U.S. patent application number 09/789440 was filed with the patent office on 2001-07-19 for device fabrication methods using charged-particle-beam image-transfer apparatus exhibiting reduced space-charge effects.
Invention is credited to Nakasuji, Mamoru.
Application Number | 20010008274 09/789440 |
Document ID | / |
Family ID | 26494583 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008274 |
Kind Code |
A1 |
Nakasuji, Mamoru |
July 19, 2001 |
Device fabrication methods using charged-particle-beam
image-transfer apparatus exhibiting reduced space-charge
effects
Abstract
Charged-particle-beam ("CPB"; e.g., electron-beam) apparatus are
disclosed that exhibit reduce image blur due to space-charge
effects. With such apparatus, a reticle pattern can be imaged on a
substrate with greater accuracy and higher throughput. Such results
can be achieved using a charged-particle source having
comparatively low emittance. An illumination-optical system directs
an illumination beam from a CPB source to a reticle defining a
pattern to be transferred to a substrate. A projection-optical
system projection-images, on the substrate, an imaging beam that
has passed through and been patterned by the reticle. The
illumination-optical system includes a beam-shaping aperture that
causes the illumination beam to have an annular transverse profile.
The reticle is illuminated with an image of a crossover of the
illumination beam. The CPB source desirably emits the illumination
beam from an annular region of a cathode. Thus, the Illumination
beam has a substantially uniform intensity distribution in the
vicinity of a crossover formed by the CPB source, and this
crossover is imaged in a plane that is optically conjugate with the
reticle.
Inventors: |
Nakasuji, Mamoru;
(Yokohama-shi, JP) |
Correspondence
Address: |
KLARQUIST SPARKMAN CAMPBELL
LEIGH & WHINSTON, LLP
One World Trade Center
121 S.W. Salmon Street, Suite 1600
Portland
OR
97204
US
|
Family ID: |
26494583 |
Appl. No.: |
09/789440 |
Filed: |
February 20, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09789440 |
Feb 20, 2001 |
|
|
|
09326483 |
Jun 4, 1999 |
|
|
|
6218676 |
|
|
|
|
Current U.S.
Class: |
250/492.3 |
Current CPC
Class: |
H01J 37/3174 20130101;
H01J 37/065 20130101; B82Y 10/00 20130101; B82Y 40/00 20130101 |
Class at
Publication: |
250/492.3 |
International
Class: |
A61N 005/00; G21G
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 5, 1998 |
JP |
10-172115 |
Jul 28, 1998 |
JP |
10-226583 |
Claims
What is claimed is:
1. An apparatus for performing a microlithographic transfer of a
pattern, defined by a reticle, to a substrate using an electron
beam, the apparatus comprising along an optical axis: (a) an
illumination-optical system situated and configured to direct an
electron illumination beam to the reticle so as to illuminate an
exposure unit of the reticle with the illumination beam and form an
imaging beam from electrons of the illumination beam passing
through the illuminated exposure unit; (b) a projection-optical
system situated downstream of the illumination optical system and
configured to project the imaging beam onto a substrate having a
sensitized surface so as to imprint the reticle pattern onto the
sensitized surface, the projection-optical system comprising a
contrast aperture; and (c) the illumination-optical system
comprising an electron gun having an electron-emission surface
configured so as to shape the imaging beam at the contrast aperture
to have an intensity distribution in which beam intensity on the
optical axis is less than off-axis beam intensity.
2. The apparatus of claim 1, wherein: the electron gun forms a gun
crossover; and the illumination-optical system illuminates the
exposure unit with an enlarged image of the gun crossover.
3. The apparatus of claim 1, wherein: the electron gun comprises a
cathode comprising an annular electron-emission surface; and the
annular electron-emission surface is defined by a defining region,
the annular electron-emission surface and the defining region
having respective work functions that are sufficiently different
from each other that electrons are emitted by the annular
electron-emission surface but not from the defining region.
4. The apparatus of claim 1, wherein the projection-optical system
comprises first and second projection lenses and a contrast
aperture situated axially between the first and second projection
lenses.
5. The apparatus of claim 1, wherein the contrast aperture is
annular.
6. The apparatus of claim 1, wherein the contrast aperture is
circular.
7. An electron-beam microlithography apparatus for performing a
projection-transfer of a pattern, defined by a reticle, to a
substrate, the apparatus comprising along an optical axis: (a) an
illumination-optical system situated and configured to illuminate
an exposure unit of the reticle with an illumination electron beam,
the exposure unit representing a respective portion of the pattern,
defined by the reticle, to be transferred to the substrate; (b) a
projection-optical system situated and configured to
projection-image, on the substrate, an imaging electron beam formed
by passage of the illumination beam through the illuminated
exposure unit of the reticle; and (c) the illumination-optical
system comprising an electron gun including a cathode having an
annular-shaped electron-emissive surface.
8. The apparatus of claim 7, wherein a portion of the illumination
beam that is of substantially uniform intensity distribution, and
that is situated in the vicinity of the gun crossover, is imaged in
a plane that is optically conjugate to the reticle.
9. The apparatus of claim 7, wherein the projection-optical system
comprises a contrast aperture situated at a position conjugate to
the cathode.
10. The apparatus of claim 7, wherein a portion of the illumination
beam having a uniform intensity distribution at the gun crossover
is imaged either on the reticle or on a plane that is optically
conjugate to the reticle.
