Illumination system and exposure apparatus having the same

An illumination system includes a first optical integrator of inside reflection type, for reflecting at least a portion of received light, with its inside surface, and for defining a surface light source at or adjacent a light exit surface thereof, a second optical integrator of wavefront division type, for dividing the wavefront of received light and for defining a plurality of light sources at or adjacent a light exit surface thereof, an imaging optical system for imaging the surface light source at or adjacent a light entrance surface of the second optical integrator, and a collecting optical system for superposing light rays from the plurality of light sources one upon another, on a surface to be illuminated, wherein the imaging optical system has a variable imaging magnification.

FIELD OF THE INVENTION AND RELATED ART
 This invention relates to an illumination system, an exposure apparatus and
 a device manufacturing method. More particularly, the invention is
 concerned with an illumination system, an exposure apparatus and a device
 manufacturing method, wherein an excimer laser for emitting light in the
 ultraviolet region is used as a light source, for illuminating uniformly
 the surface of a wafer or the surface of a reticle where a fine pattern
 such as an electronic circuit pattern is formed.
 In a semiconductor chip manufacturing process, fine patterns formed on
 different masks are sequentially transferred to and superposed on the
 surface of a wafer. To this end, an illumination system of an exposure
 apparatus illuminates a mask or reticle placed at a position optically
 conjugate with the surface of a wafer, whereby a pattern of the mask is
 projected and transferred onto the wafer surface through a projection
 lens.
 The quality of an image transferred to the wafer is largely influenced by
 the performance of the illumination system, e.g., the uniformness of the
 illuminance distribution upon the mask surface or wafer surface.
 Japanese Laid-Open Patent Application, Laid-Open No. 913/1989, No.
 295215/1989, No. 271718/1989, or No. 48627/1990 proposes an illumination
 system wherein the uniformness of the illuminance distribution is improved
 by the use of an inside reflection type integrator and a wavefront
 division type integrator.
 FIG. 13 is a schematic view of a portion of an illumination system which
 uses an inside reflection type integrator and a wavefront division type
 integrator.
 In FIG. 13, the laser beam emitted by a laser light source 101 is once
 converged by a lens system 107 at a position close to the light entrance
 surface of an optical pipe (inside reflection type integrator) 110, and
 then it is diverged such that it enters the optical pipe 110 with a
 predetermined divergence angle defined with respect to the inside
 reflection surface of the optical pipe 110.
 The laser beam divergently incident on the optical pipe is propagated
 within the optical pipe 110 while being reflected by the inside surface
 thereof. Thus, the optical pipe 110 functions to form a plurality of
 virtual images, related to the laser light source 101, on a plane (e.g.,
 plane 113) which is perpendicular to the optical axis.
 On the light exit surface 110' of the optical pipe 110, plural laser light
 fluxes which appear as coming from the virtual images, that is, as emitted
 from plural apparent or seeming light sources, are superposed one upon
 another. As a result of this, a surface light source having a uniform
 light intensity distribution is defined on the light exit surface 110' of
 the optical pipe 110.
 By means of a condenser lens 105, an aperture stop 111 and a field lens
 112, the light exit surface 110' of the optical pipe 110 and a light
 entrance surface 106 of a fly's eye lens (wavefront division type
 integrator) are placed in an optically conjugate relation with each other.
 Thus, the surface light source with a uniform intensity distribution at
 the light exit surface 110' is imaged on the light entrance surface 106 of
 the fly's eye lens. As a result, such light as having a uniform sectional
 intensity distribution enters the fly's eye lens. The fly's eye lens
 serves to define plural light sources (secondary light sources) at its
 light exit surface. Light beams from these secondary light sources are
 superposed by a condenser lens (not shown) one upon another, on the
 surface of a reticle, not shown. Thus, the pattern of the reticle as a
 whole is illuminated with a uniform intensity.
 The illumination system of FIG. 13 is provided with an aperture stop,
 disposed just after the fly's eye lens and having a fixed shape and a
 fixed diameter. Thus, the numerical aperture of the illumination system
 (the size of the secondary light source) is fixed and, therefore, the
 state of illumination is unchangeable with the size of the smallest
 pattern of the reticle.
 Further, in the illumination system of FIG. 13, if the laser light source
 101 comprises such a light source (as a certain type excimer laser)
 wherein the path of laser beam LB shifts in a direction perpendicular to
 the optical axis AX, a minute change of the optical path may cause a
 change in intensity distribution of light fluxes LF, impinging on
 respective points 106 on the light entrance surface of the fly's eye lens.
 This results in a change in the illuminance distribution upon the reticle.
 SUMMARY OF THE INVENTION
 It is a first object of the present invention to provide an illumination
 system with an inside reflection type integrator and a wavefront division
 type integrator, wherein the state of illumination is changeable.
 An illumination system according to this aspect may include a
 variable-magnification imaging optical system disposed just before a
 wavefront division type integrator. However, if the imaging magnification
 changes, the open angle NA of the light flux LF changes. Particularly,
 when the magnification decreases, the open NA of the light flux may become
 larger, excessively beyond the range allowed by the lenses of the fly's
 eye lens. On that occasion, a portion of the light entering the lens
 element is eclipsed within the lens element, such that some light does not
 emit toward the required range (direction). This causes a reduction of the
 light quantity for illuminating the reticle.
 It is accordingly a second object of the present invention to provide an
 illumination system with a wavefront division type integrator, wherein
 even in such a case (regardless of whether the system is equipped with an
 inside reflection type integrator or not) a decrease in the quantity of
 light illuminating a mask or reticle is substantially prevented.
 It is a third object of the present invention to provide an illumination
 system with an inside reflection type integrator and a wavefront division
 type integrator, wherein the illuminance distribution upon the surface
 being illuminated is unchanged even if the path of light from a light
 source shifts.
