Nitrogen Depth Profiles Research Articles

Numerical simulation of plasma implanted nitrogen depth profiles in iron

Plasma immersion ion implantation (PIII) is a relatively complex process. Utilizing plasma and TRIM simulation, plasma implanted nitrogen depth profiles in iron are simulated in this work. Many factors such as the high voltage pulse rise time, voltage amplitude, pulse duration, plasma composition, etc. have a critical influence on this depth profiles. Our results confirm the broader distribution as well as shift of the implant peak toward the surface. Compared to the pulse duration and plasma composition, the rise time of the high voltage pulse has a more predominant effect on the shape of the profile. At higher implantation voltage, the nitrogen profile resembles that of multiple energy implantation in the beam-line mode. From this perspective, broadening of the ion energy spectrum in PIII is favorable since a single process can achieve the results of multiple energy ion implantation. This is particularly useful for processes such as shallow junction formation in microelectronics.

Ultra Shallow Incorporation of Nitrogen into Gate Dielectrics by Pulse Time Modulated Plasma

ABSTRACTIn order to obtain a controllability in the nitrogen depth profile in the oxy-nitride gate dielectrics which has been known to have a strong effect on MOS reliability, micro second ordered pulse was used for the inductive coupled plasma source in our pulse time modulated plasma. The radio frequency (RF) of source plasma and pulse frequency were 12.56 MHz and 10 kHz (100 micro second), respectively. Pulse duty ratio was varied from 20 to 100 %. 1.7nm thick thermal silicon dioxide films were subjected to the pulse time modulated plasma and analyzed by SIMS to see the depth profile of nitrogen. A new finding is that both the concentration and the peak position of nitrogen atoms in silicon dioxide films depend on the pulse duty ratio and plasma radiation time. NBTI lifetime was improved with decreased interface state density. We also used this technique to high-k material and investigated process characteristics of nitridation

Irradiation Effect of Nitrogen Ion Beam on Carbon Nitride Thin Films

ABSTRACTIrradiation effect of low-energy nitrogen ion beam on amorphous carbon nitride (a-CNx) thin films has been investigated. The a-CNx films were prepared on silicon single crystal substrates by hot carbon-filament chemical vapor deposition (HFCVD). After deposition, the CNx films were irradiated by a nitrogen ion beam with energy from 0.1 to 2.0 keV. Irradiation effect on the film microstructure and composition was studied by SEM and XPS, focusing on the effect of nitrogen ion beam energy. Surface and cross sectional observations by SEM reveal that the as-deposited films show a densely distributed columnar structure and the films change to be a sparsely distributed cone-like structure after irradiation. It is also found that 2.0 keV ions skeltonize the films more clearly than 0.1 kev ions. Depth profiles of nitrogen in the films observed by XPS show that nitrogen absorption into films is more prominent after irradiation by 0.1 keV nitrogen ions than 2.0 keV ions.

Formation and Properties of Nitrogen-rich Phases in Austenitic Stainless Steel by Plasma-based Ion Implantation

Surface modification of austenitic stainless steel by plasma-based ion implantation at elevated temperatures below 450°C has been studied experimentally. The nitrogen depth profile at room temperature was similar to that obtained by TRIM code simulation, but the depth of nitrogen penetration increases with target temperature and reaches a few micrometers at a treatment condition of 450°C and an implantation time of 2 h. High-dose nitrogen implantation exceeding 1018 cm−2 at temperatures above 350°C results in the formation of expanded austenite phase (supersaturated f.c.c. phase) with little CrN precipitation, leading to remarkable enhancement of surface hardness without loss of corrosion resistance. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(4): 9–16, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10340

Effect of plasma density on the distribution of incident ions and depth profile in plasma-based ion implanted layers

