Electron-optical imaging system having controllable elements

The invention is directed to an electron-optical imaging system such as for an electron microscope. The imaging system has magnetic lenses, current and voltage sources corresponding thereto, a computer, a permanent memory and a touch panel. The electron microscope is manually calibrated when first taken into use by a discrete sequence of different operating conditions. Polynomes of the second degree are adapted to the experimentally calibrated parameter values for the lens currents. The computer polynome coefficients are stored in a permanent memory. Operating states are adjustable via the touch panel on the operating console of the electron microscope. These operating states lie between the calibrated operating states. The lens currents necessary for these operating states are computed in the computer based on the function coefficients stored in the memory and are subsequently emitted to the current sources by the computer. The step width in which the operating states are adjustable is preselectable via the keyboard independently of the position of the calibrated operating conditions.

BACKGROUND OF THE INVENTION 
In electron optics, as a rule, groups of several controllable elements, 
which act on the electron beam, are used to vary the imaging conditions. 
For example, it is known to use a two-lens or a three-lens imaging system 
(condenser system) in electron microscopes at the illuminating end so that 
the object area which can be illuminated is variable when the positions of 
the electron source and object plane are fixed. Likewise, it is known for 
a fixed object plane and a fixed projection plane that the imaging scale 
is variable with the aid of a three-lens imaging system at the imaging end 
without a rotation of the image occurring when there is a variation in 
magnification. 
The dependence of the imaging characteristics on the applied lens 
excitation can be very precisely computed for individual electron-optical 
imaging elements. However, for the combination of several imaging 
elements, mechanical tolerances and, with magnetic elements, remanences in 
the iron circuit of these magnetic elements, cause too great a deviation 
of the actual imaging characteristics (especially with respect to 
focusing, maintaining focus or image rotation) from the theoretically 
computed ones. For this reason, electron microscopes obtained in the 
marketplace up to now are therefore experimentally calibrated when first 
taken into use. That is, the excitations of the individual elements 
corresponding to various operating conditions are determined by manual 
adjustment and thereafter stored. In later use of the electron microscope, 
the excitation values which correspond to the selected operating condition 
are then read out of the memory and are applied to the current sources or 
voltage sources of the imaging elements. The very substantial calibration 
effort therefore limits the number of selectable operating conditions, 
i.e. alignment effort. The operation of the electron microscope is then 
limited to the experimentally calibrated discrete operating conditions. 
U.S. Pat. No. 4,871,912 discloses an electron microscope wherein operating 
conditions are also adjustable which were not previously calibrated. For 
this purpose, an external computer is provided which is in addition to the 
internal computer usually provided in the operator console of the electron 
microscope. If, for example, a magnification value is inputted via the 
keyboard of the external computer which lies between two calibrated 
magnification steps, then the parameter values of the imaging system 
corresponding to the two mutually adjacent calibrated magnification steps 
are read into the external computer; parameters are calculated which 
correspond approximately to the inputted magnification by means of linear 
interpolation between the calibrated parameter values; and, the electron 
microscope is then driven in correspondence to these parameter values. 
The dependency of the focal width on the lens current is nonlinear for 
magnetic lenses and is, as a rule, different for each lens because of 
remanences and manufacturing tolerances. For this reason, an adjustment of 
adequate accuracy of the imaging conditions for intermediate values is not 
ensured. In practice, defocused images occur in the final image plane. 
Furthermore, the input of the intermediate values via the external 
computer takes a great deal of time and is impractical. This input does 
not permit, for example, varying the magnification continuously or in fine 
steps preselectable by the operator while at the same time viewing the 
object image in the projection plane. However, this last-mentioned 
capability is required when the optimal magnification is to be adjusted 
for a detail of the object of interest. 
An interpolation between calibrated parameter values is also referred to in 
U.S. Pat. No. 4,851,674 in connection with a correction of astigmatism or 
deflection in an electron microscope. However, here no specific data are 
obtained with respect to the interpolation. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an electron-optical imaging 
system which makes possible a more convenient operation when utilized in 
an electron microscope. Operating conditions can be set for which the 
imaging system is not experimentally calibrated. 
The electron-optical imaging system of the invention includes a memory, a 
computer and several controllable elements which act upon an electron beam 
and these elements have corresponding current or voltage sources. Values 
are stored in the memory which are determined from experimentally 
calibrated parameter values of the imaging system for a discrete sequence 
of different experimentally calibrated operating conditions. The actual 
operating conditions are adjustable in a preselectable step width. This 
step width is independent of the calibrated operating conditions. 
