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SEM and TEM investigation of the ablation mechanisms involved in ultrashort pulsed laser drilling of silicon

SEM- und TEM-Untersuchung der beim Ultrakurzwellenimpulslaserbohren auftretenden Ablationsmechanismen
: Kaspar, J.; Luft, A.; Will, M.; Nolte, S.

Vollertsen, F. ; Wissenschaftliche Gesellschaft Lasertechnik -WLT-:
Lasers in manufacturing 2007 : Proceedings of the Fourth International WLT-Conference Lasers in Manufacturing, LIM 2007, Munich, Germany, June 18th - 22nd, 2007
Stuttgart: AT-Fachverlag, 2007
ISBN: 978-3-00-021449-3
International Conference on Lasers in Manufacturing (LIM) <4, 2007, München>
Conference Paper
Fraunhofer IWS ()
Mikrobearbeitung; Ablation; Laserbohren; Silicium; Ultrakurzwelle; Impulslaser; Rasterelektronenmikroskop; Transmissionselektronenmikroskopie; Verfahrensoptimierung; Verfahrensparameter

In the present work the ablation mechanisms involved in laser helical drilling of silicon using ultrashort laser pulses of 10 ps and 160 fs pulse width were investigated in detail by means of electron microscopy (SEM, TEM). Based on the results the following conclusions are drawn: 1) The quality of the holes drilled with ultrashort laser pulses suffers from mechanical damage of the material caused by the strong laser induced pressure surges if the applied pulse energy is chosen too high. In order to avoid detrimental mechanical effects like the formation of lattice defects, cracking and chipping the pulse energy and hence the effective fluence have to be limited. 2) Using the standard wavelength of 800 nm a pulse width of 10 ps and a pulse energy in the range between 150 microJoule and 300 microJoule are found to be optimal for high quality laser drilling of silicon because both thermal and mechanical effects can be minimised at the same time. 3) TEM investigation of the peripheral rim of the helical drilled holes provided evidence that homogeneous volume melting is involved in the ablation process rather than surface-nucleated melting and vaporising if the pulse width is reduced from 10 ps to 160 fs at 800 nm wavelength. This change in ablation mechanism is accompanied by a roughening of the wall of the hole and hence by a reduced processing quality. 4) To avoid the detrimental effects resulting from homogenous volume melting as well as to efficiently produce micro-holes of high quality and accuracy without any mechanically induced damage the 160 fs processing should be done with short wavelengths in the UV range and pulse energies that are not above 150 microJoule.
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