Showing papers by "Samuel Graham published in 2005"

A practical extension of the 3ω method to multilayer structures

Abstract: An improved data reduction method is used to extend the popular 3ω method to general layered geometries. This approach utilizes an unapproximated analytical solution, cast in terms of thermal impedance, to simultaneously measure thermal conductivity, thermal capacity, conductivity anisotropy, and interlayer contact resistance in multilayer planar structures. This method places no restrictions on the number or thickness of individual material layers, and it allows experimental measurements to be taken over a much wider range of frequencies than was previously possible. The search algorithm associated with the model is straightforward, robust, and requires no specialized software to compose. Experimental results are presented for a two-layer borosilicate glass/zeolite structure as well as a Si∕SiO2 structure. In both instances, the algorithm was able to simultaneously extract thermal conductivity and thermal diffusivity values using a single series of 3ω measurements.

Thermal transport properties of thin films of small molecule organic semiconductors

Abstract: A series of harmonic Joule-heating experiments have been employed to determine the thermal conductivities of thin films of pentacene, N,N′-diphenyl−N,N′-di(3-methylphenyl)−(1,1′-biphenyl)-4,4′-diamine, and tris(8-hydroquinolinato)aluminum, three widely used organic semiconductors Room-temperature thermal conductivity values of 051, 024, and 048W∕mK were measured for films of these three compounds, respectively These values are over two orders of magnitude lower than those of inorganic semiconductors While amorphous films were found to display only small thermal conductivity changes over the temperature range of 228–350 K, pentacene exhibited stronger variations that are typical of phonon-phonon scattering observed in polycrystalline semiconductors

Heat dissipation in high-power GaN electronics on thermally resistive substrates

Abstract: The heat dissipation in GaN devices grown on low thermal conductivity lithium gallate (LGO) substrates was investigated. The thermal conductivity of single-crystal LGO was measured utilizing the 3/spl omega/ technique for temperatures ranging from 100 K-500 K. For the GaN layer, the thermal conductivity was estimated using a phonon transport model which included dislocation density and temperature dependence. These data were then used in a finite element program to determine the thermal behavior of a heterojunction field-effect transistor. Based on a maximum junction temperature of 500 K, it was found that devices with a power dissipation of 1 W/mm were possible if the primary heat dissipation path was through the low thermal conductivity substrate. However, in using a front side cooling scheme, results suggest that it may be possible to develop devices with power dissipation in the range of 10 W/mm.

Thermal metrology of silicon microstructures using Raman spectroscopy

Abstract: The effects of temperature and stress on the Raman shift in single crystal silicon and polycrystalline silicon films were calibrated. Polysilicon films were grown by LPCVD using a range of temperatures to produce amorphous and crystalline materials followed by doping and annealing. The dependencies of the linear coefficients were related to the polysilicon microstructure using AFM surface scans to determine any possible links. Finally, the technique was utilized in measuring the temperature distribution in a thermal MEMS cantilever device with micron spatial resolution.

Thermometry of Polycrystalline Silicon Structures Using Raman Spectroscopy

Abstract: Raman spectroscopy was investigated as a method for the temperature and stress measurement in thermal MEMS devices. Calibrations of the Stokes Raman shift and the Stokes to anti-Stokes intensity ratio for doped samples were performed in order to calculate the temperature on simple polysilicon structures. Straight and serpentine micro-heaters of various sizes were fabricated from 2 micron doped polysilicon films on thick sacrificial oxide layers. Operating temperatures were measured at a range of input powers for devices attached to the oxide layer as well as released structures. Measurements show that all devices can exceed 400°C with the released devices requiring much less power, as expected. Temperature measurements using the Stokes shift method were compared to the conventional intensity ratio method in order to deduce the effects of thermal stresses on the temperature measurements. Using this method, it was found that thermal stresses could be qualitatively determined simultaneously with temperature in silicon MEMS devices. The effects of stress, however, results in less than a 10% difference in temperature over all of the input powers tested in this study.Copyright © 2005 by ASME

Heat Dissipation in High-Power GaN Electronics on

Abstract: The heat dissipation in GaN devices grown on low thermal conductivity lithium gallate (LGO) substrates was inves- tigated. The thermal conductivity of single-crystal LGO was mea- sured utilizing the technique for temperatures ranging from 100 K-500 K. For the GaN layer, the thermal conductivity was esti- mated using a phonon transport model which included dislocation density and temperature dependence. These data were then used in a finite element program to determine the thermal behavior of a heterojunction field-effect transistor. Based on a maximum junc- tion temperature of 500 K, it was found that devices with a power dissipation of 1 W/mm were possible if the primary heat dissipation path was through the low thermal conductivity substrate. However, in using a front side cooling scheme, results suggest that it may be possible to develop devices with power dissipation in the range of 10 W/mm.

Thermal design considerations in the packaging of GaN based light emitting diodes

Abstract: The temperature distribution of a dual Multi-Quantum Well (MQW) light emitting diode (LED) has been investigated using both infrared imaging and micro-Raman Spectroscopy; mean values over the device yielded temperatures ranging from 30-75°C. The InGaN/GaN based LED, grown by Metal Organic Chemical Vapor Deposition (MOCVD), was also studied using the 3ω method in order to determine an effective thermal conductivity of the MQW stack in the temperature range from 300-540K. The LED structure under investigation showed effective thermal conductivities in the range from 82-140 W/mK with the peak conductivity occurring at 440K, well above room temperature. Using temperature dependent properties determined experimentally, a numerical model of the LED structure was developed in order to study the effect that the package thermal resistance and input power has on the temperature of the device.

Characterization of Heated Atomic Force Microscope Cantilevers in Air and Vacuum

Abstract: This paper presents characterization of heated atomic force microscope (AFM) cantilevers in air and helium, both at atmospheric pressure and in a partially evacuated environment. The cantilevers are made of doped single-crystal silicon using a standard silicon-on-insulator cantilever fabrication process. The electrical measurements show the link between the cantilever temperature-dependant electrical characteristics, electrical resistive heating, and thermal properties of the heated AFM cantilever and its surroundings. Laser Raman thermometry measures temperature along the cantilever with resolution near 1 μm and 4°C. By modulating the gaseous environment surrounding the cantilever, it is possible to estimate the microscale thermal coupling between the cantilever and its environment. This work seeks to improve the calibration and design of heated AFM cantilevers.Copyright © 2005 by ASME

Extending the 3 Omega Method to Measure Thermal Conductivity Anisotropy in Three Dimensions

Abstract: A simplified method of measuring three-dimensional thermal conductivity anisotropy has been developed around the popular 3ω method. This approach utilizes a pair of surface mounted test elements as both heating and sensing probes. 3ω has historically been used to measure thermal conductivity in homogeneous, low conductivity substrates and thin surface films. The present technique employs a combination of broad band frequency measurements and directionally oriented test elements to extract property information that was previously inaccessible. Data reduction is dramatically improved through the use of a full accuracy analytical solution that replaces the heavily approximated original 3ω solution. Anisotropic degrees of freedom are optimized using an efficient gradient based algorithm that can be coded using any number of commercially available software packages. Test element design guidelines are presented to help insure that optimal experimental conditions exist during material testing. Comparative experimental results are presented for an aligned graphite fiber / epoxy composite.Copyright © 2005 by ASME