 In accordance with an aspect of the present invention, to achieve the first
 object, there is provided an illumination system, comprising: a first
 optical integrator of an inside reflection type, for reflecting at least a
 portion of received light, with its inside surface, and for defining a
 surface light source at or adjacent to a light exit surface thereof; a
 second optical integrator of a wavefront division type, for dividing the
 wavefront of received light and for defining a plurality of light sources
 at or adjacent to a light exit surface thereof; an imaging optical system
 for imaging the surface light source at or adjacent to a light entrance
 surface of said second optical integrator; and a collecting optical system
 for superposing light rays from said plurality of light sources one upon
 another, on a surface to be illuminated; wherein said imaging optical
 system has a variable imaging magnification.
 In accordance with another aspect of the present invention, to achieve the
 second object, there is provided an illumination system, comprising: a
 wavefront division type optical integrator for dividing the wavefront of
 received light and for defining a plurality of light sources at or
 adjacent to a light exit surface thereof; a light projecting optical
 system for projecting light from a light source to a light entrance
 surface of said optical integrator, and a collecting optical system for
 superposing light rays from said plurality of light sources one upon
 another, on a surface to be illuminated; wherein said light projecting
 optical system has a focal length which is changeable to cause a change of
 at least one of the size and the intensity distribution of the light, from
 the light source, upon the light entrance surface of said optical
 integrator; and wherein said light projection optical system serves to
 correct a change in an open angle of light, impinging on said wavefront
 division type optical integrator, due to the change in focal length.
 In accordance with a further aspect of the present invention, to achieve
 the second object, there is provided an illumination system, comprising: a
 first optical integrator of a wavefront division type, for dividing the
 wavefront of received light and for defining a plurality of light sources
 at or adjacent to a light exit surface thereof; a second optical
 integrator of an inside reflection type, for reflecting at least a portion
 of received light, with its inside surface, and for defining a surface
 light source having a uniform intensity distribution, at or adjacent to a
 light exit surface thereof; a third optical integrator of a wavefront
 division type, for dividing the wavefront of received light and for
 defining a plurality of light sources at or adjacent to a light exit
 surface thereof; a first imaging optical system for imaging the light
 sources as defined by said first optical integrator, on or adjacent to a
 light entrance surface of said second optical integrator; a second imaging
 optical system for imaging the surface light source as defined by said
 second optical integrator, on or adjacent to a light entrance surface of
 said third optical integrator; and a collecting optical system for
 superposing light rays from the light sources as defined by said third
 optical integrator, one upon another on a surface to be illuminated.
 In accordance with the present invention, there may be provided an exposure
 apparatus or a device manufacturing method which is based on an
 illumination system such as described above. The exposure apparatus may be
 a step-and-repeat type reduction projection exposure apparatus or a
 step-and-scan type projection exposure apparatus, having a resolution
 higher than 0.5 micron. A device which can be produced with such an
 exposure apparatus may be a semiconductor chip such as an LSI or a VLSI, a
 CCD, a magnetic sensor or a liquid crystal device.
 These and other objects, features and advantages of the present invention
 will become more apparent upon a consideration of the following
 description of the preferred embodiments of the present invention taken in
 conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 is a schematic view of a first embodiment of the present invention,
 which is applied to an illumination system for use in a step-and-repeat
 type or step-and-scan type projection exposure apparatus for manufacturing
 devices such as semiconductor chips (e.g., LSI or VLSI), CCDS, magnetic
 sensors and liquid crystal devices, for example.
 Denoted in FIG. 1 at 1 is a laser light source such as an ArF excimer laser
 (wavelength: about 193 nm) or a KrF excimer laser (wavelength: about 248
 nm), for example. Denoted at 2 is an emission angle keeping optical
 element having such a function that the emission angle (divergence angle
 or convergence angle) of light to be emitted is unchanged (maintained)
 regardless of a change in incidence light. Denoted at 3 is a condensing or
 collecting optical system, and denoted at 4 is light mixing means. Denoted
 at 5 is a zooming optical system, and denoted at 7 is a multi-flux light
 beam producing means. Denoted at 8 is another condensing optical system,
 and denoted at 9 is an object to be illuminated, such as a mask or reticle
 on which a device pattern is formed. Denoted at AX is an optical axis of
 the illumination system.
 Basically, the condensing optical system 8 and the zooming optical system
 comprise a plurality of lens elements. In some cases, they have at least
 one mirror for deflecting the light path. There may be a case wherein
 these optical components include a single lens element, respectively.
 A predetermined lens element or elements of the zooming optical system 5
 are made movable along the optical axis AX, by means of a driving
 mechanism (not shown). By moving the lens elements in a predetermined
 relationship, along the optical axis direction, the focal length is
 changed and thus the imaging magnification is changed, with the position
 of the imaging plane held fixed.
 The light mixing means 4 comprises a single optical pipe or a bundle of
 plural optical pipes. The optical pipe may comprise a solid glass rod of a
 polygonal prism shape or a polygonal pyramid shape with its top cut off,
 and it may be made of a glass material (quartz or fluorite, for example)
 having good transmissivity to the laser beam from the laser light source
 1. Alternatively, the optical pipe may comprise a hollow optical element
 such as a kaleidoscope, as can be provided by three or more flat mirrors
 disposed into a cylindrical shape, with their reflection surfaces opposed.
 Such a hollow optical element may have an outside shape of a polygonal
 prism or polygonal pyramid with its top cut off.
 The reflection surface at the side face of the optical pipe (i.e., the
 interface with the air in the case of a glass rod, and the inside
 reflection surface in the case of a hollow optical element) is flat and it
 has a high reflection factor. The light mixing means 4 functions as
 follows: it propagates the received light while at least a portion of the
 received light is reflected by the reflection surface at its side face,
 and light rays of the received light are mixed with each other, whereby a
 surface light source (light) with a uniform intensity distribution is
 formed at or adjacent the the light exit surface 4' thereof. In this
 specification, the light mixing means 4 or an element having the same
 function as the light mixing means 4 will be referred to as an "inside
 reflection type integrator".
