A Structural Investigation of the Laser Machining of Thin Bismuth Films

An experimental investigation of laser matching of thin films of Bi deposited on Mylar substrates has been carried out. Both He–Ne and argon acousto-optical intracavity modulation systems were used to effect the machining. Scanning and transmission electron microscopy were employed to determine the structural changes caused by machining. It was found that the formation of holes several microns in diameter by laser beams requires melting and probably the initiation of evaporation in the center of the illuminated region. Liquid-phase material is then transported from the center resulting in the formation of machined holes bounded by thicker rim structures. Only a few percent of the material removed needs to be in the vapor phase to achieve the maximum spot diameter for a given pulse energy. It was also found that for short optical pulses (≤ 50 nsec), which require lower machining energies to create a machined spot of a given size than with longer pulses, the substrates are undamaged.