Date of Award

Winter 2006

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Committee Director

Amin N. Dharamsi

Committee Member

Aleksey Bugayev

Committee Member

Linda L. Vahala

Committee Member

Michael J. Kelley

Abstract

There is an increasing interest in femtosecond laser micromachining of materials because of the femtosecond laser's unique high peak power, ultrashort pulse width, negligible heat conductivity process during the laser pulse, and the minimal heat affected zone, which is in the same order of magnitude of the ablated submicron spot. There are some obstacles in reaching optimal and reliable micromachining parameters. One of these obstacles is the lack of understanding of the nature of the interaction and related physical processes. These processes include amorphization, melting, re-crystallization, nucleated-vaporization, and ablation.

The focus of this Dissertation was to study the laser-matter interaction with single and two-photon excitation for optical micro-electro-mechanical system (OMEMS) applications. The laser pulse interaction mechanism was studied by performing a series of experiments including self-imaging experiments, two-photon absorption measurements, and micromachining processes characterizations.

As a result of the self-imaging experiment, it was found for both Si and GaP that the material surface reflectivity increased twice as much during the action of the laser pulse. The generation of electron-hole plasma of 1022 cm-3 density was assigned to be responsible for the reflectivity jump. The Drude damping time of the generated plasma was determined to be 0.35 fs for silicon and 0.27 fs for gallium phosphate.

Additionally, a precise measurement of the two-photon absorption (TPA) coefficient (β) was done. The TPA coefficient was found to be 0.2 cm/GW. Experimental results were in good agreement with the theoretical expectations up to a point at which the ablation started kicking off and the plasma absorption took place.

In case of a single pulse interaction with silicon, self-assembled nano-filaments of a few tens of microns' length and about 100 nm width were observed for the first time with the femtosecond single pulse interaction. The filaments were characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM).

Femtosecond micromachining parameters of silicon were then characterized. The laser beam was first cleaned up using two optical techniques: spatial filtering and expanding. Both cleaning-up techniques gave a clean beam profile; however, with spatial filtering, it was hard to maintain the laser beam quality for a long time as the pinhole in the telescope was destroyed in a matter of hours, so the second technique was used instead.

One- and two-dimensional amplitude gratings were written on the Si surface using the characterized micromachining parameters.

The properties of one-dimensional machined grating depended on laser polarization orientation with respect to laser scanning direction and pulse energy. 1-D diffraction patterns were obtained from parallel machined samples, while 2-D diffraction patterns were obtained from a perpendicular machined sample at laser energy density < 0.6 J/cm2.

The 2-D diffraction patterns resulted from secondary periodical ripples along the polarization direction. During parallel writing directions, these ripples smoothed out; in perpendicular directions, they remained and gave rise to 2-D diffraction pattern. These ripples had large periods (3-5 microns) and were perpendicular to the small periods that have been reported.

The investigations presented in this Dissertation increase the understanding of the ablation mechanism under a single laser pulse, the semiconductor materials' behavior on the femtosecond time scale, and the associated self-assemble structure process. The feasibility of short-pulse laser micromachining of semiconductor materials for the micro-electro-mechanical systems (MEMS) was also shown along with the process characterization, which provided guidelines that could help in explaining and advancing the currently existing laser materials processes.

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DOI

10.25777/9nv7-hp49

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