A two-sensor 3ω-2ω method for thermal boundary resistance measurement

We use the 3ω method to measure the effective thermal conductivity of thin films of a-Ge with thicknesses of 20–150 nm in the temperature range of 30–300 K. By using a moving shadow mask, the films are grown on the same Si (001) substrate in a single deposition run to minimize changes in the microstructure. We observe a reduction in the effective conductivity of the films with the decreasing layer thickness. From the measured data we estimate values for both the film thermal conductivity and the thermal boundary resistance (TBR) between SiO2/a-Ge/Si at the different temperatures. An experimental value of the interface resistance of 2×10−8 m2 K/W is obtained at 300 K. The temperature dependence of the TBR differs appreciably from calculations based on the diffusive mismatch model. The values derived for the intrinsic thermal conductivity of the films, kfilm(300 K)=0.64 W/mK, agree with predictions from the minimum thermal conductivity model and with values measured by Cahill and Pohl [Phys. Rev. B 37, 8773 (1988)] for thicker films.

The present work measured the thermal conductivities of the silicon nitride films prepared by lower pressure chemical vapor deposition (LPCVD) with thicknesses ranging from 100 nm to 200 nm. The measurements were made at room temperature using the transient photothermal reflectance technique, which is a non-contacting and non-destructive optical approach. The data measured were fitted by genetic algorithm to get the thermal conductivity of thin films and interfacial thermal resistance simultaneously. The results show that thermal conductivities of these films are lower than corresponding bulk material values. The interfacial thermal resistances are in the order of 10−8 m2K/W. It cannot be neglected for the very thin films. Some comparison and analysis for the results were discussed.

Solid heterostructures composed of substrates and epitaxial films are extensively used in advanced technologies, and their thermophysical properties fundamentally determine the performance, efficiency, and reliability of the corresponding devices. However, an experimental method that is truly appropriate for the thermophysical property measurement of solid heterostructures is still lacking. To this end, a three-sensor 3ω-2ω method is proposed, which can simultaneously measure the thermal conductivities of the film and the substrate, along with the film-substrate thermal boundary resistance (TBR) in a single solid heterostructure without any reference samples, showing broad applicability for miscellaneous heterostructures with film thickness ranging from 100 nm to 10 μm. In this method, three parallel metal sensors with unequal widths and distances conforming to guidelines for the three-sensor layout design are fabricated on the sample surface, in which the two outer sensors serve as heaters and the middle sensor as a detector. The respective 3ω signals of the two heaters and the 2ω signal of the detector are measured, and then the thermophysical properties of the sample are fitted within 3D finite element simulations. To verify this method, two typical wide bandgap semiconductor heterojunctions, i.e., GaN on SiC (#SiC) and GaN on Si (#Si) with ∼2.3 μm GaN epilayers, are measured. The thermal conductivity of the GaN film, the thermal conductivities of the SiC and Si substrates, and the GaN/substrate TBRs are derived, exhibiting good agreement with the literature. The proposed method will provide a comprehensive solution for the thermophysical property measurements of various solid heterostructures.

This paper reports the room temperature cross-plane thermal conductivity of pure silica zeolite (PSZ) MEL and MFI thin films. PSZ MEL thin films were prepared by spin coating a suspension of MEL nanoparticles in 1-butanol solution onto silicon substrates followed by calcination and vapor-phase silylation with trimethylchlorosilane. The mass fraction of nanoparticles within the suspension varied from 16 to 55%. This was achieved by varying the crystallization time of the suspension. The thin films consisted of crystalline MEL nanoparticles embedded in a non-uniform and highly porous silica matrix. They featured porosity, relative crystallinity and MEL nanoparticles size ranging from 40 to 59%, 23 to 47% and 55 to 80 nm, respectively. PSZ MFI thin films were made by in-situ crystallization, were b-oriented, fully crystalline and had a 33% porosity. Thermal conductivity of the PSZ thin films was measured at room temperature using the 3ω method. The cross-plane thermal conductivity of the MEL thin films remained constant around 1.02 ± 0.10 Wm−1K−1 despite increases in (i) relative crystallinity, (ii) nanoparticle size and (iii) yield as the nanoparticle crystallization time increased. Indeed, the effect of increases in these parameters on the thermal conductivity was compensated by the simultaneous increase in porosity. PSZ MFI thin films were found to have the same thermal conductivity as MEL thin films even though they had smaller porosity. Finally, the average thermal conductivity of the PSZ films was three to five times larger than that reported for amorphous sol-gel mesoporous silica thin films with similar porosity and dielectric constant.