11. The apparatus of claim 7, wherein: the electron beam between
the electron gun and the reticle forms multiple crossovers at
respective locations along the optical axis, and; the crossover
nearest the reticle is formed upstream of the reticle.
12. The apparatus of claim 7, further comprising a contrast
aperture situated at a location conjugate to the cathode.
13. The apparatus of claim 7, wherein the illumination-optical
system further comprises a field-limiting aperture that is
adjustable to independently vary one or more of an imaging
condition and a magnification ratio.
14. The apparatus of claim 7, wherein the illumination-optical
system further comprises a lens situated at a location at which an
image of the cathode is formed, the lens being adjustable to
independently vary one or more of an imaging condition and a
magnification ratio.
15. The apparatus of claim 7, wherein the annular-shaped
electron-emissive surface is defined by surrounding material on the
cathode, the electron-emissive surface having a work function that
is at least 0.6 eV less than a work function of the surrounding
material.
16. The apparatus of claim 7, wherein: the electron gun comprises
multiple electrodes including the cathode; and at least one of (i)
a location at which an image of the cathode is formed by the
illumination-optical system and (ii) a location at which a beam
crossover is formed is adjustable by varying a voltage applied to
at least one of the electrodes of the electron gun.
17. The apparatus of claim 7, further comprising a power supply
connected to the cathode, the power supply being adjustable to vary
electrical power supplied to the cathode and thus vary cathode
temperature so as to adjust brightness of the illumination
beam.
18. A method for performing a microlithographic transfer of a
pattern, defined by a reticle, to a substrate using a charged
particle beam, the method comprising: (a) generating a
charged-particle illumination beam; (b) shaping the illumination
beam to have a hollow profile of beam intensity at a contrast
aperture; (c) directing the illumination beam to a patterned
reticle so as to illuminate a region of the reticle with the
illumination beam and form an imaging beam from charged particles
of the illumination beam passing through the illuminated region of
the reticle, the illumination beam illuminating the region of the
reticle having a uniform transverse intensity distribution in the
vicinity of a crossover image formed by the illumination beam; and
(d) projecting the imaging beam onto a substrate having a
sensitized surface so as to imprint the reticle pattern on the
sensitized surface.
19. The method of claim 18, wherein step (b) comprises emitting the
illumination beam from a beam-emitting surface having an annular
profile.
20. The method of claim 18, wherein step (b) further comprises
passing the illumination beam through an annular beam- shaping
aperture.
21. A method for performing a microlithographic projection-
transfer of a pattern, defined by a reticle, to a substrate using
an electron beam, the method comprising: (a) generating an
illumination electron beam having a hollow transverse intensity
profile at a contrast aperture, the illumination electron beam
forming a gun crossover in the vicinity of which the illumination
beam has a uniform transverse intensity profile; (b) directing the
illumination beam to a patterned reticle so as to illuminate a
region of the reticle with the illumination beam and form an
imaging beam from electrons of the illumination beam passing
through the illuminated region of the reticle; and (c) projecting
the imaging beam onto a substrate having a sensitized surface so as
to imprint the reticle pattern onto the sensitized surface.
22. The method of claim 21, wherein step (a) comprises providing an
electron gun comprising a cathode having an annular
electron-emissive surface, and energizing the cathode so as to
cause the illumination beam to be emitted from the annular
electron-emissive surface.
23. The method of claim 21, wherein: step (a) further comprises
forming a gun crossover of the illumination beam emitted from the
annular electron-emissive surface; and step (b) further comprises
imaging the illumination beam, propagating from the gun crossover
and having a substantially uniform intensity distribution, in a
plane that is optically conjugate to the reticle.
24. A method for fabricating a micro-patterned device, comprising:
(a) providing a resist-coated wafer; (b) providing an electron gun
located upstream of the wafer; (c) generating an illumination
electron beam from the electron gun, the electron gun comprising an
electron-emissive surface configured so as to shape the
illumination beam to have an intensity distribution at a contrast
aperture, located downstream of the electron gun, in which beam
intensity on a propagation axis of the illumination beam is less
than off-axis beam intensity; (d) directing the illumination beam
to a patterned reticle so as to illuminate a region of the reticle
with the illumination beam and form an imaging beam from electrons
of the illumination beam passing through the illuminated region of
the reticle; (e) projecting the imaging beam onto the resist-coated
wafer so as to imprint the reticle pattern on the resist; (f)
developing the exposed resist; (g) etching the wafer; and (h)
separating remaining resist from the wafer.
25. A micro-patterned device produced by the method of claim 24.
Description
FIELD OF THE INVENTION
[0001] This invention pertains to charged-particle-beam (e.g.,
electron-beam) microlithography apparatus and methods as used in
the manufacture of semiconductor devices, displays, and the like.
More specifically, the invention pertains to such methods and
apparatus for transferring fine, high-density patterns with minimum
linewidths of 0.1 .mu.m or less at high throughput. Yet more
specifically, the invention pertains to such methods and apparatus
exhibiting reduced image blurring caused by space-charge effects,
and to semiconductor-device fabrication methods that use such
methods and apparatus.
BACKGROUND OF THE INVENTION
[0002] As used herein, a "charged particle beam" is a beam of
charged particles such as electrons or ions. For simplicity, the
following discussion is in the context of an electron beam;
however, it will be understood that the principles of the invention
can be applied with equal facility to other types of charged
particle beams.
