3ω Technique Research Articles

Thermal conductivity and interfacial thermal resistance of polymeric low k films

The effective thermal conductivity of four polymeric thin films with distinct molecular morphologies has been measured as a function of film thickness down to 70 Å using a 3ω technique. Comparing to SiO2, the intrinsic thermal conductivity of polymers is about 5–10 times smaller while the interfacial thermal resistance is about 2–10 times larger. The interfacial thermal resistances are explained in terms of the mismatches of acoustic and mechanical properties between polymers and crystalline materials. Both elastic and inelastic scatterings at the interface are examined.

Porous thin films for thermal barrier coatings

A new approach is described in the deposition of thin films for thermal barrier applications. Using controlled substrate motion, porous layers and capping layers were vacuum deposited in an alternating fashion, creating a new, multilayered film structure. Direct measurements of the thermal properties of these multilayers were made using the 3ω and Mirage techniques. In the 3ω technique, heat is introduced into the coating by an AC current flowing through an evaporated resistor with a frequency ω. A fit of resistor voltage as a function of frequency yields the thermal conductivity. In the Mirage technique, an oscillating temperature is induced immediately above the film using a pulsed laser. A second probe laser aligned parallel to the surface is deflected by these temperature variations, and the thermal diffusivity is then found by fitting amplitude and phase shift data to the solution of the three-dimensional diffusion equation. Typically, the 3ω and Mirage techniques measure thermal constants in directions normal and parallel to the substrate, respectively. Measurements using these methods led to estimates of a reduction in thermal diffusivity of as little as 9% of that of films deposited entirely at normal incidence. Thermal simulations of similar structures also predicted a substantial decrease in overall thermal conductivity. In a specific case, an improved conductivity of 18% of that of films deposited by standard techniques was estimated.

Thermal conductivity study of porous low-k dielectric materials

An experimental method based on the 3ω technique has been developed to measure thermal conductivity of porous Xerogel films as a function of porosity. The results show that the thermal conductivity of these porous dielectric films can be an order of magnitude smaller than that of SiO2. To account for the porosity dependence of thermal conductivity, two porosity weighted semiempirical models are introduced. These models suggest the scaling rule expressing the thermal conductivity as a function of porosity. The decrease observed in thermal conductivity of porous films suggests that the tradeoff between thermal and electrical performance is an important consideration when implementing porous dielectric materials as interlevel dielectrics for on-chip interconnects.

High Temperature Thermal Conductivity Measurements of Quasicrystalline Al70.8Pd20.9Mn8.3

ABSTRACTWe report measurements of the thermal conductivity on a potential high temperature thermoelectric material, the quasicrystal Al70.8Pd20.9Mn8.3. Thermal conductivity is determined over a temperature range from 30 K to 600 K, using both the steady state gradient method and the 3ω method. Measurements of high temperature thermal conductivity are extremely difficult using standard heat conduction techniques. These difficulties arise from the fact that heat is lost due to radiative effects. The radiative effects are proportional to the temperature of the sample to the fourth power and therefore can lead to large errors in the measured thermal conductivity of the sample, becoming more serious as the temperature increases. For thermoelectric applications in the high temperature regime, the thermal conductivity is an extremely important parameter to determine. The 3ω technique minimizes radiative heat loss terms, which will allow for more accurate determination of the thermal conductivity of Al70.8Pd20.9Mn8.3 at high temperatures. The results obtained using the 3ω method are compared to results from a standard bulk-thermal-conductivity-technique on the same samples over the temperature range, 30 K to 300 K.

Thermal characterization of power devices by scanning thermal microscopy techniques

The scanning thermal microscope (SThM) is used for investigating chip temperatures of power devices under operating conditions enabling the localization of failure-induced hot spots. In this paper, we present the measurement of the temperature distribution on the surface of a free-wheeling diode of an IGBT. A new modulation technique implemented into the SThM for the determination of local quantitative thermal conductivities, the so-called 3ω technique, is also used for further analyses of defective power devices. With this technique, a non-uniformity of the thermal conductivity in the micrometer range could be found at the anode side of a GTO.

Process-dependent thermal transport properties of silicon-dioxide films deposited using low-pressure chemical vapor deposition

The volumetric heat capacity and thermal conductivity of silicon-dioxide films prepared using low-pressure chemical vapor deposition (LPCVD) are measured. The measurements employ the 3ω technique, which is extended to determine the thermal conductivity anisotropy and volumetric heat capacity of thin dielectric films. The thermal conductivity of the silicon-dioxide films exhibits a significant process dependence, which cannot be attributed to highly oriented microvoids or impurities. The volumetric heat capacity, in contrast, is largely independent of processing history provided that appropriate corrections are made to account for porosity and impurity contributions. This study provides evidence that process-dependent structural disorder strongly influences the thermal conductivity of amorphous films.

Thermal characterization of anisotropic thin dielectric films using harmonic Joule heating

A technique for thermal characterization of anisotropic dielectric films is developed. The technique is applicable to a film which has thickness of the order of 1 μm and is on a substrate with high thermal conductivity. Metal lines with various widths are deposited on the film using standard IC fabrication processes and are subjected to harmonic Joule heating. Monitoring the resulting amplitude of temperature oscillations in the metal lines allows the determination of the in-plane and out-of-plane thermal conductivity. The present work performs thermometry using the electrical method known as the 3ω technique and also the thermoreflectance technique. Measurement results are reported for polyimide films on silicon substrates.

Synthesis of New Thermoelectrics Using Modulated Elemental Reactants

AbstractA series of new, metastable ternary crystalline compounds with the skutterudite crystal structure have been synthesized using modulated elemental reactants. The initial reactants are made up of multiple repeats of a ˜25Å thick unit containing elemental layers of the desired ternary metal, iron and antimony. Low temperature annealing (150°C) results in interdiffusion of the elemental layers to form amorphous reaction intermediates. Annealing these intermediates at temperatures between 200°C and 250°C results in exothermic crystallization of the desired skutterudite crystal structure. Most of the new compounds prepared are only kinetically stable, decomposing exothermically to form thermodynamically more stable mixtures of binary compounds and elements. Low angle x-ray diffraction studies show that the resulting films are exceedingly smooth. These films have an ideal geometry for measuring properties of importance for thermoelectric devices—the Seebeck coefficient and the electrical conductivity. Thermal conductivity can be measured using a modification of the 3ω technique of Cahill. Samples can be produced rapidly, allowing for systematic screening and subsequent optimization as a function of composition and doping levels.

The 3ω technique for measuring dynamic specific heat and thermal conductivity of a liquid or solid

We show how to measure dynamic specific heat and thermal conductivity of a solid or liquid sample using the 3ω technique, which is an ac-modulation method where we use a heater simultaneously as the sensor. By varying the width of the heater relative to the thermal decay length, one can choose the proper regime to measure thermal conductivity or specific heat. The technique is applied to window glass and the results confirm the validity of the method. Experimental results for potassium dihydrogen phosphate crystal demonstrate the first-order transition at the Curie point, and the dynamic specific heat of supercooled liquid potassium–calcium nitrate is shown.