Note: Descriptions are shown in the official language in which they were submitted.
1 sackground of the Invention
The present invention relates -to electron beam columns
and more particularly to electron beam column apparatus and
methods in which size and shape of a beam may be effectively
varied during the operation of the electron beam column.
Electron beam columns have been adapted for use in sys
tems for the microfabrication of large-scale integrated
semiconductor circuits. For example, U.S. Patent No.
3,644,700, issued February 22, 1972, to Kruppa et al, des-
cribes an electron beam column adapted to form or "write"
selected pa-tterns on semiconductor wafers. Such columns
have particular utility in the writing of such patterns
on photoresists, i.e., exposing selected areas on photo~
resists ~Ihich are then developed to form the photoresist
masks extensively used in a wide variety of operations dur
ing integrated circui~ fabrication. The typical electron
beam columns utilized in connection with such integrated
circuit microfabrication applications generally include
an electron beam source, condenser lenses, alignment stages,
demagnification lens stages, a projection lens, a deflec-
tion unit, and a target area, arranged in well-known fashion.
Typical electron beam columns and components thereof are
further described in U.S. Patent No. 3,949,228, Ryan,
issued April 6, 1976, and U.S. Patent No. 3,984,678, Loeffler
et al, issued October 5, 1976. Typical optical systems and
components for such columns are further described in U.S.
Patent No. 3,930,181, Pfeiffer, issued December 30, 1975,
and in the publication, "New Imaging and Deflection Con-
cept for Probe-Forming Microfabrication Systems", H.~.
Pfeiffer, J. Vac. Sci Technol., November/December 1975,
Vol. 12, No. 6, pp. 1170-1173.
FI9-77-001 -2-
7~
1 The advantage of -the square shaped electron beam
over the more traditional Gaussian round beam has been
set forth in detail in the above Pfei~fer article in the
J. Vac. Sci. Technol. as well as in the above mentioned
U.S. Patents 3,644,700 and 3,949,223. As set forth in
these teachings, resolution and current density in elec-
tron optic systems are determined by the electron optical
configuration and are effectively independent of the size
of the target image. The Pfeiffer article indicates that
a beam of relatively uniform intensity having twenty-five
times the area of the usual Gaussian spot at approximately
the same edge dose gradient can be obtained by projecting
a square shaped beam onto a target.
This result will be summarized with respect to the com-
parison of the round beam and square shaped beam. The
comparison illustrates the advantage of a square beam over
a Gaussian round beam with identical resolution. The
shape and size of a Gaussian beam spot is compared to a
s~uare shaped beam spot and a graph beneath each beam spot
layout the intensity distribution, i.e., intensity is
plotted with respect to spot area. As set forth in the
Pfeiffer article, in a conventional round-beam system, the
intensity distribution half-width (d~ equals the spatial
resolution. The resolution of a Gaussian round beam is
determined by the superposition of all n aberration disks
S i plus the demagnified Gaussian source, which typically
equals the quadrature sum of the aberrations foroptimum
current density:
FI9-77-001 -3-
dGaussian ( ~ ~i aberrations ~ Gaussian2) ~/ .
i=l
To produce a true copy of the pattern, the half-width of
the spot has to be at least five times smaller than the
smallest element of the pattern. For the square spot, re-
solution is determined by the edge slope of the intensity
distribution, caused only by the superposition of all n
aberration disks:
dsquare ( ~ 1 ~i aberrations2) 1/-2
The Gaussian disk of the source does not contribute to the
edge shape. The size of the square spot is independent
of resolution and can be chosen to match the smallest seg-
ment of the pattern. This entire pattern segment is ex-
posed at once, thereby speeding up the exposure rate by
a factor of twenty-five over a comparable round-beam system.
Brief Description of the Drawings
FIG. 1 is a generalized diagrammatic view of the
electron beam shaping apparatus in accordance with the
present invention.
FIG. 2 is a simplified diagrammatic comparison showing
the arrangement and number of exposures required to expose
a given area with respectively a round beam and a square
shaped beam in comparison with the variable shaped beam
of the present invention. The figure also shows beam plots
of the current density or the round beam spot, the square
shaped beam spot and the variable beam spot, respectively.
FIG. 3 is a diagrammatic plan view of a portion o~ an
FI9-77-001 - 4 -
1 orthogonal rectilinear target pattern showing the exposure
steps required in order to fully expose the portion of
this pattern utllizing the prior art square beam of fixed
dimensions.
FIG. 4 is the same diagrammatic plan view of the ortho-
gonal rectilinear target pa-ttern modified to show the elec-
tron beam exposure steps required when exposing the same
portion of the pattern with a variable shaped beam in ac-
cordance with the present invention~
FIG. 5 is a schematic view of a prior art square shaped
electron beam apparatus showing a linked-beam trace of the
basic imaging concept.