 Multi-flux light producing means 7 comprises a fly's eye lens having a
 number of small lenses, a lenticular lens, or a bundle of optical fibers,
 for example. It functions to divide the wavefront of received light,
 incident on its light entrance surface 7', into plural portions and to
 form a surface light source (light) consisting of plural point light
 sources, at or adjacent to the light exit surface 7" thereof. The light
 rays from these plural point light sources are superposed one upon another
 by means of the condensing optical system 8, whereby a surface light
 source (light) with a uniform intensity distribution is produced on a
 predetermined plane. In this specification, the multi-flux light producing
 means 7 or an element having the same function as the multi-flux light
 producing means will be referred to as a "wavefront division type
 integrator".
 The laser light emitted by the laser light source 1 goes by way of a light
 directing optical system which comprises a mirror or relay lens (not
 shown), and it enters the emission angle keeping optical element 2. As
 best seen in FIG. 2A, the emission angle keeping optical element 2
 comprises an aperture member 21 and a lens system 22. The emission angle
 keeping optical element 2 has a function that, even if the projected light
 shifts within a certain range, in a direction perpendicular to or
 substantially perpendicular to the optical axis AX, that is, if it changes
 from the state as depicted by light 27 (FIG. 2A) to the state as depicted
 by light 28, the emission angle (open angle) .epsilon. of the light
 emitted by the emission angle keeping optical element 2 is maintained
 constant.
 The emission angle keeping optical element 2 may be provided by a fly's eye
 lens structure, as shown in FIG. 2B, which comprises a plurality of small
 lenses 23. On that occasion, the emission angle .epsilon. depends on the
 shape of the small lens. Also, in the case of the optical element 2 shown
 in FIG. 2B, even if the projected light shifts within a certain range, in
 a direction perpendicular to the optical axis AX such that it changes from
 the state as depicted by light 27 to the state as depicted by light 28,
 the emission angle (open angle) .epsilon. of the light emitted from the
 emission angle keeping optical element 2 is maintained constant. It is to
 be noted that a wavefront division type integrator other than a fly's eye
 lens may also be used as the emission angle keeping optical element 2.
 The light emitted with an emission angle .epsilon. from the emission angle
 keeping optical element 2 (it comprises multi-flux light when a fly's eye
 lens is used), is once converged by the condensing optical system 3 at a
 position before the inside reflection type integrator. Then, it enters the
 inside reflection type integrator 4, in a divergent state. The divergent
 light beam incident on the inside reflection type integrator 4 passes
 therethrough while being multi-reflected by the inside reflection surface
 thereof, and a plurality of virtual images of the laser light source 1
 (apparent or seeming light source images) are defined on a plane
 perpendicular to the optical axis AX. Thus, at the light exit surface 4'
 of the inside reflection type integrator 4, plural light beams seemingly
 having been emitted from these Virtual images are superposed upon one
 another and, as a result, a uniform intensity distribution is produced at
 the light exit surface 4'. This phenomenon will be described later in
 detail, with reference to FIG. 4.
 The shape of the inside reflection type integrator 4 may be determined
 while taking into account (i) the divergence angle of laser light, as it
 enters the inside reflection type integrator 4 (the angle being dependent
 upon the emission angle keeping optical element 2 and the condensing
 optical system 3), and (ii) the length and width (diameter) of the inside
 reflection type integrator 4. Then, the optical path difference of
 individual laser light coming from the virtual images and impinging on the
 object 9, to be illuminated, can be made longer than the coherency length
 peculiar to the laser light. Thus, coherency of laser light with respect
 to time can be made lower, and production of speckle upon the object 9
 illuminated can be reduced.
 Referring back to FIG. 1, the surface light source (light) formed at the
 light exit surface 4' of the inside reflection type integrator 4 and
 having a uniform illuminance distribution (light intensity distribution),
 is enlarged and imaged by the zooming optical system 5 upon the light
 entrance surface 7' of the wavefront division type integrator 7, at a
 desired magnification. By this, a uniform light source image 6 is defined
 on the light entrance surface 7'.
 When uniform light source image 6 is formed on the light entrance surface
 7', the light intensity distribution on the light entrance surface 7' is
 directly transferred to the light exit surface 7" of the wavefront
 division type integrator 7. Thus, at or adjacent to the light exit surface
 7", a surface light source comprising a number of point light sources of
 substantially the same intensity and having a uniform intensity
 distribution is produced.
 With the function of the condensing optical system 8, the light fluxes
 emitted from the large number of point light sources at or adjacent to the
 light exit surface 7" illuminate the object 9 while being superposed one
 upon another. Therefore, the illuminance distribution over the object 9 as
 a whole becomes uniform.
 The words "desired magnification" mentioned above correspond to the
 magnification with which the size of the uniform light source 6 is so
 determined as to set the open angle .alpha. (emission angle) of
 illumination light impinging on the object 9 to a value best suited for
 the exposure. When the object is a mask or reticle having a fine pattern
 thereon, the "desired magnification" may be modified in accordance with
 the type of the mask pattern (i.e., the size of the smallest pattern
 linewidth).
 When the "desired magnification" is denoted by m and if the light entrance
 side numerical aperture of the zooming optical system 5, which depends on
 the open angle .beta. (emission angle) of light emitted from the inside
 reflection type integrator 4, is denoted by NA' while the light exit side
 numerical aperture of the zooming optical system 5, which depends on the
 open angle .theta. (emission angle) of light incident on the wavefront
 division type integrator 7, is denoted by NA", then a relation
 NA'=m.multidot.NA" is satisfied. Here, from the standpoint of efficient
 use of the illumination light, the magnitude of the angle .theta. should
 desirably be within a range not exceeding the light entrance side
 numerical aperture NA of the wavefront division type integrator 7 and also
 it should desirably be as close as possible to this numerical aperture NA.
