Radiant energy – Inspection of solids or liquids by charged particles – Methods
Patent
1996-06-18
1997-06-03
Berman, Jack I.
Radiant energy
Inspection of solids or liquids by charged particles
Methods
250310, G01R 3112, G01N 2792, H01J 37256
Patent
active
056357150
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention relates to a process for the characterization of an insulator and to a corresponding electron microscope.
EP-A-470,910 refers to an electron microscope usable for this purpose, firstly by implanting electric charges at the surface of the insulator and then measuring the positions of the equipotentials, which are normally circular and concentric curves at the implantation point. For this purpose the electron beam is displaced to the surface of the sample once the electric charges have been implanted, its potential being regulated to the value of the sought equipotential and a specular method is applied, whose criterion is that a transition between the absorption and the reflection of the beam is marked on arriving at the equipotential. This process makes it possible to plot a function expressing the evolution of the electric potential at the surface of the sample as a function of the inverse of the radius or the distance at the charge implantation point. This function, which in principle has a useful straight part, gives an estimate of the dielectric characteristics of the insulator and its breakdown strength simply through the gradient of the line, because the potential measured on a good insulator decreases very rapidly with the distance at the implantation point and the charges are virtually unable to move. However, this method is only valid under quasi-static conditions. However, it is well known that the resistance or strength of the insulator can be very different if a periodic, variable voltage or an isolated pulse is applied. It is then necessary to examine known characterization methods under such conditions. According to the conventional method, the samples are subject to the test voltage by electrodes and an inspection is made to establish if a breakdown has occurred. However, the disadvantage; which also applies in statics, is that the result measured is effectively not only dependent on the characteristics of the actual insulator, but also on the quality of its contact with the electrodes. Significant precautions have to be taken in order to protect the exterior of the location where the experiment is performed, because the voltages are often extremely high.
Improvements which have already been proposed by which the electric potential is created by a heat or pressure wave, which is possible with certain materials, still suffer from the disadvantage that electrodes are required for measuring the response of the insulator. These methods are also more difficult to use.
The invention is based on the finding that a satisfactory modelling of transient voltage conditions can also be obtained by using a scanning electron microscope provided that certain measures are taken, because the influence of certain phenomena are noticed whereas it would be imperceptible under static conditions.
Thus, account must be taken of the last electron flow rates for the implantation on the surface of the sample, i.e. mainly reflected electrons, more specifically be backscattering, as well as secondary electrons and electrons absorbed by the sample, but which escape therefrom.
Therefore the invention relates to a process making it possible to surmount these difficulties and perfectly simulate the placing under voltage in scanning electron microscopes. An ancillary problem which is solved is to virtually simultaneously subject the sample to calorific measurements without dismantling or displacing it.
It has in fact been found that calorific capacity variations of the sample brought about by the injection of charges make it possible to indicate if its behaviour would or would not deteriorate on aging, which is very important for buried electric cables, whose life must be very long.
The invention consequently relates to a process for the characterization of an insulator on breakdown using a scanning electron microscope by which electrons of a beam are injected at one location of the insulator, then the beam is moved in front of the insulator to measure the electric potential on the insulator at cer
REFERENCES:
patent: 5432345 (1995-07-01), Kelly
patent: 5512746 (1996-04-01), Saito
Patent Abstracts of Japan; vol. 8, No. 51; p. 259; Mar. 8, 1994.
Journal of Applied Physics; vol. 69, No. 9, pp. 6334-6339, May 9, 1991; G. Blaise et al.; Charging and Flashover Induced By Surface Polarization Relaxation Process.
International Search Report; PCT/FR94/01172; Jan. 25, 1995; Falk Heck.
Acroute Daniel
Bezille Jose
Blaise Guy
Faure Claude
Janah Hakim
A L'Energie Atomique
Alcatel Cables
Berman Jack I.
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