FIG. 6 is a schematic view of the variable shaped
electron beam apparatus of the present invention showing an
equivalent linked-beam trace for the column operation when
there is no deflection.
FIG. 7 is the same diagrammatic view of FIG. 6 show-
ing the modification of the linked-beam trace when there
is deflection of the image of the first aperture with respect
to the second aperture.
FIG. 8 is a schematic diagrammatic view of deflection
apparatus which may be utilized to laterally deflect the
image of the first aperture during beam spot shaping.
FIG. 9 is the same view of the apparatus of FIG. 8
showing the operational modifications in moving the virtual
center of deflection into coincidence with the plane
of the focused source image.
It may be seen from FIG. 2 that using the square
shaped beam, rectilinear area 27 (i.e., an area defined by
straight lines) may be totally exposed by six stepped
exposures (1 through 6) with square beam 26 while the same
FI9-77-001 - 5 -
~ ~;B~
1 rectilinear area 27' would require in the order o~ one
hundred and Eorty stepped exposures with the round Gaus-
sian beam 25.
While present shaped or square beam systems provide
significant electron beam capability in the integrated
circuit fabrication field, it is foreseeable that in future
technologies wherein portions of the patterns to be ex-
posed may have dimensions below two microns, the application
of the shaped electron beam in forming such patterns may
be limited. In such dense integrated circuits haviny aper-
ture widths and/or line widths with smallest dimensions
below two microns, the "blooming effect'~ produced by double
and greater multiple exposures to which some pattern areas
may be subject when using shaped aperture apparatus may be
beyond the dimensional tolerances o~ the integrated circuits.
The problem of double exposure will be elaborated on in
greater detail with respect to FIGS. 3 and 4. ~owever, it
may be seen in its simplest form with respect to area 27
which is exposed by the square shaped beam in FIG. 2. ~tili-
zing a beam providing a square shaped spot 26, area 27 is
exposed by six stepped exposures. Since the selected area
to be exposed does not have dimensions which are integral
multiples of spot 26, a portion 28 (crosshatched) will
be double exposed. This double exposure results in the
known "blooming effect" wherein the double exposed areas,
for example, in the photoresist develop at a faster rate
than the normal singly exposed areas during development.
This produces undercutting and edge irregularit~ in the re-
maining photoresist defining the exposed area. With -the
present integrated circuits having line widths and apertures
FI9-77-001 ~ 6 -
7~
1 with smallest pattern dimensi~ns o~ at least two mlcrons,
this "blooming effect" is within dimensional tolerances
and presents no problems. However, with the denser, more
advanced integrated circui-ts having lateral dimensions be-
low two microns, the "bloomlng effect" may produce dimen-
sional irregularities beyond the lateral tolerances.
In addition, in advanced integrated circuit technology,
it would be highly desirable if the time required for
electron beam exposure of selected patterns could be reduced
10 thereby increasing the integrated circuit fabrication through-
put.
Summary of the Invention
Accordingly, it is a primary object of the present
invention to provide a method and apparatus for exposing
selected patterns to an electron beam wherein the areas
subject to multiple exposure are minimized.
It is another object of the present invention to pro-
vide a method and apparatus for exposing selected patterns
of substrate areas to an electron beam wherein the speed
20 of the operation is increased while the areas subjected to
multiple exposure are minimized.
It is another object of the present invention to pro-
vide electron beam apparatus in which the size and the
shape of the beam spot applied to the target is readily vari-
able.
It is yet a further object of the present invention
to provide a method and apparatus for varying the size and
shape of electron beam spot at a target while matntaining
a constant beam current density at said target.
FI9-77-001 - 7 -
1 The present invention provi.des apparatus which forms
a variable shaped rectilinear beam spot, preferably a
rectangular beam spot in which the orthogonal lateral di-
mensions may be varied. With such a variable spot, the
problem of multiple exposure and the resultant "blooming
effect" may be completely avoided. Since the dimension of
the rectangular spot to which the selected target pattern
is exposed is thus variable, the dimensions of the spot at
each exposure step may be varied in order to have the maxi-
10 mum dimensions within the limi~s of the portion of the recti-
linear pattern being e~posed during said step while only
abutting but not overlapping the portion of said pattern
exposed in a previous step.
The electron beam shaping apparatus in accordance with
the present invention is provided in electron beam appara-
tus having a source of electrons and a target area or
plane toward which the electrons are directed. I'he elec-
tron beam shaping or forming means positioned along the
path from the source to the target area comprise a first
20 beam shaping member having a first spot shaping aperture
formed therein J a second beam shaping member having a second
spot shaping aperture formed therein and means for focusing
the image of the first aperture in the plane of the second
spot shaping aperture to thereby form a composite spot
shape defined by said image and said second aperture. Means
are further provided for focusing the image of the compo-
site spot in the target area. The apertures are preferably
rectangular in shape so that the compo~ite spot shape w:ill
also be rectangular.
