Femtosecond Laser Micromachining of Periodical Structures in Si <100>
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Femtosecond Laser Micromachining of Periodical Structures in Si Mohamed El-Bandrawy and Mool C. Gupta Applied Research Center Old Dominion University Newport News, Virginia, 23606 ABSTRACT A frequency doubled femtosecond Ti: sapphire laser at a wavelength of 400 nm, a pulse width of 160 fs, and a repetition rate of 1 kHz was used with a computer controlled galvo head to write periodical structures in Si . Laser pulses of ~130 nJ were focused using an objective lens of 0.65 NA. Laser parameters were optimized for efficient submicron ablation, yielding 700 nm wide by 600 nm deep lines. 1-D and 2-D periodical structures of 5 and 5x5 micron periods, respectively, were fabricated and examined using optical and atomic force microscopy. The quality of the 1-D and 2-D structures was highly depended on the light polarization orientation with respect to micromachining direction. With optimized fs laser parameters, high quality 1-D and 2-D periodical structures were obtained, which would have applications in optical devices.
INTRODUCTION Femtosecond laser technology for micromachining has attracted a lot of attention due to unique features such as ultrashort pulses, minimal heat affected zone and high peak powers [1-7]. Ngoi et al. studied the frequency doubled femtosecond laser interaction with silicon in a vacuum chamber and observed submicron structures [8]. The same group also used a frequency doubled femtosecond laser at a 300 fs pulse width to study the laser micromachining of silicon samples in air [9]. The energy of the laser pulses was varied from 50-100 nJ. A linear relation was found between the machining quality and the laser energy; however the machined lines suffered discontinuities and irregular edges. In this paper, we present the results of femtosecond laser micromachining of periodical structures in Si at the optimal micromachining parameters.
EXPERIMENTAL SETUP A Ti: sapphire laser from Spectra Physics (Spitfire) was used to conduct micromachining experiments and the experimental setup is shown in figure 1. The laser frequency was doubled using BBO crystal to increase the contrast and to obtain a tighter focus spot size. Two beam separators were used to separate the fundamental and the harmonic wavelengths with a ratio of about 99%. The laser beam was expanded using a 10X beam expander. The laser power was controlled using neutral density filters. The micromachining experiments were carried out using a 500 micron thickness silicon wafer. The sample was fixed on an X-Y stage having 10 nm resolution. The laser beam was focused using a 0.65 numerical aperture objective lens.
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Figure (1) Femtosecond Laser Micromachining Setup
DISSCUSSIONS The effect of laser power The laser power was controlled by using a set of neutral density filters to choose the laser operating power regime. Series of lines was written on the target at different laser power values and the corresponded line width was measured using an optical microscope. The obtained line width values were plotted as a function of the la
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