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Plasma immersion ion implantation of nitrogen in Si: Formation of SiO2, Si3N4 and stressed layers under thermal and sputtering effects

In the first part of this paper we present a quantitative trapping model that includes detrapping to describe the diffusion of nitrogen in austenitic stainless steel during successful nitriding. Calculated nitrogen depth profiles, assuming a diffusion pre-exponential factor D0 = 10-3 cm2/s, a diffusion activation energy Ed = 1.1 eV, and a detrapping activation energy Et2d = 1.45 eV, show good agreement with experimental nitrogen depth profiles obtained from a sample that has been subsequently nitrided with 14N and 15N. The plausibility of a trapping mechanism is also supported by a number of phenomenological and thermodynamical arguments. In the second part of this paper we present a modelling approach for the evolution of the thickness of the surface oxide layer during successful nitriding. The approach bases on the assumption that the surface oxide layer thickness is completely controlled by the interplay of sputtering and oxidation and differentiates between oxide layer growth limited by diffusion and oxide layer growth limited by the rate of oxygen supply. The applicability of this approach is supported by experimental data obtained by real-time elastic recoil detection analysis during ion nitriding at different combinations of ion energy and oxygen partial pressure.

Stainless steels show excellent corrosion resistance, which is lost during conventional hardening processes at temperatures above 500°C, like gas or plasma nitriding, to improve the low hardness and to reduce the high wear rate. For aluminium alloys it is impossible to improve the mechanical properties by traditional nitriding. Plasma immersion ion implantation (PIII) of nitrogen is successfully used for hardening both austenitic stainless steels and aluminium alloys. Compared to the untreated materials the hardness and the wear resistance can be improved significantly. For austenitic stainless steels an `expanded austenite' layer of up to several tens of micrometer thickness and for aluminium alloys an AlN layer of more than 10-m thickness were formed over a few hours. Corrosion tests showed no or only small changes in the corrosion behaviour for stainless steel. The AlN layer can be used as a supporting layer for the deposition of hard materials like CrN on aluminium alloys. It is shown that the partial pressure of oxygen in the residual gas in the vacuum chamber plays an important role for the nitriding of both stainless steel and aluminium alloys

A model for the transport of nitrogen in austenitic stainless steel at temperatures around 400°C is presented and discussed. The model considers the diffusion of nitrogen under the influence of trapping and detrapping at trap sites formed by local chromium. Nitrogen depth profiles simulated on the basis of the model with diffusion and trapping activation energies of 1.1 eV and 1.45 eV, respectively, are in good agreement with experimental nitrogen profiles.

Since the end of the 1960's spreading resistance (SR) measurements have become a routinely used technique for determining charge carrier profiles in silicon. For wide band gap semiconductors however the application of this method is difficult because of the high barrier at the interface between probe tips and the semiconductor surface. In order to lower the barrier two different approaches can be taken. First, the material of the tips could be changed with respect to its work function. But there is a limited choice because of the required mechanical properties of the tips concerning high hardness and low brittleness. The other way is to lower the barrier by influencing the surface states of the semiconductor material. This is actually more promising. A strong dependence of the SR values on the polishing material for beveling was reported in investigations for the wide band gap semiconductor GaAs [1]. For laser annealed 6H-SiC [2] and 3C-SiC [3] SR measurements have been reported, but very high measuring voltages (1 to 5 V) were used compared to the usual measurements on Si (10 mV). Ahmed et al. found an activation of more than 100% which points to problems in the interpretation of the measurement [2]. In this work the influence of mechanical processing, further annealing of the bevel and finally sputter cleaning on the SR measurements was investigated. The SiC - surface was beveled with a diamond emulsion on a rotating glass plate. Grain sizes of 1.0 µm and 0.1 µm were used. It was found that polishing with the smaller grain size leads to lower resistances. This is comparable to the results which are known for GaAs with Al2O3 emulsion from Ref. [1]. After beveling several of the samples were annealed under vacuum for 5 min at temperatures of 1300 to 1400°C. A modified thin layer (1 to 2 nm) at the surface was formed which resulted in a lowering of the barrier resistance. The measured resistance was lowered up to a factor of three but statistical variations increased

The mechanism of thermal transport during low-energy ion nitriding of aluminium has been investigated using marker and isotope sequence techniques in connection whit ion beam analysis. For an ion energy of 1 keV and a temperature of 400°C, it is shown that stoichiometric nitride grows at the surface with aluminium being supplied by diffusion from the underlying bulk.

