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BACKGROUND OF THE INVENTION
The present invention relates to an X-ray lithography apparatus which
achieves a higher precision in precision manufacturing and a practical
wafer-processing capability (a sufficient throughput).
The X-ray lithography apparatus has a higher resolution than known optical
lithography apparatuses or the direct plotting technique using electronic
beams and hence, is noted as the precision processing technology for
superhigh-integrated submicron apparatuses. It is expected that X-ray
lithography apparatus will soon replace optical lithography in the
industry as it can obviate the difficulties encountered in the electronic
beam lithography, e.g. low economic efficiency or low throughput and high
cost in apparatus manufacture.
The X-ray lithography apparatus of which application has so far been
limited to laboratories are mainly of the projection type apparatuses with
dot-type X-ray source which is generally obtained by emanating and
focusing electron beams on narrow spots on a fixed or rotary target. The
irradiated X-ray beams in this type of apparatuses become extremely
deviated from the parallel arrangement to therefore induce distortion in
transferred patterns especially on the periphery of a wafer placed below
the X-ray source. More particularly as shown in FIG. 1, a mask substrate 1
used in this type of technology comprises Si.sub.3 N.sub.4 thin film of
ca.3 .mu.m thickness in order to fully tranmit soft X-ray which is easily
absorbed. An interval S of 5-25 .mu.m is provided between the mask
substrate 1 and a wafer 2 via a spacer 4 in order to prevent damages which
might be caused by the contact with a resist surface 3 coated on the
surface of the wafer 2. Due to this interval S, a distortion .DELTA. or
the transfer error is caused in the patterns produced with the X-ray
irradiated from a dot-like source 5, which distortion becomes conspicuous
on the periphery of the wafer 2. This creates a critical defect in the
manufacture of precision apparatuses where highly precise alignment is
required. The adverse effect caused by the distortion on the periphery
becomes serious when the wafer 2 is repeatedly processed to transfer
different patterns.
SUMMARY OF THE INVENTION
In view of the above mentioned defects in the prior art, the present
invention aims at providing an X-ray transfer apparatus which can achieve
a higher precision in precision manufacturing by removing distortion in
transferred patterns and which has a sufficiently practical throughput.
For attaining such a purpose, the technical concept of this invention lies
in that a linear X-ray source is formed by line scanning of electron beams
on a target, a band-slit having slits which extend in the longitudinal
direction of the linear X-ray source and a solar-slit comprising plural
slits in the direction transverse to the above slits are interposed
between the X-ray source and the wafer in order to irradiate only a
component which is substantially perpendicular to the surface of the wafer
out of the X-ray beams from the linear source, and said wafer is moved
either continuously or stepwise to the highly collimated rectangular X-ray
irradiation region so as to conduct X-ray transfer on a predetermined
region.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 is a schematic view to show prior art technology.
FIGS. 2 through 4 are explanatory view to show embodiments of the present
invention.
FIGS. 5(a) and (b) are explanatory views to show the modes at the time of
exposure.
BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention will now be described in detail referring to
preferred embodiments thereof shown in attached drawings. As shown in
FIGS. 2 through 4 and especially in FIG. 2, an X-ray generating section 6
of an embodiment according to this invention includes a plural number, for
instance, two of electron guns 6a, 6b which are arranged in a symmetrical
position, a rotary target 6c in a cylindrical form of which inner surface
is cooled with cooling water 7, and electron beam deflectors 13a, 13b
which are connected to a known scanning power source. A linear X-ray
source 6d is formed by line scanning a region in the axial direction of
the rotary target 6c or along the Y axis in the figure with electron beams
14a, 14b shot from the electron guns 6a, 6b. An X-ray 8 irradiates the
surface of the wafer 2 which is placed on an X-Y plane. The scanning
length of the electron beams on the target 6c is defined as l. As is well
known, the permissible power of the rotary target 6c may be remarkably
increased by having electron beams used in line scanning enter the rotary
target 6c. This will alleviate the intensity attenuation of the X-ray 8
which is caused by the existence of a solar-slit 10 to be described later.
The maximum permissible power Wmax of the rotary target 6c is expressed by
the equation (1)
##EQU1##
wherein T: melting point of the material of the rotary target 6c
(.degree.C.)
