Variable curvilinear axis deflection means for particle optical lenses

An improved particle lens has an axis which is shifted to follow the central ray as it is deflected through the lens creating, in effect, a variable curvilinear optical axis for the lens. The variable curvilinear optical axis is created for the lens so that the axis varies proportionally to the magnitude of the beam deflection. The optical axis of the lens is shifted by applying a uniform field to the lens to cancel the first term of the radial field with a function that is dependent on the position along the z-axis. This function is the trajectory of the central ray of the electron beam.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to particle optical lenses, such as 
electron beam lenses, and more particularly, to a variable curvilinear 
optical axis for such lenses to correct for aberrations. 
2. Description of the Prior Art 
In light optics, it is possible to maintain low aberrations and distortions 
while utilizing a significant portion of the lens area for imaging. In 
electron optics, however, it is not practical to correct the field of a 
lens to the same extent as with light optics. This is because the electron 
optical lens is actually a magnetic field rather than a piece of optical 
glass, and it is not possible to shape the magnetic field to any desired 
shape or to the same precision that a piece of glass can be formed. The 
magnetic field must, after all, satisfy LaPlace's equation within the 
lens. This problem is typically overcome in electron optics by making the 
lens as large as possible, or practical, relative to the optical field of 
view while keeping the focal length as short as practical for the given 
application. Making the lens large relative to the application has the 
effect of approximating the field shape of an "ideal" lens, much the same 
as is done in light optics when a small portion of a large spherical 
surface is used to approximate a parabolic surface. With probe forming 
systems, this means staying as close to the lens center or optical axis as 
the off-axis distortions and aberrations will allow. It is usually the 
case that the on-axis lens errors are smaller than the off-axis errors and 
that the off-axis errors increase with the square or cube of the distance 
off axis. If higher order error terms are considered, than the errors will 
increase as the higher powers of the terms. 
It is possible to deflect an electron beam at very high speeds either 
electrostatically or magnetically or a combination of both. Thus, any 
point can be addressed within a relatively large defection field in very 
short times (on the order of microseconds or even nanoseconds). The final 
location of the beam can also be corrected during deflection by modifying 
the deflection address according to some predetermined distortion map 
acquired during system calibration and/or wafer registration. This is a 
common practice, but it only corrects the landing position of a single ray 
or small bundle of rays defining a point which is transferred from the 
object plane to the image plane. Any lens errors will still distort the 
local region about this central ray. A common practice to correct some of 
this local image distortion is to refocus and apply a stigmation 
correction to the off axis beam. The further the beam is deflected off the 
central axis, the greater the deflection aberrations will become. At some 
point, further deflection is rendered unusable due to excessive lens 
aberrations that are not correctable by methods known in the art. The 
inventions disclosed in U.S. Pat. No. 4,859,856 for a Variable Axis Lens 
(VAL) and U.S. Pat. No. 4,544,846 for Variable Axis Immersion Lens (VAIL) 
used a technique of subtracting a planar field from the lens' radial 
field. This planar field is everywhere parallel to the radius connecting 
the central z-axis and the point to which the beam is deflected. The term 
"planar" is used to refer to a field, such as that resulting from a 
deflection yoke (typically of either a Saddle or Toroidal configuration) 
where the field in any z plane is uniform, but the magnitude of the field 
may vary according to a smooth function of z as one moves from z-plane to 
z-plane. As described in the above inventions, the strength of the planar 
field subtracted from the lens radial field is proportional to the first 
derivative with respect to axial position, z, and to the distance the lens 
field is to be shifted in the radial direction. The typical method of 
applying the planar field is by means of a deflection yoke sized and 
positioned to match the negative of the first term of the radial field of 
the lens. This has the effect of shifting the optical axis laterally with 
the deflected beam so that to the beam it appears as though it is still on 
the optical axis. By this method, the off-axis errors of the lens and 
deflection system can be greatly reduced. 