Since the end of the 1960's spreading resistance (SR) measurements have become a routinely used technique for determining charge carrier profiles in silicon. For wide band gap semiconductors however the application of this method is difficult because of the high barrier at the interface between probe tips and the semiconductor surface. In order to lower the barrier two different approaches can be taken. First, the material of the tips could be changed with respect to its work function. But there is a limited choice because of the required mechanical properties of the tips concerning high hardness and low brittleness. The other way is to lower the barrier by influencing the surface states of the semiconductor material. This is actually more promising. A strong dependence of the SR values on the polishing material for beveling was reported in investigations for the wide band gap semiconductor GaAs [1]. For laser annealed 6H-SiC [2] and 3C-SiC [3] SR measurements have been reported, but very high measuring voltages (1 to 5 V) were used compared to the usual measurements on Si (10 mV). Ahmed et al. found an activation of more than 100% which points to problems in the interpretation of the measurement [2]. In this work the influence of mechanical processing, further annealing of the bevel and finally sputter cleaning on the SR measurements was investigated. The SiC - surface was beveled with a diamond emulsion on a rotating glass plate. Grain sizes of 1.0 µm and 0.1 µm were used. It was found that polishing with the smaller grain size leads to lower resistances. This is comparable to the results which are known for GaAs with Al2O3 emulsion from Ref. [1]. After beveling several of the samples were annealed under vacuum for 5 min at temperatures of 1300 to 1400°C. A modified thin layer (1 to 2 nm) at the surface was formed which resulted in a lowering of the barrier resistance. The measured resistance was lowered up to a factor of three but statistical variations increased

Time- and depth-resolved compositional analysis has been performed during the nitriding of stainless steel in order to improve the understanding of the nitriding kinetics. It is directly observed that the surface oxide layer acts as a transport barrier. The experimental data are discussed on the basis of a simple transport model that considers the removal of the surface oxide layer due to sputtering, its regeneration from the residual gas and the ballistic implantation depth of nitrogen.

Metals that form dense native surface oxide layers challenge plasma diffusion treatment techniques. Experimental results obtained during nitriding of stainless steel from real-time depth-resolved compositional analysis by elastic recoil detection give insight into the transport kinetics. In agreement with semi-quantitative considerations on the oxide removal and the oxide growth, the interplay of sputtering and oxidation emerges as a key parameter. On this background, suggestions for practical applications and optimization of the modification processes are given for different plasma diffusion treatment techniques.

In a round robin experiment a set of five polycrystalline, metallic samples is studied by low-energy ion scattering (LEIS) in five different laboratories, The energy range is 0.6-3.5 keV and He and Ne ions are used. Even though different experimental setups are used the evaluated elemental sensitivity factors agree within +/-20%. Reproducibility within single laboratories is better than 10%. In an additional study carried out in three laboratories the surface composition of an alloy, Cu55Pd45, was determined, using in situ calibration standards, These surface composition measurements agreed within +/-3 at% demonstrating that quantitative composition determination is possible using this procedure. (C) 1998 Elsevier Science B.V. All rights reserved.

High current ion nitriding of austenitic stainless steel at temperatures of about 400°C leads to the formation of a surface layer, called expanded austenite. The formation of this layer is not well understood. Deuterium induced nuclear reaction analyses have been performed on austenitic stainless steel samples that were subsequently high current ion nitrided with low energy ion implantation of 15 N and 14 N. The obtained depth profiles yield criteria for the formation process of the expanded austenite. A trapping mechanism cannot be confirmed.

Time- and depth-resolved compositional analysis has been performed during the nitriding of stainless steel in order to improve the understanding of the nitriding kinetics. It is directly observed that the surface oxide layer acts as a transport barrier. The experimental data are discussed on the basis of a simple transport model that considers the removal of the surface oxide layer due to sputtering, its regeneration from the residual gas and the ballistic implantation depth of nitrogen.

Surface modification of austenitic stainless steel by high current ion nitriding at 400°C has been studied by Slow Positron Implantation Spectroscopy (SPIS). In addition, Nuclear Reaction Analysis (NRA) has been applied to obtain information about the nitrogen depth profile. The results obtained from both methods are compared and discussed with respect to the nature of defects produced by high current ion nitriding and their influence on the formation of a surface layer, called expanded austenite.

MeV ion beam analysis (IBA) is presented as a powerful tool for in situ, real-time process diagnostics. With minor additional experimental equipment such as diferential pumping or the installation of special detectors, dynamic stoichiometric profiles can be measured during ion beam or plasma exposure. Spectrum acquisition times of the order of a few minutes or less enable the study of fast and transient diffusion effects and transient surface coverages. Examples are given addressing the diffusion and trapping of hydrogen in nickel around room temperature, the ion beam synthesis of carbon±nitrogen films, the ion beam nitriding of stainless steel, and the diagnostics of the layered structure of cubic boron nitride films. The latter example combines stoichiometric in situ analysis using dynamic ERD and structural in situ analysis using optical ellipsometry.

High current ion nitriding of austenitic stainless steel at temperatures of about 400°C leads to the formation of a surface layer, called expanded austenite. The formation of this layer is not well understood. Deuterium induced nuclear reaction analyses have been performed on austenitic stainless steel samples that were subsequently high current ion nitrided with low energy ion implantation of 15 N and 14 N. The obtained depth profiles yield criteria for the formation process of the expanded austenite. A trapping mechanism cannot be confirmed.