Thermoreflectance Technique Research Articles

Short time transient thermal behavior of solid-state microrefrigerators

We present detailed experimental and theoretical studies of the short time transient thermal behavior of SiGe superlattice microrefrigerators on a chip. Transient temperature profiles of microrefrigerator devices of different sizes are obtained using thermoreflectance technique. Thermal imaging with submicron spatial resolution, 0.1 K temperature resolution, and 100 ns temporal resolution is achieved. The dynamic behavior of the microrefrigerators shows an interplay between Peltier and Joule effects. Peltier cooling appears first with a time constant of about 10–30 μs, then Joule heating in the device starts taking over with a time constant of about 50–150 μs. The experimental results agree very well with the theoretical predictions based on thermal quadruple method. The difference in the two time constants can be explained considering the three-dimensional thermal resistances and capacitances of the microrefrigerator. In addition this shows that the Joule heating at the top metal/semiconductor interface does not dominate the microrefrigerator performance. Experimental results show that under high current pulsed operation, the microrefrigerator device can provide cooling for about 30 μs, even though steady state measurements show heating.

Thermal Properties of Ultrathin Hafnium Oxide Gate Dielectric Films

Thin-film HfO2 is a promising gate dielectric material that will influence thermal conduction in modern transistors. This letter reports the temperature dependence of the intrinsic thermal conductivity and interface resistances of 56-200-Aring-thick HfO2 films. A picosecond pump-probe thermoreflectance technique yields room-temperature intrinsic thermal conductivity values between 0.49 and 0.95 W/(mmiddotK). The intrinsic thermal conductivity and interface resistance depend strongly on the film-thickness-dependent microstructure.

Optical investigation of band-edge structure and built-in electric field of AlGaN/GaN heterostructures by means of thermoreflectance, photoluminescence, and contactless electroreflectance spectroscopy

The band-edge property and built-in electric fields of two different Al(x)Ga(1-x)N/GaN(AlGaN/GaN) heterostructures (HSs) with and without an additional AlGaN inserted layer were studied by thermoreflectance (TR), photoluminescence (PL), and contactless electroreflectance (CER) techniques. The PL spectra characterize the band-edge luminescence property of GaN. Free exciton transitions of AlGaN and GaN were probed experimentally by TR. Prominent Franz-Keldysh oscillations (FKOs) of GaN and the opposite FKO phase of AlGaN were simultaneously detected by the additional AlGaN inserted sample with CER owing to the enhancement effect of built-in electric fields of GaN and AlGaN with the same polarity direction. Optoelectronics properties of the two HSs were characterized by the experimental analyses.

Thermal Characterization of $\hbox{Si}_{3}\hbox{N}_{4}$ Thin Films Using Transient Thermoreflectance Technique

In this paper, we measure the thermal conductivities (TCs) of Si3N4 thin films prepared by lower pressure chemical vapor deposition with thickness ranging from 37 to 200 nm. The measurements were made at room temperature using a transient thermoreflectance technique. A three-layer model based on the transmission-line theory and the genetic algorithms were applied to obtain the TC of thin films and the interfacial thermal resistance (ITR). The results show that the value of the TC is 1.24-2.09 Wmiddotm-1middotK-1. The ITR between the metal layer and the thin film is about 1.2 times 10-8 m2 ldr K ldr W-1. The estimated uncertainty of the TC is less than 18%.

A High-Throughput Screening System for Thermoelectric Material Exploration Based on a Combinatorial Film Approach

A high-throughput system that consists of a combinatorial tool (a sputtering deposition tool and a pulsed laser deposition tool) and two developed property screening devices was used for thermoelectric material exploration. The thermoelectric power factor (S2σ, S = Seebeck coefficient, σ = electrical conductivity) screening device allows us to measure electrical conductivity and Seebeck coefficient of over 1000 sample-points within 6 h. The thermal effusivity measurement system using the frequency domain thermoreflectance technique allows us to screen thermal conductivity of combinatorial/conventional films. Illustrations of these applications are provided with a Co–Sn–Ce/Si(100) film for power factor determination and with a Ba2YCu3O7/SrTiO3(100) film for thermal conductivity derivation.