This technique is not a perfect solution because it corrects only to the 
first order, which is the greatest part of the errors; however, this 
approach also assumes that the effective axis of the lens remains a nearly 
straight line shifted parallel to the geometric axis of the lens. The 
electron beam is deflected prior to entering the field of the lens such 
that the beam ends up traveling coincident to the shifted axis as it 
travels through the lens. This is done so that the electron beam does not 
deviate substantially from the shifted axis and therefore does not incur 
any errors greater than is allowed by the system error budget. Such an 
approach requires a considerable spacing between lenses and deflection 
yokes; however, in a practical system design, some overlap of the beam 
deflection and lens will occur. 
The separation of the lens and beam deflection yokes cannot increase 
without penalty. The longer the path length of the electrons, the more 
Coulomb interaction between electrons will occur. As a result of the 
Coulomb interaction it would therefore be desirable for the beam 
deflection and lens to be "fully" overlapped in order to keep the optical 
path length that the electron travels to a minimum. This Coulomb 
interaction creates additional errors which add to the lens and deflection 
errors. 
The problem, therefore, is how to achieve the largest electron optical 
field of deflection with the smallest errors possible in the shortest 
optical path length possible. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved particle 
lens and beam deflection means in which the axis of the lens is shifted as 
a function of the position along the beam path so as to follow the central 
ray of the beam as it is deflected through the lens creating, in effect, a 
variable curvilinear optical axis for the lens. 
According to the invention, there is provided an electron optical lens and 
deflection means having a variable curvilinear optical axis that varies in 
its position relative to the central lens axis so as to follow the 
predetermined path of the deflected electron beam. This is accomplished by 
applying a planar field to the lens to cancel the first term of the radial 
field with a function that is dependent on the position along the z-axis. 
This function is the trajectory of the central ray of the electron beam.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
An electron optical lens, typically used in probe forming and projection 
imaging systems, is a magnetic axial symmetric lens. The description of 
the preferred embodiment of the invention will be in the context of this 
type of lens and imaging system, although the concept may be applied to 
other and different embodiments of the invention. 
Referring now to the drawings, and more particularly to FIG. 4, there is 
shown a portion of a variable axis lens electron beam projection system of 
the type disclosed in U.S. Pat. No. 4,544,846. A Variable Axis Immersion 
Lens (VAIL) is used as an example for illustration. However, the general 
Variable Axis Lens (VAL) can equally be used as an example (U.S. Pat. No. 
4,376,249). The VAIL was chosen for simplicity because the VAIL is one 
half of a VAL. At the top of the Figure, a box 3 denotes schematically an 
initial portion of the system that generates an electron beam travelling 
downward in the Figure, initially centered on the system or physical axis 
101 of the hardware, which is also the z axis of the coordinate system. 
Box 3 may be an electron gun, in the case of a probe forming system; or it 
may be the upper portion of a projection imaging system that passes the 
beam through a reticle in order to transfer the reticle "pattern" by means 
of the electron beam to a target wafer. The portion of the optical system 
shown in FIG. 4 collimates the beam emerging from box 3 and brings the 
beam to a focus on wafer or target 59 at x and y positions that are 
controlled by controller means illustrated schematically on the right of 
the figure. A general purpose computer or dedicated controller 60 sends 
signals to power supplies 61 and 63 and signal generators 65, 67 and 79, 
all collectively referred to as control means. The optical system includes 
a magnetic lens 100, extending a lens length along axis 101 and having 
pole pieces 49 and 51. Lens 100 includes an excitation coil 53 for 
applying a magnetic focussing field through pole pieces 49 and 51, 
collectively referred to as field generating means, providing telecentric 
capability, as is known in the art. Dynamic focus correction coil 69 is 
conventionally used as required by system tolerances to provide 
higher-order corrections (proportional to r.sup.2, etc.) to lens 100. 
A pair of magnetic deflection yokes 43 and 45 predeflect the projected 
electron beam, under control of the controller means, before it enters 
variable axis lens 200 in the lower portion of the figure to direct it at 
the desired x and y coordinates on wafer 59. In one example, such a system 
can be used to fabricate a reticle by scanning across the surface of the 
reticle and exposing the reticle pattern. Such a system can also be used 
for direct writing on a wafer without the use of a reticle. 
