Showing papers by "Avram Bar-Cohen published in 2007"

On-chip hot spot cooling using silicon thermoelectric microcoolers

Abstract: Thermal management of microprocessors has become an increasing challenge in recent years because of localized high flux hot spots which cannot be effectively removed by conventional cooling techniques. This paper describes the use of the silicon chip itself as a thermoelectric cooler to suppress the hot spot temperature. A three-dimensional analytical thermal model of the silicon chip, including localized thermoelectric cooling, thermoelectric heating, silicon Joule heating, hot spot heating, background heating, and conductive/convective cooling on the back of the silicon chip, is developed and used to predict the on-chip hot spot cooling performance. The effects of hot spot size, hot spot heat flux, silicon chip thickness, microcooler size, doping concentration in the silicon, and parasitic Joule heating from electric contact resistance on the cooling of on-chip hot spots, are investigated in detail.

Thermal management of high heat flux nanoelectronic chips

Abstract: Driven by the continuing Moore’s Law evolution in chip technology, power dissipation of nanoelectronics chips could exceed 300W, with heat fluxes above 150W/cm2, within the next few years, along with localized, sub-millimeter zones with heat fluxes in excess of 1kW/cm2. New and novel cooling techniques, with the ability to selectively cool sub-millimeter “sun spots” while providing effective global cooling for high heat flux chips are needed. Several promising approaches, including the application of miniaturized silicon and BiTe thermoelectric coolers and direct cooling with dielectric liquids through thin film evaporation, will be described.

Mini-Contact Enhanced Thermoelectric Cooling of Hot Spots in High Power Devices

Abstract: Cooling hot-spots with high heat flux (e.g., >1000 W/cm2) is becoming one of the most important technical challenge in today's integrated circuit industry. More aggressive thermal solutions, than would be required for uniform heating, are highly desired. Recently, solid state thermoelectric coolers (TECs) have received more attention for hot-spot thermal management. However, present day TECs typically have cooling flux much lower than heat flux in the hot-spots. In this work, we reported an innovative technique-TE Mini-contact-to significantly increase cooling flux of TECs for the application in hot-spot cooling. A chip package featuring a TE Mini-contact cooler integrated with conventional integrated heat spreader and heat sink is designed. The cooling performance of such chip package has been investigated by using a 3-D numeric model. It is found that the cooling in the hot-spot (1250 W/cm2, 400 mum by 400 mum) can be about 19 degC better in the proposed package than that achieved in the conventional chip package without TEC. The effects of trench, die thickness, and TEC misalignment on the cooling of the hot-spot are also discussed.

Modeling and Prediction of Two-Phase Refrigerant Flow Regimes and Heat Transfer Characteristics in Microgap Channels

Abstract: This keynote lecture will open with a brief review of the primary two-phase flow regimes and their impact on thermal transport phenomena in tubes and channels The Taitel and Dukler flow regime mapping methodology will then be described and applied to the two-phase flow of refrigerants and dielectric liquids in microgap channels The effects of channel diameter, as well as alternative transition criteria, on the prevailing flow regimes in microgaps will be explored along with available criteria for microchannel behavior Available microgap data will then be shown to reflect the dominance of annular flow and to display a characteristic heat transfer coefficient curve in such configurations It is found that the heat transfer coefficients in the low-quality annular flow segment of this locus can be predicted by available, microtube correlations, but that the moderate-quality transition to the axially-decreasing segment occurs at substantially

Energy Efficiency of Low Form Factor Cooling Devices

Abstract: With increasing attention to the energy efficiency of consumer and commercial products, thermal engineering and science community is devoting greater effort and attention to the design and implementation of energy-efficient cooling solutions. This study focuses on the cooling potential and Coefficient of Performance, (COP), achievable with three distinct meso-scale cooling technologies, applicable to a wide range of electronics cooling challenges. The thermo-fluid and thermodynamic characteristics of synthetic jets, piezo-driven vibrating blades, and compact muffin fans will be addressed. We are dedicating this paper to Prof. Kakac for his contributions to heat transfer science and technology, developing young scientists, writing highly valuable heat transfer textbooks, and most importantly for his kindness and friendship.Copyright © 2007 by ASME

