Process and device for the ionic analysis of an insulating sample

The invention provides a process and device for the ionic analysis of an insulating sample brought to a given negative potential, of the type in which a target on the surface of the sample to be analyzed is bombarded by means of a primary electron beam and negative ions emitted by the bombarded target are used for producing an ion image of the sample. An electron beam whose normal speed component cancels out just at level of the surface of the target is directed perpendicularly to the target. The device comprises for this purpose a filament, brought substantially to the same negative potential as the sample, which emits the electron beam. The electron beam, after emission, is deflected by a magnetic prism so as to be brought into coincidence with the optical axis of the negative ion beam emitted by the target.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process and device for the ionic analysis of an 
insulating sample. 
Conventionally, in spectroscopy, a target at the surface of the sample to 
be analyzed is bombarded by a beam of primary ions. The target then emits 
negative ions which are collected by a system which gives the distribution 
pattern of the elements present at the surface of the sample. 
This technique is satisfactory for samples which are sufficiently 
conducting. 
But, a large number of samples examined by the ionic analyser are 
insulating: polished rock sections, sections of teeth or bones, sections 
of biological tissues, oxide inclusions, oxide layers, passivation layers 
. . . Observation of these samples causes problems because the electric 
charges flow badly. Even when a balance is reached between the flows of 
charges received, emitted and evacuated by low conductivity, charge 
excesses or defects may exist locally and create potentials which stagger 
the energy distributions of the secondary ions, deform the paths of the 
secondary ions which will form the images and modifiy the primary 
bombardment conditions. Furthermore, the electric fields may be locally 
very intense and induce the migration of a number of elements. This 
phenomenon is particularly visible when implantation or diffusion profiles 
in silica layers on semi-conductors are plotted. 
2. Description of the Prior Art 
This problem has been partially solved in the case where, for removing the 
positive ions emitted by the target, a metal grid is deposited by 
evaporation on the surface. The insulating areas of the sample received, 
in addition to the primary ion beam, low energy secondary electrons 
emitted by the bars of the grid and high energy electrons produced on the 
extraction electrode and attracted by the sample. Control of the surface 
potential of the sample is then obtained by suitably adjusting the density 
of the primary bombardment. 
On the other hand, when it is a question of negative secondary ions, on the 
one hand the electrons produced on the extraction electrode are pushed 
back by the sample and, on the other hand, the electrostatic field 
extracts at the same time the secondary electrons emitted by the target: 
the emission of secondary electrons is much more intense then the ionic 
emission, so that, whatever the sign of the primary bombardment ions, a 
positive charge always appears at the surface. This emission is further 
increased when the output work is lowered by a Cs.sup.+, K.sup.+ 
bombardment or by Cs vapor blowing. Moreover on heterogeneous samples, 
this charge may vary from one place to another depending on the secondary 
electron output of the location considered. Experience shows that this 
positive charge is such that any ionic microscopy from negative secondary 
ions is impossible with conventional procedures. 
SUMMARY OF THE INVENTION 
The aim of the invention is to remove this impossibility by providing a 
process for effectively suppressing charge effects. 
This aim is reached in accordance with the invention by directing, 
perpendicularly to the target, an electron beam whose normal speed 
component is cancelled out just at the level of the surface of the target. 
Thus, any positive charge appearing at the surface is immediately 
neutralized without fear of an excess negative charge for, since the 
electrons are slow, they are pushed back as soon as the surface becomes 
slightly negative. Another major advantage of the invention is then to 
supply the surface of the sample with electrons without these latter 
having a harmful interaction with the sample (such as the problems of 
diffusion of some elements under electron bombardment of too high an 
energy, or cracking problems.) 
Advantageously, a filament brought substantially to the same negative 
potential as the sample emits said electron beam. 
Advantageously, the electron beam is, after emission, deflected by a 
magnetic prism so as to be brought into coincidence with the optical axis 
of the negative ion beam emitted by the target. 
Advantageously, electrons leaving the target along the optical axis are 
used for obtaining an electron image of the sample. 
The invention also provides a device for the ionic analysis of an 
insulating sample brought to a predetermined negative potential, of the 
type which comprises a primary bombardment ion source directed on a target 
at the surface of the sample to be analyzed and, in the optical axis of 
the device, a mass spectrograph. 
According to the invention, said device also comprises a source brought 
substantially to the potential of the sample emitting an electron beam and 
means for directing said beam on said target perpendicularly to the 
bombardment surface. 
