Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

/pdf/nanoscale-thermal-transport-1frv881kzm.pdf

Nanoscale thermal transport

A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of ∼1 nm, the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interface...

/pdf/nanoscale-thermal-transport-ii-2003-2012-4upcs4bq86.pdf

Nanoscale thermal transport. II. 2003–2012

Heat transport in 20–300 nm thick dielectric films is characterized in the temperature range of 78–400 K using the 3ω method. SiO2 and SiNx films are deposited on Si substrates at 300 °C using plasma enhanced chemical vapor deposition (PECVD). For films >100 nm thick, the thermal conductivity shows little dependence on film thickness: the thermal conductivity of PECVD SiO2 films is only ∼10% smaller than the conductivity of SiO2 grown by thermal oxidation. The thermal conductivity of PECVD SiNx films is approximately a factor of 2 smaller than SiNx deposited by atmospheric pressure CVD at 900 °C. For films <50 nm thick, the apparent thermal conductivity of both SiO2 and SiNx films decreases with film thickness. The thickness dependent thermal conductivity is interpreted in terms of a small interface thermal resistance RI. At room temperature, RI∼2×10−8 K m2 W−1 and is equivalent to the thermal resistance of a ∼20 nm thick layer of SiO2 .

Heat transport in thin dielectric films

We review recent advances in experimental methods for high spatial-resolution and high time-resolution thermometry, and the application of these and related methods for measurements of thermal transport in low-dimensional structures. Scanning thermal microscopy (SThM) achieves lateral resolutions of 50 nm and a measurement bandwidth of 100 kHz: SThM has been used to characterize differences in energy dissipation in single-wall and multi-wall carbon nanotubes. Picosecond thermoreflectance enables ultrahigh time-resolution in thermal diffusion experiments and characterization of heat flow across interfaces between materials; the thermal conductance G of interfaces between dissimilar materials spans a relatively small range, 20<G<200 MW m -2 K -1 near room temperature. Scanning thermoreflectance microscopy provides nanosecond time resolution and submicron lateral resolution needed for studies of heat transfer in microelectronic, optoelectronic and micromechanical systems. A fully-micromachined solid immersion lens has been demonstrated and achieves thermal-radiation imaging with lateral resolution at far below the diffraction limit, <2 μm. Microfabricated metal bridges using electrical resistance thermometry and joule heating give precise data for thermal conductivity of single crystal films, multilayer thin films, epitaxial superlattices, polycrystalline films, and interlayer dielectrics. The room temperature thermal conductivity of single crystal films of Si is strongly reduced for layer thickness below 100 nm. The through-thickness thermal conductivity of Si-Ge and GaAs-AlAs superlattices has recently been shown to be smaller than the conductivity of the corresponding alloy. The 3ω method has been recently extended to measurements of anisotropic conduction in polyimide and superlattices. Data for carbon nanotubes measured using micromachined and suspended heaters and thermometers indicate a conductivity near room temperature greater than diamond.

https://nanoheat.stanford.edu/sites/default/files/publications/A57.pdf

Thermometry and Thermal Transport in Micro/Nanoscale Solid-State Devices and Structures

Photothermal effects in plasmonic nanostructures have great potentials in applications for photothermal cancer therapy, optical storage, thermo-photovoltaics, etc. However, the transient temperature behavior of a nanoscale material system during an ultrafast photothermal process has rarely been accurately investigated. Here a heat transfer model is constructed to investigate the temporal and spatial variation of temperature in plasmonic gold nanostructures. First, as a benchmark scenario, we study the light-induced heating of a gold nanosphere in water and calculate the relaxation time of the nanosphere excited by a modulated light. Second, we investigate heating and reshaping of gold nanoparticles in a more complex metamaterial absorber structure induced by a nanosecond pulsed light. The model shows that the temperature of the gold nanoparticles can be raised from room temperature to >795 K in just a few nanoseconds with a low light luminance, owing to enhanced light absorption through strong plasmonic r...

Nanosecond Photothermal Effects in Plasmonic Nanostructures

The vertical thermal conductivities of thermally grown (TG) and chemical vapor deposited (CVD) silicon‐dioxide layers 20 to 200 nm thick are measured using a simple, noncontact photothermal technique. The conductivities of TG and CVD layers are less by as much as 18% and 30%, respectively, than the conductivity of bulk fused silicon dioxide. No significant thickness dependence is observed. The thermal boundary resistance between the oxide layers and silicon is shown to be negligibly small. The boundary resistance of gold layers sputtered directly onto TG oxide is considerably larger than that of gold layers evaporated on TG oxide with a 20‐nm chromium adhesion layer, and is comparable to internal resistances of the oxide layers.

Thermal conduction in metallized silicon‐dioxide layers on silicon

We report spatial mapping of temperature fields in semiconductor devices with sub-microsecond temporal resolution. The measurements are performed at a facility that integrates scanning laser-reflectance thermometry with electrical stressing capability. Data for SOI LDMOS transistors investigate transient heat diffusion within the buried silicon dioxide and capture large temperature gradients in the drift region, which result from the spatially-varying impurity concentration. The new thermometry facility is promising for the study of transistor and interconnect thermal failure due to electrostatic discharge (ESD).

Short-timescale thermal mapping of semiconductor devices

Passive diamond layers fabricated using chemical vapor deposition can improve thermal conduction in electronic microstructures. The benefit of using diamond depends strongly on the thermal boundary resistance between active semiconducting regions, where heat is generated, and the diamond. Two independent experimental methods measure the total thermal resistance for conduction normal to 0.2, 0.5, and 2.6 μm thick diamond layers deposited on silicon, providing an upper bound for the effective silicon‐diamond boundary resistance. The data agree with predictions that couple the local phonon scattering rate in the diamond to the grain size.

Thermal conduction normal to diamond‐silicon boundaries

Passive chemical‐vapor‐deposited diamond layers have the potential to improve thermal conduction in electronic microstructures because of their high thermal conductivities. The thermal resistances for conduction normal to the boundaries of diamond layers, which must be small in order to realize this potential, have not been measured. This research develops two independent experimental methods that measure the total thermal resistance for conduction normal to diamond layers thinner than 5 μm on silicon substrates, yielding an upper bound for the thermal resistance of the diamond‐silicon boundary. The data for layers as thin as 0.2 μm agree with predictions that account for phonon scattering on layer boundaries and couple the local scattering rate in the diamond to the grain size. The agreement lends support to the conclusion that the effective diamond‐silicon boundary resistance is dominated by a highly localized volume resistance in the diamond near the interface.

https://nanoheat.stanford.edu/sites/default/files/publications/A14.pdf

Experimental investigation of thermal conduction normal to diamond‐silicon boundaries

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Thermal conduction in metallized silicon‐dioxide layers on silicon

Short-timescale thermal mapping of semiconductor devices

Thermal conduction normal to diamond‐silicon boundaries

Experimental investigation of thermal conduction normal to diamond‐silicon boundaries