Thermal analysis in the field of Thin Film Technology
The physical properties of thin films have gained increasing importance across various industries and applications, including phase-change materials, optical disk media, thermoelectric materials, light-emitting diodes (LEDs), fuel cells, phase-change memories, flat panel displays, and the semiconductor industry at large.
In these diverse sectors, single or multi-layer setups are utilized to imbue devices with specific functions. However, thin films exhibit significantly distinct physical properties compared to their bulk counterparts, primarily due to their reduced thickness and unique deposition techniques. These differences necessitate the use of specialized characterization devices to obtain thickness and temperature-dependent properties accurately. Moreover, the high aspect ratios and deposition methods often introduce additional boundary and surface scattering effects, further impacting the transport properties of thin films.
Given these disparities, tailored metrology techniques are required for the measurement of thin film materials. Notably, the thermal conductivity and electrical conductivity of thin films are typically lower than those of their bulk counterparts, sometimes exhibiting dramatic reductions. For instance, at room temperature, the thermal conductivity (lambda) of a 20 nm Si film or nanowire can be as much as five times smaller than that of its bulk single-crystalline counterpart. In the case of a 100 nm-thick Au film, the transport properties can be reduced by nearly half.
These reductions in thermal conductivity can be attributed to two primary mechanisms. First, many thin film synthesis methods introduce more impurities, disorder, and grain boundaries compared to bulk single crystals, all of which contribute to decreased thermal conductivity. Second, even atomically perfect thin films are expected to have reduced thermal conductivity due to boundary scattering, phonon leakage, and other related interactions. Furthermore, these mechanisms often affect in-plane and cross-plane transport differently, resulting in anisotropic thermal conductivity in thin films, even for materials that are isotropic in their bulk forms.
For reference:
[1] Li, Deyu, et al. “Thermal conductivity of individual silicon nanowires.” Applied Physics Letters 83.14 (2003): 2934-2936.
[2] Linseis, V., Völklein, F., Reith, H., Nielsch, K., and Woias, P. 2018. “Thermoelectric properties of Au and Ti nanofilms, characterized with a novel measurement platform.” Materials Today: Proceedings, ECT2017 Conference Proceedings.
[3] Linseis, V., Völklein, F., Reith, H., Hühne, R., Schnatmann, L., Nielsch, K., and Woias, P. 2018. “Thickness and temperature-dependent thermoelectric properties of Bi87Sb13 nanofilms measured with a novel measurement platform.” Semiconductor Science and Technology.