The solid-liquid interfacial thermal resistance is getting more and more important as various solid-liquid systems are utilized in nanoscale, such as micro electro-mechanical systems/nano electro-mechanical systems (MEMS/NEMS) with liquids and nanoparticle suspension in liquids. The present paper deals with the transport of thermal energy through the solid-liquid interfaces, and the goal is to find a molecular-scale mechanism that determines the macroscopic characteristics of the transport phenomena. Nonequilibrium molecular dynamics simulations have been performed for systems of a liquid film confined between atomistic solid walls. The two solid walls have different temperatures to generate a steady thermal energy flux in the system, which is the element of macroscopic heat conduction flux. Three kinds of liquid molecules and three kinds of solid walls are examined, and the thermal energy flux is measured at the control surfaces in the liquid film and at the solid-liquid interfaces. The concept of boundary thermal resistance is extended, and it is defined for each degree of freedom of translational motion of the molecules. It is found that the interaction strength between solid and liquid molecules uniformly affects all boundary thermal resistances defined for each degree of freedom; the weaker interaction increases all the resistances at the same rate and vice versa. The boundary thermal resistances also increase when the solid and liquid molecules are incommensurate, but the incommensurability has a greater influence on the boundary thermal resistances corresponding to the molecular motion parallel to the interface than that for the normal component. From these findings it is confirmed that the thermal resistance for the components parallel to the interface is associated with the molecular-scale corrugation of the surface of the solid wall, and that the thermal resistance for the component normal to the interface is governed by the number density of the solid molecules that are in contact with the liquid.

The influence of interfacial thermal boundary resistance (TBR) on the thermal performance of heterojunction devices is becoming increasingly significant, making TBR measurements a key area of interest, not only for the advancement of next-generation semiconductor devices but also for the development of precise physical measurements. In this study, we optimized the design of an integrated test chip with heat source temperature measurement separation to have higher temperature sensitivity characteristics. Using this chip, TBR was measured at the gallium nitride/Al2O3 interface. The measurement results are in good agreement with the theoretical and experimental values reported in the literature with the similar magnitude, validating the reliability of the proposed method. This research method has advantages in measuring the TBR of low thermal conductivity substrates and high optical transparency materials, providing valuable insights for thermal management and precise characterization of advanced semiconductor heterojunctions.

Thermal conductivity of amorphous and crystalline thin films by molecular dynamics simulation

Integrating high thermal conductivity cost-effective SiC to the Si-active-layer film is an effective solution to enhance the thermal performance of silicon-on-insulator (SOI) and maintain its distinct electrical performance. Although direct bonding of Si films to SiC substrates demonstrates many advantages over epitaxial growth, different bonding techniques may yield disparate bonding qualities and heat dissipation abilities. Here, a comparative analysis from thermal-mechanical-structural perspectives of different Si/SiC interfaces, prepared by hydrophilic bonding (HB) and surface-activated bonding (SAB) methods, is systematically conducted. It was found that the amorphous interlayer thickness and thermal boundary resistance (TBR) of the SAB-Si/SiC heterojunction can be significantly modulated by annealing, accompanied by a beneficial decreased TBR at high working temperatures. Moreover, the residual stress within the Si film by SAB is greatly reduced to an order of magnitude lower than that by HB. In general, the SAB-Si/SiC interface exhibits a tunable microstructure, comparably low TBR, and nearly fully relaxed stress, potentially outperforming the HB-Si/SiC interface when considering the comprehensive performance. This work puts forward an important thermal-mechanical perspective to evaluate interface bonding quality for practical applications, and removes a headache barrier toward wafer-scale high-yield integration of Si active layers onto SiC substrates to greatly enhance SOI devices' thermal management.

The photoacoustic (PA) technique is one of many techniques for measuring thermal conductivity of thin films. Compared with other techniques for thermal conductivity measurement, the photoacoustic method is relatively simple, yet is able to provide accurate thermal conductivity data for many types of thin films and bulk materials. In this work, the PA measurement in a high frequency range is made possible by a newly developed PA apparatus, which extends the limit of the PA technique. Thermal conductivities of SiO2 with thicknesses from 0.05 to 0.5 μm on Si wafer, e-beam evaporated thin nickel film on Si wafer, and thermal barrier coatings are obtained. In addition to the commonly used phase shift fitting, which is only appropriate for thermally-thin films, an amplitude fitting method is developed and employed for measuring both thin films and bulk materials with smooth or rough surfaces. Comparing results by amplitude fitting to those obtained by other methods and reference values shows good agreements. Applications and limitations of the photoacoustic technique are discussed.