In accordance with a more particular aspect of the
invention, the apparatus further includes means for defle~t-
FI9-77-001 8
~ ;~ B ~
1 ing the image of the first aperture laterally with respect
to the second aperture and to thereby provide the capabllity
of varying the shape and dimensions of the composite spot.
A more specific aspect of the present invention includes
means for ensuring that the image of the source in no way
interferes with the composite spot shape defined by the
combination of the second aperture and the image of the
first aperture. This is accomplished by focusiny the image
of the source in a plane which is outside of the depth of
focus o~ said first aperture image.
In accordance with another more specific aspect of the
present invention, the means for deflecting the image of the
first aperture laterally are electrostatic deflection means
positioned along the beam path between the first and second
apertures.
Further1 in accordance with a particularly significant
aspect of the present invention, means are provided for en
suring that during the lateral deflection of the first aper-
ture image with respect to the second aperture that all
images of the source remain at a fixed location along the
identical axial path of the beam. In order for the electron
beam apparatus to function effect:ively, the shaped beam
path below the second aperture must remain aligned with re-
spect to the demagnification lens system and the projection
lens system which serve to focus the image of the composite
spot produced by the combined apertures on the target. If
the beam path does not remain thus constantly aligned, the
beam path may move to positions within the demagniEication and
projection lens systems which are oEf cen-ter. Since only
FI9~77-001 - 9 -
1 the central portions of such lens systems have sufficient
quality to provide optimum focus of the beam, any movement
of the beam path away from such optimum posi-tions will de-
grade the edge resolution of the focused spot. Furthermore,
such constant alignment of the beam path is important where
the electron beam apparatus utilizes a circular aperture
plate along the beam path below the second aperture. Such
a plate is standard in some electron beam apparatus such
as that described in U.S. Patent 3,644,700 (Plate 20, FIG. 1),
where it serves to restrict beam fringes so that only elec-
trons passing through the center of the demagnification
lenses are used and spot distortion is thus minimized.
When such a round aperture is used, if the beam path is not
in constant alignment with respect to this aperture, such
a round aperture may intercept a portion of the beam which
will reduce the effective aperture angle and hence the cur-
rent density.
In order to maintain the constant beam path when the
image of the first aperture is being deflected laterally,
means are provided for focusing the image of the source
in a plane between the first and second shaping apertures
and for bringing the virtual center of said deflection in-
to coincidence with the plane of the focused source image
source.
The foregoing and other objects, features and advan-
tages of the invention will be apparent from the following
more particular description of the preferred embodimen-ts
of the invention, as illustrated in the accompanying draw-
ings.
FI9-77-001 - 10 -
3 ~6~7~6
l Description of the Preferred Embodiments
_
With reference to FIG. 1, the apparatus for shaping
the beam spot will be described in general. An electron
source 10 directs a beam of electrons 11 along an a~is 12
in an electron beam column toward a target which is not
shown. The beam ls shaped into a variable rectangular
spot by first passing the beam through square aperture
14 in shaping member 13. Condenser lens 15 simultaneously
focuses the image 14' in the plane of square aperture 16
of shaping member 17 and focuses the image of the source
at a position 18 in a plane coincident with the center of
deflection provided by deflection means l9 which move the
focused image 14' of the first aperture 14 laterally with
respect to aperture 16. In the embodiment of FIG. 1, de-
flecting means 19 are conventional electrostatic deflection
plates with plates 20 and 20' acting to deflect image ].4'
in the X direction while plates 21, 21' act to deflect
image 14' in the Y direction. For optimum results, image
14' has the same dimensions and shape as aperture 16. The
final beam spot shape is determined by that portion 22 of
image 14' which is not blocked by the plate of shaping
member 17 and passes through aperture 16 as shaped com-
posite image 23. While the operation has been illustrated
with respect to a deflection in the X direction, it will,
of course, be understood that with various combinations of
deflections of image 14' in the X and Y direction a wide
variety of rectangular shapes may be achieved for compo-
site spot 23.
The variable spot shaping apparatus shown in general
in FIG. 1 may be used in combination with standard electron
beam columns such as those described in the above mention-
ed Pfeiffer article in the J. Vac. Sci. Technol. or in the
FI9-77-001 - 11 -
7B~
1 Kruppa et al patent, 3,644,700. When used in such columns,
the composite spot 23 may then be passed through ~emagni-
fication lenses and projection lenses to project the imaye
of spot 23 onto a target. Also, the beam may be deflected
in the conventional manner for scanning purposes wi-th re-
spect to such targets. Standard demagnification, projec-
tion and deflection equipment such as that described in
U.S. patent 3,644,700 and the Pfeiffer et al article may
be used for this purpose.