Variable axis immersion lens 200, in the lower portion of FIG. 4, includes 
an upper pole piece 13 and a lower pole piece 14. Excitation coil 41 
activates immersion lens 200 and produces magnetic field lines that run 
from upper pole piece 13, having a non-zero bore that permits passage of 
the beam, to lower pole piece 14. A leg 18 of the immersion lens magnetic 
circuit includes alternating magnetic and non-magnetic sections so as to 
shield metal parts of lens 200 from changing yoke fields while at the same 
time preventing the magnetic lines of force of the lens from being 
short-circuited. The magnetic circuit is also shaped at section 19 of the 
lower pole piece to permit flux to pass to zero bore section 14 with 
minimal reluctance and fringing. A single magnetic compensation yoke 11 
provides a magnetic field that is proportional to the first derivative of 
the axial magnetic field produced by immersion lens 200. Yoke 11 and its 
associated signal or current generator may be referred to as axis shifting 
means. 
FIG. 4 also illustrates the target holding, handling, and stepping stage. 
Target 59 is mounted to a target holder 16 for providing accurate 
registration of the target within the electron beam projection system. A 
target handler arm 20 inserts the target into lens 200. A target stepper 
table 17 is employed for x-y movement of the target. Since the magnetic 
projection field produced by upper pole piece 13, lower pole piece section 
14 and excitation coil 41 has zero slope in the vicinity of the target 
area, the compensation field produced by compensation yoke 11 has zero 
field strength in the vicinity of the target area. Accordingly, no eddy 
currents are generated by compensation yoke 11 in or around the target 
area. 
Astigmatism and field curvature are corrected using dynamic correction. As 
shown in FIG. 4, a power supply 61 is connected to excitation coil 53, and 
a power supply 63 is connected to excitation coil 41. A computer 
controlled driver 65 supplies excitation signals to deflection yokes 43 
and 45. Deflection yokes 43 and 45 have two sets of magnetic coils that 
cooperate to generate a magnetic field in the x-y plane, perpendicular to 
axis 101, to deflect the electron beam in both an x and a y direction. 
Deflection yokes 43 and 45 are typically comprised of a plurality of yokes 
of saddle or toroidal configuration. Driver 65 also activates magnetic 
compensation yoke 11 which consists of a pair of x-y magnetic deflection 
yokes. Magnetic compensation yoke 11 may comprise a simple saddle coil 
because of its smaller outside diameter as compared to a toroidal yoke of 
the same deflection sensitivity. The x-y current sent to magnetic 
compensation yoke 11 is proportional to the x-y currents sent to 
deflection yokes 43 and 45 and may be supplied by the same driver 65. A 
driver 79 connected to dynamic focus correction coils 71 and 73 receives 
an input signal which is proportional to r, the deflection distance of the 
shifted axis, and generates a signal proportional to r.sup.2. 
In the first approximation, the field of the axis-shifting yoke 11 
compensates the radial component of the lens 200 field along a line 
parallel to the symmetry axis of the lens and having an x-y position 
proportional to the current into the compensation yoke. This line 
represents the shifted electron optical axis because the radial component 
of the field has become zero there. 
In operation, yokes 43 and 45 will be driven to steer the beam to selected 
points on wafer 59. Compensating coil 11 will be driven to center the 
optical axis of lens 200 above the desired position on the wafer. 
The magnetic field of an axially symmetric lens can be uniquely described 
anywhere inside the lens by the distribution of the magnetic flux density 
on the axis of the lens. The normal lens axis 101 defined by the 
mechanical center of the lens will also be referred to as the system axis 
and the shifted or variable axis will be referred to as the "variable 
axis". The radial component (designated B.sub.r) of the field anywhere in 
the lens is described by the infinite series 
##EQU1## 
where z is the position along the system axis. On the axis, r=0 and 
therefore, B.sub.r =0. If B.sub.r (r,z) is set equal to zero at some 
non-zero r by setting all the terms of Equation (1) equal to zero, then 
the axis of the lens is shifted to this new location of r. The series of 
the Equation (1) converges very rapidly, and the first term is the 
dominant term of the series. The first term of this series is 
##EQU2## 
Thus, if this first term is set equal to zero at a specific value of r by 
adding a planar field in the direction of r that is of equal magnitude to 
Equation (2) for the given value of r but of opposite sign, then in effect 
the axis of the lens is shifted magnetically to this new location of r. In 
the prior art variable axis electron optical lens system as shown in FIG. 
