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BACKGROUND OF THE INVENTION
The invention relates generally to x-ray lithography and more particularly
to soft x-ray projection lithography, and x-ray optical devices for
performing same.
After its initial demonstration in 1972, proximity print x-ray lithography
(PPXRL) appeared to be the lithography of choice for future submicron
work. PPXRL uses "hard" x-rays (wavelengths of 0.3 to 2 nm) to expose a
mask consisting of an x-ray absorber pattern (usually gold or tungsten) on
an x-ray transparent membrane (silicon, silicon nitride, boron nitride,
etc) at some finite distance (5 to 50 microns) from a resist coated wafer.
Unfortunately, PPXRL has several fundamental constraints arising from
diffraction effects, penumbra and secondary photoelectron range which may
limit replications to linewidths greater than 200 nm. Even with these
limitations, it appeared that PPXRL would be the primary lithographic tool
for linewidths from 200 nm to 1 micron and would meet lithographic needs
for many years to come.
However, PPXRL has not reached expectations. There are three primary
reasons for this: (1) a high brightness x-ray source was needed to obtain
high wafer throughput, (2) the hard x-rays required masks with thick
absorber patterns and high-aspect-ratio submicron structures which are
difficult to produce and (3) the thin, x-ray transparent membranes have
had severe distortion and lifetime problems. While solutions to these
problems were pursued, optical lithography has advanced its capabilities
so that it can now replicate 500 nm linewidths. This has reduced the
immediate need for PPXRL. With the fundamental resolution limitations of
PPXRL and some mask issues still unresolved, it is questionable if PPXRL
will ever meet original expectations.
New advances in the field of x-ray optics have been responsible for many
new x-ray optical components such as normal incidence soft x-ray mirrors,
beamsplitters and highly dispersive multilayer mirrors. These new optical
components have made it possible to design and build new instruments such
as x-ray microscopes, telescopes, waveguides and interferometers. It is
highly desirable to apply these new x-ray optical components to produce a
soft x-ray projection lithography (XRPL) system which is capable of
projecting a magnified or demagnified image of an existing pattern from a
mask onto a resist coated substrate.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide method and
apparatus for performing soft x-ray projection lithography.
It is also an object of the invention to provide a soft x-ray reduction
camera which projects a mask pattern onto a resist coated substrate.
It is another object of the invention to use soft x-rays for projection
lithography.
It is a further object of the invention to provide method and apparatus for
performing submicron, and even sub-100 nm, x-ray lithography.
It is also an object of the invention to provide a soft x-ray reduction
camera with 1-10x demagnification.
It is another object of the invention to relax the mask requirements for
x-ray lithography.
It is a further object of the invention to provide a soft x-ray reduction
camera with high resolution, a large depth of field, and a flat-field
image over large areas.
The invention is method and apparatus for performing soft x-ray projection
lithography. An x-ray reduction camera is formed of a pair of spherical
x-ray mirrors positioned in a spaced apart relationship having a common
center of curvature; a camera could also be formed with aspherical
mirrors. The convex surface of the shorter radius (primary) mirror and the
concave surface of the larger radius (secondary) mirror are coated with
periodic multilayers of alternating high index/low index materials, e.g.
Cr/C, Mo/Si or B/Ru, to provide high x-ray reflectivity at near normal
incidence. A transmissive or reflecting mask is positioned relative to the
mirrors so that x-rays incident on the mask are transmitted through or
reflected by the mask onto the primary mirror which reflects the x-rays to
the secondary mirror which reflects the x-rays to an image plane. A laser
generated plasma source or a synchrotron can be used to produce soft
x-rays. A condenser system is used to provide uniform illumination of the
mask by the source. The transmission mask can be used in an on-axis
embodiment in which the mask is aligned on a common axis with the two
mirrors, or in an off-axis embodiment which provides higher collection
efficiency. A reflection mask off-axis embodiment is preferable since mask
requirements are easier, e.g. a patterned multilayer on a thick substrate.
