Thickness-dependent thermal properties of amorphous insulating thin films measured by photoreflectance microscopy
Introduction
Thermal and electronic conductivities are strongly correlated in most materials. However, many applications demand the maximization of one of these properties while minimizing the other. In microelectronics for instance, good electrical insulation is essential (capacitors, interconnects), but low-k dielectrics usually come with poor thermal conductivity, hampering heat dissipation. Conversely, high electrical conductivity and thermal insulation are crucial for thermoelectric conversion, in order to avoid Joule heating while preserving the temperature gradient [1], [2]. Nanostructured materials offer a new way to act on these antagonistic requirements, since nanoscale thermal properties can significantly differ from bulk values [3], [4]. A lot of attention has been focused recently on understanding the underlying physics, like phonon scattering [5] and heat transport phenomena [6], [7].
In this paper, we investigate the thermal properties of two electrical insulators, SiO2 and Al2O3 thin films. SiO2 is essential to microelectronics and other industrial applications. It has therefore received a lot of attention, and its thermal properties are relatively well known. Some research groups have studied the thermal conductivity of Al2O3 amorphous thin films [8], [9], [10], [11], but the evaluation of their interfacial thermal resistances is still very incomplete [12]. Al2O3 amorphous thin films are promising, since they can reduce electronic recombination losses in solar cells by the passivation of silicon surfaces, thus enabling higher efficiency [13]. Moreover, thin amorphous Al2O3 films are good thermal insulators as well as excellent moisture barriers [14] that can be fabricated at low temperatures [15], [16], making them highly desirable in electronic components [17].
A broad range of experimental methods is available in order to determine the thermal properties of materials. They essentially differ in their heat generation process (optical, Joule, …), in the property which is probed (temperature of the surface, sample or air, acoustic waves, etc.…), and in the probing mechanism (refractive index, thermal emission, interferometry, fluorescence, electrical resistance…). Temporally, various strategies have also been developed: steady state, transient or modulated. Several reviews of thin films characterization techniques have been proposed [18], [19]. Among these techniques, modulated photoreflectance microscopy has the advantages of being contactless, non-destructive and, owing to the high spatial resolution of visible light microscopy, allows measurements on relatively small samples (> 10 μm). It is based on the generation of thermal “waves” by intensity-modulated optical excitation. This technique was first proposed by A. Rosencwaig et al. [20], and then widely used to determine the thermal properties of bulk materials [21], [22], grains [23], coatings and thin films [24], [25]. In this work, the frequency domain photoreflectance method is used to study the effect of thickness on the thermal properties of amorphous SiO2 and Al2O3 thin films. The method requires the deposition of a gold layer to opacify the surface, but is well adapted to this kind of study, where different nanoscale layers have to be distinguished. A 3D heat diffusion model was used to extract the thermal properties of each material independently [22].
Section snippets
Experimental
The SiO2 thin films with different thicknesses were fabricated by Kelvin Nanotechnology Ltd. (KNT), in collaboration with Glasgow University. The starting material is a thick layer of SiO2 grown by thermal oxidation on a p-type Si wafer. Repeated photolithography steps, followed by timed hydrofluoric acid (HF) etching, were performed to obtain the required thicknesses of 12, 30, 65, 145, 237, 530 and 950 nm, as depicted in Fig. 1a). The layer thickness was measured by white light interferometry
Results and discussion
The thermal diffusivity D of an optically and thermally thick, isotropic, bulk material can be straightforwardly extracted from the slope of the phase lag P of the surface temperature rise with respect to the excitation at a distance x from the excitation, where f is the excitation frequency [30]. For a multilayered sample, there is no such simple relation, since the phase slope.
is then a function of the diffusivities and conductivities of the different layers [29]. However, the
Conclusion
In summary, we presented a study of the thermal properties of SiO2 and Al2O3 thin films as a function of thickness. For low thicknesses, 12 nm for SiO2 and 2–5 nm for Al2O3, we measured reductions of the effective thermal conductivity by 30% and 70% compared to the values obtained for micrometric layers of SiO2 and Al2O3 respectively. The interfacial thermal resistance was small in both cases, suggesting good thermal transport across the interfaces. The diffusivity and conductivity are not
Acknowledgments
This work was funded by the European Union Seventh Framework Program FP7-NMP-2013-LARGE-7 under grant agreement number 8604668 “Quantiheat”. The authors would like to thank L. Aigouy and L Billot for the gold coatings deposition.
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