Note: Descriptions are shown in the official language in which they were submitted.
386
_ckground of the Invention
This invention relates to an apparatus and a method
for fabricating microminiature devices and, more particularly,
to a variable-spot raster scanning technique for use in an
electron beam exposure system.
U.S. Patent No. 3,900,737, which issued to R.J.
Collier and D.R. Herriott on August 19, 1975, describes an
electron beam exposure system (EBES) that is a practical tool
for generating high-quality fine-featured integrated circuit
masks. The system is also capable of exposing patterns
directly on resist-coated semiconductor wafers. EBES combines
continuous transla-tion of the mask or wafer substrate with
periodic deflection of the electron beam in a raster-scan
mode of operation.
The EBES exposure process requires a beam of
electrons emitted from a cathode to be focused to a sub-
micron-size spot on an electron-sensitive resist layer. In
practice, the diamèter of the spot is also the address
dimension of the system. In one particular practical embodi-
ment of EBES, the electron beam is focused to a spot 0.5
micrometers (~m) in diameter on the resist layer and is
modulated on and off as the spot is successively scanned in
raster fashion across a subregion of the layer. Each scan
line of the raster has a width of one address dimension and
a length of 256 address dimensions. Such a system meets
important current needs for moderate-resolution devices
(about 2 ~m linewidths with 0.5 ~m resolution) but does not
illustrate the ultimate limits of the capabilities of EBES.
`'.-;Y
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~9~8~
Various modifications of EBES are possible to
adapt it to meet the increasing demand for devices with
still smaller features. For example, if it is desired
to write 1 ~m minimum features with 0.25 ~m resolution,
using the EsES scanning mode described in the aforecited
U.S. Patent No. 3,900,737, an electron spot 0.25 ~m in
diameter can be employed. However, the penalty that is
thereby incurred is that the time required to expose a given
area of the resist is increased by a factor of four. For
many proposed applications of practical importance this is
an economically burdensome penalty which is not acceptable.
Thus, considerable effort has been directed at trying to
devise a way in which to decrease the resolution or address
dimension of EsES without at the same time increasing the
pattern-writing speed of the system. Moreover, it was
recognized that such a way, if available, would also
increase the pattern-writing speed of EBES in its afore-
mentioned moderate or 0.5 ~m resolution mode.
Summary of the_Invention
Accordingly, an object of the present invention
is an improved raster scanning technique for an electron
beam exposure system.
More specifically, an object of this invention
is a raster scanning technique in which the writing spot
dimensions of the electron beam are varied rapidly during
the scan.
Briefly, these and other objects of the present
invention are realized in an electron column designed for
variable-spot raster scanning. In such a column, an
illuminated aperture is demagnified to form the writing
spot. By imaging a first aperture upon a second aperture
and rapidly deflecting the image of the first aperture,
the portion of the second aperture that is illuminated by
the electron beam is altered. In that way, the writing
spot size normal to the direction of scanning is
selectively varied in a high-speed way during the raster
scanning process.
In accordance with an aspect of the invention there is
provided apparatus for irradiating a surface of a
workpiece comprising first means for scanning a charged
particle beam relative to said surface to traverse in
sequence a plurality of address positions, and second
means for varying, in dependence upon a control signal,
the extent of the beam at the address positions as said
scanning occurs, in which the second means includes a
first mask plate having a single beam-transmitting
aperture arranged to be illuminated in its entirety by a
charged particle beam, a second mask plate having a single
beam-transmitting aperture disposed to transmit an image
of the beam transmitted through the aperture in the first
mask plate only where that image overlies the aperture of
the second mask plate, and means for deflecting the image
with respect to the aperture in the second mask plate to
vary the registration therebetween, in dependence upon the
control signal, to vary said extent of the beam, in which
a main longitudinal axis of said apparatus extends
perpendicular to said first and second mask plates, and in
which the cross sections of said apertures as viewed along
said axis are non-coincident.
Brief Description of the Drawings
A complete understanding of the present invention and
of the above and other objects may be gained from a
T~ - 3 -
86
consideration of the following detailed description
presented hereinbelow in connection with the accompanying
drawings, in which:
FIG. 1 is a diagrammatic representation of an electron
beam column which is adapted to achieve variable-spot
raster scanning;
FIGS. 2 and 3 are specific illustrative depictions of
the respective geometries of two apertures that may be
included in the FIG. 1 column;
FIGS. 4 through 11 are respective superimposed
showings of the apertures of FIGS. 2 and 3 to represent
the result of deflecting the image of the FIG. 2 aperture
with respect to the FIG. 3 aperture;
FIG. 12 shows a portion of a pattern written by the
herein-described variable-spot raster scanning technique
using the column of FIG. 1 equipped with the apertures of
FIGS. 2 and 3;
FIGS. 13 and 14 are specific illustrative depictions
of the respective geometries of two additional apertures
that may be included in the FIG. 1 column; and
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86
FIG. 15 shows a portion of a particular pattern
written by the herein-described variable-spot raster
scanning technique using the column of FIG. 1 equipped
with the specific apertures of FIGS. 13 and 14.