The mask substrate is curved to produce a flat image. A resist coated
wafer is placed at the image plane so that a reduced image of the mask is
transferred thereto.
Using x-ray optical components in accordance with the invention, a soft
x-ray reduction camera (XRRC) with 1-10x demagnification and capable of
producing sub-100 nm lines can be built. An XRRC has many advantages over
a PPXRL system including superior resolution and ease of mask fabrication.
In a preferred XRRC design, the x-rays reflect off a mask pattern on a
thick substrate rather than transmit through a thin membrane. The mask
fabrication technology for the XRRC system has already been demonstrated
(the masks are patterned multilayer mirrors). In addition, since the XRRC
demagnifies the original mask pattern, optical lithography can be used to
generate a mask suitable to produce 100 nm linewidth patterns at the image
plane of a 5x reduction system.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a schematic view of an x-ray reduction camera using a
transmission mask and on-axis imaging.
FIG. 2 is a schematic view of an x-ray reduction camera using a
transmission mask and off-axis imaging.
FIG. 3 is a schematic view of an x-ray reduction camera using a reflection
mask and off-axis imaging.
FIG. 4 is a schematic view of the optics of an off-axis reflective mask
x-ray reduction camera.
FIGS. 5 A, B are graphs of theoretical normal incidence x-ray mirror
performance of a C/Cr multilayer.
FIGS. 6 A, B, C compare a conventional x-ray lithography mask, a
transmission mask for an x-ray reduction camera, and a reflective mask for
an x-ray reduction camera, respectively.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
I. GENERAL DESCRIPTION
The soft XRRC design of the invention utilizes normal incidence reflecting
spherical (or aspherical) mirrors for imaging. Normal incidence soft x-ray
multilayer mirrors are presently being fabricated. Reflectivities in
excess of 50% at 13 nm have been measured and theoretical reflectivities
in excess of 75% are predicted at various wavelengths. An initial XRRC
design using spherical mirrors and an x-ray transmissive mask is shown in
FIG. 1 (the condenser optics and source are not shown) and can be
described as an inverse cassegrainian system. A properly designed
reduction system with spherical imaging mirrors can be free of all
significant wavefront aberration over an extended field of view.
In the x-ray reduction camera of FIG. 1, soft x-ray radiation is incident
substantially normally to and transmitted through mask 10 onto the convex
surface of a primary multilayer spherical x-ray mirror 12 which reflects
the x-rays onto the concave surface of a concentric secondary multilayer
spherical x-ray mirror 14 which reflects the x-rays to an image plane 16
at which a resist coated wafer 18 is placed. The system of FIG. 1 uses
on-axis imaging; secondary mirror 14 is positioned on the axis between
mask 10 and primary mirror 12. Aperture 20 is provided in secondary mirror
14 so that x-rays transmitted through mask 10 are incident on primary
mirror 12. A variation of the camera of FIG. 1 which utilizes a
transmission mask and off-axis imaging is shown in FIG. 2. Mask 10 is
placed off-axis so that secondary mirror 14 does not extend into the path
between mask 10 and primary mirror 12. The x-ray radiation is incident on
mask 10 at an angle so that it is transmitted to primary mirror 12 without
having to pass through secondary mirror 14. Mirrors 12, 14 are spherical
multilayer x-ray mirrors having a common center so that x-rays from the
convex surface of mirror 12 are reflected by the concave surface of mirror
14 to image plane 16 at which resist coated wafer 18 is placed.
A preferred embodiment of the invention is an off-axis imaging camera with
reflecting mask as shown in FIG. 3. X-rays are incident at an angle onto a
reflecting mask 22 which reflects the x-rays onto the convex surface of a
primary multilayer spherical x-ray mirror 12 which reflects the x-rays
onto the concave surface of a commonly centered secondary multilayer
spherical x-ray mirror 14 which reflects the x-rays to an image plane 16
at which a resist coated wafer 18 is positioned.
