Costas A. Charitidis

The effect of substrate temperature and biasing on the mechanical properties and structure of sputtered titanium nitride thin films

The mechanical properties of titanium nitride (TiNx) thin films have been investigated using depth sensing nanoindentation tests. The effects of substrate temperature (Ts) and of substrate biasing (Vb) on the mechanical properties and the microstructure of the TiNx films were studied. Ts and Vb have strong effect on the films microstructural characteristics such as density, grain size and orientation. It was found that deposition at high Ts and Vb promotes the growth of (002) oriented films with density close to the bulk density of stoichiometric TiN, indicating the absence of voids and the growth of stoichiometric TiN. The film hardness and elastic modulus were measured using the continuous stiffness measurements technique. It was found that there exists a direct correlation between the films mechanical properties and microstructure. The films that exhibit the best mechanical performance are those grown along the (002) orientation and being denser and stoichiometric.

Insights on the deposition mechanism of sputtered amorphous carbon films

Low energy Ar+ ion bombardment (LEIB) during growth of amorphous carbon (a-C) films deposited with magnetron sputtering (MS), results to dense films, rich in sp3 C–C bonds, and exhibit high hardness and compressive stress. We present here a preliminary study of the growth mechanism of a-C films deposited with negative bias voltage (LEIB) in terms of their composition, density and mechanical properties. The experimental results showed that stress and hardness are directly related with the sp3 C–C bonding in the film and described well with the so far proposed models on the formation mechanism of tetrahedral carbon. However, the film density, that is a composite property, was found to depend not only on the sp2 and sp3 content but also on a new, denser than graphite, carbon phase when the Ar+ ion energy is above ∼130 eV.

Combined electrical and mechanical properties of titanium nitride thin films as metallization materials

Titanium nitride (TiNx) thin films, ∼100 nm thick, were deposited on Si(100) substrates by dc reactive magnetron sputtering. The effects of the substrate bias voltage and deposition temperature on their optical, electrical, and mechanical properties have been studied. It was found a strong correlation between the electrical and mechanical properties of the films which are significantly improved with increasing the substrate bias voltage and the deposition temperature. The low resistivity (43 μΩ cm), combined with the high hardness and elastic modulus values, suggest the TiNx as a promising metallization material in Si technology.

Structure, mechanical and tribological properties of sputtered TiAlBN thin films

TiAlBN films were produced by simultaneously sputtering from TiAl and TiB 2 targets in Ar/N 2 mixtures at a substrate temperature of 150 °C. The goal of this investigation was to study the influence of the nitrogen content (0‐40 at.%) on two sets of four coatings with B/Al ratios of approximately 1 and 5. All coatings showed very dense structures and were mainly found to be only partially crystalline. Evidence of Al, AlN, Ti, TiB 2 , TiN and BN bonding was observed in the XPS spectra, and correlation of the compositional and mechanical data showed that film hardness and elastic modulus decreased with the presence of a ‘‘soft’’ phase (Al, Ti or amorphous BN ). Hardness and elastic modulus values of up to 25 and 250 GPa, respectively, were found for TiAl 0.3 B 1.7 coatings. Coatings with a similar elastic modulus to that of the steel substrate showed the best adhesion. Friction coeYcient values between approximately 0.5 and 0.7 were recorded in sliding wear experiments against SAE52100 and WC/6%Co balls. Although friction coeYcients could not be correlated to the amorphous BN contents, it was noted that low combined ball and disc wear rates were found for coatings containing large amounts of BN. Coatings with B/Al ~5 and elastic moduli of 290 GPa showed two orders of magnitude better sliding wear resistance against WC/6%Co balls compared to commercially available TiN.

STRESS RELAXATION AND STABILITY IN THICK AMORPHOUS CARBON FILMS DEPOSITED IN LAYER STRUCTURE

We have developed amorphous carbon films in sequential layers and studied their intrinsic stress, composition, and stress relief mechanisms. The films were deposited by sputtering either thin, with fixed bias voltage Vb or thick, with alternative (positive-layer A/negative-layer B) Vb. In situ spectroscopic ellipsometry and stress studies were used and supported by nanoindentation and x-ray reflectivity measurements. The films deposited with fixed negative (positive) Vb exhibit a thickness-dependent increase (decrease) of compressive stresses up to 200 A and saturate at 6(1) GPa. In thick films deposited with alternative Vb the results demonstrate that: (1) the same modulation in stress values and sp3 content versus film thickness exists, supporting their direct interrelation; (2) the A layers (rich in sp2 sites) promote the stress relaxation of the films during a compositional rearrangement when a B layer is deposited; and (3) this process develops thick and stable films with lower stresses.

Pre-osteoblastic cell response on three-dimensional, organic-inorganic hybrid material scaffolds for bone tissue engineering.