[0003] In conventional electron-beam microlithography apparatus, an
electron beam is produced by an electron gun. The electron beam
passes through an "illumination-optical system" to illuminate a
portion of a patterned reticle. The reticle defines the pattern
(e.g., a layer of an integrated circuit) to be transferred to a
sensitized substrate (e.g., semiconductor wafer). The beam between
the electron gun and the reticle is termed an "illumination beam."
After passing through the illuminated portion of the reticle, the
beam (now termed an "imaging beam" or "patterned beam") passes
through a "projection-optical system" to form a corresponding image
on the surface of the substrate. The substrate surface is
"sensitized" by a previously applied layer of a suitable resist
that is responsive in an image-forming way to exposure to charged
particles of the imaging beam. For exposure, the dosage of charged
particles impinging on the surface of the substrate can be
increased or decreased by increasing or decreasing, respectively,
the beam current.
[0004] In electron-beam microlithography systems, if the beam
current is increased (e.g., in an effort to increase throughput),
the electron density in the beam is correspondingly increased,
which results in correspondingly increased Coulomb repulsion
between electrons in the beam. Such Coulomb repulsion, also termed
a "space-charge effect," causes the beam to spread out, which
causes blurring of the image transferred by the beam.
[0005] Certain types of electron-beam microlithography apparatus
are termed "critical illumination" systems in which an enlarged
image of a crossover produced by the electron gun is formed on a
downstream beam-shaping aperture in the illumination-optical
system. In conventional illumination-optical systems intended for
critical illumination, the transverse intensity profile of the
electron beam (i.e., intensity profile of the beam in a plane
perpendicular to the optical axis of the illumination-optical
system or projection-optical system) exhibits a Gaussian
distribution. In the Gaussian distribution of the beam in
conventional systems, the center of the beam has the highest
intensity, with intensity falling off rapidly with increasing
distance from the beam center. For example, the portion of the beam
at the center of the distribution where the beam intensity is flat
to within.+-.1% has a diameter of 1/8or less of the total beam
diameter. As a result, with critical illumination, an exposure area
that is the same as that obtainable with Kohler illumination cannot
be obtained without increasing the current supplied to the electron
gun. But, as noted above, increasing the gun current increases the
beam current, which increases space-charge effects. (In a Kohler
illumination system, the beam diverging from a crossover is
incident to a field-limiting aperture, and the crossover is imaged
in the entrance pupil of the projection-optical system.)
[0006] A beam having a Gaussian intensity distribution with a
center peak intensity is termed a "solid" beam. In a solid beam,
space-charge effects are a major problem.
[0007] "Hollow" beams are known. According to the reference Ura,
Katsumi, Electron Optics, Kyoritsu, 1979, a hollow beam exhibits
less beam spreading due to space-charge effects. A hollow electron
beam can be generated, for example, using an electron gun having a
frusto-conical cathode, wherein the conical surface of such a
cathode is the electron-emitting surface. A gun crossover is
typically located just downstream of the cathode of such an
electron gun. The current density at the gun crossover of an
electron gun with a frusto-conical cathode exhibits a Gaussian
distribution. Certain of such electron guns also have an associated
gun lens. So long as any aberrations generated by the gun lens are
small, the angular distribution of the electron beam emerging from
the crossover will be characteristic of a hollow beam. If the gun
lens or any other portion of the electron gun exhibits excessive
aberration, however, the edges of the beam-intensity distribution
become blurred, producing a beam that is no longer clearly hollow.
Such aberrations are extremely difficult to correct or control at
the gun. Even though a blurred hollow beam can be shaped to some
extent by passing the beam through an annular aperture, this remedy
alone is inadequate for forming the desired quality of hollow beam.
In addition, attempting to "correct" a blurred beam using an
annular aperture in such a manner results in blocking a large
proportion of the beam from propagating downstream of the annular
aperture to, e.g., the reticle. Consequently, a very large beam
current is required which further aggravates space-charge effects
and causes an excessive temperature rise of the annular
aperture.
[0008] With certain conventional electron-beam microlithography
systems, the reticle pattern is divided into multiple exposure
units (e.g., stripes, subfields, or the like, wherein an "exposure
unit" is the area on the reticle that is illuminated, and thus
exposed, by the beam at any given instant of time). Each exposure
unit defines a respective portion of the overall pattern defined by
the reticle. The exposure units typically exhibit differing feature
densities from one exposure unit to another and can exhibit
substantial differences in feature density within individual
exposure units. Differences in feature density result in
corresponding differences in downstream beam current. As a result,
under such conditions, points of best focus of the beam at the
substrate are not in the same plane.
SUMMARY OF THE INVENTION
[0009] The present invention was derived so as to solve the
problems of conventional systems summarized above. More
specifically, apparatus according to the invention exhibit, inter
alia, reduced image blurring due to space-charge effects. Apparatus
according to the invention also allow larger exposure units of the
reticle to be exposed per "shot," even using an electron-beam
source having a comparatively low emittance (an emittance of no
more than approximately 1 mm.multidot.mrad). Hence,
microlithographic pattern transfer can be performed with greater
accuracy and throughput than with conventional systems.
