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Measuring Film Thicknesses and Surface Profiles by Frequency-Modulated Laser Beams

IP.com Disclosure Number: IPCOM000042926D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 2 page(s) / 44K

Publishing Venue

IBM

Related People

Zapka, W: AUTHOR

Abstract

A frequency-modulated laser beam is split into its carrier and sideband components which are used as physically separated measurement and reference beams, respectively, whose phase difference is determined by electronic frequency mixing. In the figure, the output beam of CW-laser 1 with a frequency 10 is modulated by electro-optical modulator 2 generating two sidebands 1+1 = 10 + 1M and 1-1 = 10 - 1M, 1M being the modulation frequency generated in RF generator 15. The modulated beam with its direction of polarization in the drawing plane (arrow 20) passes polarizing beam splitter 3 and an optical phase shifter 4 (g/4 plate), impinging upon Fabry-Perot interferometer 5.

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Measuring Film Thicknesses and Surface Profiles by Frequency-Modulated Laser Beams

A frequency-modulated laser beam is split into its carrier and sideband components which are used as physically separated measurement and reference beams, respectively, whose phase difference is determined by electronic frequency mixing. In the figure, the output beam of CW-laser 1 with a frequency 10 is modulated by electro-optical modulator 2 generating two sidebands 1+1 = 10 + 1M and 1-1 = 10 - 1M, 1M being the modulation frequency generated in RF generator 15. The modulated beam with its direction of polarization in the drawing plane (arrow 20) passes polarizing beam splitter 3 and an optical phase shifter 4 (g/4 plate), impinging upon Fabry-Perot interferometer 5. The optical constants of interferometer 5 are chosen such that the carrier is transmitted to another g/4 plate 6 (which orients the polarization vector back to the drawing plane (arrow 23)), mirrors 7a, 7b and 8, and polarizing beam splitter 3 from where it exits as reference beam 9. The sidebands, however, are reflected at interferometer 5, so that their polarization is vertical to the drawing plane (arrow
21) after they have passed g/plate 4 twice. The sidebands are thus deflected at polarizing beam splitter 3, exiting as measuring beam 10 which is parallel to reference beam 9. The distance L between beams 9 and 10 can be adjusted by displaceable mirror 8. By optical system 11, both beams 9 and 10 are focused on to transparent probe 12 which is sho...