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Time domain thermoreflectance is a very well-suited technique for thermophysical property measurements. The heterodyne or asynchronous optical sampling versions of the technique allow transient temperature measurements from hundreds of femtoseconds up to tens of nanoseconds. The corresponding spectral material response expands from 100 MHz up to 10 THz. However, the ultimate bandwidth of the technique is not limited by optical sampling but by the presence of the metal transducer deposited on top of the material of interest. In this work, we investigated the thermal transparency of several metal transducer thin films. We implemented and solved numerically the two-temperature model for a set of Al, Au, Ag, Cu, Cr, and Pt thin (50 and 150 nm) layers. This numerical study showed that Al thin films are better suited for spectroscopic lattice temperature measurements up to 10 GHz, while noble metals like Au allow the measurement of hot carrier dynamics up to 5 THz.

Decay of plasmons to hot carriers has recently attracted considerable interest for fundamental studies and applications in quantum plasmonics. Although plasmon-assisted hot carriers in metals have already enabled remarkable physical and chemical phenomena, much remains to be understood to engineer devices. Here, we present an analysis of the spatio-temporal dynamics of hot electrons in an emblematic plasmonic device, the adiabatic nanofocusing surface-plasmon taper. With femtosecond-resolution measurements, we confirm the extraordinary capability of plasmonic tapers to generate hot carriers by slowing down plasmons at the taper apex. The measurements also evidence a substantial increase of the “lifetime” of the electron gas temperature at the apex. This interesting effect is interpreted as resulting from an intricate heat flow at the apex. The ability to harness the “lifetime” of hot-carrier gases with nanoscale circuits may provide a multitude of applications, such as hot-spot management, nonequilibrium hot-carrier generation, sensing, and photovoltaics., Knowledge of the electron-gas dynamics in nanometric hot spots is of importance for hot-carrier technologies. Here Lozan et al. present a theoretical and experimental analysis of the spatio-temporal dynamics of hot electrons in a nano-focusing surface-plasmon polariton taper.

International audience; We have studied the thermal conductivity of Ge and Si allotrope heterostructured nanowires (NWs) synthesized by phase transformation. The NWs are composed of successive hexagonal 2H and cubic diamond 3C crystal phases along the 〈111〉 axis. Using 3ω-scanning thermal microscopy on NWs embedded in a silica matrix, we present the first experimental evidence of thermal conductivity reduction in such allotrope 2H/3C heterostructured NWs. In Ge heterostructured 2H/3C NWs, similarly to homogeneous 3C NWs, we show a thermal conductivity reduction when the NW diameter decreases. In addition, in Si and Ge NWs, we observe a reduced thermal conductivity due to the heterostructuration 2H/3C. We evidence that the temperature of phase transformation, which influences the size and the number of 2H domains, can constitute an efficient parameter to tune the thermal conductivity.

The slowdown of propagating electromagnetic waves by engineering waveguide structures is receiving considerable attention in nanophotonics. Two types of structures are independently investigated, namely, diffraction-limited photonic-crystal waveguides and highly confined plasmonic waveguides. Here we propose a unified viewpoint on slow waveguide structures by analyzing both approaches within a single theoretical framework. This allows us to clarify the physical mechanisms underlying slowness—thereby highlighting their inherent differences—and to remove some inaccurate ideas on the topic. We further discuss the strengths and weaknesses of plasmonic and photonic slow waveguides, assess the technical and scientific challenges recently solved, and critically identify those that still stand in the way.

International audience; Room temperature flexible heat harvesters based on conducting polymers are ideally suited to cover the energy demands of the modern nomadic society. The optimization of their thermoelectric efficiency is usually sought by tuning the oxidation levels of the conducting polymers, even if such methodology is detrimental to the Seebeck coefficient (S) as both the Seebeck coefficient and the electrical conductivity (σ) are antagonistically related to the carrier concentration. Here we report a concurrent increase of S and σ and we experimentally derive the dependence of Seebeck coefficient on charge carrier mobility for the first time in organic electronics. Through specific control of the conducting polymer synthesis, we enabled the formation of a denser percolation network that facilitated the charge transport and the thermodiffusion of the charge carriers inside the conducting polymer layer, while the material shifted from a Fermi glass towards a semi-metal, as its crystallinity increased. This work sheds light upon the origin of the thermoelectric properties of conducting polymers, but also underlines the importance of enhanced charge carrier mobility for the design of efficient thermoelectric polymers.