Detailed Description
FIG. 1 depicts a specific illustrative litho-
graphic apparatus for controllably moving a variable-size
electron spot to any designated position on the top surface
of an electron-resist 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-resist
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 resist layer 10 in a highly ac-
curate and high-speed manner, it is possible to make
integrated circuit masks or to write directly on a resist-
coated silicon wafer to fabricate extremely small and
precise low-cost integrated circuits. Some suitable resists
for use as the layer 10 are described, for example, in a
two-part article by L.E. Thompson entitled "Design of
Polymer Resists for Electron Lithography", Solid State
Technology, part 1: July 1974, pages 27-30; part 2:
August 1974, pages 41-46.
The apparatus of FIG. 1 may be considered to
comprise two main constituents. One is an electron beam
column to be described in detail below, which is
characterized by highly accurate high-speed deflection
and blanking capabilities similar to those exhibited by
the column described in U.S. Patent No. 3,801,792, issued
April 2, 1974 to L.H. Lin. Additionally, in accordance
~9~
with the principles of the present invention, the depicted
column is further characterized by a variable-spot-size
scanning capability. This last-mentioned capability in
particular will be described in detail below.
The other main constituent of the FIG. 1 apparatus
comprises control equipment 14. Illustratively, the equip-
ment 14 is of the type described in the aforecited U.S.
Patent No. 3,900,737. The equipment 14 supplies electrical
signals to the described column to systematically control
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, as described in the aforecited
U.S. Patent No. 3,900,737.
The specific illustrative electron column of
FIG. 1 includes an electron source 22 (for example, a tung-
sten filament), a grid 24 and an accelerating anode 26
which comprises a cylindrical metal cap with a central
aperture in the bottom flat end thereof maintained at
ground potential. In -that case the source 22 is maintained
at a relatively high negative potential (for example, 10
kilovolts below ground).
The initial trajectories of electrons supplied
by the source 22 of FIG. 1 are represented in the drawing
by dashed lines. In the vicinity of the aforementioned
aperture in the anode 26 these trajectories go through a
so-called crossover or source image point 28 which, for
example, is 35 ~m in diameter. Thereafter the electron
paths successively diverge and converge as the electrons
travel downstream along longitudinal axis 30 toward the work
surface 10. Successive crossovers or images of the source
8~;
point 28 are represented in FIG. 1 by dots 32, 34, 36
and 38 disposed along the axis 30.
Advantageously, the electron column of FIG. 1
includes coils 40 by means of which the electron traject-
ories emanating from the aforedescribed source point 28
may be exactly centered with respect to the longitudinal
axis 30. Thereafter the electron beam is directed at a
plate 42 which contains a precisely formed aperture 44
therethrough. The beam is designed to uniformly illuminate
the full extent of the opening or aperture 44 in the plate
42 and to appear on the immediate downstream side of the
plate 42 with a cross-sectional area that corresponds
exactly to the configuration of the aperture 44.
A top view of one advantageous geometry for the
aperture 44 in the plate 42 of FIG. 1 is shown in FIG. 2.
Illustratively the plate 42 comprises a disc of molybdenum
in which the depicted aperture 44 is formed in a high-
precision way by, for example, conventional laser machining
techniques.
The dashed lines within the opening 44 of FIG. 2
are included simply to facilitate subsequent discussion.
In actuality the opening 44 is a single continuous aperture
having straight edges as indicated by the solid straight
lines. Illustratively, the single aperture 44 may be
regarded as composed of six square segments, each defined
by one or more solid straight lines and one or more dashed
straight lines. In FIG. 2 the square segments are
designated Ml through M6. In one particular illustrative
embodiment of the present invention, each of the squares
Ml through M6 measures 100 ~m on a side~ When the plate
6 --
of FIG. 2 is mcunted in th.e FIG. 1 column, the longitudinal
axis 30 of the column is perpendicular to and extends
through the mid-point of the square M3 shown in FIG. 2.
