Papers

AVS2004 Session CT+TF-MoM: Thermal Transport in Thin Films and Nanostructured Materials

Monday, November 15, 2004 8:20 AM in Room 303B

Monday Morning

Start	Invited?	Item
8:20 AM		CT+TF-MoM-1 Impact of Electron-Phonon Coupling on Thermal Boundary Resistance by Molecular-Dynamics Simulation R.J. Stevens, P.M. Norris (University of Virginia) With the growing interest in ULSI circuits and superlattices, an increasing need to understand thermal transport mechanisms across interfaces has become necessary. As the density of interfaces rapidly increases, device level thermal management is no longer dominated by the thermal properties of the individual layers but rather the thermal boundary resistances (TBR). Unfortunately, our current understanding of room temperature TBR is not adequate for proper thermal design of interface dense devices. Most TBR theoretical work has been an extension of the acoustic mismatch theories and has been limited to phonon elastic scattering processes for perfect interfaces. Other transport mechanisms have been considered such as electron-phonon (e-p) scattering and inelastic phonon scattering. There has been very little effort to systematically measure room temperature TBR and verify the proposed theories. Unfortunately, measuring TBR is quite difficult, although there has been some success using ultrafast spectroscopy techniques. Alas, it is problematical to systematically fabricate and fully characterize a series of interfaces and validate the proposed models. Molecular-dynamics simulations (MDS) can enhance existing experimental work by allowing analysis of controlled and well-defined interfaces. MDS enable the ability to alter material properties and atomic-level structure of the interface, so the mechanisms of TBR can better be understood. In this work, we perform MDS of the energy transport through an interface of a Si â?" metal system described by semi-empirical potentials (Stillinger-Weber and Embedded Atom Method). The electronic heat conduction in the metal film and e-p coupling are included by using a recently developed model that combines MD with a continuum description of the evolution of the electron temperature. The electron scattering contribution to TBR is determined by altering the e-p coupling strength in the bulk and at the interface.
8:40 AM		CT+TF-MoM-2 Thin Film SiGe Superlattice Micro Refrigerators Flip-Chip Bonded with IC Chips Y. Zhang, A. Shakouri (University of California, Santa Cruz); G. Zeng (University of California, Santa Barbara); P. Wang, A. Bar-cohen (University of Maryland) Thin film SiGe-based superlattice micro refrigerators, with device sizes ranging from 40-150 micron in diameter, have demonstrated cooling by 4.5C at ambient temperature and cooling power density exceeding 500W/cm2. In this talk we present theoretical and experimental study of these thin film refrigerators flip-chip bonded underneath 50 micron thick silicon substrate. The idea is to evaluate the effectiveness of these refrigerators to eliminate hot spots in IC chips without modifying the IC processing steps. Even though the 50 micron thick silicon has relatively high thermal conductivity and the cooling of micro refrigerators is spread over larger areas, theoretical studies show that localized cooling by 1-2C with cooling power density exceeding 100W/cm2 should be possible. Experimentally three micron thick gold-to-gold bonding is used to attach the two wafers. Cooling on top of the silicon heat load wafer is measured using microthermocouples. Thin film resistor heaters are used to evaluate the cooling power density. We have achieved cooling power density ~40W/cm2. It is interesting to note that even though bare microrefrigerators have an optimum size for maximum cooling on the order of 70 microns in diameter and largest cooling power density is obtained with the smallest devices, in the two-chip bonded configuration, the biggest coolers have the largest cooling and the cooling power density is not a strong function of the size. 3D electrothermal simualtions are used to explain the measured results and to evaluate maximum cooling performance under various ideal and non-ideal conditions.
9:00 AM		CT+TF-MoM-3 Interfaces, Functionalization and Heat Flow in Nanoscale Materials. S. Shenogin, A. Bodapati, L. Xue, P. Keblinski (Rensselaer Polytechnic Institute) The influence of the interface resistance on heat exchange between carbon nanotubes (CNs), fullerenes and embedding soft material medium was studied by means of molecular dynamics simulation. Due to a weak coupling between thermal vibrations of stiff carbon nanostuctures and soft organic matrix, the inclusion-matrix interface has high thermal resistance (Kapitza resistance). Recent experiments and simulations showed that the resistance of such interfaces is equivalent to the matrix layer with thickness 5 to 20 nm¹,². High boundary resistance considerably reduces thermal conductivity of the nanotube-based polymer composites and limits potential heat management applications. Our simulation shows that chemical functionalization of the nanoparticles with short organic chains reduces the interface thermal resistance by enhancing interactions between nanoparticels and the matrix. Detail dynamical analysis demonstrates that functionalization widens the overlap between the vibrational spectra of carbon nanostructures and the matrix. Remarkably, in the case of fullerenes, functionalization with a single chain reduces interfacial resistance by a factor of ~ 5. In the case of CN the interface resistance is reduced 4 times when 7 or more % of carbon atoms are functionalized. However, the functionalization decreases the intrinsic high thermal conductivity along the nanotube. The selection of the optimal side group length and concentration will be discussed. ¹ S.T Huxtable et. al., Nature Materials, 2, 731 (2003). ² S.Shenogin et. al, J.Appl.Phys 95 (12), (2004).
