Laurent Boudou

Molecular mobility in amorphous and partially crystalline PEN after isothermal crystallization

Poly(ethylene-2,6-naphthalene dicarboxylate) (PEN), a new aromatic polyester, presents high-performance physical and chemical properties and may be considered as a worthy substitute for polyethylene terephthalate (PET). Dynamic dielectric measurements were performed under isochronal conditions for twelve different frequencies between 1 and 10/sup 5/ Hz. The aim was to follow the dielectric properties and their dependence on temperature, ranging from -100 to 200/spl deg/C, with a heating rate of 2/spl deg/C/min. Based on experimental considerations by using differential scanning calorimeter (DSC), a thermal cycle of crystallization was carried out. Different specimens were obtained in this way starting with as-received amorphous polymers. Crystallinity saturation, accompanied by a microstructure change, was observed with a second melting peak and a dual lamellar stack model was adopted. Amorphous and semi-crystalline PEN samples were compared. The thermal instability of PEN may be shown through dielectric relaxation before melting. A study was undertaken to understand the different dielectric relaxations present in PEN and the effect of thermal treatment on these relaxations. PEN mobility has been characterized by the presence of four relaxations: /spl beta/, /spl beta/*, /spl alpha/ and /spl rho/. When frequency increased, the /spl beta/ and /spl alpha/ relaxations moved towards higher temperature, while the /spl beta/* process disappeared gradually under the a maximum. The two secondary relaxations B and /spl beta/* were found to obey Eyrings law while the primary one obeyed the empirical Vogel-Fulcher-Tamman (VFT) relation. Characteristic of the amorphous PEN relaxation is the presence of the /spl rho/ peak, at a temperature>T/sub g/. In this temperature range, it was concluded that this peak, not observed in semicrystalline specimens, was due to cold crystallization of the amorphous structure. To assign the occurrence of this peak to the mechanism of detrapping of free charges in the material seems inappropriate.

Silver nanoparticles embedded in dielectric matrix: Charge transport analysis with application to control of space charge formation

This work presents a study on the charge injection and control of dielectric charging phenomenon in thin dielectric layers with tailored interfaces. Single layer of silver nanoparticles (Ag-NPs) was deposited on thermally grown SiO2 layer and covered with plasma deposited thin organosilicon SiCO:H layer. Thus, the Ag-NPs layer can be located at different distances from the surface with Ag-NPs representing artificial deep traps for the charges injected from the surface. The space-charge diagnostic was performed by Kelvin Force Microscopy (KFM) - a diagnostic method derivative from Atomic Force Microscopy (AFM) and giving the possibility to probe the surface potential and the quantity of charges injected previously by an AFM tip under bias voltage. The obtained results reveal that presence of Ag-NPs close to the dielectric surface significantly modifies the electric field distribution; thus a single layer of Ag-NPs can efficiently be used to block the electrical charge injection in thin dielectric layers.

Dielectric charging by AFM in tip-to-sample space mode: overview and challenges in revealing the appropriate mechanisms

The study of charge distribution on the surface and in the bulk of dielectrics is of great scientific interest because of the information gained on the storage and transport properties of the medium. Nevertheless, the processes at the nanoscale level remain out of the scope of the commonly used diagnostic methods. Atomic force microscopy (AFM) is currently applied for both injection and imaging of charges on dielectric thin films at the nanoscale level to answer the increasing demand for characterization of miniaturized components used in microelectronics, telecommunications, electrophotography, electrets, etc. However, the mechanisms for dielectric charging by AFM are not well documented, and an analysis of the literature shows that inappropriate mechanisms are sometimes presented. It is shown here that corona discharge, frequently pointed out as a likely mechanism for dielectric charging by AFM in tip-to-sample space mode, cannot develop in such small distances. Furthermore, a review of different mechanisms surmised to be at the origin of dielectric charging at the nanoscale level is offered. Field electron emission enhanced by thermionic emission is identified as a likely mechanism for dielectric charging at the nanoscale level. Experimental validation of this mechanism is obtained for typical electric field strengths in AFM.

Effects of a modified interface by silver nanoparticles/SiOC:H barrier layer against space charge injection under HVDC

Large number of studies describes phenomena associated with space charge build-up in polymeric materials and the incited consequences when high voltage direct current is imposed. Currently, the studies are oriented towards possibilities to minimize this drawback by addition of nanometric size inclusions into the bulk of the polymer material. One alternative approach is to tailor the interface of the insulating material with deep traps in order to limit and control charges injection before their penetration into the bulk. The barrier layer is composed of a mono plan of silver nanoparticles (AgNPs) embedded in an organosilicon matrix (SiOC:H) issued from hexamethyldisiloxane plasma polymerization and deposited at the surface of a low density polyethylene (LDPE) film. We present a comparative study of charge density distribution between reference LDPE films and a one-face tailored sample. The impact of the barrier layer was evaluated for applied fields in the range 10 to 50 kV/mm. Results show a high efficiency of the AgNPs/SiOC:H barrier layer to limit injection of space charge at the LDPE interface, for both positive and negative charges.

