Corpuscular beam microscope for ring segment focusing

A corpuscular beam microscope for ring segment focusing is provided with a ondenser and an objective magnetic lens system which is disposed substantially axially symmetrically about the microscope axis for the purpose of generating two field maxima separated by a distance not exceeding five times the arithmetic means of half the half height widths of the component magnetic fields forming the maxima. The provision of such a magnetic lens system facilitates the elimination of aperture aberrations of the first and second order and the elimination of chromatic aberrations of zero order and partly of the first order as well as certain extra-axial defects.

The present invention relates to a corpuscular beam microscope comprising 
means for the production of at least one corpuscular beam which is 
confined to one segment of an annular zone substantially concentrically 
surrounding the microscope axis, and which is inclined at an angle of at 
least 30.degree. to the microscope axis at the point thereon where an 
object of which a magnified image is desired is located, a corpuscular 
optical lens system which in the path of the corpuscular beam contains at 
least one illuminating condenser preceding the object and a further 
reproducing lens system following the object, which latter system includes 
at least one objective (and usually also an intermediate lens and a 
projector lens), at least one stigmator system for correcting image 
defects in the ring segment section corpuscular beam, an object molder and 
an image recording means disposed in the final image plane. 
A corpuscular beam microscope of such a kind is described in the as-filed 
published specification of German Patent Application No. 21 65 089. The 
instrument is intended more particularly for taking several images of an 
object at angles of irradiation distributed over as large a solid angle as 
possible. From such photographs the three-dimensional density distribution 
of the object can be reconstructed, as described for instance in the 
papers published by W. Hoppe and D. J. DeRosir et al. (for instance in 
Naturwissenschaften 55 (1968), pp.333 to 336; Optik 29 (1969) pp.617 to 
621; Nature 217, (1968) pp.130-133 and J. Mol. Biol. 52 (1970), 
pp.355-369). 
In order to permit a high image resolution to be obtained image defects 
must be corrected as far as may be possible. This can be done without 
major difficulties by providing in each path of the beam of the different 
irradiating directions a substantially independent corpuscular optical 
reproducing system. On the other hand, with a view to simplifying the 
complexity of the apparatus it is desirable to provide a common 
corpuscular optical reproducing system for all the paths of the beam and 
to make separate provision in each path merely for correcting the image 
defects. 
The present invention contemplates a corpuscular beam microscope for ring 
segment focusing, as above specified, and seeks to improve such a 
corpuscular beam microscope with respect to the correction of image 
defects. 
This object is achieved by the inventive features defined in claim 1. 
By the provision of a condenser and an objective lens system having at 
least two closely consecutive field maxima in a corpuscular beam 
microscope for ring segment focusing it is possible more particularly to 
eliminate aperture aberrations of the first and second order, chromatic 
aberration of zero order and partly of the first order as well as certain 
extra-axial defects. In embodiments providing a plurality of paths for a 
beam confined in cross section to ring segments, the said paths together 
defining the surface of a cone, the invention yields a very compact and 
relatively simple structural design which is of particular advantage when 
making use of super-conducting shielding lenses.

The following particular description relates to electron beam microscopes 
although the scope of the invention is not intended to be limited to this 
type of microscope, since it is clearly just as readily applicable to ion 
microscopes. 
In FIG. 1 a path of the electron beam in an embodiment of the electron 
microscope according to the invention is schematically shown. The 
illustrated electron microscope comprises an electron source 10 which is 
only diagrammatically shown, and which may be of conventional kind, 
preferably containing a field emission cathode. The electron source 10 
emits an electron beam 12 which along the length of the microscope axis 
consecutively passes through a first magnetic condenser 14, a second 
magnetic condenser 16, an illuminating field diaphragm stop 18, an 
electrostatic deflecting system 20, only two of the four deflecting plates 
being shown, permitting the electron beam to be deflected in any desired 
direction away from the microscope axis 11 into any one of a number of 
different discrete paths, an illumination stigmator 22 which in practice 
is needed when a very small part of a specimen (less than 1 micron 
diameter) is to be illuminated at apertures of 10.sup.-4 to 10.sup.-3, a 
magnetic adapter lens 24, an illuminating condenser 26, an objective 28, 
an intermediate lens 30 and an image projector 32. The image projector 32 
projects a magnified image of an object 34 disposed between the 
illuminating condenser 26 and the objective 28 in a final image plane 36 
in which image recording equipment is provided, such as a photographic 
plate or an electronic picture recording device comprising an image 
converter or a television camera tube and the like as is conventional the 
electron beam travels in a vacuum. 
