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23 October 2001 High-resolution measurement of thin metallic films and multilayers by femtosecond laser pulses
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Proceedings Volume 4400, Microsystems Engineering: Metrology and Inspection; (2001) https://doi.org/10.1117/12.445590
Event: Lasers in Metrology and Art Conservation, 2001, Munich, Germany
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.
© (2001) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Jacqueline Vollmann, Dieter Michael Profunser, and Jurg Dual "High-resolution measurement of thin metallic films and multilayers by femtosecond laser pulses", Proc. SPIE 4400, Microsystems Engineering: Metrology and Inspection, (23 October 2001); https://doi.org/10.1117/12.445590
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