Instrument for very high resolution ionic micro-analysis of a solid sample

A sample (EC) is mounted on a sample carrier (PO). Vertically thereabove a common optical portion (5) receives a beam of primary ions derived from an ion source (S10) together with a beam of primary electrons derived from an electron source (S30). The secondary electrons and ions due to the sample (EC) being bombarded by said primary ions and electrons are retrieved by said common optical portion (5). The electrons are detected by electron detection means (D40). The secondary optical system (2) transmits the secondary ions to a mass spectrometer (SP20). This instrument is capable of providing ion and electron images of a single sample simultaneously.

The invention relates to analyzing elements and isotopes from very small 
volumes of material taken from the surface of a solid sample. 
BACKGROUND OF THE INVENTION 
Such measurements have previously been performed using an instrument such 
as the IMS-3F ion microscope sold by the French CAMECA corporation. The 
following patents relate to said instrument: French patent No. 1,240,658 
filed July 30th, 1959; U.S. Pat. No. 3,585,383, British patent No. 
1,183,310 and French patent application Ser. No. 2,542,089 filed Jan. 
14th, 1983 corresponding to U.S. patent application Ser. No. 575,828 filed 
Jan. 2, 1984. 
Operation of an ion microscope may be summed up as follows: a target is 
bombarded with a beam of primary ions which are all of the same nature, 
and at a controlled energy, said ions covering the entire observed 
surface. Under the effect of this bombardment, each point of the target 
releases the atoms located at said point. A considerable portion of these 
atoms may be spontaneously ionized during the ejection process and thus 
constitute positive or negative secondary ions which are characteristic of 
the chemical and isotopic composition of the eroded volume. The beam of 
secondary ions created in this way is taken and used to create an ion 
image of the sample, and for sequential analysis by means of a mass 
spectrometer. 
This prior instrument has enabled considerable progress to be made by 
virtue of its qualities of detection sensitivity and of mass resolution, 
and also by its ability to form an "ion image" for each element present in 
the solid sample being analyzed. 
There now arises a need to further improve resolution, and in particular to 
improve spatial resolution, and also to improve detection sensitivity 
while enabling several elements to be detected simultaneously so that very 
small volumes of material can be analyzed using a method which is 
inherently destructive. It is also essential to dispose of independent and 
non-destructive means for initial observation of the sample. 
Preferred implementations of the present invention solve the above problem. 
One of the aims of the invention is to focus a narrow and intense probe of 
primary ions (1 .mu.m at 200 .ANG.) in a manner which is compatible with 
highly efficient collection of the secondary ions which are emitted over a 
very large solid angle and which are dispersed over an energy band of 
several tens of electron-volts. 
Another aim of the invention is to allow simultaneous observation of the 
sample by means of a scanning electron microscope using the secondary 
electrons or else using transmitted electrons, or else using backscattered 
electrons. 
A third aim of the invention is to provide apparatus which scans the 
primary electrons and ions together in such a manner as to simultaneously 
produce the electron image of the scanning microscope and the ion images 
each of which gives the distribution of one of the elements present at the 
surface of the sample. 
Such scanning must be compatible with simultaneous analysis (in parallel) 
of the various secondary ions released by each of the scanned points of 
the target, while retaining high sensitivity and high mass resolution. 
(high resolution and simultaneous multiple detection mass spectometer). 
The scanner must also be compatible with the beam of primary ions being 
focussed into a narrow probe regardless of its position on the surface. 
Furthermore, the scanning must retain the necessary qualities in the 
secondary beams of ions and of electrons. 
Another aim of the invention is to allow four beams having different 
energies to pass simultaneously along the same optical axis in a manner 
which is compatible with independent adjustments being performed on each 
of said beams. 
Yet another aim of the invention is to provide an instrument which is 
compact. This is important for reasons of mechanical stability and because 
of the high vacuum which is required for proper operation of the 
instrument. 
SUMMARY OF THE INVENTION 
The invention provides an instrument which is comparable with the 
above-mentioned CAMECA instrument in that it comprises, within an 
evacuated enclosure: 
a moving sample carrier suitable for receiving the sample to be analyzed; 
a source of primary ions; 
first electrostatic optical means for causing a beam of primary ions from 
said source to bombard said sample; and 
second electrostatic optical means for collecting the secondary ions 
emitted by the sample in response to the bombarding primary ions, and for 
conveying the beam of secondary ions collected in this manner to the 
inlets of an ion analyzer including a mass spectrometer. 
According to a first aspect of the invention, an instrument in accordance 
with the invention has the following features: 
Said first and second electrostatic optical means comprise a common and 
coaxial optical portion having an axis which is perpendicular to the 
sample carrier and which extends to the vicinity thereof in order to 
provide highly efficient collection of secondary ions while using the same 
optical components to simultaneously process the beam of primary ions and 
the beam of secondary ions; 
Said first and second optical means include, at the end of said common 
optical portion furthest from said sample carrier, means in the form of a 
transverse electrostatic field for separating primary ions from secondary 
ions and means suitable for causing said beam of primary ions to scan, and 
for dynamically and synchronously correcting in said beam of secondary 
ions the effects which result from displacing the emissive zone on the 
sample surface, the pivot center of the primary scanning lying on the axis 
of the common optical portion; and 
Autonomous electron observation means are provided comprising magnetic 
optical members incorporated coaxially with said common optical portion 
but adjustable in a manner which is substantially decoupled from said 
first and second electrostatic optical means, together with means for 
applying a transverse magnetic field perpendicular to the above-mentioned 
electrostatic field in order first to cancel the deflection which said 
electrostatic field would otherwise produce on the primary electron beam, 
and second to separate the electron beam and the beam of secondary ions, 
thereby enabling an ion image of the sample and an electron image of the 
sample to be obtained simultaneously. 
Preferably, the autonomous observation means is disposed as a scanning 
electron microscope having its electron scanning beams incorporated in 
said common optical portion. 
According to another aspect of the invention, the common optical portion 
includes an electrostatic lens for bringing the pivot center of the 
primary scanning to the center of a diaphragm which delimits the angular 
aperture of the primary beam, together with a near optical system suitable 
for co-operating with the sample: 
firstly to focus a reduced image of the ion source (the ion probe) on the 
surface of the sample, which focussing may be modulated in synchronism 
with the scan in order to correct focussing as a function of the position 
of the probe on the surface (dynamic focussing correction); and 
secondly to collect both secondary ions and electrons widely and 
efficiently in order to form electron and ion images in the vicinity of 
said diaphragm. 