One important aspect of the apparatus of FIG. 1 which
will be discussed in greater detail subsequently with
respect to FIGS. 6 - 9 i5 the optical separation of the
image of source 10 from the image of aperture 14. The
image 14' of the aperture is focused in the plane of aper-
ture 16 by condenser lens 15 while the same lens focuses
the image of source 10 in a plane near the center of de-
flection produced by deflection means 19. There will sub-
sequently be described with respect to FIGS. 6 - 9 how
the plane of the image 18 of source 10 is brought into co-
incidence with the center of deflection. In any event,
by such optical separation of the focused images, the il-
lumination, i.e., current density of the beam spot at
the target, which is dependent on the position of the source
image remains constant. This constant illumination is en-
sured because the source image is not deflected and re-
mains essentially aligned with the electron beam column
axis. This alignment also provides for maximum edge re-
solution of the beam spot at the target after conventional
demagnification and projection stages.
While the aperture shaping expedient of the present
FI9-77-001 - 12 ~
7~
l invention is being described with respec-t to conventional
electron beam apparatus using raster scanning of the beam
with respect to the target, it should be clear that the
expedient is equally applicable to apparatus employing
other modes of scanning, e.g., vector scanning.
With such a variable shaped beam, the multiple ex-
posure and consequent "blooming problems" are clearly
avoidable. This may be simply understood again with re-
ference to FIG. 2. The overlapped region 28 resulting
from the use of the square shaped beam is avoided when
the vaxiable shaped beam is used to fill in the illustra-
tive area 27" which is equivalent to area 27. The expo-
sure is achieved in two steps: exposure by a first shaped
beam 29 followed by an exposure second shaped beam 30 which
abuts the area exposed by beam 29. ~s may be seen from
the accompanying profile of current density, there is no
overlap which could lead to increased exposure or "blooming",
i.e,, total current density at interface 31 between the
two exposed areas does not exceed the current density or
intensity of illumination of the beam spot which remains
constant irrespective of the shape of the beam.
It should be noted from the beam profiles in FIG. 2
that the resolution of the variable shaped beam remains
substantially constant. Resolution is described in the
above mentioned Pfeiffer article in J. Vac. Sci. Technol.
as the half-width of the intensity distribution (d) of a
round or Gaussian beam having the selected intensity, i.e.,
current density. The Pfeiffer article further indicates
that the square shaped beam will have the same resolution
as the round beam provided that the edge slope of the square
shaped beam equals the half-width (d) of the round beam.
FI9-77-001 - 13 -
~ ~6~
l We have further found with respect to the variable shaped
beam that so long as the smallest dimension of the pat-
tern being subjected to stepped exposure by the variable
shaped beam is at least five times the resolution (d) of
the round shaped beam of the selected intensity, then the
pattern will be subject to the same resolution as the one
exposed by a plurality of round shaped beams. For example,
with reference to FIG. 2, although beam spot 30 has one
dimension of only 4d, the total dimension of the pat-tern
is 14d. Thus, it will be subject to substantially the
same resolution as pattern 27' which is exposed to the round
beam.
The advantages of the variable shaped beam both with
respect to the elimination of multiple exposure and in-
creased speeds for exposure, i.e., increased throughput,
will be more readily understood with reference to FIGS. 3
and 4. Section 32 in FIG. 3 is the layout of an ortho-
gonal rectilinear pattern to be exposed by an electron beam
column. The pattern is to be formed in a photoresist, for
example, and is to define metallization lines. I~ pattern
element 32 were to be exposed by a square beam in accor-
dance with conventional practice, this square beam would
be limited to a spot 33 having a width, W, which is no
greater than the minimum dimension of the entire pattern
being exposed. With such a square beam spot 33, it would
require twenty-seven stepped exposures to completely ex-
pose portion 32 of the pattern. Because of the geometric
limitations, areas 34 which are crosshatched would be
doubly exposed and at least one region 35 would be quad-
ruply exposed. Such multiple-exposed areas would be sub-
ject to "blooming effects".