[0010] According to one aspect of the invention, apparatus are
provided for performing a microlithographic transfer of a pattern,
defined by a reticle, to a substrate using a charged particle beam
(e.g., electron beam). A representative embodiment of such an
apparatus comprises, along an optical axis, an illumination-optical
system and a projection-optical system. The illumination-optical
system directs an electron illumination beam to the reticle so as
to illuminate an exposure unit of the reticle with the illumination
beam. The illumination-optical system also forms an imaging beam
from electrons of the illumination beam passing through the
illuminated exposure unit. The projection-optical system is
situated downstream of the illumination optical system and includes
a contrast aperture. The projection-optical system projects the
imaging beam onto a substrate having a sensitized surface so as to
imprint the reticle pattern onto the sensitized surface. The
illumination-optical system comprises an electron gun having an
electron-emission surface configured to have an annular profile
about the optical axis. Thus, the imaging beam is shaped at the
contrast aperture to have an intensity distribution in which beam
intensity on the optical axis is less than off-axis beam
intensity.
[0011] The annular electron-emission surface is conveniently
defined on the cathode by a "defining region" typically made of a
different material than the electron-emission surface. To such end,
the annular region and the defining region desirably have
respective work functions that are sufficiently different from each
other that electrons are emitted by the annular region but not from
the defining region.
[0012] The electron gun can be a type that forms a gun crossover,
wherein the illumination-optical system illuminates the exposure
unit with an enlarged image of the gun crossover.
[0013] The projection-optical system desirably comprises first and
second projection lenses, wherein the contrast aperture is situated
axially between the first and second projection lenses. The
contrast aperture can be, for example, annular or circular. A
transverse profile of the illumination beam on the contrast
aperture can be controllable.
[0014] The illumination-optical system desirably comprises first
and second condenser lenses and a beam-shaping aperture situated
axially between the first and second condenser lenses.
Alternatively, the beam-shaping aperture is situated in the
projection-optical system. The beam-shaping aperture can be
annular, and desirably is situated so as to be conjugate with the
contrast aperture.
[0015] The apparatus can include a field-limiting aperture situated
upstream of the reticle. In such an instance, the
illumination-optical system can comprise first and second lenses
situated between the CPB source and the field-limiting
aperture.
[0016] According to another aspect of the invention, electron-beam
microlithography apparatus are provided for performing
projection-transfer of a pattern (defined by a reticle) to a
substrate. A representative embodiment of such an apparatus
comprises, along an optical axis, an illumination-optical system
and a projection-optical system. The illumination-optical system
illuminates an exposure unit of the reticle with an illumination
electron beam. (The exposure unit represents a respective portion
of the reticle pattern to be transferred to the substrate.) The
illumination-optical system comprises an electron gun including a
cathode that has an annular-shaped electron-emissive surface. The
projection-optical system projects an imaging electron beam (formed
by passage of the illumination beam through the illuminated
exposure unit of the reticle) onto the substrate. Thus, an image of
the illuminated exposure unit is formed on the substrate.
[0017] In the foregoing embodiment, it is preferred that at least a
portion of the illumination beam be of substantially uniform
intensity distribution in the vicinity of the crossover formed by
the electron gun. Such a beam is desirably imaged in a plane that
is optically conjugate to the reticle.
[0018] Because the portion of the reticle receiving a substantially
uniform flux is relatively large in diameter, a larger exposure
area can be accommodated. Also, the portion of the electron beam
that is not used for exposure is correspondingly reduced. This
enables the power supply for the electron gun to be reduced in size
and output, and reduces the cooling load of the microlithography
apparatus. The "substantially uniform intensity distribution"
referred to above corresponds, according to one example described
herein, to an intensity distribution exhibiting a variation in
current density of no more than.+-.1% from the peak value. In
addition, except for cathode/reticle image portions, the
illumination beam and imaging beam are hollow beams, which reduces
the influence of space-charge effects.
[0019] The electron gun desirably forms a gun crossover, wherein
the portion of the illumination beam having a substantially uniform
intensity distribution (and that is situated at or in the vicinity
of the gun crossover) is imaged in a plane that is optically
conjugate to the reticle. A portion of the illumination beam having
a uniform intensity distribution at (or in the vicinity of) the gun
crossover can be imaged either on the reticle or on a plane that is
optically conjugate to the reticle.
[0020] The annular-shaped electron-emissive surface can be defined
by surrounding material ("defining region") on the cathode. The
electron-emissive surface desirably has a work function that is at
least 0.6 eV less than a work function of the defining region. With
such a difference in work function, the illumination beam is
emitted substantially only from the annular electron-emissive
surface, to thereby produce a hollow illumination beam. Also, the
illumination beam can be provided with the required brightness
without having to use a large electron-gun current. By way of
example, the electron-emissive surface can be relatively small
(e.g., 4-12 mm.sup.2 area) compared to the surface of the cathode
that is 8 mm in diameter.
[0021] The electron gun can comprise multiple electrodes including
the cathode. Such a configuration can allow at least one of the
following to be adjusted: (a) a location at which an image of the
cathode is formed, and (b) a location at which a beam crossover is
formed. The adjustment can be made by varying a voltage applied to
at least one of the electrodes of the electron gun.
[0022] A power supply is connected to the cathode. The power supply
can be adjustable to vary electrical power supplied to a heating
element in the cathode. Thus, cathode temperature can be changed as
required to change the brightness of the illumination beam.
