The present invention relates generally to optically systems
and exposure apparatuses, and more particularly a mechanism and to a method for
retaining a mirror used in an exposure apparatus. The present invention is suitable,
for example, for an illumination optical system and projection exposure apparatus
using an extreme ultraviolet ("EUV") region having a wavelength of 200 nm to 10
nm or an X-ray region.
Reduction projection exposures using ultraviolet have been
conventionally employed to manufacture such a fine semiconductor device as a semiconductor
memory and a logic circuit in photolithography technology. The critical dimension
to be transferred by the reduction projection exposure is proportionate to a wavelength
of light used for transfer, and inversely proportionate to the numerical aperture
("NA") of a projection optical system. In order to transfer a finer circuit pattern,
a shorter wavelength of used ultraviolet ("UV") light has been promoted from an
ultra-high pressure mercury lamp i-line with a wavelength of about 365 nm to KrF
excimer laser with a wavelength of about 248 nm and ArF excimer laser with a wavelength
of about 193 nm.
However, the lithography using the UV light has the limit
to satisfy rapidly promoting fine processing of a semiconductor device, and a reduction
projection exposure apparatus using EUV light with a wavelength of about 10 to 15
nm much shorter than that of the ultraviolet has been developed to efficiently transfer
a very fine circuit pattern of 0.1 µm or less.
The EUV light source uses, for example, a laser plasma
light source. The laser plasma light source irradiates a highly intensified pulse
laser beam to a target in a vacuum chamber, and generates high-temperature plasma,
emitting EUV light with a wavelength of about 13 nm. The target uses a metallic
thin film, inert gas, droplet, etc., and is supplied to the vacuum chamber by such
means as a gas jet. In order to raise an average intensity of the emitted EUV light,
the pulse laser preferably has a higher repetitive frequency, and is usually driven
by the repetitive frequency of several kHz.
Absorption in an object of EUV light region is so large
that a refraction optical system that uses a lens may lower throughput, while it
is usually used for visual light and UV light. Therefore, exposure apparatuses that
use EUV light usually include a catoptric optical system. For example, isotropically
emitted EUV light from the laser plasma is then condensed by a first condenser mirror
in an illumination optical system, and emitted to the next mirror to illuminate
The laser plasma light source generates not only the EUV
light, but also flying particles called debris, which causes contamination, damages
and lowered reflectance of an optical element. While some methods have been disclosed,
for example, in document
, which prevent debris from reaching an optical element from the target,
there has not been proposed a method for effectively preventing debris from reaching
the first stage mirror, particularly close to the target, in the illumination optical
system. As a result, the debris adheres to a surface on the first mirror and lowers
its reflectance over exposure time. The first mirror should thus be replaced regularly
when the reflectance lowers down to a certain level. A method for facilitating an
exchange and maintenance of the mirror has been proposed, for example, in documents
(corresponding to document
A description will be given of a conventional mirror replacement
method proposed in document
, with reference to FIGS. 9 and 10. Here, FIG. 9 is a schematic partial
section of a vacuum chamber that accommodates an illumination system of an exposure
apparatus. FIG. 10 is a flowchart for explaining a conventional mirror replacement
method. A first mirror 4 is retained by a mirror holder 2 fixed in a vacuum chamber
1 that accommodates an illumination system of an exposure apparatus. The vacuum
chamber 1 has an openable door 6. A water cooling tube 8 is connected to the mirror
holder 2 and cools it. The water cooling tube 8 is connected to the door 6, and
receives cooling water from the outside of the door 6.
In exchanging the mirror 4, the vacuum chamber 1 is returned
to the atmospheric pressure (step 1002), the door 6 is opened (step 1004), and the
water cooling tube 8 is dismounted from the door 6 (step 1006). Then, a hand is
inserted from the door 6, and the mirror 4 is dismounted from the mirror holder
2 (step 1008), a new mirror 4 is mounted onto the mirror holder 2 and its reflective
surface is optically and mechanically positioned (step 1010). Then, the water cooling
tube 8 is attached to the door 6 (step 1012), and the door 6 is shut (step 1014),
followed by the step of drawing a vacuum (step 1016). Thus, the conventional exchange
of the mirror 4 requires a large maintenance space in the exposure apparatus and
a long maintenance time, disadvantageously lowering exposure throughput and contaminating
mirrors, such as an illumination optical system, and the chamber 1 due to a long
opening time of the vacuum chamber 1.
discloses a mirror retaining method that uses part of a mirror for a partition
of the vacuum chamber. This method may shorten an exchange time, because when the
mirror is attached to the vacuum chamber, the mirror itself is simultaneously positioned.
