Stefan Dilhaire

Precise control of thermal conductivity at the nanoscale through individual phonon-scattering barriers

The ability to precisely control the thermal conductivity (kappa) of a material is fundamental in the development of on-chip heat management or energy conversion applications. Nanostructuring permits a marked reduction of kappa of single-crystalline materials, as recently demonstrated for silicon nanowires. However, silicon-based nanostructured materials with extremely low kappa are not limited to nanowires. By engineering a set of individual phonon-scattering nanodot barriers we have accurately tailored the thermal conductivity of a single-crystalline SiGe material in spatially defined regions as short as approximately 15 nm. Single-barrier thermal resistances between 2 and 4 x 10(-9) m(2) K W(-1) were attained, resulting in a room-temperature kappa down to about 0.9 W m(-1) K(-1), in multilayered structures with as little as five barriers. Such low thermal conductivity is compatible with a totally diffuse mismatch model for the barriers, and it is well below the amorphous limit. The results are in agreement with atomistic Greens function simulations.

Laser beam thermography of circuits in the particular case of passivated semiconductors

Abstract Surface temperature changes upon integrated circuits can be observed by measuring the corresponding reflectance variation. We presented this method for temperature measurement in earlier work [1, 2] for Si integrated circuits with no passivation layer. We show in this presentation how the passivation oxide layer upon integrated circuits increases, with a factor depending upon the oxide thickness, the reflectance response resulting from a temperature change. Absolute temperature changes are derived from reflectance measurements through a temperature coefficient. This coefficient, known for silicon, is calculated for different oxide thickness.s. We have built a laser probe for reflectance measurements upon integrated circuits. The probe, which includes a visualisation set-up, has a lateral resolution of 1 μm, the size of the laser spot. Absolute temperature changes from 0.05 to 500 K can be determined and followed as a function of time as the detection system covers the DC-125 MHz range. The laser probe allows also the precise measurement of the oxide thickness upon the semiconductor component. This is performed through the measurement of the reflectance at two different angles of incidence.

Dynamic Surface Temperature Measurements in ICs

Measuring techniques of the die surface temperature in integrated circuits are reported as very appropriate for failure analysis, for thermal characterization, and for testing modern devices. The paper is arranged as a survey of techniques oriented towards measuring the temperature dynamics of the circuit surface and presenting and discussing both the merits and drawbacks of each technique with regard to the accuracy, reliability and efficiency of the measurements. Two methods are presented in detail: laser probing methods, based on interferometry and thermoreflectance, and embedded CMOS circuit sensors. For these techniques, the physical principles, the state of the art in figures of merit and some application examples are presented

Calibration procedure for temperature measurements by thermoreflectance under high magnification conditions

We show that, in thermoreflectance measurements under high focusing conditions, the signal is not only due to the surface temperature variations. Indeed, the reflected light from the device under test interferes with the incoming one, making a parasitic Fabry-Perot. Besides, the motion of the sample surface induced, for example, by Joule heating, can produce an additional signal superimposed on the thermoreflectance one. Consequently, reliable thermoreflectance measurements under high focusing conditions, and particularly their calibration, demand a control of the distance between the objective and the sample. Using a servo loop to maintain the distance between the objective and the device constant, thermoreflectance measurements have been made in transient regime on an electronic device and the thermoreflectance coefficient of gold has been deduced.

Scanning thermal microscopy of individual silicon nanowires

Thermal imaging of individual silicon nanowires (Si NWs) is carried out by a scanning thermal microscopy (SThM) technique. The vertically aligned 1.7 μm long Si NWs are fabricated combining nanosphere lithography and metal-induced wet chemical etching. A thermal model for the SThM probe is then presented with two steps: a model out of contact which enables a calibration of the probe, and a model in contact to extract thermal parameters from the sample under study. Using this model and the experimental thermal images, we finally determine a mean value of the tip-to-sample thermal contact resistance and a mean value of the Si NWs thermal conductivity. No significant thermal conductivity reduction in comparison with bulk Si is observed for Si NWs with diameters ranging from 200 to 380 nm. However, the technique presented here is currently the only one available to perform thermal measurements simultaneously on an assembly of individual one-dimensional nanostructures. It enables to save time and to make a sta...

