ICMCTF2006 Session B3: CVD Coatings and Technologies

Transition metal diborides are metallic ceramic materials that have melting temperatures ~ 3000°C, hardnesses > 20 GPa and low electrical resistivities. However, they have been largely overlooked in thin film science and technology because of difficulties associated with conventional deposition methods. We recently developed a successful thermal CVD approach using impurity-free single source precursors; the resulting diboride coatings are extremely conformal, highly conductive, and super hard.

We discuss the growth of HfB₂ films from the precursor Hf(BH₄)₄. By employing line-of-sight mass spectrometry, and by analyzing the coating profiles on trench structures, we show that the growth kinetics follow a Langmuirian surface reaction mechanism. Using low growth temperature and high precursor pressure, 100% step coverage is achieved on trench structures with an aspect ratio of 20:1 (depth:width). At growth temperatures of 200-300°C, the films are X-ray amorphous with resistivity as low as 400µΩ-cm. The nanoindentation hardness of the as-deposited film is 20 GPa; annealing to 700°C for 30 minutes transforms the film into a nanocrystalline phase and raises the hardness to 40 GPa. The above attributes make the CVD growth of HfB₂ films attractive for applications such as diffusion barriers for Cu metallization in microelectronics, and wear-resistant hard coatings.

We will also report methods to tailor the film mechanical properties and to achieve super-conformality with the addition of a secondary flux. For example, we add atomic nitrogen from a remote plasma source to obtain ternary Hf-B-N thin films, which have lower hardness and elastic modulus than HfB₂ film. By modulating the N₂ flow, we grow binary-ternary multilayer films with precise control of layer thickness and periodicity. The hardness and elastic modulus of these nanolaminate structure can be fine tuned, e.g. to preserve high hardness while matching the elastic modulus of the substrate.

Titanium nitride (TiN) coatings have been extensively used for tribological applications, corrosion protection, decoration, biomaterials, etc. However, for certain applications the properties of single TiN coatings must be improved; generally the hardness and the adhesion to the substrate need to be increased and the coating porosity reduced. Among other strategies, this can be done by the synthesis of multilayered coatings in which the TiN layers alternate with metal layers, or by the formation of nanocomposite coatings where a phase of TiN is combined with other nitride phases (for example, AlN or SiN_x).

In this work we have studied the synthesis of TiN/SiN_x multilayer coatings by Chemical Vapor Deposition in a Fluidized Bed Reactor at Atmospheric Pressure (AP/FBR-CVD). It is well known that the good heat and mass transfer of the fluidized bed system yield very homogeneous coatings. Moreover, deposition of two immiscible phases such as TiN and SiN_x is a good strategy to overcome limitations related to interdiffusion between the several layers. The coatings were deposited from TiCl₄, SiCl₄ and NH₃ by AP/FBR CVD at 850°C. The relations between deposition conditions, coating substructure and mechanical properties are presented.

Multilayer coatings of TiC, TiN and alumina are often formed on cemented carbide cutting tools by chemical vapour deposition (CVD) in order to increase the wear resistance of the tools. The thickness of the coatings is typically in the range of 3-10 µm. Two of the alumina polymorphs, alpha and kappa, are most commonly used in the multilayers. There is also a third alumina phase that is occasionally present in CVD coatings, gamma.

This paper deals with the detailed microstructure of kappa alumina multilayers separated by thin intermediate TiN layers. Deposition of the experimental coatings was carried out in a computer-controlled hot-wall CVD reactor. Commercial cemented carbide inserts with a composition of 85.5 % WC and 5.5 % Co and 9 % cubic carbides were used as substrates. A 3 µm thick intermediate layer of Ti(C,N) was deposited directly onto the cemented carbide substrate, followed by a κ-bonding layer, which promotes nucleation and growth of the kappa phase. Then kappa alumina and TiN layers were deposited in sequence until the coatings were composed of 8 individual kappa alumina layers.

The coatings were pre-examined by scanning electron microscopy (SEM) using a Leo Ultra 55 FEG. The main part of this work was carried out by transmission electron microscopy (TEM) using a Philips CM 200 FEG working at 200 kV. Cross-section TEM specimens of thin foils were prepared using a combined FIB (Focused Ion Beam)/SEM and standard ion milling. These methods produce electron transparency in all layers and interfaces, allowing all parts of the multilayer coating to be viewed simultaneously. The detailed microstructure, such as defects, twinning, grain size, porosity and epitaxy will be described. Special emphasis is put on the alumina/TiN interfacial structures and the occasional presence of porosity and gamma alumina. Comparisons with first-principles calculations of the interfacial structure will be made.