Characterization of Single Barrier Microrefrigerators at Cryogenic Temperatures

The experimental characterization of single barrier heterostructure thermionic cooling devices at cryogenic temperatures is reported. The device studied was a cylindrical InGaAs microrefrigerator, in which the active layer was a 1 μm thick In0.527Al0.218Ga0.255As heterostructure barrier with n-type doping concentration of 6.68 × 1016 cm−3 and an In0.53Ga0.47As emitter/collector of 5 × 1018 cm−3 n-doping. A full field thermoreflectance imaging technique was used to measure the distribution of temperature change on the device’s top surface when different current excitation values were applied. By reversing the current direction, we studied the device’s behavior in both cooling and heating regimes. At an ambient temperature of 100 K, a maximum cooling of 0.6 K was measured. This value was approximately one-third of the measured maximum cooling value at room temperature (1.8 K). The paper describes the device’s structure and the first reported thermal imaging at cryogenic temperatures using the thermoreflectance technique.

Examining Interfacial Diffuse Phonon Scattering Through Transient Thermoreflectance Measurements of Thermal Boundary Conductance

Today’s electronic and optoelectronic devices are plagued by heat transfer issues. As device dimensions shrink and operating frequencies increase, ever-increasing amounts of thermal energy are being generated in smaller and smaller volumes. As devices shrink to length scales on the order of carrier mean free paths, thermal transport is no longer dictated by the thermal properties of the materials comprising the devices, but rather the transport of energy across the interfaces between adjacent materials in the devices. In this paper, current theories and experiments concerning phonon scattering processes driving thermal boundary conductance (hBD) are reviewed. Experimental studies of thermal boundary conductance conducted with the transient thermoreflectance technique challenging specific assumptions about phonon scattering during thermal boundary conductance are presented. To examine the effects of atomic mixing at the interface on hBD, a series of Cr/Si samples was fabricated subject to different deposition conditions. The varying degrees of atomic mixing were measured with Auger electron spectroscopy. Phonon scattering phenomena in the presence of interfacial mixing were observed with the trends in the Cr/Si hBD. The experimental results are reviewed and a virtual crystal diffuse mismatch model is presented to add insight into the effect of interatomic mixing at the interface. The assumption that phonons can only transmit energy across the interface by scattering with a phonon of the same frequency—i.e., elastic scattering, can lead to underpredictions of hBD by almost an order of magnitude. To examine the effects of inelastic scattering on hBD, a series of metal/dielectric interfaces with a wide range of vibrational similarity is studied at temperatures above and around materials’ Debye temperatures. Inelastic scattering is observed and new models are developed to predict hBD and its relative dependency on elastic and inelastic scattering events.

アルミナ焼結体の微小領域における熱浸透率／熱伝導率

Thermal properties of sintered alumina at the micrometer-scale were quantitatively measured with a thermal microscope using thermo-reflectance and periodic heating techniques. The average value of thermal conductivity of 28.9 W·m−1K−1 was obtained from a 50 μm×50 μm area, and the average value was also in good agreement with that measured by a laser flash technique.The anomalous data were found by the measurement with the thermal microscope in some regions, which were correlated with the surface pores or inside defects of the specimen. The portions with very low thermal conductivities, 5 to 10 W·m−1K−1, were found to correspond to around the pore presents.In addition, unshared 10 square areas of 10 μm×10 μm were randomly selected and were scanned by an area scanning mode in order to calculate the average conductivities at the areas. The conductivity values were in the range of 24.9 to 37.7 W·m−1K−1 and the variation in the conductivities was estimated about 13 W·m−1K−1. The area with maximum thermal conductivity was consisted of large grains and the value was considered to be almost equal to that of one grain. On the other hand, the area including several small grains indicated the minimum conductivity. The decrease of the conductivity was due to the influence of the grain boundaries.

