The Seebeck coefficient of sputter deposited metallic thin films : the role of process conditions

Because of their reduced dimensions and mass, thin film thermocouples are a promising candidate for embedded sensors in composite materials, especially for application in lightweight and smart structures. The sensitivity of the thin film thermocouple depends however on the process conditions during deposition. In this work, the influence of the discharge current and residual gas impurities on the Seebeck coefficient is experimentally investigated for sputter deposited copper and constantan thin films. The influence of the layer thickness on the film Seebeck coefficient is also discussed. Our observations indicate that both a decreasing discharge current or an increasing background pressure results in a growing deviation of the film Seebeck coefficient compared to its bulk value. Variations in discharge current or background pressure are linked as they both induce a variation in the ratio between the impurity flux to metal flux towards the growing film. This latter parameter is considered a quantitative measure for the background residual gas incorporation in the film and is known to act as a grain refiner. The observed results emphasize the importance of the domain size on the Seebeck coefficient of metallic thin films

Impurity dominated thin film growth

Magnetron sputter deposition was applied to grow thin metal films in the presence of impurities. These impurities are ambient gas molecules and/or atoms from the residual gas present in the vacuum chamber. Seven materials were investigated: four single element metals (Al, Ag, Cu, and Cr), two widely applied alloys (Cu55Ni45 and Ni90Cr10), and one high entropy alloy (CoCrCuFeNi). The thin films were analyzed using X-ray diffraction to determine the domain size, the film texture, and the lattice parameter. The same trend for all studied materials is observed. When the ratio between the impurity and metal flux towards the substrate is low, the domain size is not affected by the presence of the impurities. In this regime, the incorporation of the impurities affects the lattice parameter. At high flux ratios, the change of the domain size can be described by a power law with the exponent equal to -1/2 for all studied materials. A kinetic Monte Carlo code is used to demonstrate this observed trend

Sputter deposited metal layers embedded in composites : from fundamentals to applications

Due to the low heat flux towards the substrate, magnetron sputter deposition offers the possibility to deposit thin films on heat sensitive materials such as fiber-reinforced polymers, also known as composite materials. Passive thermal probe measurements during the sputter deposition of metal layers show indeed that the temperature increase remains well below 25 degrees C for film thicknesses up to 600 nm. The latter thickness threshold is based on the influence of embedded metal films on the adhesion of the composite plies. Films thicker than this threshold deteriorate the mechanical integrity of the composite. The introduction of the uncured composite in the vacuum chamber strongly affects the base pressure by outgassing of impurities from the composite. The impurities affect the film properties as illustrated by their impact on the Seebeck coefficient of sputter deposited thermocouples. The restrictions to embed thin films in composites, as illustrated by both the heat flux measurements, and the study on the influence of impurities, are however not insurmountable. The possibility to use embedded thin films will be briefly demonstrated in different applications such as digital volume image correlation, thermocouples, and de-icing

Modeling reactive magnetron sputtering : opportunities and challenges

The complexity of the reactive magnetron sputtering process is demonstrated by four simulation examples. The examples, commonly encountered during the application of this process for thin film deposition, are described by a numerical model for reactive sputter deposition. A short description of the current model precedes these case studies. In the first example, redeposition of sputtered atoms on the target is studied by its effect on the hysteresis behavior often observed during reactive sputtering. Secondly, the complexity of current-voltage characteristics during reactive magnetron sputtering is treated. The influence of substrate rotation and the pulsing of the discharge current illustrate the time dependence of the reactive sputtering process. As a conclusion, the two main challenges for a further improvement of the model are discussed

The Seebeck Coefficient of Sputter Deposited Metallic Thin Films: The Role of Process Conditions

Because of their reduced dimensions and mass, thin film thermocouples are a promising candidate for embedded sensors in composite materials, especially for application in lightweight and smart structures. The sensitivity of the thin film thermocouple depends however on the process conditions during deposition. In this work, the influence of the discharge current and residual gas impurities on the Seebeck coefficient is experimentally investigated for sputter deposited copper and constantan thin films. The influence of the layer thickness on the film Seebeck coefficient is also discussed. Our observations indicate that both a decreasing discharge current or an increasing background pressure results in a growing deviation of the film Seebeck coefficient compared to its bulk value. Variations in discharge current or background pressure are linked as they both induce a variation in the ratio between the impurity flux to metal flux towards the growing film. This latter parameter is considered a quantitative measure for the background residual gas incorporation in the film and is known to act as a grain refiner. The observed results emphasize the importance of the domain size on the Seebeck coefficient of metallic thin films

On the electrical properties of sputter deposited thin films : the role of energy and impurity flux

The energy available for an adatom diffusing on the substrate surface is an important parameter with regard to the morphological and structural properties of a thin film. A change of the available energy during film growth can be achieved by a modification of the process parameters. However, quite often the implied variation causes also an alteration of the impurity-to-metal impingement flux ratio on the substrate. In this work, the influence of the energy per arriving atom and the impurity-to-metal impingement flux ratio with respect to the resistive properties of chromel thin films deposited by direct-current magnetron sputtering is discussed. It is shown that an evaluation of the impurity-to-metal ratio is essential in order to properly disentangle energy effects from impurity effects. A correlation between the film resistivity and the variation in deposition conditions was found, and initially assigned to the variation in the energy per arriving atom. However, when impurity effects were excluded, the correlation between the available energy per atom and the resistivity could no longer be confirmed. At high levels of contamination, the effect of the energy variation under influence of target-to-substrate distance is fully absorbed by the effect of an increased impurity-to-metal impingement flux ratio. The increased impurity incorporation during growth of the film acts as a grain refiner and increases the film resistivity mainly due to a dominant contribution of grain-boundary scattering