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Proceedings Paper

High-resolution measurement of thin metallic films and multilayers by femtosecond laser pulses
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Paper Abstract

This investigation deals with various new aspects of the sensitivity improvement of a pump-probe laser based acoustic method. A short laser pulse is used to excite a mechanical pulse thermo-elastically. Echoes of these mechanical pulses reaching the surface are causing a slight change of the optical reflectivity. The surface reflectivity is scanned versus time with a probe pulse. Thus the time of flight of the acoustic pulse is measured. The quantity to be measured i.e. the optical reflectivity change DR caused by acoustic pulses, is rather small. A set-up having an estimated sensitivity DR/R of about 10(superscript -5 has shown to be sufficient to detect up to the 5th echo in a 50 nm aluminum film on sapphire substrate. A key challenge is the reduction of optical and electrical cross talk between the excitation and the detection. Therefore the concepts of double-frequency modulation, cross-polarization, and balanced-photo-detection are implemented. Practical aspects like beam guiding, modulation techniques, beam focus-minimization, beam focus-matching, and the variation of the pump-probe power ratio are discussed. Measurements for single and multi-layer metallic films demanding higher sensitivity are presented.

Paper Details

Date Published: 23 October 2001
PDF: 8 pages
Proc. SPIE 4400, Microsystems Engineering: Metrology and Inspection, (23 October 2001); doi: 10.1117/12.445590
Show Author Affiliations
Jacqueline Vollmann, Swiss Federal Institute of Technology/Zuerich (Switzerland)
Dieter Michael Profunser, Swiss Federal Institute of Technology/Zuerich (Switzerland)
Jurg Dual, Swiss Federal Institute of Technology/Zuerich (Switzerland)

Published in SPIE Proceedings Vol. 4400:
Microsystems Engineering: Metrology and Inspection
Christophe Gorecki; Werner P. O. Jueptner; Malgorzata Kujawinska, Editor(s)

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