4, the value of r is constant, thus shifting the axis laterally as a 
straight line parallel to the original axis. This planar field is easily 
approximated very well by a typical deflection yoke field. 
When the electron beam has been deflected prior to entering the imaging 
lens 200, so that the beam is travelling parallel to the system axis, then 
the electrons entering on the shifted axis will continue on this axis 
traveling in a straight line. This deflection prior to the lens is not 
easy to achieve in practice because the deflection magnetic field and the 
lens magnetic field do not end abruptly but, rather, taper to a small 
value over relatively large distances and it is not practical to space the 
deflection coils far enough away that the fields do not overlap. The more 
the overlap, the more the electron beam will be off the variable axis 
through a longer portion of the lens. As the overlap increases, the lens 
and deflection errors also increase. Thus, as a given system is made more 
compact, the error reduction benefits of using a variable axis lens become 
vanishingly small. 
The present invention addresses the problem caused by the overlap, by 
making the shifted axis curvilinear, rather than straight. This is 
achieved by applying the planar correction field to the lens field in 
order to cancel the first term of the radial component of the lens field 
using a non-constant function of the z-position, R(z), in Equation (2) 
instead of the constant term, r. This function is the r-coordinate of the 
trajectory of the central ray of the electron beam. Referring now to FIG. 
2, two curves are displayed to illustrate the relationship of the lens 
field and its derivative in a multi-lens system. The solid line 60 in FIG. 
2 is the combined axial flux density of the lenses, peaking at L1, the 
center of lens 1 and again at L2, the center of lens 2. The dashed line 62 
is the first derivative of the field with respect to z. The boxes on the 
side of the Figure represent a set of coils for generating correction 
fields. It can readily be seen that the field of lens 2 penetrates within 
the volume defined by the coils of lens 1, i.e. the fields overlap. To use 
this lens as a variable curvilinear axis lens system according to this 
invention, a planar field from a deflection yoke that varies in magnitude 
along the z-axis, proportional to the dashed curve is superimposed onto 
the lens magnetic field. The function R(z) is proportional to the radial 
coordinate of the trajectory of the central ray of the electron beam. Thus 
the magnetic axis of the variable curvilinear axis lens is made to follow 
the central electron trajectory of the electron beam. The physical 
implementation of the z-dependence is effected by the use of a plurality 
of compensation yokes of the same type as coil 11 in FIG. 4 to provide a 
contigually smooth displacement of the variable axis so as to follow the 
beam trajectory. The greater the number of compensation yokes, the better 
the approximation to produce the magnetic field which establishes the 
desired curvilinear axis. 
Referring now to FIG. 1, there is shown in partially pictorial, partially 
schematic form a projection electron beam system employing the present 
invention, the bottom portion of which corresponds to FIG. 4. The 
invention applies to any deflection lens system, such as a probe-forming 
system, a shaped-beam system, or a scanning electron microscope. The 
projection system is chosen merely as an example and also because it 
requires the most demanding Image fidelity over a large field of view, not 
just a probe or a small shaped spot. At the top of the figure, box 1 
represents schematically the electron gun that generates a slightly 
diverging beam, having an energy of illustratively 100 keV and travelling 
along the z-axis 101. Controller 60' and signal generator 70, analagous to 
boxes 61, 63, 65, 67 and 79 of FIG. 4, perform similar functions of 
controlling lens coils and deflectors. Deflectors 5 and 7 are used 
together as is known in the art to deflect the beam so that the 
extrapolated intersection point of the beam trajectory with the z-axis 101 
is located between the two yokes 5 and 7. This point is the conjugate 
image point of aperture 275 discussed below. The current in yokes 5 and 7 
are nearly equal and are adjusted during system setup so that no lateral 
motion of the beam is observed at aperture 275, thereby decreasing the 
variation of beam current impinging the target wafer 59. Variations of 
beam current of less than 1% can produce intolerable critical dimension 
errors within the circuit pattern transferred from the reticle to the 
wafer. The combination of deflectors 5, 7, 55 and 57 is set as is known in 
the art to illuminate the nth subfield of reticle 375 with a shaped beam 
that impinges the reticle plane with nearly perpendicular landing. 