To: temperature of the cooling water (.degree.C.)
k: thermal conductivity of the material of the rotary target 6c at its
melting point (gr-cal.multidot.sec.sup.-1 .multidot.cm.sup.-1
.multidot..degree.C..sup.-1)
2.delta.: width of the scanning electron beams on the target 6c (cm)
c: specific heat of the material of the rotary target 6c
(cal.multidot.gr.sup.-1 .multidot.C..sup.-1)
v: velocity of the surface of the rotary target 6c (cm.multidot.sec.sup.-1)
.rho.: density of the material of the rotary target 6c
(gr.multidot.cm.sup.-3)
If it is assumed that a typical soft X-ray (Al-k.alpha.) the
physio-chemical constants of aluminum (Al) are used, the diameter of the
rotary target 6c is 15 cm, the revolution rate thereof is 2500 rpm, and
the width of the electron beams is 0.05 cm, the maximum permissible power
Wmax will be obtained from the equation (1)
Wmax=42.times.10.sup.3 .times.l (2)
Accordingly, if the scanning length is set at l=2 cm for practical reasons,
Wmax will become: Wmax=84 kW. This is equivalent to the case where two
sets of electron guns 6a, 6b generating ca. 1 A of electron beam current
with 40 KV of acceleration voltage are used. This technology therefore is
sufficiently applicable to practice. The X-ray conversion efficiency .eta.
of Al-k.alpha. beams against the electron beams of acceleration voltage of
40 KV is ca. 1.5.times.10.sup.-3 to enable an extremely strong and
powerful X-ray 8. Although tungsten is generally used as a filament in an
electron gun for X-ray source, as the electron beam current obtainable
from this type of filament is limited to ca. 400 mA, the electron guns
according to this invention use lanthanum hexaboride (LaB.sub.6) which can
easily provide electron beam current of 1 A or higher.
As shown in detail in FIG. 3, the band-slit 9 has a slit (an aperture) 9a
extending in the direction of Y axis or in the longitudinal direction of
said linear X-ray. The band slit 9 is positioned below the linear X-ray
source 6d in order to restrict the irradiation angle 2.theta. of the X-ray
8 toward the mask substrate 1. The region on the substrate 1 irradiated by
the X-ray therefore remains within a band having the width B. The
distortion in the transferred patterns on the periphery of the wafer 2 is
given by the equation .DELTA.=.theta..multidot.S assuming that the
interval distance between the mask substrate 1 and the wafer 2 is S.
In order to illustrate an example, it is assumed that a submicron pattern
of the line width of 0.5 .mu.m is transferred. If the allowable distortion
.DELTA.=0.05 .mu.m and the interval distance S=5 .mu.m,.theta. will be
.theta.=1.times.10.sup.-2 rad. If the distance between the X-ray source 6c
and the substrate 1 is selected to be D.sub.1 =300 mm and the distance
between the X-ray source 6 and the band-slit 9 is D.sub.2 =100 mm as
suitable for components of a practical apparatus, the width B and width
B.sub.s of the slit (aperture) 9a will respectively be 6 mm
(=2.theta..times.D.sub.1) and 2 mm (=2.theta..times.D.sub.2).
The blur (umbra) w of transferred patterns which is attributable to the
width 2.delta. of the electron beams of the X-ray source 6 is given by the
equation, w=[(2.delta.s)/D.sub.1 ]. By substituting above mentioned
parameters, w=0.0083 .mu.m, which is completely negligible value in the
lithography of the submicron patterns having the line width of 0.5 .mu.m.
As detailedly shown in FIG. 4, a solar-slit 10 comprises a plural number of
oblong slit plates 10a which are arranged at an interval d in the
direction of Y axis and along the direction perpendicular to the slit
(aperture) 9a. The solar-slit 10 is interposed between the band-slit 9 and
the mask substrate 1. By appropriately setting a value for the interval d,
the angle .theta.' of the X-ray 8 on the substrate 1 may be limited to
obtain a desired resolution .DELTA.'. The opening angle .theta.' and the
resolution .DELTA.' herein are to define the opening angle and the
resolution at the point P on the substrate 1 directly below the middle
point of the distance d of the slit plate 10a. If the thickness t of the
slit plate 10a is less than the distance d, the angle and the resolution
at the point Q on the mask substrate 1 directly below the slit plate 10a
will be 2.theta.' and 2.DELTA.' respectively. More particularly, when the
X-ray 8 is irradiated through the solar-slit 10, as the intensity of the
X-ray 8 changes in proportion to the opening angles on respective points
on the mask substrate 1, the X-ray intensity distribution of the transfer
patterns 11 will change with a cycle corresponding to the distance d as
illustrated in FIG. 4. Therefore, if the transfer operation is conducted
with a wafer 2 suspended, the transfer pattern might be added with stripe
patterns caused by the changes in said X-ray intensity distribution to
thereby deteriorate the image quality. This can be obviated by moving a
stage (not shown) which carries the wafer 2 continuously during the
operation in the direction of Y axis in the figure to average intensities
at respective points.