However, actually, the vacuum chamber is likely to deform and the mirror also undesirably
deforms along with a deformation of a wall surface of the vacuum chamber after the
mirror is positioned by attaching it to the chamber. A mechanism for retaining and
positioning a mirror in a vacuum chamber having a wall with an opening, comprising
the features summarized in the preamble of claim 1 is known from document
DE-A-40 07 622
. This document does not explicitly disclose that the chamber having the
wall is a vacuum chamber. However, the chamber disclosed in this document can be
assumed to be evacuated.
In the known mechanism, the coupling means for elastically
connecting the mirror to the lid comprises an elastic ring extending along the edge
of the mirror and located directly between the mirror and the lid. The coupling
means furthermore comprises an O-ring for holding the mirror to the lid. The mount
of the known mechanism is a ring like structure having a supporting surface against
which a reflecting surface of the mirror is pressed by the elastic forces exerted
by the elastic ring.
Due to the design of the known mechanism, the mirror might
be deformed when it is pressed against the supporting surface of the mount so that
the positioning accuracy of the mirror is impaired. Moreover, when the mirror is
to be exchanged, foreign particles on the supporting surface might further impair
the positioning accuracy.
It is an object of the present invention to provide a mechanism
for retaining and positioning a mirror which facilitates an exchange of the mirror
and achieves a high positioning accuracy of the mirror in the vacuum chamber. Furthermore,
it is an object of the present invention to provide a method for retaining and positioning
a mirror, a method for exchanging a mirror, and an illumination apparatus, an exposure
apparatus and a device fabrication method comprising and using, respectively, the
According to the invention, the first-mentioned object
is achieved by the mechanism defined in claim 1. The further object is achieved
by the subject-matters of claims 6 to 8, 10 and 11.
Advantageous further developments of the invention are
defined in the dependent claims.
The devices manufactured by the device fabrication method
of the present invention cover devices as intermediate and final products. Such
devices include semiconductor chips like an LSI and VLSI, CCDs, LCDs, magnetic sensors,
thin film magnetic heads, and the like.
The objects and features of the present invention will
become readily apparent from the following description of the preferred embodiments
with reference to accompanying drawings.
- FIG. 1 is a schematic sectional view showing a mechanism for retaining and exchanging
a mirror of a first embodiment according to the present invention.
- FIG. 2 is a schematic perspective view of the mechanism shown in FIG. 1.
- FIG. 3 is a schematic sectional view showing a mechanism for retaining and exchanging
a mirror of a second embodiment according to the present invention.
- FIG. 4 is a schematic perspective view of the mechanism shown in FIG. 3.
- FIG. 5 is a schematic plane view of an exposure apparatus according to the present
- FIG. 6 is a flowchart showing an inventive method for exchanging a mirror.
- FIG. 7 is a flowchart for explaining how to fabricate devices (such as semiconductor
chips such as ICs and LSIs, LCDs, CCDs, and the like).
- FIG. 8 is a flowchart for Step 4 that is a wafer process shown in FIG. 7.
- FIG. 9 is a schematic sectional view for explaining a conventional mirror retaining
- FIG. 10 is a flowchart showing a conventional method for exchanging a mirror
shown in FIG. 9.
A description will now be given of an exemplary exposure
apparatus 100 of one embodiment according to the present invention, with reference
to accompanying drawings. In each figure, the same reference numeral denotes the
same element. FIG. 5 is a schematic plane view of the exposure apparatus 100. The
exposure apparatus 100 is an exposure apparatus that uses EUV light (e.g.,
with a wavelength of 13.4 nm) as exposure light for step-and-scan exposure.