 Thus, in the illumination system of this embodiment, the value of angle
 .theta. is set constantly to be kept at an optimum angle suited for the
 entrance side numerical aperture of the wavefront division type integrator
 7, regardless of changes in the value of the magnification m.
 Namely, if the exposure condition such as the type of mask changes and thus
 the value of the optimum magnification m of the zooming optical system 5
 should be changed to an extent that cannot be disregarded, the value of
 the open angle .beta. of light to be emitted from the inside reflection
 type integrator 4 is also changed to prevent a decrease of utilization
 efficiency of the illumination light.
 Once an optimum magnification m for an exposure process under a certain
 condition is determined, an optimum angle for the open angle .beta.
 (emission angle .beta.) of light emitted from the inside reflection type
 integrator 4 can be appropriately determined, on the basis of equation
 (1).
 The illumination system of this embodiment is based on that: the value of
 angle .beta. is equal to the incidence angle .phi. of light impinging on
 the inside reflection type integrator 4 and also the incidence angle .phi.
 is dependent upon the open angle (emission angle) .epsilon. of light from
 the emission angle keeping optical element 2. Thus, in accordance with the
 exposure condition, the emission angle keeping optical element 2 is
 changed by another having a different emission angle .epsilon. and, by
 this, the value of angle .theta. can be maintained constant or
 substantially constant. As a result of this, the entrance side numerical
 aperture of the wavefront division type integrator is maintained
 substantially constant.
 Switching the emission angle keeping optical element 2 will be described in
 detail, with reference to FIGS. 3A and 3B.
 In FIGS. 3A and 3B, denoted at 2a is an emission angle keeping optical
 element having a smaller emission angle .epsilon. (=.epsilon.a), and
 denoted at 2b is another emission angle keeping optical element having a
 larger emission angle .epsilon. (=.epsilon.b). The remaining reference
 numerals like those of FIG. 1 are assigned to corresponding elements.
 Generally, in an illumination system of a semiconductor chip manufacturing
 projection exposure apparatus, it is required that the open angle
 (incidence angle) .alpha. of light incident on the pattern bearing surface
 of a mark or reticle (which corresponds to the object 9 to be illuminated)
 is set to an optimum angle and also that a high light utilization
 efficiency (light quantity) is maintained for the projected light. In
 consideration of this, in the illumination system of this embodiment, a
 zooming optical system and a plurality of emission angle keeping optical
 elements 2 are prepared, and zooming and optical elements are switched as
 required, such as in response to a change in the type of the mask used,
 for example.
 FIG. 3A illustrates a case wherein the incidence angle .alpha. of light
 incident on the mask 9 is relatively small (this is called a "smaller
 .sigma. state"). It corresponds to a case wherein the smallest linewidth
 of a circuit pattern of the mask 9 is relatively large, although it is
 within the range of submicrons. Here, .sigma. means the ratio between the
 light exit side numerical aperture Ni of the illumination optical system
 and the light entrance side numerical aperture Np of the projection
 optical system, that is, it corresponds to the ratio Ni/Np.
 In order to accomplish the state for smaller .sigma., the light exit
 surface 4' of the inside reflection type integrator 4 (or the surface
 light source at or adjacent to it) should be imaged, at a small
 magnification, upon the light entrance surface 7' of the wavefront
 division type integrator 7. While this can be accomplished by making the
 magnification of the zooming optical system 5 smaller, as described above,
 the incidence angle .theta. has to be maintained at an optimum angle which
 is dependent upon the structure of the wavefront division type integrator
 4. Thus, when the system is to be changed into the smaller .sigma. state,
 the magnification of the zooming optical system is changed to one
 corresponding to the value of the incidence angle .alpha. and,
 additionally, in order to assure that the incidence angle .theta. is kept
 at an optimum value, the emission angle keeping optical element 2b having
 an emission angle .epsilon.b (&gt;.epsilon.a) is replaced by the emission
 angle keeping optical element 2a having an emission angle .epsilon.a.
 FIG. 3B shows a case wherein the incidence angle .alpha. of light incident
 on the mask 9 is relatively large (this is called a "larger .sigma.
 state"). It corresponds to a case wherein the minimum linewidth of the
 circuit pattern of the mask 9 is relatively small, within the range of
 submicrons. In order to provide the state for a larger .sigma., the light
 exit surface 4' of the inside reflection type integrator 4 (or the surface
 light source at or adjacent to it) should be imaged, at a large
 magnification, upon the light entrance surface 7' of the wavefront
 division type integrator 7. While this can be accomplished by making the
 magnification of the zooming optical system 5 larger, as described above,
 the incidence angle .theta. has to be maintained at an optimum angle which
 is dependent upon the structure of the wavefront division type integrator
 4. Thus, when the system is to be changed into the larger .sigma. state,
 the magnification of the zooming optical system is changed to one
 corresponding to the value of incidence angle .alpha. and, additionally,
 in order to assure that the incidence angle .theta. is kept at an optimum
 value, the emission angle keeping optical element 2a having an emission
 angle .epsilon.a (&lt;.epsilon.b) is replaced by the emission angle keeping
 optical element 2b having an emission angle .epsilon.b.
 Although in the above-described example the imaging magnification of the
 zooming optical system and the emission angle keeping optical elements are
 switched or changed by two steps, the structure may be modified so that
 the imaging magnification of the zooming optical system and the emission
 angle keeping optical elements are changed by three steps or more. Since
 the magnification of the zooming optical system can be changed
 continuously within a predetermined range, changing the magnification by
 three steps or more is easy. Thus, it can be used without modification.