Reticle 375 represents the pattern on a layer of an integrated circuit and 
is divided into subfields, illustratively on the order of 1 mm on a side 
and carrying the pattern that is to be imaged on wafer 59 at the bottom of 
the figure. The beam will step in sequence through the subfields on the 
reticle, the totality of which represents the pattern of the integrated 
circuit. Such a system is described in U.S. Pat. No. 5,466,904. 
Illumination lens 50, which may be a curvilinear axis lens according to 
the invention or a conventional variable axis lens, forms an image of the 
emitting surface of the electron source or cathode on the nth subfield by 
means of deflectors 5, 7, 55 and 57. This has the effect of illuminating 
the reticle pattern. The beam passes through the reticle to curvilinear 
variable axis collimator lens 100, positioned along axis 101 at a distance 
from the reticle nearly equal to its focal length and which produces an 
image of the reticle at or near infinity so that there will be the minimum 
amount of Coulomb interaction between the electrons in the beam and also 
to minimize the chromatic effects due to deflection. 
The beam is then deflected back toward axis 101 by the combination of 
deflectors 105, 107 and 205, crossing the axis at aperture 275, while a 
set of axis compensation yokes 150-1 to 150-n located inside lens 100 and 
distributed along the beam path in the axial direction (parallel to axis 
101) shifts the variable axis so as to be coincident with the central beam 
trajectory along its entire length thereby establishing a curvilinear 
field axis. Illustratively, yokes 150 are placed between deflection yokes 
105 and 107 and the magnetic pole pieces of lens 100. They are shown 
displaced in the drawing for ease of illustration. Each of the yokes 150-1 
to 150-n is excited with a control current that is proportional to the 
magnitude of deflection and of a fixed relative ratio to each of the other 
compensation yokes. This fixed relative ratio is determined from the lens, 
yoke, and central beam trajectory physical relationships such that the 
best fit of the desired compensation field is applied, as described below. 
The use of a fixed ratio is an approximation for convenience and system 
simplicity. In the most general case, a fit could be made for each 
subfield of the reticle, with the results stored in the controller means. 
The current ratios in the coils would then be determined by this stored 
value. By "fixed" is meant that the ratio does not vary as a function of x 
and y when using a first order approximation. 
After passing through the reticle 375, some of the electrons comprising the 
illuminating beam will be slightly scattered because they passed through 
the patterned portion of the reticle. These scattered electrons are 
absorbed in plate 277 which contains aperture 275 while aperture 275 
passes the unscattered electrons in the beam down to the wafer 59 to form 
the image of the reticle pattern. Reticle 375 may be either a "stencil" 
reticle, having openings for radiation to pass through, or it may be a 
"differential" reticle, having areas of relatively low and high scattering 
cross section, as described in U.S. Pat. No. 5,466,904. 
Lens 200, forms a reduced image of the nth subfield of reticle 375 on wafer 
59. In this embodiment of the invention, lens 200 is a curvilinear 
variable axis lens (CVAL) that is not an immersion lens. A curvilinear 
variable axis immersion lens (CVAIL) having a plurality of axis-shifting 
coils and a plate similar to plate 14 of FIG. 4 could be used if the 
system designer preferred. Those skilled in the art will readily 
appreciate that the use of an immersion lens results in a strong field at 
the wafer surface, while making it difficult to satisfy the doublet 
condition between lenses 100 and 200 referred to below; and that the use 
of a non-immersion lens results in a smaller fringe field at the wafer 
surface and permits easier satisfaction of the doublet condition. System 
designers will select one or another system configuration depending on the 
usual design tradeoff considerations. At the bottom of the figure, box 17' 
represents schematically the associated wafer support, positioning 
mechanism, and the like shown in FIG. 4. Illustratively, the reduction 
ratio has a conventional magnitude in the range between 3:1 to 5:1. This 
demagnification ratio is achieved by the combination of lenses 200 and 100 
which form a doublet pair as described in U.S. Pat. No. 5,466,904. 