In an advantageous embodiment, said near optical system comprises, going 
from the diagraphm towards the sample: 
a grounded annular electrode; 
a collection-controlling electrode connected to a singal of the same 
polarity as the sample; 
a focussing electrode connected to a signal of the opposite polarity as the 
sample and 
an annular electrode placed in the vicinity of the sample to delimit the 
sample surface liable to disturb the focussing electric fields, said 
electrode being connected to a signal of the same polarity as the sample, 
which polarity is opposite to the polarity of the incident primary ions. 
Typically for positive primary ions and negative secondary ions, the energy 
of the primary ion beam is 10 keV upstream from the near optical system 
and rises to 15 keV by said near optical system co-operating with the 
sample which is taken to a potential of -5 keV, and the energy of the 
secondary ions is equal to 5 keV at the diaphragm. 
The instrument advantageously further includes a magnetic lens in the 
vicinity of the diaphrgm of the near optical system. 
More generally, the near optical system constitutes a portion of an optical 
system for forming the ion probe and for collecting the secondary ions, 
said optical system further including an electrostatic lens upstream from 
the diaphragm. 
Overall, the common coaxial portion comprises, upstream from said optical 
system for forming the probe and for collecting the secondary ions: 
a beam combiner/separator comprising a pair of electrostatic plates and 
defining the pivot center; 
an optical system for shaping the ion and the electron beams; and 
electron beam scanning means. 
In its scanning electron microscope application, the shaping optical system 
comprises a magnetic lens followed by two electrostatic lenses which are 
polarized by opposite voltages, and the scanning means comprise two 
electromagnetic windings disposed one behind the other. 
The ion scanning means on the primary beam and the synchronous correction 
means on the secondary beam preferably comprise two pairs of plates 
disposed after two electrostatic sectors on the primary ion side and two 
pairs of plates preceding two electrostatic sectors on the secondary ion 
side. 
Advantageously, the instrument then includes firstly an electrostatic lens 
for providing coupling between the two primary electrostatic sectors, and 
secondly an energy filtering slot followed by an electrostatic lens for 
providing coupling between the two secondary electrostatic sectors, the 
assembly being arranged to produce ion deflections which are substantially 
achromatic to the first order. 
Similarly, the first optical means include, after the source of ions, an 
optical system for adjusting the diameter of the ion probe. 
Thus, the first optical means form successively smaller images of the 
source of ions, while both the ion and the electron secondary images 
formed in the vicinity of the diaphragm are at substantially unity 
enlargement. 
At the other end of the instrument, the ion analyzer advantageously 
includes a transfer optical system followed by a high-aperture mass 
spectrometer capable of simultaneous multiple detection. 
Like the source of primary ions, the instrument may include, coaxially with 
said common optical portion and upstream therefrom, an electron gun, 
followed by a magnetic lens for adjusting the diameter of the electron 
probe. 
Typically, the energy of the primary electrons is about 30 keV. 
In a variant of the invention, with an insulating sample, the energy of the 
primary electrons is reduced so that they can be reflected from the 
vicinity of the surface with substantially zero energy. This arrangement 
eliminates charge effects due to the ion bombardment (and is described in 
the above-mentioned French patent application Ser. No. 8,300,538). 
In another variant, the energy of the primary electrons is about 100 keV, 
and the sample is thin and is observed by transmission. 
Finally, the instrument preferably includes, starting from the further end 
of the common portion from the sample, another electrostatic sector which 
retrieves the secondary electrons and conveys them towards a detector.

DETAILED DESCRIPTION 
FIGS. 1 to 3 show that the instrument described includes a source of ions 
S10 and a mass spectrometer SP20 which is preceded by a transfer optical 
system O21. 
In a preferred embodiment of the invention, the source of ions is the 
source described in U.S. Pat. Ser. No. 730,172 filed on May 3, 1985 and 
entitled "A source of ions operating by surface ionization, in particular 
for providing an ion probe". Similarly, the mass spectrometer and its 
transfer optical system are as described in U.S. Pat. Ser. No. 695,240 
filed on Jan. 28, 1985 under the title "A high-aperture mass spectrometer 
capable of multiple simultaneous detection". The contents of each of these 
two prior patent applications is incorporated in the present description 
in order to add to the description and to the definition of the invention. 
Further, the present invention makes use both of a beam of ions and of a 
beam of electrons. By an extension of meaning which is now conventional, 
the words "optical" and "optics" are used to cover devices which operate 
on such beams in the same way as corresponding devices operate on beams of 
photons. Ion optics makes use of electrostatic means, for the most part; 
the primary electron optics makes use of magnetic fields (from magnetic or 
electromagnetic means), for the most part; and the lower energy secondary 
electrons are subject to the effects both of the magnetic fields and of 
the electrostatic means. 
OVERALL DESCRIPTION 
The instrument in accordance with the invention shown in FIGS. 1 to 3 lies 
inside an enclosure EN in which a high vacuum is maintained by cryogenic 
pumps PC1 and PC2. 
An adjustable sample carrier PO supports a sample EC and is capable of 
displacing the sample parallel to its free surface. 
In the center, perpendicularly to the surface of the sample, the instrument 
is disposed like the column of a scanning electron microscope. 
In accordance with an important feature of the invention, this column has a 
common and coaxial optical portion for the beams of primary and secondary 
ions and electrons. This common and coaxial portion is referenced 5 in 
FIG. 1 and extends as far as the sample which has an effect on the optical 
properties thereof. 
In order to clarify the description, assume that the primary ions are 
positive (and preferably Cs.sup.+ or K.sup.+). In theory, the secondary 
ions are negative. The impact of the primary ions also creates secondary 
electrons. Finally, a primary electron beam (an electron microprobe), 
striking the same region of the sample as the primary ion beam (ion 
microprobe) also generates secondary electrons thereon. Under such 
conditions (see FIG. 1): 
The primary ions emitted by the source of ions S10 are conveyed to the 
sample by primary electrostatic optical means given an overall reference 1 
and including the common optical portion 5; 
The secondary ions ejected from the sample EC are conveyed to the mass 
spectrometer SP20 by second electrostatic optical means 2, including the 
common optical portion 5; 
Primary electrons are produced by an electron gun S30, followed by an 
electron beam adjusting optical system R31 for adjusting the geometry of 
the electron beam, and in particular for adjusting its diameter, said 
primary electrons are then conveyed towards the sample through the common 
optical portion 5; 
Finally, the secondary electrons emitted from the sample EC pass also 
through the common optical portion 5 and are retrieved by a detector 
member D40 on leaving said common portion. 
It is immediately apparent that the functions required of the common 
optical portion 5 are complex since it must: 
Provide optical functions for four beams (primary/secondary, ion/electron) 
and this must be done in a manner which is compatible with all four beams, 
and in addition the beams must be independently adjustable; 
These functions must continue to work properly even though the beams of 
ions and of electrons are made to scan and are subjected to synchronous 
corrections; and 
The above must be achieved while meeting the aim of having a small (i.e. 
fine) probe at the same time as highly efficient collection of secondary 
ions. 
