C. Villeneuve-Faure

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.

Towards 3D charge localization by a method derived from atomic force microscopy: the electrostatic force distance curve

Charges injection and accumulation in the dielectric remains a critical issue, mainly because these phenomena are involved in a great number of failure mechanisms in cables or electronic components. Achieving a better understanding of the mechanisms leading to charge injection, transport and trapping under electrical stress and of the relevant interface phenomena is a high priority. The classical methods used for space charge density profile measurements have a limited spatial resolution, which prevents them being used for investigating thin dielectric layers or interface processes. Thus, techniques derived from atomic force microscopy (AFM) have been investigated more and more for this kind of application, but so far they have been limited by their lack of in-depth sensitivity. In this paper a new method for space charge probing is described, the electrostatic force distance curve (EFDC), which is based on electrostatic force measurements using AFM. A comparison with the results obtained using kelvin force microscopy (KFM) allowed us to highlight the fact that EFDC is sensitive to charges localized in the third-dimension.

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.

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.

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.

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.

Charges injection investigation at metal/dielectric interfaces by Kelvin Probe Force Microscopy

Charges injection at metal/dielectric interface and their motion in silicon nitride layer is investigated using samples with embedded lateral electrodes and surface potential measurement by Kelvin Probe Force Microscopy (KPFM). Bipolar charge injection was evidenced using this method. From surface potential profile, charge density distribution is extracted by using Poissons equation. The evolution of the charge density profile with polarization bias and depolarization time was also investigated.

Dielectric engineering of nanostructured layers preventing electrostatic charging in thin dielectrics

New dielectric-engineering concept is developed intending a net improvement of the performance of dielectric layers under electrical stress. Instead of synthesis of new dielectric materials a new class of dielectric layers that gain their performance from design rather than from composition is established. Two kinds of nanostructured dielectric layers are presented here: (i) silicon oxynitride layers (SiOxNy:H) with gradual variation of their properties (discrete or continuous), and (ii) SiO2 layers with tailored interfaces; a single layer of silver nanoparticles (AgNPs) is embedded in the vicinity of the dielectric free surface. The nanostructured layers exhibit much shorter charge retention times and appear promising candidates for general applications where surface charging of dielectrics must be avoided, in particular for implementation in RF MEMS capacitive switches with electrostatic actuation.

Impact of press-molding process on chemical, structural and dielectric properties of insulating polymers

The macroscopic dielectric response of insulating materials, like apparent conductivity or space charge features is known to be sensitive to electrode and interface properties in a general sense. To unravel the detailed properties (chemical, topological, microstructural) controlling electronic properties, specific methods need to be implemented. The present contribution addresses the electrical, as well as the chemical and structural features of low density polyethylene material (LDPE), brought by the use of different protective films during compression moulding. The bare materials were studied by Photoluminescence (PL), Infrared Spectroscopy (IR) and Atomic Force Microscopy to characterize their morphology and chemical properties. Electrical properties were determined by charging/discharging currents and space charge measurements. We show that when using polyethylene terephthalate (PET) as protective cover layer, extra-signatures appear in PL and IR spectra related to diffusion of oxidized groups-presumably decomposition products from PET into the LDPE surface. Also changes in surface roughness appear dependent on the nature of the cover layer. The different press-molding conditions did not reveal substantial changes in the formation of space charge, and the nature of electrode remains the most influencing factor for space charge and conduction features.

Sensitivity analysis of the electrostatic force distance curve using Sobol’s method and design of experiments

Previous studies have demonstrated that the electrostatic force distance curve (EFDC) is a relevant way of probing injected charge in 3D. However, the EFDC needs a thorough investigation to be accurately analyzed and to provide information about charge localization. Interpreting the EFDC in terms of charge distribution is not straightforward from an experimental point of view. In this paper, a sensitivity analysis of the EFDC is studied using buried electrodes as a first approximation. In particular, the influence of input factors such as the electrode width, depth and applied potential are investigated. To reach this goal, the EFDC is fitted to a law described by four parameters, called logistic law, and the influence of the electrode parameters on the law parameters has been investigated. Then, two methods are applied—Sobols method and the factorial design of experiment—to quantify the effect of each factor on each parameter of the logistic law. Complementary results are obtained from both methods, demonstrating that the EFDC is not the result of the superposition of the contribution of each electrode parameter, but that it exhibits a strong contribution from electrode parameter interaction. Furthermore, thanks to these results, a matricial model has been developed to predict EFDCs for any combination of electrode characteristics. A good correlation is observed with the experiments, and this is promising for charge investigation using an EFDC.