The retained dose and compositional depth profile were studied in the context of cylindrical target with different plasma density treated by plasma-based ion implantation (PBII). Nitrogen was implanted into silicon wafer clamped on the samples in order to acquire high quality profiles. Auger electron spectroscopy (AES) was used to acquire the nitrogen depth profile at the middle of Si wafer. A method, that combined fluid dynamic model to simulate plasma sheath expansion during high voltage pulse and TRIM code to simulate incident ion distribution in the solid was presented to simulate the experimental results. Both retained dose and N depth profile were compared with the results of theoretical simulation. The agreement between them for all three cases is good; that is, the model can give a good prediction and explanation to the experimental results. The retained dose for cylinder increases with increasing plasma density. The continuously distributed energy of incident ions and low N +/N 2 ratio in the plasma shift the N depth profile nearer to the surface and reduce the range significantly.

Phase depth distribution characteristics of the plasma nitrided layer on AISI 304 stainless steel

The phase depth distribution of a plasma nitrided layer on AISI 304 stainless steel at 400–420 °C was investigated by glancing-angle X-ray diffraction through step-wise electrio-polishing removal. The nitrogen depth profile was measured by energy-dispersive X-ray. The results confirm the dominant phase in the nitrogen-rich layer as an expanded austenite. The amount of the lattice expansion relative to the substrate austenite decreases from the layer surface inwards, due to the decrease in nitrogen content. The amount of lattice expansion from planes of (2 0 0) is always larger than that from (1 1 1) at a specific depth. However, the amount of distortion does not decrease monotonically with the decrease in nitrogen content with depth. It is related to the different arrangement of the nitrogen atoms in the fcc lattice at various nitrogen contents. Observations in situ on the surface morphology were performed by scanning electron microscopy. When the strain resulting from supersaturating nitrogen is sufficient to initiate the slip system activity, the slip bands are observed on the surface of the nitrided layer.

Effect of grain orientation on the nitriding rate of a nickel base alloy studied by electron backscatter diffraction

Electron backscatter diffraction is a rather new and powerful technique that provides local orientation. In this paper we present an investigation on cross-sections of a nickel base alloy (Inconel 690) treated by low temperature plasma assisted nitriding. The studied alloy presents non-uniform nitrided layer thickness from grain to grain. A linear relationship is found between the thickness of the nitrided layer within a surface grain and the minimum angle between nitriding direction and the 〈100〉 crystal direction. This angle characterizes the orientation of the grain beneath the nitrided layer. Deeper diffusion is observed in grains with orientation close to 〈100〉 than in those grains with orientation close to 〈111〉. An anisotropic dependence of the stress on the strain is proposed to explain these phenomena. The consequences of the interpretation of X-ray diffraction pattern and nitrogen depth profiles of the nitrided layer are discussed.

SIMS and high-resolution RBS analysis of ultrathin SiO xN y films

Nitrogen depth profiles in ultrathin silicon oxynitride films (∼2.6 nm) are measured by secondary ion mass spectroscopy. The results are compared with the profiles measured by high-resolution Rutherford backscattering spectroscopy (HRBS) to see if SIMS can be accurate in the topmost 1–2 nm region. Using a constant relative sensitivity factor for SIMS analysis, agreement between SIMS and HRBS is not satisfactory. SIMS underestimates the nitrogen concentration at larger concentrations probably due to matrix effects. The empirical composition-dependent RSF is derived from the observed results. Correcting the SIMS result with the empirical RSF, the nitrogen depth profiles agree with the HRBS results reasonably well except for the very surface region ( d<0.3 nm).

Quantitative depth profiling of nitrogen in ultrathin oxynitride film with low energy SIMS

It was shown that the thickness of ultrathin gate oxides measured with low energy O 2 + SIMS shows linear relationships with medium energy ion scattering spectroscopy (MEIS) and HRTEM results in the range of 9–2.5 nm. For quantitative N depth profiling, N profiles in ∼3 nm Si oxynitrides were measured by low energy O 2 + and Cs + SIMS and calibrated with MEIS analysis results of the thickness and the N areal density. N SIMS profiles observed for low energy O 2 + and Cs + SIMS showed some difference in the shape and distribution.