In order to do this, the input point (the top point, supposing the column 
to be vertical) to the common optical portion 5 is a pivot center for the 
primary ion scan (B13). In contrast, the primary electron scan takes place 
in a member B56 which is incorporated in the common optical portion. 
Preferably, the instrument is folded. To this end, the primary ions are 
deflected through slightly less than 180.degree. by a deflector D12 prior 
to arriving at their scanning member B13. The deflection is brought up to 
180.degree. by electrostatic plates B543 (see FIG. 3) which constitute a 
part of a beam combiner/separator member CS54, which constitutes the top 
end of the common optical portion 5. 
Reciprocally, on leaving the member CS54, the secondary ions enter a device 
B23 for synchronous dynamic correction of the primary scan effect, 
followed by a deflector D22 which brings the deflection of the secondary 
ions to 180.degree. and which performs energy filtering (at Fe22, see FIG. 
3). 
On the primary side, the first electrostatic optical means begin with an 
optical system R11 located between the source S10 and the deflector D12 
for adjusting the primary ion beam. 
On the secondary side, the second electrostatic optical means end with an 
optical system O21 for transferring the secondary ions from the outlet 
from the deflector D22 to the inlet to the mass spectrometer SP20. 
DESCRIPTION OF A TICULAR EMBODIMENT 
(USING POSITIVE PRIMARY IONS) 
The ion microprobe (optical means 1) 
The source of ions S10 is a source of cesium Cs.sup.+ of the kind described 
in U.S. Pat. Ser. No. 730,172 filed on May 3, 1985, using the version 
which is rich in axial ions having a nominal energy of 10 keV. 
The adjusting optical system R11 begins with three electrostatic lenses 
L110, L111, and L112 disposed in cascade and positively polarized. These 
lenses are centered on the axis of the source and they form reduced images 
I1-P and I2-P (see FIG. 4A). The lenses are followed by centering plates 
B0113, and an astigmatism corrector CA114 which is adjusted to be 
effective right up to the sample. 
The primary ion beam then enters the first electrostatic spherical sector 
S121 of the deflector D12. After being deflected through 90.degree., the 
image of the source is located at I3-P (FIG. 4A). An electrostatic lens 
L123 then conveys the ion beam to a second electrostatic spherical sector 
S122 which deflects it through 84.degree.. On leaving the second deflector 
sector, which gives an image I4-P, the beam is directed towards the pivot 
center of plates B543, which bring the beams into the optical axis of the 
common optical portion 5 by further deflection through 6.degree.. 
The function of the lens L123 is to combine the achromatic focal lengths f 
S121, S122 and B543 so that the overall deflection of 180.degree. produced 
by said optical systems is first order insensitive to possible variations 
in the ion accelerating voltage at the source, in the initial energy 
dispersion of the ions, and/or in the voltages applied to the 
electrostatic sectors (which are preferably the same for both sectors). It 
is also possible to perform energy filtering on the primary beam by means 
of a filter slot, not shown. 
Between S122 and B543, but off the axis of the common optical portion, the 
primary ions are made to scan at B13 by two pairs of plates B131 and B132 
disposed in cascade, said pairs of plates causing the beam to pivot in 
practice about the pivot center of B543 (see FIG. 5A). The resulting scan 
over the sample extends over about 10 micrometers. 
Going along the common optical portion 5, and beyond the plates B543, the 
primary ions encounter: the electrostatic lens L554 which is positively 
polarized and which constitutes a portion of the shaping optical system 
MF55; and the near optical system OR57 which constitutes a part of the 
focussing/collecting terminal assembly FC57. The other components of the 
common optical portion have little or no effect on the beam of primary 
ions. 
The lens L554 gives a penultimate image I5-P from the image I4-P. The near 
optical system OR57 makes the final reduced image on the surface of the 
sample. FIG. 4A shows the images. FIG. 4B should be read in conjunction 
with FIG. 4A and shows the corresponding crossover points CO1-P to CO6-P 
(i.e. the points where aperture diaphragms could be installed). The 
crossover points are where the trajectories of the ions which were 
initially parallel to the optical axis subsequently pass through said 
optical axis. These crossover points and the trajectories interconnecting 
them are important in obtaining a satisfactory optical system. 
Near optical system OR57 
FIG. 6 shows the near optical system OR57 in greater detail. It comprises a 
diaphragm D570 and electrodes L572 to L575, which electrodes are connected 
to various polarities. 
The electrode L572 is grounded. The electrodes L573 and L575 are polarized 
with the same sign as the sample, i.e. the opposite sign to the primary 
ions. Only the electrode L574 is polarized with the same sign as the 
primary ions. 
Thus, supposing the primary ions to be positive, the sample EC, the 
electrode L575 and the electrode L573 are all negatively polarized while 
the electrode L574 is positively polarized. 
The electrode L554 forms an image of the pivot center (in B543) at the 
center of the diaphragm D570 which diaphragm acts as an aperture diaphragm 
for the beam of primary ions FP. 
The disposition used for scanning the beam of primary ions ensures that 
scanning takes place about said pivot center without acting on the beam of 
primary electrons, nor on the secondary beams. However, the scanning takes 
place at a point where the image of the source is not small enough so that 
subsequent reduction in image size also has the effect of reducing the 
amplitude of the displacement of the ion microprobe over the sample 
(.+-.10 .mu.m). 
Finally, focus on the sample is adjusted by means of the positive lens L574 
of the near optical system OR57. This focussing may be dynamically 
corrected by modulating the polarization of L574 synchronously with the 
scan, in such a manner as to ensure that the image of the ion source is 
continuously in focus on the surface of the sample regardless of the 
position of the microprobe. 
Finally, it should be observed that the distance between the front of the 
near optical system and the sample is very small. The compact construction 
at the end of the optical system OR57 ensures that the last lens for 
reducing the ion microprobe has fairly low aberration coefficiets: both 
the chromatic aberration coefficient Cc and the spherical (or aperture) 
aberration coefficient Cs are about 20 millimeters. 
As a result, the acceleration voltages and the energy V1 of the primary 
ions may be limited, making it possible to work with a medium brightness 
source without having an unacceptable effect on the intensity conveyed by 
the ion microprobe. Starting from a value of V1=10 keV, the near optical 
system OR57 raises the pimary ions to an impact energy of 15 keV. 
These questions are dealt with in greater detail below after the 
aberrations have been described in greater detail. 
It may be observed that the optical system OR57 includes a circular 
electrode L575 which is very close to the sample EC and at the same 
potential as the sample. This electrode is analagous to the control grid 
of an electron gun. The portion of the sample EC which is hidden by this 
electrode has no effect on the electric fields set up by the electrode 
L574 (whose potential is of opposite polarity), thereby allowing the 
sample to be moved without disturbing the processing of the primary or 
secondary ions. 