 Further, as regards the emission angle keeping optical elements, three or
 more emission angle keeping optical elements having mutually different
 focal lengths may be prepared. It is to be noted here that the structure
 is such that, independently from an interchange of the emission angle
 keeping optical elements, the position of convergence of laser light (in
 this embodiment, it corresponds to the absolute position of a real image
 or virtual image of the light emitting portion which is at infinity) is
 maintained constant.
 As regards the zooming optical system, different types of imaging optical
 systems having different imaging magnifications (focal lengths) may be
 prepared, and one of them may be selectively disposed between the two
 integrators 4 and 7. On the other hand, as regards the emission angle
 keeping elements, a zooming optical system having lens elements movable
 along the optical axis direction may be used.
 Next, how the illuminance distribution upon the light exit surface 4' of
 the inside reflection type integrator 4 is made uniform, will be explained
 with reference to FIG. 4.
 It is assumed that in the example of FIG. 4, the inside reflection type
 integrator 4 comprises a glass rod of a hexagonal prism shape. FIG. 4 is a
 side sectional view, containing the optical axis AX.
 Laser light from the condensing optical system 3, not shown in this
 drawing, is once converged (imaged) at a focal point P.sub.0. From there,
 it advances as divergent light having a divergence angle .phi.. Here, if
 the laser light comprises excimer laser light, since the intensity is
 generally high, an enormous energy density is produced in the neighborhood
 of the focal point P.sub.0. There is a possibility that it damages or
 breaks the coating material (anti-reflection film) on the light entrance
 surface of the inside reflection type integrator 4 or the glass material
 itself of the integrator 4. In such a case, therefore, the inside
 reflection type integrator 4 is disposed at a small distance from the
 focal point P.sub.0, as illustrated.
 The divergent light impinging on the inside reflection type integrator 4
 passes therethrough while being repeatedly reflected (it may be subjected
 to total reflection) by the inside reflection surface. After this, the
 light goes out of the inside reflection type integrator 4 while
 maintaining the same divergence angle 41 as having been incident. Here,
 since the light beam having been reflected at respective portions of the
 inside reflection surface of the inside reflection type integrator 4 is
 still divergent after being reflected, the light fluxes reflected by
 respective portions define virtual images P.sub.1, P.sub.2, P.sub.3,
 P.sub.4, P.sub.5, P.sub.6, P.sub.7, P.sub.8, P.sub.9 and P.sub.10 behind
 it, as depicted by broken lines. Although not shown in the drawing, in the
 case of a hexagonal prism glass rod, actually there are similar virtual
 image groups defined additionally by the function of the remaining two
 pairs of inside reflection surfaces.
 Thus, at the light exit surface 4' of the inside reflection type integrator
 4, a large number of light fluxes which seemingly appear as having been
 emitted from a large number of virtual images are superposed one upon
 another, by which the illuminance distribution is made uniform.
 FIG. 5 shows an array of virtual image groups (seeming light source image
 groups) as produced by the inside reflection type integrator of FIG. 4, as
 viewed from, in the arrangement of FIG. 3A, for example, the light exit
 surface of one small lens which constitutes the wavefront division type
 integrator 7. In FIG. 5, denoted at 51 is a small lens of the wavefront
 division type integrator 7, and denoted at P.sub.1 -P.sub.10 are virtual
 images of FIG. 4. As seen from FIG. 5, when the inside reflection type
 integrator 4 comprises an optical pipe of a hexagonal prism shape, the
 groups of virtual images have a honeycomb-like array. When, on the other
 hand, the inside reflection type integrator comprises an optical pipe of a
 rectangular prism shape, the groups of virtual images have a rectangular
 grid-like array. These virtual images are images of convergent points
 (point light sources) of laser light as formed between the condensing
 optical system 3 and the inside reflection type integrator 4.
 Each of the emission angle keeping optical elements 2a and 2b of the
 illumination system of this embodiment comprises a fly's eye lens having
 small lenses of a number "m.times.n" (m.gtoreq.2 and n.gtoreq.2). Thus, an
 individual virtual image in the virtual image groups is provided by plural
 images, being divided into a number about "m.times.n". Therefore, virtual
 images as provided by a honeycomb array of these divided images are seen,
 and they correspond to a single small lens of the wavefront division type
 integrator 7.
 Thus, in the illumination system of this embodiment, when the light fluxes
 from the plural point light sources (effective light sources) as formed at
 or adjacent to the light exit surface 7" of the wavefront division type
 integrator 7 are superposed one upon another by the condensing optical
 system 7 to illuminate the object 9, the number of such point light
 sources (effective light sources) is made quite large. This enables, for
 illumination of the object 9, provision of a more uniform illuminance
 distribution over the whole object 9.
 Further, as has been described with reference to FIG. 2B, even if the light
 from the laser light source 1 shifts minutely due to external disturbance,
 the emission angle .epsilon. of light from the emission angle keeping
 optical element 2a or 2b can be maintained constant. Only each of the
 divided images shown in FIG. 5 shifts minutely, and there is no change in
 the virtual image groups constituting the honeycomb array. Thus, when the
 whole virtual images within the small lenses 51 of the wavefront division
 type integrator 7 are viewed macroscopically, there is substantially no
 change. Therefore, the effect upon the illuminance distribution on the
 object 9 being illuminated is very small and it can be disregarded.
 In summary, the illumination system of this embodiment can be said to be a
 system with a constantly stable performance, independently of a shift of
 laser light from the laser light source 1.
 FIG. 6 shows an embodiment wherein the illumination system of the
 above-described embodiment is incorporated into a step-and-repeat type or
 step-and-scan type projection exposure apparatus for the manufacture of
 semiconductor devices such as LSI or VLSI, CCDs, magnetic sensors or
 liquid crystal devices, for example.
