• PDF / 1,619,613 Bytes
• 8 Pages / 612 x 792 pts (letter) Page_size
• 65 Downloads / 204 Views

DOWNLOAD

REPORT

---

Silicon Surface Morphologies after Femtosecond Laser Irradiation Brian R. Tull, James E. Carey, Eric Mazur, Joel P. McDonald, and Steven M. Yalisove Abstract In this article, we present summaries of the evolution of surface morphology resulting from the irradiation of single-crystal silicon with femtosecond laser pulses. In the first section, we discuss the development of micrometer-sized cones on a silicon surface irradiated with hundreds of femtosecond laser pulses in the presence of sulfur hexafluoride and other gases. We propose a general formation mechanism for the surface spikes. In the second section, we discuss the formation of blisters or bubbles at the interface between a thermal silicon oxide and a silicon surface after irradiation with one or more femtosecond laser pulses. We discuss the physical mechanism for blister formation and its potential use as channels in microfluidic devices. Keywords: laser ablation, morphology, oxide, Si.

Introduction Over the past several decades, ultrashort-pulse laser irradiation of silicon surfaces has been an active area of materials science research that has led to a number of unexpected observations and the formation of new materials. The basic physics of this interaction is fully described in the article by Reis et al. in this issue of MRS Bulletin. Starting in the late 1970s, picosecond studies1–8 and later femtosecond pump-probe studies9–21 have been used to elucidate the specific mechanism of many processes, including electron–hole plasma formation,9,10,13,14 melting,5,9,10,12 ablation,19–21 and ultrafast melting.16–18 Ultrafast melting—the disordering of a “cold” lattice within 100 fs of excitation due to covalent bond weakening upon excitation of more than 15% of the valence electrons22–25 —is a phenomenon unique to irradiation with high-intensity femtosecond laser pulses, because these pulses are shorter than the electron–phonon relaxation time. Technologically, ultrashort laser irradiation offers an alternative method for annealing ion-implanted semiconductors.6,7

626

Early research on the surface morphology resulting from picosecond laser irradiation near the melting threshold revealed the formation of ripples on the surface with a wavelength related to the wavelength of the laser.4,26 These socalled laser-induced periodic surface structures (LIPSS)27–30 are similar to ripple structures observed on a variety of materials after irradiation with one or more pulses from a wide range of laser systems (including femtosecond, picosecond, and nanosecond pulses) and are well understood. In short, when the laser pulse is energetic enough to fully melt the surface, the incident pulse interferes with light scattered from defects at the surface, setting up an inhomogeneous melt depth and the formation of capillary waves, which freeze in place.30 Recently, a number of groups have reported the formation of micro- and nano-sized structures resulting from irradiation with femtosecond laser pulses.31–41 The majority of this research deals with the interaction of a single l