On the other hand, with reference to FIG. 4, if the
FI9-77-001 - l~ -
1 portlon of the rectilinear pattern 42 which is equiva-
lent in shape and dimensions to pattern portion 32 in
FIG. 3 is exposed to a varlable shaped beam formed in
accordance with the present invention, then only eight
exposure steps are necessary to expose the total pat-
tern portion. Five difEerent beam shapes would be in-
volved. The first beam shape would be used in exposing
areas 43, 43A; a second shape for areas 43s and 43C;
a third shaped spot to expose area 44; a fourth shaped
spot to expose area 45, and a fifth shaped spot used to
expose area 46. The total exposure of pattern portion
42 is accomplished without any overlap; each of the step-
ped exposed regions abuts the adjacent ,region. There
is no need for any exposed region to overlap the ad
jacent exposed region. In addition, the total exposure
for the same pattern portion is accomplished by the
shaped beam apparatus of the present invention in one-
fourth as many stepped exposures as with the fixed square
shaped aperture. Thus, the total time for exposure of
substrate patterns in integrated circuit fabrication
would be substantially reduced, thereby greatly enhancing
throughput.
The electron beam shaping apparatus o~ the present
invention will be more clearly understood, particular-
ly with respect to the applicat.ion of principle of op-
tical separati.on during the beam shaping operation, when
reference is made to FIGS. 5, 6 and 7. FIG. 5 which is
FI9-77-001 ~ 15
a diagrammatic vlew of a conventional electron beam colurnn
as described in the above mentioned Pfeiffer article in the
J Vac Sci. Technol. com~rises an electron source 50, a
condenser lens 51, a square spot shaping aperture 52,
blanking plates 53 which operate in the manner described in
U.S. Patent No. 3,644,700, a demagnification sys-tem
comprising a first demagnification lens 54 and a second
demagnification lens 55. The apparatus further includes a
circular aperture plate 56 for defining the axial portions
of the beam which have the maximum intensity in the manner
described in U.S. Patent 3,644,700 as well as a standard
projection lens 57 having a central deflection yoke 58 and
standard dynamic elements 49 for correcting field curvature
and axial and deflection astigmatismO This projection lens
and yoke are a standard structure as described in detail in
the Pfeiffer article. They may also have the structure
described in the above identified U.S. Patent 3,930,181 or
3,98~,678.
The column directs a beam at a target 59, e.g.,
photoresist covered semiconductor wafer on which a
rectilinear pattern is to be exposed.
The linked-beam trace shown in FIG. 5 is the same as
that shown in the above mentioned Pfeiffer article. The
design of the column in FIG. 5 optimizes the beam current
density, i.e., intensity distribution, simultaneously wi-th
the square shaping of the spot by implementing the
linked-beam imaging concept of A. Koehler, Z. Wiss.
Mikroskopie, 10, 433 (1893), further
FI9-77-001 -16-
.
1 described in H.C~ Pfeiffer and K. H. I,oeffler, Pro-
ceedings of the 7th Internation l Conference on Elec-_
tron Microscop~, Grenoble ~1970), p. 63. As the
drawn linked-beam trace shows, condenser lens 51 im-
ages the source 50 into the entrance pupil of the
first demagnification lens 54 of the demagnification
sectio~, to thereby provide the most efficient and
uniform illumination for the beam, i.e., constant cur-
rent density. (The trace of -the source beam is indi~
cated by the widely spaced cross lines while the pro-
jection of the image of square aperture 52 is indica-
ted by the narrow spaced cross lines.) Projection lens
57 generates the electron beam spot 60 by projecting
the image of aperture 52 demagnified by demagnifica-
tion lenses 54 and 55 onto the target wafer 59.
In the prior art structure of FI~. 5, the con-
denser lens 51 and the projection lens 57 are the
critical elements of the respective square aperture
image and source beam tracing; demagnification lenses
54, 55 establish the link between the condenser lens
51 and the projection lens 57. The square aperture 61
which is being imaged to form the beam spot is formed
in a thin metal plate 52. In the apparatus shown, the
image of square aperture 61 is demagnified in two steps
through lenses 54, 55. For example, with an aperture
61 in the order of 400 microns square, the demagnifi-
cation system will reduce to a final beam spot about
2.5 microns square. While the image of aperture 61 is
FI9-77-001 - 17 -
1 ~ 7 ~ ~
l ~eing thus demagnified, the first demagnification lens
54 simultaneously creates a magnified image of the
source in the plane of circular aperture 63 centered
about the electron beam column axis 62. The second
demagnification lens 55 images the round aperture 63
at the center 64 of the projection lens and defines
the semi-angle of convergence. Thus, uniform beam
current density is provided since circular aperture
63 admits only the central or axial portion of the
source beam trace which minimizes aberrationsO For
a given round aperture size, the second demagnifi-
cation lens determines the final beam convergence
angle and consequently the required brightness, e.g.,
about 3 x 105 A/cm2-sterad is necessary to achieve
a target current of 3 microamps. The final or pro-
jection lens provides the required working distance
deflection yoke 58 in order that the beam may be de-
flected over the target field to be exposed which
is in the order of 5 millimeters square. The beam
is deflected by a conventional deflection yoke. Dur-
ing the writing of patterns by a stepped electron
beam, the intensity of the beam spot 60 may be modula-ted
by electrostatic beam blanking plates 53 which oper-
ate essentially in the manner described in Patent 3,644,700.