 Denoted in FIG. 6 at 91 is a beam shaping optical system for rectifying
 parallel light from a laser light source 1, comprising an ArF excimer
 laser or a KrF excimer laser, for example, into a desired beam shape.
 Denoted at 92 is an incoherency transformation optical system for
 transforming coherent laser light into incoherent light. Denoted at 93 is
 a projection optical system for projecting a unit-magnification image of a
 reduced image of a circuit pattern of a mask 9. Denoted at 94 is a wafer
 which comprises a substrate (silicon or glass) having a photosensitive
 material applied thereto. The elements corresponding to those shown in
 FIG. 1 are denoted by like numerals, and a duplicate explanation therefor
 will be omitted.
 As regards the laser light from the laser light source 1, when the
 projection optical system 93 is one not having been chromatic-aberration
 corrected, the spectral half width may be band-narrowed to about 1-3 pm.
 When the projection optical system 93 is one having been
 chromatic-aberration corrected, the spectral half width may be
 band-narrowed to a certain value not less than 10 pm. When the projection
 optical system 93 is one having been chromatic-aberration corrected, in
 some cases, the laser light not band-narrowed may be used.
 As regards the projection optical system 93, an optical system provided by
 lens elements only, an optical system provided by lens elements and at
 least one concave mirror, or an optical system provided by lens elements
 and at least one diffractive optical element such as a kinoform, may be
 used. For correction of chromatic aberration, lens elements made of glass
 materials having different dispersion powers (Abbe constants) may be used
 or, alternatively, the diffractive optical element described above may be
 arranged to produce dispersion in the opposite direction to the lens
 elements.
 The laser light emitted by the laser light source 1 goes along a light
 directing optical system comprising a mirror or relay lens (not shown),
 and it impinges on the light shaping optical system 91. This shaping
 optical system 91 comprises plural cylindrical lenses or a beam expander,
 for example, and it functions to transform the lateral-longitudinal ratio
 in the size of a sectional shape of the laser light (perpendicular to the
 optical axis AX) into a desired value.
 The light having its sectional shape rectified by the shaping optical
 system 91 enters the incoherency transformation optical system 92, for
 preventing interference of light upon the wafer 94 which leads to
 production of speckle. By this optical system 92, the light is transformed
 into incoherent light with which speckle is not easily produced.
 The incoherency transformation optical system 92 may be one such as shown
 in Japanese Laid-Open Patent Application, Laid-Open No. 215930/1991, that
 is, an optical system including at least one returning system arranged so
 that: at a light dividing surface, the received light is divided into at
 least two light beams (e.g., P-polarized light and S-polarized light) and,
 after this, an optical path difference larger than the coherency length of
 the laser light is applied to one of the divided light beams; then, the
 one light beam is re-directed to be superposed with the other light beam
 and, thereafter, these light beams are emitted.
 The incoherency transformed light from the optical system 92 enters the
 emission angle keeping optical element 2. Subsequently, in accordance with
 the procedure as having been described with reference to FIGS. 1-5, light
 fluxes emitted from small regions (small lenses) of the wavefront division
 type integrator 7 are superposed one upon another by the condensing
 optical system 8 to illuminate the mask 9, such that the mask 9 is
 uniformly illuminated with a uniform illuminance distribution produced
 over the whole circuit pattern of the mask 9 to be projected. Thus, the
 circuit pattern of the mask 9 is projected and imaged on the wafer 94 by
 the projection optical system 94, and the circuit pattern (image) is
 printed on the photosensitive material of the wafer 94. The wafer 94 is
 held fixed on an X-Y-Z movable stage (not shown) through vacuum
 attraction, for example. The X-Y-Z movable stage has a function for
 translation motion in upward/downward directions as well as
 leftward/rightward directions as viewed in the drawing, and this movement
 is controlled by use of distance measuring means such as a laser
 interferometer, not shown. Since this is well known in the art, a detailed
 description thereof will be omitted.
 In FIG. 6, there is no aperture stop for illumination in the light path on
 the light exit side of the wavefront division type integrator 7. However,
 plural aperture stops corresponding to different .sigma. values may be
 provided in a disk member (turret) which may be rotated in association
 with zooming of the zooming optical system and interchange of emission
 angle keeping optical elements such that an aperture stop of a desired
 .sigma. value may be inserted into the light path on the light exit side
 of the wavefront division type integrator 7.
 As for the shapes of such aperture stop members, ordinary circular shape
 openings or ring-like openings, or a combination of four openings off the
 optical axis as disclosed in Japanese Laid-Open Patent Application,
 Laid-Open No. 329623/1992, may be used.
 Another embodiment of an illumination system according to the present
 invention will be described with reference to FIGS. 7A-8B.
 FIGS. 7A-8B are schematic views, respectively, of an illumination system
 which is suitably usable in a step-and-scan type projection exposure
 apparatus for the manufacture of devices such as semiconductor chips
 (e.g., LSI or VLSI), CCDs, magnetic sensors and liquid crystal devices,
 for example.
 FIGS. 7A and 7B shows a case wherein the illumination system of this
 embodiment is in the smaller .sigma. state as described. FIG. 7A shows the
 illumination system, as viewed in the scan direction (hereinafter "z
 direction"), and FIG. 7B shows the illumination system as viewed in a
 direction (hereinafter "y direction") perpendicular to the scan direction.
 FIGS. 8A and 8B show a case wherein the illumination system of this
 embodiment is in the larger .sigma. state as described. FIG. 8A shows the
 illumination system in the z direction, and FIG. 8B shows the illumination
 system as viewed in the y direction. In FIGS. 7A-8B, the section which
 contains the optical axis AX and an axis extending in the y direction from
 the optical axis AX will be referred to as the "x-z section", and the
 section which contains the optical axis AX and an axis extending in the z
 direction from the optical axis AX will be referred to as the "x-z
 section".
