Didier Marty-Dessus

On-line measurements are developed to monitor space charge evolution in a polymeric insulation film by the mean of a (Focused) Laser Intensity Modulation Method set-up. The purpose of this paper is to describe how, starting from experimental on-line (F)LIMM currents, one is able to develop a global procedure leading to extract space charge profiles in dedicated test samples. Objects of study are 25$-\mu$m films of PTFE submitted to a protocol of poling starting from OV to 2.5 kV, alternating 6 volt-off and 6 volt-on measurements on each step of 500 V. Each measurement consists in recording a(F)LIMM pyro-current spectrum from 10Hz to 10 kHz hence a whole protocol is composed of 63 acquisitions of such spectra. Once measurements performed on PTFE sample hereinabove described, a normalization process has allowed first to confirm that there was no charge evolution (injection) between two successive Volt-on to Volt-off measurements, and secondly to launch a thermal calibration of the bench that is needed in (F)LIMM experiment. Then a mathematical correction on Volt-on current was operated in order to extract the useful part linked to the internal field produced by the space charge. Finally, significant space charge representations and evolutions were obtained in our test samples by using a mathematical deconvolution procedure.

For Polyimides (PI) used as insulating inter-layers or passivation layers in microelectronics and power electronics, reliability and failure mechanisms are intimately linked to phenomena such as non-linear conduction, distortion of field distribution and electrical ageing that have all some relation to space charge processes. Knowledge about these space charge distributions is therefore of importance to optimize structures and designs involving PIs. However, in the thickness scale of interest for such applications, of the order of 10 μm, no results were reported for PI, and only scarce studies for other materials. Space charge measurements carried out on PI using the laser intensity modulation method (LIMM) are reported in this work. PI-containing metal-insulator-semiconductor structures were pre-stressed under DC fields up to 2 MV/cm. The impact of the doping type of the substrate (either n-type or, p-type silicon) has been evaluated to address the mechanisms of space charge formation. The nature of electrode substrate substantially impacts the measured charge amount and charge nature, pointing towards charge injection as the origin of space charge build-up. The interpretation is substantiated by results obtained using a SiO2 layer as a barrier against electronic injection from the substrate into the PI layer.

An evolution of a conventional thermal wave technique for space charge investigation in insulations is presented. Based on a laser diode as source of heat, the set-up has been modified in order to be able to stress electrically samples under study with an external electric field while doing measurements for space charge investigation. After a short presentation of the fundamentals related to the technique, its evolutions towards “on-line” studies are evoked, in particular the fundamental equations associated in this new configuration. In the experimental part of the text are described the new challenges arising with the modified set-up and the solutions proposed. At the end, the conclusion is two folds: the modified bench is not only consistent with the classical one, but also allows a thermal calibration of the technique that will enhance the accuracy of results to come.

To improve the spatial resolution of space charge distribution using thermal measurements, an accurate profile of the temperature in the sample is needed. The measurement of both sample surface temperature profile and space charge induced signal is a first crucial step for this knowledge. The described setup allows both measurements simultaneously and gives good signal-to-noise ratios.

The implementation of space charge measurements based on thermal perturbation on thin films requires an improvement of the temperature distribution estimation at the surface and in the depth of dielectric materials for getting reliable space charge profile measurements. Absolute temperature variations are needed, both in time and space. The present contribution addresses surface temperature measurements based on either thermoelectric or bolometric effects. Both responses have been measured on copper-coated silicon nitride layers and gold-coated polypropylene films heated with a Nd:YAG laser pulse. It is shown that high temporal resolution thermal response can be obtained through the bolometric response and that the information appears nearly independent on the nature of the coating electrode. The setup developed provides good signal to noise ratio for heated electrodes of a few ohm resistance. Strategies are still to develop to get the temperature profile in the insulating sample layer.

A new technique for the determination of space charge profiles in insulating polymers is proposed. Based on the evolution of an existing thermal wave technique called Focused Laser Intensity Modulation Method ((F)LIMM), it allows non-contact measurements on thin films exhibiting an internal charge to be studied. An electrostatic model taking into account the new sample-cell geometry proposed was first developed. It has been shown, in particular, that it was theoretically possible to calculate the internal charge from experimental measurements while allowing an evaluation of the air layer appearing between the sample and the electrode when non-contact measurements are performed. These predictions were confirmed by an experimental implementation for two thin polymer samples (25 μm-polyvinylidenefluoride and 50 μm-polytetrafluoroethylene (PTFE)) used as tests. In these cases, minimum air-layer thickness was determined with an accuracy of 3% and 20%, respectively, depending on the signal-to-noise ratio during the experimental procedure. In order to illustrate the reachable possibilities of this technique, 2D and 3D cartographies of a negative space charge implanted by electron beam within the PTFE test sample were depicted: like in conventional (F)LIMM, a multidimensional representation of a selectively implanted charge remains possible at a few microns depth, but using a non-contacting way of measurement.