The diffracted limited resolution of an imaging system is approximately 1.2
.lambda.f#. A preferred 5x reduction design will use soft x-rays, because
high reflectivity (R>50%) multilayer coatings are achievable at these
wavelengths and the range of secondary photoelectrons (which could degrade
the resolution in the exposed resist) is small (about 5 nm). The design
(and the invention in general) utilizes 2 nm-250 nm wavelength radiation
(particularly 2 nm-150 nm radiation) and normal incidence reflecting
optics which are highly reflective for 2 nm-250 nm wavelengths. The
inverse cassegrainian system will require an f#<18.5 at the image plane
(f#92.5 at the object plane) to produce sub-100 nm lines. The
depth-of-field (X(f#).sup.2) at the image plane of this system would be
greater than 1 micron.
The preliminary design in FIG. 1 has two major limitations which can be
easily corrected. The first is that on-axis radiation will travel through
the mask, strike the first mirror and be reflected out of the system
without being collected by the second spherical mirror. This can lead to
non-uniform illumination at the image plane but can be corrected by
repositioning the mask and illuminating it at a slight angle, as shown in
FIG. 2. The second limitation is a curved image plane due to the spherical
optics. Uncorrected, this would limit the field-of-view of the system. A
flat-field image plane over large (about cm.sup.2) areas is accomplished
by using a reflection mask (as shown generally in FIG. 3) comprised of a
patterned x-ray multilayer mirror on a curved substrate, as shown in FIG.
4. By appropriately choosing the mask curvature, a flat-field image is
obtained with no loss in resolution across the entire image plane. A
reflective mask on a thick substrate has several advantages over a
transmissive mask on a thin membrane such as: (1) reduced mask distortion;
(2) durability (less radiation induced damage); (3) handling ease; (4)
better temperature control (efficient cooling) and (5) higher contrast
(greater than 1000:1). In addition, a demagnifying XRRC requires a mask
with large (about a micron) linewidths and low aspect ratios as compared
to the submicron linewidths and high aspect ratios needed for a PPXRL
mask, making the mask fabrication for the XRRC relatively easy.
II. ILLUSTRATIVE EXAMPLE - 5x DESIGN
preferred and illustrative embodiment of the invention is a 5x demagnifying
XRRC suitable for sub-150 nm lithography and large field exposures. This
exemplary system has a 0.5 cm diameter (image area=0.2 cm.sup.2) and f#=25
at the image plane. Theoretically, this system should have a diffraction
limited resolution of about 135 nm and a depth-of-field of 2.8 microns for
4.5 nm radiation. Calculations indicate that the field-of-view can be
extended to greater than 0.5 cm.sup.2 areas without a degradation in
resolution. The image area is rectangular because the mask is displaced in
one dimension (as shown in FIG. 3). For clarity, the XRRC design is
subdivided into the following subsections: (A) Imaging Optics; (B)
Multilayer Coatings; (C) X-ray Source; (D) Mask; (E) Condenser Optics and
(F) Alignment of Optics. In some instances, the sections are interrelated
and design decisions in one section utilize information from other
sections.
(A) Imaging Optics
The x-ray imaging and reduction optics are two spherical, multilayer coated
mirrors having a common origin or center of curvature, as shown in FIGS.
3, 4. Various combinations of mirror radii can be used to obtain
demagnification factors from 1.5 to 1000 with no aberrations up to third
order. The primary mirror is the first mirror that x-rays reflected from
the mask will strike and the remaining mirror is the secondary mirror. The
5x reduction camera requires a primary mirror with a radius of curvature
of 16.66 cm and a secondary mirror with a radius of curvature of 66.66 cm
for a mask to primary mirror distance of 50 cm. A small error in the
radius of curvature of either mirror will affect the system magnification
and the location of the image plane, but will not severely affect system
resolution. Therefore, mirrors with slightly incorrect radii of curvature
can be used if they are accurately measured.
