Showing papers by "Avram Bar-Cohen published in 2016"

Superlattice-based thin-film thermoelectric modules with high cooling fluxes

Abstract: In present-day high-performance electronic components, the generated heat loads result in unacceptably high junction temperatures and reduced component lifetimes. Thermoelectric modules can, in principle, enhance heat removal and reduce the temperatures of such electronic devices. However, state-of-the-art bulk thermoelectric modules have a maximum cooling flux qmax of only about 10 W cm−2, while state-of-the art commercial thin-film modules have a qmax <100 W cm−2. Such flux values are insufficient for thermal management of modern high-power devices. Here we show that cooling fluxes of 258 W cm−2 can be achieved in thin-film Bi2Te3-based superlattice thermoelectric modules. These devices utilize a p-type Sb2Te3/Bi2Te3 superlattice and n-type δ-doped Bi2Te3−xSex, both of which are grown heteroepitaxially using metalorganic chemical vapour deposition. We anticipate that the demonstration of these high-cooling-flux modules will have far-reaching impacts in diverse applications, such as advanced computer processors, radio-frequency power devices, quantum cascade lasers and DNA micro-arrays. Current thermoelectric modules provide cooling fluxes that are insufficient for high-heat flux applications. Here, the authors demonstrate thin-film-based thermoelectric modules capable of providing cooling fluxes more than double that of the current state-of-the-art.

Near-Junction Microfluidic Cooling for Wide Bandgap Devices

Abstract: GaN has emerged as the material of choice for advanced power amplifier devices for both industrial and defense applications but near-junction thermal barriers severely limit the inherent capability of high-quality GaN materials. Recent “embedded cooling” efforts, funded by Defense Advanced Research Projects Agency Microsystems Technology Office (DARPA-MTO), have focused on reduction of this near-junction thermal resistance, through the use of diamond substrates and efficient removal of the dissipated power with convective and evaporative microfluidics. An overview of the accomplishments of the DARPA Near-Junction Thermal Transport (NJTT) program and recent results from the on-going DARPA Intra-Chip Embedded Cooling (ICECool) program are provided. It is shown that growth or bonding of diamond to GaN epitaxy has enabled a 3-5× increase in power handling capability per transistor unit area, while use of microfluidic cooling has enabled heat fluxes of 30 kW/cm2 at the transistor level and 1 kW/cm2 at the die-level, for a 3-6× improvement in the total RF output power of GaN power amplifiers. These demonstrations provide near-term validation of the large improvement in output power gained through embedded cooling and confirm the potential for well above a 6× improvement in GaN power amplifier output power to the electrical, rather than thermal, limits of GaN.

Method for predicting junction temperature distribution in a high-power laser diode bar.

Abstract: A hybrid experimental/numerical method is proposed for predicting the junction temperature distribution in a high-power laser diode (LD) bar with multiple emitters. A commercial water-cooled LD bar with multiple emitters is used to illustrate and validate the proposed method. A unique experimental setup is developed and implemented first to measure the average junction temperatures of the LD bar emitters. After measuring the heat dissipation of the LD bar, the effective heat transfer coefficient of the cooling system is determined inversely from the numerical simulation using the measured average junction temperature and the heat dissipation. The characterized heat dissipation and effective heat transfer coefficient are used to predict the junction temperature distribution over the LD bar numerically under high operating currents. The results are presented in conjunction with the wall-plug efficiency and the center wavelength shift.

Unlocking the True Potential of 3-D CPUs With Microfluidic Cooling

Abstract: 3-D integration is a promising technology to sustain transistor density scaling in the future, as well as facilitating new architectural designs that were not possible with traditional integration techniques. However, 3-D integration comes with some serious challenges, chief among them heat removal. A promising technology for thermal issues is microfluidic (MF) cooling. In this paper, we perform a design space analysis study on 3-D CPUs. We show that aggressive cooling solutions such as MF cooling are necessary to unlock the true potential of 3-D ICs. Without such cooling the thermal feasibility region of the design space is significantly reduced. We observe that interactions between thermal, electrical, and physical aspects of 3-D CPUs with MF cooling are substantial, and must be cooptimized during our analysis to correctly identify optimal design points. We simulate a spectrum of 3-D CPU architectures which offer vast improvements to performance, but are energy inefficient and thermally infeasible with air cooling. Furthermore, we show a $2.30\times $ ( $1.59\times $ ) improvement in performance (energy efficiency) when MF cooling and floorplan cooptimization are added to our design space analysis simulation flow.