 In FIGS. 7A-8B, denoted at 20a and 20b are emission angle keeping optical
 elements having different open angles (emission angles) of emitted light.
 Denoted at 40 is an inside reflection type integrator, and denoted at 40'
 is the light exit surface of this inside reflection type integrator.
 Denoted at 70 is a wavefront division type integrator, and denoted at 70'
 and 70" are light entrance surfaces of this wavefront division type
 integrator. Denoted at 200y is the length of an illumination region on the
 mask, in the y direction. Denoted at 200z is the length of the
 illumination region of the mask, in the z direction. The elements of this
 embodiment corresponding to those shown in FIGS. 1-6 are denoted by like
 numerals as those of FIG. 3.
 The basic structure and function of the illumination system of this
 embodiment shown in FIGS. 7A-8B are essentially the same as those of the
 illumination system of the preceding embodiment shown in FIGS. 1-6. The
 illumination system of this embodiment differs from that of the preceding
 embodiment of FIGS. 1-6, in the structure and function of the emission
 angle keeping optical system, inside reflection type integrator and
 wavefront division type integrator. Thus, only the difference of this
 embodiment over the preceding embodiment will be explained below.
 In step-and-scan type projection exposure apparatus, an illumination region
 of a rectangular slit-like shape, extending in the y direction (length is
 larger in the y direction than in the z direction) should be effectively
 defined on the mask 9.
 In consideration of this, in this embodiment, as regards the emission angle
 keeping optical elements, those elements 20a and 20b each comprising a
 fly's eye lens with small lenses having a rectangular shape, in section
 (y-z section) perpendicular to the optical axis, being elongated in the y
 direction, are used. As regards the inside reflection type integrator, the
 integrator 40 comprising a rectangular prism optical pipe having a shape,
 in section (y-z section) perpendicular to the optical axis, which shape is
 represented by a pair of straight lines extending in the y direction as
 well as a pair of straight lines extending in the z direction, is used.
 Further, as regards the wavefront division type integrator, the integrator
 70 comprising a fly's eye lens with small lenses having a rectangular
 shape, in y-z section, being elongated in the y direction, is used.
 The small lenses constituting the emission angle keeping optical elements
 20a and 20b each has a numerical aperture in the x-y section which is
 larger than the numerical aperture in the x-z section. Thus, as regards
 the relation of the open angle (emission angle) of light between these
 sections, the emission angles .epsilon.ay and .epsilon.by in the x-y
 section are larger than the emission angles .epsilon.az and .epsilon.bz in
 the x-z section. Therefore, with regard to the open angles (emission
 angles or incidence angles) .phi.y, .phi.z, .beta.y, .beta.z, .theta.y,
 .theta.z, .gamma.y, .gamma.z, .alpha.y, and .alpha.z of light as
 illustrated, there are relations .phi.y&gt;.phi.z, .beta.y&gt;.beta.z,
 .theta.y&gt;.theta.z, .gamma.y&gt;.gamma.z, and .alpha.y&gt;.alpha.z. Here, since
 .gamma.y&gt;.gamma.z, on the mask 9, an illumination region of a rectangular
 slit-like shape elongated in the y direction is produced.
 Further, similar to the preceding embodiment, in dependence upon the
 magnitude of .sigma., there are relations .epsilon.ay&lt;.epsilon.by and
 .epsilon.az&lt;.epsilon.bz. Also, in dependence upon the property of the
 optical pipe of a prism-like shape, there are relations .phi.y=.beta.y and
 .phi.z=.beta.z.
 As regards the emission angle keeping optical elements 20a and 20b, a fly's
 eye lens with small lenses having a focal length in x-y section smaller
 than the focal length in x-z section and being arrayed two-dimensionally
 along the y-z section, may be used. Further, as regards the stop 21 shown
 in FIG. 2A, an element having a slit opening extending in the y direction,
 may be used. It is to be noted that the small lenses constituting the
 fly's eye lens may be provided by ordinary lenses or a diffractive optical
 element (e.g., a Fresnel lens).
 FIG. 9 illustrates an array of virtual image groups (seeming light source
 image groups) produced by the inside reflection type integrator 40, as
 viewed from the light exit surface from a single small lens of the
 wavefront division type integrator 70. In FIG. 9, denoted at 220 is a
 small lens of the wavefront division type integrator 70, and denoted at
 Y1-Y12 and Z1-Z12 are virtual images.
 As seen from FIG. 9, since the inside reflection type integrator 40
 comprises an optical pile of a rectangular prism shape, the virtual image
 groups are arrayed in a grid, along the y direction and z direction. Since
 the incidence angle of divergent light, impinging on the inside reflection
 type integrator 40, differs between the x-y section and the x-z section,
 the number of times of reflection at the inside reflection surface differs
 between the x-y section and the x-z section. As a result, the number of
 virtual images differs between the y direction and the z direction. It is
 to be noted that these virtual images are images of convergence points
 (point light sources) of laser light as formed between the condensing
 optical system 3 and the inside reflection type integrator 40.
 In the illumination system of this embodiment, each of the emission angle
 keeping optical elements 20a and 20b shown in FIGS. 7A-8B comprises a
 fly's eye lens having small lenses of a number "m.times.n" (m.gtoreq.2 and
 n.gtoreq.2). Thus, an individual virtual image in the virtual image groups
 is provided by plural images, being divided into a number about
 "m.times.n". Therefore, virtual images as provided by a grid array of
 these divided images are seen, and they correspond to a single small lens
 of the wavefront division type integrator 70.
 Thus, also in the illumination system of this embodiment, when the light
 fluxes from the plural point light sources (effective light sources) as
 formed at or adjacent to the light exit surface 70" of the wavefront
 division type integrator 70 are superposed one upon another by the
 condensing optical system 8 to illuminate the object 9, the number of such
 point light sources (effective light sources) is made quite large. This
 enables, for illumination of the object 9, the provision of a more uniform
 illuminance distribution over the whole object 9.