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International audience; We report on the observation of strongly anisotropic surface acoustic wave propagation on nanostructured thin films. Two kinds of tungsten samples were prepared by sputtering on a silicon substrate: a conventional thin film with columns normal to the substrate surface, and an oriented columnar architecture using the glancing angle deposition (GLAD) process. Pseudo-Rayleigh waves (PRWs) were imaged as a function of time in x and y directions for both films thanks to a femtosecond heterodyne pump-probe setup. A strong anisotropic propagation as well as a high velocity reduction of the PRWs were exhibited for the GLAD sample. For the wavevector k /2 π = 3 × 10 5 m −1 the measured group velocities v x and v y equal 2220 m s −1 for the sample prepared with conventional sputtering, whereas a strong anisotropy appears ( v x = 1600 m s −1 and v y = 870 m s −1 ) for the sample prepared with the GLAD process. Using the finite element method, the anisotropy is related to the structural anisotropy of the thin film’s architecture. The drop of PRWs group velocities is mainly assigned to the porous microstructure, especially favored by atomic shadowing effects which appear during the growth of the inclined columns. Such GLAD thin films constitute a new tool for the control of the propagation of surface elastic waves and for the design of new devices with useful properties.

International audience; Since 980-nm GaAs-based laser diodes are now implemented in space missions, there is a huge need to study their thermal behavior under vacuum in order to evaluate their reliability. Then spatially resolved temperature variations have been evaluated by thermoreflectance on the front facet under atmospheric and vacuum conditions, and for two different unpumped windows lengths. We show that, in all cases, vacuum induces a front facet temperature variation increase of the laser diodes, making them more sensitive to a catastrophic optical damage.

International audience; We report on the characterization and long-term compensation of additive timing jitter introduced by a femtosecond ytterbium regenerative amplifier with a 100 kHz repetition rate. A balanced optical cross-correlation technique is used to generate a jitter error signal. This approach is well suited to characterize the additive timing jitter of Yb amplifiers seeded by narrow spectrum Yb oscillators. The balanced optical cross-correlator is in a noncollinear configuration allowing a background free coindence detection. This setup enables the measurement of additive timing jitter from the amplifier, with a noise floor of 300 as integrated from 10 Hz to 10 kHz. The measured additive timing jitter level is about 5 fs, integrated from 0.1 Hz to 10 kHz. The amplifier timing drift characterization and control are performed for more than an hour.

Due to the rising need for clean energy, thermoelectricity has raised as a potential alternative to reduce dependence on fossil fuels. Specifically, thermoelectric devices based on polymers could offer an efficient path for near-room temperature energy harvesters. Thus, control over thermoelectric properties of conducting polymers is crucial and, herein, the structural, electrical and thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT) thin films doped with p-toluenesulfonate (Tos) molecules were investigated with regards to thin film processing. PEDOT:Tos thin films were prepared by in-situ polymerization of (3,4-ethylenedioxythiophene) monomers in presence of iron(III) p-toluenesulfonate with different co-solvents in order to tune the film structure. While the Seebeck coefficient remained constant, a large improvement in the electrical conductivity was observed for thin films processed with high boiling point additives. The increase of electrical conductivity was found to be solely in-plane mobility-driven. Probing the thin film structure by Grazing Incidence Wide Angle X-ray Scattering has shown that this behavior is dictated by the structural properties of the PEDOT:Tos films; specifically by the thin film crystallinity combined to the preferential edge-on orientation of the PEDOT crystallites. Consequentially enhancement of the power factor from 25 to 78.5 μW/mK2 has been readily obtained for PEDOT:Tos thin films following this methodology.