9:20 AM		CT+TF-MoM-4 Thermal Transport in Nanostructured Materials D. Cahill (University of Illinois, Urbana-Champaign) The thermal conductance of interfaces is a key factor in controlling the thermal conductivity of materials with high densities of internal interfaces, e.g., nanocomposites, nanocrystalline ceramics, and short-period multilayer films and superlattices. Low interface conductance puts a lower limit on the size of nanoparticles that can be used as fillers in thermal interface materials and limits the increases in the the thermal conductivity that can be achieved in carbon nanotubes composites. We study these effects using high-precision measurements of thermal conductivity using the 3-omega method and psec transient absorption measurements of the thermal decay time of carbon-nanotubes suspended in micelles in water. We have also recently advanced the state-of-the-art of time-domain-thermoreflectance (TDTR) measurements of thermal transport and are using TDTR to study heat transport across individual interfaces and the thermal conductivity of sputtered multilayers. The thermal conductance of epitaxial interfaces between similar materials approaches the high values predicted by simple theory. Nanolaminates of dissimilar materials show remarkable reductions in thermal conductivity when the layer thickness is a few nm; this approach provides a novel materials with ultra-low thermal conductivity without sacrificing strength or environmental protection.
10:00 AM		CT+TF-MoM-6 Thin Film Micro Refrigerators for on Chip Thermal Management A. Shakouri (University of California at Santa Cruz) In this talk, we review design considerations for high cooling power density thermoelectric/thermionic coolers. Conventional bismuth telluride-based thermoelectric modules have a maximum cooling of about 70° C, however the cooling power density is low, on the order of 1-10 W/cm2. The micro and nanoscale electronic devices can generate thousands of watts per centimeter square heating, which is far beyond the capability of current TE modules. The maximum cooling power density of a TE module is inversely proportional to the length of its elements (distance between hot and cold junctions). Thus it is possible to increase the cooling power density with the use of thin film material. 100 micron thick Peltier modules with cooling power density exceeding 100W/cm2 have been demonstrated. Further increase requires significant improvement in metal-semiconductor contact resistance and in heat sink thermal resistance. An alternative solution is to use the thermoelectric properties of silicon or III-V substrate material. Heat and current spreading in 3D electrode configuration, allow removal of hot spots in IC chips. Furthermore, addition of a 1-5 micron thick superlattice can improve the cooling performance by increasing the selection between hot and cold carrier transport via thermionic emission and by reducing thermal resistance between hot and cold junctions. Several III-V and silicon heterostructure integrated thermionic (HIT) microcoolers have been fabricated and characterized. They have achieved cooling, on the order of 4.5° C at room temperature and 12° C at 200° C ambient temperature. Cooling power density was also characterized and values ranging from 100-680W/cm2 were measured. Finally, an optical technique based on thermoreflectance imaging was used to obtain temperature distributions on top of devices with sub micron spatial resolution and <0.1° C temperature resolution.¹ ¹ Work supported by DARPA and ONR MURI.
10:40 AM		CT+TF-MoM-8 Micro- and Nanoscale Thermal Phenomena in Magnetic Recording Heads Y.S. Ju (UCLA) Advances in magnetic recording heads are critical in enabling continued growth in areal densities of magnetic data storage. In the present talk we review studies of thermal characteristics of recording heads at micro- and nanoscales, which strongly affect their performance and reliability. Geometric scaling of GMR (giant magnetoresistance) and TMR (tunneling magnetoresistance) sensors leads to significant increase in temperature rise per unit power, which limits maximum permissible bias current and increases electrostatic discharge (ESD) susceptibility. This motivates fundamental studies of thin film thermal conductivity, thermal interface resistance, and heat generation due to electron tunneling. Reliability of magnetic recording heads is also compromised by thermal protrusion and thermal creep flow induced by Joule heating in write heads. Spatial distribution of eddy current heating in magnetic yoke structures and heat transfer across air bearing are important factors influencing thermomechanical behavior of recording heads and reliability of head-disk interface. Fundamental understanding of thermal phenomena in magnetic recording heads will also play an important role in the development of thermally-assisted recording, which is a promising approach to extending magnetic recording beyond the superparamagnetic limit of conventional recording media.
11:20 AM		CT+TF-MoM-10 Transport and Mechanics in Hard and Soft Nanomaterials A. Majumdar (University of California, Berkeley) Hard and soft materials are characterized by the ratio of their respective binding energies (Eb) with respect to thermal fluctuations that are characterized by kT. Mechanics and dynamics of hard materials (Eb >> kT) are generally unaffected by kT, except when undergoing irreversible processes such as transport phenomena or inelastic deformations. On the other hand, fluctuations dominate the behavior of soft materials (Eb ~ kT) such as liquids and biomolecules, where entropic forces are critical in their mechanics. As part of this lecture, I will focus on two topics, both of which relate to the interplay between entropic and elastic forces: (i) Transport of heat and fluid in solid nanostructures such as nanotubes and nanowires. I will share some of our recent discoveries of how heat and charge transport in such nanostructures can be manipulated by size confinement and interface engineering; (ii) Actuation of mechanical devices such as cantilever beams using reactions of biomolecules (eg. DNA hybridization, antigen-antibody binding). I will also discuss the implications of our work on energy conversion and biomedical technologies.