A. Straboni

Zirconium nitrides deposited by dual ion beam sputtering : physical properties and growth modelling

Abstract Zirconium nitrides reveal interesting optical and electrical properties which highly depend on the nitrogen stoichiometry. Indeed, the material exhibits a transition from the stable metallic ZrN (optical index for bulk at 633 nm: N=0.5−i3.2) to the metastable semi-transparent insulating Zr3N4 (N=3.2−i0.4). This work deals with the elaboration of homogeneous ZrN-like and Zr3N4-like coatings. These have been prepared using reactive Dual Ion Beam Sputtering (DIBS) using a Zr target and N2 or N2+Ar reactive gas. The influence of different elaboration parameters (ion energy, gas composition of the reactive beam and substrate temperature) on the nitrides composition and on their optical and electrical properties was particularly studied. A model was proposed to explain the influence of energy and temperature on the nitrogen composition. The nitrogen stoichiometry was shown to be controlled by a competitive mechanism between implantation of excess nitrogen amount in the subsurface and their elimination by exodiffusion. The first phenomenon is mainly controlled by the ion energy whereas the second one is enhanced by a high temperature and a high irradiation defects density. Therefore, the Zr3N4-like nitrides were obtained with low temperature and high energy (200 eV) conditions whereas high temperature and low energy led to ZrN-like materials.

Nitrogen and oxygen transport and reactions during plasma nitridation of zirconium thin films

Zirconium nitride (ZrN) is a refractory material with good mechanical and thermal properties. It is therefore a good candidate for hard surface treatment at high temperature. In this work, we report the growth and characterization of ZrN by plasma assisted thermal nitridation of zirconium films in a NH3 atmosphere. The process was monitored by in situ monochromatic ellipsometry and the nitrides grown were profiled and analyzed by Auger electron spectroscopy. By using temperatures in the 700–800 °C range, the material obtained is quite close to ZrN, but, depending on experimental conditions, residual oxygen (impurities) can be easily incorporated by reaction with zirconium. The analysis of the ellipsometric data has shown that the nitridation did not occur by simple growth of nitride on zirconium. Auger profiles confirmed the presence of an oxidized zirconium layer localized between the nitrided surface and the remaining metal. This oxidation was observed to occur preferentially during temperature ramping,...

High temperature plasma based ionic implantation of titanium alloys and silicon

Abstract Plasma based ionic implantation (PBII) of refractory materials is an alternative technique to conventional beam line implantation which appears to be very promising in the field of aeronautics, biomaterials and semiconductor electronics. In order to monitor sample temperature independently of the plasma discharge and of the pulsed high voltage conditions, we have developed a new thermally assisted PBII set-up. The thermally assisted plasma immersion implantation reactor (TAPIIR) which enables plasma implantation in the 0.5–60 keV range at controlled temperature between 200 and 1000 °C. Thermochemical treatments like nitriding of titanium and silicon were studied with a separated control of implantation and diffusion parameters. This paper describes implantations made in TAPIIR at elevated temperatures (500–900 °C) on titanium. The new results are presented and discussed by considering transport mechanisms during implantation at high temperature.

New trends in PBII technology: industrial perspectives and limitations

Abstract The two general specifications required for plasma-based ion implantation are low pressure large size plasmas and high voltage high current pulse generators. Due to the wide ion sheath expansion (up to a few tens of cm), large volumes of plasma are mandatory around the substrate. Multipolar discharges, which produce a peripheral ionization facing the substrate and can be easily scaled up, are well suited to PBII processing and begin to be widely used. However, hot filaments to sustain plasmas of reactive gases in multipolar magnetic field structures must be ruled out in favor of distributed electron cyclotron resonance (DECR) plasma sources. In order to produce the high voltage high current pulses necessary for PBII processing, generators using pulse transformers, where the voltage at the primary is provided by transistor switches and where the energy is stored at a low voltage level, appear particularly well-adapted to fulfill most of the PBII requirements in terms of reliability, compactness, cost and safety. At the industrial level, a very great advantage of PBII over ion beam implantation lies in achieving sequential processing in the same reactor, such as cleaning, etching and deposition prior to, during, or after the implantation process. As examples, thermochemical processing can be performed via PBII with or without external independent heating. More generally, the combination of DECR plasmas and magnetron discharges in the same reactor opens new possibilities for complex treatments such as PBII/CVD (chemical vapor deposition) in DECR plasmas or PBII/PVD (physical vapor deposition) in hybrid DECR-magnetron reactors. However, the transfer of processes from the laboratory to industry is mainly limited to very specific and low energy applications. In fact, mass production using high voltage PBII processing requires production tools still under development. Due to huge secondary electron emission and sheath thickness above 100 kV pulse voltages, large volume reactors (a few cubic meters) on one hand, high power pulse supplies (100 kV–1000 A/100 MW) on the other hand, are mandatory for the rise of PBII at the industrial scale.

A new thermally assisted, plasma based, ionic implantation system of treatment and deposition.

Abstract Plasma based ionic implantation (PBII) is a new alternative to conventional ion implantation to produce near-surface treatments, layer growth or semiconductor doping, with the advantage of being non-directional. Furthermore, it can be used for improving the corrosion, friction and wear resisting properties of materials. This paper describes the development of a thermally assisted plasma immersion implantation reactor (TAPIIR). The system aimed at treating samples in the 0.5–60 keV range, with temperature regulation up to 1000°C. Thermochemical treatments, like the nitriding of steels or aluminium, are studied with a separate implantation and diffusion parameter control.