 As in the preceding embodiment, in the illumination system of this
 embodiment, when the smaller .sigma. state or larger .sigma. state is to
 be established in accordance with the type of the mask 9 used, the imaging
 magnification of the zooming optical system 5 is switched between a larger
 value and a smaller value and, additionally, the emission angle keeping
 optical elements 20a and 20b are interchanged. This enables changing the
 values of angles .alpha.y and .alpha.z (=.alpha.y) while the values of
 angles .theta.y and .theta.z are maintained constant or substantially
 constant. Thus, the value .sigma. can be changed without loss of light
 utilization efficiency. Further, even if the laser light from the laser
 light source shifts, non-uniformness of illuminance is not produced on the
 mask 9 surface.
 FIG. 10 shows an embodiment wherein the illumination system shown in FIGS.
 7A-9 is incorporated into a step-and-scan type projection exposure
 apparatus, for example, for manufacture of semiconductor devices such as
 LSI or VLSI, CCDs, magnetic sensors or liquid crystal devices, for
 example.
 Denoted in FIG. 10 at 91 is a beam shaping optical system for rectifying
 parallel light from a laser light source 1, comprising an ArF excimer
 laser or a KrF excimer laser, for example, into a desired beam shape.
 Denoted at 92 is an incoherency transformation optical system for
 transforming coherent laser light into incoherent light. Denoted at 93 is
 a projection optical system for projecting a unit-magnification image of a
 reduced image of a circuit pattern of a mask 9. Denoted at 94 is a wafer
 which comprises a substrate (silicon or glass) having a photosensitive
 material applied thereto. The elements corresponding to those shown in
 FIGS. 7-9 are denoted by like numerals, and a duplicate explanation
 therefor will be omitted.
 The laser light emitted by the laser light source 1 goes along a light
 directing optical system comprising a mirror or relay lens (not shown),
 and it impinges on the light shaping optical system 91. This shaping
 optical system 91 comprises plural cylindrical lenses or a beam expander,
 for example, and it functions to transform the lateral-longitudinal ratio
 in the size of a sectional shape of the laser light (perpendicular to the
 optical axis AX) into a desired value.
 The light having its sectional shape rectified by the shaping optical
 system 91 enters the incoherency transformation optical system 92, for
 preventing interference of light upon the wafer 94 which leads to
 production of speckle. By this optical system 92, the light is transformed
 into incoherent light with which speckle is not easily produced.
 The incoherency transformation optical system 92 may be one such as shown
 in Japanese Laid-Open Patent Application, Laid-Open No. 215930/1991,
 having been described hereinbefore.
 The incoherency transformed light from the optical system 92 enters the
 emission angle keeping optical element 20a or 20b. Subsequently, in
 accordance with the procedure as having been described with reference to
 the first embodiment in relation to FIGS. 1-5, light fluxes emitted from
 small regions (small lenses) of the wavefront division type integrator 70
 are superposed one upon another by the condensing optical system 8 to
 illuminate the mask 9, such that the mask 9 is uniformly illuminated with
 a uniform illuminance distribution produced over the whole circuit pattern
 of the mask 9 to be projected. Here, an illumination region (light) of a
 rectangular slit-like shape is formed on the mask 9. Then, the circuit
 pattern of the mask 9 is projected and imaged on the wafer 94 by the
 projection optical system 93, and the circuit pattern (image) is printed
 on the photosensitive material of the wafer 94.
 The wafer 94 is held fixed on an X-Y-Z movable stage (not shown), being
 movable in x, y and z directions, through vacuum attraction, for example.
 Also, the mask 9 is held fixed on another x-y-z movable stage (not shown),
 being movable in the x, y and z directions, through vacuum attraction, for
 example. The motion of these stages is controlled by the use of distance
 measuring means such as a laser interferometer, not shown. These x-y-z
 stages are moved with a rectangular slit-like illumination region defined
 at an end portion of the circuit pattern of the mark 9, so that the mask 9
 is scanned in the z direction while the wafer 94 is scanned in the -z
 direction. By this, the whole circuit pattern of the mask 9 is projected
 on the wafer 94, and the whole circuit pattern is transferred and printed
 on the wafer 94. It is to be noted that, when the projection optical
 system 93 has a projection magnification M and the scan speed of the mask
 9 is V, the scan speed of the wafer 94 should be "-M.times.V".
 FIG. 11 is a flow chart for explaining the processes for the manufacture of
 devices such as LSI or VLSI (semiconductor chips), for example, by use of
 one of the exposure apparatuses as described hereinbefore. The exposure
 apparatus according to any one of the preceding embodiments is used for
 the "wafer process" at step 4.
 FIG. 12 is a flow chart for explaining details of the wafer process of FIG.
 11. The exposure apparatus according to any one of the preceding
 embodiments is used in the "exposure" process at step 16.
 In accordance with any one of the embodiments of the present invention as
 described above, there is provided an illumination system with an inside
 reflection type integrator and a wavefront division type integrator, by
 which the state of illumination can be changed.
 Also, in accordance with any one of the embodiments of the present
 invention as described above, there is provided an illumination system
 with a wavefront division type integrator, by which substantially no
 decrease occurs in the quantity of light, irradiating an object such as a
 mask or reticle, even if the state of illumination is changed.
 Further, in accordance with any one of the embodiments of the present
 invention as described above, there is provided an illumination system
 with an inside reflection type integrator and a wavefront division type
 integrator, by which no change occurs in the illuminance distribution on
 the surface of an object such as a mask or reticle, even if the path of
 laser light from a laser light source shifts.
 While the invention has been described with reference to the structures
 disclosed herein, it is not confined to the details set forth and this
 application is intended to cover such modifications or changes as may come
 within the purposes of the improvements or the scope of the following
 claims.