Extracting secondary ions 
A consequence of the above description is that the energy V2 of the 
secondary (negative) ions after acceleration will be not more than 5 keV, 
by virtue of the potential difference between the electrode L572 and the 
sample. 
The position of the electrode L574 close to the sample and its opposite 
polarity relative thereto has the advantage of setting up a high ion 
extractor field close to the surface of the sample. This field is slightly 
attenuated by the presence of the electrode L575, but the electrode L575 
provides an equipotential curving effect which helps focussing the 
secondary ions. 
The secondary ions are focussed by the electrode L573, taking account of 
the effect of the other lenses and in such a manner as to ensure that the 
trajectories of secondary ions emitted over a solid angle of 2.pi. 
steradians from a point on the surface of the sample EC come together in 
the plane of the diaphragm D570 (ignoring aberrations). 
The secondary ion image I1-S (FIGS. 7A and 7B) formed in this way in the 
plane of diaphragm D570 is at about unity enlargement. The secondary ions 
can thus pass through this diaphragm without being attenuated regardless 
of the point of emission on the scanned area of the sample, so long as the 
scanned point is not too far from the axis. 
It has been observed that the intensity in a given microprobe depends on 
the diameter of B570. Further, the number of pixels (i.e. elementary image 
points) which can be acquired in given time (and with acceptable 
statistical fluctuations) depends on the intensity of the microprobe. 
The number of pixels thus depends on the diameter of the diaphragm D570. 
However, the number of pixels is also related, for a given size of 
microprobe, to the area of the field scanned on the sample. This area is 
limited by the diameter of the diaphragm D570. It should also be observed 
that the image of an emitting point is full of aberrations and that it is 
located in the plane of said diaphragm. 
The microprobe can thus be scanned only insofar as the aberrations do not 
fill the entire open area of the diaphragm B570, otherwise vignetting is 
produced in the secondary electron and ion images created by the ion 
bombardment. (Vignetting is a reduction in the intensity of illumination 
near the edges of an optical instrument's field of view caused by rays 
being obstructed by the edge of an aperture). 
Secondary particles whose initial energy (on being ejected) does not exceed 
about 20 electron volts have aberrations which are much smaller than the 
diameter of the diaphragm D570, even when the smallest diaphragm diameters 
are used in order to produce finer microprobes (since in that case the 
scanned field is also smaller). 
For initial energies of more than 20 eV, account must be taken of the 
energy passband (i.e. the chromatic passband) of the mass spectrometer, 
and also of the various attenuations which are related to the limited 
acceptance of the mass spectrometer. 
The person skilled in the art will understand that the near optical system 
OR57 which solves the above-described problems is an essential element of 
the invention. 
Ion microprobe and optical aberrations 
FIGS. 8, 8A and 8B are diagrams showing the first electrostatic optical 
means 1, in which said means are represented by a single electrostatic 
lens L accompanied by a diaphragm D.sub.o. 
The output from the source of ions is a restricted gap S.sub.o of diameter 
D.sub.o. The kinetic energy of the primary ions is V, and the (chromatic) 
dispersion is .+-..delta.V. An approximation to the average brightness B 
of a thermal source using surface ionization can be written: 
EQU (J.sub.o /.pi.) (V/.DELTA.V) 
Where J.sub.o is the current emitted per unit area of the ionizer and 
.DELTA.V=2..delta.V. 
The diaphragm D.sub.o delimits an "object" aperture half angle 
.alpha..sub.o, and .alpha..sub.i and d.sub.i respectively denote the 
aperture half angle and the Gaussian diameter of the microprobe on the 
"image" side of the lens L. 
If V.sub.o and V.sub.i designate the energy of the ions in the object and 
the image spaces, respectively, considerations of optical symmetry give: 
##EQU1## 
The angular enlargement MA=.alpha..sub.i /.alpha..sub.o, and the linear 
enlargement (which in fact is a reduction) is ML=d.sub.i /d.sub.o. When 
V.sub.i =V.sub.o, MA.ML=1. 
Aperture aberration 
Aperture aberration is principally spherical aberration (FIG. 8A) 
characterized by a diameter d.sub.s of the intersection of a caustic type 
curve with the beam aperture limits (.+-..alpha..sub.i) Thus: d.sub.s =1/2 
ML.Cs..alpha..sub.o.sup.3 
Where Cs is the spherical aberration coefficient which may be written: 
EQU Cs=Cs.sub.0 +Cs.sub.1. 1/ML+Cs.sub.2.(1/ML).sup.2 +Cs.sub.3.(1/ML).sup.3 
+Cs.sub.4.(1/ML).sup.3 
The coefficients Cs.sub.j (j=0 to 4) depend only on the geometrical and 
electrical characteristics of the lens L. As a function of j, and for 
V.sub.i =V.sub.o, it follows that when ML&lt;&lt;1, then: 
EQU d.sub.s =1/2(ML.sup.4.Cs).alpha..sub.i.sup.3 
.apprxeq.1/2(CS.sub.4..alpha..sub.i.sup.3) 
This relationship is used to determine the aperture aberration of the 
lenses of the primary ion optics 1 (so long as the condition Vi=Vo is 
satisfied). For the lens L574 in the near optical system OR57, V.sub.o (10 
keV) is less than V.sub.i (15 keV). In this case the applicable 
coefficient is Cs.sub.4 (V.sub.i /V.sub.o).sup.3/2 which is applicable 
rather than Cs.sub.4 on its own. 
Chromatic aberration 
FIG. 8B shows "chromatic" aberration. S.sub.i ',S.sub.i,S.sub.i " are the 
images of S.sub.o for energies V- V,.delta.V, and V+.delta.V, 
respectively. The diameter of the chromatic aberration spot (at S.sub.i) 
is given by: 
EQU d.sub.c =Cc..alpha..sub.i..DELTA.V/V 
where .DELTA.V=2..delta.V 
A "real" diameter d is then associated with the microprobe where: 
EQU d.sup.2 =d.sub.g.sup.2 +d.sub.s.sup.2 +d.sub.c.sup.2 
Where d.sub.g is the Gaussian diameter in the absence of aberration. For a 
given diameter d it has been observed that there exists a pair of values 
(d.sub.g,.alpha..sub.i) for which the intensity of the probe passes 
through a maximum, which it is desirable to achieve. The smaller the 
aberration coefficients Cs.sub.4 and Cc, in particular by virtue of using 
short focal length lenses, the greater the maximum. 