Referring to FIG. 5, the exposure apparatus includes a
vacuum chamber 110, an illumination optical system 120, a catoptric reticle or mask
150, an alignment optical system 160, a projection optical system 170, a reticle
stage 154, a wafer stage 184, an EUV light source 200, and accommodates the illumination
optical system 120, 184 and the elements and systems therebetween the wafer stage
in the vacuum chamber 110.
The EUV light source 200 uses, for example, a laser plasma
light source. The laser plasma light source irradiates a highly intensified pulse
laser beam from pulsed laser 204 through a condenser lens 205 to a target 203 supplied
by a target supply unit 202 accommodated in the vacuum chamber 110, thus generating
high-temperature plasma 206 for use as EUV light with a wavelength of about 13 nm
emitted from this. The target 203 uses a metallic thin film, inert gas, droplet,
etc., and is supplied to the vacuum chamber 110 by the target supply unit 202 such
as a gas jet. In order to raise an average intensity of the emitted EUV light, the
pulsed laser 204 preferably has a higher repetitive frequency, and is usually driven
by the repetitive frequency of several kHz. Alternatively, a discharge plasma light
source is used, which discharges gas around an electrode arranged in the vacuum
chamber 110, applies pulsed voltage at the electrode to create discharge, and generates
high-temperature plasma 206, from which the EUV light is emitted, for example, with
a wavelength of about 13 nm.
The illumination optical system 120 propagates the EUV
light, and illuminates the mask (reticle) 150. The illumination optical system 120
includes first to third mirrors 122, 126 and 128, an optical integrator124, and
an aperture 127. The first mirror 122 collects approximately isotropically emitted
EUV light. A multilayer film is made of alternately layered, two types of materials
having different optical constants, such as molybdenum (Mo) and silicone (Si). For
example, a Mo layer has a thickness of about 2 nm, and a Si layer has a thickness
of about 5 nm. The number of layers is about 20 pairs. An addition of two thicknesses
of two types of materials is referred to as a film period. In the above example,
the film period is 2 nm + 5 nm = 7 nm. The multilayer film that includes layered
20 pairs exhibits reflectance close to 70 % in the EUV area, each pair having a
film thickness of about 7 nm. A method for retaining and exchanging this mirror
is applied to an exchange of this mirror. The optical integrator 124 serves to evenly
illuminate the reticle 150 with a predetermined NA. The aperture 127 is provided
at a position conjugate with the reticle 150 in the illumination optical system
120, and limits an area to illuminate the reticle 150 to an arc shape.
A retention and exchange mechanism of the mirror 122 includes,
as shown in FIGs. 1 and 2, a flange 114 provided on a wall 112 of the vacuum chamber
110 so that the flange 114 may open and close, a cooling mechanism 130 for the mirror
122, elastic columns (elastic members) 140, fixing shafts 142, and a kinematic mount
146. Here, FIGs. 1 and 2 are schematic sectional and perspective views for explaining
the retention and exchange mechanism for the mirror 122.
The flange 114 serves as a lid provided on the wall 112
of vacuum chamber 110, and is sealed by an O-ring 118 when it is closed. The O-ring
118 may maintain the atmosphere in the vacuum chamber 110 airtight. A metal ring
or conflate would be used for higher vacuum.
The cooling mechanism 130 includes a cooling plate 132,
a pair of water cooling tubes 134, a pair of water cooling tubes 136, and a seal
138. The cooling plate 132 is adhered to the mirror 122, and cools the mirror 122
using heat conduction. Coolant, e.g., cooling water is supplied to the cooling
plate 132 by the water cooling tubes 134 and 136. The water cooling tubes 134 and
136 are connected to a channel 115 in the flange 114. Cold water is supplied from
one of the water cooling tubes 134 and 136, and drained from the other through the
cooling plate 132. For instance, cold water is supplied from the upper water cooling
tubes 134 and 136, and drained from the lower water cooling tubes 134 and 136. Each
water cooling tube 134 is formed as a flexible tube compatible with high vacuum
to be flexible to movements to some extent. A channel for coolant formed in the
cooling plate 132 may use any shape known in the art, and a detailed description
will be omitted.