This paper reports, for the first time, the cross-plane thermal conductivity of amorphous and crystalline templated cubic mesoporous titania thin films from 30 to 320 K. Both sol-gel and nanocrystal-based films were synthesized by evaporation-induced self-assembly with porosity of 30% to 35%, respectively. The pore diameter in sol-gel mesoporous films ranged from 14 to 25 nm and film thickness from 120 to 370 nm. Crystalline domains in crystalline mesoporous films were 9 to 13 nm in diameter. The thermal conductivity was measured between 30 and 320 K using the 3ω method. The experimental setup and the associated analysis were first validated by comparing experimental measurements with data reported in the literature for high purity silicon substrate and thermal oxide films over the temperature range considered. The thermal conductivity of sol-gel dense and mesoporous TiO2 films was found to increase with increasing temperature. The thermal conductivity of polycrystalline dense film was strongly dependent on temperature while that of dense amorphous and mesoporous films increased slowly with increasing temperature. The amorphous mesoporous TiO2 films featured very small thermal conductivity due to the fact that heat was mainly transferred by very localized non-propagating vibrational modes. Despite the particles crystallinity, the nanocrystal-based film showed significantly lower thermal conductivity than that of the sol-gel polycrystalline mesoporous thin films due to the strong phonon scattering at the nanocrystal boundaries.

Thermal conductivity of thin diamond films grown from d.c. discharge

Summary form only given. In this talk we present a self-consistent model for calculating the lattice thermal conductivity in semiconductor thin films and nanowires. The model is based on the solution of the phonon Boltzmann equation. It rigorously takes into account modification of the acoustic phonon dispersion in low-dimensional structures with the lateral feature size of 10 nm - 30 nm, and change in the non-equilibrium phonon distribution due to partially diffuse scattering from rough boundaries and interfaces. It is shown that the presence, of distinctive boundaries in such structures leads to a transformation of the acoustic branches of phonon dispersion to quasi-optical branches characterized by a low group velocity and different cut-off frequencies. This transformation affects both in-plane and cross-plane thermal conduction in low-dimensional structures. In this talk we also analyze three-phonon Umklapp scattering processes in thin films and nanowires and compare them with those in bulk semiconductors. The predictions. of the thermal conductivity. values based on our model are in good agreement with available experimental data for silicon thin films and nanowires. The described changes in phonon transport in semiconductor thin films and nanowires bear important consequences for nanostructure and device simulation since neither Fourier beat theory nor Debye approximation are accurate at nanometer length scale. The observed modification of the thermal resistance of the low-dimensional structures has to be taken into account in simulation of thermal transport in deep-submicron devices since, it strongly affects the electrostatic, discharge voltage (EDS) and other reliability characteristics. The change in the acoustic phonon dispersion and group velocity may be partially responsible for the experimentally observed drop of EDS in devices based on thin-film silicon-on-insulator (SOI) wafers.

Thermal conductivity is an intriguing physical property in the nanometer or quantum regime. In this study, we report the growth and thermal conductivity of β-FeSi2 thin film and single crystalline nanowires. The sample structures and chemical composition were identified by standard microscopy techniques. The temperature-dependent thermal conductivity of the thin film was measured by the 3ω method, whereas that of the nanowires was conducted by a suspended pattern technique. Temperature-dependent thermal conductivity was found to increase gradually from 300 to 500 K due to the effect of surface scattering. The experimental evidence presented herein is the suppression of Umklapp transport at the nanoscale near room temperature and the potential application of nanoengineered β-FeSi2 for future design of thermoelectric materials.

Thermal conductivity assessment in a low dimension structure

Semiconducting â-Zn4Sb3and ZrNiSn-based half-heusler compound thin films were prepared by co-evaporation for the application of thermoelectric (TE) materials. High-purity solid zinc and antimony were evaporated by electron beam to grow the â-Zn4Sb3thin film while high-purity zirconium powder and nickel tin powders were evaporated by electron beam to grow the ZrNiSn-based half-heusler compound thin film. Rutherford backscattering spectrometry (RBS) was used to analyze the composition of the thin films. The grown thin films were subjected to 5 MeV Si ions bombardments for generation of nanostructures in the films. We measured the thermal conductivity, Seebeck coefficient, and electrical conductivity of these two systems before and after 5 MeV Si ions beam bombardments. The two material systems have been identified as promising TE materials for the application of thermal-to-electrical energy conversion, but the efficiency still limits their applications. The electronic energy deposited due to ionization in the track of MeV ion beam can cause localized crystallization. The nanostructures produced by MeV ion beam can cause significant change in both the electrical and the thermal conductivity of thin films, thereby improving the efficiency. We used the 3ù-method measurement system to measure the cross-plane thermal conductivity ,the Van der Pauw measurement system to measure the cross-plane electrical conductivity, and the Seebeck-coefficient measurement system to measure the cross-plane Seebeck coefficient. The thermoelectric figures of merit of the two material systems were then derived by calculations using the measurement results. The MeV ion-beam bombardment was found to decrease the thermal conductivity of thin films and increase the efficiency of thermal-to-electrical energy conversion.