Threshold behavior of Si films melted with a cw yttrium aluminum garnet laser

When polycrystalline silicon films deposited on fused quartz are heated with 1.06 μm radiation from a cw yttrium aluminum garnet laser, melting occurs at a sharp threshold of laser power and time. This threshold behavior is interpreted as a thermal runaway phenomenon. The initially weak absorption of the laser beam slowly heats the film thereby increasing the absorption. This positive feedback results in abrupt melting after a substantial delay. A one-dimensional analysis of the heat flow predicts the essential features of this effect.