With reference to the near optical system OR57, it may be observed that: 
The front distance between the sample EC and the electrode L574 is small, 
and aberrations in the primary beam are therefore likewise small, allowing 
the intensity of the probe to be increased; 
The electric field accelerates or slows down the primary beam depending on 
the point under consideration, but does not deflect the primary beam 
thereby eliminating risks of instability in probe position; 
The behavior would be different if the primary beam were at an angle to the 
object, and in addition, there would be a shadow effect. 
Dynamic focussing correction 
FIG. 8C is simplified in a similar manner to FIGS. 8 to 8B and shows how 
dynamic correction is performed by the lens close to the sample. It is 
recalled that an image of the pivot center of the primary beam is set up 
at the diaphragm D570 (D.sub.o in the FIG. 8C diagram). 
Because of the scanning (see FIG. 8C) and because of aberration in the 
lens' field curvature, the probe would normally scan a portion of a sphere 
rather than a plane surface, which portion is shown as being tangential to 
the sample EC in the figure. 
The polarization of the control electrode of the lens L in FIG. 8C (lens 
L574 in FIG. 6) is adjusted as a function of the scan position so that the 
probe is always focussed on the plane surface of the sample EC regardless 
of the scan position. 
Secondary ion optics (optical means 2) 
On leaving the surface of the sample EC, the secondary ions have energies 
lying in the range zero to about a hundred electron volts, and the 
distribution of energies has a maximum round about 5 to 10 eV. The ions 
are ejected in directions distributed over a solid angle of 2.pi. 
steradians in an approximately cosine distribution centered on the axis. 
In the optical system OR57, the electrode L574 may be at about +15 kV 
relative to the sample EC, for example. 
The resulting accelerating electric field on the negative secondary ions 
opposes the natural divergence of the initial trajectories of these ions, 
and reduces the initial energy dispersion, in relative terms. This 
electric field imparts a kinetic energy to said ions (5 keV for single 
charge ions leaving L572) which is much greater than their initial average 
energy. In this respect, the sample EC is an integral part of the near 
optical system 57. For the purposes of simplification, the emissive 
surface of the sample EC is assumed to be conductive, plane, and at a 
uniform potential. 
FIG. 9 is a simplified diagram in which the collection optics FC57 and the 
shaping optics MF55 are lumped together as a single lens 57 and the device 
B543 is represented by two scanning plates. Two particular optical items 
are associated with this collection optical system, namely the 
"cross-over" and the image which it creates. 
Regardless of which point is emitting, trajectories which are normal to the 
surface of the sample pass through the optical axis at the same point. 
This is the cross-over point and it is advantageous to make this point 
coincide with the pivot center for dynamically correcting secondary ions 
(plates B234 and B235). However, the image is a little further away (see 
FIG. 9, and also CO3-S and I3-S in FIGS. 7A and 7B). 
When collecting a scanning ion microprobe, merely recovering the secondary 
ions suffices in this case. The cross-over does not need to be limited in 
diameter in order to obtain fine localization of the emitting point (as is 
required for an ion microscope). The image of an object point (A or B) is 
so distorted by aberrations that the term "image" is hardly descriptive of 
the structure of the beam. 
For example, if the meridian trajectories from an object point are limited, 
the distance r and the slope r' of the trajectories relative to a given 
point on the optical axis (see FIG. 9A) may be plotted along the X-axis 
and the Y-axis respectively. 
If the optics were perfect, a straight line would be obtained passing 
through the origin. On FIG. 9A, this is true for 2.5 eV. However, as soon 
as the initial energy rises, the errors produced by aperture aberrations 
increase within increasing energy (see the 20 eV curve in FIG. 9A), i.e. 
the ions have trajectories which are further and further from the axis. 
Meanwhile, chromatic defects in the collection optics give rise to 
different slopes at the origin for each energy. 
The person skilled in the art will understand that although the first 
secondary ion image (I1-S, FIG. 7A) is incapable of spatially localizing 
the emission point, this point is nevertheless localized by the primary 
bombardment of the sample. 
In the common optical portion 5, the lenses L556 and L555 (see FIG. 3) form 
intermediate images I2-S and I3-S (see FIG. 7A). These lenses are used, in 
particular, to ensure that the beam of secondary ions does not move too 
far away from the optical axis before reaching the plates B543, in 
particular when the emitting point is situated far from the axis, as can 
occur when scanning a sample over a wide field. 
In the shaping optical system MF55, the positively polarized lens L554 
provides only little convergence. 
Tests have shown the following: 
The beam of secondary ions from the impact area, after being shaped, must 
have a structure which is compatible with the acceptance of the mass 
spectrometer SP20, whose separating power is high; 
However, a wide range of initial energies must be accommodated, 
furthermore, trajectories from the impact area are spread over an extended 
"cross-over" and over an image which is spoiled by rather large 
aberrations, the beam coming from a single point thus covers the entire 
geometrical extent which can be accepted by the spectrometer; and 
Consequently the beam coming from an adjacent point during scanning will 
not completely enter the spectrometer. 
In order to solve this problem, the secondary ion beam is, in accordance 
with the invention, scanned synchronously with the primary beam, and this 
scanning operation is referred to as synchronous dynamic correction. 
FIG. 10 shows how the primary and secondary ion beams FP and FS are 
separated at the plates B543. 
FIG. 11 shows the same phenomenon in greater detail. Since the primary ions 
have an energy V1=10 keV at this point whereas the secondary ions have an 
energy V2=5 keV, the primary ions are deflected through 6.degree. whereas 
the secondary ions are deflected through 12.degree.. 
In the example described, the primary ions are Cs.sup.+ or K.sup.+, and the 
secondary ions are negative since implanting cesium or potassium gives 
rise to the emission of negative ions. Conversely, primary I.sup.- ions 
would have the same effect for positive secondary ions. The separator 
works in both cases and it even works when the energies of the primary 
ions and the secondary ions are interchanged (V1=5 keV; V2=10 keV). 
It should also be observed that the plates B543 can be used to separate 
ions having the same sign, in which case they would be deflected in the 
same direction. 
Downstream from the plates B543, the secondary ion beam heads towards the 
deflector D22 which is similar to the deflector D12 (see FIGS. 1 to 3). 
However, the first electrostatic spherical sector S223 of the deflector D22 
operates through 78.degree. only (compare FIGS. 3 and 11). The sector S223 
is coupled by the lens L227 to the second sector S224 which operates 
through 90.degree.. 
At this point, it is necessary to solve a problem related to the wide 
energy range of secondary ions: any electrostatic dispersion gives rise to 
energy dispersion, since the deflection angle depends on the energy of the 
particles. 
FIG. 12 shows two trajectories which differ by an angle .alpha..sub.c, due 
to the fact that the trajectories correspond to energies V2 and V2+v. The 
trajectory "V2" is inclined at .theta. to the axis from the pivot center 
F.sub.c '. When the angle 8 is small: 
EQU .alpha..sub.c .apprxeq..theta.(v/V2) 
For the spherical sector S224 which provides a deflection through 
90.degree., 
EQU .alpha..sub.c .apprxeq.v/V2. 
