Jungwan Cho

Kinetics of copper drift in PECVD dielectrics

We quantified the drift of Cu ions into various PECVD dielectrics by measuring shifts in capacitance-voltage behavior after subjecting Cu-gate MOS capacitors to bias-temperature stress. At a field of 1.0 MV/cm and temperature of 100/spl deg/C, Cu ions drift readily into PECVD oxide with a projected accumulation of 2.7/spl times/10/sup 13/ ions/cm/sup 2/ after 10 years. However, in PECVD oxynitride, the projected accumulation under the same conditions is only 2.3/spl times/10/sup 10/ ions/cm/sup 2/. These findings demonstrate the necessity of integrating drift barriers, such as PECVD oxynitride layers, in Cu interconnection systems to ensure threshold stability of parasitic field n-MOS devices.

Low Thermal Resistances at GaN–SiC Interfaces for HEMT Technology

The temperature rise in AlGaN/GaN high-electron-mobility transistors depends strongly on the GaN-substrate thermal interface resistance (TIR). We apply picosecond time-domain thermoreflectance measurements to GaN-SiC composite substrates with varying GaN thickness to extract both the TIR and the intrinsic GaN thermal conductivity at room temperature. Two complementary data extraction methodologies yield 4-5 for the GaN-SiC TIR and 157-182 for the GaN conductivity. The GaN-SiC interface resistance values reported here, as well as the TIR experimental uncertainties documented in this letter, are substantially lower than those reported previously for this material combination.

Improved Thermal Interfaces of GaN–Diamond Composite Substrates for HEMT Applications

High-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs) requires efficient heat removal through the substrate. GaN composite substrates, including the high-thermal-conductivity diamond, are promising, but high thermal resistances at the interfaces between the GaN and diamond can offset the benefit of a diamond substrate. We report on measurements of thermal resistances at GaN-diamond interfaces for two generations (first and second) of GaN-on-diamond substrates, using a combination of picosecond time-domain thermoreflectance (TDTR) and nanosecond transient thermoreflectance techniques. Two flipped-epitaxial samples are presented to determine the thermal resistances of the AlGaN/AlN transition layer. For the second generation samples, electrical heating and thermometry in nanopatterned metal bridges confirms the TDTR results. This paper demonstrates that the latter generation samples, which reduce the AlGaN/AlN transition layer thickness, result in a strongly reduced thermal resistance between the GaN and diamond. Further optimization of the GaN-diamond interfaces should provide an opportunity for improved cooling of HEMT devices.

Anisotropic and inhomogeneous thermal conduction in suspended thin-film polycrystalline diamond

While there is a great wealth of data for thermal transport in synthetic diamond, there remains much to be learned about the impacts of grain structure and associated defects and impurities within a few microns of the nucleation region in films grown using chemical vapor deposition. Measurements of the inhomogeneous and anisotropic thermal conductivity in films thinner than 10 μm have previously been complicated by the presence of the substrate thermal boundary resistance. Here, we study thermal conduction in suspended films of polycrystalline diamond, with thicknesses ranging between 0.5 and 5.6 μm, using time-domain thermoreflectance. Measurements on both sides of the films facilitate extraction of the thickness-dependent in-plane ( κr) and through-plane ( κz) thermal conductivities in the vicinity of the coalescence and high-quality regions. The columnar grain structure makes the conductivity highly anisotropic, with κz being nearly three to five times as large as κr, a contrast higher than that report...

Cooling Limits for GaN HEMT Technology

The peak power density of GaN HEMT technology is limited by a hierarchy of thermal resistances from the junction to the ambient. Here we explore the ultimate or fundamental cooling limits made possible by advanced thermal management technologies including GaN-diamond composites and nanoengineered heat sinks. Through continued attention to near-junction resistances and extreme flux convection, power densities that may exceed 50 kW/cm2 - depending on gate width and hotspot dimension - are feasible within 5 years.

Thermal transport: Cool electronics

Although heat removal in electronics at room temperature is typically governed by a hierarchy of conduction and convection phenomena, heat dissipation in cryogenic electronics can face a fundamental limit analogous to that of black-body emission of electromagnetic radiation.

Electromigration properties of electroless plated Cu metallization

The electromigration properties of electroless plated copper films have been evaluated under DC stress conditions. The formation of microvoids and the diffusion of copper through the seed layer caused an increase of the line resistance in the initial stage of the stressing. The current density dependence and the activation energy of the lifetime were determined.<<ETX>>

Fundamental Cooling Limits for High Power Density Gallium Nitride Electronics

The peak power density of GaN high-electron-mobility transistor technology is limited by a hierarchy of thermal resistances from the junction to the ambient. Here, we explore the ultimate or fundamental cooling limits for junction-to fluid cooling, which are enabled by advanced thermal management technologies, including GaN-diamond composites and nanoengineered heat sinks. Through continued attention to near-junction resistances and extreme flux convection heat sinks, heat fluxes beyond 300 kW/cm2 from individual 2-μm gates and 10 kW/cm2 from the transistor footprint will be feasible. The cooling technologies under discussion here are also applicable to thermal management of 2.5-D and 3-D logic circuits at lower heat fluxes.

Analysis and characterization of thermal transport in GaN HEMTs on Diamond substrates

The emergence of Gallium Nitride-based High Electron Mobility Transistor (HEMT) technology has proven to be a significant enabler of next generation RF systems. However, thermal considerations currently prevent exploitation of the full electromagnetic potential of GaN in most applications, limiting HEMT areal power density (W/mm2) to a small fraction of electrically limited performance. GaN on Diamond technology has been developed to reduce near junction thermal resistance in GaN HEMTs. However, optimal implementation of GaN on Diamond requires thorough understanding of thermal transport in GaN, CVD diamond and interfacial layers in GaN on Diamond substrates, which has not been thoroughly previously addressed. To meet this need, our study pursued characterization of constituent thermal properties in GaN on Diamond substrates and temperature measurement of operational GaN on Diamond HEMTs, employing electro-thermal modeling of the HEMT devices to interpret and relate data. Strong agreement was obtained between simulations and HEMT operational temperature measurements made using two independent thermal metrology techniques, enabling confident assessment of peak junction temperature. The results support the potential of GaN on Diamond to enable a 3X increase in HEMT areal dissipation density without significantly increasing operational temperature. Such increases in HEMT power density will enable smaller, higher power density Monolithic Microwave Integrated Circuits (MMICs).

Thermal characterization of GaN-on-diamond substrates for HEMT applications

High-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs) requires efficient heat removal through the substrate. GaN composite substrates including high-thermal-conductivity diamond are promising, but high thermal resistances at the interfaces between the GaN and diamond can offset the benefit of a diamond substrate. We report on measurements of the thermal resistances at the GaN-diamond interfaces for two generations (1st and 2nd) of GaN-on-diamond substrates using a combination of picosecond time-domain thermoreflectance (TDTR) and nanosecond transient thermoreflectance (TTR) techniques. Two flipped-epitaxial samples are presented to determine the thermal resistances of the AlGaN/AlN transition layer. For the 2nd generation samples, electrical heating and thermometry in nanopatterned metal bridges confirms the TDTR results. This paper demonstrates that the latter generation samples, which reduce the AlGaN thickness by 75%, result in a strongly-reduced thermal resistance between the GaN and diamond. Further optimization of the GaN-diamond interfaces should provide an opportunity for improved cooling of HEMT devices.