As the slit plates 10a absorb the X-ray 8 which is emanated thereon, the
solar-slit 10 unavoidably attenuates the X-ray 8. If the line elements of
the X-ray source 6d expected at the points P, Q are respectively .DELTA.lP
and .DELTA.lQ, the X-ray exposure at the points P, Q will be attenuated to
.DELTA.lP/l and .DELTA.lQ/l respectively from the exposure obtained
without the solar-slit 10. But if the stage is moved continuously as
mentioned above, the line elements of the X-ray source 6d corresponding to
each point may have a uniform averaged value
.DELTA.l=[(.DELTA.lP+.DELTA.lQ)/2] to make the X-ray attenuation ratio
.DELTA.l/l.
If it is assumed that the resolution of a transfer pattern is 2.DELTA.'=0.1
.mu.m, the equation holds as 2.theta.'=2.times.10.sup.-2 rad because the
distance between the mask substrate 1 and the wafer 2 is assumed as S=5
.mu.m. Therefore, if the upper surface of the solar-slit 10 is positioned
at 150 mm above the mask substrate 1, the distance d between slit plates
10a becomes d=150 mm.times..theta.=1.5 mm to cause no problems whatsoever
in manufacture. If the distance is assumed to be D=300 mm as mentioned
above, the line elements .DELTA.lP and .DELTA.lQ will become 6 mm and 3 mm
respectively. The averaged attenuation ratio of the X-ray intensity will
be .DELTA.l/l=4.5/20=0.225.
The minimum length L min of a slit plate 10a of the solar-slit 10 is easily
determined by the conditions sufficient to shield the irradiation of the
X-ray 8 from the slit plate 10a. As shown in FIG. 4, if the angle formed
by the point P and the upper ends of adjacent slit plates 10a is defined
as .tau., and if the position of the solar-slit 10 is set as
aforementioned, L min=d/.tau. to make L min=100 mm.
As the X-ray 8 is emanated from a linear source 6d toward the mask
substrate 1 via a band slit 9 in this embodiment, as illustrated in FIGS.
5(a) and (b), the irradiation region 12 of the X-ray 8 is defined as a
rectangular region having a width B and the length l' (which is
substantially similar to the length l of the linear X-ray source 6d). As
the rectangular irradiation region 12 is usually smaller than the surface
areas of the mask substrate 1 and the wafer 2, it is necessary to move the
substrate 1 and the wafer 2 in respect of the region 12 in order to
transfer all the patterns on a single piece of wafer 2.
A mask substrate (therefore a wafer) is divided into plural bands
.circle.1 , .circle.2 , .circle.3 . . . of said irradiation region (of
width B and length l') as shown in FIGS. 5(a) and (b). The band width B is
6 mm, the length l' is 20 mm or substantially similar to the length l of
the X-ray source on the rotary target in this embodiment. If it is assumed
here that transfer is made to a 4-inch wafer and more particularly to the
region of 60 mm.times.60 mm at the center thereof, the number of bands
will be 30 as shown in FIG. 5. In this embodiment, the mask substrate 1
and the wafer 2 therefore are superposed on a stage (not shown) so as to
be conveyed in the X-Y plane. At the time the transfer pattern is exposed,
they are moved consecutively over the irradiation region 12 by the stage
as shown by arrows and numerals .circle.1 , .circle.2 and .circle.3
. . . in FIGS. 5(a) and (b). FIG. 5(a) shows the case where the
irradiation region 12 is moved in the direction of the Y axis or of the
slit plate 10a arrangement while FIG. 5(b) shows the case where the
irradiation region 12 is moved in the direction of X axis or the direction
perpendicular to the direction of FIG. 5(a).