Conversely, if an energy dispersed beam having appropriate angles 
.alpha..sub.c is caused to converge towards F.sub.c (see FIG. 11), the 
axis of the beam will leave following a single radius. Chromatic 
dispersion is cancelled. 
Thus, in accordance with the invention, the lens L227 is placed in such a 
manner as to conjugate the achromatic focal length resulting from the 
combined actions of B543, S223, and of the sector S224, by applying 
angular magnification suitable for cancelling the chromatic effects at the 
output from S224, at least to the first order. 
When the probe scans the surface of a sample, the axis of the secondary 
beam pivots about its cross-over CO1-S (see FIG. 7B). The same is true for 
the images of the cross-over, namely CO2-S, and CO3-S (see FIGS. 5B, 7B, 
and 11). 
Synchronous dynamic correction on the secondary beam is performed by two 
pairs of plates B234 and B235 (see FIG. 3). These two plates are disposed 
in such a manner that the pivot center resulting from their combined 
influence is located at (CO3)', which is the image of CO3-S as given by 
the plates B543. (see FIG. 11). 
Then, by applying voltages to the plates B234 and B235, synchronously with 
the voltages applied to the plates B131 and B132 acting on the primary 
beam, rotation about (CO3)' can be cancelled. 
The secondary beam is thus stationary on entering the deflector D22. As a 
result, the mass spectrometer SP20 always receives a secondary beam having 
the same geometrical characteristics regardless of the scan point from 
which it originates. 
Meanwhile, an energy filtering slot FE22 placed at the cross-over CO4-S 
(FIG. 11) limits the band of energies transmitted to the spectrometer to 
the range 0 eV to 20 eV. 
The transfer optics 021 and the spectrometer SP20 are preferably as 
decsribed in French patent application Ser. No. 84 01 332, with 
corresponding items being shown in the following table: 
______________________________________ 
The present application 
US SN 695 240 
______________________________________ 
Lens L218 Lens LE11 
Plates B216 Plates PC12 
Slotted lens LF211 Slotted lens LF13 
Lens L219 Lens LE14 
Slot Fe21 Slot FE20 
Post-acceleration 1 
Post-acceleration 1 
(not shown) Hexapole HP22 
Spectrometer SP20 References 23 to 25 
(together with 
optical preceding 
letters) 
______________________________________ 
In outline, the lenses L218 and L219 co-operate to produce a beam 
restriction at the slot Fe21, which is the inlet slot to the spectrometer. 
The plates B216 center the beam. The slotted lens LF211 makes the beam 
parallel, and ensures a vertical section at the inlet slot F21. In some 
applications, a post-acceleration stage may be provided. 
ELECTRON OBSERVATION MEANS 
The electron beams are now described. These beams enable the samples to be 
observed by means other than the ion microprobe, and the resulting 
observations are complementary to the ion observations in several ways. 
In the embodiment described so far, the scanning electron microscope 
operates by means of re-emission of secondary electrons. 
The electron microprobe 
The beam of primary electrons, at an energy of 30 keV for example, is 
conveyed by the magnetic lens M31 to the axis of the plates B543 of the 
combiner/separator CS54. A magnetic field perpendicular to the plane of 
the figure (see FIGS. 13, 13A, and 13B) is superposed on the electric 
field due to the plates. 
The magnetic field is adjusted, taking account of the energy of the primary 
electrons, in order to exactly compensate the deflection which the 
electric field would produce on the electrons if it were acting alone. The 
primary electrons therefore continue along a rectilinear path. 
It should immediately be observed that in the reverse direction, the 
secondary electrons at a different energy level are, in contrast, 
deflected towards a detector device D40 which includes an electrostatic 
sector S405 followed by a detector D406 (see FIG. 13B), for example an 
electron multiplier of the X919AL type as sold by R. T. C. A more 
elaborate version consists in post-accelerating the secondary electrons 
and in using a scintillator as the detector. An additional stage may be 
passed by modifying the structure of the electron beam to give access to a 
real cross-over image. Means may then be provided, if so desired, to make 
use of a portion only of the cross-over (or indeed of two or more 
different portions thereof in parallel in two or more distinct detection 
paths) in order to modify the contrast of the electron image or to show up 
different contrasts. This is because a cross-over contains all the 
information on the initial angular distribution of the electrons. 
Extracting just a portion of the cross-over thus has the effect of giving 
more weight to a solid angle of emission which is more or less populated, 
depending on the relief of the sample surface, for example, or on small 
local non-uniformities in the electric or the magnetic fields due to some 
feature of the very structure of the sample: contrast varies with the 
region of the cross-over taken into account. Thus, by taking electrons 
from two distinct regions of the cross-over in parallel makes it possible 
to perform image processing in order to better distinguish the sources of 
contrast. 
The magnetic lens M552 serves to transfer the primary electron beam (taking 
due account of the presence of the electrostatic lenses) to a final 
reducing magnetic lens M571. In the main object plane of said lens, the 
diaphragm D570 limits the aperture of the primary electron beam. A 
conductive non-magnetic diaphragm may be immersed in the magnetic field of 
the lens to limit the aperture without disturbing processing. (This is not 
true of electrostatic lenses). 
Between M552 and M571, a system B56 of coils essentially comprising four 
coils B56a and four coils B56b, disposed in pairs in two orthogonal planes 
serves to center and scan the electron probe, while astigmatism is 
corrected by eight coils (not shown) disposed halfway between the coils 
B56a and B56b. The magnetic field due to these coils has practically no 
influence on the trajectories of the primary and secondary ions. 
It is thus possible to analyze a single sample both with an ion microprobe 
and with an electron microscope having comparable resolution,, which 
resolution may be as small as about ten nanometers. 
Collecting secondary electrons 
When the sample is negatively polarized (for use with positive primary 
ions) the primary electron beam gives rise to a beam of re-emitted 
secondary electrons. 
The near optical system OR57 thus need to extract the secondary electrons 
in order to accelerate them in substantially the same way as it 
accelerates the negative secondary ions (to the same energy, but with less 
chromatic aberration). The first secondary ion image I1-S is formed close 
to the diaphragm D570 (see FIG. 7A). 
However, the diaphragm is immersed in the magnetic field of the lens M571 
and is located in the main object plane thereof (for the primary 
electrons). Further: 
The lens M571 exerts a much greater focussing action on the secondary 
electrons (at 5 keV) than it does on the primary electrons (at 30 keV); 
The leakage field from the magnetic lens M571 reaches as far as the sample 
EC, and also penetrates through the electrostatic lens L576 (see FIG. 3); 
Finally, the magnetic optics rotates the electron images around the optical 
axis. 
