Variable-spot scanning in an electron beam exposure system

An attractive high-throughput technique for writing microcircuit patterns with a scanning electron spot of variable size is described in application Ser. No. 855,608, filed Nov. 29, 1977. In such an electron beam exposure system, two spaced-apart apertured mask plates with a deflector therebetween are included in the electron column of the system. As described herein, a third apertured mask plate and an associated deflector are serially added to the aforenoted components in the column. In this way, the throughput and other performance characteristics of such a system are significantly enhanced.

TECHNICAL FIELD 
This invention relates to an apparatus and a method for fabricating 
microminiature devices and, more particularly, to a variable-spot scanning 
technique for use in an electron beam exposure system designed for 
fabricating large-scale-integrated devices. 
BACKGROUND ART 
It is known to increase the pattern-writing speed of an electron beam 
exposure system (EBES) by varying the writing spot dimensions of the 
electron beam during the process of scanning the beam over the surface of 
a resistcoated workpiece. Such a variable-spot scanning technique is 
described in a commonly assigned copending application, Ser. No. 855,608, 
filed Nov. 29, 1977. In one specific illustrative mode of operation of the 
described system, two spaced-apart mask plates in the electron column of 
the system contain respectively different apertures therethrough. By 
interposing a high-speed deflector between the mask plates, it is feasible 
to rapidly deflect the image of the first electron-beam-illuminated 
aperture thereby to alter the portion of the second aperture that is 
illuminated by the beam. In turn, the beam propagated through the second 
aperture is demagnified to form a variable-size writing spot on the 
surface of a resist-coated workpiece. 
In one particular embodiment described in the aforecited copending 
application, four scan lines at a time are traversed by the beam in a 
raster mode of operation. At each address position along such a 
four-line-at-a-time scan, any specified one of sixteen different 
combinations (each comprising zero through four electron spots) is formed 
by the mask plate apertures and transmitted to the workpiece surface. In 
that way, the pattern-writing speed of an EBES system is significantly 
increased. 
Considerable interest exists on the part of workers in the microelectronics 
field in trying to still further increase the throughput capabilities of 
an EBES-type machine. In attempting to achieve this goal with the 
aforedescribed two-mask-plate system, the swath width or the number of 
scan lines traversed during one scan may be increased. But, as this width 
is increased, the size and complexity of the aperture configurations in 
the mask plates tend as a practical matter to become excessive and the 
design of the illumination system becomes more complicated. Moreover, as 
this width is increased, the number of address positions over which the 
image of the first aperture must be deflected to achieve different 
variable-spot combinations also becomes excessive. In turn, this 
complicates the deflection system design and entails longer deflection 
times, which are undesirable because they impose a limitation on the 
overall operating speed of the system. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is an improved scanning 
technique for an electron beam exposure system. 
More specifically, an object of this invention is a variable-spot scanning 
technique that improves the throughput and other operating characteristics 
of an electron beam lithographic system. 
Briefly, these and other objects of the present invention are realized in a 
specific illustrative electron column designed for variable-spot scanning. 
Such a column comprises three spaced-apart apertured mask plates. A first 
high-speed deflector is interposed between the first and second mask 
plates, and a second high-speed deflector is positioned between the second 
and third plates. The aperture in the first plate is fully illuminated 
with an electron beam. The image of that illuminated aperture is then 
selectively deflected with respect to the aperture in the second plate. In 
turn, the image propagated through the second plate is selectively 
deflected with respect to the aperture in the third plate. Finally, the 
resulting image transmitted through the third plate is demagnified to form 
a writing spot on the surface of a workpiece. In that way, a variable-size 
electron spot is generated in a high-speed manner to facilitate precision 
patterning of an electron-sensitive medium.

DETAILED DESCRIPTION 
FIG. 1 depicts a specific illustrative lithographic apparatus for 
controllably moving a variable-size electron spot to any designated 
position on the top surface of an electron-sensitive layer 10 supported on 
a substrate 12. In turn, the substrate 12 is mounted on a conventional 
x-y-movable table 16. 