The imaging system of the invention makes it possible to adjust operating 
conditions for which no experimental calibration was made. In contrast to 
the state of the art, however, specific operating conditions must not be 
inputted via the computer keyboard. The change of the operating conditions 
is instead possible in a stepwise manner with the step width being 
adjustable as desired. The advantage is here provided that the operating 
conditions such as the magnification can be varied successively by the 
operator while at the same time viewing the image of the object. This 
advantage is especially provided with the use of such an imaging system in 
an electron microscope. It is then especially advantageous when the change 
of the operating conditions is possible directly at the operating console 
of the electron microscope and especially via up and down switches or 
buttons. Also, the selection of the step width should be possible directly 
at the operator console. 
The control signals of the current or voltage sources, which correspond to 
the actual adjusted operating conditions, are computed from the stored 
values in the computer. In an advantageous embodiment, the stored values 
are coefficients of approximation functions. The control signals for the 
current or voltage sources are computed in the computer based on these 
coefficients and the corresponding approximation function. Depending upon 
how well the approximation function approximates the actual curve of the 
current or voltage characteristic of the parameter to be varied, the 
better the computed control signals match the ideal control signals 
corresponding to the same operating conditions. The ideal control signals 
would be obtained from an experimental calibration under these conditions. 
Suitable approximation functions must be determined only once during 
calibration of the imaging system, that is, when the imaging system is 
taken into service. The approximation functions themselves are then later 
reconstructed with little computer effort based on a few stored 
coefficients. 
The approximation functions are each intended to be continuous over an 
interval with two mutually adjacent approximation functions each 
exhibiting intervals of overlapping continuity. Definitive control signals 
can be computed over a large range of different operating conditions such 
as different magnifications by means of the continuous approximation 
functions and the overlap of the continuity intervals. 
Polynomes of the second degree have proven to be especially advantageous 
for the approximation functions. On the one hand, these functions 
approximate the magnification current characteristic of the magnetic 
lenses over an interval with adequate accuracy. Magnetic lenses are 
usually utilized in electron microscopes. On the other hand, a polynome of 
the second degree fits uniquely to each of three mutually adjacent 
calibrated parameter values with little computer effort. This polynome 
passes through these calibrated parameter values. Furthermore, the number 
of function coefficients to be stored is low. Three coefficients are 
stored for each calibrated parameter value. With the aid of these 
coefficients, any desired intermediate values between the experimentally 
calibrated parameter values can thereafter be computed. 
The invention is usable not only for a single imaging system. Instead, the 
invention is also applicable to several imaging systems coupled to each 
other, for example, to the imaging system in an electron microscope which 
is at the condenser end and at the imaging end. In the so-called constant 
brightness mode, an automatically coupled magnification change of the 
condenser system is possible when there is a change of the projective 
magnification so that the brightness of the viewing screen remains 
constant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The electron microscope shown in FIG. 1 includes an electron-optical column 
1 having the electron-optical components which are here not shown in 
detail. The electron-optical column 1 and the operating console 2 
conjointly define a unit. The projected image of the specimen can be 
observed on a fluorescent screen (not shown) through a window 3 in the 
projection chamber of the electron microscope. A commercial computer 
having a permanent memory such as a hard-disc memory is integrated into 
the operating console 2. The corresponding keyboard is identified by 
reference numeral 5 and the monitor by reference numeral 6. Furthermore, 
the current and voltage sources for the electron-optical elements of the 
column 1 are contained in the operating console. A touch panel 7 is 
arranged on the operating console 2 close to the projection chamber so 
that an observer viewing the projection chamber through the window 3 can 
simultaneously change the operating conditions of the electron microscope 
via the touch panel 7. In electron microscopes according to the state of 
the art, only such operating conditions can be adjusted via a touch panel 
7 of the kind referred to above wherein the electron microscope had been 
previously experimentally calibrated. In contrast, the touch panel 7 of 
the electron microscope according to the invention permits inputting 
operating conditions which lie between such experimentally calibrated 
operating conditions. 
As shown in greater detail in FIG. 2, the electron column 1 includes an 
electron source 10 having a current and voltage source 20 corresponding 
thereto. The object 15 is mounted in the center of the pole shoe gap of 
the condenser-objective single-field lens 14. The object 15 is illuminated 
by the electron beam emanating from the electron source 10. Three 
additional magnetic condenser lenses (11, 12, 13) are mounted between the 
electron source 10 and the condenser-objective single-field lens 14. The 
excitation currents of the condenser lenses (11, 12, 13) can be varied via 
their current sources (21, 22, 23). By varying the excitation currents, 
the aperture of the electron beam in the TEM-mode can be varied or the 
magnitude of the electron probe on the object 15 can be varied in the 
scanning mode. A condenser diaphragm 13a is mounted between the third 
condenser lens 13 and the condenser-objective single-field lens 14. The 
condenser diaphragm 13a can be configured as a multiple diaphragm for 
adjusting different illuminated areas in the TEM-mode or for adjusting 
various probe apertures in the scanning mode. The deflection systems 
required for the diaphragm selection are not shown in FIG. 2. Details of 
the selection of illuminated areas are disclosed in U.S. Pat. No. 