The ion image (I1-2, FIG. 7A) and the electron image (I'A, FIG. 13B) are 
thus not superposable, nor are they situated at exactly the same place. 
It turns out that this problem can be solved in a satisfactory manner by 
acting on the polarization of the electrode L573 in order to bring the 
electron image I'1 into the plane of the diaphragm B570 without moving the 
ion image I1-S so far that it gives rise to a vignetting effect. 
As a result, the ion image and the electron image are clearly separated 
(compare I2-S in FIG. 7A with I'2 in FIG. 13B). 
The secondary electrons first encounter the scanning coils B56. They are 
deflected about 2.5 times as much by the scanning coils than are the 
primary electrons. A compromise may be obtained by setting up the distance 
relative to the sample so that the image has turned through 90.degree. at 
the coils. The lenses L576 and L555 can then shape the beam of secondary 
electrons so that it passes through the lenses L554 and M552 without 
attenuation due to electrons being too far from the axis. Thus, a 
secondary electron image I'3 is to be found at lens M552. 
The secondary electrons are then separated from the primary electrons by 
the member CS54 and are finally detected, as described above, in order to 
create an electron image in synchronization with the electron scan (which 
image may be a video image). 
Electron observation variants 
It is clear that if the secondary electrons are due solely to ion 
bombardment (with no primary electrons being simultaneously provided by 
the electron gun), the lenses M571 and M552, and also the scan B556 may be 
inactivated. The plates B543 and the magnetic field then extend far enough 
to separate the secondary ions from the secondary electrons. 
If the sample is thin, electron observation thereof may be performed by 
transmission electron microscopy. The energy of the primary electrons then 
needs to be raised, for example up to 100 keV. A detector similar to those 
already used in transmission electron microscopy (advantageously of a type 
suitable for analyzing the energy losses to which the electrons are 
subjected) is then located behind the sample EC within the sample carrier 
PO. The secondary ions due to the ion microprobe may also be observed, as 
described above, together with the secondary electrons which the sample 
may continue to emit under the effect of the primary electron bombardment. 
General operation of the instrument 
It is important to observe that the selection of acceleration energies for 
the various particles in combination with the sign difference between the 
primary and the secondary ions are used to make it possible to adjust each 
beam in a manner which is substantially independent from the adjustment of 
the other beams. 
In particular, the magnetic lenses of the microscope which focus the 
primary electrons have very little effect on the ion beams. 
The positively polarized electrostatic lenses act on the positive primary 
ions but their focussing effect on the negative secondary ions (or on the 
secondary electrons) is much smaller. Their focussing effect on the 
primary electrons is practically negligible. 
The negatively polarized lenses act on the negative secondary ions and on 
the secondary electrons (which are accelerated to the same energy) but 
have relatively little effect on the positive primary ions and practically 
no effect on the primary electrons since their energy is too high (30 
keV). 
When using negative primary ions and collecting positive secondary ions, 
adjustments can still be made more or less independently. However, in this 
case it is not possible to simultaneously obtain an ion image and a 
secondary electron image since the secondary electrons are retained on the 
target by its positive polarization. When observing thick samples, there 
nevertheleness remains the possibility of using backscattered electrons, 
so long as their energy is high enough to escape the attraction of the 
sample. 
When examining thin samples in transmission microscopy, it is always 
possible to simultaneously obtain electron images in conjunction with 
secondary ions which may be positive or negative. 
Finally, charge effects which could appear on insulating samples when 
collecting negative secondary ions may be eliminated by using the primary 
electron beam in a special way. If the energy of the primary electrons is 
reduced to 5 keV, they are reflected with very low energy close to the 
surface of the sample. As a result positive charges which tend to develop 
on the insulator, in part because of secondary electron emission, are 
automatically cancelled without an excess of negative charge developing 
since the electron beam would then automatically be deflected elsewhere 
(French patent application No. 83 00 538). 
Electronic auxiliary equipment for the instrument 
Reference is now made to FIG. 2 while describing the instrument's auxiliary 
equipment, which equipment makes the above-described operation possible. 
A block AS30 feeds the electron gun in known manner. 
The scanning electron microscope lenses (M31, magnetic field at B543, M552, 
M571) are controlled from a feed block AM31. 
A feed block AD supplies high tension to the achromatic deflectors D21 and 
D22, or more exactly to their spherical sectors. 
A feed block ACS controls the feed and the adjustment of the beam separator 
CS54, and a block AMF controls said beam by means of the shaping optics 
MF55. 
The diameter of the ion probe is adjusted by R11 under the control of a 
circuit AR11, and the ion source S10 is powered and adjusted by means of a 
circuit AS10. 
A circuit AFC controls the focussing/collecting terminal assembly FC57. 
A circuit CG monitors and controls the vacuum and the safety aspects 
related thereto. 
A computer or a central processor unit UCT associated with a mass storage 
device MEV is provided to collect the measurements, and preferably to 
assist in the adjustments provided by the other circuits. 
In any event, the computer controls the image scanning system SBI which 
acts directly on the ion scanning means B12 and B23, on the electron 
scanning means B56, and on the motion of the sample carrier PO. The system 
SBI acts indirectly on the lens L574 (FIG. 6) and on the optical system 
FC57 (FIG. 2) or more precisely on the near optical system OR57 (FIG. 6), 
in order to provide dynamic focussing. This action takes place via a 
circuit AFC for collecting secondary particles and for performing dynamic 
focussing, which circuit controls the optics FC57 (except for its magnetic 
lens). 
A unit CSM under the control of the computer UCT exercises control on the 
mass spectrometer SP20. 
The measurements from the spectrometer SP20 and from the electron detector 
D40 are acquired in an acquisition circuit MAD, which also has an effect 
on the scanning system SBI. 
Finally, the computer UCT collects the measurements and stores them in the 
mass memory MEV as a function of the scan. 
PRACTICAL IMPLEMENTATION OF THE INSTRUMENT 
Some other aspects of the invention are now described using as a specific 
example an ion probe having a diameter of 0.1 .mu.m and an intensity of 
7.10.sup.-12 A (on the sample). 
The effect of emission noise 
If an image point or pixel is the same size as the probe diameter, then a 
pixel receives an average of n=440 ions per 10.sup.-5 seconds. 
Assuming that five atoms are extracted from the sample per incident ion, 
irradiation for 10.sup.-5 sec. will remove a layer of material having a 
thickness of about 5.10.sup.-2 Angstroms (this is the destructive effect 
of ion analysis). 
About ten secondary electrons are generated per incident ion, so the 
electron signal from a pixel may be detected, but it is accompanied by 
statistical noise of about .+-.5%. 
Under such conditions, obtaining a secondary electron image which is 
renewed at the television field rate (25 images per second) implies that 
the scanned field must be limited to 4.10.sup.3 pixels. In order to 
increase the field, the current intensity of the probe must be increased. 