Thermal Isolation within High-Power 2.5D Heterogenously Integrated Electronic Packages

Abstract: We propose a two-pronged approach to reducing the impact of thermal cross-talk between components of disparate thermal operating points within a heterogeneously integrated electronic package. First, a low thermal conductivity interposer enhanced with an array of conductive thermal vias is employed to provide a high degree of lateral thermal isolation while providing adequate conduction in the vertical direction. Second, a "differential" embedded microfluidic cooling strategy is proposed that provides high rates of heat removal in the isolated high-power regions while using a low pumping cost background cooling for low-power regions. At the end of this paper, a case study is presented where required device separation is reduced three-fold and required pumping power is reduced by an estimated 40% compared to a silicon interposer with a uniform aggressive cooling strategy.

Flow regimes and evaporative heat transfer coefficients for flow boiling in smooth and internally-grooved tubes

Abstract: This paper outlines the development of a physics-based heat transfer coefficient model that recognizes the role played by two-phase flow structures in enhancing two-phase thermal transport within internally-grooved tubes. Flow regime data, obtained with dynamic total-internal-reflection measurements, and heat transfer coefficient data, obtained with infrared thermography, are presented and analyzed for two-phase HFE-7100 flow in horizontal 8.84 mm diameter smooth and internally-grooved tubes, with mass fluxes from 25–300 kg/m2s, heat fluxes from 0–56 kW/m2, and vapor qualities approaching 1. This data, along with data from the literature, is then compared to the new flow regime map and associated heat transfer coefficient correlation. The model is shown to predict the current data set and data from several independent researchers, with a marked improvement in predicting performance near the transition from Stratified-Wavy to Annular flow in internally-grooved tubes.

Modeling Thermal Microspreading Resistance in Via Arrays

Abstract: As thermal management techniques for three-dimensional (3D) chip stacks and other high-power density electronic packages continue to evolve, interest in the thermal pathways across substrates containing a multitude of conductive vias has increased. To reduce the computational costs and time in the thermal analysis of through-layer via (TXV) structures, much research to date has focused on defining effective anisotropic thermal properties for a pseudohomogeneous medium using isothermal boundary conditions. While such an approach eliminates the need to model heat flow through individual vias, the resulting properties are found to depend on the specific boundary conditions applied to a unit TXV cell. More specifically, effective properties based on isothermal boundary conditions fail to capture the local “microspreading” resistance associated with more realistic heat flux distributions and local hot spots on the surface of these substrates. This work assesses how the thermal microspreading resistance present in arrays of vias in interposers, substrates, and other package components can be properly incorporated into the modeling of these arrays. We present the conditions under which spreading resistance plays a major role in determining the thermal characteristics of a via array and propose methods by which designers can both account for the effects of microspreading resistance and mitigate its contribution to the overall thermal behavior of such substrate–via systems. Finite element modeling (FEM) of TXV unit cells is performed using commercial simulation software (ansys).

Nonintrusive Optical Validation of Two-Phase Flow Regimes in a Small-Diameter Tube

Abstract: While the majority of studies examining the relationship between flow regime and thermofluid performance rely on subjective flow regime determination, objective techniques are needed to more firmly establish the nature and repeatability of these phenomena. This study describes a nonintrusive optical flow regime characterization methodology used to study horizontal, adiabatic, two-phase flow of water–water vapor and water–nitrogen gas in small (8.84 mm) diameter tubes. The method relies on shining a fiber-optic light source through the top of a borosilicate glass tube at the outlet of a smooth copper tube, using a CMOS camera to capture light rings resulting from total internal reflection at the liquid–vapor interface, and extracting a film thickness profile from successive images. Using these unique temporally varying film profiles, quantitative identification measures were developed for the primary flow regimes, including the ability to explain and quantify the more subtle transitions that exist between ...

Inverse approach to characterize die-attach thermal interface of light emitting diodes

Abstract: An inverse approach is developed and implemented to quantify the resistance of the die-attach thermal interface (DTI) in high power light emitting diodes (LEDs). The transient time domain dominated by the resistance of the DTI is selected first using a hybrid analytical/numerical solution. Then, the resistance of the DTI is determined inversely from the experimental data over the predetermined transient time domain using numerical modeling. The results confirm that the proposed approach offers a measurement accuracy of 0.01 K/W.