The cooling plate 132 and water cooling tubes 134 are provided
between the mirror 122 and flange 114, and may be introduced into and taken out
of the vacuum chamber 110 with the mirror 122 when the flange 114 opens and closes.
It is convenient that the mirror 122 is made replaceable without dismounting the
water cooling tubes 134.
The flange 114 is connected to three elastic columns 140
through projections 116. Each elastic column 140 elastically supports the mirror
122 via one of the fixing shafts 142. Each elastic column 140 is made of a coil
spring in the instant embodiment, which supports weights of the mirror 122 and cooling
plate 132 and forces them toward a light source direction. Alternatively, the elastic
column 140 may use a spring, other than a coil spring, which applies a compression
force in the light source direction, and a vacuum-compatible direct acting cylinder.
Although the instant embodiment couples the elastic columns 140 to the mirror 122
via the cooling plate 132 connected to the mirror 122, the elastic columns 140 may
be directly coupled to the mirror 122 as in another embodiment which will be described
later with reference to FIGs. 3 and 4. Since the elastic columns 140 couple the
flange 114 to the mirror 122, the mirror 122 is taken out of the vacuum chamber
110 when the flange 114 opens and introduced into the vacuum chamber 110 when the
flange 114 closes. Advantageously, one action serves as plural functions, and shortens
an exchange time. In addition, the elastic columns 140 enable the mirror 122 to
be softly positioned.
Three fixing shafts 142 each having a hemispheric tip 144
are fixed onto the cooling plate 132 through three connection parts 143. The three
fixing shafts 142 have the same shape and are arranged at an interval of 120°
around the cooling plate 132. Irrespective of this same shape, tip shapes of members
146a to 146c that are engaged with them are different, as distinguished in FIG.
2. The fixing shafts 142 are positioned relative to the mirror 122 so as to provide
joints that maintain distances from the spherical tips 144 to the mirror 122 surface
and relative arrangement among them. While these joints that maintain distances
and arrangement require a special jig, these joints reproduce positions without
specific optical adjustments in exchanging a mirror, and improve workability.
The kinematic mount 146 includes the three cylindrical
members (mount members) 146a to 146c having different shaped tips. Referring to
FIG. 2, the member 146a has a cone groove tip, the member 146b has a sectionally
V-shaped tip, and the member 146c has a flat shaped tip. The members 146a to 146c
restrain the fixing shafts 142 so that the member 146a restrains three axes, the
member 146b restrains two axes, and the member 146c restrains one axis. Thus, the
members 146a to 146c restrain the fixing shafts 142 having hemispherical tips 144
with respect to six axes, and positions the fixing shafts 142. When the fixing shafts
142 are brought into contact with the kinematic mount 146, the mirror 122 is fixed
more easily and quickly than fixed by other fixing means, such as a bolt.
If necessary, known debris removing means may be provided
between the high-temperature plasma 206 and the mirror 122. A laser plasma method
may be used that uses a metal target, such Cu, formed into a tape that uses a metal
target, such as Cu, formed into a tape shape, and feeds the tape by a reel to use
a new surface. The light source 200 may use a discharge method, such as Z pinch
method, a plasma focus, a capillary discharge, and hollow cathode triggered Z pinch.
FIGs. 3 and 4 show a variation of FIGs. 1 and 2. Here,
FIGs. 3 and 4 are schematic sectional and perspective views for explaining another
retention and exchange mechanism of the mirror 122. According to the retention and
exchange mechanism of the instant embodiment, the connection parts 143 are replaced
with connection parts 148 connected to the mirror 122. Fixing shafts 147 correspond
to fixing shafts 142, and tips 149 correspond to the tips 144. This configuration
may relatively easily maintain distances and arrangement from the hemispherical
tips 149 of the three shafts 147 to the mirror 122 surface, and improve precision
by dispensing with the cooling plate 132. Although FIGS. 3 and 4 show the cooling
plate 132, the cooling plate 132 may be omitted when a heat problem is solved. Thus,
the inventive effects of facilitating an exchange and retention of the mirror 122
may be maintained even when the connection parts 148 are coupled to the mirror 122.