Referring now to FIG. 6, the operation of the
beam shaping apparatus of the present invention will
be described by showing the trace of the .I.inked-beam
concept by which optimum optical separation of the
source beam from the shaping aperture image is achieved.
FI9-77-001 - 18 -
~ ~$~
1 As will be seen, such separation is hiyhly desirable in
that it permits a uniform current density irrespective
of the shape of the spot. In the column of FIG. 6,
the lower portion of the apparatus has subs-tantially the
same structure and operational characteristics as the
apparatus described in FIG. 5. First and second de-
magnification lenses 64 and 65 are respectively equiv-
alent to demagnification lenses 54 and 55 in FIG. 5.
Round aperture plate 66 is substantially the same as
round aperture plate 56 in FIG. 5. Projection lens
67 performs the same function as lens 57 in FIG. 5,
deflection yoke 68 performs the equivalent function
of deflection yoke 58 in E'IG. 5, and dynamic correc-
tion elements 98 pexform the same functions as elements
49 in FIG. 5. Target 69 is a photoresist wafer on
which a rectilinear pattern is to be formed. Also,
blanking plates 63 in the column of FIG. 6 performs
the conventional blanking function as described with
respect to plates 53 in FIG. 5.
In performing the beam shaping function of the
present invention which was previously described in
general with respect to FIG. 1, source 70 directs a
beam of electrons along the axis of the electron beam
column 71. Following the imaging concept used in FIG.
5 and in the Pfeiffer article in the J. ac. Sci. Technol.,
the trace of the source imaging is indicated by the
widely spaced cross lines wh:ile the trace of the first
aperture image and of the composite image formed by
the first and second apertures is shown by the narrow
FI9-77-001 - 19 ~
7t~
1 spaced cross lines. The beam is shaped into a variable
rectangular spot by first passing the beam throuyh a
square aperture 72 formed in plate 73. con~enser lens
74 which may suitably be a magnetic lens of convention-
al design in the electron beam art, performs two func-
tions. It focuses an image of aperture 72 in the plane
of square aperture 75 formed in plate 76. In addition,
condenser lens 74 focuses the image 77 of source 70 at
a point along the axis and in the center of the image
deflection means provided by electrostatic plates 78 and
78'. This pair of plates has the capability of deflect-
ing the focused image 79 of the first square aperture
72 with respect to second square aperture 75 during the
beam shaping operation. There is, of course, a second
pair of plates not shown in FIG. 6 but shown in FIG. 1
which act to deflect the beam laterally in the other
orthogonal direction during the shaping operation. The
deflection of the image 79 of the first aperture with
respect to the second aperture 75 is shown in FIG. 7.
For the optimum operation of the electron beam column
of the present invention, the focused image 77 of source
70 must be at the virtual center of deflection of the
deflection means provided by electrostatic plates 7' and
78' as well as the corresponding pair of plates for
deflection in the other orthogonal direction. The fo-
cal length of lens 74 is determined primarily to focus
the aperture image 7g in the plane of aperture 75.
Therefore, the attendant focusing of source image 77
wil] not necessarily occur at the center of deflection.
FI9-77-001 - 20 -
B 6
1 While it may be possible to move deflection plates 78
and 78' along the axis of the column to position
source image 77 at the center o~ deflection, this is
not considered to be very practical. There will sub-
sequently be described with respect to FIGS. 8 and 9
a suitable expedient for movin~ the center of de-
flection of an electrostatic deflection system into
coincidence with the plane of the focused image 77 of
the source without physically moving plates 78 and 78'.
The significance of having the source image 77
at the center of deflection becomes apparent when one
consider.s the action of condenser lens 80 which may
be any standard magnetic condenser lens within which
the second aperture plate 76 is disposed. Condenser
lens 80 acts to image source image 77 into the en-
trance pupil of the first demagnification lens 64 in
much the same manner that condenser lens 51 in FIG. 5
acted to image the source itself, 50, into the en-
trance pupil of first demagnification lens 54. When a
voltage differential is applied between electrostatic
plates 78 and 78' in order to deflect the image 79 of
the first aperture with respect to the second aperture
75 as shown in FIG. 7 (FIG. 7 is a schematic of the
linked-beam trace of the column of FIG. 6 showing the
trace where the deflection apparatus operates to de
flect), source image 77 is not deflected; it remains
stationary because it is at the center of deflection.