Surface temperature measurements were performed with a scanning thermal microscope mounted with a thermoresistive wire probe of micrometric size. A CMOS device was designed with arrays of resistive lines 0.35 mum in width. The array periods are 0.8 mum and 10 mum to study the spatial resolution of the SThM. Integrated circuits (ICs) with passivation layers of micrometric and nanometric thicknesses were tested. To enhance signal-to-noise ratio, the resistive lines were heated with an ac current. The passivation layer of nanometric thickness allows us to distinguish the lines when the array period is 10 mum. The results raise the difficulties of the SThM measurement due to the design and the topography of ICs on one hand and the size of the thermal probe on the other hand.

International audience; An innovative method to perform femtosecond time-resolved interferometry in reflection mode is proposed. The experiment consists in the combined use of a pump-probe setup and of a fully passive in-line femtosecond common-path interferometer. The originality of this interferometer relies on the use of a single birefringent crystal first to generate a pair of phase-locked pulses and second to recombine them to interfere. As predicted by analytical modeling, this interferometer measures the temporal derivative of the ultrafast changes of the complex optical reflection coefficient of the sample. Working conditions are illustrated through picosecond opto-acoustic experiments on a thin film.

We propose in this paper a new backside imaging technique. Due to the constant increase ofnumerous metal layers, active areas can no longer be characterized through the frontside component. Nowadays, the most advanced imaging and failure analysis techniques require a modified backside component to allow probing. We propose a technique, where sample preparations are minimized. An optical time gating is used to reduce artefacts coming from the backside surface.

International audience; An in-depth study related to a new method of characterizing properties in thermoelectrics is proposed in this paper. This technique is appropriate for single or multi-layered thermoelectric devices. A modulated laser beam is used as a heater in order to generate a Seebeck electromotive force (EMF). The laser beam, line shaped, can be focused at any location along the sample surface, allowing spatially resolved measurements. Seebeck EMF measurements, associated with a versatile model based on the thermal quadrupoles method, allow determination of the sample Seebeck EMF profile and identifying of the sample thermal contact resistances, and should be useful for identification of devices and material thermoelectric properties.

We present experimental and calculation results demonstrating the generation of longitudinal and shear acoustic waves in picosecond laser ultrasonics in an isotropic aluminium plate of 0.54 μm thickness. The waves are detected by an interferometric set-up. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; The objective of this paper is the determination of the thermal properties of micrometric layers of electronic devices using a thermoreflectance probe. Unlike classical thermoreflectance methods, the main point of the method presented in this paper is to be able to quantify the heating energy (by Joule effect) and the effective temperature response (by calibration). It is then possible to estimate the thermal conductivity (in W m−1 K−1) instead of the thermal diffusivity (in m2 s−1). A semi-analytical thermal 3D-periodic model then enables to identify a few thermal properties of the layers of the device, and in particular the thermal conductivity of the passivation layer. This methodology has been applied to the study of an industrial device containing interconnect test structures made of copper lines on a silicon wafer with a few micrometers BCB (BenzoCycloButene) polymer passivation layer. The BCB thermal conductivity and the metal heat capacity are obtained using this method.

Mechanical characterization of electronic devices is often quite uneasy; most of the techniques used require a contact with the sample under study. In this paper, we propose an optical noncontact interferometric imaging method to study the thermomechanical behavior of running devices, and in particular, to deduce the corresponding elastic strain energy. This analysis will permit us to localize the zones of fragility of the device. Results obtained on a power MOS transistor to detect the region of maximum elastic strain energy are presented. It is in particular adapted in microelectronics applications to detect stress accumulation due to dilation coefficient mismatches when assembling microchips.

We present in this paper the comparison of two imaging techniques dedicated to the detection of faults in integrated circuits. These techniques are based on thermoelectric effects occurring when a laser beam heats junctions on integrated circuits or when current flows through the same junction. The first one called Seebeck Effect Imaging (SEI) consists in measuring the thermoelectric voltage appearing when a laser beam heats the circuit surface while in the second the laser is used to measure the Peltier temperature field induced by the current flow through thermoelectric junctions. We have compared these two complementary techniques based on reciprocal phenomena. We present a simple analytical model to explain the SEI and PEI signals and relate it to reliability analysis.