The projection optical system 170 includes a projection
system first mirror 172, a projection system second mirror 174, a projection system
third mirror 176, and a projection system fourth mirror 178, and images a pattern
on a wafer surface. While the use efficiency of the EUV light improves as the number
of mirrors reduces, a correction to aberration becomes difficult. The number of
mirrors necessary to correct aberration is from about four to about six. The mirrors
have a convex or concave spherical or aspheric reflective surface. NA is about 0.1
to about 0.2. The mirrors are formed by polishing and grinding a plate made of a
material having high rigidity and hardness and a small coefficient of thermal expansion,
such as low-expansion glass and silicon carbide, and creating a predetermined reflective
shape, and forming a multilayer, such as molybdenum / silicon, on its reflective
The reticle stage 154 and the wafer stage 184 each include
a mechanism for scanning synchronously at a speed ratio in proportion to a reduction
ratio. Here, "X" is a scan direction in the reticle 150 surface or the wafer 180
surface, "Y" is a direction perpendicular to "X", and "Z" is a direction perpendicular
to the reticle 150 surface or the wafer 180 surface.
The reticle 150 forms a desired pattern and is held on
a reticle chuck 152 on the reticle stage 154. The reticle stage 154 has a mechanism
for moving in the direction X, and a fine adjustment mechanism in the directions
X, Y, Z, and rotational directions around each axis for positioning the reticle
150. A position and orientation of the reticle stage 154 are measured by a laser
interferometer, and controlled based on the measurement results.
The wafer 180 is held onto the wafer stage 184 by the wafer
chuck 182. Similar to the reticle stage 154, the wafer stage 184 has a mechanism
for moving in the direction X, and a fine adjustment mechanism in the directions
X, Y, Z, and rotational directions around each axis for positioning the wafer 180.
The position and orientation of the wafer stage 184 are measured by a laser interferometer,
and controlled based on the measurement results.
The alignment detection optical system 160 measures a positional
relationship between the position of the reticle 150 and the optical axis of the
projection optical system 170, and a positional relationship between the position
of the wafer 180 and the optical axis of the projection optical system 170, and
sets positions and angles of the reticle stage 154 and the wafer stage 184 so that
a projected image of the reticle 150 may be positioned in place on the wafer 180.
A focus detection optical system 165 measures a focus position in the direction
Z on the wafer 180 surface, and control over a position and angle of the wafer stage
184 may always maintain the wafer 180 surface at an imaging position of the projection
optical system 170 during exposure.
Once a scan exposure finishes on the wafer 180, the wafer
stage 184 moves stepwise in the directions X and Y to the next start position for
scan exposure, and the reticle stage 154 and the wafer stage 184 synchronously scan
in the direction X at a speed ratio in proportion to the reduction ratio of the
projection optical system.
While the reduced, projected image of the reticle 150 is
thus formed on the wafer 180, synchronously scanning between them is repeated (step-and-scan
manner). As a result, a transferred pattern on the reticle 150 is transferred onto
the whole area of the wafer 180.
In order to prevent gas from absorbing the EUV light, and
to prevent those molecules including carbon which remain in the space that accommodates
an optical element onto which the EUV light is irradiated, the space that propagates
the EUV light and accommodates the optical element should be maintained at a certain
reduced pressure. In other words, the light source, optical elements of illumination
optical system 120 and projection optical system 170, reticle 150 and wafer 180
are accommodated in the vacuum chamber 110, which is exhausted to meet predetermined
degree of vacuum.
A description will now be given of an inventive mirror
exchange method. Here, FIG. 6 is a flowchart for explaining the inventive mirror
exchange method. First, the vacuum chamber 110 is opened to the atmospheric pressure
(step 502), and the mirror 122 is taken out of the chamber 110 simultaneous with
opening of the flange 114 (step 504). Then, the mirror 122 is exchanged (step 506).
