Etching-enhanced Ablation and the Formation of a Microstructure in Silicon by Laser Irradiation in an SF 6 Atmosphere
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J.D. Budai Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6056 (Received 25 April 2001; accepted 5 February 2002)
Sequential pulsed-laser irradiation of silicon in SF6 atmospheres induced the formation of an ensemble of microholes and microcones. Profilometry measurements and direct imaging with an intensifying charge-coupled device camera were used to study the evolution of this microstructure and the laser-generated plume. Both the partial pressure of SF6 and the total pressure of an SF6-inert gas mixture strongly influenced the maximum height that the microcones attained over the initial surface. The cones first grew continuously with the number of pulses, reached a maximum, and then began to recede as the number of laser pulses increased further. The growth of the cones was closely connected with the evolution of the laser-generated plume.
I. INTRODUCTION
Mechanical spallation, expulsion of small droplets from a transiently laser-melted layer, desorption of atoms and ions, and evaporation have been identified as some of the processes responsible for laser-induced ablation.1 When a reactive atmosphere is present, ablation can be enhanced if volatile compounds within the target material are produced and/or if the reaction yields a surface layer that is easier to remove by laser irradiation than the target material itself. In this paper this process is identified as etching-enhanced laser ablation. Generally, part of the material removed from the target can be collected on a substrate and this constitutes the basic step for the deposition of thin films by pulsed laser ablation. In this process, laser irradiation can produce very drastic changes in the topography of the target. Commonly, these changes strongly decrease the deposition rate. When silicon is irradiated in air, or more generally in O 2 -rich atmospheres, with 1000 laser pulses at a power of 130 MW/cm2, a dense array of high aspect ratio columns forms on the surface. These columns are 20 to 70 m high and 2–3 m in diameter.2–10 An array of conical microstructures is produced when laser irradiation is performed under an SF6 atmosphere.11–13 Formation of conelike structures due to pulsed-laser irradiation is well documented for a wide range of materials.14 In many instances preferential etching could a)
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J. Mater. Res., Vol. 17, No. 5, May 2002 Downloaded: 16 Mar 2015
explain the formation of these structures.14 In the case of the formation of cones under sulfur hexafluoride (SF6) atmospheres in silicon this explanation is not sufficient because the cones protrude above the initial surface.11,12 Moreover, we have studied laser-induced silicon microstructuring and have observed that it strongly depends on the irradiation atmosphere.2–5,12 By contrast, under the same irradiation conditions of wavelength, pulse fluence, and accumulated fluence, no columns are formed if the irradiation is performed under 1 atm of high-purity nitrogen or argon.2,5 We have propo
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