Various positive and negative electron-sensitive materials suitable for use 
as the layer 10 are well known in the art. By selectively scanning the 
electron spot over the surface of the layer 10 in a highly accurate and 
high-speed manner, it is possible to make integrated circuit masks or to 
write directly on a coated wafer to fabricate extremely small and precise 
low-cost microminiature devices. Some suitable electron-sensitive 
materials for use as the layer 10 are described, for example, in a 
two-part article by L. F. Thompson entitled "Design of Polymer Resists for 
Electron Lithography", Solid State Technology, part 1: July 1974, pages 
27-30; parts 2: August 1974, pages 41-46. 
The electron beam apparatus of FIG. 1 may be considered to comprise two 
main constituents. One is the column itself and the other is equipment 14 
connected to the column for controlling the operation of various elements 
in the column. The column is characterized by highly accurate high-speed 
deflection and blanking capabilities generally similar to those exhibited 
by the columns described in U.S. Pat. No. 3,801,792, issued Apr. 2, 1974 
to L. H. Lin, in U.S. Pat. No. 3,900,737, issued Aug. 19, 1975 to R. J. 
Collier and D. R. Herriott, and in the aforecited copending application. 
But, in accordance with the principles of the present invention, the 
column depicted in FIG. 1 is further characterized by a variable-spot-size 
scanning capability that is a significant and unique extension of the 
technique described in the Collier-Thomson application. This extended 
capability in particular will be described in detail below. 
The other main constituent of the FIG. 1 apparatus comprises control 
equipment 14. Illustratively, the equipment 14 is of the type described in 
the aforecited Collier-Herriott patent. The equipment 14 supplies 
electrical signals to the described column to systematically control 
deflecting, scanning and blanking of the electron beam. Moreover, the 
equipment 14 supplies control signals to the x-y table 16 to mechanically 
move the work surface 10 during the electron beam scanning operation, in a 
manner now well known in the art. 
The specific illustrative electron column of FIG. 1 includes a conventional 
electron source 18. For example, the source 18 comprises a standard 
lanthanum boride electron emitter. In the immediate downstream vicinity of 
the source 18, the trajectories of electrons emanating from the source 18 
go through a so-called crossover or source image point 20 which, for 
example, is about 50 micrometers in diameter. Thereafter the electron 
paths successively diverge and converge as the electrons travel downstream 
along longitudinal axis 22 toward the work surface 10. 
Illustratively, the electron column of FIG. 1 includes standard coils 24 by 
means of which the electron trajectories emanating from the crossover 
point 20 may be exactly centered with respect to the longitudinal axis 22. 
Thereafter the electron beam is directed at a mask plate 26 which contains 
a precisely formed aperture 28 therethrough. (Actually, in the first of 
several specific illustrative embodiments to be described hereinbelow, the 
aperture 28 comprises four separate spaced-apart apertures. In other 
embodiments, however, the aperture 28 comprises only a single opening. 
Thus, the schematic depiction in FIG. 1 of the mask plate 26 is to be 
regarded as a general representation of a plate having either a single or 
multiple apertures therethrough.) The beam is designed to uniformly 
illuminate the full extent of the opening or aperture 28 in the plate 26 
and to appear on the immediate downstream side of the plate 26 with a 
cross-sectional area that corresponds exactly to the configuration of the 
aperture 28. 
By way of example only, the mask plate 26 of FIG. 1 is shown mounted on and 
forming an integral unit with an electromagnetic field lens 30. Inclusion 
of the lens 30 in the FIG. 1 column is not always necessary. And, even 
when included, the lens 20 may if desired be separate and distinct from 
the plate 26. If included, the lens 30 is not usually designed to magnify 
or demagnify the cross-sectional configuration of the electron beam on the 
downstream side of the plate 26. But, in combination with a next 
subsequent downstream lens, to be described later below, the lens 30 
serves to maximize the transmission of electrons along the depicted column 
and to selectively control the locations of successive crossover points on 
the axis 22. 