A 2.5 cm diameter mask requires that the diameter of the primary optic be
about 2.5 cm and the diameter of the secondary optic be about 7.5 cm. The
system is specifically designed to keep the size of the optics as small as
possible. To achieve diffraction limited resolution will require spherical
optics with .lambda./8 (about 1 nm) figure of merit over the entire
imaging surface. Fortunately, optics with about 6 nm figures are
commercially available and 1 nm figures appear possible over the small
areas required.
The XRRC design places a pupil 24 in the same plane as the center of
curvature of the two spherical mirrors 12, 14, as shown in FIG. 4. This
correctly balances aberrations over large fields so the resolution over
the entire image area is constant. The chief rays are defined as passing
through the center of the pupil. Imaging the chief rays to a point will
require a mask on a curved substrate, but this will produce a distortion
free image in this monocentric system.
(B) Multilayer Coatings
X-ray multilayer mirrors on flat and curved surfaces have been fabricated.
These mirrors provide high normal incidence reflectivities (>50%) for soft
x-rays (2 nm-250 nm). In brief, alternating layers of high index and low
index materials are deposited onto a smooth substrate. The choice of
materials is determined by the x-ray wavelength and the thicknesses of the
individual layers is determined by the wavelength and the angle of
incidence between the radiation and the mirror surface. Multilayer mirrors
are defined by their "d-spacing" which is the total thickness of two
adjacent layers or one period, and by their "Y" which is the ratio of the
thickness of the high index material to the period. A "d-spacing"
uniformity better than 0.25% over a 75 mm diameter wafer has been
demonstrated and can probably be maintained over much larger areas.
High normal incidence reflectivities (R>50%) at 4.5 nm wavelength are
predicted for a carbon and chromium multilayer mirror with a "d-spacing"
of about 2.25 nm, Y of about 0.35 (Cr=0.8 nm; C=1.45 nm) and 200 layer
pairs, FIGS. 5 A, B. Unfortunately, the spectral bandwidth of this mirror
is small (<1%) and may cause practical difficulties in the XRRC (a small
error in the "d-spacing" of one mirror relative to the second will reduce
the overall system performance). A suitably designed mirror fabrication
facility will be able to maintain multilayer "d-spacing" control within
this narrow bandwidth.
(C) X-ray Source
A number of different x-ray sources were evaluated and a laser produced
plasma source may be the most appropriate. However, other sources such as
a synchrotron could also be used. In an ideal imaging system, the entendue
(also known as the optical invariant), defined as the product of the
collection solid angle, d.OMEGA., and the source size, dA, is conserved.
The present design requires that at the final image plane and all
intermediate image planes, the entendue is constant. The entendue at the
image plane in the XRRC design is:
Entendue=d.OMEGA.dA=(1.3.times.10.sup.-3).times.(0.2
cm.sup.2)=2.5.times.10.sup.-4 sr-cm.sup.2 where the solid angle,
d.OMEGA.=.pi./4(f#).sup.2). To produce this image, a source with an
entendue equal to or greater than the entendue at the image plane is
required. A laser produced plasma has a large solid angle
(d.OMEGA.=2.pi.sr) and can have sufficient entendue for illuminating the
reduction camera. A typical laser produced plasma is 100-300 microns in
diameter, producing a source entendue of 5.times.10.sup.-4 to
4.times.10.sup.-3 sr-cm.sup.2. A 300 micron source diameter and a
condenser system (FIG. 4) which collects 0.35 sr of the source emission to
match the entendue at the image plane are typically used.
