Method to detect carbon in silicon crystals in the concentration range down to 5 × 10¹⁴ cm⁻³ by Fourier transform infrared absorption at room temperature

Abstract

The procedure of Fourier transform infrared absorption (FT-IR) at room temperature for quantifying C impurities in Si crystals was reexamined to improve the detection limit down to 5×1014 cm-3. Since the substitutional C absorption peak overlaps with a strong two-phonon absorption, the difference spectroscopy is necessary with using a C-lean reference sample. The baseline flatness less than 0.0005 and the thickness uniformity less than 0.001 mm are required to realize the detection limit of 5×1014 cm-3 for 2 mm thick samples. To check the flatness we define the Si/Si baseline which is the difference in the absorbance spectra measured twice with the removal and attachment of the same Si sample. Four organizations participated in FT-IR round robin test for ten samples with the C concentration ranging from 3.6×1014 to 3.3×1015 cm-3. The obtained C concentrations were almost within 30% deviation from the values determined by reliable secondary ion mass spectroscopy.