Femtosecond Laser Microprocessing of Aluminum Films and Quartz

Authors

David Doerr, University of Nebraska - LincolnFollow

Date of this Version

August 2007

Document Type

Article

Comments

A DISSERTATION Presented to the Faculty of the Graduate College at the University of Nebraska in Partial Fulfillment of Requirements for the Degree of Doctor of Philosophy. Major: Interdepartmental Area of Engineering (Electrical Engineering). Under the Supervision of Professor Dennis R. Alexander
Lincoln, Nebraska: August, 2007
Copyright © 2007 David Doerr.

Abstract

Laser micro and nano-machining using sub-picosecond laser pulses has become a very active area of research, driven by the development of more user-friendly lasers. An improved understanding of the advantages and limitations of using these lasers when processing various materials is desirable.

Ablation of 20 nm thick aluminum films was carried out using pulse widths from 300 fs to 6 ns to achieve sub-micron ablation diameters. Resolution and quality of the resulting craters is compared and discussed. The minimum crater diameter obtained with complete ablation of the film was 130–260 nm for a 400 nm wavelength, 400 fs pulse focused with a microscope objective with numerical aperture of 0.85. A large window of pulse energies exists where clean ablation of 20 nm Al films can be obtained with minimal damage to the substrate for pulses shorter than 4 ps. This pulse energy range was a factor of 2.6-7.3 times the ablation threshold for pulses less than 4 ps and decreased with increasing laser pulse width to ≤1.2 for 6 ns pulses. Nanoscale protusions (nanobumps and nanospikes) with heights from 20-140 nm were created at fluences just below the ablation threshold. The laser damage threshold of the film is also measured and compared to a theoretical model.

The quality of fs-laser micromachined quartz crystals was also examined. Fluence was varied and the entrance and exit side examined for microcracking to determine optimal processing parameters. Cuts with a high quality laser entrance side without microcracking could be obtained for fluence ranges from 2.5-13 J/cm2 while microcracking at the cut entrance is observed at 16 J/cm². Damage on the exit side of the sample was observed within a distance of 50 μm from the center of the cut and runs parallel to the laser cut.