Nitrogen depth profiles in plasma implanted stainless steel

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.

High current density nitrogen implantation of an austenitic stainless steel

Surface modification of AISI 304L austenitic stainless steel under high flux and low energy nitrogen ion implantation has been carried out at moderate temperatures in the range 270–400 °C with 1.2 keV ions and current densities between 0.5 and 1 mA/cm 2. The influence of temperature, current density and fluence on the nitrogen transport and the microstructure of the nitrided layer have been investigated. The nitrogen depth profiles have been determined by nuclear reaction analysis, the microstructure of the modified layers has been analysed by X-ray diffraction and transmission electron microscopy. It is shown that the processing temperature and the ion flux have a major influence on both the profile shapes and the unusually deep penetration depth of nitrogen. The results suggest that above a critical temperature the nitrogen transport increases rapidly giving rise to a flat depth profile. The formation of the well known expanded austenite γ N has been observed and its structure identified to a f.c.c. lattice containing a high density of stacking faults induced by the high level of internal stresses. The X-ray photoelectron spectroscopy study of the Cr2p 3/2, Fe2p 3/2 and N1s binding states demonstrates clearly the preferential bonding of chromium with nitrogen. The atomic transport of nitrogen and the profile shapes are discussed in relation with both the specific role of Cr and additional processes such as the formation of surface vacancies and adatoms.

Mass transport mechanisms during excimer laser nitriding of aluminum

Surface layers of aluminum nitride were formed by irradiating pure aluminum substrates in nitrogen atmosphere with a pulsed excimer laser. The beam was focused on the sample placed inside a chamber filled with nitrogen gas. The irradiation was carried out at various laser fluences, nitrogen gas pressures, and numbers of pulses in order to investigate the influence of each parameter on the nitrogen incorporation and the mass transport mechanisms. X-ray diffraction showed the formation of polycrystalline AlN phase with the wurtzite structure, and the analysis of the nitrogen depth profiles by means of resonant nuclear reaction Analysis revealed a monotonic increase of the nitrogen concentration with the ambient gas pressure and the number of laser shots. It has been found that the laser fluence directly determines the temperature of the substrate and strongly changes the transport mechanism. The thermal simulations and the experimental evidence show that for fluences higher than 3 J/cm 2 the temperature of the substrate exceeds 2900 K. This value is higher than the dissociation temperature (∼2400 K) and close to the melting point (∼3070 K) of AlN, which can therefore dissociate or melt. The atomic nitrogen can rapidly diffuse to greater depths in the liquid Al matrix or it can degas (outgas) through the surface of the sample, leading to the formation of rather homogeneous concentration profiles. For fluences lower than 3 J/cm 2 the temperature of the substrate is not sufficient to destroy the nitride phase and the AIN grains can move inside the molten Al. In this case, the material transport can be attributed to Brownian motion and thermophoretic drift, which in turn are correlated with the chemical and thermal gradient, respectively.

Phase transformation and corrosion behavior of stainless steel bombarded by pulsed energetic ion beams

The treatment of stainless steel by pulsed energetic nitrogen ion irradiation in the millisecond regime may cause phase transformations close to the surface. The characterization of the phases was done by measurements of Mößbauer and grazing incidence X-ray diffraction. The results indicate transformations of the austenitic phase into Cr 2N, Fe 2N and the γ N-phase (paramagnetic and magnetic) in dependence on the bombardment conditions and the nitrogen depth profiles measured by nuclear reaction analysis. Some samples were studied by depth selective conversion electron Mößbauer spectroscopy. One of these samples contained three new phases: Fe 2N, γ N2-phase (paramagnetic) and γ N1-phase (magnetic). The magnetic phase is found predominantly in the surface region, whereas the Fe 2N and the γ N2-phase (paramagnetic) reach their maximum concentration at a depth of approximately 100 nm. The other sample contained four new phases: the three phases mentioned above and additionally α-(Fe,Ni)—martensite, indicating the formation of Cr 2N. This phase reaches its maximum concentration at a depth of approximately 40 nm. Thus, by interaction of different mechanisms, i.e. segregation of Cr and formation of Fe–Ni rich clusters by radiation enhanced diffusion, the α-(Fe,Ni)-phase is formed on the expense of the other phases. Hardness was measured as Vickers hardness and current-density/potential-curves in a 5 N H 2SO 4-solution were performed as corrosion tests. The influence of the different surface phases on mechanical and chemical properties are discussed.