[0023] The illumination-optical system can be configured to form
multiple crossovers of the illumination beam at respective
locations along the optical axis. In such a configuration, the
crossover nearest the reticle is formed upstream of the reticle.
The illumination-optical system can comprise multiple lenses and a
field-limiting aperture. The field-limiting aperture can be
adjustable to independently adjust one or more of an imaging
condition and a magnification ratio of the illumination beam.
Furthermore, the illumination-optical system can comprise a lens
situated at a location at which an image of the cathode is formed.
Such a lens is adjustable to independently adjust one or more of an
imaging condition and a magnification ratio of the illumination
beam.
[0024] The projection-optical system can comprise a contrast
aperture, as summarized above, desirably situated at a position
conjugate to the cathode. Placing the contrast aperture in a
location that is conjugate to the cathode forms a hollow imaging
beam without having to use a special beam-shading aperture. In a
direction downstream of the contrast aperture, an image of the
cathode can be formed before a crossover. A crossover can be formed
between the contrast aperture and the substrate.
[0025] The foregoing and additional features and advantages of the
invention will be more readily understood from the following
detailed description, which proceeds with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 schematically depicts a charged-particle-beam (CPB)
microlithography apparatus according to a first representative
embodiment of the invention, and also depicts certain imaging
relationships in the apparatus.
[0027] FIG. 2 includes plots showing the peak portions of
transverse beam-intensity distributions for respective pairs of
Gaussian distributions having the same intensity at the same
instant in time but separated from each other along the optical
axis.
[0028] FIG. 3 schematically depicts a CPB microlithography
apparatus according to a second representative embodiment, and also
depicts certain imaging relationships in the apparatus.
[0029] FIG. 4 is a block diagram of a device fabrication
process.
DETAILED DESCRIPTION
[0030] The invention is described below in connection with multiple
representative embodiments.
[0031] A first representative embodiment is depicted in FIG. 1 that
schematically shows certain imaging relationships associated with
the embodiment. An electron beam EB (as an exemplary charged
particle beam) is produced by an electron gun 1. The electron beam
EB propagates downstream of the electron gun 1 along the optical
axis A. The electron gun 1 in this embodiment comprises three
electrodes: a cathode 1a (including an annular electron-emitting
region 1b), a Wehnelt 1c, and an anode 1d. The cathode 1a desirably
is made of a lanthanum hexaboride (LaB.sub.6) monocrystalline rod
approximately 4-8 mm in diameter. The annular electron-emitting
region 1b is situated on the downstream-facing surface of the
cathode 1a.
[0032] The entire downstream-facing surface of the cathode 1a,
except the annular electron-emitting region 1b, desirably is coated
with a "defining region" of, e.g., carbon or rhenium (to a
thickness, e.g., of 100 nm as achieved by, e.g., sputtering) to
allow electron emission only from the annular region 1b. In other
words, the defining region defines the shape and dimensions of the
annular region 1b. At a temperature greater than approximately
1000K, LaB.sub.6 reacts with most metals, causing corrosion of the
LaB.sub.6. However, LaB.sub.6 does not react with carbon or
rhenium. Also, because the work functions of carbon and rhenium are
much higher than the work function of LaB.sub.6 (about 2.5 eV),
these elements emit substantially no electrons at temperatures
lower than approximately 1500K at which LaB.sub.6 emits electrons.
Therefore, only the annular region 1b emits electrons, and the
resulting beam has a "hollow" transverse intensity profile.
[0033] A negative acceleration voltage (e.g., -100 KV) is applied
to the cathode 1a. The anode 1d, which defines an axial aperture
through which the electron beam EB passes, is normally at zero
volts ("ground"). A negative voltage (e.g., -100.02 KV) is applied
to the Wehnelt 1c, which forms a ring-shaped field around the
cathode 1a. The Wehnelt 1c acts upon the electron beam EB emitted
from the annular region 1b to urge the beam to propagate downstream
along the optical axis A. The resulting hollow electron beam EB
emitted from the electron gun 1 forms a gun crossover 5 immediately
downstream of the anode 1d. ("Hollow" means that the beam has an
intensity distribution in which beam intensity on the propagation
axis of the beam is less than beam intensity off-axis.) The hollow
beam exhibits substantially reduced space-charge effects compared
to a beam produced by a conventional system. As summarized above, a
conventional system employs an annular aperture, located between
the electron gun and the reticle, for producing the desired quality
of hollow beam. According to the embodiment described above, the
gun itself generates the desired quality of hollow beam for
illuminating the reticle.
[0034] Downstream of the electron gun 1 is an illumination-optical
system. In the FIG. 1 embodiment, the illumination-optical system
comprises first and second condenser lenses 6, 7, respectively,
that collectively form a two-stage condenser lens. Co-positioned
with the second condenser lens 7 is a beam-shaping aperture 8. The
beam-shaping aperture 8 desirably comprises a round center plate 8a
surrounded by an outer ring 8c. Thus, the center plate 8a and outer
ring 8c define a circumferential gap therebetween configured as an
annular aperture 8b. The center plate 8a can be supported as
required at multiple locations around its circumference by support
members 8d. The beam-shaping aperture 8 shapes the hollow electron
beam EB to have a more uniform intensity around its propagation
axis. Whereas, in conventional systems, an annular aperture located
downstream of the electron gun blocks passage therethrough of most
of the electron beam, the beam-shaping aperture 8 in this
representative embodiment merely trims the beam and allows most of
the beam to pass through. As a result, compared to conventional
systems, a substantially lower beam-emission current can be used
(which reduces space-charge effects) and the beam-shaping aperture
8 exhibits a much reduced rise in temperature.