Investigating Coherent Zone-Folded Acoustic Phonons in Si/SiGe Superlattices by Transient Thermoreflectance Technique

AbstractIn this paper, we present a systematic study of coherent phonons in Si/SiGe superlattices with two different periods. The superlattice periodicity affects the acoustic properties of the structure. Transient thermoreflectance (TTR) technique is used to perform picosecond ultrasonics experiments and then, investigate coherent zone-folded acoustic phonons in different Si/SiGe superlattice structures. Several classes of coherent phonons are produced in the superlattice, whose generation mechanisms are different: Brillouin oscillations, coherent longitudinal-acoustic phonon Bragg reflection and impulsive stimulated Raman scattering (ISRS).

Thermal Conductivity Measurement of the AlN Ceramics at the Grain Scale Using Thermoreflectance Technique

The thermal conductivity at the micrometer-scale of AlN ceramics with eliminated grain boundary phase was measured by the thermoreflectance technique with periodic heating. Thermal conductivities were ranged from 160 to 260 W/m•K and an average value of 201 W/m•K was obtained from a 22 m2 area. The variation in the thermal conductivities was related to the individual AlN grains and grain boundary characteristics.

Macro- and Micro-Scale Thermal Conductivities of SiC Single Crystal and Ceramic

Thermal properties of SiC at the micrometer-scale were measured quantitatively with a thermal microscope using thermo-reflectance and periodic heating techniques. In this study, SiC single crystal and polycrystal were investigated. The small values of standard deviation suggest that the SiC single crystal had constant thermal conductivities. For the single crystal, the average value of the thermal conductivity at the micrometer-scale was in good agreement with the macro-scale thermal conductivity value obtained by the laser flash technique. On the other hand, thermal conductivity of the polycrystal was heterogeneous at the micrometer-scale. An average thermal conductivity value of 257 Wm-1K-1 was obtained within an area of 50 m ×100 µm. The highest and lowest values of the thermal conductivity from the polycrystal were 300 and 220 Wm-1K-1, respectively.

Thermal Conductivity and Interfacial Thermal Resistance: Measurements of Thermally Oxidized SiO2 Films on a Silicon Wafer Using a Thermo-Reflectance Technique

This article describes the development of a method to measure the normal-to-plane thermal conductivity of a very thin electrically insulating film on a substrate. In this method, a metal film, which is deposited on the thin insulating films, is Joule heated periodically, and the ac-temperature response at the center of the metal film surface is measured by a thermo-reflectance technique. The one-dimensional thermal conduction equation of the metal/film/substrate system was solved analytically, and a simple approximate equation was derived. The thermal conductivities of the thermally oxidized SiO2 films obtained in this study agreed with those of VAMAS TWA23 within ± 4%. In this study, an attempt was made to estimate the interfacial thermal resistance between the thermally oxidized SiO2 film and the silicon wafer. The difference between the apparent thermal resistances of the thermally oxidized SiO2 film with the gold film deposited by two different methods was examined. It was concluded that rf-sputtering produces a significant thermal resistance ((20 ± 4.5) × 10−9 m2·K·W−1) between the gold film and the thermally oxidized SiO2 film, but evaporation provides no significant interfacial thermal resistance (less than ± 4.5 × 10−9 m2·K·W−1). The apparent interfacial thermal resistances between the thermally oxidized SiO2 film and the silicon wafer were found to scatter significantly (± 9 × 10−9 m2·K·W−1) around a very small thermal resistance (less than ± 4.5 × 10−9 m2·K·W−1).

