Quantitative Depth Profiling of Argon in Tungsten Films by Secondary Ion Mass Spectrometry

Abstract

Depth profiling of Ar in Ar-implanted tungsten (W) films with an excellent detection limit was investigated by secondary ion mass spectrometry (SIMS). Depth profiles of Ar with the detection of Ar⁺ and ArCs⁺ secondary ions, which were produced by O₂⁺ and Cs⁺ primary ions, respectively, were compared in view of the detection limit and the depth resolution. The detection limit of Ar monitoring Ar⁺ was limited by the carbon- and oxygen-containing molecular ion (C₂O⁺) in the sample as well as in the SIMS instrument. It was observed that some of the Ar⁺ ions were produced in the vacuum above the sample surfaces, whereas the ionization of almost all C₂O⁺ occurred at the samples. By using different energy spectra between Ar⁺ and C₂O⁺, we showed that the energy-filtering technique is advantageous for suppressing C₂O⁺ ion detection. It is also confirmed that the ArCs⁺ secondary ion is only slighting by the C₂OCs⁺ mass-interference ion. A detection limit of 4 × 10¹⁸ cm^-3 for monitoring Ar⁺ and 3 × 10¹⁶ cm^-3 for monitoring ArCs⁺ was achieved under a primary-ion current density of 0.16 mA/cm². The detection of ArCs⁺ ion rather than Ar⁺ was found to be superior in the detection limit and the depth resolution. We conclude that SIMS is useful for the determination of the Ar depth distribution in W films.