[0035] The electron beam EB diverging from the gun crossover 5 is
diverged even more by passage through the first condenser lens 6.
The beam then passes through the second condenser lens 7 and the
beam-shaping aperture 8. Downstream of the beam-shaping aperture 8,
the beam forms a first crossover CO1 just upstream of a condenser
lens 9 and a second crossover CO2 at a field-limiting aperture
10.
[0036] The field-limiting aperture 10 trims the outer edge of the
illumination beam as required so as to illuminate a desired
exposure unit on a downstream reticle 12. A condenser lens 11 forms
an image of the field-limiting aperture 10 on the selected exposure
unit of the reticle 12.
[0037] The size of the second crossover CO2 can be controlled by
changing the "magnification" factor by a "zoom" adjustment of the
condenser lenses 7 and 9. Because an image of the second crossover
CO2 is illuminated onto the reticle 12, illumination can be made
uniform over the entire selected exposure unit even when the
beam-current density as emitted from the cathode 1a is not uniform.
I.e., at the crossover CO2, the beam currents emitted from
different loci on the cathode converge and are averaged, resulting
in a Kohler-like illumination of the reticle. Also, because the
cathode 1a is operated under temperature-limited conditions, beam
brightness as illuminated on the selected exposure unit can be
adjusted by adjusting the cathode temperature. (In a
"temperature-limited" condition, the emission current is controlled
by the temperature of the emission surface rather than electrode
potential.)
[0038] Although not shown in FIG. 1, a selection deflector is
situated downstream of the field-limiting aperture 10. The
selection deflector sequentially scans the illumination beam
primarily in the horizontal direction (in the figure) so as to
achieve sequential illumination of all the exposure units of the
reticle 12 within the field of the illumination-optical system. In
addition, the reticle 12 and substrate 16 are individually mounted
on respective stages (not shown) that are scannably moved as
required in the horizontal direction (in the figure) to increase
the lateral range of exposure to a width wider than the field of
the optical system of the apparatus.
[0039] Situated downstream of the reticle 12 is a
projection-optical system that, in the FIG. 1 embodiment, comprises
first and second projection lenses (objective lenses) 13, 14,
respectively, a contrast aperture 15, and a deflector (not shown).
As discussed above, the exposure units on the reticle 12 are
individually and sequentially illuminated by the illumination beam.
As each exposure unit is illuminated, the resulting imaging beam
acquires an ability to form an image of the illuminated exposure
unit. As the imaging beam passes through the projection lenses 13,
14, the imaging beam is demagnified and caused to form an image of
the illuminated exposure unit at the proper location on the
substrate 16. (As used herein, "demagnified" means that the image
formed on the substrate 16 is smaller than the corresponding
illuminated exposure unit by a pre-determined "demagnification
ratio.") The substrate 16 (e.g., a semiconductor wafer) is coated
with an appropriate resist that is sensitive to a dosage of the
electron beam such that the images of the illuminated exposure
units are imprinted on the substrate.
[0040] The contrast aperture 15 is situated between the first and
second projection lenses 13, 14. Thus, the contrast aperture 15
effectively divides the axial distance between the projection
lenses 13, 14 according to the demagnification ratio. The contrast
aperture 15 is situated in a plane that is optically conjugate with
the plane of the cathode surface. The contrast aperture 15
desirably comprises a round center plate 15a surrounded by an outer
ring 15c so as to define a circumferential gap (annular aperture
15b) therebetween. The annular aperture 15b desirably extends from
8 mrad (inside radius) to 10 mrad (outside radius) relative to a
point on the optical axis A corresponding to the plane of the first
projection lens 13. The center plate 15a can be peripherally
supported using support members 15d.
[0041] The contrast aperture 15 functions, inter alia, as a shield
to block non-patterned portions of the imaging beam (i.e.,
electrons scattered by the reticle 12) from reaching the substrate
16. The contrast aperture 15 is desirably annular in view of the
annular emission surface 1b of the cathode 1a. Hence, the contrast
aperture 15 efficiently removes scattered electrons. In
conventional systems, in contrast, an annular scattering aperture
is not used (although an open round aperture is sometimes used,
which is much less effective with a hollow beam).
[0042] The embodiment shown in FIG. 1 and described above provides
an electron-beam microlithography system in which image blur due to
space-charge effects are reduced. This allows microlithographic
pattern transfer to be performed with greater accuracy and
throughput than achieved with conventional systems.
[0043] According to the invention, even though the electron beam is
not emitted from the center of the cathode 1a, the beam as emitted
from the electron gun 1 exhibits a simple Gaussian intensity
distribution at crossovers (with a center maximum intensity located
on the axis A). However, as viewed within respective successive
planes axially displaced from the crossover, the transverse
intensity profile of the beam is the sum of multiple component
Gaussian distributions. Each component Gaussian distribution has a
respective peak intensity at a respective radial distance from the
optical axis. Representative transverse intensity distributions
were calculated, and peak portions of the resulting distributions
are plotted in FIG. 2. (In FIG. 2, the abscissa is lateral position
wherein "1" is the optical axis and the ordinate is "y" calculated
using Equation (1) below.)
