Stéphane Holé

Quantitative thermal imaging by synchronous thermoreflectance with optimized illumination wavelengths

Using thermoreflectance microscopy with a camera, we have designed a system which delivers submicronic images of the alternative temperature variations in integrated circuits working in a modulated regime. A careful choice of the illumination wavelength permits us to highlight the heating in chosen parts of the sample and to optimize the thermoreflectance signal. We measure and explain the modifications of the photothermal response which are induced by the presence of a passivation layer. A calibration conducted on various materials with a thermocouple gives access to the absolute alternative temperature variations in integrated circuits working at frequencies between 0.1 Hz (quasipermanent regime) and 5 MHz.

Measuring and predicting the thermoreflectance sensitivity as a function of wavelength on encapsulated materials

Thermoreflectance microscopy can deliver thermal images with high spatial resolutions by measuring the variations of the reflection coefficient with temperature. It is therefore a unique tool to measure the temperature mapping of integrated circuits with submicronic features. However, integrated circuits are usually protected by a dielectric encapsulation layer which is transparent to visible light. The optical interference which occurs in these layers strongly modifies the reflectivity and can even forbid thermoreflectance measurements at some wavelengths. For each series of circuits, it is therefore necessary to determine the illumination wavelengths for which thermoreflectance will deliver optimal signals. A sequential wavelength scan can deliver this information but it is time consuming and therefore subject to drifts. We have developed a CCD camera-based thermoreflectance microscope coupled to a grating which disperses white light directly onto the CCD. This instrument gives the complete spectra of the reflection coefficient and its temperature dependence, R(λ) and dR/dT(λ), on one or several materials with only one acquisition. The optimal wavelength for thermoreflectance measurements can therefore be measured within minutes on any sample. A model taking into account multiple reflections and the thermal expansion of the encapsulation layer has been developed to explain the spectra R(λ) and dR/dT(λ) measured on encapsulated circuits. It can be used to predict qualitatively the optimal working wavelength.

Pressure wave propagation methods: a rich history and a bright future

The pressure wave propagation (PWP) method was developed 25 years ago to study space charges in solid dielectrics. This method has been a powerful tool in this field of research but has also proved its efficiency in adjacent fields for the characterization of materials or processes. In this paper the history of this method and its principle are recalled. The pulsed electro-acoustic (PEA) method, which can be physically described in a similar way and for which data can be analyzed in an equivalent fashion, is also presented. Examples of applications and developments are given which indicate the direction which these methods might take in the future

Resolution of direct space charge distribution measurement methods

Spatial resolution is a key parameter for the measurement of any distribution. In the case of space charge distribution measurements, it is difficult to compare technique performances since each technique has its own resolution definition most of the time. In this paper the resolution, in terms of position accuracy and charge discernment, is determined on the basis of a unique definition for the thermal, the pressure-wave- propagation and the electro-acoustic methods and their derived techniques. It is shown that spatial resolution is similar throughout the sample in the case of the pressure- wave-propagation or the electro-acoustic methods, except if attenuation and dispersion of elastic waves are important, but decreases as the charge position inside the sample in the case of thermal method. Elastic wave methods are therefore preferable for a given signal to noise ratio as soon as the sample thickness is larger than twice the sound velocity times the excitation rise time.

Measurement of space-charge distributions in insulators under very rapidly varying voltage

In this paper we describe a new instrument for real-time measurements of space charge distributions in insulators submitted to rapidly varying voltages. It is illustrated with the pressure wave propagation method but can also be adapted for use with the pulsed electro-acoustic method. A new acoustic generator has been designed to produce high amplitude pressure pulses, at a high rate, and with good reproducibility. This generator, made of a thin transducer coupled with a waveguide and a backing medium, responds with a pressure pulse when a step voltage is applied. A special sample holder with a signal decoupling circuit, and a storage unit have been developed. The described instrument allows for measurements at a rate of 3200 Hz. Its potentialities are illustrated by studying the evolution of space charge in polyethylene samples stressed by both pulsed and 50 Hz ac voltages.

Analytical capacitive sensor sensitivity distribution and applications

Robustness, versatility, high level of performance and low cost are some of the characteristics that make capacitive sensors well suited for industrial applications. They consist only in electrodes and measurement circuits but give access to position information and/or material properties. The design of capacitive sensors is however not so obvious so that simple structures are generally used to avoid time-consuming calculations and developments. We propose an analytical method to determine the sensitivity distribution of any capacitive sensor structure. This method makes it possible to rapidly optimize structures in order to increase the sensor sensitivity to one parameter or to render it less sensitive to another. Comparisons between the sensitivity map of known sensors and those obtained with the analytical method proposed in this paper show a great accordance.

A preliminary study of space charge distribution measurements at nanometer spatial resolution

The continuous reduction in size of devices, such as integrated circuits or micro-electro-mechanical systems (MEMS), results in the need to control with better and better resolution the materials involved, and in particular the electrical properties of the insulating and semiconducting parts. In this paper we propose an approach applicable to space charge measurement methods for improving the resolution to the nanometer range by using femtosecond laser pulses. It is shown that a resolution of about 60 nm in SiO2 can be achieved with thermal and pressure wave propagation methods. Concerning the pulsed electro-acoustic method, the interfacial displacement as small as 100 fm can be measured at THz rate

Space charge behaviour in epoxy laminates under high constant electric field

The development of space charge in insulating materials is one of the main causes of their electrical ageing. The pulsed electro-acoustic method is often used to determine space charge distribution, but the signal analysis in the case of laminate structures is much more complex to analyse. In this paper the authors describe and use a simulated signal in order to study laminates made of epoxy resin and fibre mat. The relatively large conductivity of the fibres compared with that of the resin seems to produce a rapid charge dissociation and recombination in the fibres. Under voltage the presence of fibres close to an electrode seems to promote charge injection.

Space charge distribution measurement methods and particle loaded insulating materials

In this paper the authors discuss the effects of particles (fillers) mixed in a composite polymer on the space charge measurement techniques. The origin of particle-induced spurious signals is determined and silica filled epoxy resin is analysed by using the laser-induced-pressure-pulse (LIPP), the pulsed-electroacoustic (PEA) and the laser-induced-thermal-pulse (LITP) methods. A spurious signal identified as the consequence of a piezoelectric effect of some silica particles is visible for all the methods. Moreover, space charges are clearly detected at the epoxy/silica interface after a 10-kV/mm poling at room temperature for 2 hours.

The influence of filler particles on space charge measurements

The effects of filler particles on the signal measured using the pressure-wave-propagation method to determine the space charge distribution are analysed. A special test sample geometry is used to classify the influence of particles, and in particular, the role of piezoelectricity is discussed. Applications with Epoxy resin samples loaded either by silica or alumina particles are presented. Silica particles are shown to exhibit piezoelectricity which strongly modifies the measured signals.