ICMCTF2000 Session B4: Emerging Technologies and Critical issues in Vapor Deposition

Plasma nitriding at temperatures at or below 450°C offers the opportunity to enhance the mechanical properties of austenitic stainless steels without affecting the excellent corrosion resistance of this material. In this case a metastable phase named S-Phase or saturated austenite with outstanding properties is formed. Nitrogen remains in solid solution inside this new phase instead of removing Chromium from the austenitic structure by precipitation of CrN. papragraph In this paper, we report on a series of experiments designed to study the influence of plasma nitriding on the mechanical properties of austenitic stainless steel. Firstly the formation and the microstructure of the modified layer will be highlighted followed by the results of hardness measurement, adhesion testing, wear resistance and fatigue life. The modified surface has been analysed directly after plasma nitriding as well as in the depth profile.

The hardness after plasma nitriding is increased up to 1400 HK0.01, that is a factor of five higher compared to the untreated material (280 HK0.01). The adhesion is examined by Rockwell indentation and scratch test. No delamination of the treated layer could be observed. The wear rate after plasma nitriding is significantly reduced compared to the untreated material. Plasma nitriding produces compressive stress inside the modified layer which can be easily derived by the bending of one side treated thin metal foils. The compressive stress is examined by XRD-residual stress measurement. The treatment influences the fatigue life which can be raised by a factor of ten at low stress level (230 MPa).

Magnetron sputtering is an established vacuum deposition process which has some limitations with respect to target utilisation and sputtering from thick ferrous targets. It will be shown in this paper that these limitations can be overcome by using the helicon-assisted sputtering process.

The helicon is an RF generated plasma wave resulting from the interaction between a radio-frequency antenna with a magnetic field. Efficient coupling of the plasma wave with the plasma electrode results in a high ion density plasma. The resulting plasma stream can then be steered using electromagnets towards the target.

This paper will describe the process route and the potential advantages helicon-assisted sputtering can bring to a number of applications.

Nowadays, almost all products are deliberately coated, however, at least they have native oxides on the surface. In particular, sprayed coatings, ECD- , PVD-, and CVD-coatings are applied to manifold application areas, substrate materials and service conditions. The performance of such coating/substrate systems refers at least to one functional feature, i. e. a mechanical, optical, electro-magnetic, or chemical property that has to be guaranteed under service conditions over life time. In addition and depending on the application, sufficient adhesion and required thickness uniformity have to be realised. Moreover, deposition technology has to meet various substrate requirements (e.g. deposition temperature). Coating properties may strongly vary for the same deposition process if the substrate material is changed.

At any rate, localised coating defects have to be avoided prior to and as a result of service. Degradation effects, such as thermally (diffusion), chemically (corrosion) or electrically (migration) induced localised failure, inhomogeneities of the coating (microstructure, stoichiometry, density, thickness), mechanical (tensile or compressive film stress) and/or chemical (solid state reactions) mismatch between coating and substrate material, surface or interface properties (roughness, waviness), coating imperfections (inclusions, scratches) may cause coating damage for a given application, i.e. a loss of functionality.

Besides an overview on degradation mechanisms, typical failure pattern are presented and examples of damage analysis are discussed for the above mentioned coating systems. The major points of effort, i.e. material selection and combination, the choice of process parameters, the design of the coating/substrate system may strongly differ depending on the specific application and the coating technology. Prevention of coating failure and damage prediction are still underestimated, in particular as long as no failure or damage has been occurred. Reliability issues and derived quality assurance measures are discussed.

Reactive magnetron sputtering is a powerful and widely used technique for deposition of metallurgical coatings or ceramic films of high physical quality. However, in industrial plants, an unstable behaviour of the process is often observed due to the largeness of the reactor walls owing to the suction capacity of the pumping system. Several authors have proposed more or less sophisticated means to overcome the negative effect of this instability (i.e. a low deposition rate of the stoichiometric film), by increasing the pumping speed or by decreasing the area of the reactor walls, by regulating the convenient electrical parameter of the discharge or by monitoring the process via a closed loop control system.

In previous papers, we proposed a new way to increase the deposition rate of stoichiometric ceramic films by modulating at low frequency the discharge current. In this paper, we present the main trends of the modulation characteristics allowing a high rate deposition of stoichiometric films and we discuss the mechanisms at the origin of this high rate deposition of stoichiometric ceramic films.

In a first part, the temporal evolutions of the sputtered metal flux, measured via optical emission spectroscopy, and of the reactive gas partial pressure, measured by means of an absolute gauge, are presented. Then the effect of a square wave low-frequency modulation of the discharge current is discussed, in particular, the characteristic times of target periodic poisoning and cleaning which ensure the high rate deposition of stoichiometric ceramic films even if a slight under stoichiometric layer is deposited at the end of the step high of the modulation. Finally, we conclude on the different means of process stabilisation depending on the reactivity of the considered metal-metalloid system.

Reactive magnetron sputtering is a widely used technique for deposition of high quality ceramic films among which titanium nitride and titanium oxide occupy a key position. However, due to the large area of the chamber walls of industrial plants associated with a too low pumping speed, an unstable behaviour is often observed which prohibits the synthesis of stoichiometric ceramic films with a high deposition rate.

In a previous paper, we pointed out the beneficial effect of increasing the target temperature in so called unstable sputtering conditions, which allowed the deposition of over stoichiometric titanium nitride films in elemental sputtering mode, whereas with a cooled target it was not possible to reach the stoichiometry of the film. However, because of thermal contact problems, the discharge voltage associated with hot target sputtering (HTS) was significantly higher than that related to the cold target sputtering (CTS), implying that the average sputtering yield was significantly higher in HTS than in CTS.

In this paper, we investigate more in detail the intrinsic effect of the target temperature during sputtering of a titanium target in argon-nitrogen or argon-oxygen reactive mixtures.

In a first part, the design of a new target dedicated to HTS is presented. This new design yields a discharge voltage in HTS comparable to that of CTS for a same discharge current. Moreover, it allows an excellent reproducibility of the target temperature, measured along its radius via an infrared pyrometer, as a function of the discharge current. In a second part, we discuss the effect of an increase of the sputtering rate, either by increasing the discharge current or the discharge voltage, on the nitrogen or oxygen content of the coatings before runaway toward the reactive sputtering mode. Finally, the effect of the target temperature is presented and discussed owing to the diffusion kinetics of nitrogen or oxygen into the bulk target.

A.S.Zolkin. Metal vapor Sources for Scientific Research and Thin Film Technology. A Review. J.Vac.Sci. and Tech. A15