Deflectors 205 and 207 guide the beam through aperture 275 and on to the 
nth subfield position on wafer 59, also performing "stitching" when 
required to correct for a finite separation of the subfields on reticle 
375 by shifting the images so that they are contiguous. The invention may 
be used with reticles that have both contiguous or non-contiguous 
subfields. The axis-shifting coils for lens 200 are illustrated as boxes 
250-1 to 250-n and function to position the variable axis of lens 200 
according to the invention as described below. 
Referring now to FIG. 3, line 160 represents the axial flux density of a 
doublet lens system. For generality, the lens system illustrated is not 
necessarily the one used in FIF. 1. The boxes at the sides of the figure 
represent correction coils that shift the axis. No particular number of 
correction coils is required and the correction coils are not necessarily 
limited in the z-direction by the lens pole pieces. They may extend a coil 
length along the system axis 101 that may be greater than, equal to, or 
less than a lens length between the pole pieces. A first lens L1, not 
shown in the Figure, is centered at the mark L1 and a second lens L2, also 
not shown in the Figure, is centered at a corresponding mark on the right 
of the drawing, showing the effect of its fringing fields. The magnitude 
of the flux peaks midway between the positions of the pole pieces and 
tapers off above and below. The dashed line 162 is the first derivative of 
the axial flux with respect to z. To use this lens as a variable 
curvilinear axis lens system according to the invention, a planar field 
that is uniform within any plane perpendicular to the z-axis and that 
varies in magnitude along the z-axis, proportional to the dashed curve, is 
applied to the lens field. 
FIG. 3 also shows an added solid line 165 that is an example of the radial 
position of a typical trajectory that may be of interest in an optical 
system. There could be any number of trajectories used as an example. This 
trajectory represents the central ray of a deflected electron beam used in 
a projection lithography system. The x's on the trajectory are located at 
the central point of the corresponding coils. As can be seen by its 
relationship to the lens field (line 160), a straight shifted axis 
parallel to the z-axis would at best be at the same distance from the axis 
at only a very small portion of its length or even at only one point. 
However, by modulating the first derivative of the lens (the dashed line) 
by the trajectory or R(z), the distance from the z-axis, a new function 
Y(z) of curve 167 is obtained (the dash-dot-dashed line 167). This 
function Y(z) replaces the first term of Equation (2) as follows: 
##EQU3## 
This is a function of z and determines the magnitude of the field to be 
superimposed on the lens field as a function of z. As before, this field 
is achieved with typical deflection yokes (either of the saddle type or 
toroidal wound type). The terms deflection yoke or coil and compensation 
yoke or coil are used interchangeably here because the coils implemented 
to apply a field in any x-y plane can be used for deflection and/or axis 
shifting by means of superposition of the fields using the appropriate 
control signals applied to each axis of a single coil. 
The function Y(z) is the curvilinear axis yoke field to be applied to the 
lens in order to curve the axis of the lens to follow the beam path 
through the lens, thus forming a curvilinear axis lens. Any of the higher 
order terms could also be applied if accuracy requirements dictated, 
forming additive functions in z containing higher powers of R(z), 
R(z).sup.3, R(z).sup.5, etc. Typically, however, only the first order term 
is applied for simplicity and because it is by far the dominant term. 
Generally, each successive term is two orders of magnitude smaller than 
the preceding term for magnetic lenses of the type used in the 
illustrative projection system. Line 167 in FIG. 3 shows an actual yoke 
field that can be practically achieved for this example. The particular 
associated line 165 was selected as an example to show that when the 
trajectory passes through the system axis, curve 167 is forced to zero, 
even though the derivative 162 (used in the prior art) is quite large at 
this point. 
It can be seen in FIG. 3 that the boxes representing coils are touching. As 
applied to an actual system, the term "adjacent" will be taken to mean 
close together or nearly touching, since coils are not simple boxes that 
can be butted together. It can also be seen that there are two gaps in the 
line of coils where line 167 passes through zero. When the magnitude of 
the compensation field is small (at such points) there is very little 
effect on the accumulated error of a beam by omitting the compensation 
field. In general, it would be possible to reduce costs in a particular 
application by placing coils only where the magnitude of the counterpart 
curve to line 167 is large enough to justify compensation. Such a 
distribution will be referred to as "nonuniform" for purposes of this 
application. System designers will readily be able to calculate the number 
and spacing of coils required to reduce errors in a particular application 
so that the position of the coils generally coincides with peaks in curve 
167. 
