Nitrogen depth profiles in plasma implanted stainless steel

Abstract

Nitrogen plasma immersion ion implantation (PIII) is a useful technique to enhance the surface properties of stainless steels and the in-depth distribution of the implanted nitrogen is a crucial parameter. A comparison of the nitrogen depth profiles in AISI 304 stainless steel reported in the literature and observed in our laboratory with the one simulated using a plasma sheath model and TRIM shows a discrepancy. The simulated profile is non-Gaussian and shallower due to the non-perfect high voltage pulses whereas the experimental profile is a better fit to a Gaussian distribution. Since most PIII equipment is not designed for ultra-high vacuum (UHV) operation and the plasma is highly reactive in this environment, the surface of the implanted samples is easily contaminated by a large amount of atmospheric species such as oxygen and carbon from the residual vacuum in the processing chamber, thereby converting the materials surface into an oxidized and carburized form. The change in the matrix composition in the near surface skews and translates the nitrogen depth profile obtained by Auger electron spectroscopy. By normalizing the nitrogen signal point-by-point with the combined (Fe+Cr+Ni) signal, a more accurate depth profile can be obtained. This type of normalization, albeit common in secondary ion mass spectrometry (SIMS) data quantification, is seldom implemented in the plasma community when dealing with nitrogen depth profiles acquired by Auger electron spectroscopy. Our results indicate that the excessively high surface contamination renders the raw nitrogen depth profile inaccurate and a proper normalization measure must be adopted.