Space charge probing in dielectrics at nanometer scale by techniques derived from atomic force microscopy

Charges accumulation and injection in dielectric material remains critical because it is related to a lot of applications or issues. A deep understanding of interfaces phenomena is needed, but classical space charges techniques exhibit less resolution than the required one. Atomic Force Microscopy (AFM) because of its sensitivity to electrostatic force and its high resolution (close to nanometer) appears to be the best method to characterize charges at nanoscale. Here, two techniques are investigated and compare: Kelvin Force Microscopy (KFM) and Electrostatic Force Distance Curve (EFDC). KFM is used to measured surface potential modification induced by charges. However vertical localization of charges seems difficult to attempt. EFDC follows electrostatic force as function of tip-surface distance. This technique appears promising because of its high resolution, sensitivity to charges localization and distance dependance.

Charge injection in thin dielectric layers by atomic force microscopy: influence of geometry and material work function of the AFM tip on the injection process.

Charge injection and retention in thin dielectric layers remain critical issues for the reliability of many electronic devices because of their association with a large number of failure mechanisms. To overcome this drawback, a deep understanding of the mechanisms leading to charge injection close to the injection area is needed. Even though the charge injection is extensively studied and reported in the literature to characterize the charge storage capability of dielectric materials, questions about charge injection mechanisms when using atomic force microscopy (AFM) remain open. In this paper, a thorough study of charge injection by using AFM in thin plasma-processed amorphous silicon oxynitride layers with properties close to that of thermal silica layers is presented. The study considers the impact of applied voltage polarity, work function of the AFM tip coating and tip curvature radius. A simple theoretical model was developed and used to analyze the obtained experimental results. The electric field distribution is computed as a function of tip geometry. The obtained experimental results highlight that after injection in the dielectric layer the charge lateral spreading is mainly controlled by the radial electric field component independently of the carrier polarity. The injected charge density is influenced by the nature of electrode metal coating (work function) and its geometry (tip curvature radius). The electron injection is mainly ruled by the Schottky injection barrier through the field electron emission mechanism enhanced by thermionic electron emission. The hole injection mechanism seems to differ from the electron one depending on the work function of the metal coating. Based on the performed analysis, it is suggested that for hole injection by AFM, pinning of the metal Fermi level with the metal-induced gap states in the studied silicon oxynitride layers starts playing a role in the injection mechanisms.

Dissociating space charge processes from orientation polarization in poly(ethylene naphthalate) films

Thermo-stimulated depolarization current (TSDC) measurements and space charge measurements were performed on poly(ethylene naphthalene 2,6-dicarboxylate) (PEN), an aromatic and polar polyester. The aim is to develop an understanding of the dipolar and conduction processes at play in this material and in particular to understand the effects of temperature. For the TSDC measurements, when polarizing at 130 and 170 °C, the sub-glass transition and the glass transition relaxations are observed. However, in the case of a polarization temperature of 170 °C, one more current peak, labelled ρ peak, is observed at temperatures above the glass transition. This peak is not only of dipolar origin and could be associated with charge detrapping in the material. To unravel the mechanisms behind this process, a TSDC was combined with space charge measurements using the pulsed electroacoustic method (PEA) and the partial heating method was used. It is shown that the ρ peak is predominantly associated with the release of the negative charge build-up in the material.

Current conduction instabilities in polyethylene during heat cycles

Both low density and cross-linked polyethylene 1.5 mm-thick virgin samples have been submitted to a HVDC (20 kV) at room temperature. When the steady-state current was reached, the temperature was increased up to 70degC (ramp: 0.5degC/min). After a stay at 70degC, the temperature was then reduced. During this heat treatment, the external current was recorded. Analysis of this external current revealed instabilities (peak current); these instabilities has been correlated with some changes occurring in the structure of the studied polymers and revealed by TSC.

Methodology for extraction of space charge density profiles at nanoscale from Kelvin probe force microscopy measurements

To understand the physical phenomena occurring at metal/dielectric interfaces, determination of the charge density profile at nanoscale is crucial. To deal with this issue, charges were injected applying a DC voltage on lateral Al-electrodes embedded in a SiN x thin dielectric layer. The surface potential induced by the injected charges was probed by Kelvin probe force microscopy (KPFM). It was found that the KPFM frequency mode is a better adapted method to probe accurately the charge profile. To extract the charge density profile from the surface potential two numerical approaches based on the solution to Poissons equation for electrostatics were investigated: the second derivative model method, already reported in the literature, and a new 2D method based on the finite element method (FEM). Results highlight that the FEM is more robust to noise or artifacts in the case of a non-flat initial surface potential. Moreover, according to theoretical study the FEM appears to be a good candidate for determining charge density in dielectric films with thicknesses in the range from 10 nm to 10 μm. By applying this method, the charge density profile was determined at nanoscale, highlighting that the charge cloud remains close to the interface.

Electrical characterization of PEN films using TSDC and PEA measurements

Thermo-stimulated depolarization current (TSDC) measurements have been performed on poly(ethylene naphthalene 2,6-dicarboxylate) (PEN), an aromatic polyester. The aim is to develop the understanding of trapping mechanisms at play in this material, and particularly to understand the effect of temperature. Experimental results of TSDC are interpreted with the help of space charge measurements using the pulsed electroacoustic method (PEA) on PEN samples. For TSDC measurements, samples were polarized at temperatures of 130°C and 170°C. In both cases, the sub-glass transition and the glass transition relaxations are observed. However, in the case of a polarization temperature of 170°C, one more TSDC peak, so-called ρ-peak is observed at temperatures above the glass transition. From space charge results, it is shown that the ρ-peak has not a dipolar origin; it has been associated to charge detrapping in the material.