Gilbert Teyssedre

Space charge measurement in solid dielectrics by the pulsed electro-acoustic technique

Born in 1985, the PEA technique consists in detecting acoustic waves generated by space charges when they are subjected to an impulse of electric field. Its principle is relatively simple to understand and its use is very flexible. Compared to other techniques as LIPP (Laser Induced Pressure Pulse) method and TSM (Thermal Step Method), the PEA technique is the only one being not based on the relative displacement of the space charge in respect to the electrodes. We show here how consistent information on space charge can be extracted from a simple physical equation and we discuss with the help of a simple modeling the effect of numerical processes such as Gaussian filter.

Evidence of hot electron-induced chemical degradation in electroluminescence spectra of polyethylene

Unlike semiconducting organics, insulating polymers exhibit electroluminescence features that cannot be interpreted on the basis of the photophysical properties of the material. In particular, it is shown for the first time that the spectral components observed in electroluminescence of polyethylene can only be reproduced when the material is irradiated by an electron beam. This shows that hot electron impact is a driving process in electroluminescence and that the excited states decay route goes along the chemical pathway ending with molecular fragmentation. From these results, electroluminescence and electrical degradation can be associated in an implicit scheme, opening the way for defining safety limits in terms of electric stresses applied to a material for a given application.

Charge injection and charge separation as revealed by dynamic space charge measurement in poly(propylene-ethylene) copolymer films

The time resolved fast pulsed electroacoustic method is used to investigate the space charge characteristics in an additive-free poly(propylene-ethylene) copolymer in film form. The 190 μm thick films, sandwiched by aluminum (at the lower electrode) and semiconducting material (at the upper electrode), were submitted to dc and low frequency ac stress for various durations at various temperatures. The results show that under dc stress, homocharges accumulate near the lower electrodes and heterocharge near the upper electrode at 25 °C whereas at 50 °C, besides the similar space charge behavior as at 25 °C in the vicinity of the electrodes, a region of symmetric separation of charges of opposite polarity is detected in the bulk. In addition, the time for the symmetric separation emergence under high voltage application depends on the field intensity, being shorter for higher field. Similar charging characteristics were also found at 50 °C under low frequency (5 mHz) square 50 kV/mm rms stress, with surprisin...

Dielectric Engineering of Nanostructured Layers to Control the Transport of Injected Charges in Thin Dielectrics

A new concept concerning dielectric engineering is presented in this study aiming at a net improvement of the performance of dielectric layers in RF MEMS capacitive switches with electrostatic actuation and an increase of their reliability. Instead of synthesis of new dielectric materials, we have developed a new class of dielectric layers that gain their performance from design rather than from composition. Two kinds of nanostructured dielectrics are presented. They consist of 1) silicon oxynitride layers (SiOxNy:H) with gradual variation of their properties (discrete or continuous) and 2) organosilicon (SiOxCy:H) and/or silica (SiO 2) layers with tailored interfaces; a single layer of silver nanoparticles (AgNPs) is embedded in the vicinity of the dielectric free surface. The nanostructured dielectric layers were deposited in a plasma process. They were structurally characterized and tested under electrical stress and environmental conditions typical for RF MEMS operation. The charge injection and decay dynamics were probed by Kelvin force microscopy. Modulation of the conductive properties of the nanostructured layers over seven orders of magnitude is achieved. Compared to dielectric monolayers, the nanostructured ones exhibit much shorter charge retention times. They appear to be promising candidates for implementation in RF MEMS capacitive switches with electrostatic actuation, and more generally for applications where surface charging must be avoided.

Space charge distributions in Polyimide thin films determined by FLIMM

Polyimides (PI) are used in microelectronics mainly as insulating and passivating layers. In these applications space charge-related processes such as conduction, field distortion and ageing phenomena are important to consider for reliability and failure purpose. It is therefore of great interest to estimate these distributions in order to optimize PI-containing structures and designs. However, no space charge measurements have been reported for PI films of the order of 10 μm thickness, and anyway such results are scarce whatever the material considered. In this preliminary work, space charge distribution in PI thin film has been measured after the application of a dc electric field using metal-insulator-semiconductor structure by Focused Laser Intensity Modulation Method. Different types of electrode substrate (n-type Si and p-type Si) have been considered to evaluate their influence on space charge formation. Experimental results demonstrate that the amount of charge and the polarity of the dominant injected charges exhibit an important dependence on the electrode substrate.

Dielectric layers with gradual properties

This work presents results from a study of thin dielectric layers organized in a structure that behaves as an unified layer with gradual properties. A better understanding of dielectric charging phenomena is aimed at in order to control the conductive properties of the multi-layer system. We first characterize each mono-layer deposited singly on a substrate before the characterization of our multi-layer system. The presented results are for the material and electrical properties of the layers. They are obtained from different diagnostic methods. It was found that such a concept allows modulation of the conductive properties of dielectric materials.

Experimental Evidence of Hot Electrons in Insulating Polymers

Insulating polymers used in Electrical Engineering age under the combined effects of electrical, thermal, mechanical and environmental stresses. Of particular relevance is the electric stress that can initiate material degradation reactions through energetic processes such as partial discharges and hot electrons. Although the degradation can take specific form depending on the time scale and the specific situation (micro-void formation or enlargement, decohesion between fillers and matrix in the case of multiple-internal interfaces, formation of conducting paths by percolation of aged domains, electrical treeing), excited states generated by hot electrons and other sources could be a common feature in the early stage of electrical ageing. We present recent experimental results advocating hot electrons processes at applied field away from the breakdown field.

Electroluminescence in saturated polyesters: Temperature dependence and correlation with space charge measurements

This paper reports experimental investigations of poly(ethylene terephthalate) dealing with the electro-luminescence (EL) measurements at variable temperature under both DC and AC stress, with an over-pressure of nitrogen. The changes in the emission yield are discussed based on the temperature dependencies of photoluminescence and transport properties of these materials in correlation with space charge measurement.

Space charge and orientational polarization within epoxy as probed by the Pulsed Electro-Acoustic technique

The Pulsed Electro-Acoustic technique, which allows probing the space and time dependence of charge density within insulators, has been applied to probe polarization and space charge phenomena within an epoxy resin under direct current stress. The response appears as a superposition of orientational polarization, due to the polar nature of epoxy resins, and space charge accumulation, both exhibiting a dynamic character considering either polarization or depolarization experiments. Attempts have been made to separate these two contributions to the space charge profile and to reconcile transient currents that can be estimated from the time dependence of space charge and those obtained from external circuit measurements. The influence of the nature of the electrodes that are used for achieving measurements on the nature of space charge is also discussed in this work.

Space charge formation in polyimide films and polyimide/SiO2 double-layer measured by LIMM

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.