Enhanced Thermoelectric Cooler for On-Chip Hot Spot Cooling

Abstract: Due to shrinking feature size and increasing transistor density, combined with the performance demanded from next-generation microprocessors, on-chip hot spots, with their associated high heat fluxes and sharp temperature gradients, have emerged as the primary driver for thermal management of today’s IC technology. This paper describes the novel use of thermoelectric coolers for on-chip hot spot cooling through the use of a copper mini-contact pad, which connects the thermoelectric cooler and the silicon chip thus concentrating the thermoelectric cooling power. A package-level numerical simulation is developed to predict the local on-chip hot spot cooling performance which can be achieved with such mini-contacts. Attention is focused on the hot spot temperature reduction associated with variations in mini-contact size and the thermoelectric element thickness, as well as the parasitic effect of the thermal contact resistance introduced by the mini-contact enhanced TEC. This numerical model and simulation results are validated by comparison to spot cooling experiments with a uniformly heated chip serving as the test vehicle. The experimental results demonstrate that a copper mini-contact pad can improve spot cooling performance by 80 ∼ 115% on a 500μm thick silicon chip under optimum operating conditions and that larger power dissipation on the chip leads to better spot cooling performance.Copyright © 2007 by ASME

Thermo-Optic Effects in Polymer Bragg Gratings

Abstract: This book encompasses a broad area of microand opto-electronic engineering materials: their physics, mechanics, reliability, and packaging, with an emphasis on physical design issues and problems. The editors tried to bring in the most eminent engineers and scientists as chapter authors and put together the most comprehensive book ever written on the subjects of materials, mechanics, physics, packaging, functional performance, mechanical reliability, environmental durability and other aspects of reliability of microand opto-electronic assemblies, components, devices, and systems. University professors and leading industrial engineers contributed to the book. The contents of the book reflect the state-of-the-art in the above listed fields of applied science and engineering. The intended audience are all those who work in microand opto-electronics, and photonics; electronic and optical materials; applied and industrial physics; mechanical and reliability engineering; electron and optical devices and systems. The expected and targeted readers are practitioners and professionals, scientists and researchers, lecturers and continuing education course directors, graduate and undergraduate students, technical supervisors and entrepreneurs. The book can serve, to a great extent, as an encyclopedia in the field of physics and mechanics of microand opto-electronic materials and structures. In the editors’ opinion, it can serve also as a textbook, as a reference book, and as a guidance for selfand continuing education, i.e., as a source of comprehensive and in-depth information in its areas. The book’s chapters contain both the description of the state-of-the-art in a particular field, as well as new results obtained by the chapter authors and their colleagues. We would like to point out that many methods and approaches addressed in this book extend far beyond microelectronics and photonics. Although these methods and approaches were developed, advanced and reported primarily in application to microand opto-electronic systems, they are applicable also in many related areas of engineering and physics. The editors are proud of the broad scope of the book, and of the quality of the contributed chapters, and would like to take this opportunity to deeply acknowledge, with thanks, the conscientious effort of the numerous contributors.

Thermo-optical modeling of an intrinsically heated polymer fiber Bragg grating

Abstract: A fully concatenated thermo-optical model is presented to predict the thermo-optical behavior of an intrinsically heated polymer fiber Bragg grating (PFBG) Coupled-mode theory and heat-conduction theory are first used to determine the axial heat generation and temperature distribution of a PFBG and the transfer matrix method (TMM) is subsequently employed to predict its thermo-optical behavior The validity of the TMM is corroborated experimentally using an externally heated glass fiber Bragg grating (FBG) with an axially decaying temperature field The verified model is utilized to investigate the thermo-optical behavior of a poly(methyl methacrylate) (PMMA) FBG The counteracting thermally driven changes in the refractive index and the grating pitch, respectively, are found to be of comparable magnitude and to result in very modest net shifts in the Bragg wavelengths despite the considerable temperature changes induced by the absorption of the incident light

Fiber Bragg Grating Sensor to Characterize Curing Process-dependent Mechanical Properties of Polymeric Materials

Abstract: An integrated technique using a fiber Bragg grating (FBG) sensor is proposed to measure critical mechanical properties of polymeric materials. In the method, a polymer of interest is first cured around a glass FBG and the Bragg wavelength (BW) shift is measured and documented while polymerization progresses at the curing temperature. After complete polymerization, the BW shift is monitored continuously as the temperature of the cured polymer changes. The critical properties are then found from the relationship between the Bragg wavelength shift and the deformation of the polymer caused by the changes in the material properties through an over-deterministic inverse approach.