Two field correctors 38 and 40 and one stigmator system 42 are associated 
with the objective 28. The field correctors 38 and 40 contain a plurality 
(e.g. 8 or 16) small magnet coils disposed with their axes distributed 
radially about the microscope axis 11 and designed to generate a multipole 
field. The correction relates to the totality of the circular magnetic 
lenses forming the objective and is therefore the same for all beam paths 
that can be selected by control of the deflecting system 20. In practice 
the electron beam will always be deflected by the deflecting system 20 at 
the same angle to the microscope axis but in different azimuthal 
directions about the microscope axis, so that the centre axes of the 
different paths intersect at a specific point in the object 34 and define 
the surface of a cone which has its verbex at the said point of 
intersection. 
Contrary to the field correctors 38 and 40 the stigmators 22 and 42 which 
are only schematically shown in FIG. 1 serve for individually correcting 
the electron beam in the different paths. The stigmators 22 and 42 in the 
embodiment according to FIG. 1 contain a plurality of electrodes which may 
have the form of axially oriented strip-like zones on the circumferential 
surface of a cylinder, and which in conventional manner are electrically 
energisable to produce an electrostatic multipole field. The illumination 
stigmator 22 is preferably an electric octopole, whereas the stigmator 42 
likewise comprises at least 8, but preferably 16 or more electrodes. 
The potentials at the electrodes of the stigmators 22 and 42 are 
transferred from electrode to electrode jointly with the deflection of the 
beam by the deflecting system 20 and they are calculated for each path so 
that the desired beam correction is achieved. 
FIGS. 2 and 3 illustrate in somewhat greater detail an embodiment of that 
part of the electron microscope in FIG. 1 which comprises the illumination 
stigmator 22, the adapter lens 24, the illuminating condenser 26, the 
objective 28, the object 34, the field correctors 38 and 40 as well as the 
stigmator 42. The illuminating condenser 26 and the objective 28 are 
formed by a superconductor shielding lens 44 containing two annular gaps 
46 and 48 hich produce a magnetic field B(z), i.e. a field strength B in 
Tesla (T) units as a function of z (the z-coordinate), having two maxima 
50 and 52 (FIG. 3). The shielding lens comprises a casing 54 of 
superconducting material, such as a sinter body of Nb.sub.3 Sn, or some 
other hard superconductor. The gaps 46 and 48 are separated by an 
intermediate superconducting ring 56. In practice the gaps 46 and 48 are 
sealed in vacuum tight manner by annular members 58 and 60 (FIG. 3) which 
consist of a material that is not superconducting at operating temperature 
(e.g. copper). Apart from forming a seal the members 58 and 60 also serve 
for mechanically supporting and for cooling the intermediate ring 56. 
The casing 54 contains a system of magnetic coils which superconduct during 
operation, preferably comprising at least the principal coils 62 and 64, 
usually codirectionally energised, and at least one correcting coil 66. By 
an appropriate choice of the size, disposition and energisation of the 
coils 62, 64 and 66 in conjunction with the design dimensions of the gaps 
46 and 48 and of the intermediate ring 56, a large number of parameters 
are available to enable a desired field distribution to be achieved. 
FIG. 4 shows part of a different embodiment of an electron microscope 
according to the invention. The illustrated portion corresponds roughly to 
the zone extending from the second condenser 16 to the intermediate lens 
30 in FIG. 1 and contains a ferromagnetic circular lens 16' which 
corresponds to the second condenser lens 16 in FIG. 1, an electromagnetic 
deflecting system 20' containing four deflecting coils, an exchangeable 
and adjustable illumination field diaphragm 18', two consecutive 
ferromagnetic circular lenses 26', 28' which provide the magnetic field 
for the illuminating condenser and the objective, an object holder 34', 
field correctors 38' and 40', as well as an electromagnetic stigmator 
system 42' which will be later described in greater detail with reference 
to FIGS. 5 and 6. The circular lenses 26' and 28' share a common field 
plate 27' and thus form a "double-gap lens" similar to that in FIGS. 2 and 
3, permitting a magnetic field having two closely neighbouring maxima to 
be generated, as illustrated in FIG. 3. 