As stated above, the cross-sectional configuration
5 of the electron beam that passes through the mask plate 42
of FIG. 1 is determined by the geometry of the aperture 44.
In turn, this beam configuration propagates through a
conventional electromagnetic lens 46 (for example, an annular
coil with iron pole pieces) which forms an image of the
aforedescribed aperture 44 on a second mask plate 48
(FIG. 1). Plate 48 contains a precisely formed aperture 52.
Illustratively, the plate 48 is mounted on and forms an
integral unit with electromagnetic field lens 49. The lens
49 is not designed to magnify or demagnify the cross-
sectional configuration of the electron beam on theimmediate downstream side of the plate 48. But in comb-
ination with a next subsequent downstream lens, to be
.described later below, the lens 49 serves to maximize the
transmission of electrons along the depicted column. With
that next lens, the lens 49 is effective to image the
crossover point 28 to the center of a beam-limiting aperture
59.
A predetermined quiescent registration of the
image of the aperture 44 on the plate 48 of FIG. 1 is
assured by including registration coils 51 in the depicted
column.
In accordance with the principles of the present
invention, the location of the image of the illuminated
aperture 44 on the plate 48 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
,~ ..,s.,
86
surface 10. This is done by means of deflectors ~0
positioned, for example, as shown in FIG. 1 to move the
beam in the x and/or _ directions. Advantageously, the
deflectors 50 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 50 may also
be utilized to achieve registration of the image of
aperture 44 in the second mask plate 48. This is done by
applying a steady-state centering signal to the deflectors
50. In such a case the separate registration coils 51
may, of course, be omitted from the column.
sefore proceeding to describe further the
components included in the electron column of FIG. 1, it
will be helpful to specify the nature of the mask plate
48 and to illustrate the effect of moving the location
of the image of the aperture 44 on the plate 48. A top
view of a specific illustrative element suitable for
inclusion in the FIG. 1 column as the mask plate 48 is
shown in FIG. 3. Aperture 52 in the plate 48 may, for
example, have the shape shown in FIG. 3. In one particular
embodiment of this invention, the aperture 52 is a laser-
machined opening measuring 100 by 300 ~m. Centrally
located dot 54 in FIG. 3 indicates the location of the
longitudinal axis 30 of FIG. 1 when the plate 48 is
mounted in the column of FIG. 1.
Quiescently, the aperture 44 of the mask
plate 42 is imaged by the lens 46 of FIG. 1 onto the center
of the mask plate 48. Illustratively, the image
.
, ~ i
projected by the lens 46 onto the plate 48 corresponds
exactly in size with the dimensions of the aperture 44.
(If desired, the lens 46 may, of course, be designed to
achieve other than a 1:1 projection of the aperture 44.
Or in some cases of practical interest, the lens 46 may
be omitted altogether.) By means of the coils 51 the
image so projected is precisely centrally registered on
the plate 48, as indicated in FIG. 4.
From FIG. 4 it is apparent that only the
segments M2, M3 and M6 of the projected image of the
illuminated aperture 44 are transmitted through the
rectangular aperture 52 in the mask plate 48. Accordingly,
for the depicted registration, the electron beam appearing
immediately downstream of the plate 48 has a cross-sectional
area corresponding exactly to the geometry of the aperture
52. Hence, for the particular illustrative case in which
the aperture 52 constitutes an opening 100 ~m wide and 300 ~m
high, the cross-section of the electron beam immediately
downstream of the plate 48 exhibits the same dimensions.
Subsequently, the cross-sectional area of the
electron beam transmitted through the plate 48 of the
electron column of FIG. 1 is demagnified. This is done
by means of three conventional electromagnetic lenses 54,
56 and 58. 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 trans-
mitted by the mask plate 48 and to image a reduced
_ g
O~f~
counterpart thereof on the work surface 10. For an overalldemagnification of 400, and for the specific illustrative
case in which the cross-section of the beam immediately
downstream of the plate 48 measures 100 by 300 ~m, the
electron spot imaged on the surface 10 will quiescently be
a rectangle 0.25 ~m wide and 0.75 ~m high.
The other elements included in the column of
FIG. 1 are conventional in nature and may, for example,
be identical to the corresponding parts included in the
column described in the aforecited Lin patent. These
elements include the beam-limiting aperture 59, electro-
static beam blanking plates 60 and 62, an apertured
blanking stop plate 64 and electromagnetic deflection coils
65 through 68.
