Silicon Phononic Nanowires Enable Ultra-Low Thermal Conductivity Measured by Raman Spectroscopy

We present a novel device structure and technique for measuring the remarkably low thermal conductivity of phononic crystal (PnC) in a micro-platform supported by PnC nanowires under vacuum conditions (~20 mTorr). This microelectromechanical systems (MEMS) structure is extensively utilized in photonic pixel applications, including electrically heated micro-hotplates and blackbody emitters. Our approach involves the application of a Raman spectroscopy system to gauge the thermal conductivity (
$k$
) of a silicon MEMS pixel. The system is meticulously calibrated by heating the pixel in a vacuum condition and using Raman spectral shift to determine the temperature at various spots within the pixel. As the pixel undergoes thermal heating, temperatures at different locations are harnessed to derive
$k$
at varying temperature levels. We determine a markedly low
$k$
of 1.1 W/(
$\text{m}\cdot \text{K}$
) with a holey fill factor (0.46). The emphasis lies not only on the ultra-low thermal conductivity achieved, but also on the efficacy of the Raman spectroscopy method in this measurement.