In a magnetic lens, the image is rotated as it passes through the lens, as 
is illustrated in FIG. 1. This rotation complicates the normal imaging 
process of the lens. In addition, in a fixed-axis lens, the image of the 
deflection field will rotate about the fixed axis. This requires 
calculation of the effect of the rotation and corresponding adjustment to 
the deflection field applied to produce a desired result. 
When a curvilinear axis is applied to the beam deflection, according to the 
invention, no rotation of the deflection field about the system axis 
occurs; though the image or beam still rotates about the curvilinear axis. 
Thus, if an electron beam formed an image at the image plane and the beam 
was on the axis of the lens, it would be rotated by some amount relative 
to the object plane. If this beam was then deflected with a curvilinear 
axis, the image would still be rotated at the image plane, but the entire 
curvilinear axis would lie in a plane that also contains the z-axis and 
the deflected central ray of the beam. 
The fact that the variable axis of the lens follows the central ray of the 
beam makes application of the deflection fields and the curvilinear axis 
yoke fields straight forward, since the curvilinear axis lies in a 
vertical plane, and the deflection field and the variable axis-shifting 
field are mutually orthogonal. In the case where the beam travels in the 
x-z plane, for example, the variable axis correction field will be 
parallel to the X axis and the deflection will be parallel to the Y axis. 
The deflection field is applied to one axis of a yoke, and the curvilinear 
correction is applied to the other (yokes conventionally have coils that 
apply a field in two orthogonal directions in the x-y plane). In general, 
the deflection may be in a plane other than those containing the two field 
axes of the yoke. Thus the fields and therefore the currents to energize 
each axis of the yoke must be coupled according to the sine and cosine of 
the azimuthal angle of the deflected beam relative to the yoke axes. 
The function R(z) is determined by calculating the electron trajectory for 
a given deflection yoke arrangement, as is known in the art. It is not 
even necessary to include the lens field in this calculation since the 
central ray of the deflected beam is unaffected by the lens in a 
curvilinear axis lens. The lens axial field and its derivative is usually 
found by computer simulation of the lens. The curvilinear axis function 
Y(z) of line 167 can then be derived by multiplying the derivative of the 
lens field by R(z). This function would then be scaled proportional to the 
magnitude of the deflection currents in each axis of the deflection yoke, 
and then the curvilinear axis correction currents would be applied to the 
orthogonal axes of the deflection yoke or, alternatively, to separate 
correction yokes. 
In the example of FIG. 1, separate deflection yokes 205 and 207 and axis 
compensation yokes 250-1 to 250-n were shown for maximum clarity. A more 
compact arrangement can be effected by using single set of yokes to 
provide both axis compensation and deflection. In that case, the computer 
for the control means calculates the combined current to be applied to 
both the X-coils and the Y-coils in each of yokes 250-1 to 250-n to 
produce the required net field. 
This arrangement of a plurality of yokes distributed along the z-axis has 
the further advantage that a smaller current in each of a number of 
distributed coils will produce the same deflection as a larger current in 
a single coil or a few coils, so that the current can be reduced and/or 
the number of turns (and inductance) can be reduced, in order that the 
speed of the deflection response will be greater. In addition, the use of 
a number of smaller distributed deflection yokes permits a better 
approximation to the curve representing the compensation field. In a 
particular example, a lens designed for a 100 keV electron system had a 
total length along the z-axis of 600 mm and used 13 axis-shifting coils. 
Those skilled in the art will appreciate that electrostatic means can be 
used to produce the same result as magnetic means in many circumstances 
albeit with more difficulty, and the terms deflectors, compensation beam 
and the like are intended to include electrostatic deflection systems as 
well as magnetic ones. 
While the invention has been described in terms of a single preferred 
embodiment, those skilled in the art will recognize that the invention can 
be practiced with modification within the spirit and scope of the appended 
claims.