Micromachining and Image Recording on Thin Films by Laser Beams

Abstract

A theoretical and experimental description of laser machining of thin metallic films is given. Calculations are carried out for the temperature rise of a thin film in response to a pulse of incident light energy and for the dependence of the temperature rise on the different thermal constants of the substrate and film and on the illumination conditions. Experiments have been conducted on the pulsed machining of thin bismuth films with glass and mylar as substrates using a lowest-order transverse mode argon laser with an average power output capability of 20 milliwatts. A fast intracavity acoustooptical modulation system produced optical pulses with durations controllable from 25 ns to several ms. The pulse repetition rate could be varied from a single pulse up to several MHz. The peak power depended on the duty cycle but was limited to 2 watts for low duty cycles. Experiments were done with both front and back illumination. (In back illumination the laser beam is incident on the film through the substrate.) For optimal machining conditions in which the optical beam diameter is adjusted to produce the maximum diameter of transparent area for a given pulse energy, less than 50 percent of the removed material left the surface of the substrate. The remainder was displaced so as to leave some areas free of bismuth while the thickness of bismuth in other areas was increased. With a pulse duration of 25 ns and a 600-Å-thick bismuth film on mylar, the peak power required to machine a 6-µm-diameter spot was about 0.7 watt. Variable amplitude light pulses produced by the intracavity modulation system with a 1-MHz repetition rate and duration of 25 ns were used to write images on a 600-Å-thick bismuth film, deposited on a mylar substrate, by deflecting the laser beam in raster fashion over the surface of the bismuth film. The average laser power output was 20 mW. The film area was 8 × 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Both positive and negative continuous-tone images were recorded. The images consisted of an array of 1200 by 2000 completely independent spots of varying diameter and a spot density of 4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Images of documents which were originally of size 8½ × 11 inches showed a limiting resolution of over 175 lines per inch when magnified to the original size.