If the beam blanking plates 60 and 62 of FIG. 1
are activated, the electron beam propagating along the
axis 30 is deflected to impinge upon a nonapertured
portion of the plate 64. 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 65
through 68 to appear at any desired position in a
specified sub-area of the work surface 10. Access to
other sub-areas of the surface 10 is gained by mechani-
cally moving the surface~by means for example, of acomputer-controlled micromanipulator, as described in the
aforecited U.S. Patent No. 3,900,737.
As specified above, the rectangular electron
spot provided by centrally positioning the image of the
aperture 44 on the mask plate 48 is controlled by the
-- 10 --
8~
column of FIG. 1 to impinge or no-t onto a specified
location of the work surface 10.
A demagnified version of the rectangular area
composed of segments M2, M3 and M6 (FIG. 4) is shown in
FIG. 12 and designated by reference numeral 70. For ease
of conceptualizatlon and discussion, the rectangular
electron spot 70 that impinges on the work surface 10 of
FIG. 12 is shown divided into three square segments M2', -
M3' and M6'. These segments correspond respectively to
portions M2, M3 and M6 of FIG. 4. For the particular
illustrative case assumed above in which the overall
demagnification is 400, each of the segments M2', M3' and
M6' measures 0.25 ~m on a side~
Scanning of the beam provided by the electron
column of FIG. 1 is represented in FIG. 12 as occurring
from right to left in the -x direction along center
line 72. Illustratively, 512 equally spaced-apart
address positions are assumed to lie along the scanning
center line 72. The location of the first several ones
of these address positions are indicated in FIG. 12 by
arrows designated APl though AP9. At each address
position during the linear scan, 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 60 and 62 at, for
example, a 10 megahertz rate. This modulation rate
~9~
corresponds with a single-address exposure time of
100 nanoseconds, which is compatible with the sensitivities
of available electron resist materials.
At the completion of each scan line, the electron
beam is rapidly deflected to an initial position to start
a next adjacent scan line. In the particular case
illustrated in FIG. 12, such deflection or "fly back"
positions the ~eam above address position APl on a new
scanning center line 74 which is parallel to and 0.75 ~m
above the line 72. In this way, successive lines of a
subregion of the work surface 10 are selectively irradiated
in a raster-scan fashion. This raster scan mode of
operation (without the variable-spot-size feature) is
described in detail in the aforecited U.S. Patent No.
3,900,737.
In accordance with the principles of the present
invention, the geometry of the electron spot directed at
the surface 10 is varied during scanning in a high-speed
way in response to control signals applied by the equip-
ment 14 (FIG. l) to the deflectors 50. Thus, for example,by applying appropriate deflection potentials to the
deflectors 50, the image of the aperture 44 on the mask
plate 48 may be moved in the x and/or _ directions. The
effect of doing so is illustrated in the next-described
set of figures.
FIG. 5 represents the case wherein the projected
image of the aperture 44 has been deflected by the array 50
100 ~m in the +y direction and lO0 ~m in the -x direction.
The effect of this relative disposition is that only the
illuminated segment M4 of the projected image is
transmitted through the opening 52 in the mask plate 48.
In turn, a demagnified version (M4') of the segment M4 is
projected onto the wor]c surface 10. This 0.25 by 0 25 ~m
version is shown in FIG. 12 as being located at the address
position AP2.
In one illustrative mode of operation that is
characteristic of the present invention, the deflection
signals applied to the deflectors 50 of FIG. 1 are changed
(if necessary) while the scanning electron beam is
approximately midway between adjacent address positions.
Establishment of the new deflection signals is carried out
in a high-speed way. Thus, Eor example, for the case
assumed above wherein each single-address exposure time is
100 nanoseconds, the deflection signals required to
achieve a specified spot size are, for example, established
by the control equipment 14 in about 10 nanoseconds or
less.
The geometrical superposition illustrated in
FIG. 5 is achieved, for example, by changing the
deflection signals applied to the deflectors 50 of FIG. 1
while the scanning electron beam is about midway between
the address positions APl and AP2 (FIG. 12). In practice,
voltage swings of about 5 to 10 volts applied to electro-
static deflection plates are sufficient to change the
quiescent representation shown in FIG. 4 to the deflected
condition represented in FIG. 5. Such changes can be
realized by means of ultra-high-speed amplifiers in about
5 to 10 nanoseconds.
FIG. 6 represents the case wherein the projected
image of the aperture 44 has been deflected 100 ~m in the
+y direction by the deflectors 50. As a result,
illuminated segments M3 and M6 of the projected image are
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86
transmitted through the opening 52 in the mask plate 48.
In turn, a demagnified version (M3', M6') of the segments
M3 and M6 is projected onto the work surface 10. This
0.25 by 0.5 ~m version is shown in FIG. 12 located at
the address position AP3.