Next follows closing of the flange 114 simultaneous with introduction of the mirror
122 into the vacuum chamber 110 and positioning using the above positioning mechanism
(144 and 146 etc.) (step 508). The last step draws a vacuum in the vacuum chamber
110 (step 510). According to this method, the steps 504 and 508 serve as two or
more actions, and enable an exchange of the mirror 122 to end in a shorter time
than the conventional arrangement that performs these actions separately. While
FIGs. 1 and 2 show a perpendicular structure, the present invention may achieve
similar effects even in a horizontal arrangement.
Referring now to FIGs. 7 and 8, a description will be given
of an embodiment of a device fabricating method using the above exposure apparatus.
FIG. 7 is a flowchart for explaining a fabrication of devices (i.e., semiconductor
chips such as IC and LSI, LCDs, CCDs, etc.). Here, a description will be given of
a fabrication of a semiconductor chip as an example. Step 1 (circuit design) designs
a semiconductor device circuit. Step 2 (mask fabrication) forms a mask having a
designed circuit pattern. Step 3 (wafer preparation) manufactures a wafer using
materials such as silicon. Step 4 (wafer process), which is referred to as a pretreatment,
forms actual circuitry on the wafer through photolithography using the mask and
wafer. Step 5 (assembly), which is also referred to as a posttreatment, forms into
a semiconductor chip the wafer formed in Step 4 and includes an assembly step (e.g.,
dicing, bonding), a packaging step (chip sealing), and the like. Step 6 (inspection)
performs various tests for the semiconductor device made in Step 5, such as a validity
test and a durability test. Through these steps, a semiconductor device is finished
and shipped (Step 7).
FIG. 8 is a detailed flowchart of the wafer process in
Step 4 in FIG. 7. Step 11 (oxidation) oxidizes the wafer's surface. Step 12 (CVD)
forms an insulating film on the wafer's surface. Step 13 (electrode formation) forms
electrodes on the wafer by vapor disposition and the like. Step 14 (ion implantation)
implants ion into the wafer. Step 15 (resist process) applies a photosensitive material
onto the wafer. Step 16 (exposure) uses the exposure apparatus to expose a circuit
pattern on the mask onto the wafer. Step 17 (development) develops the exposed wafer.
Step 18 (etching) etches parts other than a developed resist image. Step 19 (resist
stripping) removes disused resist after etching. These steps are repeated, and multilayer
circuit patterns are formed on the wafer. The device fabrication method of this
embodiment neither spends a long time in exchange nor exposes the inside the vacuum
chamber 110 to the atmospheric pressure for a long time. Therefore, the device fabrication
method of this embodiment may manufacture, with desired throughput, a higher quality
device than the conventional method.
Further, the present invention is not limited to these
preferred embodiments, and various variations and modifications may be made without
departing from the scope of the present invention. For example, the first mirror
122 is not limited to one mirror system, but is applicable to plural mirrors for
increase a concentration ratio of exposure light. The present invention may exhibit
similar effects with similar structure for the plural mirrors.
As discussed, an exposure apparatus that uses a plasma
light source according to the instant embodiment facilitates a quick exchange of
a first mirror in a small space, and prevents a deformation of the mirror in positioning
of the mirror by providing three pairs of cooperating fixing shafts and mount members
with V-shaped, flat and cone tips of the kinematic mount on a illumination system
frame and avoiding excessive constraints of the mirror. Stable positioning of the
first mirror to the same position in the illumination optical system may be quickened
in a small space by maintaining the relationship of the mirror surface and three
tips of the three fixing shafts. Improved workability shortens an exchange time
of the mirror, and improves throughput as a whole.
Each water cooling tube for supplying coolant between the
flange and the cooling plate is formed as a flexible tube. This configuration may
reduce the restraint force applied by a tube to the mirror, save laborious piping
in the vacuum chamber, and improves workability. This also remarkably shortens maintenance
time, improves throughput as a whole, and miniaturizes the exposure apparatus due
to the small maintenance space. A configuration that arranges three fixing shafts
from the cooling plate directly onto the mirror would also provide similar effects
and enable more precise positioning.
The present invention may provide a mirror retainer, a
mirror retaining method, and a mirror exchange method, which may facilitate an exchange
of a mirror in an illumination optical system, maintain initial positioning accuracy,
and shorten an exchange time.