A bottom view of one advantageous geometry for the apertured mask plate 26 
of FIG. 1 is shown in FIG. 2. Illustratively, in accordance with one 
specific illustrative embodiment made in accordance with the principles of 
the present invention, the plate 26 comprises a disc of molybdenum in 
which four apertures 31 through 34 are formed in a high-precision way by, 
for example, conventional laser machining techniques. 
The dashed lines within the openings 31 and 33 of FIG. 2 are included 
simply to facilitate subsequent discussion. In actuality, of course, each 
of the openings 31 and 33 is a single continuous aperture having straight 
edges as indicated by the solid straight lines. Thus, the aperture 31 may 
be regarded as composed of eight basic rectangular segments each defined 
by two or three solid straight lines and one or two dashed straight lines. 
Similarly, the aperture 33 may be regarded as composed of two basic 
rectangular segments each defined by three solid straight lines and a 
common dashed straight line. Each of the apertures 32 and 34 comprises a 
single basic rectangular segment. 
In FIG. 2 the segments included in the aperture 31 are designated A1 
through A8. The aperture 32 comprises segment A9, the aperture 33 
comprises segments A10 and A11, and the aperture 34 comprises segment A12. 
In one particular illustrative embodiment of the present invention, each 
of the rectangles A1 through A12 measures 100-by-200 micrometers (.mu.m). 
When the plate 26 of FIG. 2 is mounted in the FIG. 1 column, the 
longitudinal axis 22 of the column is perpendicular to and extends through 
the midpoint of the square formed by the segments A4 and A5 shown in FIG. 
2. 
The cross-sectional configuration of the electron beam that passes through 
the mask plate 26 of FIG. 1 is determined by the geometry of the apertures 
31 through 34. In turn, this beam configuration propagates through a 
conventional electromagnetic lines 36 (for example, an annular coil with 
iron pole pieces) which forms an image of the aforedescribed apertures on 
a second mask plate 40. The plate 40 contains a precisely formed aperture 
42 and, illustratively, is mounted on and forms an integral unit with 
electromagnetic field lines 44. 
A predetermined quiescent registration of the image of the aperture(s) in 
the mask plate 26 on the plate 40 of FIG. 1 is assured by, for example, 
including registration coils 46 in the depicted column. 
The location of the image of the electron-beam-illuminated aperture(s) 26 
on the second mask plate 40 of FIG. 1 is selectively controlled in a 
high-speed way during the time in which the electron beam is being scanned 
over the work surface 10. This is done by means of deflectors 48 
positioned, for example, as shown in FIG. 1 to move the beam in the x 
and/or y directions. Advantageously, the deflectors 48 comprise two pairs 
of orthogonally disposed electrostatic deflection plates. Electromagnetic 
deflection coils may be used in place of the electrostatic plates, but 
this usually leads to some loss in deflection speed and accuracy. Whether 
electrostatic or electromagnetic deflection is employed, the deflectors 48 
may also be utilized to achieve registration of the image of the 
aperture(s) in the plate 26 on the second mask plate 40. This is done by 
applying a steady-state centering signal to the deflectors 48. In that 
case the separate registration coils 46 may, of course, be omitted from 
the column. 
In one specific illustrative embodiment of the principles of this 
invention, the aperture 42 formed in the second mask plate 40 has the 
particular configuration shown in FIG. 3, which is a bottom view of the 
plate 40 included in the FIG. 1 column. For subsequent ease of discussion, 
the opening 42 is represented as being divided into multiple constituent 
rectangular segments designated B1 through B36. In one particular 
embodiment of this invention, each such segment measures 100-by-200 .mu.m. 
Centrally located dot 50 in FIG. 3 indicates the location of the 
longitudinal axis 22 of FIG. 1. 
