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SIMS21, Poland 2017 - Rafał Jakieła abstract

Rafał Jakieła oral presentation (SN1-Mon2-3-3)

Ultra-high Sensitivity SIMS Analysis of Oxygen in Silicon

Rafał Jakieła1, Adam Barcz1,2, Jerzy Sarnecki3, George K Celler4

1 Institute of Physics Polish Academy of Sciences, Lotników 32/46, 02-668 Warsaw, Poland
2 Institute of Electronics Technology, Lotników 32/46, 02-668 Warsaw, Poland
3 Institute of Electronic Materials Technology, Wólczyńska 133, 01-919 Warsaw, Poland
4 IAMDN and Materials Science Dept - Rutgers Univ, 607 Taylor Road, NJ NJ 08854 Piscataway, United States

Oxygen, an element with 46% abundance in the Earth's crust, plays an important role in physics and technology of silicon-based semiconductors. Perhaps the most significant feature of oxygen in this context is its ability to form stable planar SiO2 films that serve as insulators, masks and gate barriers in MOS transistors. The natural O content in CZ Si varies from 7 to 10×1017at/cm3, and it is ~1016at/cm3 in FZ Si. Here we report on our decade+ of experience in detecting and profiling of 16O isotope in various structures produced by several European institutions. Detection of oxygen as well as other “atmospheric” species requires special procedures simply because these elements constitute residual gases in the analysis chamber that adsorb on the sample surface, thereby imposing limitations on the ultimate detectability. Using a magnetic IMS 6F Cameca SIMS spectrometer and applying a very high primary Cs+ ion flux, prolonged pre-sputtering, extensive vacuum chamber baking, titanium sublimation pump, and an LN trap, we have reached a detection limit of ~1015 O atoms/cm3 in CVD epitaxial Si films. This value appears to be at least 10 times lower than in any published or unpublished source known to the authors, including the reference sensitivities listed by the instrument manufacturer. Most likely, the key improvement that has allowed us to drive the detection limit to 1015 at/cm3 is the use of an ion pump in the analysis chamber instead of a turbomolecular one as routinely installed in spectrometers of this type. Thanks to this outstanding sensitivity, we were able to detect a surprisingly low O content in gas-phase epitaxial Si.

This paper demonstrates optimized analytical conditions for the oxygen measurements in Si, as a function of depth: (i) very shallow profiles are practically impossible to measure accurately because of native oxide at the surface. (ii) shallow-to-medium range profiles, up to ~20 µm, are the most amenable to SIMS measurements. Typically, this range covers the out-diffusion of oxygen from CZ Si as well as diffusion into FZ Si from the external thermal oxide. The latter case is related to the research, in cooperation with CERN, on the radiation-hard detectors for the LHC collider [1]. Profiling of the top 20 µm can also provide data on internal gettering, denuded zone formation, and SIMOX fabrication. (iii) medium-to-deep (~20-50 µm) range is required to follow interdiffusion and segregation in epitaxial layers when the oxygen-free layer is grown on a CZ Si substrate. In principle, such deep profiling should be done by sample beveling and step-by-step measurement on the exposed surface. We noticed, however, that continuous sputtering with secondary ion monitoring is easier to perform and it does not appreciably degrade the depth resolution, especially if the O distribution is ascending with depth. (iiii) extremely deep profiles – up to full thickness of the wafer, definitely necessitate beveling.

[1] A Barcz, A Panas and R Jakiela; Semicond. Sci. Techn. 19 (2004) 1311