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Nanoengineered Heat Sink Materials
 

Advanced thermal materials will radically improve the performance of devices and instruments such as high-performance computers and high power optical components used in exploration hardware.

Benefit

There exists a crucial need for the development of effective and robust thermal management technologies, including novel thermally conductive materials and structures. Advances in thermal management technology are required for the vast amounts of systems impacted by the need for thermal control.Spacesuits, habitats, and mobile systems will need multi-zone, reconfigurable thermal control systems.This platform technology can be adapted to a wide range of form factors as we have also demonstrated similar technology on a flexible metallic substrate.

Research Overview

Fabrication of the Heat Sink Composite Interface

Our laboratory is equipped with pilot-scale and wafer scale plasma growth chambers for synthesizing the vertically aligned CNTs. Following nanotube synthesis, copper filling between individual MWNTs (hereafter known as nanotube trenches) was accomplished through electro-deposition (Figure 1).In a three-electrode setup with a MWNT array as the working electrode, a Saturated Calomel Electrode (SCE) was used as the reference electrode, and a one square inch platinum foil as the counter electrode (CE), set in parallel with the MWNT sample.Both the metal substrate and MWNTs serve as electrodes during the electrodeposition.

Cross-sectional electron micrographFigure 1. Cross-sectional electron micrograph view of the as-fabricated thermal interface materials. A film of carbon nanotube/copper composite has been shown to be an effective, reusable heat sink material for integrated circuit cooling applications.

Various additives were employed in the solution to achieve optimum gap filling into the high-aspect-ratio "forest-like" CNT arrays.

Interfacial Thermal Characteristics

An apparatus consisting of two copper blocks, four resistive cartridge heaters (not shown) embedded in the upper block, and a cooling bath connected to the lower block was used to measure the thermal resistance of a given material.The upper copper block is surrounded by insulation to minimize heat loss to the ambient, with the exception of the one square inch section designed to contact the material to be measured.The clamping pressure on the sample is controlled by pneumatically manipulating the upper block.Heat is delivered to the system by applying a constant power to the resistive heaters.The steady state temperature difference ( D T=T B -T C ) between the two blocks (and consequently, the sample) was measured.From this data, the thermal resistance of the sample is calculated.Figure 2 shows the thermal resistance as a function of applied power.

Thermal contact resistance vs pressure for MWNT-Cu composite film at input power of 27.2 WFigure 2. Thermal contact resistance vs pressure for MWNT-Cu composite film at input power of 27.2 W. The upper curves show the measurement results considering the Microfaze A6 and single silicon interface with the copper block, respectively. The lower curve represents the interfacial resistance of the composite film with the copper block. The solid lines represent the best fit data trends.

The preliminary data indicate that there is a definite incentive in using CNTs and CNT-Cu composite films as efficient heat conductors.Our study confirms that these novel films can accomplish effective heat conduction by increasing contact area.In addition, it has been shown that CNTs provide the added benefit of high mechanical stability and reusability.This method can be integrated into various packaging processes and designs for providing highly efficient device cooling, such as a package shown in Figure 3.

Background

With the progress in the ultra-large-scale-integration (ULSI) of integrated circuits (ICs), microelectronic components and devices are becoming increasingly more dense and compact.State-of-the-art ICs for microprocessors operated at high frequencies are routinely characterized by power densities on the order of 50 W/cm 2 . Such a large density leads to highly localized heating of ICs ("hot spots").

An alarming rise in power density and the increased number of "hot spots" on the surface of high power chips have been observed in many mainstream microprocessor technologies.The ability to solve this problem is imperative for the next-generation IC packages. Likewise, cooling other high power devices such as lasers and detectors in Space exploration hardware is critical for achieving the highest possible performance and instrument resolution.

fabricated thermal heat sinkFigure 3. A fabricated thermal heat sink device for use in integrated circuit packaging technology. The Cu/CNT composite is shown on the backside of the chip.

Carbon nanotube (CNT) based systems are able to provide a solution to the problem of effective thermal management of high power devices. CNT arrays and CNT-based composite structures have demonstrated their superior thermal conductance and therefore have a tremendous potential for providing the most efficient heat transfer. Particularly, it has been shown, that CNTs exhibit a very high "axial" thermal conductivity. For a discrete multiwalled nanotube (MWNT), the thermal conductivity is expected to surpass 3000 W/m-k along the tube axis.Using a DC-biased plasma-enhanced chemical vapor deposition (PECVD) technique, one can fabricate vertically aligned MWNT arrays on silicon wafers or copper substrates, and the fabricated structure can be successfully employed as an effective heat-sink, which is able to remove large amounts of heat away from critical "hot spots" in ICs.Additional advantages of a CNT-based heat sink design are that the CNT structures (modules) of the type in question are reusable.