An unexpected effect has been observed when a laser is used to melt a hole in an aluminum layer. Consider a thin film (0.7 µm) of aluminum alloy on a silicon wafer. If an argon-ion laser is focused to a 1.3-µm diam (FWHM) spot on the surface it takes some minimum light power, typically ~1.5 W, to melt a hole in the layer when the beam continually illuminates the metal. With a short pulse (say 1 ms) it naturally takes more power—~2.5 W. However, if the surface is exposed to a series of pulses (several hundred) it takes less than half the power, 0.5 W, to generate the same damage during the same dwell time as the cw beam. This is exactly contrary to what would be expected—a chopped beam delivers less total energy to the surface than cw (18% in this case) and hence should require a higher laser power to cause the same damage. The pulse duration does not appear to matter over the studied range of 0.04–1 ms or does the time between laser pulses; only the number of pulses is important.

© 1989 Optical Society of America

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