Quiescently, that is, in the absence of deflection signals applied to the 
unit 48 of FIG. 1, images of the apertures 31, 32 and 34 of the mask plate 
26 (see FIG. 2) are transmitted through portions of the aperture 42 of the 
plate 40 of FIG. 3. Illustratively, the aperture image projected by the 
lens 36 (FIG. 1) onto the plate 40 corresponds exactly in size with the 
dimensions of the apertures 31 through 34. (If desired, the lens 36 may, 
of course, be designed to achieve other than a 1:1 projection of the 
apertures 31 through 34. Or, in some cases of practical interest, the lens 
36 may be omitted altogether.) By means of the coils 46, the image so 
projected is precisely centrally registered on the plate 40. More 
specifically, in that central or quiescent registration, the image of the 
aperture 31 corresponds exactly in size and directly overlies the portion 
of the aperture 42 composed of segments B19 through B26. Accordingly, the 
image defined by segments A1 through A8 is transmitted in its entirety 
through the mask plate 40. In addition, the images of the apertures 32 and 
34 directly overlie the segments B34 and B36, respectively, of the 
aperture 42 and are, accordingly, also quiescently transmitted through the 
plate 40. But, quiescently, the image of the aperture 33 is projected onto 
a nonapertured portion of the plate 40 and, hence, is not propagated 
through the plate 40. 
In accordance with the principles of the present invention, the electron 
image transmitted through the mask plate 40 of FIG. 1 propagates 
downstream through electromagnetic lens 52 and deflectors 54 to impinge on 
a third apertured mask plate 56, which is, for example, mounted on field 
lens 58 to form an integral unit therewith. Plate 56 includes aperture 60 
therethrough. 
In the absence of signals applied to the deflectors 54, the aforedescribed 
quiescent image transmitted through segments B19 through B26, B34 and B36 
of FIG. 3 is projected onto the mask plate 56 to achieve a quiescent 
registration therewith. (For this purpose, registraton coils 47 may 
advantageously by included in the column of FIG. 1.) In that condition, 
the image emanating from the segments B19 through B26 of the aperture 42 
of FIG. 3 directly overlies the aperture 60 in the mask plate 56 of FIG. 4 
and is, accordingly, quiescently transmitted through the plate 56. 
In FIG. 4, the aperture 60 is indicated as being divided into eight 
equal-sized rectangular segments designated C1 through C8. In one 
particular embodiment of this invention, each such segment measures 
100-by-200 .mu.m. Centrally located dot 62 in FIG. 4 indicates the 
location of the longitudinal axis 22 of FIG. 1. 
The cross-sectional area of the electron beam transmitted through the 
apertured plate 56 of the electron column of FIG. 1 is subsequently 
demagnified. This is done by means of three conventional electromagnetic 
lenses 64, 66 and 68 positioned downstream of the plate 56. In one 
specific illustrative embodiment of the principles of the present 
invention, these lenses are designed to achieve an overall demagnification 
of the beam propagated therethrough by a factor of 400. More particularly, 
these lenses are selected to demagnify the aforementioned cross-sectional 
area of the beam transmitted by the mask plate 56 and to image a reduced 
counterpart thereof on the work surface 10. For an overall demagnification 
of 400, and for the specific illustrative case in which the cross-section 
of the beam immediately downstream of the plate 56 measures 200-by-800 
.mu.m, the electron spot imaged on the surface 10 will quiescently be a 
rectangle 0.5 .mu.m wide and 2.0 .mu.m high. 
The other elements included in the column of FIG. 1 are conventional in 
nature. Except for one deflector unit, these elements may, for example, be 
identical to the corresponding parts included in the columns described in 
the aforecited patents and application. These elements include a 
beam-limiting apertured plate 70, electrostatic beam blanking plates 72 
and 74, an apertured blanking stop plate 76 and electromagnetic deflection 
coils 78 through 81. 
If the beam blanking plates 72 and 74 of FIG. 1 are activated, the electron 
beam propagating along the axis 22 is deflected to impinge upion a 
nonapertured portion of the plate 76. In that way the electron beam is 
blocked during prescribed intervals of time from appearing at the surface 
10. If the beam is not so blocked, it is selectively deflected by the 
coils 78 through 81 to appear at any desired position in a specified 
subarea of the work surface 10. Access to other subareas of the surface 10 
is gained by mechanically moving the surface by means, for example, of a 
computer-controlled micromanipulator, as is known in the art. 
In addition, the column of FIG. 1 includes deflectors 82. The purpose of 
these deflectors will be described later below. 
