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Molecular dynamics simulation was used to study the effects of each grain dimension and of grain boundary characteristics on the inter-grain thermal boundary resistance (TBR) and intragrain thermal conductivity of nanocrystalline diamond. The effect of the grain boundaries perpendicular to the heat flow was studied using a multiple slab configuration, which greatly reduced the artifacts associated with the heat source/sink. The TBR between the slabs was found to be more sensitive to the atomic arrangement at the boundary than to the tilt angle between the slabs. When the atomic arrangement at the interface was altered from the minimum energy configuration, the TBR increased by a factor of three, suggesting that a sub-optimal interface quality between the grains could play a large role in reducing the thermal conductivity of nanocrystalline diamond. The thermal conductivity between the boundaries was found to be similar to the bulk value, even when the boundaries were only 25 nm apart. The effect of grain boundaries parallel to the heat flow was found to have a large dependence on the microstructural details. Parallel boundaries which were 2 nm apart reduced the thermal conductivity of defect-free diamond by between one third and a factor of ten. [ABSTRACT FROM AUTHOR]

Polycrystalline diamond (PCD) was grown onto high-k dielectric passivated AlGaN/GaN-on-Si high electron mobility transistor (HEMT) structures, with film thicknesses ranging from 155 to 1000 nm. Transient thermoreflectance results were combined with device thermal simulations to investigate the heat spreading benefit of the diamond layer. The observed thermal conductivity (κDia) of PCD films is one-to-two orders of magnitude lower than that of bulk PCD and exhibits a strong layer thickness dependence, which is attributed to the grain size evolution. The films exhibit a weak temperature dependence of κDia in the measured 25–225 °C range. Device simulation using the experimental κDia and thermal boundary resistance values predicts at best a 15% reduction in peak temperature when the source-drain opening of a passivated AlGaN/GaN-on-Si HEMT is overgrown with PCD. [ABSTRACT FROM AUTHOR]

The article discusses issues that must be addressed by advanced thermal solutions, particularly for radio frequency (RF) power amplifiers. It focuses on the chemical vapor deposition (CVD) diamond as a heat spreader to reduce overall package thermal resistance. It mentions the functional considerations includes electrical conductivity of the heat spreader, thermal simulation to optimize the heat spreader configuration and metallizations.