ICMCTF2005 Session BP: Symposium B Poster Session

Electron beam (e-beam) ionization is an efficient method of producing plasma and is readily scalable to large area (square meters). Furthermore, ionization and dissociation by e-beams is not limited to low energy threshold processes as in conventional materials processing plasmas, and therefore provides wider chemistry domains than typical pulsed or continuous plasma deposition tools. NRL has developed multi-kilovolt e-beam sheets to generate similarly sized (up to 1/2 square meter) high density plasmas (10¹⁰-10¹² cm^-3) that possess low electron temperatures (<0.5 eV) and allow unique chemistries due to the e-beam plasma generation mechanism. These systems have been used to deposit films of silicon dioxide for use in flexible electronics as the primary dielectric and barrier layer, from mixtures of TEOS and oxygen precursors in multiple configurations. The film electrical and optical properties with respect to gas mixtures, substrate type, temperature and ion energy with the characterization of the plasma environment (using in situ time-resolved electrostatic probes and mass spectrometry) will be presented. Subsequent steps to optimize and more fully understand the deposition process in these plasma systems will also be discussed. The deposition of additional films pertinent to flexible electronics (amorphous Si and SiN_x) will also be presented and discussed.

Work supported by the Office of Naval Research.

Composite transition metal boride (and also nitride or carbide) based materials with special properties show great promise for a wide range of tribology applications. For example, the wear and friction properties of PVD hard films can in some cases be enhanced significantly by introducing some amount of solid lubricant material into the coating. Complex multi-element composite materials are able to combine the advantages of several constituent phases to survive under varying wear conditions - by exhibiting "adaptive" or "chameleon" properties.

Sputtering of multi-component coatings from blended powder targets offers an easy, rapid and cost-effective way of varying the elemental composition of the target and hence of the deposited film. It also solves some overheating problems associated with the sputtering of brittle ceramic targets. Furthermore, pulsed magnetron sputtering can have a significant beneficial impact on PVD coating structure and properties.

In this study, high quality multicomponent chromium boride based coatings, with Ti, C and MoS_X alloying additions, were deposited by pulsed magnetron sputtering of blended powder targets. The results of investigations of structure, composition, mechanical & tribological properties and corrosion resistance of these coatings are presented.

The main objective of this study is to improve the formation of a viable interface for CVD diamond film growth on steel substrates by the use of a boride interlayer between the steel substrate and diamond coating.

Boronizing is a thermochemical diffusion surface treatment in which boron atoms diffuse into the surface of the samples to form iron boride (FeB and Fe₂B). The substrate material used for this study was AISI D2 tool steels and the boronizing was carried out using a salt bath consiting of boron carbide (B₄C), alumina (Al₂O₃), potassium fluoborate (KBF₄) and silicon carbide (SiC). Borinding treatments were performed at 850°C and 950°C followed by an annealing procedure in air. Boron concentration, boronizing time, annealing time and temperature were varied to optimize the formation of a Fe₂B interlayer.

Diamond deposition was performed in runs of 4 hours using a conventional hot-filament CVD reactor. Diamond deposition was carried out utilizing a fixed CH₄ / CF₄ / H₂ volume ratio of 1.5 / 0.5 / 98.5%, respectively, at a pressure of 50 torr. Substrate temperature was varied to enhance diamond film adhesion on the Fe₂B interface. Runs with a higher temperature (around 850°C) during the nucleation process and a lower temperature (around 650°C) during growth resulted in a higher adherence. The CF₄ addition enhances the growth conditions at the lower growth temperature.

Cross-sectional metallographic analysis was performed to study the borided interface thickness and structure. The diamond films were characterized by X-Ray diffraction (X-RD), scanning electron microscopy (SEM), micro-Raman spectroscopy (Raman) and the adherences were analyzed by Rockwell C indentation.

Nanostructured coatings are of increasing importance for mechanical applications due to the rather high level of hardness and low brittleness associated with their nano-sized microstructure. In particular, titanium- or zirconium-based coatings are almost studied in the literature. Among the techniques available to produce such films, reactive arc deposition is suitable because of its efficiency owing to the deposition rate and adhesion of the coatings on their substrates.