[0044] The distribution for two Gaussian distributions having the
same intensity but having respective peaks separated from each
other by a given distance "a" in a radial direction from the
optical axis, is given by Equation (1): 1 y = 1 2 [ exp ( - x 2 2 )
+ exp ( - ( x + a ) 2 2 ) ] ( 1 )
[0045] Respective plots for three different values of "a" are shown
in FIG. 2 (i.e., a=2.0, 2.1, and 2.2, respectively), wherein "a"
has the same units as "x". In FIG. 2, .DELTA.x represents the range
of x over which, in the central portion of the overall intensity
distribution, the intensity is within.+-.1% of the central value.
For a value of a=0 (i.e., where the distribution is strictly
Gaussian, not shown), the value of .DELTA.x is 0.282. In
comparison, .DELTA.x=1.2 for a=2.0 (the top curve in the figure),
.DELTA.x=1.55 for a=2.1 (the middle curve), and .DELTA.x=0.62 for
a=2.2 (the lower curve). All three of these values of "a" provide a
wider lateral uniform beam intensity than provided when a=0. For
example, at a=2.1, the value of AX is increased by a factor of 5.5
over the value of .DELTA.x at a=0. This represents a substantial
increase in the size of the region of uniform beam-intensity
distribution. These results are achieved by the combination, in
FIG. 1, of items 1 and 6-11.
[0046] According to the above, a uniform high-intensity electron
beam is obtained that is transversely wider than obtained from
prior-art apparatus. The wider beam is produced by combining
particles from the beam originating from multiple locations within
a ring-shaped electron-emission source.
[0047] FIG. 3 depicts the overall configuration and certain imaging
relationships of a second representative embodiment of an
electron-beam microlithography system according to the invention.
The electron beam is produced by an electron gun 101 situated at
the most upstream location in the system. The electron beam
propagates from the electron gun 101 in a downstream direction
along an optical axis A.
[0048] The electron gun 101 comprises three electrodes: a cathode
101a, a first (or control) anode 101b, and a second anode 101c. The
cathode 101a comprises a plate (desirably made of hafnium (Hf) and,
by way of example, 12 mm in diameter). Except for an annular
(ring-shaped) exposed region 102, the cathode 101a has a surficial
coating 103 of an element such as iridium (Ir). The coating 103
serves as a "defining region" that defines the annular exposed
region 102. The work function of Hf is 3.6 eV, and that of Ir is
5.3 eV (providing a work-function difference of 1.7 eV). Therefore,
during operation of the electron gun 101, electrons are emitted
essentially only from the annular region 102 of the cathode 101a
because the annular region 102 is not coated. Hence, the annular
region 102 is the electron-emissive surface of the cathode 101a. By
way of example, the annular region 102 is 2 mm wide and has an
outer diameter of 10 mm.
[0049] As an alternative to the coating 103 being a defining
region, the annular region 102 can be layered onto the layer
103.
[0050] During operation, a negative voltage (e.g., -100 KV) is
applied by a power supply 104 to the cathode 101a. The second-anode
101c, which defines a center aperture through which the electron
beam passes, is normally at 0 V ("ground"), as controlled by the
power supply 104. The control anode 101b, having a configuration
similar to that of the second anode 101c, is situated between the
cathode 101a and the second anode 101c. In this example, a negative
voltage (e.g., -82 KV) is applied to the control anode 101b by the
power supply 104. The electron gun 101 emits a "hollow" electron
beam from the annular region 102. According to simulation studies,
and by way of example, a virtual image of the cathode 101a (i.e.,
of the annular region 102) is formed at an axial position 115
located 63 mm upstream of the cathode 101a. An image of the cathode
is formed at an axial location denoted "C.O.1 " situated 185 mm
downstream of the cathode 101a.
[0051] Situated downstream of the electron gun 101 is an
illumination-optical system. The illumination-optical system
comprises a four-stage condenser-lens assembly including the
condenser lenses 105, 106, 107, and 109. A field-limiting aperture
108 is situated at the same axial position as the third condenser
lens 107. By way of example, the first (most upstream) condenser
lens 105 is situated 200 mm from the cathode 101a. The
field-limiting aperture 108 is used to define the outer
(peripheral) profile of the illumination beam. An image of the
field-limiting aperture 108, as formed on the reticle 110,
encompasses one exposure unit (e.g., a subfield) of the pattern
defined on the reticle 110. The dimensions of the image of the
cathode 101a at the field-limiting aperture 108 are controlled by
the magnification imparted to the beam by the condenser lenses 105,
106 that act concertedly in a "zoom" manner.
[0052] Situated downstream of the field-limiting aperture 108 is a
selection deflector (not shown). The selection deflector scans the
illumination beam to individually illuminate the exposure units of
the reticle 110 in a sequential manner. Such scanning is primarily
within a defined range (corresponding to the optical field of the
illumination-optical system) in the horizontal (left-right)
direction in the figure. To extend the scanning range beyond the
optical field, each of the reticle 110 and substrate 114 are
individually mounted on respective movable stages (not shown).
[0053] In FIG. 3, although only one exposure unit on the reticle
110 is shown, it will be understood that the reticle 110 extends
further in a plane perpendicular to the optical axis A and
typically includes a large number of exposure units.