Microscale and Nanoscale Thermal Characterization Techniques

Miniaturization of electronic and optoelectronic devices and circuits and increased switching speeds have exasperated localized heating problems. Steady-state and transient characterization of temperature distribution in devices and interconnects is important for performance and reliability analysis. Novel devices based on nanowires, carbon nanotubes, and single molecules have feature sizes in 1–100 nm range, and precise temperature measurement and calibration are particularly challenging. In this paper we review various microscale and nanoscale thermal characterization techniques that could be applied to active and passive devices. Solid-state microrefrigerators on a chip can provide a uniform and localized temperature profile and they are used as a test vehicle in order to compare the resolution limits of various microscale techniques. After a brief introduction to conventional microthermocouples and thermistor sensors, various contact and contactless techniques will be reviewed. Infrared microscopy is based on thermal emission and it is a convenient technique that could be used with features tens of microns in size. Resolution limits due to low emissivity and transparency of various materials and issues related to background radiation will be discussed. Liquid crystals that change color due to phase transition have been widely used for hot spot identification in integrated circuit chips. The main problems are related to calibration and aging of the material. Micro-Raman is an optical method that can be used to measure absolute temperature. Micron spatial resolution with several degrees of temperature resolution has been achieved. Thermoreflectance technique is based on the change of the sample reflection coefficient as a function of temperature. This small change in 10−4–10−5 range per degree is typically detected using lock-in technique when the temperature of the device is cycled. Use of visible and near IR wavelength allows both top surface and through the substrate measurement. Both single point measurements using a scanning laser and imaging with charge coupled device or specialized lock-in cameras have been demonstrated. For ultrafast thermal decay measurement, pump-probe technique using nanosecond or femtosecond lasers has been demonstrated. This is typically used to measure thin film thermal diffusivity and thermal interface resistance. The spatial resolution of various optical techniques can be improved with the use of tapered fibers and near field scanning microscopy. While subdiffraction limit structures have been detected, strong attenuation of the signal reduces the temperature resolution significantly. Scanning thermal microscopy, which is based on nanoscale thermocouples at the tip of atomic force microscope, has had success in ultrahigh spatial resolution thermal mapping. Issues related to thermal resistance between the tip and the sample and parasitic heat transfer paths will be discussed.

Influence of Interfacial Mixing on Thermal Boundary Conductance Across a Chromium/Silicon Interface

The thermal conductance at solid-solid interfaces is becoming increasingly important in thermal considerations dealing with devices on nanometer length scales. Specifically, interdiffusion or mixing around the interface, which is generally ignored, must be taken into account when the characteristic lengths of the devices are on the order of the thickness of this mixing region. To study the effect of this interfacial mixing on thermal conductance, a series of Cr films is grown on Si substrates subject to various deposition conditions to control the growth around the Cr∕Si boundary. The Cr∕Si interfaces are characterized with Auger electron spectroscopy. The thermal boundary conductance (hBD) is measured with the transient thermoreflectance technique. Values of hBD are found to vary with both the thickness of the mixing region and the rate of compositional change in the mixing region. The effects of the varying mixing regions in each sample on hBD are discussed, and the results are compared to the diffuse mismatch model (DMM) and the virtual crystal DMM (VCDMM), which takes into account the effects of a two-phase region of finite thickness around the interface on hBD. An excellent agreement is shown between the measured hBD and that predicted by the VCDMM for a change in thickness of the two-phase region around the interface.

Thermal Properties of Metal-Coated Vertically Aligned Single-Wall Nanotube Arrays

Owing to their high thermal conductivities, carbon nanotubes (CNTs) are promising for use in advanced thermal interface materials. While there has been much previous research on the properties of isolated CNTs, there are few thermal data for aligned films of single wall nanotubes. Furthermore, such data for nanotube films do not separate volume from interface thermal resistances. This paper uses a thermoreflectance technique to measure the volumetric heat capacity and thermal interface resistance and to place a lower bound on the internal volume resistance of a vertically aligned single wall CNT array capped with an aluminum film and palladium adhesion layer. The total thermal resistance of the structure, including volume and interface contributions, is 12m2KMW−1. The data show that the top and bottom interfaces of the CNT array strongly reduce its effective vertical thermal conductivity. A low measured value for the effective volumetric heat capacity of the CNT array shows that only a small volume fraction of the CNTs participate in thermal transport by bridging the two interfaces. A thermal model of transport in the array exploits the volumetric heat capacity to extract an individual CNT-metal contact resistance of 10m2K1GW−1 (based on the annular area Aa=πdb), which is equivalent to the volume resistance of 14nm of thermal SiO2. This work strongly indicates that increasing the fraction of CNT-metal contacts can reduce the total thermal resistance below 1m2KMW−1.