International audience; A High-Throughput Time-Domain ThermoReflectance (HT-TDTR) technique was developed to perform fast thermal conductivity measurements with minimum user actions required. This new setup is based on a heterodyne picosecond thermoreflectance system. The use of two different laser oscillators has been proven to reduce the acquisition time by two orders of magnitude and avoid the experimental artefacts usually induced by moving the elements present in TDTR systems. An amplitude modulation associated to a lock-in detection scheme is included to maintain a high sensitivity to thermal properties.We demonstrate the capabilities of the HT-TDTR setup to perform high-throughput thermal analysis by mapping thermal conductivity and interface resistances of a ternary thin film silicide library FexSiyGe100-x-y (20 < x, y < 80) deposited by wedge-type multi-layer method on a 100 mm diametersapphire wafer offering more than 300 analysis areas of different ternary alloy compositions.

Ge was deposited as thickness gradient films at temperatures up to 800C by direct current (DC) and high power pulsed magnetron sputtering (HPPMS). Structural characterization shows increased crystallization with increasing substrate temperature and film thickness. Thermal conductivity was measured by a novel high-throughput time-domain thermo-reflectance method. Thermo-electrical properties correlate to the degree of crystallization. Conductivities increase with increasing substrate temperature up to 500C. For higher temperatures the trend reverses. A room temperature deposited/ annealed film displays smaller crystallites (10 nm) and lower thermal conductivity (5Wm-1 K-1) compared to 25Wm-1K-1 for hot DC deposition. Compared to DC, HPPMS films show higher thermal conductivities up to 45Wm-1K-1.

International audience; This paper presents an overview of the work done so far related to the use of temperature sensors as performance monitors for RF and MMW circuits with the goal to implement built-in testing or self-calibration techniques. The strategy is to embed small temperature sensors on the same silicon die as the circuit under test, taking advantage of empty spaces in the layout. This paper reviews the physical principles, and presents examples that reveal how temperature sensors can be used as functional built-in testers serving to reduce testing costs and enhance yield as part of self-healing strategies.

To date, there is no experimental characterization of thermal conductivity of semiconductor polymeric individual nanowires embedded in a matrix. This work reports on scanning thermal microscopy measurements in a 3ω configuration to determine how the thermal conductivity of individual nanowires made of a model conjugated polymer (P3HT) is modified when decreasing their diameters. We observe a reduction of thermal conductivity, from λNW = 2.29 ± 0.15 W K-1 m-1 to λNW = 0.5 ± 0.24 W K-1 m-1, when the diameter of nanowires is reduced from 350 nm to 120 nm, which correlates with the polymer crystal orientation measured by WAXS. Through this work, the foundations for future polymer thermal transport engineering are presented. © 2014 the Partner Organisations., M. M. R wants to acknowledge JAE-PreDoc for its financial support. ERC 2008 Starting Grant “Nano-TEC” number 240497 and Nanotherm Consolider CSD-2010-00044 projects are acknowledged for financial support.

International audience; GigaHertz (GHz) thermoreflectance technique is developed to measure the transient temperature of metal and semiconductor materials located behind an opaque surface. The principle is based on the synchronous detection, using a commercial THz pyrometer, of a modulated millimeter wave (at 110 GHz) reflected by the sample hidden behind a shield layer. Measurements were performed on aluminum, copper, and silicon bulks hidden by a 5 cm thick Teflon plate. We report the first measurement of the thermoreflectance coefficient which exhibits a value 100 times higher at 2.8 mm radiation than those measured at visible wavelengths for both metallic and semiconductor materials. This giant thermoreflectance coefficient κ, close to 10−3 K−1 versus 10−5 K−1 for the visible domain, is very promising for future thermoreflectance applications.

International audience; A compact femtosecond dual-oscillator pump-probe setup with 48 MHz-repetition rate, relying on asynchronous optical sampling, is presented. The relative timing jitter between both lasers over the whole pump-probe delay range is of the order of or lower than 500 fs. We demonstrate that both a picosecond temporal resolution and a 48 MHz spectral resolution combined with the fast acquisition rate inherent to the asynchronous optical sampling allow performing broadband opto-acoustic imaging with a spectrum covering more than two decades from 300 MHz to 150 GHz. As an illustration, the opto-acoustic response of a supported thin film is investigated, revealing high frequency acoustic echoes close to the epicenter as well as low GHz surface acoustic waves propagating up to 40μm away from the epicenter. Semi-analytical calculations have been carried out and perfectly reproduce the dispersion of the surface acoustic waves experimentally observed.

