Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2003-02-26
2004-05-04
Berman, Jack (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S306000, C250S307000, C250S492300, C324S452000
Reexamination Certificate
active
06730908
ABSTRACT:
TECHNICAL DOMAIN
The present invention relates to a process for charging a structure comprising an insulating body and a charging device for such a structure. Control of conditions for charging an insulating body, in other words knowledge of the quantity of the charge and the distribution of the charges then makes it possible to study the potential decay phenomenon from when it starts, and the potential return with time after the structure has discharged. These studies then determine the electrical properties of the body such as the electronic mobility of the insulating material, its conductivity and its dielectric constant. Knowledge of these properties is essential to determine the aptitude of new insulating materials for industrial use, for example in capacitors, electrical cables, semiconductors, electronic tubes.
STATE OF PRIOR ART
The state of prior art is illustrated by documents [1] to [12] listed at the end of this description.
For the purposes of studying the behavior of insulating materials subjected to strong fields, an understanding of charge injection phenomena within the volume of the material and the associated transport mechanisms is essential. In order to characterize these transient properties, a large number of articles suggest that one face of a sample of insulating material can be charged to a given electrical potential and then the variation of this potential can be monitored with time. The observed decay, called the “potential decay” is a natural phenomenon involving several physical processes such as the injection of charges in volume, polarization or conduction as described in documents [1] and [2]. In this case, it is particularly important to be able to use a process to perfectly control the initial conditions of the decay (quantity and nature of charges, spatial distribution) as to determine the injection and mobility of the charges correctly.
The samples must be previously charged before a potential decay experiment can be carried out. It is usually assumed that this charge is initially close to the surface of the sample. It is very critical to respect this condition in order to study the decay of the potential from its starting point, in other words for a maximum field. Consequently, the charge time must be practically instantaneous compared with the decay time. The potential is usually measured using a slaved potential probe (contact free measurement). Different charging techniques have been used in the past; using the corona effect described for example in document [3], using an electron beam described for example in document [4], or by contact described for example in document [5].
Studies carried out starting from corona discharges have enabled Ieda et al. in document [6] and then other authors later, for example in document [7], to confirm the existence of charges injection into a volume with a high electric field, by indirect effects. However, use of the corona effect is difficult to the extent that it uses a large number of gas ionization and ion deposition processes on the surface of the sample. The nature of the deposited charge and its distribution is then difficult to control. All that can be imposed precisely is the surface potential, without any guarantee about the nature and distribution of the charges. Different combinations of these parameters can give the same surface potential. Furthermore, since the experiment frequently takes place in an ambient atmosphere, a recombination of surface charges with ions in air contributes to the decay, which complicates application of the experiment.
The charge may be directly injected by using a high energy electron beam. With this type of technique, Watson characterized the energy level of traps in which the injected charges are located, in document [4]. More recently, Coelho et al. developed a device in the patent document [8] to measure the mobility of charges injected in an insulating material.
This technique is based on the use of the electron microscope beam to charge the sample. In document [9], Coelho also proposed to use the electrostatic mirror described in patent document [10] for local study of the potential decay on films a few tens of micrometers thick.
The use of an electron beam actually controls the quantity and type of carriers involved. However, the charge is not actually on the surface but is distributed over a thickness that depends very much on electron injection conditions (energy, current, focus, etc.). This thickness is difficult to control.
Furthermore, penetration of electrons imposes the use of samples that are much thicker than the electron stop depth. Consequently, this technique cannot be applied for studying thin layers.
Finally, an excessively high secondary electronic emission can create complex distributions between positive and negative charges. The use of this technique requires thorough knowledge of charge trapping phenomena in insulating materials, which is not always easy to understand.
In order to overcome the problem of electron penetration, charges can be injected by contact with a charged electrode (using an electron beam or a voltage generator). In this case the charge must overcome an energy barrier before penetrating into the material. The result is slower potential decay as described in document [6]. In document [11], Coelho suggested a model to describe this phenomenon. This technique has the advantage that it takes account of the influence of the insulating material/electrode interface in the injection process. This configuration is more representative of electrotechnical applications. It can also be used to study thin layers.
However, when the electron beam is directed directly onto the electrode, the effective energy of the beam reduces as a function of the increase in the potential of the electrode. However, the number of electrons actually remaining on the electrode depends directly on the beam energy. Consequently, the electron beam current can no longer be considered as being constant and may vary considerably during injection until it is cancelled out. The initial potential decay conditions (quantity and distribution of charges) are then not known precisely.
Therefore, regardless of the method used for charging, the quantity and nature of deposited charges are difficult to control satisfactorily. This distorts interpretation of the potential decay and consequently the validity of the associated transport models.
PRESENTATION OF THE INVENTION
The purpose of the charging process according to the invention is to overcome the disadvantages mentioned above in order to control the quantity and distribution of charges at the end of the charge and therefore at the beginning of the potential decay.
More precisely, the process according to the invention is a process for charging a structure formed from an insulating body sandwiched between two electrodes. It comprises the following steps:
a Faraday cage is placed in contact with one of the electrodes in the structure, the other electrode being made equal to a reference potential;
electrons originating from a controlled electron emission device are introduced into the Faraday cage, the electrons reaching the electrode with which it is in contact in order to charge the structure.
The structure and the Faraday cage can be placed in a vacuum chamber particularly to prevent recombination of electrons participating in the charge with ions in the atmosphere around the structure.
During the charge, the potential of the electrode in contact with the Faraday cage can be measured.
It is preferable to measure a secondary emission of electrons, if any, close to the Faraday cage to make sure that all electrons emitted by the controlled emission device actually participate in the charge.
At the end of the charge, the potential of the electrode in contact with the Faraday cage can be measured at different times, this potential variation representing a potential deca
Bigarre Janick
Hourquebie Patrick
Matallana Jérôme
Berman Jack
Commissariat a l'Energie Atomique
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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