Mechanics analysis of femtosecond laser-induced blisters produced in thermally grown oxide on Si(100)

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M.D. Thoulessb) Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109; and Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109

Steven M. Yalisove Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 (Received 18 October 2009; accepted 16 March 2010)

Blister features produced by laser-induced delamination of silicon dioxide from silicon substrates were analyzed with thin-film buckling mechanics. These analyses revealed the role of the interaction between the material and the femtosecond (fs)-pulsed laser on blister formation. In particular, it was deduced that the magnitude of the compressive residual film stress within the irradiated region appeared to exceed the intrinsic residual stress obtained from wafer curvature techniques. This apparent increase in the compressive stress after fs-pulsed laser irradiation may be caused by a modification of the oxide, which resulted in a local rarefaction of the film. The results demonstrated important features of the interaction between materials and fs-pulsed laser, including the presence of subtle modification thresholds and the limited role of thermal effects.

I. INTRODUCTION

Femtosecond (fs)-pulsed lasers have emerged as a powerful tool for high precision machining a wide range of materials, including dielectrics,1–4 semiconductors,5–16 metals,17–24 and thin films.25–30 Essential to this precision is the ultrashort nature of the energy impulse, which is shorter than most physical phenomena associated with material ablation.31 Focused ultrashort energy impulses lead to high intensities, which allow access to nonlinear effects of energy absorption,32 providing for micromachining of dielectrics and semiconductors where the band gap energy exceeds the energy of a single photon. Furthermore, subsurface micromachining and modification of glasses have been demonstrated for the purposes of producing fluidic networks3,33,34 and waveguides.35–39 In this work, the interaction between ultrashort light pulses and materials is further investigated through a mechanical analysis of thin-film blister features produced by fs-pulsed laser irradiation of silicon dioxide on the surface of silicon. The surface morphology of ablation features produced by fs laser pulses onto Si(100) substrates with thermally Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2010.0146 J. Mater. Res., Vol. 25, No. 6, Jun 2010

grown and plasma-enhanced chemical vapor deposited oxide films have been described earlier.14,40,41 Two primary ablation morphologies were observed; one in which the oxide film was delaminated from the underlying substrate but remained intact, and the other in which an area of the oxide film was completely removed. Here, we focus on the former case in which the oxide film is delaminated from the underlying substrate producing a thin-film buckling event. Previous work attributed laserinduced