Production of stable and metastable phases of zirconium nitrides by NH3 plasma nitridation and by double ion beam sputtering of zirconium films

Abstract Nitrided surfaces and nitrogen composition gradients in thin films exhibit interesting mechanical, electrical and optical properties. Metal, semiconductor or oxide surfaces can be transformed into a nitrided compound via interactions of nitrogen species issued from a plasma or an ion beam. The thermal activation is a key factor in both cases to ensure chemical reactions and short/long-range diffusion necessary to allow the growth of stable or metastable structures. In this work, we focus our attention on zirconium nitrides prepared under controlled temperature through reaction and diffusion, in Zr films, of low energy NH x species produced in NH 3 plasma and through the implantation–diffusion of energetic N + 2 ions during Zr deposition by using double ion beam sputtering. Zirconium nitrides show optical and electrical properties that depend on the conditions and on kinetics of the nitrogen take-up; the material exhibits a transition from the stable metallic ZrN to a metastable phase Zr 3 N 4 that appears transparent and insulating. The influence of the energy of the nitriding species and of the temperature on nitride compositions and phases are addressed. A model using coupled implantation and thermal diffusion mechanisms is proposed to explain the phases produced. In relation with the described phenomena, a temperature-controlled plasma-immersion ion-implantation system is proposed for tailoring in-depth stable/metastable ceramic structures such as nitrides, oxides and carbides.

Structure of ZrO2 optical thin films prepared by dual ion beam reactive sputter deposition

Zirconia thin films have been produced at room temperature and at 600 K using a dual ion beam reactive sputter deposition technique. A Zr target was sputtered with 1.2 keV Ar+ ions and the growing films were continuously bombarded with 100 eV O2 + ions. The chemical composition of the films was determined by Rutherford backscattering spectrometry (RBS) and the microstructural state by transmission electron microscopy (TEM). Density and thickness were deduced from X-ray reflectometry experiments. The refractive index was measured by ellipsometry. A maximum refractive index of 2.16 was obtained for films deposited onto heated substrates.

Improved nitridation efficiency and mechanical property of stainless steel surface after N2-H2 plasma nitridation at low temperature

Abstract In the thermochemical process of stainless steel nitridation, the improvement of mechanical properties is governed by the way the nitrogen diffusion profiles extend into the material. The efficiency of conventional thermal or ionic nitridation is substantially reduced at temperatures lower than 550 °C because of low nitrogen diffusivity. The present study shows that improved nitrogen transport can be obtained after nitridation in N 2 –H 2 plasma at floating potential, that is without cathodic bias on the samples. Such cold conditions allow for the iron matrix to be nitrided in a depth range of 10 μm at a temperature as low as 430 °C. Under these conditions, the top surface hardness was increased by a factor of three. Using the nanoindentation technique on cross-sections prepared on plasma nitrided samples, we could determine the profiles of hardness and Young modulus. Unexpectedly, the Young modulus was shown to be almost unaffected by the treatment. Furthermore, hardness profiles correlated very well with the nitrogen chemical profile and the results showed that the interface between the treated layer and the untreated part of the sample was very abrupt. The high level of surface strengthening and the increased nitridation efficiency at low temperature are thought to be a consequence of an activation of surface transport and diffusion using plasma (cold) conditions at floating potential.

Improved nitrogen transport in Fe–C alloys during NH3 plasma nitridation

Abstract In the thermochemical process of Fe–C alloy nitridation, improvements of mechanical properties are governed by the way the nitrogen diffusion profiles extend into the material. Up to now, thermal or ionic nitridation cannot be achieved at temperature lower than 550°C because of the strong lowering of nitrogen diffusivity. The present study shows that improved nitrogen transport can be obtained after nitridation in NH3 plasma without cathodic bias on the samples. Such cold conditions allow the iron matrix to be nitrided in a depth range of 100–400 μm at a temperature as low as 350°C. The top surface hardness was shown to be improved by a factor of 3. This surface strengthening, markedly superior to that obtained with the classical treatments, is a well-known consequence of the temperature lowering that avoids coarsening of nitride microprecipitates. In addition, using this particular process, no growth of compound layer was observed at the surface. This absence of diffusion barrier is clearly beneficial to the improvement of the nitrogen transport. The high nitridation efficiency obtained at low temperature may be explained by an enhanced grain boundary diffusion due to defects generated by hydrogenous radicals produced in the plasma.

Evidence of ω-phase in ion beam sputtered zirconium thin films

Abstract Zirconium thin films were sputtered at room temperature with Ar + ion beam (1200 eV, 80 mA) on different substrates (Si, oxidized Si, amorphous quartz,...). Although classical 2 atom hcp α -phase was expected at normal pressure conditions, present paper proves the occurrence of the high pressure phase of zirconium: three atom hexagonal ω -phase. TEM diffraction patterns and grazing incidence X-ray diffraction spectra clearly show the existence of a phase different from the α -one. The unexpected phase was identified as the ω -one by means of CEEXAFS experiments. Moreover, results show that significant fraction of ω -phase coexists with α -grains which present a considerable increase of the interplanar distances. Substrate curvature induced by the deposit reveals an important compressive stress in the film. This residual stress is explained by conditions of non equilibrium maintained during the growth by the continuous ion bombardment of the subsurface region. Annealings performed at 700°C show that the ω -phase relaxes in the α -phase.