The influence of nitrogen and oxygen additions on the thermal characteristics of aluminium-based thin films
Introduction
Thin films based on metal nitrides and oxides are established materials with great interest to the academic communities due to their numerous industrial applications and still with a promising future [1], [2]. The industrial importance of these materials keeps growing rapidly, not only in the well-established applications based on the strength and refractory nature of these materials such as cutting tools and abrasives, but also in new and promising fields such as electronics, optoelectronics and medical device applications [3], [4], [5], [6], [7], [8], [9]. More recently and using the idea of tailoring the film's properties between those of metal nitrides, MeNx, and those of the correspondent insulating oxides, MeOy, a new class of materials gained more importance in several technological applications: the metal oxynitrides MeNxOy, where Me can be Al [10], [11], [12], [13], Cr [14], Ta [15], Nb [16], Zr [17][18], Mo [19], Ti [20], [21], among others. These ternary materials allow, in principle, merging the benefits of the basic characteristics of both metal nitrides and oxides. Their relevance arises from the fact that the addition of oxygen and nitrogen to metallic elements allows a vast number of stoichiometries, opening the possibility of tuning the band gap, the electrical conductivity, and the crystallographic order between nitride and oxide and hence the electronic and (micro)structural properties of the materials, and thus the overall set of properties [2].
An example of such systems is the ternary aluminium oxynitride (AlNxOy), which might combine the advantages of metallic aluminium with those of the correspondent binary systems, AlNx and AlOy. The wide difference between the properties of these three base materials opens the possibility to combine some of their advantages by simply changing the CN+O/CAl atomic ratio of the film. In previous works the authors carried out a systematic study of the AlNxOy system, taking as reference the corresponding AlNx and AlOy base binary systems. It was showed that by using different pressures of reactive gas (N2 and/or O2), in the magnetron sputtering deposition process [12], it was possible to tune the films composition with different structural and morphological characteristics. These features induced a wide range of electrical [22], optical [11] and electrochemical [23] properties, making this films potentially useful for biomedical sensors or solar applications [23]. Nevertheless, in order to scan the possibility of using this type of films in or near heat sources, such as solar power systems [24], a detailed knowledge about the thermal transport properties and their correlation with the deposition conditions and resulting microstructural features is fundamental for the successful development of such materials.
In order to study the thermal transport properties of the films, modulated IR radiometry (MIRR), also called modulated photothermal radiometry, is ordinarily applied, since it's a non-destructive and contactless thermal wave method appropriate for the characterization of the thickness and the thermal properties of thin films [25], [26]. In terms of thermal behaviour, this technique can be useful to determine the thermal diffusivity and the thermal effusivity of the materials. The thermal diffusivity reflects the capacity of the materials to spread out the thermal energy and thus it is a very important parameter since a very low value can lead to a high localized heating, eventually damaging the sample. Thermal effusivity can be seen as a measure of thermal inertia and it is crucial in controlling heat propagation between different media. In particular, the thermal effusivity ratio between the film and the substrate is a fundamental parameter to understand the way heat propagates in layered materials. Furthermore, if the effusivity of the substrate is known, the effusivity of the film material can be calculated.
In this work, the thermal parameters, such as the thermal effusivity ratio between film and substrate, the thermal diffusion time and the thermal diffusivity of AlOy, AlNx and AlNxOy films were calculated based on MIRR measurements. Since the thermal properties of these materials rely on their composition and microstructure, which depend on the deposition conditions, a detailed analysis of these interdependencies is also a major concern. In section 2 experimental details related to the film's production and characterization are given, whereas the basic description of the experimental setup for thermal properties, the description of the two-layer model and interpretation of signals amplitude and phase of the MIRR can be found in section 3. In section 4 the target potential evolution and the growth rate of the three systems of films are compared each other and some correlations with (and between) the composition and microstructure are also analysed with detail. The results of the thermal behaviour of the films are presented in section 5, along with some correlations with other basic characteristics and physical properties (electrical and optical) of the films.
Section snippets
Details of thin films deposition and characterization
The thin films were produced by reactive DC magnetron sputtering, in a laboratory-sized deposition system [22], using silicon wafers with <100 > orientation (used for structural, morphological and composition analysis) and glass lamellae (ISO 8037) (used for thermal characterization). The substrates were placed in a grounded holder at 70 mm from the target, in a rotation mode-type (9 r.p.m.), and kept at a constant temperature of 100 °C before discharge ignition by using a Joule effect
Basic description of the experimental setup for thermal measurements
Radiometry is a non-destructive technique, belonging to the class of photothermal techniques, which rely on the principle of thermal waves generation [30]. Thermal waves are induced in the samples by means of excitation with a modulated energy beam, usually a laser, which heats the sample causing a temperature variation with the same frequency as the excitation [6]. The infrared radiation emitted by the sample is then analysed. Controlling the modulation frequency will allow the selective
Target potential evolution
The target potential during the deposition of the films was monitored using an acquisition system which revealed an equilibrium state a few minutes after discharge ignition. Fig. 2 shows the evolution of the equilibrium target potential as a function of the partial pressure of the reactive gas used in each one of the three systems studied for comparison.
In the case of the AlOy system, produced with an Ar/O2 mixture, it is possible to identify two different regimes for the target. The first
Conclusions
Modulated IR Radiometry, a non-contact and non-destructive technique based on the excitation of thermal waves by modulated laser beam heating and on the IR detection of the thermal wave response, has proved to be very efficient in the thermal characterization of thin films. In this work, this method was applied in order to study qualitative and quantitatively the thermal response of AlOy, AlNx AlNxOy thin films deposited by magnetron sputtering. The deposition characteristics of the films were
Acknowledgements
This research is sponsored by FEDER funds through the program COMPETE – Programa Operacional Factores de Competitividade – and by National funds through FCT – Fundação para a Ciência e a Tecnologia –, under the projects PEST-C/FIS/UI607/2013, PEst-C/EME/UI0285/2013. J. Borges also acknowledges the support by the European social fund within the framework of realizing the project “Support of inter-sectoral mobility and quality enhancement of research teams at Czech Technical University in Prague
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