The column shown in FIG. 1 may be controlled by equipment 14 to operate in 
its so-called raster-scan mode of operation. This mode, which is described 
in the aforecited Collier-Herriott patent and in the Collier-Thomson 
application, involves successively scanning the beam on the work surface 
10 along parallel equally spaced-apart scan lines. Illustratively, each 
such scan line may be considered to comprise multiple equally spaced-apart 
address positions. At each address position during traversal of a scan 
line, the electron beam is blanked or not in the manner described above. 
Additionally, in accordance with the principles of the present invention, 
the area of the beam that impinges upon the work surface 10 at each 
address position is selectively controlled. 
As the variable-size electron spot is deflected along a row of the scan 
field, the spot is intensity modulated by the beam blanking plates 72 and 
74 at, for example, a 20 megahertz rate. This modulation rate corresponds 
with a single-address exposure time of 50 nanoseconds, which is compatible 
with the sensitivities of available electron resist materials. 
In the particular illustrative example specified herein, the maximum size 
of the rectangular electron spot imaged onto the surface of the layer 10 
of FIG. 1 is, as specified above, 0.5 .mu.m wide and 2.0 .mu.m high. For 
this particular case, the aforementioned scan lines in the raster mode of 
operation are usually spaced apart 2 .mu.m from each other and successive 
address positions along a scan line are spaced 0.5 .mu.m apart. 
For purposes of a specific example, it will be assumed that the system 
represented in FIG. 1 is programmed to form any one at a time of 
sixty-eight different electron spot configurations. In accordance with the 
principles of this invention, this is done by selectively applying signals 
to the deflection units 48 and 54. The result of applying no deflection 
signals (i.e., an all-zero deflection set) to these units was specified 
above in connection with the description of FIGS. 2 through 4 and is 
represented in FIG. 7(a). The result of applying one other particular set 
of deflection signals to the units 48 and 54 will be specified immediately 
below with the aid of FIGS. 5 and 6. Further, the sixty-six additional 
different spot configurations achievable by respectively applying 
sixty-six other deflection signal sets to these enits will be specified 
below in connection with the representations of FIGS. 7 through 12. 
By means of the deflection unit 48 shown in FIG. 1, the image transmitted 
through the apertured mask plate 26 and projected onto the plate 40 can be 
deflected in the X and/or Y directions. Assume, for example, that, with 
respect to the x and y axes shown in FIG. 5, the transmitted image is 
deflected zero half-address positions in the positive x direction and two 
half-address positions in the negative y direction. The resulting 
registration of the A1 through A12 image with respect to the aperture 42 
in the mask plate 40 is indicated in FIG. 5. 
As specified above, successive address positions are illustratively spaced 
apart 0.5 .mu.m on the surface of the workpiece 10. Accordingly, a 
so-called half-address position equals a distance of 0.25 .mu.m on the 
workpiece 10 and 100 .mu.m in the plane of the mask plate 40 or in the 
plane of the plate 56. 
Hereinafter, the x,y deflection imparted to the electron beam by the 
deflection unit 48 will be designated D1:x,y, where x and y are integers 
indicating the number of half-address positions that the beam 
configuration has been moved in the x and y directions, respectively. 
Accordingly, in FIG. 5 the designation D1:0,-2 has been included to 
represent the particular aforespecified deflection amplitude imparted to 
the image transmitted through the plate 26 and projected onto the plate 
40. 
As seen in FIG. 5, only the rectangular segments A1, A2, A7 and A8 of the 
image formed by the apertured mask plate 26 are transmitted through the 
aperture 42 in the second mask plate 40. In turn, if this image is 
deflected by the unit 54 two half-address positions in the negative x 
direction and two half-address positions in the positive y direction, the 
registration of the segments A1, A2, A7 and A8 on the third apertured mask 
plate 56 is as depicted in FIG. 6. It is apparent that only the segments 
A1 and A2 are transmitted through the aperture 60 toward the surface of 
the workpiece 10. In FIG. 6, and in the representations of FIGS. 7 through 
12, the stippled portion or portions of the aperture 60 indicate the 
particular part(s) thereof illuminated by the propagating electron beam. 