Mass Transport Driven by Atomic Relocations Under High Flux Ion Irradiation at Elevated Temperatures

Experiments with a nitrogen torch at atmospheric pressure have been performed in order to identify the role of surface processes in the mechanism of nitrogen transport during nitriding of stainless steel AISI 304. The unusually thick (~175 μm) layers of supersaturated nitrogen solid solution fcc phase were obtained after treatment for 10 min at 450°C. A radically different structure occurs in specimens treated at 550°C. Scanning electron microscopy (SEM) surface and cross-sectional micrographs reveal that the surface topography indicates the degree of modification occurring in the nitrided layer. Surface vacancies generated by surface instabilities move deeply into the bulk at elevated temperatures and form a highly defected layer with pores and microcracks. The transport of nitrogen in austenitic stainless steel is driven by the flux es of matrix atoms directed to stabilise surface instabilities. Nitrogen depth profiles simulated on the basis of the model with the surface atom relocation process and an activation energy of 1·1-1·5 eV, and including balanced flux es of atoms in the bulk for relax ation of surface energy are in quantitative agreement with ex perimental results.

Effect of ion implantation on corrosion resistance and high temperature oxidation resistance of Ti deposited 316 stainless steel

Corrosion and oxidation studies of Ti coated on 316 SS have been performed after 30 keV N 2 + ion implantation. Thirty nanometer of Ti was coated on 316 SS by thermal evaporation method. The implantations were carried out at doses ranging from 1×10 16 to 5×10 17 ions/cm 2. Glancing angle X-ray diffraction was used to check nitride formation after ion implantation. RBS and secondary ion mass spectroscopy were employed to find concentration and depth profile of Ti and nitrogen. The aqueous corrosion studies were carried out in 0.5 N H 2SO 4 and 0.5 N HNO 3 solutions. The oxidation resistance studies were carried out in air at 800 °C using continuous and discontinuous method. The oxidation and corrosion resistance increase after nitrogen implantation. The oxidation resistance is maximum at a dose of 5×10 16 ions/cm 2. It is also observed that the dose affects the initial period of nucleation. In case of aqueous corrosion it was found that corrosion resistance has improved substantially with respect to the untreated substrates. Saturation in corrosion improvement is noticed at higher doses.

Chemical bonding of nitrogen in low energy high flux implanted austenitic stainless steel

AISI 304L austenitic stainless steel was implanted at 400 °C with 1.2 keV nitrogen ions using a high beam current density of 1 mA/cm2. The nitrogen depth profile, structure, and chemical composition in the modified surface layer were determined by nuclear reaction analysis (NRA) and x-ray diffraction (XRD). The chemical bonding of Fe, Cr atoms with nitrogen was investigated by x-ray photoelectron spectroscopy (XPS). For a treatment time of 1 h, the formation of a thick nitrided layer of about 3.5 μm with a high nitrogen content (∼20 at. %) is observed by NRA. The nitrogen depth profile is characterized by a nearly flat shape over a thickness of 2.5 μm followed by an abrupt decrease. XRD spectra show the formation in the nitrided layer of a phase usually called expanded austenite γN, which corresponds fairly well with a nitrogen solid solution of the fcc structure containing a high density of stacking faults. The XPS study of the Cr 2p3/2, Fe 2p3/2, and N 1s binding states indicate clearly the preferential bonding of chromium with nitrogen with a binding energy of about 1 eV. This value, which is lower than the expected one for chromium nitride CrN, would be characteristic of the binding energy of nitrogen with Cr in the γN expanded austenite phase. Moreover, it has been found that the atomic ratio N/Cr in the nitride layer deduced from both NRA and XPS is very close to 1. These experimental results support the specific role of chromium in the mechanisms of atomic transport of nitrogen over long distances at moderate temperature in austenitic stainless steels.

