-sible blockage of the hollow tip due to material depositionfrom the machined surface, and limited throughput. Anotherapproach involves illuminating the tip of a scanning tunnel-ing microscope or an atomic force microscope with a pulsedlaser. 4 Nanostructures with lateral dimensions below 30 nmand therefore well below half of the laser wavelength havebeen produced underneath the tip, but with very low serialthroughput.An approach that may lead to massively parallel nano-structuring was demonstrated by using an array of micro- ornanospheres as a mask to pattern a solid substrate. 5–7 If thediameter of the sphere is greater than the wavelength of theincident laser beam, the transparent sphere may act as a lensto focus the laser beam onto the substrate for surface modi-fication. If the diameter of the sphere is equal to or smallerthan the laser wavelength, near-field enhancement may playan important role in nanostructuring the substrate surface. Inthis letter, we report a Marangoni effect in nanosphere-enhanced laser direct nanopatterning of a solid silicon sur-face.Silicon wafers of n-type, ~100! crystal orientation wereused as the substrate. This type of silicon wafer has a nativeoxide layer of 2 to 3 nm thick. Silica nanospheres with adiameter of 640 nm were used in this work. The nanosphereswere monodispersed in an aqueous solution. We applied thissolution to the silicon surface. After evaporation of the sol-vent, a monolayer of hexagonally close-packed nanosphereswas formed on the silicon due to capillary forces ~Fig. 1!,measured by scanning electron microscopy ~SEM!. Thesample was then irradiated with a KrF excimer laser ~l5248nm, full width at half-maximum510 ns!. A lens of 50.8-mmfocal length was used to focus the laser beam onto thesample mounted on a three-dimensional precision stage. Dif-ferent laser intensities were used to study the laser energydependence of the nanostructures. The laser energy was mea-sured by a joulemeter located between the sample and thelens. All experiments were performed under ambient condi-tions.The laser-patterned silicon surface was characterized bySEM, as shown in Fig. 2, and verified by atomic force mi-croscopy ~AFM!. The spheres disappeared in the laser-irradiated area with nanostructures formed with the samehexagonal pattern on the silicon surface. Large-scale imagesindicated that nanodent structures were produced underneaththe spheres. It is interesting to see that bowl-type dent struc-a! Electronic mail: scchen@mail.utexas.eduFIG. 1. SEM image of a hexagonally close-packed monolayer of silicananospheres of 640 nm in diameter on a ~100! silicon substrate. The scalebar indicates 1 m m.APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 23 9 JUNE 20034143 0003-6951/2003/82(23)/4143/3/$20.00 © 2003 American Institute of Physics