Laser produced plasma sources have been well characterized and described. A
short pulse laser (usually <1 nsec) is focused onto a solid target,
typically gold or another high atomic number material. Laser power
densities of 2 to 10.sup.15 watts/cm.sup.2 on the target will produce a
high density, high temperature plasma. The x-ray emission from these
plasmas is essentially black-body radiation with characteristic
temperatures of 50-200 ev. Shorter wavelength lasers can couple more
efficiently to the plasma and heat it more effectively. As a result, much
work has been done with YAG lasers (1.06 microns) and through frequency
conversion crystals at higher harmonics of the YAG laser (0.53 microns and
0.26 microns). The x-rays are emitted into 2 .pi.sr, and the power emitted
is given by:
P(watts/sr)=2(kt)(.DELTA..lambda.)cA/.lambda..sup.4
where:
.lambda.=4.5 nm
.DELTA..lambda.=0.05 nm
kt=black-body temperature
c=3.times.10.sup.10 cm/sec
A=area
In the preferred design, a 10 joule, 1 nsec/pulse YAG laser is focused onto
a solid target (300 micron diameter spot size) to produce a power density
of 1.4.times.10.sup.13 watts/cm.sup.2. Experiments have shown that, under
similar conditions, a plasma whose x-ray emission is characteristic of
about a 100 ev black-body radiator (kt=1.6.times.10.sup.-17 joules) will
be produced. Using the above equation, the x-ray emission during the time
that the laser irradiates the target and within the approximately 1%
bandwidth of the multilayer mirrors is about 8.times.10.sup.7 watts/sr or
about 80 millijoules/sr per laser pulse.
(D) Mask
The masks for the XRRC design will be a patterned x-ray multilayer mirror
fabricated on a thick substrate, 2.5 cm in diameter to produce an image
area 0.5 cm in diameter. In this design, the mask is part of the condenser
system (FIG. 4). A mask on an appropriately curved substrate can eliminate
the field curvature at the image plane. The required ideal mask substrate
is ellipsoidal with a radius of curvature of 11.08 cm and a conic constant
of -0.69 (a conic constant of -1 defines a parabola). Fortunately, the
figure of the ellipsoidal surface must be accurate within the
depth-of-field of the imaging system. At the mask, the depth-of-field for
the XRRC design is about 70 microns. Such tolerances may allow the surface
to be approximated with an appropriate spherical surface.
Molybdenum/silicon x-ray multilayer mirrors have been patterned by a number
of techniques to produce highly dispersive multilayer mirrors and this
technology will be used to pattern the required x-ray masks. For
carbon/chromium multilayer mirrors, it appears that either reactive ion
etching or ion beam etching can be used to pattern the multilayer. The
most appropriate mask fabrication procedure may be to (1) deposit the
multilayer mirror onto the substrate, and (2) pattern the multilayer with
a focused ion beam system. This mask-less fabrication sequence eliminates
any problems with patterning on a curved substrate.
The contrast of the x-ray mask will be determined by the ratio of the x-ray
reflectivity from the multilayer mirror surface versus the reflectivity
from regions where the mirror is removed (reflectivity from bulk
material). At 4.5 nm, the x-ray reflectivity from a polished silicon or
glass substrate is less than 10.sup.-4, as compared to a 50% reflectivity
from the multilayer mirror. This gives a theoretical mask contrast greater
than 1000:1. In practice, mask contrasts of at least about 100:1 are
desired. For comparison, in PPXRL, mask contrasts are typically only 10:1.
This increase in mask contrast will improve image definition and can relax
the requirements on the x-ray resist.
A comparison of a conventional x-ray lithography mask (PPXRL), a
transmission mask for reduction camera, and a reflection mask for
reduction camera is shown in FIGS. 6 A, B, C. The conventional mask is
formed on a thin substrate, e.g. 2 micron Si, with submicron width lines
of Au or W with a high aspect ratio of about 3-4, e.g. 0.5-1.0 micron
height, with 10:1 contrast for 10 A x-rays. The XRRC transmission mask has
a thin substrate, e.g. 2 micron diamond, with micron width lines of Au or
W of low aspect ratio (about 0.1), e.g. 0.1 micron height, with 10:1
contrast for 45 A x-rays. The reflection mask is the easiest to fabricate,
on a thick solid substrate (which may be curved), with micron width lines
of patterned multilayers (Cr/C, Mo/Si or B/Ru) 0.2-0.4 microns high, with
an aspect ratio of about 0.3 and a contrast of about 1000:1 for 45 A
x-rays.