Four different approaches for the measurement of IC surface temperature: application to thermal testing

Silicon die surface temperature can be used to monitor the health state of digital and analogue integrated circuits (IC). In the present paper, four different sensing techniques: scanning thermal microscope, laser reflectometer, laser interferometer and electronic built-in differential temperature sensors are used to measure the temperature at the surface of the same IC containing heat sources (hot spots) that behave as faulty digital gates. The goal of the paper is to describe the techniques as well as to present the performances of these sensing methods for the detection and localisation of hot spots in an IC.

Laser probing of thermal behaviour of electronic components and its application in quality and reliability testing

Abstract We have developed two optical laser probes for the contactless characterisation of microelectronic components and ICs. The first is a high resolution interferometer for the measurement of dilatations, absolute values over 11 decades are obtained ranging from 10 -3 to 10 -14 m. The second is a reflectance probe for the absolute measurement of surface temperature variations upon ICs. The instrument is a thermometer for surface micrometric analysis able to measure temperature variation in the 10 -3 to 500 °K range. The outstanding performances of these probes have been the starting point of the development of new investigation methods in the field of quality and reliability measurements. We show results of hot points detection upon integrated circuits with micrometric lateral resolution. We also present a method for homogeneity analysis of current density inside power MOS transistors. Finally we present a method for absolute temperature mapping upon metallic lines used in accelerated tests of current stress to study their reliability with regard to electromigration.

Fabrication of Bi2Te3 nanowire arrays and thermal conductivity measurement by 3ω-scanning thermal microscopy

Bi2Te3 is well-known for its utility in thermoelectrical applications and more recently as topological insulator. Its nanostructuration has attracted plenty of attention because of its potential capacity to reduce thermal conductivity. Here, we have grown a composite sample made of a Bi2Te3 nanowires (NWs) array embedded in an alumina matrix. We have then performed scanning thermal microscopy (SThM) in a 3ω configuration to measure its equivalent thermal resistance. Using an effective medium model, we could then estimate the mean composite thermal conductivity as well as the thermal conductivity of the NWs to be, respectively, (λC) = (1.68 ± 0.20) W/mK and (λNW) = (1.37 ± 0.20) W/mK, showing a slight thermal conductivity reduction. Up to now, there have been two main techniques reported in literature to evaluate the thermal conductivity of nanostructures: the use of a thermal microchip to probe a single NW once its matrix has been dissolved or the probing of the whole NWs array embedded in a matrix, obtai...

Thermal exchange radius measurement: Application to nanowire thermal imaging

In scanning thermal microscopy (SThM) techniques, the thermal exchange radius between tip and sample is a crucial parameter. Indeed, it limits the lateral spatial resolution but, in addition, an accurate value of this parameter is necessary for a precise identification of thermal properties. But until now, the thermal exchange radius is usually estimated but not measured. This paper presents an experimental procedure, based on the 3omega-SThM method, to measure its value. We apply this procedure to evaluate the thermal exchange radius of two commercial probes: the well-known Wollaston one and a new probe constituted of a palladium film on a SiO(2) substrate. Finally, presenting silicon nanowire images, we clearly demonstrate that this new probe can reach a spatial resolution better than 100 nm whereas the Wollaston probe hardly reaches a submicronic spatial resolution.

Nanoscale Block Copolymer Ordering Induced by Visible Interferometric Micropatterning: A Route towards Large Scale Block Copolymer 2D Crystals

We have overcome the cost and time consumption limitations of common lithography techniques used to control the self-assembly of block copolymers into highly ordered 2D arrays through the use of a guiding pattern created from a polymeric sub-layer. The guiding pattern is a sinusoidal surface-relief grating interferometrically inscribed onto an azobenzene containing copolymer sub-layer leading to a defect-free single grain of block copolymer domains.