Consequently, irrespective of the deflection of aper~
ture image 79 in the X or Y direction, focused source
image 77 remains stationary, and the image of source
FI9-77-001 - 21 -
~ ~67~
1 image 77 projected by condenser lens 80 into the en-
trance pupil of first demagnification lens 64 will
remain constant in posi-tion at the axis of column 71.
The demagnification system and projection sys-
tem of the column in FIG. ~ ac~ on the composite image
determined by laterally deflected image 79 and aper-
ture 75 in essentially the same manner that the demag-
nification and projection system of FIG. 5 acted upon
the image of square aperture 61. Thus, in the columns
of FIGS. 6 and 7, the composite image is demagnified
in two steps through demagnification lenses 64 and 65.
(In FIG. 6, the composite image is identical with the
second aperture 75). While the composite image is
being thus demagnified, first demagnification lens 64
simultaneously creates a magnified image of the source
in the plane of circular aperture 81. This image of
the source is, of course, dapendent on the position of
source image 77. Since source image 77 remains sta-
tionary irrespective of the deflection in forming the
composite aperture image, focused image 82 of the source
remains centered about the column axis at aperture 81.
Thus, substantially uniform current density is pro-
vided by circular aperture 81 which admits only the
central or axial portion o~ the Gaussian source being
traced and minimizes aberrations generated in the final
lens.
FI9-77-001 - 22 -
B 8 '~
"~
1 The second demagnification lens 65 and projection
lens 67 operate in the same fashion as lenses 55 and
57 in FIG. 5 in accomplishing this. Likewise, deflec-
tion yoke 68 provides for ~he deflection of the com-
posite beam spot 83 across the tar~et field in the man-
ner previously described with respect to yoke 5~ of
FIG. 5. In addition, since the imaye of the source pro
jected upon the entrance pupil of demagnification lens
64, will be centered about the axis irrespective of the
deflection, only the central portions of the lenses in
the demagnification system and of the projection lens
will be primarily utilized. Thus, degradation of beam
spot edge resolution which would result if the source
image projected upon demagnification and projection
lenses were off center is avoided. It should be noted
that this latter effect is significant in systems which
do not utilize a round beam forming aperture like aper-
ture 81.
In this connection, it should be noted that in elec-
tron beam columns, it is possible to eliminate physical
apertures like round aperture 81 which restricts the beam
diameter in the demagnification and projection stages
of the column by using properly scaled images of the
source itsel~. However, the scaled image approach has
its shortcomings in that there is non-uniform current
density distribution of the source which Jeads to higher
aberrations f~r the same total beam current or bright-
ness.
Referring now to FIGS. 8 and 9, appara-tus in accor-
dance with the present invention associated with the
FI9-77-001 - 23 -
1 deflection means is provided for moviny the center of
deflection into coincldence with the plane in which
the source image is focused. The apparatus will be
described with respect to one pair of electrostatic
deflection plates 88 and 88'. However, it should be
understood that the same adjustment may be made with
the pair of electrostatic deflection plates which move
the aperture laterally in the other direction. The
movement of the center of deflection into coincidence
with the plane in which the image of the source is foc-
used is customarily carried out before the apparatus is
actually in operation. It may be conveniently accom-
plished during the calibration period that is described
in U.S. Patent 3,644,700 for an elec-tron beam column~
Once the center of deflection is adjusted into coinci-
dence with the focused source :image, the further modi-
fication should usually be unnecessary during the opera-
tion of the electron beam column irrespective of the
number or nature of lateral deflections carried out to
change the beam spot shape during the normal operation.
With reference first to FIG. 8, the voltage dif-
ferential between plates 88 and 88' is provided by the
eonventional push-pull circuit through which deflec-
tion is achieved by applying a signal to amplifier 84,
the output of which is applied to plate 88' and to
amplifier 85. The level and the sense (positive or
negative) of the signal will determine the extent of
deflection. The output of amplifier 85 is, in turn, ap-
plied to plate 88 to provide a push-pull eircuit in
~I9-77-001 - 24 -
7~;
1 which the voltage level of plate 88 swings negative when
plate 88' is positive and vice versa. This portion of
the structure represents a conventional arrangement for
developing the voltage dif-~erential between a pair of
electrostatic plates. In addition, the present struc-
ture comprises a pair of auxiliary plates 89 and 89'.
Au~iliary plates 89 and ~9' are each connected to the
outputs of amplifiers 84 and 85 respectively through
the variable resistors 90 and 90'. Thus, let us con-
sider the initial condition, shown in FIG. 8 when a differential voltage is applied between primary plates 88
and 88'. Since the contacts 91 and 91' respectively
between plates ~9 and 89' and balanced resistors 90 and
90' are centered with respect to such resistors, plates
89 and 89' will be at the identical voltage level (half-
way between voltage levels of plates 88 and 88'), and
the deflection of the path of the first aperture image
diagrammatically represented by line 92 will be deflec-
ted as shown. Also, as the beam trace 93 of the source
image indicates, the source image will be focused at point
94. The virtual center of deflection 95 of the appara-
tus is determined b~ the intercept of the extension 96
of the beam path as deflected and axis 97 which was the
original path of beam 92 before deflection.