5,013,913 incorporated herein by reference and details as to the selection 
of the aperture can be obtained from U.S. Pat. No. 5,483,073 which is 
likewise incorporated herein by reference. 
The object 15 is imaged magnified on the fluorescent screen 30 with the aid 
of the condenser-objective single field lens 14 and four downstream 
magnification stages configured of magnetic lenses (16, 17, 18, 19) and 
their corresponding current sources (26, 27, 28, 29). This magnified image 
of object 15 can be viewed through the window 3 in the projection chamber. 
Basically, the excitations of all magnetic lenses (11, 12, 13, 14, 16, 17, 
18, 19) are adjustable independently of each other. The excitation 
parameters of the magnetic lenses are, however, coupled to each other for 
maintaining the defined illumination and imaging conditions. The control 
of the current and voltage sources (22 to 29) via a computer 31 having 
hard-disc memory 32 connected thereto always guarantees maintaining 
defined illuminating and imaging conditions. The input and change of the 
operating conditions take place in part via a touch panel 7 connected to 
the computer and, on the other hand, via a computer keyboard 5. 
The first experimental calibration of the electron microscope is made by 
factory personnel. This calibration takes place in correspondence to the 
flowchart shown in FIG. 3. 
In a first step 41, the lens excitations corresponding to the operating 
conditions which are to be calibrated are first adjusted in discrete steps 
via the touch panel 7 in such a manner that the electron beam has the 
desired beam path within the electron column 1. As soon as the desired 
beam path is adjusted, the parameters characterizing the adjusted 
operating state are inputted via the computer keyboard 5 in a second step 
42. These parameters include magnification, electron energy, illuminating 
field diameter, aperture, et cetera. After completing this input, the 
calibrated parameter values, which were adjusted on the current and 
voltage sources (20 to 29), are read out to the computer 31 and, in a next 
step 44, are stored on the hard-disc memory 32 and are assigned to the 
inputted operating parameters. 
In a next step 45, an inquiry is made as to whether a further operating 
condition is to be experimentally calibrated. In the event that further 
operating conditions are to be experimentally calibrated, the function 
steps (41, 42, 43, 44, 45) are run through for each operating condition to 
be calibrated. 
The electron microscope is experimentally calibrated in an adequate number 
of operating conditions. Thereafter, in function step 45, the inquiry as 
to whether the manual calibration of an additional operating condition is 
desired is answered in the negative. The computer 31 then computes, in 
step 46, the coefficients of the polynome of the second degree which 
passes through three sets of calibrated parameter values. This computation 
is made for each calibrated operating condition from the corresponding 
temporarily stored calibrated parameter values and the corresponding 
stored calibrated parameter values of the two operating conditions having 
directly adjacent operating parameters. If In(k) is the current of the nth 
lens in the kth operating condition and if k-1 and k+1 identify the 
operating conditions next to k to which the electron microscope is 
experimentally calibrated, then In(k) is defined by equation (1) as 
follows: 
EQU In(k)=a(k+b).sup.2 +c (1) 
By applying equation (1) to the lens currents In(k+1) and In(k-1) of the 
nth lens corresponding to the neighbored operating conditions k+1 and k-1 
and by mutually substituting the three equation systems which result in 
this manner, the function coefficients a, b, c can be computed pursuant to 
equations (2), (3) and (4) as shown below: 
##EQU1## 
The computation of the function coefficients (a, b, c) corresponding to the 
equations (2), (3), (4) is carried out in function step 46 for each lens 
of the electron microscope and for each operating condition k for which 
the electron microscope was experimentally calibrated. If the electron 
microscope has been calibrated in total in K operating conditions, then 
8.times.K sets of function coefficients a, b and c result for the eight 
magnetic lenses of the electron microscope which can be adjusted 
independently of each other. These 8.times.K sets of function coefficients 
are stored on the hard-disc memory 32 in the next storage step 47. After 
storing the function coefficients, the experimental calibration of the 
electron microscope is ended and the program moves out of the calibration 
program in step 48. 
When the electron microscope is switched on later (function step 51 shown 
in FIG. 4), the function coefficients are loaded from the hard-disc memory 
32 into the computer 31 in an initialization step 52. The input or change 
of the operating conditions is performed via the touch panel 7 in step 53. 
The step width for the finely graduated change can be inputted via 
keyboard 5. For a freely adjusted step width (.delta.), the lens current 
of the nth lens In(k+m.multidot..delta.) is computed corresponding to 
equation (5). 
