Ultrafast optical characterization of nanoscale thermal properties [electronic resource]
Keywords: Mechanical Engineering

Ultrafast thermoreflectance is a powerful technique designed to measure thermal properties in films less than a micrometer thick. Careful sample design and control over the measurement timescale allow spatial and temporal confinement of the measurement to a region of interest. This work explores the capability of nanosecond and picosecond thermoreflectance in capturing the thermal properties of a host of exotic materials used in next generation electronic devices. These include the phase change material Ge2Sb2Te5 (GST), diamond substrates for high electron mobility transistors (HEMT), and multilayer Mo/Si mirrors for extreme ultraviolet wavelengths (EUV). Nanosecond and picosecond thermoreflectance were used to determine the thermal properties of the phase change material, GST, along with several candidate electrode films (C, Ti, TiN, W, and WNx) and novel electrode multilayers (C-TiN and W-WNx). These results offer a material selection roadmap for device designers seeking to tune the thermal properties of their PCM cell. This work also reports picosecond thermoreflectance measurements of GST films sandwiched between TiN electrode layers and annealed at multiple temperatures. Thermal conductivity of the hexagonal close-packed (HCP) phase exceeds that of the face centered cubic (FCC) phase due to the addition of electron thermal conduction. Electron interface transport is shown to be negligible, implying that the addition of electrons as energy carriers does not significantly affect thermal boundary resistance (TBR). Thermal spreading analysis of a representative HEMT structure on diamond and SiC substrates shows that a device-substrate thermal interface resistance in excess of 20 m2 K GW-1 negates the benefits of diamond as a substrate material. Picosecond thermoreflectance measurements on multiple diamond samples were performed to determine the thermal conductivity, thermal anisotropy, and boundary resistance of diamond on AlN substrates. Further measurements on the top and bottom surfaces of a suspended diamond films demonstrated the thermal conductivity of the coalescence region (80 W m-1 K-1) and high quality layer (1350 W m-1 K-1) of a single diamond film. Using a two-layer model of the diamond film, we predict the thickness of the coalescence region and show it to be less than 1 [micrometer]. The operating temperatures of Mo/Si multilayers used in EUV lithography affect their lifetimes. Predicting the mirror/mask damage threshold fluence requires accurate knowledge of the mirror thermal properties. This study reports high temperature thermal properties of the TaN masking film, the MoSi2 intermetallic, and the room temperature properties of the Mo/Si multilayer. The thickness dependent electrical conductivity of TaN estimates the mean free path of electrons in the film unhindered by the material interfaces (~ 30 nm). Measurements on MoSi2 demonstrate the change in thermal conductivity due to crystallization, from 1.7 W m-1 K-1 in the amorphous phase to 2.8 W m-1 K-1 in the crystalline phase. Mo/Si results demonstrate thermal conductivity (1.1 W m-1 K-1) significantly lower than previous literature assumptions (4-5 W m-1 K-1). A finite element thermal model uses these results to predict the maximum EUV fluence allowed on a Mo/Si mirror for a single shot and for a one billion pulse lifetime before causing a reflectance loss of 1%.