Process for the characterization of an insulator and the corresponding electron microscope

Process and apparatus for the characterization of an insulator (5) at breakdown with the aid of an electron microscope (1), in which the electron flow rate of the beam (3) is adjusted as a function of in particular backscattered, lost, secondary or absorbed electrons. An automatic controller (9) sensitive to certain sensors (10,12) is provided for this purpose. The electron flow rate is proportional to the voltage to be simulated in the insulator.

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 certain distances from the injection 
location, said process being characterized in that it includes a 
measurement of the flow rates of electrons reflected from the insulator 
and an adjustment of the flow rate of the beam as a function of said 
reflected electron flow rate using an automatic controller for injecting a 
given flow rate of electrons which are not reflected, in accordance with a 
given time function. The time function is preferably identical to the time 
voltage of an electric voltage to be simulated on the insulator, to within 
a proportionality coefficient. Finally, it is advantageous for the process 
to include calorific capacity measurements with respect to the insulator 
before or after injection by placing it on a calorimeter positioned in the 
microscope. 
It also relates to an electron microscope for performing this process and 
which includes means for measuring the flow rates of electrons reflected 
from the sample and for adjusting the electron beam flow rate as a 
function of said reflected rates in order to obtain a flow rate of 
electrons which are not reflected or returned. According to the invention, 
it is used for implanting electric charges at a fixed point of the sample 
in accordance with a giventime function, unlike in the prior art of 
DE-A-4,020,806, which relates to an electron microscope, whose electron 
flow rate is dependent on the flow rate of secondary electrons so as not 
to burden the sample. This process is only useful for a conventional 
utilization of the microscope on observing a sample scanned by a mobile 
beam, which encounters insulating zones, which must not be charged because 
otherwise an excessive brightness would be obtained, because the electrons 
are only slightly diffused and remain substantially in situ. Thus, it is 
preferable to reduce the incident electron flow rate.

FIG. 1 shows an electron microscope 1, which in the usual way incorporates 
magnetic means 2 for directing and accelerating an electron beam 3 emitted 
by an electron gun 4 at the top of the column of the electron microscope 
1. The sample 5 is placed on a support 6 and is mobile under the action of 
means such as a push rod /, to which it is connected by not shown means 
enabling it to also be pulled. Thus, it is possible to vary the electron 
impact point on the sample 5. 
A vacuum pump 8 makes it possible to suck up the gaseous content at the 
electron microscope 1. An automatic controller 9 adjusts the electron flow 
mate of the beam 3. It makes use of a detector for secondary and 
backscattered electrons 10, oriented towards the sample 5, as well as an 
absorbed current measuring circuit 11 connected to the support 6 and 
provided with an ammeter 12 or an equivalent means. The detector 10 and 
the circuit II are both connected to the automatic controller 9, whose 
function is to add to the initially provided electron flow rate a 
supplementary rate equal to the sum of the rates corresponding to the 
secondary, backscattered and absorbed electrons. Thus, a perfectly 
controlled electron flow rate is injected at a fixed point of the sample 5 
and stays there. 
This flow rate is proportional to the voltage which it is wished to 
simulate and consequently corresponds to a signal of the same form 
(sinusoidal, square, pulse-type, etc.) and of the same frequency or 
duration. The sampling step of the signal, corresponding to the reaction 
time of the automatic controller 9, is a few microseconds with known 
electron systems or even much less, which is sufficient for supplying 
perfect or almost perfect simulations for ordinary situations. 
When the simulation signal is completely supplied, use is again made of the 
known method for estimating the dielectric qualities of the sample 5, 
involving the displacement of the electron beam 3 after raising it to an 
electric potential which is different on each occasion, in order to 
measure the distance between the impact point and the equipotential on the 
sample 5 at the sample potential as the electron beam 3, as a result of 
the transition between the reflection and the absorption of the electron 
beam 3, which then appears. The curves of FIG. 2 give voltages on the 
abscissa and inverses of the distance (or the radius of the equipotential) 
on the ordinate, expressed in arbitrary units. Curve 20 is representative 
of a continuous signal and curve 21 of a signal formed from pulses lasting 
two microseconds. As curve 21 has a smaller gradient, it can be deduced 
that the flow of the charges at the surface of the sample 5 is more 
pronounced when the charge is applied in a continuous manner and that 
therefore the breakdown resistance of the insulator is interior. 
The electric potential of the electron beam 3 is freely chosen during the 
implantation of the charges and can in practice be 30 to 40 kV. 
The electron microscope 1 finally incorporates a Calvet microcalorimeter 13 
and heating or cooling means 14 located in the support 6 and whose 
function is to be heat or cool the sample 5 by the Joule or Peltier 
effect. They can be constituted by a heating coil in the first case, which 
brushes the sample 5, or a pair of electrodes in the second case, whereof 
one touchs the sample 5 and the other is enveloped in the support 6. 
The means 14 and the microcalorimeter 13 enable the performance of 
measurements of the specific heat of the insulator. If this heat decreases 
following the injection of the charges, it is deduced therefrom that there 
is a rearrangement of the internal energy at the sample 5 and it can be 
assumed that it will not or will only scarcely age in time, i.e. its 
breakdown characteristics will not deteriorate. The measurement is made 
possible by sliding the sample 5 from the upper surface of the support 6 
to that of the microcalorimeter 13, which is continuous therewith. It is 
pointed out that the vacuum is established in the microscope 1, which 
eliminates convection-based heat losses. It is consequently possible to 
modify a conventional microcalorimeter, where the sample would be 
installed in a closed cavity, by placing the measuring means (a thermopile 
in the case of a Calvet microcalorimeter) on the upper surface of the 
apparatus. The thermopile is then sensitive to the temperature difference 
between the sample 5 placed thereon and the remainder of the apparatus. It 
carries the reference 15 in FIG. 1 and the electric wires by which the 
current flows expressing the temperature difference are designated 16,