Laser-induced surface perturbations in silicon
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In this paper it is shown that the initial stages in the laser-induced roughening in silicon is independent of the atmosphere used, whether it is Ar, vacuum, or SF6. It is also shown that the morphology that results after a few hundred laser pulses strongly depends on the crystallographic orientation of the surface. The morphological features that appear in this first stage have been related to the nature of the solidification process that follows laser melting. A second stage in the roughening process with a dramatic change in morphology takes place when a surface with deep depressions and hills is further irradiated in SF6. Very deep etching occurs in the depressions promoting the formation of microholes that with further irradiation lead to cone formation. It is further shown that the distance between microholes is equal to the distance between the depressions that formed as the initial perturbations developed. Then the wavelength of the initial perturbation and by extension the distance between microholes has been estimated.
I. INTRODUCTION
Pulsed laser irradiation can alter the topography of substrates of different materials under a variety of processing conditions. For instance, when tape-cast Al2O3 is laser irradiated at a laser fluence of 1 to 2 J/cm2, the surface is smoothed.1 The initial surface is made of micron-size particles that melt upon irradiation. To decrease the surface free energy of the system the melt spreads laterally producing a significantly smoother surface. For the same reason aluminum films deposited on vias for electronic circuits can be planarized as a result of laser melting. On the other hand, when smooth polished surfaces of single crystals of Si and Ge are exposed to pulsed-laser irradiation, a pronounced surface relief develops.2–7 A single laser pulse can produce on the surface of semiconductors a highly uniform and periodic surface roughness.8–10 In this last instance, the fluence is slightly larger than the melting threshold and can be as low as to only induce partial melting of the substrate. The more commonly accepted explanation of this phenomenon is that the interaction between the refracted beam and the electromagnetic field scattered by surface roughness produces a nonhomogeneous energy deposition.11,12 This roughness is much less than the laser wavelength, and it is assumed to exist in as-polished substrates, prior to any treatment. The nonhomogeneous deposition of energy
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e-mail: [email protected] J. Mater. Res., Vol. 16, No. 12, Dec 2001
http://journals.cambridge.org
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produces melting in discrete regions of the surface and gives rise to the observed periodic surface relief. The diffraction patterns observed for this phenomenon depend on the angle of incidence and polarization of the laser beam.11 These changes are a function of the laser fluence, the irradiation atmosphere, and the number of pulses. Similar surface modifications have been observed at the surface of metallic targets when subjected to laser irradiation.13 In this work we st
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