Exposing large areas (about 1 cm.sup.2) and sub-100 nm linewidths will
require large masks which must be illuminated with a large range of angles
(>.+-.10.degree.). Under these conditions, it may be necessary to reduce
the number of multilayers on the mask to increase its angular bandwidth
because a carbon/chromium multilayer mirror with 200 layer pairs will only
reflect x-rays within .+-.5.degree. of normal incidence. A decrease in the
number of layers will increase the angular bandwidth with a corresponding
loss in peak reflectivity, resulting in an increase in the exposure time.
At longer wavelengths, about 13 nm, the angular bandwidth of the mirrors
is sufficiently large that this will not be a problem.
(E) Condenser Optics
A condenser system is used to direct an x-ray beam from the x-ray source to
the mask. As shown in FIG. 4, the x-ray source (laser produced plasma) 30
is placed at the focus of a primary condenser lens 32 which is formed of a
concave spherical x-ray mirror (various x-ray mirrors in the XRRC are
sometimes referred to as lenses since they provide the same function).
X-rays from source 30 incident on lens (mirror) 32 are reflected onto
secondary condenser lens 34 which is formed of a concave spherical x-ray
mirror in a spaced relation to primary lens 32. The condenser optics
(lenses 32, 34) are positioned so that an x-ray beam from source 30 is
directed onto mask 26 from which it is reflected into the camera imaging
optics (mirrors 12, 14). The XRRC design employs a Kohler-type condenser
system. The advantage of this system over a single lens critical
illumination design is that every point in the source fully illuminates
the mask and produces a uniform illumination. The disadvantage of the
Kohler system is that it requires additional x-ray reflecting surfaces
which will increase exposure times because of the finite mirror
reflectivities. The condenser lens will need to collect 0.35 sr (f#=1.5)
of emitted radiation from the laser produced plasma to match the entendue
required at the image plane. At first, this may seem to be a difficult
task because a high quality x-ray optic with such a large solid angle has
never been fabricated. Fortunately, the resolution of an imaging system is
nearly independent of the imaging characteristics of the condenser lens.
Therefore, the condenser lens can be fabricated in several small sections
36 and assembled together, FIG. 4. Fabricating the condenser lens in
sections has the added advantage that the multilayer "d-spacing" can be
varied along the surface of the condenser lens to achieve optimum
performance.
FIG. 4 shows one possible condenser lens configuration. Other lens
configurations are possible, such as a three lens condenser system, which
may have some advantage. A blast-shield 38 between the laser produced
plasma and the primary condenser lens protects the condenser lens from
debris. The x-rays from the laser produced plasma are re-imaged by the
primary lens at an intermediate location between the two lenses and then
collected by the second condenser lens and directed onto the mask. Because
the mask is an ellipsoid, the second condenser lens should also be an
ellipsoid of similar design (radius of curvature=11.08 cm, conic
contrast=-0.69). Again, the ellipsoid can be of very poor quality because
the resolution of the imaging system is determined only by the final
imaging optics. The tolerances on the mask and condenser lens ellipsoids
can easily be met with todays technologies.
(F) Alignment of Optics
The spherical imaging optics have a common center of curvature and the
multilayer coatings will reflect both x-ray and optical radiation. This
feature is critical in that it allows the system to be aligned at visible
or near visible wavelengths. A laser interferometer can be used to
position a spherical mirror to within a few microns longitudinally and a
fraction of a micron laterally. The x-ray imaging optics are mounted on a
rotary table whose axis of rotation coincides with the focal point of a
long path length focusing interferometer. A small sphere is located on the
axis of rotation of the rotary table. This sphere is used to align the
interferometer to the center of rotation of the table. The spherical
imaging optics are individually aligned to the interferometer by rotating
each optic into the interferometer and adjusting the optic until alignment
is achieved. In this manner, both imaging mirrors can be accurately
positioned with respect to the interferometer focal point. The accuracy of
this system is consistent with alignment tolerances of about 10 microns
longitudinal and about 3 microns lateral displacement of the primary and
secondary imaging mirror centers of curvature.
