Increasing power densities and decreasing transistor dimensions are hallmarks of modern computer chips. Both trends are increasing the thermal management challenge within the chip and surrounding packaging, as well as accelerating research progress on high-conductivity materials. This article reviews recent materials advances with a focus on novel composite substrates and interface materials, including those composites leveraging the high conductivities of carbon nanotubes. Furthermore, attention is given to the special properties of one-dimensional structures that are likely to be of increasing importance in future applications.
Thermal management is widely recognized to be an important aspect of computer design, with device performance being significantly affected by temperature. In addition, device lifetime can be decreased drastically because of large thermal stresses that occur especially at interfaces. The ability of a structure to remove heat is best quantified by its thermal resistance, which is given by the temperature difference divided by input power. In microprocessor design, the allowable temperature drop between the transistor (where most of the heat is generated) and the ambient air is constant. As a result, the challenge for thermal management is to develop high-conductivity structures that can accommodate this fixed temperature drop with the increasing power densities that characterize new generations of microprocessors.
We review the recent progress in thermal characterization and sensor applications of one-dimensional nanostructures employing microelectromechanical system (MEMS) devices. It was found by thermal measurements that the thermal conductance of a single wall carbon nanotube (SWCNT) was very close to the ballistic thermal conductance of a 1-nm-diameter SWCNT without signatures of phonon−phonon Umklapp scattering, a high thermoelectric figure of merit can potentially be obtained in bismuth telluride (BixTe1-x) nanowires with an optimized atomic ratio of x, and the thermal conductivity of metal oxide nanobelts was suppressed by increased phonon-boundary scattering. We further suggest that dielectrophoresis and other directed-assembly methods can be used for the large-scale integration of nanowires with MEMS to obtain ultrasensitive, stable, and selective sensor systems.
We have observed experimentally that the thermal conductance of a 2.76-μm-long individual suspended single-wall carbon nanotube (SWCNT) was very close to the calculated ballistic thermal conductance of a 1-nm-diameter SWCNT without showing signatures of phonon−phonon Umklapp scattering for temperatures between 110 and 300 K. Although the observed thermopower of the SWCNT can be attributed to a linear diffusion contribution and a constant phonon drag effect, there could be an additional contact effect.
For an electrodepositedbismuth telluride (BixTe1−x)nanowire from one batch with x found to be about 0.46, the Seebeck coefficient (S) was measured to be 15%–60% larger than the bulk values at temperature 300K. For four other nanowires from a different batch with x≈0.54, S was much smaller than the bulk values. The electrical conductivity of the nanowires showed unusually weak temperature dependence and the values at 300K were close to the bulk values. Below 300K, phonon-boundary scattering dominated phonon-phonon Umklapp scattering in the nanowires, reducing the lattice thermal conductivity.
In this paper, we have designed and fabricated a microfluidic channel to focus biological cells using dielectrophoresis for cytometry applications. The device consists of an elliptic-like channel fabricated by isotropic etching of soda lime glass wafers and a subsequent wafer-bonding process. Microelectrodes are patterned on the circumference of the channel to generate ac fringing fields that result in negative dielectrophoretic forces directing cells from all directions to the center of the channel. An analysis using a thin shell model and experiments with microbeads and human leukemia HL60 cells indicate that biological cells can be focused using an ac voltage of an amplitude up to 15 Vp-p and a frequency below 100 kHz, respectively. This design eliminates the sheath flow and the fluid control system that makes conventional cytometers bulky, complicated, and difficult to operate, and offers the advantages of a portable module that could potentially be integrated with on-chip impedance or optical sensors into a micro total analysis system.
A three-dimensional electrothermal model has been developed to investigate the spatial resolution of the scanning thermoelectric microscopy (SThEM). We found that if the electrical resistivity of the sample changes abruptly, the SThEM will measure a voltage close to the local thermoelectric voltage where electrical resistivity is relatively low, rather than a simple weighted average of the thermoelectric voltage distribution based on the temperature profile. This is due to the presence of internal currents in the sample. The spatial resolution of the Seebeck profiling is limited by the finite value of the phonon mean free path of the sample and the tip size of the microscopy. With a tip size around 1 nm, the scanning thermoelectric microscopy can achieve a spatial resolution of the physical limit defined by the statistical nature of the charge carrier and phonon behavior in a very small region.
We have probed the local thermoelectric power of semiconductor nanostructures with the use of ultrahigh-vacuum scanning thermoelectric microscopy. When applied to a p-n junction, this method reveals that the thermoelectric power changes its sign abruptly within 2 nanometers across the junction. Because thermoelectric power correlates with electronic structure, we can profile with nanometer spatial resolution the thermoelectric power, band structures, and carrier concentrations of semiconductor junctions that constitute the building blocks of thermoelectric, electronic, and optoelectronic devices.
We have measured the thermal conductivities of a 53-nm-thick and a 64-nm-thick tin dioxide (SnO2)nanobelt using a microfabricated device in the temperature range of 80–350 K. The thermal conductivities of the nanobelts were found to be significantly lower than the bulk values, and agree with our calculation results using a full dispersion transmission function approach. Comparison between measurements and calculation suggests that phonon–boundary scattering is the primary effect determining the thermal conductivities.