[0054] Situated downstream of the reticle 110 is a
projection-optical system. The projection-optical system comprises
first and second projection lenses 111 and 113, a contrast aperture
112, and a deflector (not shown). As each exposure unit on the
reticle 110 is illuminated by the electron beam (i.e., by the
illumination beam) the resulting imaging beam is deflected by the
deflector and demagnifyingly projected by the projection lenses
111, 113 to form an image of the illuminated exposure unit at a
prescribed location on the substrate 114. The deflector downstream
of the second projection lens 113 adjusts the lateral position of
the imaging beam so that the image of the illuminated exposure unit
is formed at the desired location on the substrate 114. The
substrate 114 is surficially coated with an appropriate resist so
as to be imprinted with the respective images of the illuminated
exposure units if exposed with a sufficient dosage of
electrons.
[0055] The contrast aperture 112 is situated between the projection
lenses 111, 113 in an axial location that effectively divides the
axial distance between the projection lenses 111, 113
proportionately to the demagnification ratio. Thus, the contrast
aperture 112 is situated in a plane that is optically conjugate to
the field-limiting aperture 108 described above. The contrast
aperture 112 has a profile that is a geometric analog of an
exposure unit on the reticle 110. Thus, the contrast aperture 112
acts as a shield that blocks non-patterned portions of the imaging
beam (i.e., electrons scattered by the reticle 110) from reaching
the substrate 114.
[0056] In FIG. 3, regions in which the transverse beam-intensity
distribution is essentially uniform are located within the area
bounded by the dot-dash lines 117, and regions in which an image of
the cathode 101a can be formed are located within the area bounded
by the dashed lines 118. The uniform-distribution regions are
located on both sides of each of respective crossovers C.O.1-C.O.5.
Since a crossover is a location where space-charge effects are
greatest, it is desirable that the crossover C.O.5 be located
relatively near the substrate 114. That is, the beam current at the
crossover C.O.4 is higher than at the crossover C.O.5 because, at
C.O.5, the beam does not include scattered electrons blocked by the
contrast aperture 112. It is important to reduce space-charge
effects in a beam about to be incident on the substrate 114. Also,
the FIG. 3 system is configured so that a uniform beam-intensity
distribution desirably occurs at an object position 116 of the
condenser lens 105.
[0057] The field-limiting aperture 108, configured to allow the
electron beam to illuminate only one exposure unit at a time on the
reticle 110, is situated at the same axial location as the
condenser lens 107. In this illumination-optical system, the
principal plane of the condenser lens 107 is optically conjugate to
the imaging-location 116 at which the transverse intensity
distribution of the beam is uniform. As a result, changes in beam
intensity at the condenser lens 107 do not affect imaging
conditions at the imaging location 116.
[0058] The location of the image of the crossover C.O.1 can be
changed by changing the voltage applied to the control anode 101b.
This makes it possible to adjust the imaging conditions without
changing the lens conditions.
[0059] Therefore, an electron-beam microlithography system is
provided that can illuminate a larger exposure unit using an
electron beam produced by an electron gun having lower emittance
than achievable with conventional electron-beam microlithography
systems.
[0060] Consequently, image degradation due to space-charge effects
is reduced compared to conventional systems.
[0061] FIG. 4 is a flow chart of steps in a process for
manufacturing a semiconductor device such as a semiconductor chip
(e.g., an integrated circuit or LSI device), a display panel (e.g.,
liquid-crystal panel), or CCD, for example. In step 1, the circuit
for the device is designed. In step 2, a reticle ("mask") for the
circuit is manufactured. In step 3, a wafer is manufactured from a
material such as silicon.
[0062] Steps 4-12 are directed to wafer-processing steps,
specifically "pre-process" steps. In the pre-process steps, the
circuit pattern defined on the reticle is transferred onto the
wafer by microlithography. Step 13 is an assembly step (also termed
a "post-process" step) in which the wafer that has passed through
steps 4-12 is formed into semiconductor chips. This step can
include, e.g., assembling the devices (dicing and bonding) and
packaging (encapsulation of individual chips). Step 14 is an
inspection step in which any of various operability and
qualification tests of the devices produced in step 13 are
conducted. Afterward, devices that successfully pass step 14 are
finished, packaged, and shipped (step 15).
[0063] Steps 4-12 also provide details of wafer processing. Step 4
is an oxidation step for oxidizing the surface of a wafer. Step 5
involves chemical vapor deposition (CVD) for forming an insulating
film on the wafer surface. Step 6 is an electrode-forming step for
forming electrodes on the wafer (typically by vapor deposition).
Step 7 is an ion-implantation step for implanting ions (e.g.,
dopant ions) into the wafer. Step 8 involves application of a
resist (exposure-sensitive material) to the wafer. Step 9 involves
microlithographically exposing the resist so as to imprint the
resist with the reticle pattern, as described elsewhere herein.
Step 10 involves developing the exposed resist on the wafer. Step
11 involves etching the wafer to remove material from areas where
developed resist is absent. Step 12 involves resist separation, in
which remaining resist on the wafer is removed after the etching
step. By repeating steps 4-12 as required, circuit patterns as
defined by successive reticles are superposedly formed on the
wafer.
[0064] Whereas the invention has been described in connection with
multiple representative embodiments, it will be understood that the
invention is not limited to those embodiments. On the contrary, the
invention is intended to encompass all alternatives, modifications,
and equivalents as may be included within the spirit and scope of
the invention as defined by the appended claims.
* * * * *