11 August 2008 Laser confocal feedback profilometry
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In this paper, a novel profiling method, Laser Confocal Feedback Profilometery (LCFP), combining quasi-common-path heterodyne phase detection with laser confocal feedback technology, is proposed. A microchip Nd:YAG laser emits 1064nm laser, which passes through a pinhole and frequency shifter, and is focused onto the sample surface. The reflected light is coupled back to the microchip laser cavity and forms the frequency shifted feedback light, causing the laser intensity modulation. When the sample is scanned laterally, its surface height variation changes both the phase and strength of the feedback light. LCFP then extracts both the amplitude and phase information out of the laser intensity modulation to determine the integral and fractional number of half laser wavelengths contained in the height variation of two points on the sample surface. LCFP can thus overcome the half-laser-wavelength limit of phase measurement in the axial direction. The high sensitivity of microchip laser to feedback light makes LCFP able to measure samples with very low reflectivity. The LCFP experimental setup is built, and it has successfully measured the height of the stages on a glass-substrate grating. The current performances of LCFP are as followed: the axial resolution is better than 2nm, the axial range about 5μm, and the detectable reflectivity as low as 10-9. Due to its direct traceability to laser wavelength, LCFP can potentially be used as the metrology standard of small-scale features.
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Xinjun Wan, Xinjun Wan, Shulian Zhang, Shulian Zhang, Zhou Ren, Zhou Ren, } "Laser confocal feedback profilometry", Proc. SPIE 7063, Interferometry XIV: Techniques and Analysis, 70630J (11 August 2008); doi: 10.1117/12.792752; https://doi.org/10.1117/12.792752

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