In FIG. 6 and hereinafter, the x,y deflection imparted to the electron beam 
by the deflection unit 54 is designated by the format D2:x,y, where x and 
y are integers indicating the number of half-address positions that the 
beam has been moved in the x and y directions, respectively. Accordingly, 
FIG. 6 includes the designation D2:2,2. 
Each solid line rectangle included in FIGS. 7 through 12 may be considered 
to represent the aperture 60 in the third mask plate 56 in FIG. 1. 
Associated with each such rectangle is a pair of deflection signal 
indicators designated D1, D2, which are formatted as specified above. And, 
as indicated earlier, the stippled portion of each rectangle represents 
the part thereof illuminated by the propagating electron beam. 
The limited number of spot configurations represented in FIGS. 7 through 12 
is simply a specific illustrative set formulated in accordance with 
certain criteria established therefor. Thus, for example, this particular 
set was devised based on the specification that constituent parts 
simultaneously emanating from the aperture 60 must be at least 200 .mu.m 
apart in the x direction in the plane of the downstream side of the third 
mask plate 56. For an example of a configuration in which the constituents 
are exactly 200 .mu.m apart, see FIG. 7(j). Additionally, the design rules 
for this set specify that only the topmost and/or bottommost rectangular 
segments of a spot configuration can be as small as 100 .mu.m in the x 
direction. See FIG. 9(h) for an example of a configuration which includes 
two such 100 .mu.m-high constituents. All other permitted spot 
constituents in the set depicted in FIGS. 7 through 12 measure at least 
200 .mu.m in the x direction. Significantly, the set of spot 
configurations shown in FIGS. 7 through 12 exhibits 0.25 .mu.m precision 
in the x direction at the surface of the workpiece 10. 
It is emphasized that the particular set of electron beam configurations 
represented in FIGS. 7 through 12 is illustrative only. In accordance with 
the principles of this invention, various modifications thereof can be 
devised. 
By applying appropriate deflecting and/or centering signals to the column 
of FIG. 1, variations of the particular configurations described above can 
easily be achieved. For example, the beam configurations transmitted 
through the third mask plate 56 of FIG. 1 can be designed to extend in the 
y direction less than the full width of the aperture 60. In addition, 
these configurations can be designed to extend in the x direction by less 
than the particular above-specified one-quarter-micron address size. 
Several such limited-extent spots are represented in FIGS. 13(b) through 
13(j). Each of these spots may be considered a variant of the previously 
specified configuration depicted in FIG. 7(a), which is also shown in FIG. 
13(a). 
In accordance with an aspect of the principles of the present invention, 
the y or scanning-direction extent of the electron spot directed at the 
workpiece 10 can be varied during the scanning process. This capability is 
the basis for selectively varying the electron exposure level at the 
workpiece surface, thereby improving the sharpness of feature edges that 
extend in the x direction. 
Ordinarily, the leading and trailing edges of an irradiated feature exhibit 
a ramp exposure profile. In turn, this leads as a practical matter to a 
lack of sharpness or definition in these edges. But, in accordance with 
the principles of this invention, edge sharpness in exposed features is 
significantly improved by controlling the herein-described system to 
achieve exposure profiles that are rectangular in nature. This is done by 
applying time-varying signals (as well as constant signals to achieve a 
specified spot configuration) to the unit 54 (FIG. 1) between masks 40 and 
56 to continuously deflect the image transmitted through the aperture 42 
in the negative y direction during selected intervals at the beginning and 
end of a feature exposure. Illustratively, the velocity imparted to the 
writing spot on the surface of the workpiece 10 by the time-varying 
deflection signal applied to the unit 54 is equal and opposite to that 
imparted to the spot by the scanning coils 78 through 81. Initially, at 
time t.sub.1, at the leading edge of a feature, both the constant and 
time-varying components of the deflection signals applied to the unit 54 
are turned on, whereby the image transmitted by the aperture 42 is 
projected onto the mask plate 56 directly adjacent the aperture 60. As the 
coils 78 through 81 sweep the writing spot over the workpiece 10 in the 
positive y direction, the time-varying component of the deflection signals 
continuously moves the image transmitted through the aperture 42 in the 
negative y direction into the aperture 60 until at t.sub.2 the aperture 60 
is filled with the configuration specified by the plates 26 and 40 and 
their respectively associated deflectors. Between t.sub.2 and t.sub.3 only 
the constant deflection signal required to produce the desired spot shape 
is applied to the unit 54. The beam, of course, continues to scan over the 
feature. At time t.sub.3, the time-varying component of the deflection 
signals is turned on again thereby once more sweeping the image of the 
aperture 42 in the negative y direction, this time out of the aperture 60. 
