D. Juvé

Electron beam charging of insulators: A self-consistent flight-drift model

Electron beam irradiation and the self-consistent charge transport in bulk insulating samples are described by means of a new flight-drift model and an iterative computer simulation. Ballistic secondary electron and hole transport is followed by electron and hole drifts, their possible recombination and/or trapping in shallow and deep traps. The trap capture cross sections are the Poole-Frenkel-type temperature and field dependent. As a main result the spatial distributions of currents j(x,t), charges ρ(x,t), the field F(x,t), and the potential slope V(x,t) are obtained in a self-consistent procedure as well as the time-dependent secondary electron emission rate σ(t) and the surface potential V0(t). For bulk insulating samples the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and σ=1. Especially for low electron beam energies E0<4keV the incorporation of mainly positive charges can be controlled by the potential VG of a vacuum grid in front of the target surface. For...

Electron Beam Charging of Insulators with Surface Layer and Leakage Currents

The electron beam induced self-consistent charge transport in layered insulators (here, bulk alumina covered by a thin silica layer) is described by means of an electron-hole flight-drift model and an iterative computer simulation. Ballistic secondary electrons and holes, their attenuation and drift, as well as their recombination, trapping, and detrapping are included. Thermal and field-enhanced detrapping are described by the Poole–Frenkel effect. Furthermore, an additional surface layer with a modified electric surface conductivity is included which describes the surface leakage currents and will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate. As a main result, the time-dependent secondary electron emission rate σ(t) and the spatial distributions of currents j(x,t), charges ρ(x,t), field F(x,t), and potential V(x,t) are obtained. For bulk full insulating samples, the time-dependent distributions approach the final stationary state with j(x,t)=c...

Nickel-alumina bonds: Mechanical properties related to interfacial chemistry

Abstract Mechanical properties of solid state bonded Ni-Al2O3 interfaces have been investigated as a function of several bonding parameters (temperature, pressure, time) and purity of the starting alumina. It has been shown that they depend upon the plastic deformation of the metal, on the damages induced at the ceramic surface and on both the nature and amount of interfacial phase that appears during the bonding process. According to the processing conditions used, the nickel aluminate spinel has not been found at the interface, but it has been shown that bonding arises from the magnesium aluminate spinel or sodium magnesium silicate and that nickel suicide is formed on the nickel side of the bond near by the interface.

Joining of AlN with metals and alloys

The use of aluminium nitride in a wide range of applications depends on the capability to form strong AlN/AlN and AlN/metals (or alloys) joints. It has been shown that the different methods of joining, successfully used for alumina, solid-state bonding, liquid-state bonding and direct copper bonding (DCB, brazing), can be used for AlN. An optimisation of the different bonding parameters is required to obtain strong bonds. The influence of oxygen, nitrogen and the role of reactions products have been outlined.

Silver-alumina solid state bonding: Study of diffusion and toughness close to the interface

Abstract The study of Ag/Al 2 O 3 interfaces achieved by solid state bonding shows that silver diffuses in alumina during the joining process and later annealings. This diffusion, particularly along grain boundaries, is correlated to the decrease of the toughness of alumina observed at the same time close to the metal-ceramic interface.

Conduction and trapping mechanisms in monocrystalline titanium dioxide through the mirror method

The present work deals with the use of the Scanning Electron Microscope Mirror method (SEMM method) for characterizing the conduction and trapping mechanisms in the monocrystalline rutile (TiO/sub 2/). First, the SEMM characterizations shows that monocrystalline rutile traps strong quantity of electric charges, depending on the preliminary thermal and mechanical treatments, like for sapphire. However, one of the most interesting results is the unusual shape of the mirror image. Indeed, instead of being circular the mirrors images are elliptic, leading to the idea that the anisotropy of the material could play an important role on the conduction and trapping mechanisms. This idea is confirmed by the evolution of the curved part of the typical mirror plot 1/d=f(V). This curved part, interpreted for example in a model of multipole approximation, clearly shows an anisotropic shape of the distribution geometry of the charges. Finally, correlations with the presence of dislocations, as preferential way of electron conduction and (or) traps will be made to explain this anisotropic shape.