Reactive vacuum arc deposition has then been used for deposition of zirconium based boronitride films from Zr-ZrB₂ composite targets inserted into a conventional titanium multiarc target and evaporated in argon-nitrogen reactive mixtures. After a short description of the deposition device and of the target geometry, we first compare the reactive evaporation mechanisms depending on the fraction of zirconium diboride into the target. The second part of this paper deals with the chemical and structural characteristics of in relation with their deposition conditions. Finally, surface energy and mechanical properties of the coatings are investigated as a function of their composition and structure and are discussed in relation with potential applications in the field of protection of steel parts against aggressive environment such as in the presence of molten glass.

It is well known that incorporation of Al atoms into TiN and CrN films shows the phase transitions from the cubic to hexagonal structures at certain Al amount. In our previous works, the micro-hardness of cubic-type Ti_xAl_yN changed from 20 GPa to 32 GPa, which corresponded to changes in lattice parameters. The Cr_xAl_yN films showed the maximum hardness of 27 GPa with Y=0.6 similar to those of Ti_xAl_yN films.

The Cr_xAl_yB_zN films were synthesized by the cahodic arc method using Cr-Al-B alloy targets with differing Z values from 0 to 0.2 and investigated the changes in crystal structures, lattice parameter and micro-hardness. The microhardness of Cr_xAl_yB_zN films increased from 27(Z=0) to 33 GPa (Z=0.1) corresponding to the decrease in lattice parameter. Further, the XRD measurement showed that the Cr_xAl_yB_zN films had the cubic structure without hexagonal segregations up to 900°C.

In this study, microhardenss, microstructures and thermal stability of Cr_xAl_yB_ZN were characterized based on X-ray and transmission electron microscopy analysis.

Time-resolved investigation of an asymmetric bipolar pulsed magnetron discharge for the deposition of MgO films has been done by Langmuir double probe measurements and optical emission spectroscopy (OES).

The magnetron was equipped with a 4" circular Mg target and operated in an Ar/O₂ atmosphere. The discharge was powered by a Pinnacle Plus unit with typically 100 W. Pulse frequencies of 100 - 300 kHz were used. All measurements were made on the axis of symmetry of the discharge.

The Langmuir probe measurements show a typical temporal evolution of the charge carrier density. Immediately after switching the discharge "on" two sharp maxima are observed, before a transition into a stationary state occurs. In dependence on the pulse parameters the height and the temporal position of the extrema are changed. After switching the pulse "off" the charge carrier density decays exponentially with a time constant of 0.5 micros which was practically independent of the pulse parameters.

With OES the most intensive Ar and Mg lines have been measured. Their temporal evolution I(t) was similar to that of the charge carrier density. In particular the exponential decay and the second maximum were confirmed while the first maximum appeared only very weak. A careful comparison showed that different Ar lines may differ significantly in their temporal behaviour. Considering the excitation mechanisms it comes out that the Ar line at 750.4 nm is best suited for a comparison to the charge carrier density. The other lines show broader I(t) traces and larger decay constants. Another interesting feature occurs after switching from "on" to "off", where an additional maximum is observed which may be caused by high energy electrons due to the fast switching potential.

The neutral gas temperature is an important process parameter in low pressure plasmas. On the one hand, chemical reactions may be thermally driven. On the other hand the contact of the process gas with the substrate cannot be avoided so that in particular for thermally sensitive substrates the knowledge of the gas temperature is essential.

The neutral gas temperature is often determined by evaluating the shape of rotational spectra of diatomic molecules present in the plasma. In low pressure plasmas the light emission takes place before the thermalisation of the rotational level population of the electronically excited state. Thus, the observed population of the rotational levels is determined by the excitation mechanisms and the rotational level population of the electronic states which the upper electronic state of the chosen molecule band is populated from.

We used the first negative system of the nitrogen molecule ion for temperature determination in different kinds of low pressure discharges. The spectra were evaluated by fitting. We will show that the spectra can be fitted best assuming that they are composed of two contributions representing two populations of the nitrogen molecule ion with distinct different rotational temperatures. Thus, at least two excitation channels of the upper electronic state have to be considered. One excitation channel is the electron impact on the ground state of the neutral nitrogen molecule connected with ionisation and excitation. In this process the rotational level distribution remains essentially unchanged and the rotational temperature of the so generated molecule ions reflects the gas temperature. The second excitation channel is the impact of nitrogen molecules in high vibrational states on the ground state nitrogen molecule ion. This is connected with rotational excitation and thus the rotational temperature of the so generated excited molecule ions is much higher than the gas temperature.