In this work, we study the heat dissipated at metal surfaces by the electromagnetic field scattered by isolated subwavelength apertures in metal screens. In contrast to our common belief that the intensity of waves created by local sources should decrease with the distance from the sources, we reveal that the dissipated heat at the surface remains constant over a broad spatial interval of a few tens of wavelengths. This behavior which occurs for noble metals at near infrared wavelengths is observed with nonintrusive thermoreflectance measurements and is analyzed with an analytical model which underlines the intricate role played by quasi-cylindrical waves in the phenomenon. Additionally, computational results reveal that the absorption plateau is an intrinsic property that occurs for a large variety of topologies, irrespective of aperture shapes., 5 pages and 4 figures

This paper presents a review of some of the recent works that we have done on thermal characterisation of running electronic devices by laser probing. Both the single point probing and the surface imaging methodologies are considered. Besides temperature mapping, laser point probing allows fault detection in integrated circuits. Electronic speckle pattern interferometry and shearography metholologies are presented, and examples of images of running power devices and this relation to the underlying thermomechanical stress are shown.

We present thermoreflectance measurements upon a 980 nm wavelength laser diode. Using the energy conservation law, we are able to determine the temperature increase near the active region due to surface absorption. In this paper, we have studied the influence of this surface source contribution for three different configurations of the input current: below the threshold current ITh, for a moderate current above ITh and finally for a current far above ITh. We can analyse frome these reliability issues for the diode in terms of carastrophic optical facet damage (COFD)

International audience; In this work we present an original method for detection and localisation of heat sources at microscopic scale upon integrated circuits. The methodology is based upon thermoreflectance and interferometry, which allows us the measurement of surface temperature and dilatation with resolution better than micrometer. The heat sources are transistors electrically activated. The optical probes we have build act as a thermal antenna sensitive to the presence of thermal wave emitted by the heat source. By a set of three measurements of the phase of the thermal wave we are able to localise the heat source which generates the wave. This methodology is applied to microelectronics to detect and localise faults in CMOS integrated circuits.

We present a scanning imaging system using galvanometric mirrors coupled to a heterodyne interferometric probe. This setup is dedicated to the off-plane surface displacement imaging, in particular for microelectronic devices. The use of galvanometric mirrors coupled to the heterodyne probe makes it a sensitive and fast measurement system. We present results on a microelectronic device, and we compare the performances of this system with point and CCD interferometry imaging setups.

International audience; We present an original optical approach for the thermomechanical study of electronic devices. We have applied it to image the deformation undergone by a MOS power transistor due to its operation. This imaging method allows the derivation of the three components of the displacement vector of each point of the surface of the component while heated by Joule effect while running. The method has a nanometric resolution for the displacement measurement and is based on the analysis of the speckle structure of the device while illuminated by coherent light. A high-resolution interferometer is also used to record the transient behavior of the normal surface displacement of a point of the surface. These optical approaches provide interesting quantitative information about strain and stress in electronic power devices and allow testing of finite element simulations. These techniques can be compared to Moiré thermomechanical studies but with better resolution and sensitivity.

International audience; New methods for fault detection in ICs are needed due to new technological trends. The detection of heat generated by faults offers interesting perspectives in this respect. Periodic heat released at the surface or inside ICs generates a surface thermal wave that can be detected by appropriate laser probing at distances up to 500 μm from the source. We propose in this paper a goniometric laser probing method allowing the determination of the direction and distance of a fault with respect to the probing point.