FIGS. 7 through 10 illustrate other geometrical
superpositions that may be achieved in accordance with the
principles of the present invention. The demagnified
electron spots that respectively correspond to the super-
positions depicted in FIGS. 7 through 10 are shown inFIG. 12 at the address positions AP4 through AP7. Each
such superposition is achieved in the manner described
above by the deflectors 50 deflecting the image of the
aperture 44 100 ~m in the x and/or y directions.
As described above, blanking of the electron
beam is achieved in the column of FIG. 1 by means of the
plates 60 and 62 and the blanking stop plate 64.
Alternatively, blanking may be achieved by activating
the deflectors 50 to deflect the projected image of the
aperture 44 sufficiently far with respect to the opening
52 in the mask plate 48 that no portion of the projected
image overlies the opening 52.
This condition, which is represented in FIG. 11, requires
that the deflectors 50 deflect the noted image 200 ~m in
the -x direction. (Of course a deflection of 200 ~m in
the +x direction would suffice also). In some cases of
practical interest, such a relatively large deflection
can be achieved by the deflectors 50 sufficiently rapidly
so as to make this alternative blanking technique a
feasible one. In such cases the elements 60, 62 and 64
can, of course, be omitted from the Fig. 1 column.
- 14 -
9~8~
As mentioned above, the particular confiyurations
of the apertures included in the mask plates 42 and 48 of
FIGS. 2 and 3 are illustrative only. It is apparent that
a variety of other configurations may be selected to
achieve the selective superposition that is the basis for
the herein-described variable-spot-size technique. Two
such other aperture geometries are shown in FIGS. 13 and
14. Mask plate 80 with aperture 82 therethrough (see
FIG. 13) may be substituted for the mask plate 42 shown
in the column of FIG. 1. And mask plate 84 with aperture
86 therethrough (FIG. 14) may be substituted for the mask
plate 48 in FIG. 1. By way of example, each of the segments
M7 through M14 of the aperture 82 of FIG. 13 is assumed
to measure 100 ~m on a side, and the rectangular
aperture 86 of FIG. 14 is assumed to be 100 ~m wide and
400 ~m high.
By deflecting the projected image of the
aperture 82 of FIG. 13 with respect to the aperture 86 of
FIG. 14, it is possible to form a variety of electron spot
sizes on the surface 10. In turn, this capability makes
it possible to irradiate high-resolution patterns on the
suxface 10 in a high-speed manner.
A portion of a chevron pattern irradiated in
accordance with the principles of the present invention
and employing the aperture pair 82 and 86 is shown in
FIG. 15. Lines 90 and 92 are the actual idealized
boundaries of the pattern to be defined on the surface 10.
The depicted grid formed of horizontal and vertical lines
spaced 0.25 ~m apart is not actually included on the
surface 10 but is shown only to facilitate understanding
of Fig. 15. Lines 94 and 96 are scanning center lines
corresponding to the lines 72 and 74 of Fig. 12.
Assume that scanning of the surface 10 of
- FIG. 15 occurs from right to left, first along the center
line 94 and then along the line 96. Those squares of the
grid that are irradiated by the electron beam during
variable-spot-size raster scanning of the surface are
shown shaded. Each such shaded square is designated with
a primed symbol to indicate which corresponding demagnified
portion of the illuminated aperture 82 actually impinges
on the surface 10. The variable-height rectangular spots
shown in FIG. 15 at respective address positions are
achieved by successively deflecting the image of the
aperture 82 by 100 ~m in each of the x and/or y directions.
As specified above, an illustrative electron beam
exposure system made in accordance with the principles of
the present invention may have an address length of 0.25 ~m
and a spot dimension that can, for example, be varied from
a square 0.25 ~m on a side to a rectangle 0.25 ~m by l um.
Eor a given writing spot exposure time, such a system can
expose areas at a rate about four times as fast as can be
achieved with conventional raster scanning of a fixed-size
spot 0.25 ~m in diameter. Or a system equipped for variable-
spot raster scanning as specified h~rein can write patterns
with 0.25 ~m resolution as fast as a conventional exposure
system writes patterns with 0.5 ~m resolution.
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. For example,
although primary emphasis herein has been directed to the
- 16 -
case of varying the extent of an electron spot in the
direction normal to the scanning direc-tion, it is
emphasized that the principles of the present invention
also encompass the case wherein spot size or location is
varied in the direction of scanning. Moreover, these
principles are also applicable to radiant beams other
than electron beams (for example light, x-ray and ion
beams).
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