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Electron microscopic investigation of structural changes in single crystalline silicon induced by short pulse laser drilling

Elektronenmikroskopische Untersuchungen der Strukturänderung von Silizium-Einkristallen, die durch die Bearbeitung mit kurzen Laserpulsen hervorgerufen werden
: Kaspar, J.; Luft, A.

Michel, B.; Winkler, T. ; Deutscher Verband für Materialforschung und -prüfung e.V. -DVM-, Berlin:
Micro Materials. Micro Mat '97. Proceedings : April 16 - 18, 1997, Berlin, Germany
Berlin: DVM, 1997
ISBN: 3-932434-05-6
S.539-542 : Ill., Lit.
Micro Materials (Micro Mat) <2, 1997, Berlin>
Fraunhofer IWS ()
Kurzpulslaser; laser induced material damage; laser micro-machining; laserinduzierte Werkstoffschädigung; Lasermikrobearbeitung; microstructural characterisation; plastic deformation; plastische Verformung; short pulse laser; silicon; Silizium; Strukturuntersuchung; transmission electron microscopy; Transmissionselektronenmikroskopie

Short laser pulses can be used for micromachining a great variety of materials. Using short and ultra short pulses one should expect to reduce thermal damage onto the material but on the other hand to increase mechanical damage. This paper gives an experimental approach to proof these rather theoretical predictions by electron microscopic analysis of micro-holes drilled in silicon wafers with different laser systems (copper vapour laser in MOPA configuration, mode-coupled Nd:YLF laser and titanium:sapphire laser). The primary aim of the study was to establish the influence of pulse duration (50 ns, 40 ps, 200 fs) and pulse energy (0. 1 mJ, and 1.3 mJ) on the laser induced structural changes during laser drilling. The experiments were carried out on (001)and (1 1 1) oriented single crystalline silicon wafers with a thickness of 410 mu m and 250 mu m respectively. The entrance diameter of the laser drilled holes was in the range form 5 mu m to 60 mu m depending on the depth of the hole a nd the pulse energy chosen. Cross sections in different depths below the surface were thinned to electron transparency by a sequence of grinding, abrasion and polishing on a dimple grinder followed by ion polishing. The different stages of preparation were examined by SEM. TEM was used to directly observe the defects resulting from laser drilling. TEM revealed two typical arrangements of dislocations and cracks. The first kind of damage which was found in the upper part of the hole developed during heating and cooling of the hole wall and is closely related to material redeposition. The second kind of damage that was typically found at the bottom of the holes can be regarded as a result of the action of the high laser induced pressures surges on the material. The results showed that the use of smaller puls energies favours the redeposition of ablated material in the hole independent on the pulse duration. A reduction in pulse duration as well as an increase in pulse energy led to a hig her pressure in the drill channel and therefore to a higher mechanical load upon the material. Although the laser damaged zone is only in the range of a few micrometers the laser system used and the parameters have to be carefully matched to the processing task.