Advantageously, said means consist of a magnetic prism and magnetic 
compensation is provided for the deflection by said prism of the path of 
the negative ions emitted by the target. 
Advantageously, the prism is a double magnetic prism, deflecting the 
electrons which leave the target along the optical axis and a second 
analyser is provided delivering an electron image of the sample.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 can be seen a source 1 of primary ions brought to a potential of 
10 kV bombarding, with an ion beam 2, a target at the surface of sample 3 
brought to a potential of -4 500 V. 
The secondary negative ion beam emitted by the target is accelerated by the 
extractor electrode 4, at ground potential, passes through a collecting 
optical system 5 then through diaphragm 6 situated at the crossover. Along 
the same optical axis 7 is disposed the mass spectrograph 8 to which the 
present invention does not relate. 
According to the invention, between diaphragm 6 and spectrograph 8 is 
inserted a system 11 for suppressed charge effects. This system 11 
comprises an electron source 12 situated on an axis 13 intersecting the 
general optical axis 7 at an angle which, in the Figure is 90.degree.. The 
electrons are provided by a filament brought to a potential of -4 500 V; 
they are accelerated by an extractor electrode at 0 potential and thus 
through a conventional centering device 14. They are deflected by a 
magnetic prism 15 which thus brings them back along the optical axis. At 
the output of prism 15, the electron beam passes through diaphragm 6, a 
common cross-over for the ion beam and the electron beam; then it passes 
through the emission lens 5 and arrives in the zone situated between the 
object 3 and the extractor electrode 4, in which zone reigns the electric 
field for accelerating the negative particles emitted by the surface. This 
electric field then brakes the electrons, their energy decreases gradually 
and is cancelled out substantially at the level of the surface. The 
emission lens 5 acts on the electron parts as it does on those of the 
negative ions. The result is that the electrons arrive at the surface with 
a lateral speed component which is very small, their normal component at 
the surface being zero or practically zero (and in any case not creating 
any harmful interaction with the sample). The electrons which are not used 
for neutralizing the surface of the sample turn back, with secondary 
electrons emitted by the surface of the sample (and naturally with the 
negative ions). 
The magnetic prism 15 used for deflecting the discharge electrons through 
90.degree. is advantageously double; it deflects in the reverse direction 
the electrons leaving the target, thus making possible observation of the 
electron image formed from the back scattered or reflected electrons, or 
else that of the image obtained with the secondary electrons produced 
either by primary ionic bombardment or by impact of the incident 
electrons. To this end, an associated analyzing device, which is known per 
se, is provided in the path of the deflected electron beam 16: selection 
plates 7, electrostactic centering 18, a post-accelerating optical system 
19, a projection optical system 20, a screen 21. 
The presence of prism 15 in the optical axis 7 also produces a slight 
deflection through angle .theta. of the path of the secondary ions. An 
additional magnetic system 22 is therefore provided for compensating this 
rotation whatever the mass of the secondary ions. 
FIG. 2a shows a general ion image--that is to say without mass filtering of 
the ions--obtained by ion bombardment of an ion die. The dark spots 
correspond to disturbances of the secondary ion emission induced by the 
presence of small alumina precipitates (respective dimensions about 10 and 
20 micrometers) having very low electric conductivity. 
FIG. 2b shows an ion image obtained under the same ion bombardment 
conditions but, this time, with the use of a low energy electron beam (&lt;10 
eV) in accordance with the invention. The image of the alumina 
precipitates becomes perceptible, giving information about the real 
dimensions of these objects and showing for each of them the existence of 
an internal structure. 
FIG. 3a is a general ion image obtained, as before, by bombarding an iron 
target--a metal which is rich in insulating alumina precipitates. 
In FIG. 3b, the slow electron beam has this time been localised on the 
alumina precipitate situated on the right of the preparation. The effect 
of the electron bombardment can be discretely felt at the level of the 
precipitate situated in the centre of the image. One of the merits of this 
procedure for neutralizing the electrostatic charges is that it is 
self-adjustable. 
Adaptation of this device for discharging insulating samples to a 
conventional ion analyzer allows the analysis to be made, impossible up to 
now, of negatively charged oxygen ions emitted from insulating material, 
such as most of the compounds of oxygen, oxides and silicates, and makes 
possible the pin point analysis of the iosotopic composition of this 
element in objects of interest in the astrophysical sphere: key mineral 
phases of meterorite, interplanetary dust. 
Moreover, the process of the invention, which in no wise modifies the 
physical or chemical appearance of the sample analyzed, allows all sorts 
of in situ analyses whose non destructive character is of primary 
importance.