There are only few studies on mechanical properties of CVD Al₂O₃ coatings. Usually CVD α-Al₂O₃ and κ-Al₂O₃ coatings have been dealt with. However, the earlier studies have been performed on α-Al₂O₃, which was formed as a result of the κ - α phase transformation. This kind of α-Al₂O₃ layers were typically composed of relatively large, equiaxed grains with a high defect density. Modern CVD α-Al₂O₃ layers produced using controlled nucleation are typically composed of defect-free columnar grains with enhanced mechanical properties. In addition, the growth textures of modern Al₂O₃ coatings can be predetermined by optimised nucleation and growth processes. The following preferred growth directions have been reported: <012>, <104>, <110> and <300>. The aim of this study was to compare the mechanical properties of nucleated α-Al₂O₃ with κ-Al₂O₃ and transformed κ-Al₂O₃. For this purpose the following Al₂O₃ layers were deposited:

a) κ-Al₂O₃

b) α-Al₂O₃ with random texture (transformed κ-Al₂O₃)

c) Nucleated α-Al2O3 with <012> texture

d) Nucleated α-Al₂O₃ with <104> texture

e) Nucleated α-Al₂O₃ with <001> texture.

The experimental coatings were deposited to thickness of about 8 µm and were investigated using XRD and SEM before hardness measurements were carried out. The hardness and modulus of the Al₂O₃layers were measured using nanoindentation technique. The indentations were performed on polished tapered cross-sections.

All the nucleated α-Al₂O₃ layers showed superior mechanical properties (hardness and modulus) as compared with κ-Al₂O₃ and α-Al₂O₃, which was formed as a result of the phase transformation. The <001> textured α-Al₂O₃ coating exhibited the highest hardness and modulus.

The aim of this study is to investigate the feasibility of modifying aluminium coating with small additions of hafnium (Hf) on ferritic steel (P-91, P-92) at temperatures below 600°C by Chemical Vapour Deposition by means of fluidized bed reactor (CVD-FBR), to increase the coating properties and their high temperature durability in oxidising and corrosive environments.

Thermochemical calculations were made before the experimental study to investigate the conditions for the formation of gas precursor for deposition. This technique is based onthe reaction of the powder of aluminium (Al) and few amount of Hf with chloride of hydrogen (HCl) for the formation of gas precursor necessary for deposition of Al-coatings. These coatings were characterized by different techniques such as X-ray diffraction (XRD), optical microscopy (OM), Scanning Electron Microscopy (SEM) and Energy Dispersion Spectroscopy (EDS). The preliminary results obtained are discussed in this work.

Catalytic chemically vapour deposited (Cat-CVD) is now becoming a mature technique in the field of thin silicon (Si) film deposition. In Cat-CVD, Si deposition takes place upon catalytic decomposition of the reactant gases at the surface of a hot filament heated at temperatures in the range of 1500-2000°C.

In this study, we present mechanical stress evolution of Cat-CVD polycrystalline Si films during post-deposition thermal cycling. Polycrystalline Si films were deposited into oxidized monocrystalline silicon wafers by Cat-CVD in an ultra-high vacuum multichamber deposition system by optimizing the gas pressure, flow rate and filament temperature. Mechanical stresses in films were evaluated from the measured curvature change using the Stoney equation. In situ wafer curvature was measured between room temperature and 530°C using a laser interferometer with an external thermo-chamber.

The overall mechanical stress state is determined by the superposition of two primary effects. The intrinsic (athermal) stress is generated during the deposition and is strongly related to the process parameters. The thermal stress only results from the temperature change between deposition and characterization and the difference in thermal expansion coefficients of the film and the wafer. It is found that the Cat-CVD polycrystalline Si film stresses strongly depend on the processing history and thermal stresses annihilation are identified as the major mechanisms to control the mechanical behaviour of the films. The dependence of residual stress on temperature is non-linear with significant hysteresis. This hysteresis reduced significantly during the subsequent thermal cycle. Thermal cycling is shown to result in major plastic deformation of the film and a switch from a compressive to a tensile state of stress; both athermal and thermal components of the net stress alter in different ways during cycling.