Finally, at time t.sub.4 the sopt is blanked by this action. 
In writing patterns with an EBES machine, a deliberate overlapping of 
feature stripes by, for example, a few tenths of a micrometer is sometimes 
called for in connection with the butting of adjacent stripes. But, 
because of the deliberate double exposure in the overlap region, a 
broadening occurs which tends to bridge small gaps between parallel edges 
crossing the butt. 
In accordance with one aspect of the principles of the present invention, 
broadening effects during purposeful overlapping of stripe features in a 
lithographic process can be eliminated by selectively reducing the 
exposure only in the region of overlap. This is done, for example, by 
generating a reduced-width overlap beam tab at the beginning and/or end of 
each stripe to be overlapped. Thus, for example, a beam configuration of 
the general type depicted in FIG. 13(b) may be utilized at the beginning 
and/or end of a stripe having terminal portions which are to overlap other 
stripe portions. During nonoverlapping portions of the stripe scan, 
however, the beam configuration is advantageously of the full-width type 
depicted in FIG. 13(a). In that way, the electron exposure in overlapping 
regions may be controllably reduced relative to that achieved with 
constant-width beams, thereby to minimize broadening effects in the 
overlapping regions. 
Embodiments of the present invention are advantageously suited to perform 
lithographic processing in a raster scan mode of operation of the type 
described in the aforecited Collier-Herriott patent. But the principles of 
this invention are also applicable to a lithographic system adapted to 
operate in a modified raster scan mode in which the beam also is vector 
scanned over a limited field or in effect stepped along the raster lines. 
Or these principles are also applicable to a system operated in a standard 
vector scan mode without any raster scanning. In these other modes, the 
deflection unit 82 shown in FIG. 1 may be utilized during specified 
intervals to exactly counter the y-direction deflection otherwise imparted 
to the electron beam by the scanning coils 78 through 81. Accordingly, 
during those intervals the variable-size writing electron spot is in 
effect held stationary on the surface of the workpiece 10. A feature to be 
defined on the surface is formed by abutting successive such stationary 
spots in a mosaic-like manner. 
A particularly advantageous alternative pair of apertured mask plates 
adapted for inclusion in the illustrative column of FIG. 1 is depicted in 
FIGS. 14 and 15, respectively. Plate 27 with aperture 29 therethrough 
(FIG. 14) may be substituted for the plate 26 in FIG. 1, and plate 41 with 
aperture 43 (FIG. 15) may be substituted for the plate 40. In that 
particular case, it is further advantageous to configure the third 
apertured mask plate as shown in FIG. 16. 
Plate 57 with aperture 61 therethrough (FIG. 16) is designed to be 
substituted for the plate 56 of FIG. 1 and to form a set with the 
particular pair of mask plates shown in FIGS. 14 and 15. Illustratively, 
the aperture 61 comprises a square whose size is sufficient to pass the 
largest image capable of being produced by the apertures 29 and 43. One 
particular advantage achieved by utilizing the specific set of apertured 
mask plates shown in FIGS. 14 through 16 is that triangular writing spots 
are thereby formable. As a result, it is possible in, for example, 
small-field vector scanning to more accurately pattern slant 
line-to-straight line junction regions. 
Finally, it is to be understood that the above-described arrangements are 
only illustrative of the application of the principles of the present 
invention. In accordance with these principles, numerous other 
arrangements may be devised by those skilled in the art without departing 
from the spirit and scope of the invention.