Engineering artificial scaffolds that enhance cell adhesion and growth in three dimensions is essential to successful bone tissue engineering. However, the fabrication of three-dimensional (3D) tissue scaffolds exhibiting complex micro- and nano-features still remains a challenge. Few materials can be structured in three dimensions, and even those have not been characterized for their mechanical and biological properties. In this study, we investigate the suitability of three novel materials of different chemical compositions in bone tissue regeneration: a hybrid material consisting of methacryloxypropyl trimethoxysilane and zirconium propoxide, a hybrid organic-inorganic material of the above containing 50 mole% 2-(dimethylamino)ethyl methacrylate (DMAEMA) and a pure organic material based on polyDMAEMA. More specifically, we study the mechanical properties of the aforementioned materials and evaluate the biological response of pre-osteoblastic cells on them. We also highlight the use of a 3D scaffolding technology, Direct femtosecond Laser Writing (DLW), to fabricate complex structures. Our results show that, while all three investigated materials could potentially be used as biomaterials in tissue engineering, the 50% DMAEMA composite exhibits the best mechanical properties for structure fabrication with DLW and strong biological response.

Elastic properties of hydrogen-free amorphous carbon thin films and their relation with carbon–carbon bonding

Abstract The hardness and elastic modulus of thin, free of hydrogen, amorphous carbon (a-C) films, deposited with sputtering, were measured using the nanoindentation, conventional and continuous stiffness measurements (CSM), depth-sensing techniques. It is shown that the CSM technique is quite efficient in characterizing very thin (∼30 nm) a-C films. The sp2 and sp3 content and thickness of a-C films were determined with spectroscopic ellipsometry. Films deposited with ion bombardment during growth exhibit high hardness and elastic modulus due to the formation of high fraction of sp3 sites supporting an interrelation between elastic properties and C–C bonding. We have also found that a-C films deposited with alternating layers, rich in sp2/rich in sp3 content, exhibit even higher hardness and elastic modulus and above those so far reported for sputter-produced a-C films. The enhancement of the elastic properties is possibly due to either the formation of compositional smooth interfaces between the two type of layers or due to the curved and cross-linked graphite planes in the sp3/sp2 rich interfacial region.

Comprehensive study on the properties of multilayered amorphous carbon films

Abstract Amorphous carbon (a-C) multilayered films consisting of sequential layers rich in sp2 (A) and sp3 (B) content have been developed by magnetron sputtering. We study here the effect of thickness d of the A layer in developing stable thick films with controllable stress and elastic properties. In situ spectroscopic ellipsometry is used to calculate the thickness and the composition of the individual layers. The latter were compared with those obtained by depth profiling X-ray photoelectron spectroscopy, which also provides the different chemical bonding of the multilayers in depth. The stress and hardness of the deposited a-C films were found to be related to the thickness of the Aj layers and the relative ratio dAj/dB of thicknesses. The possible mechanisms for the stress control, stability and enhancement of elastic properties of multilayered a-C films are discussed.

Superlinearity of synthetic quartz: Dependence on the firing temperature

Abstract In the present work, the sensitivity and thermoluminescence (TL) dose response behaviour of glow-peaks of synthetic quartz are studied as a function of the firing temperature between 300°C and 900°C in steps of 100°C. Complete TL dose response curves in the dose region, 0.1 up to 170 Gy were obtained at each firing temperature. Each TL dose response curve consists of two dose regions, which on a log–log scale have different slopes, k, indicating two different degrees of superlinearity. In the case of the glow-peak at 110°C, the TL dose response curves were fitted with an analytical expression. From the fitting procedure the superlinearity index g(D) was evaluated as a function of dose. The results show that the firing temperature influences the TL dose response behaviour of all glow-peaks. However, the effects are more profound on the glow-peak at 110°C. The superlinearity index g(D) is strongly reduced as the firing temperature increases.

Mechanical performance and growth characteristics of boron nitride films with respect to their optical, compositional properties and density

Abstract We present a study of the growth processes of sputtered BN films, deposited on c-Si (001) substrates, at room temperature (RT) by rf magnetron sputtering. Nanoindentation (hardness ∼21 GPa), density (∼2.6 g/cm 3 ) and roughness by X-ray reflectivity and stress measurements indicated the existence of sp 3 -bonded BN with properties similar to those of crystalline BN being superior for optical applications due to their homogeneity offering a single refractive index and smooth surfaces. The growth mechanism of sputtered BN films is sensitive to the Ar partial pressure and the bias voltage applied to the substrate during deposition inducing bombardment of the film with Ar + ions. There is a narrow ion energy window where sp 3 -bonded BN is predominant providing dense, hard- and wear-resistant films. The fact that they are produced at RT and they are homogeneous with no gradient refractive index and smooth surfaces are correct for processes, such as optical applications on polymers or soft substrates.