Accelerated reliability tests of aluminum interconnections with respect to electromigration are often performed by high current density stressing of particular structures designed to promote failures of this kind. In order to derive predictions of the interconnect lifetime under normal operating conditions, it is essential to know the temperature of the test structure when high current-density stressing conditions are applied. In this paper, we present the first experimental temperature measurements with two high spatial resolution laser probes of metal lines. The two optical probes provide an indirect access to the temperature of the metal line. The calibration of the probes for temperature measurements is performed by comparison to electrical temperature measurements. The probes have a lateral resolution of 1 /spl mu/m. Two types of test structures, used in accelerated electromigration analysis, are studied. The measurements are compared with two theoretical simulations. Good agreement is obtained only when a particular value of the thermal conductivity of the submicrometer oxide insulation layer of the metal line is used. This is not surprising, since the thermal properties of submicrometer layers are size dependant. Our experimental method provides an elegant way to determine these parameters.

Testing techniques based on the functional behaviour, the propagation delay and the levels of quiescent current have been used with great success for the last two decade technologies. However, the efficiency of such techniques is dubious for future technologies, characterised by huge mixed-mode complex circuits and very low supply voltage levels. In this paper the feasibility of using internal thermal sensors to detect heat sources provoked by structural defects are considered and evaluated.

We present a new laser probe, a differential polarimetric interferometer, which is dedicated to the study of common failure mechanisms in microelectronic interconnects. Our investigation is mainly concentrated on the study of thermomechanical stress build-up and electromigration in metal lines. In the differential interferometer, two laser beams, separated by a few microns, are reflected from the surface of the device under test. Reflectance, phase and polarization changes between the two beams can be observed, this allows surface temperature and surface bending measurements together with the observation of stress induced in the silicon dioxide layer by Joule thermal expansion. The laser probe provides unique and useful information about local thermal effects due to electromigration. It also shows, for the first time to our knowledge, induced local stress effects due to the mismatch of thermal expansion coefficients of the metal, Si and SiO 2 involved in running interconnect lines.

We have developed optical techniques for thermal characterisation at micrometric scale based upon the detection and the analysis of the modifications of a reflected laser beam. The modifications are induced by a well controlled thermal excitation originating from electric heating. According to this principle we have developed a very compact, high sensitivity and high resolution optical bench. It includes a homodyne stabilised Michelson interferometer, a reflectometer and a differential interferometer. It is capable of detecting a temperature variation as small as 10−3 K and a thermal dilatation as small as 10−15 m. It has a large bandwidth ranging from DC to 125 MHz. The optical bench also includes a microscope and a visualisation system allowing analysis at micrometric scale. In the field of thermal studies in microelectronics, we have been able to determine the temperature of running microelectronic components in order to estimate their quality and reliability, to study the temperature distribution and to detect hot spots in integrated circuits and to study thermooptical and thermophysical properties of materials used in microelectronics.

Nous proposons l'etude d'une methode originale de formation d'image au travers des milieux diffusants ou fortement absorbants. Une mire de resolution est placee derriere un wafer de silicium de 600 μm d'epaisseur. Nous avons cherche a developper des outils optiques performants qui permettent une observation en profondeur et l'obtention d'images bidimensionnelles tomographiques. D'autres images de la partie active d'un circuit microelectronique en fonctionnement a travers le substrat de silicium ont ete realisees. Les images obtenues, ont une resolution spatiale laterale d'environ 12 μm a travers un wafer de 600 μm d'epaisseur.

The thermomechanical behavior of microelectronic interconnect lines under pulsed electric excitation is studied by laser interferometry and reflectometry. An original data processing method and an analytical stationary thermoelastic model are proposed to derive data of interest. The experimental techniques allow one to measure the local transient temperature change at surface and the normal surface displacement produced by Joule heating in the interconnects under normal operating conditions. The proposed data processing method exploits the temporal behavior of the surface temperature change of the interconnects as an “identifying characteristic’’ to analyze the surface displacement. It separates the thermal dilatation which follows “instantaneously’’ the temperature change from the one related to the heat diffusion in the resultant normal surface displacement. After the separation operation, a stationary thermoelastic model for a metallic line bonded to a thick and rigid substrate subject to a uniform temperature change is used to interpret the transient surface displacement measurement. Consequently, the operating temperature, one of the major factors limiting the quality and reliability of the interconnects and other related parameters can be estimated in a simple way.