Nitriding of an austenitic stainless steel in plasma torch at atmospheric pressure

Experiments with a nitrogen torch at atmospheric pressure have been performed in order to identify the role of surface processes in the mechanism of nitrogen transport during nitriding of stainless steel AISI 304. Unusually thick (∼175 μm) layers of supersaturated N solid-solution f.c.c. phase have been obtained for 10 min at 450 °C. Samples treated at 550 °C have a radically different structure. The scanning electron microscopy (SEM) surface and cross-sectional micrographs reveal that surface topography is an indicator of the degree of modification occurring in the nitrided layer. Surface vacancies generated by surface instabilities move deeply into the bulk at elevated temperatures and form a highly defective layer with pores and microcracks. The transport of nitrogen in austenitic stainless steel is driven by the fluxes of matrix atoms directed to stabilize surface instabilities. Nitrogen depth profiles simulated on the basis of a model with a surface-atom relocation process and activation energy of 1.15 eV, and including balanced fluxes of atoms in the bulk for relaxation of surface energy, are in qualitative agreement with experimental results.

Nitrogen and hydrogen depth profiling with MaRPel

We report on the first measurements carried out with the new accelerator facility MaRPel (Materials Research Pelletron) in Göttingen. Samples of pure tantalum were irradiated with a pulsed nanosecond excimer laser in the UV wavelength range in a nitrogen atmosphere. The laser-nitrided tantalum samples were perfectly suited for testing a low-level set-up for resonant nuclear reactions analysis. The samples also allowed for a recalibration of the mass separator magnet, which was necessary after moving the 3 MV NEC-Pelletron from the Max Planck Institut für Kernphysik in Heidelberg to the University of Göttingen. In addition, it is shown that the low-level set-up allows high resolution hydrogen depth profiling.

Defects and Magnetic Phases at Nitrided Iron Surfaces

The influence of imperfect and annealed structure on the phase composition of α-iron after nitrogen implantation is studied. The surface phase composition of α-iron specimens is investigated by the glancing angle X-ray diffraction and Conversion Electron Mossbauer Spectroscopy (CEMS). The nitrogen depth profile is obtained by Rutherford backscattering spectroscopy. The high density of surface defects induced by grinding and polishing facilitates the formation of a predominantly paramagnetic e-Fe 2 N phase during and after nitrogen implantation. The structure of the annealed surface does not trap such a high content of implanted nitrogen atoms and the ferromagnetic Fe-N phases with lower nitrogen concentrations are detected as dominant in the surface layers. Similar results are obtained for the mechanically treated surface after electrochemical etching that removes partly the defect structure.

A Study of the Decomposition of GaN during Annealing over a Wide Range of Temperatures

ABSTRACTAnnealing experiments were carried out on gallium nitride layers, which were grown on sapphire through Metal Organic Chemical Vapor Deposition (MOCVD). Rutherford Backscattering Spectrometry (RBS) was performed on as-grown and annealed GaN samples using a 2 MeV proton beam to study the stoichiometric changes in the near-surface region (750 nm) with depth resolution better than 50 nm. No decomposition was measured for temperatures o up to 800 °C. Decomposition in the near-surface region increased rapidly with a further increase o of temperature, resulting in a near-amorphous surface-region for annealing at 1100 °C. The depth profiles of nitrogen and incorporated oxygen in the decomposed GaN are extracted from the nanoscale RBS data for different annealing temperatures. The surface roughness of the GaN layers observed by atomic force microscopy (AFM) is consistent with RBS decomposition measurements. We describe the range of annealing conditions under which negligible decomposition of GaN is observed, which is important in assessing optimal thermal processing conditions of GaN for both conventional and nanoscale optoelectronic devices.