Although either one of the moving methods may be employed, there is the
following difference between two. In the case shown in FIG. 5(a), as the
direction of wave in the X-ray intensity distribution 11 (refer to FIG. 4)
coincides with the moving direction of the substrate 1 and the wafer 2,
the X-ray intensity distribution 11 will be averaged without further
manipulation. In the case shown in FIG. 5(b), as the above mentioned two
directions do not coincide with each other, it is necessary to
horizontally vibrate the solar-slit 10 or the mask substrate 1 integrally
with the wafer 2 in the direction of the Y axis.
The exposed (transfer) time in the embodiment shown in FIG. 5 is estimated
below. If the time necessary to expose a region 12 is defined as t.sub.B,
the moving speed V.sub.s of a stage is expressed by the following equation
(3).
##EQU2##
Therefore, the time T required for exposing all the regions will be given
by the equation below.
##EQU3##
The time t.sub.B is expressed by the equation (5).
##EQU4##
wherein J: resist sensitivity (joul/cm.sup.2)
.eta.: X-ray conversion efficiency
h.mu.: photon energy (eV)
E: electron energy (eV)
W: electron beam input (W)
D: distance between the X-ray source 6 and the wafer 2 (cm)
l: scanning length (cm)
.DELTA.l: average width between lines of X-ray source 6 (cm)
When Al-k.alpha. beam is used as the X-ray 8 if above
.eta..congruent.1.5.times.10.sup.-3, h.mu.=1.5.times.10.sup.3 stand,
parameters of this embodiment are used as E=40.times.10.sup.3,
W=84.times.10.sup.3, D=30, l=2 and .DELTA.l=0.45, and J=1.times.10.sup.-3
is substituted as a highest feasible resist sensitivity in the equation
(5), the time t.sub.B will be 0.84 sec (t.sub.B =0.84 sec). The time T
will be obtained if the value is substituted in the equation (4), or T=40
t.sub.B =33.6 (sec). The time T (=33.6) is a throughput which is
sufficiently feasible in the practice of X-ray lithography apparatus for
submicron apparatuses.
Although the stage is assumed to be moved continuously in this embodiment,
it is not limited to that. But it may be moved in the step-and-repeat
system which is generally employed in the electron beam exposure
apparatuses. However, if that system is employed, the time required for
moving the stage between irradiation regions 12 will be wasted as loss
time.
The positional relation between the band-slit 9 and the solar-slit 10 may
be reverse to the above stated one. The band-slits may be positioned on
both sides of the solar-slit. Said solar-slit 10 may not necessarily
comprise layers of planes, but may be other structures, e.g. a grid-like
formation comprising one or more plates arranged arranged in the
longitudinal direction and layers of planes similar to the aforementioned,
or a bundle of plural cylindrical members. In the above embodiment, the
influence from the spatial change of the X-ray intensity which is denoted
by numeral 11 in FIG. 4 is removed by slightly vibrating the mask
substrate of the solar-slit in the longitudinal direction of the
band-slits, and the trouble-some operation of generating vibration could
be avoided if the transfer is conducted at a speed which sufficiently
expose the resist coated on the wafer in the region where the X-ray
intensity is minimal (the point P in FIG. 4).
As described referring to the preferred embodiment in the foregoing
statement, the present invention can remarkably reduce the distortion on
the lithography patterns, can conduct transfer at a higher precision and
can provide sufficient amount of X-ray by forming the X-ray source in a
linear form even if some of the X-ray is unavoidably attenuated by said
slits, limiting the divergence of the X-ray in the longitudinal direction
of the source as well as in the direction perpendicular thereto so that
the largest possible amount of X-ray may be irradiated at a right angle on
the surface of the mask substrate and the wafer. The throughput thus
obtained in the apparatus is fully practical and usable. As the relative
position between the slits and the mask substrate or the wafer can be
adjusted, the influence from the X-ray intensity change on the substrate
may be easily removed. As a plural number of electron guns are positioned
in a symmetrical manner in respect of the X-ray source, the input power of
the electron beams can be maintained uniform over the whole regions of the
X-ray source without using special or complex electron beam scanning
system even though the electron beams are emanated from the direction
which is inclined against the target.
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