Using a time-resolved Langmuir probe ², the temporal evolution of electron energy distribution function and plasma parameters is investigated under various frequencies from 5 kHz to 50 kHz and various duty cycles from 10 to 90% at 20 kHz in unipolar pulsed dc magnetron discharge. With the substrate grounded, the measured electron distributions show a bi-Maxwellian distribution with high-energy electron tail during the pulse-on irrespective of the driving frequency. In the after glow after the pulse-off, the decay rates of the electron temperature and the electron density are characterized by the two characteristic decay times of the fast decay of order of few μmsec and the slower decay of order of few tens μmsec. On the other hand, the existence of the high-energy primary electrons of which energy reaches to a third of the cathode voltage is observable during few μmsec after the pulse-on. These phenomena will be explained by considering the electron transport under the magnetic field.

In the constant-voltage mode at 20 kHz, the averaged electron temperature in the pulse-on period increases as the duty cycle is reduced, 4.1 eV at 10% and 2.7 eV at 90 %. However, the averaged electron temperature weakly depends on the driving frequency in the present frequency range. From the detailed measurements of the electron energy distribution functions, the increased electron temperature is caused by the decrease of low-energy electrons and the increase of energetic electrons with decreasing the duty ratio.

¹ The authors would like to thank the Engineering and Research Center (grant no. R11-2000-086-03007-0) for their financial support of this research.

² The company Plasmart Ltd. is specially acknowledged for the technical support and the supply of probe acquisition system.

The formation of superhad nanocomposites nc-TiN/a-Si₃N₄ with a high thermal stability and oxidation resistance was explained by strong, thermodynamically driven phase segregation¹. Later on, it was suggested that this segregation is of spinodal nature and, therefore, results in the formation of a stable nanostructure by self-organization (e.g.²), but this has been questioned by several researchers. In this paper we present thermodynamic calculations which clearly show that the second derivative of the free energy of the formation of the mixed Ti_xSi_1-xN phase with the composition x is negative, i. e. it meets the condition of the spinodal nature of the phase segregation within the whole range of nitrogen pressure that was used during the deposition of these nanocomposites by Veprek et al. For the calculation, we proposed an empirical thermodynamic model based on the sublattice model³ and then the model was used for interpretation of the phase segregation in the ternary Ti-Si-N systems. The results show that the phase segregation in the ternary Ti-Si-N systems is of typical spinodal nature at the nitrogen pressure typically used during the deposition of these coatings (i.e. about 1mbar for plasma CVD and 0.002 mbar for reactive sputtering) and deposition temperature of 550°C (823 K). Only at much lower nitrogen pressure and much higher temperature, the spinodal decomposition can not occur. The important role of the nitrogen activity during the deposition will be illustrated by several examples. The theoretical calculations are supported by experimental results available so far.

¹S.Veprek and S.Reiprich, Thin Solid Films 268(1995)64

² S.Veprek and A.S.Argon, J. Vac. Sci. Technol. B 20(2002)650

³ M Hillert, Phase equilibria, Phase phase Transformations: Their Thermodynamic Basis, Cambridge University Press, United Kingdom, 1998

Silicon nitride thin films can be deposited by several chemical and physical methods. In this work we study the Chemical Vapor Deposition in Fluidized Bed Reactors at Atmospheric Pressure (AP/FBR-CVD) for the deposition of silicon nitride coatings on AISI316 steel. The unique thermal and transport properties of the fluidized bed minimize some of the limitations of conventional CVD methods, such as inhomogeneities that lead to variations in thickness, and sometimes difficulty in mantaining the substrate at uniform temperature. Additionally, the particles of the bed can be activated through alternating reaction steps to promote the formation in situ of highly reactive silicon-containing species and thus increase the deposition rate. It is suggested that these thin and very homogeneous FBR-CVD ceramic films could be used as a part of a multilayer strategy to build more sofisticated coatings.

The coatings were obtained by the reaction of silicon tetrachloride and ammonia in a reducing atmosphere at temperatures in the range 700 °C to 800 °C. The composition and structure of the coating were determined by means of X ray Diffraction (XRD), Scanning Electronic Microscopy (SEM), Energy Dispersive X-ray analysis (EDX), Infrared Spectroscopy (FT-IR), Glow Discharge Optical Emission Spectroscopy (GD-OES) and X-ray Photoelectronic Spectroscopy (XPS). Silicon nitride films were amorphous and substoichiometric. We found that the deposition temperature has a great influence in their morphology and mechanical properties. An underlayer containing chromium nitride was found at the coating-steel interface.