We have batch-fabricated a microdevice consisting of two adjacent symmetric silicon
nitride membranes suspended by long silicon nitride beams for measuring thermophysical
properties of one-dimensional nanostructures (nanotubes, nanowires, and nanobelts)
bridging the two membranes. A platinum resistance heater/thermometer is fabricated on
each membrane. One membrane can be Joule heated to cause heat conduction through
the sample to the other membrane. Thermal conductance, electrical conductance, and
Seebeck coefficient can be measured using this microdevice in the temperature range of
4–400 K of an evacuated Helium cryostat. Measurement sensitivity, errors, and uncertainty
are discussed. Measurement results of a 148 nm and a 10 nm-diameter single wall
carbon nanotube bundle are presented. @DOI: 10.1115/1.1597619#
We discuss the mesoscopic experimental measurements of electron energy dissipation, phonon thermal transport, and thermoelectric phenomena in individual carbon nanotubes. The temperature distributions in electrically heated individual multiwalled carbon nanotubes have been measured with a scanning thermal microscope. The temperature profiles along the tube axis in nanotubes indicate the bulk dissipation of electronic energy to phonons. In addition, thermal conductivity of an individual multiwalled nanotube has been measured using a microfabricated suspended device. The observed thermal conductivity is two orders of magnitude higher than the estimation from previous experiments that used macroscopic mat samples. Finally, we present thermoelectric power (TEP) of individual single walled carbon nanotubes using a novel mesoscopic device. A strong modulation of TEP as a function of the gate electrode was observed.
This paper presents a technique, scanning thermal wave microscopy (STWM), which can
image the phase lag and amplitude of thermal waves with sub-micrometer resolution by
scanning a temperature-sensing nanoscale tip across a sample surface. Phase lag measurements
during tip-sample contact showed enhancement of tip-sample heat transfer due
to the presence of a liquid film. The measurement accuracy of STWM is proved by a
benchmark experiment and comparison to theoretical prediction. The application of
STWM for sub-surface imaging of buried structures is demonstrated by measuring the
phase lag and amplitude distributions of an interconnect via sample. The measurement
showed excellent agreement with a finite element analysis offering the promising prospects
of three-dimensional thermal probing of micro and nanostructures. Finally, it was
shown that the resolving power of thermal waves for subsurface structures improves as
the wavelengths of the thermal waves become shorter at higher modulation frequencies.
The thermal conductivities of individual single crystalline intrinsic Si nanowires with diameters of 22, 37, 56, and 115 nm were measured using a microfabricated suspended device over a temperature range of 20–320 K. Although the nanowires had well-defined crystalline order, the thermal conductivity observed was more than two orders of magnitude lower than the bulk value. The strong diameter dependence of thermal conductivity in nanowires was ascribed to the increased phonon-boundary scattering and possible phonon spectrum modification.
We describe a thermoelectric devicestructure that confines the thermal gradients and electric fields at the boundaries of the cold end, and exploits the reduction of thermal conductivity at the interfaces and the poor electron-phonon coupling at the junctions. The measuredtemperature–current and voltage–current characteristics of a prototype cold point-contactthermoelectric cooler based on a p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.9Se0.1material system indicate an enhanced thermoelectric figure-of-merit ZT in the range of 1.4–1.7 at room temperature.
We have measured the thermal conductivity of an individual multiwalled carbon nanotube (MWNT) using a microfabricated suspended device. The observed thermal conductivity is more than 3000 W/K-m at room temperature, which is two orders of magnitude higher than the estimation from previous experiments that used macroscopic mat samples. In addition, the temperature distributions in electrically heated MWNTs have been measured with a scanning thermal microscope. The temperature profiles along the tube axis in MWNTs indicate the bulk dissipation of electronic energy to phonons, which suggests diffusive electronic transport.
We have experimentally investigated the heat transfer mechanisms at a 90610 nm diameter point contact between a sample and a probe tip of a scanning thermal microscope (SThM). For large heated regions on the sample, air conduction is the dominant tipsample heat transfer mechanism. For micro/nano devices with a submicron localized heated region, the air conduction contribution decreases, whereas conduction through the solid-solid contact and a liquid meniscus bridging the tip-sample junction become important, resulting in the sub-100 nm spatial resolution found in the SThM images. Using a one dimensional heat transfer model, we extracted from experimental data a liquid film thermal conductance of 6.761.5 nW/K. Solid-solid conduction increased linearly as contact force increased, with a contact conductance of 0.7660.38 W/m2-K-Pa, and saturated for contact forces larger than 38611 nN. This is most likely due to the elasticplastic contact between the sample and an asperity at the tip end. @DOI: 10.1115/1.1447939# Keywords: Contact Resistance,
The thermal conductivity and thermoelectric power of a single carbon nanotube were measured using a microfabricated suspended device. The observed thermal conductivity is more than 3000 W/K m at room temperature, which is 2 orders of magnitude higher than the estimation from previous experiments that used macroscopic mat samples. The temperature dependence of the thermal conductivity of nanotubes exhibits a peak at 320 K due to the onset of umklapp phonon scattering. The measured thermoelectric power shows linear temperature dependence with a value of 80μV/K at room temperature.
One of the key issues related to studies in microscale heat transfer is the ability to measure temperature at small scales. In the recent past, rapid and significant progress has enabled temperature measurements to be made with unprecedented high spatial and temporal resolutions. This has allowed heat transfer research to enter a new regime which was previously inaccessible. This article reviews recent developments and discusses future directions, indicating the variety of opportunities for research that are of scientific and technological importance.
We have designed and batch-fabricated thin-film thermocouple cantilever probes for scanning thermal microscopy (SThM). Here, we report the use of these probes for imaging the phonon temperature distribution of electrically heated carbon-nanotube (CN) circuits. The SThM images reveal possible heat dissipation mechanisms in CN circuits. The experiments also demonstrate that heat flow through the tip-sample nanoscale junction under ambient conditions is dominated by conduction through a liquid film bridging the two surfaces. With the spatial resolution limited by tip radius to about 50 nm, SThM now offers the promising prospects of studying electron-phonon interactions and phonon transport in low dimensional nanostructures.