Because it is desired that the center of deflection
be moved into coincidence with the focused beam at 94,
the apparatus will be adjusted as indicated in FIG. 9 to
bring such coincidence about. Since the center of de-
FI9-77-001 - 25 -
I ~B7~G~;
1 flection 95 produced by voltage drop between primary
plates 88 and 88' is above ~ocused ima~e 94 of the
source, the center of de~lection 95 is moved down by
applying a potential dif~erence between auxiliary
plates 89 and 89' in the same sense as the potential
difference between primary plates 88 and 88', i.e., if
primary plate 88' has a positive voltage with respect
to plate 89. This is accomplished by moving variable
resistor contact gl' as indicated in FIG. 9 so
that the portion of var.iable resistor 90' between plates
89' and 88' is reduced to move the voltage level of
plate 89' toward that of plate 88'. Similarly, vari-
able resistor contact 91 is moved to the position in-
dicated to reduce the portion of variable resistor 90
between plate 89 and plate 88 whereby voltage level
on plate 89 approaches that of plate 88. This shifts
the path 92 of the aperture image as shown whereby
extension 96 crosses a~is 97 as shown to produce a vir-
tual center of deflection 95 in coincidence with focus
source image 94.
Conversely, if it is desired to move the center of
deflection 95 upward, then variable resistor contact 91
is moved to increase the resistance between auxiliary
plate 89' and primary plate 88' and to thus diminish
the resistance between auxiliary plate 89' and primary
plate 88, At the same time, variable resistor contact
91 may be moved to increase the resistance between au~
iary plate 89 and primary plate 88, and to thus diminish
the resistance between auxiliary plate 89 and primary
plate 88'. As a result, a voltage drop is produced be~
tween auxiliary plates 89 and 89' which i5 in the opposite
FI9-77-001 - 26 -
) ~6~7~
1 sense to the voltage drop between -the primary plates.
This l~as -the ef~ect of bucking or opposing the de-
flection action of the primary plates and thereby
moving the center of deflection upward.
In suitable operating conditions, for the de-
flection apparatus of FIGS. ~ and 9, the voltage
drop which may be applied between auxiliary plates
89 and 89' may be in the order of about ten per cent
of the voltage drop across the primary plates, e.g.,
when the voltage swing between the primary plates is
in the order of about twenty volts, the voltage swing
between the auxiliary plates would be in the order
of about two volts.
While the description in FIGS. 8 and 9 was di-
rected to moving the center of deflectio~ with re-
spect to means for deflecting the aperture image in
one lateral direction, similar center of deflection
adjusting apparatus may be used in connection with
the electrostatic deflection plates which deflect the
beam in the other lateral direction.
As a practical matter, the movement of the center
of deflection into coincidence with the focused image
of the source is accomplished during electron beam
calibration by first measuring the current density of
the shaped beam spot at the target using any standard
measurement technique under conditions where there is
no lateral deflection of the first aperture image with
respect to the second beam shaping aperture. Then,
if the center of deflection is coincident with the
source image, the current density will remain constant
FI9-77-001 27 -
7 ~ ~;
1 irrespective of the lateral deflection of the Eirst
aperture image in the X and Y directions. Accordiny-
ly, after the ini-tial reading as to current density,
the first aperture image is de~lected in the X and/or
Y direction and the auxiliary plates are "tuned'l by
moviny variable resistor contacts 90 and 91' until the
constant current density at the initial level is a-
chieved. This indicates that the center o deflection
of the deflection means is in coincidence with the
source image. Once this coincidence is achieved by
the initial "tuning'l, the current should remain con-
stant thereafter. No additional changes should be
necessary during the operation of the electron beam
column when the beam shape is changed from step to step.
Thus, the apparatus of the present invention, provides
for rapid change in beam aperture size and shape during
the electron beam column operation to effectively ex-
pose rectilinear regions in a target without any multiple
exposure or exposure overlap and with increased through-
put.
~ lthough electrostatic deflection apparatus has beendisclosed for the preferred embodiment as the means for
deflecting the image of the first aperture and for moving
the center of deflection, it will be clear that other
deflection apparatus such as magnetic deflection appara
tus may be used for the same purpose.
While the invention has been particularly shown and
described with reference to the preferred embodiments
thereof, it will be understood by those s]~illed in the
art that ~arious changes in form and details may be made
therein without departing from the spirit and scope of
the invention.
FI9-77-001 - 28 -