Consequences of anisotropy in electrical charge storage: application to the characterization by the mirror method of TiO2 rutile

This paper is devoted first, to anisotropic distributions of stored electric charges in isotropic materials and second, to charge trapping and induced electrostatic potential in anisotropic dielectrics.On the one hand, we examine the case of anisotropic trapped charge distributions in linear homogeneous isotropic insulators, obtained after an electron irradiation in a scanning electron microscope. This injection leads to the formation of a mirror image. We first establish the characteristics of the mirror image obtained from such anisotropic distribution by linking the mirror diameter to the curvature tensor of the equipotentials thanks to the geometric optics ansatz (GOA). Second, the equipotentials induced by the presence of an anisotropic charge distribution in such isotropic dielectrics have been determined in the case of homeoidal (ellipsoidal) distributions that generalize the classical spherical distributions. Then, for these homeoidal distributions in isotropic dielectrics, the features of the mirror image have been deduced from the previous GOA estimation. Elliptic mirrors can be obtained and calculated in the limit cases of such homeoidal distributions.On the other hand, we consider the non-trivial case of a point charge lying at the interface between the vacuum and a linear homogeneous orthotropic anisotropic dielectric and the determination of its corresponding potential seen from the vacuum. This problem has already been solved in the case of transversal isotropic dielectrics (ex = ey = er, er ≠ ez), but we extend in this paper the classical dielectric image problem to the more general case where ex ≠ ey ≠ ez. The equivalent charge and the induced electrostatic potential are evaluated. For these anisotropic insulators, the equipotentials created by a point charge at the interface are found to be ellipsoids and this leads to an elliptic mirror image. The ratio between the two main axis values of the elliptic mirror is proportional to the square root of the ratio of the permittivities values in the plane of the interface. Finally these calculations are used to explain the experimental results obtained by the mirror method on a TiO2 sample that is known to be an anisotropic dielectric.

Analysis of electron transfer between electron irradiated metallic ball and insulators in vacuum: A specific alternative to the mirror method

In order to improve the knowledge of dielectric properties of insulators, we have imagined an original method of characterization of the charge buildup. Electrons of an electron beam are implanted through a metallic ball directly in contact with the insulator in a scanning electron microscope. By calculating and modeling the capacitance and the electrostatic force between the ball and the insulator plane, it has been possible to determine the relationship between the injected charges in the metallic ball and its surface potential. The major role of the dielectric thickness has been evidenced when the insulator is placed on a grounded metallic plane. At high potential values, a dielectric breakdown of the medium surrounding the sphere occurs and electrical charges are transferred from the ball to the dielectric sample. This transfer has been evidenced and quantified in the case of sapphire and quartz. Analytical calculations and numerical simulations using the finite-element method have been performed for interpreting these experimental results.

Electron beam probing of insulators

Electron beam irradiation and charge injection associated by selfconsistent charge transport in insulating samples are described by means of an electron-hole flight-drift model (FDM) implemented by an iterative computer simulation [1,2]. Ballistic scattering and transport of secondary electrons and holes is followed by electron and hole drift, their possible recombination and/or trapping in shallow and deep traps. Furthermore a detrapping by the temperature- and field-dependent Poole-Frenkel-effect becomes possible allowing even a charge hopping transport. In this context a special surface layer has been installed to investigate surface leakage currents, see Fig.1.

Injection, transport and trapping of electrons in overlayered insulators

This work is focused on the charging behavior of bulk dielectric sample covered with a surface layer. Indeed, thanks to a flight-drift model of electrons and holes in insulators and an iterative computer simulation, it is possible to understand the charge transport and trapping in bulk insulating samples. However the presence of a surface layer modifies the electrical charging of the whole sample, as previously experimentally shown by other teams. The aim of the present work is to show by simulation and experimental measurements, the influence of an overlayer on the electron and hole injection, drift and trapping, on the secondary electron emission and on the spatial charging of overlayered dielectrics. A special effective layer method is introduced in the model to describe the electron scattering and straggling in the layered sample. The surface leakage currents on the top surface are also taken into account, as well as the effects of the temperature on the trapping-detrapping processes by a Poole-Frenkel like approach. The numerical and experimental results will be presented in the case of a bulk alumina which surface has been modified by femtosecond laser irradiation.