The expression "closely neighbouring" is intended to mean that the distance 
between the field maxima 50, 52 does not exceed three to five times the 
arithmetic mean of half the half height widths of the magnetic component 
fields forming the maxima. These component magnetic fields are each 
produced in an embodiment according to FIGS. 2 and 3 by a gap 58, 60, and 
in the embodiment according to FIG. 4 by the circular lenses 26' and 28'. 
The component magnetic fields are represented in FIG. 3 by the chain line 
curves. 
Between the circular lens 16' forming the second condenser and the circular 
lens 26' two additional arrays 68 and 70 may be provided, as shown in FIG. 
4, for the creation by each of a magnetic multipole field, functionally 
roughly analogous to the illumination stigmator 22 in FIG. 2. 
In FIG. 5 the coils of the stigmator 42' are shown in a section across the 
principal microscope axis. Each coil is connected to the corresponding 
terminal of a programme-controlled power source 72 which sends a specified 
current through each coil in respect of each path of the beam that has 
been preselected by the deflecting system 20. For each path the current is 
separately controlled and in routine operation the programme-controlled 
power source 72 will then supply exactly the correct current for each path 
of the beam. Moreover, in FIG. 5, a possible magnetic potential field is 
illustratively shown which provides a magnetic quadrupole having the 
desired parameters when the electron beam is in position 12. 
FIG. 6 is another possible magnetic potential distribution for the 
production of a stigmator quadrupole. 
In the Table at the end of the specification ten sets of parameters for a 
system of two magnetic circular lenses of the kind described with 
reference to FIGS. 2 and 4 are listed. 
The lens system is described by the magnetic field strength B.sub.(z) along 
the microscope axis 11, viz. by 
##EQU1## 
For values of .tau. between 1 and 3 a field of this geometry can be well 
realised with ferromagnetic lenses or superconducting shielding lenses 
having several gaps. The position z.sub.B of for instance the first 
stigmatic intermediate image (cf. FIG. 1) is 150 mm .multidot. f.sub.L. 
The values in the Table are calculated for an accelerating voltage V=100 
kV. 
The relativistically corrected accelerating voltage is V.sup.* =V+eV.sup.2 
/2m.sub.o c.sup.2. For V=100 kV this gives V.sub.100 *=109.885 kV. 
For V.sub.x *.noteq.100 kV it follows that V.sub.x 
*=109.885.multidot.f.sub.V *; (f.sub.V *=V.sub.x* /V.sub.100*. The 
meanings of the symbols in the Table are as follows: 
B.sub.oi =maximum field strength along the axis of the component field 
produced by the gap in question ("gap component"); 
z.sub.Li =position of the i.sup.th field maximum of the i.sup.th gap 
component related to the first maximum (z.sub.L l=0) in the direction of 
the beam; 
h.sub.i =half the half height width of the i.sup.th gap component; 
.tau..sub.i =shape parameter of the i.sup.th gap component (cf. FIG. 8); 
.sigma..sub.o =angle of the beam axis to the microscope axis at the object; 
V=electron accelerating voltage; 
z.sub.Q=location of source image (illuminating field diaphragm 18). 
The systems having the data given in the Table yield an image which is free 
from aperture aberrations to inclusive of the 2nd order and from chromatic 
aberration of zero order. The specified systems can be modified by 
imparting values different from unity to the factors f.sub.L, f.sub.B and 
f.sub.V * contained in the lengths, inductions and accelerating voltages, 
whilst simultaneously keeping the expressions f.sub.L .multidot.F.sub.B 
/.sqroot.f.sub.V= 1. The modified systems have the same properties as the 
original systems. 
System No. is so calculated that the object can be illuminated by a light 
source (illumination field stop 18) at z=-500 mm.multidot.f.sub.L * at an 
angle of 45.degree. by cutting down to a narrow pencil of light. In the 
other system z.sub.Q is substantially closer to the first lens of the 
illuminating field condenser-objective system. 
A comparison of the parameters of the system Nos. 1 to 5 shows that the 
above conditions can also be satisfied by varying the shape parameters 
.tau..sub.i, the angle .sigma..sub.o or the object plane Z.sub.G. This 
enables an image to be adjusted having a minimum of defects, even if the 
data stated in the Table are not exactly attained because of manufacturing 
tolerances and other sources of error. 
