Extending the 3w-method to the MHz range for thermal conductivity measurements of diamond thin films

With diamond becoming increasingly important for high-power and high-speed electronics, optical, and micro-electromechanical system (MEMS) applications, we are developing experimental techniques to measure the thermal conductivity of this material. Thermal conductivity values reported in the literature cover a wide range depending on the particular crystalline structure of these thin films which range from nano to poly (micro) crystalline structures. A special challenge is the measurement of the thermal conductivity of very thin diamond films. In this paper we summarize our implementation of the so-called 3w method [D. G. Cahill, R. O. Pohl, Phys. Rev. B 35 (1987) 4067.; D. G. Cahill, H. E. Fischer, T. Klitsner, E. T. Swartz, R. O. Pohl, J. Vac. Sci. Technol. A7 (1989) 1259.; D. G. Cahill, Ref. Sci. Instrum. 61 (2) (1990) 802.; D. G. Cahill, M. Katiyar, J. R. Abelson, Phys. Rev. B 50 (1994) 6077. [1 -4]]. Our technique aims at measuring the thermal conductivities of the entire range of diamond materials including thin films. The experiments are designed to measure at driving frequencies in the megahertz range to keep the thermal wavelength shorter than the film thickness. In addition the appropriate data analysis technique for fitting the results in this frequency range has been developed. To test the method we performed measurements on thick glass disks (1.5 W/nvK) and silicon dioxide thin films (thicknesses between 300 nm and 3mum, 1.6W/m-K). The first successful measurements on diamond films were performed on a 16pm thick ultra-nano crystalline thin film, which yielded 26W/m-K.