However, taking account of the fact that the minimum observable contrast C 
is equal to K times the noise: 
##EQU2## 
Where n2 and n1 are the numbers of incident ions on two adjacent areas and 
n is the average number of incident ions. Putting K=5, N=4.10.sup.3 and 
n=440 ions, the image must have at least 25% contrast on emission. This 
holds for 25 frames per second, but operating at one frame per second 
makes it possible to observe contrasts of 5%. 
Further, the image will change under the effect of ion erosion. It may be 
supposed that the topography of the sample will not change perceptively 
before the thickness of material removed therefrom is equal to about one 
third of the probe diameter (i.e. about 300 .ANG.ngstroms). In the example 
given, this corresponds to about four minutes of observation, which is 
compatible with observation of the secondary electron image at a TV 
frequency. The use of potassium would reduce the erosion rate on the 
sample and increase the observation period. 
However, other factors may have an influence on the quality of the electron 
image, in particular: 
The implantation of primary ions (esium). enhances secondary electron 
emission, and this effect may vary from one crystal grain to the next, and 
it may also vary during transient secondary electron emission conditions 
as a function of primary ion bombardment; and 
This effect may also enhance fixing molecules of the residual gas in the 
evacuated enclosure, and thereby have an influence on the efficiency of 
secondary electron emissions. 
The secondary ion image (as obtained by scanning the sample with the ion 
microprobe) is more difficult to obtain since each incident primary ion 
creates fewer secondary ions than it creates secondary electrons. 
Generally speaking, the useful yield of secondary ions (i.e. the ratio 
between the number of secondary ions which reach the detector (the 
spectrometer) divided by the number of atoms ejected from the sample) 
depends on the probability of ion formation during the ejection process 
and on the efficiency with which the extractor system couples to the mass 
spectrometer collects said ions. 
Thus, with an average yield of one secondary ion per 50 ejected atoms, and 
an atom concentration of 25%, the image obtained in 1/25 of a second over 
4.10.sup.3 points per image using a probe having a diameter of 0.1; 
microns is 10 ions per pixel on average. As a result the statistical noise 
is .+-.30%. 
In order to reduce this noise, a greater thickness of material must be 
eroded during the image acquisition time, i.e. the probe must be of 
greater intensity. 
Using a probe which is 40 times as intense (for the same pixel size) the 
statistical noise is the same as in the electron image at TV frequency, 
however the thickness eroded per image is 2 .ANG.ngstroms. 
In order to achieve the same statistical noise using the initial probe (0.1 
microns, 7.10.sup.-12 Amps, and 0.05 .ANG.ngstroms eroded per image) the 
number of pixels per image must be reduced to 10.sup.2 (at 25 images per 
second). 
However, the invention offers another possibility: 
The secondary electron image due to ion bombardment may be used to adjust 
ion probe focussing at a video frequency (25 images/second); 
Once that has been done, there is no need to observe the secondary ion 
image at the video frequency. 
The influence of detection noise 
Account must also be taken of the background noise in the ion detector, 
which is the mass spectrometer in this case. 
For an ion image having 4.103 pixels of 0.1 micron diameter, data may be 
accumulated over a period corresponding to eroding a thickness of 300 
.ANG.ngstroms (in 4 minutes). 
After four minutes, an element present at a concentration of 40 ppm will 
have produced about ten characteristic ions per pixel. 
Putting the average background noise of the spectrometer at 0.1 
strikes/second, the noise will be about 25 strikes over four minutes. The 
ratio of the secondary ion signal to the noise due to detection is greater 
than 10.sup.3, which is satisfactory. 
The influence of the quality of the vacuum 
Finally, there remains the effect of the residual gas in the evacuated 
enclosure. The presence of water vapor, carbon dioxide, and other residues 
may induce, even at a very low pressure, various types of ion to be 
emitted, and in particular H.sup.-, C.sup.-, OH.sup.-, and OH.sup.-. 
For a vacuum at 5.10.sup.-9 Torr, each square micron of the sample surface 
receives 1.5.times.10.sup.4 molecules per second. 
If all these molecules fix to the surface, and if their ionization yield is 
the same as for atoms of the sample, the ratio between the number Nv of 
atoms due to the residual gas on the sample (e.g. oxygen) to the number NA 
of ejected atoms per unit time may be written: 
EQU Nv/NA=(1.5.times.10.sup.4)/.rho..multidot..epsilon..perspectiveto.50p/.epsi 
lon. 
Where .rho. is the atomic density of the sample (number of atoms per 
micro-cube), is the eroded thickness in microns per second, and p is the 
partial pressure expressed in Torr. 
In the present example (p=5.10.sup.-9 Torr, probe diameter =0.1 microns 
over 4.10.sup.3 pixels), .epsilon.=1.25.times.10.sup.-4 microns per 
second, giving Nv/NA.perspectiveto.2.10.sup.-3. 
The influence of the residual gas is thus equivalent to a concentration of 
matter in the sample of 0.2%, i.e. it is 50 times above the threshold of 
detection (40 ppm per pixel). 
It is thus necessary to improve the vacuum, or to work with a higher 
intensity probe, and/or to scan a smaller area. 
Let C be the minimum observable contrast, Np be the probe current in ions 
per second, N be the number of pixels per image, t be the exposure time 
for the electron image, t be the atomic density of the sample, S be the 
ion yield from the sample, and e be the eroded thickness. 
It has been observed that: 
##EQU3## 
This shows that the observable contrast is essentially limited: 
by the thickness e which may be eroded without reducing resolution (as 
given by the diameter d); and 
by the yield S. 
This information can be related to that given above since: 
EQU .epsilon.=e/t=Np.S/.rho..N.d.sup.2 
whence Nv/NA=50p(.rho.Nd.sup.2 /Np.S) 
Putting N=10.sup.4 and p=5.10.sup.-9, the following table summarizes the 
results. 
______________________________________ 
Nv/Na 
Probe diameter d 
Probe current Np 
(5.10.sup.-9 Torr; 
(microns) (ions/second) 
10.sup.4 pixels 
______________________________________ 
0.02 4.10.sup.5 0.5 
0.05 4.10.sup.6 0.05 
0.1 4.10.sup.7 0.005 
0.5 3.10.sup.9 6.10.sup.-5 
______________________________________ 
These values suppose that the beam scan rate is high enough to ensure that 
erosion takes place as though the bombardment was uniform in spite of it 
occurring in the form of a TV type frame scan. 
Various means can be used to reduce these effects, in particular, the 
scanning may be digitally controlled, thereby enabling a higher degree of 
flexibility, or enabling the beginning of emission to be eliminated (and 
consequently avoiding the transient conditions associated therewith).