EQU In(k+m.multidot..delta.)=a.multidot.((k+m.multidot..delta.)+b).sup.2 
+In(k)-a.multidot.(k+b).sup.2 (5) 
Here, (m) is an integer positive or negative number and .delta.&lt;1 is an 
integer fraction of 1, i.e. 1 divided by an integer. This computation is 
carried out for all eight lenses of the electron microscope and, 
thereafter, the lens currents so computed are transmitted to the current 
sources (21, 22, 23, 24, 25, 26, 27, 28, 29) in step 55. The excitation of 
all lenses is then changed simultaneously. The function k coefficients 
relating to operating condition are used for the computation based on 
equation (5) for all lens currents between In(k-1) and In(k+1) as a 
central support point of the parameters. 
If lens currents result via the up and down buttons (7a, 7b) on the touch 
panel 7 which are less than In(k-1) or greater than In(k+1), then, in the 
newly occurring interval, the lens currents are computed on the basis of 
an equation corresponding to equation (5) in which, however, In(k-1) or 
In(k+1) is used in lieu of In(k) and the function coefficients 
corresponding to operating conditions k-1 and k+1 are used in lieu of a, b 
and c. In this way, overlapping intervals are obtained. A jump of current 
values at boundary points is thereby avoided. The step width for .delta. 
can be selected to be as small as desired because no further experimental 
calibration is required. 
To minimize off-axis image errors (distortion, radial chromatic 
aberration), projective systems are operated up to approximately average 
magnification with virtual intermediate imaging and only in the higher 
magnification range do all lenses generate a real intermediate image. 
Discontinuities in the lens current curves occur in the transition from 
virtual to real imaging. In FIG. 5, the lens currents I.sub.1 and I.sub.2 
of two lenses are plotted over a magnification range of 1,000X to 
500,000X. The transition from the virtual intermediate image to the real 
intermediate image takes place at approximately 25,000X magnification. The 
largest magnification step is identified by (v). At this magnification, 
the electron microscope is usually still driven at virtual intermediate 
imaging. The smallest magnification step is identified by r=v+1 at which a 
real intermediate image is the norm. In order to adjust any desired 
magnification values between v and r, the electron microscope is 
additionally calibrated for a virtual intermediate step v' and a real 
intermediate step r'. The magnification value of the virtual intermediate 
step v' is equal to the magnification value of the first real step r and 
the magnification value of the real intermediate step r' is equal to the 
magnification value of the last virtual step v. The intermediate steps do 
not serve as central support points; instead, they serve only for 
computing function coefficients in adjacent support points. The selection, 
which intermediate step r' or v' forms the basis for magnifications 
between v and r, is dependent upon from which end (v or r) the 
magnification is adjusted in the range between v and r. In this way, a 
discontinuous change of the lens excitation for fine adjustment is also 
avoided in this intermediate range. 
No knowledge of the focal width or the path equations of the electron beam 
is required for computing the function coefficients and for computing of 
the lens currents from these function coefficients. The invention is 
therefore applicable also for parameter values of the lens currents which 
have been determined strictly empirically. The computation of the 
intermediate values corresponding to the invention minimizes hysteresis 
effects of the lenses which can occur especially for large focal length 
variations. 
The invention is not only applicable for an individual imaging system but 
also for the combination of several imaging systems. Such a combination of 
two imaging systems is defined in FIG. 2 by the illuminating end condenser 
lenses (11, 12, 13, 14), on the one hand, and the imaging end lenses (14, 
16, 17, 18, 19) on the imaging end. If, for example, the so-called 
constant brightness mode is adjusted via the touch panel 7 and the imaging 
end magnification is varied with the aid of the up and down buttons (7a, 
7b), then lens current values for the lenses (11, 12 and 13) are computed 
automatically by the computer 31 based on the loaded function coefficients 
so that the image brightness on the viewing screen 30 is constant. 
To simplify the example, it was assumed for the equations (1) to (5) that 
the calibrated operating conditions are equidistant and therefore 
m.multidot..delta. is always between -1 and 1. This is, however, not 
required for applying the invention. If, for example, a spacing x.sub.1 
lies between the calibrated conditions k-1 and k and a spacing x.sub.2 
lies between the calibrated conditions k and k+1, then the current of the 
nth lens can be defined in all conditions between k-1 and k+1 by the 
following equations (1a) and (1b) which are analogous to equation (1): 
EQU In(k+x.sub.2)=a(k+x.sub.2 +b).sup.2 +c (1a) 
EQU In(k-x.sub.1)=a(k-x.sub.1 +b).sup.2 +c (1b) 
Together with the equation (1) for In(k), a system results, in turn, of 
three equations for the three unknowns a, b and c and from this results an 
equation analogous to the equation (5) for computing the intermediate 
states. 
The invention is not only applicable for making an adjustment of 
magnifications lying between calibrated magnifications but also for other 
variable parameters such as electron current density, image rotation, et 
cetera. 
It is understood that the foregoing description is that of the preferred 
embodiments of the invention and that various changes and modifications 
may be made thereto without departing from the spirit and scope of the 
invention as defined in the appended claims.