III. EXPOSURE TIMES
The exposure time will depend upon the x-ray throughput of the imaging
system, the number of x-rays produced by the laser produced plasma and the
sensitivity of the x-ray resist. The x-ray throughput is defined as the
amount of x-ray energy focused in the image field divided by the x-ray
energy emitted by the source within the bandwidth of the optics. The x-ray
throughput can be calculated based upon the properties of the multilayer
mirrors. However, the total amount of x-rays produced by the source will
depend upon the target, the power density at the focus and the average
power from the laser.
The present design utilizes x-rays generated by the laser produced plasma
at 4.5 nm and within a 1% bandwidth. X-rays outside this bandwidth will
not be reflected by the mirrors and will not contribute to the image
formation. The x-rays within the mirror bandwidth will travel through the
blast shield twice (assume 75% transmission per pass), reflect off the two
condenser lenses, the x-ray mask and the two imaging lenses to be imaged
in the 0.5 cm diameter field (area=0.2 cm.sup.2). The XRRC will image
about 1.5% (0.55.times.0.752) of the x-rays that are collected by the
first condenser lens within its 0.35 sr solid angle. Assuming that the
laser system (10 Joules/pulse) can produce a 300 micron diameter, 100 ev
black-body radiator, calculations show that the source will emit 80
millijoules/sr per laser pulse within the desired bandwidth. The optics
will collect 0.35 sr and focus 420 microjoules per laser pulse onto the
image field. To expose an x-ray resist with a sensitivity of 20
millijoules/cm.sup.2 will require 10 laser pulses. Therefore, a 100 watt
average power laser will expose the field in 1 second or a 3-inch wafer in
3.8 minutes.
Improvements in exposure times can be realized by increasing the solid
angle of the condenser lens or by changing the x-ray wavelength. For
example, increasing the collection solid angle to 1 sr will decrease the 3
inch wafer exposure times to about 1 minute. Changing the x-ray wavelength
to coincide with higher reflectivity mirrors will also decrease the
exposure times. For example, reflectivities in excess of 70% are predicted
at a wavelength of 12.4 nm compared to the 50% that are predicted at 4.5
nm. With five reflective surfaces, this increase in reflectivity will
decrease the exposure times by more than a factor of five. Together, these
two changes could increase the system throughput to five 3-inch wafer
exposures per minute. The disadvantage is that, for a fixed f#, the longer
x-ray wavelength will decrease the diffraction limited resolution. An XRRC
operating with 12.4 nm radiation will need an f# of about 10 at the image
plane to obtain equivalent diffraction limited resolution as the system
described herein. In addition, the x-ray absorption depth in the resist at
this longer wavelength is decreased to about 0.3 microns which will
require a bi-level resist process to produce high aspect ratio structures.
IV. CONCLUSION
An x-ray reduction camera utilizing normal incidence spherical x-ray
mirrors and a reflective x-ray mask can be used to perform soft x-ray
projection lithography. This system should have about 135 nm resolution
and a large exposure field. The advantages of the reflective masks on a
thick substrate versus a transmissive mask on a thin membrane are many and
include stability, durability, ease of handling and fabrication. The
technology to build a prototype system for 1/4 micron lithography is
currently available, although the x-ray imaging mirror substrates need to
be improved to produce diffraction limited (sub 100-nm lithography)
results.
An alternate design utilizes a flat (planar) x-ray reflecting mask and
aspheric imaging lenses to obtain a flat image plane and high resolution.
Calculations indicate that the required aspheric surfaces can be easily
fabricated by controllably depositing a thin layer of material on a
previously measured surface, such as depositing SiO.sub.2 onto a polished
spherical glass surface in such a way as to obtain the necessary aspheric
surface.
Changes and modifications in the specifically described embodiments can be
carried out without departing from the scope of the invention which is
intended to be limited only by the scope of the appended claims.
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