Systems 1 to 5 in the Table use fields of the type indicated in FIG. 7 by 
the full line curve. This type of field has an exponential distribution 
and it corresponds to a value .tau. roughly equal to 1. The systems 6 to 
10 use a Glaser bell field, as represented in FIG. 7 by the chain line 
curve. Such a field is obtained for .tau..fwdarw.0. 
FIG. 8 shows the relationship between the shape parameter .tau. and the 
dimensional parameters b, l, s and w of a magnetic ring lens having a 
single gap. The meaning of the dimensional parameters will be understood 
from the diagram in FIG. 8 on the right. FIG. 8 holds only as a first 
approximation for a circular lens having several gaps, since the fields 
due to the different gaps do to some extent mutually interfere. 
In a calculation of the system parameters, as shown in the Table, a model 
field which satisfies the prescribed imaging conditions is determined 
first. Based on the field parameters (field strength, spacing and half 
height width of the field maxima) the design parameters of the lenses, for 
instance the shape of the superconducting surfaces of the shielding lens 
in FIGS. 2 and 3 are then determined by a potential programme. 
TABLE 
__________________________________________________________________________ 
Parameter families for a system comprising two magnetic circular lenses 
using conical 
intercept focusing and providing freedom from zero order chromatic 
aberration 
The lens system is defined by the induction B.sub.(z) on the scale: 
##STR1## 
System 
Accelerating voltage of the beam electrons 100 kV. 
No. z.sub.G /mm 
.sigma..sub.O 
B.sub.O1 /T 
B.sub.O2 /T 
z.sub.L1 /mm 
z.sub.L2 /mm 
h.sub.1 /mm 
h.sub.2 /mm 
.tau..sub.1 
2 
__________________________________________________________________________ 
1 -0.16000 . f.sub.L 
45.degree. 
0.89375 . f.sub.B 
1.19021 . f.sub.B 
0.0 8.4631 . f.sub.L 
4.0000 . f.sub.L 
2.2535 . 
1.05b.L 
1.10 
2 0.0 45.degree. 
0.93335 . f.sub.B 
1.163045 . f.sub.B 
0.0 8.27313 . f.sub.L 
4.0000 . f.sub.L 
2.35964 . 
1.5ub.L 
1.5 
3 0.0 40.degree. 
0.992103 . f.sub.B 
1.399417 . f.sub.B 
0.0 9.76606 . f.sub.L 
4.0000 . f.sub.L 
2.1893 . f.sub.L 
4 0.0 45.degree. 
0.95655 . f.sub.B 
1.212478 . f.sub.B 
0.0 8.44976 . f.sub.L 
4.0000 . f.sub.L 
2.22646 . 
1.1ub.L 
1.2 
5 0.0 50.degree. 
0.888582 . f.sub.B 
1.016263 . f.sub.B 
0.0 7.42979 . f.sub.L 
4.0000 . f.sub.L 
2.28583 . f.sub.L 
6 0.0 34.degree. 
0.89449 . f.sub.B 
1.60756 . f.sub.B 
0.0 19.02918 . f.sub.L 
4.0000 . f.sub.L 
1.96096 . 
0.sub.L 
0 
7 0.0 40.degree. 
0.855973 . f.sub.B 
1.446301 . f.sub.B 
0.0 13.34168 . f.sub.L 
4.0000 . f.sub.L 
1.94969 . 
0.sub.L 
0 
8 0.0 45.degree. 
0.821607 . f.sub.B 
1.276782 . f.sub.B 
0.0 10.05662 . f.sub.L 
4.0000 . f.sub.L 
1.95589 . 
0.sub.L 
9 0.0 50.degree. 
0.783505 . f.sub.B 
1.075348 . f.sub.B 
0.0 7.64739 . f.sub.L 
4.0000 . f.sub.L 
1.99376 . 
0.sub.L 
0 
10 0.0 55.degree. 
0.73329 . f.sub.B 
0.855829 . f.sub.B 
0.0 5.94836 . f.sub.L 
4.0000 . f.sub.L 
2.10648 . 
0.sub.L 
0 
__________________________________________________________________________