The mechanism of thin film Si nanomachining using femtosecond laser pulses

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 81-85).Femtosecond (fs) laser ablation has been the subject of intense recent research. The pulse time ('width') is shorter than the electronic relaxation time, resulting in a decoupling of the period of laser illumination and the melting of the substrate. Since the laser does not directly heat the substrate, and there has been no directly observed heat affected zone (HAZ) nor vaporization, fs laser ablation is sometimes considered to be a direct solid-vapor phenomenon. Recent research indicates that the phenomenon is not as straightforward as assumed. Time-of-flight spectroscopy used to measure the reflectivity indicates that molten material is present for a few hundred picoseconds, well after the laser pulse. A material-modification threshold has been observed that is below the ablation threshold. This indicates that the laser can affect the substrate without ablation occurring. However, many scanning electron microscopy (SEM) studies have been performed, but material change in the substrate have not been observed. Transmission electron microscope (TEM) studies have also been done, but they have all been carried out in plane-view, so that it is difficult to separately observe bulk and surface effects. In this study, cross-sectional TEM analysis of holes drilled in single crystal silicon films in silicon-on-insulator (SOI) structures have been carried out, and have allowed direct observation of the subsurface modified material. Samples were prepared using a focused ion beam (FIB) system, and a metal mask was applied to protect the surface from the ion beam. Through transmission electron microscopy,(cont.) electron diffraction analysis and energy dispersive x-ray (EDX) analysis, a surface layer of clearly modified material was identified as amorphous silicon (a-Si). This demonstrates that conventional heating of adjacent material occurs during femtosecond pulsed laser ablation. Furthermore, cross sectional transmission electron microscopy allows direct measurement of the extent of the heat-affected zone. Secondary effects were also observed in the SOI structures. An a-Si layer was observed below the insulator (SiO2) layer, indicating that the SiO2 is transparent to the laser beam and that the a-Si formed without ablation or recondensation. There was also undercutting of the top layer of silicon as well as the formation of bubble in the oxide layer. These observations also provide evidence for the nature and extent of heating that occurs during femtosecond pulsed laser ablation.by Jimmy Yi-Jie Jia.S.M