The Thermal Conductivity of Silicon Dioxide

Silicon dioxide (SiO₂) is one of the most common and also one of the most important materials in the world as it is the basis for our windows, beaches and wine glasses. It is also a source of silicon, of course. As such, the thermal conductivity (TC) of the bulk material is well documented. However, in many cases of interest to the thermal management of semiconductors (for example, power devices, MEMS, and low-k dielectrics), SiO₂ is used in the form of a thin layer. Unfortunately, the TC of SiO2 is about two orders of magnitude less than that of Si. Hence, even the influence of a very thin layer can be significant. For example, in the case of SOI (Silicon-On-Insulator), thicker oxide layers result in faster devices, but at the cost of a decrease in reliability due to higher temperature (gradients).

Since the thermal properties of thin films may differ significantly from the bulk value, it is a must to repeat the measurements. This is by no means a trivial task and accordingly literature data vary considerably. A major reason is that the effective TC depends on the way the layer is created and its thickness.

A recent publication [1] discussed the discrepancies and claimed more accurate results using a thermoreflectance technique, showing that the intrinsic TC was independent of the thickness.

Another paper on thermally grown SiO₂ [2] showed a clear dependence of the effective TC on the oxide thickness, attributed to an increase in the contribution of the interface. Obviously a distinction should be made between intrinsic values and values that include the interfacial resistance between layer and substrate. A final remark on the temperature dependence: the TC shows an increase of about 10% over the range 0-200° C.

The Table shows a survey of published values for both bulk values and thin layers.

Table 1. Survey of published values for both bulk values and thin layers.

* Effective values for particle bed

References

1. Burzo, M., Komarov, P., Raad, P., Thermal Transport Properties of Gold-Covered Thin-Film Silicon Dioxide, IEEE CPMT 26, 2003, pp. 80-88.
2. Delan, A., Rennau, M., Schulz, S.E., Gessner, T., Thermal Conductivity of Ultra Low-k Dielectrics, Microelectronics Engineering 70, 2003, pp. 280-284.