It has been suggested that low-energy-x-ray fluorescence (LEXRF) could be a surface analysis technique used for measuring elemental composition and thickness of thin films on a Si substrate. Conventional XRF, due to constraints on primary beam flux, requires close coupling of the sample to the source and illumination of a large area, making measurements inconvenient with limited spatial information. Polycapillary optics capture a large solid angle of x rays from a source and efficiently focus them to a sub-millimeter spot on the wafer with significant flux gains. Polycapillary optics do not suffer from the 1/r↑(2) losses in going to longer source to sample distances. A prototype focusing optic has been constructed and characterized at Cu L (0.93 keV) and Al K (1.49 keV). The optic collects 0.043 sr of x rays from a point source and focuses the beam to a point 0.46 mm and 0.42 mm in diameter (FWHM) for the Cu L and AlK source, respectively. The optic is 76 mm in length and has an input and output focal length of 4 mm. Compared to the direct beam 100 mm away from the source, the average flux gains within the output focal spot (FWHM) are 94 and 168 times greater for Cu L and AlK, respectively. It is estimated that coupling this optic with a 18 W (1.8 kV 10 mA) Cu source would allow a 4 nm SiO↓(2) layer on a Si wafer to be measured within 1 minute with a statistical accuracy ofσ=2 %.
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