Thermal processes in high-power laser bars investigated by spatially resolved thermoreflectance

The implementation of the thermoreflectance technique for the analysis of facet heating in commercial high-power laser bars is demonstrated. The technique provides highly spatially resolved information about facet temperature along the active stripes of all the emitters in the bar. The recorded maps reveal multiple hot-spots localized in the active area region of the laser structure. Apart from the thermoreflectance, thermal properties of the laser bar are studied by two complementary thermometric techniques: IR thermography which provides fast information about temperature profiles and hot spot location, and wavelength tuning technique which is used to determine the time-resolved bulk temperature for all emitters in the bar. Analysis of the results obtained by different methods indicates that the observed local overheating is related to defects starting at the laser facet or very close to the facet. The thermal data for non-degraded emitters, on the other hand, shows a moderate facet overheating of 5−12 °C (for the laser bar operating under quasicontinuous-wave conditions).

Thermal Conductivity Measurement of Submicron-Thick Aluminium Oxide Thin Films by a Transient Thermo-Reflectance Technique

Thermal conductivity of submicron-thick aluminium oxide thin films prepared by middle frequency magnetron sputtering is measured using a transient thermo-reflectance technique. A three-layer model based on transmission line theory and the genetic algorithm optimization method are employed to obtain the thermal conductivity of thin films and the interfacial thermal resistance. The results show that the average thermal conductivity of 330–1000 nm aluminium oxide thin films is 3.3 Wm−1K−1 at room temperature. No significant thickness dependence is found. The uncertainty of the measurement is less than 10%.

Thermal conductivity measurement at the micrometer scale of ceramics using thermoreflectance technique -High-thermal-conductivity AlN with reduced amount of grain boundary phase-

High-thermal-conductivity AlN ceramic with eliminated grain boundary phase was prepared by long-term sintering with Y2O3 addition at 1900°C for 100 h in a reducing N2 atmosphere. The thermal conductivity of polished surfaced of obtained ceramic was quantitatively measured at the micrometer-scale by using a thermoreflectance technique with periodic heating. This equipment measures the phase lag, which is the delay between the signal of periodic heating laser and the reflectance signal of detecting laser, to calculate thermal effusivity using the calibration curve obtained from standard materials. The conductivity of polished surface of the ceramic is calculated from obtained thermal effusivity, density and specific heat. The thermal conductivities on the densified and polished surface of the ceramic were ranged from 219 to 318 W/m·°C. Low-thermal-conductivity region (< 20 W/m·°C) relating to presence of hidden pores was observed in thermal conductivity map of the polished surface. Furthermore, high-resolution map of thermal conductivity demonstrated no significant reduction in thermal conductivities due to presence of grain boundaries.

サーモリフレクタンス法によるＴｉＮｘ薄膜の熱拡散率の測定

Thermal diffusivity of titanium nitride (TiNx) films normal to the film surface was measured quantitatively using a thermoreflectance technique. TiNx films were deposited on alkali free glass substrate by reactive rf magnetron sputtering using a Ti metal target and mixture gas of Ar and N2. Nitrogen compositions of the film were determined from plasma energy using Bendavid's relation. The ratio of nitrogen to titanium atoms increased from 0.8 to 1.2 when reactive N2 gas composition changed from 4.7% to 100% at the deposition process. For the stoichiometric film, the thermal diffusivity shows peak value, as well as the electrical conductivity. In contrast, when nitrogen composition becomes to over- or under-stoichiometry, both thermal diffusivity and electrical conductivity rapidly decrease, respectively. The relationship between the thermal diffusivity and the electric conductivity suggests that the free electron contribution is dominant to the heat conduction in the TiNx films.

Laser scanning thermoreflectance imaging system using galvanometric mirrors for temperature measurements of microelectronic devices

We present a thermoreflectance imaging system using a focused laser sweeping the device under test with a scanner made of galvanometric mirrors. We first show that the spatial resolution of this setup is submicrometric, which makes it adapted to microelectronic thermal measurements. Then, we studied qualitative temperature variations on two dissipative structures constituted of thin (0.35 microm) dissipative resistors, the distance between two resistors being equal to 0.8 or 10 microm. This technique combines sensitivity and speed: it is faster than a point classical thermoreflectance technique and, in addition, more sensitive than a charge-coupled device thermoreflectance imaging technique.