Thermofluid Characteristics of Two-Phase Microgap Coolers

Abstract: The thermofluid characteristics of a chip-scale microgap cooler, including single-phase flow of water and FC-72 and flow boiling of FC-72, are explored. Heat transfer and pressure drop results for single phase water are used to validate a detailed numerical model and, together with the convective FC-72 data, establish a baseline for microgap cooler performance. Experimental results for single phase water and FC-72 flowing in 120 μm, 260 μm and 600 μm microgap coolers, 31mm wide by 34mm long, at velocities of 0.1 – 2 m/s are reported. “Pseudo-boiling” driven by dissolved gas and flow boiling of FC-72 are found to provide significant enhancement in heat transfer relative to theoretical single phase values.Copyright © 2007 by ASME

Thermal Performance Measurements of Thermal Interface Materials Using the Laser Flash Method

Abstract: Thermal interface materials are used to reduce the interfacial thermal resistance between contacting surfaces inside electronic packages, such as at the die-heat sink or heat spreader-heat sink interfaces. In this study, the change in thermal performance was measured for elastomeric gap pads, gap fillers, and an adhesive throughout reliability tests. Three-layer composite structures were used to simulate loading conditions encountered by thermal interface materials in actual applications. The thermal resistance of the thermal interface material, including contact and bulk resistance, was calculated using the Lee algorithm, an iterative method that uses properties of the single layers and the 3-layer composite structures, measured using the laser flash method. Test samples were subjected to thermal cycling tests, which induced thermomechanical stresses due to the mismatch in the coefficients of thermal expansion of the dissimilar coupon materials. The thermal resistance measurements from the laser flash showed little change or slight improvement in the thermal performance over the course of temperature cycling. Scanning acoustic microscope images revealed delamination in one group of gap pad samples and cracking in the putty samples.Copyright © 2007 by ASME

Passive Immersion Cooling of 3-D Stacked Dies

Abstract: Three-dimensional die stacking increases integrated circuit (IC) density, providing increased capabilities and improved electrical performance on a smaller printed circuit board (PCB) footprint area. However, these advantages come at the expense of higher volumetric heat generation rates and decreased thermal and mechanical access to the die areas. Passive immersion cooling, allowing for buoyancy-driven fluid flow between stacked dies, can provide high heat transfer coefficients directly on the die surfaces, can easily accommodate a wide variety of interconnect schemes, and is scalable to any number of dies. A methodology for the optimization of immersion cooled 3-D stacked dies is presented, including the effects of confinement on natural convection and channel boiling. Optimum die spacings for both single and two phase cooling with saturated FC-72 are found to be on the order of 0.5mm for typical microelectronics geometries and to yield heat densities of 10–50 W/cm3 in natural convection and 100–500 W/cm3 in channel boiling.Copyright © 2007 by ASME

Optimization of Pool Boiling Heat Sinks Including the Effects of Confinement in the Interfin Spaces

Abstract: A finite element analysis (FEA) approach is developed and used to efficiently evaluate and optimize the boiling performance of longitudinal rectangular plate fin heat sinks, including the explicit dependence of fin spacing on boiling heat transfer coefficients and CHF. Polished silicon heat sinks are shown to dissipate nearly five times the CHF limit of the unfinned base area and outperform comparable aluminum heat sinks by a factor of two. For optimum heat sink geometries, over the parameter ranges explored, fin thickness is found to be approximately equal to the fin spacing, and the relationship between optimum thickness and spacing is demonstrated to be relatively insensitive to fin thermal conductivity. Results suggest that even greater performance enhancements may be gained with appropriately-selected advanced materials.Copyright © 2007 by ASME

Simplified Thermal Model of Silicon Thermoelectric Microcooler for On-Chip Hot Spot Remediation

Abstract: Thermal management of on-chip hot spots has become an increasing challenge in recent years because such localized high flux hot spots can not be effectively removed by conventional cooling techniques. The authors have recently explored the novel use of the silicon chip itself as a solid state thermoelectric micrcooler (μTEC) for hot spot thermal management. This paper describes the development and application of a thermo-electric design tool based on closed-form equations for the primary variables. This tool can be used to effectively reduce the complexity and required time for the design and optimization of the silicon microcooler geometry and material properties for on-chip hot spot remediation.Copyright © 2007 by ASME