Exposure method and device producing method using the same

A reticle pattern is split into a first pattern which defines a configuration in the shorter dimension (X direction) and a second pattern which defines a configuration in the longer dimension (Y direction). The length in the longer dimension of each light-blocking pattern element of the first pattern is set longer than the length in the Y direction of the original reticle pattern. The second pattern has two opening pattern elements arrayed in the Y direction at a predetermined interval. The interval is set to a distance which is not shorter than the length in the Y direction of the original reticle pattern and is shorter than the length of each light-blocking pattern element. The image of the first pattern and the image of the second pattern are superimposed on one another by overlay exposure.

BACKGROUND OF THE INVENTION
 The present invention relates to an exposure method used for transferring a
 mask pattern onto a photosensitive substrate in lithography processes to
 produce, for example, semiconductor devices, liquid-crystal display
 devices, or thin-film magnetic heads. The present invention also relates
 to a device producing method using the exposure method.
 Semiconductor devices or other similar devices are produced by using
 one-shot exposure type projection exposure apparatuses (e.g. steppers) in
 which a pattern formed on a reticle as a mask is projected onto a
 photoresist-coated wafer (or a glass plate or the like) through a
 projection optical system, or scanning exposure type projection exposure
 apparatuses such as the step-and-scan type. The degree of integration of
 semiconductor devices is becoming increasingly higher, and patterns to be
 formed are becoming even finer. Consequently, projection exposure
 apparatuses are required to provide even higher resolution.
 In general, resolution is proportional to the wavelength of exposure light
 and inversely proportional to the numerical aperture (NA) of the
 projection optical system. Therefore, straightforward methods of improving
 the resolution are to use exposure light of shorter wavelength and to
 increase the numerical aperture of the projection optical system.
 Accordingly, excimer laser light, e.g. KrF excimer laser light
 (wavelength: 248 nanometers) or ArF excimer laser light (wavelength: 193
 nanometers), is also used as exposure light these days. However, because
 depth of focus is inversely proportional to the square of the numerical
 aperture, the depth of focus becomes excessively shallow if the numerical
 aperture is merely increased. In a case where a projection optical system
 with a large numerical aperture is used, when defocus occurs, the pattern
 length varies to a considerable extent owing to the effect of wavefront
 aberration, although the reduction in pattern length is not remarkable at
 the best focus position.
 To improve resolution, it has been proposed that illumination optical
 systems should adopt annular zone illumination which uses an annular
 zone-shaped aperture stop, or modified illumination which uses an aperture
 stop consisting essentially of a plurality of small decentered apertures.
 Accordingly, attempts have recently been made to combine a projection
 optical system having a relatively small numerical aperture and the
 annular zone illumination to transfer fine patterns. Even when such a
 technique is used, the conventional practice is to transfer one pattern by
 a single exposure operation.
 To transfer a fine periodic pattern having a small pitch, for example,
 there has been proposed a technique whereby the periodic original-plate
 pattern is split into a plurality of separate patterns each having a pitch
 larger than that of the original pattern, and the images of the separate
 patterns are superimposed on one another by multiple exposure.
 Among the conventional techniques as described above, the technique whereby
 a projection optical system with a small numerical aperture (NA) and the
 annular zone illumination are combined to transfer a fine pattern requires
 a large amount of light exposure because only a part of diffracted light
 generated from the fine pattern can pass through the projection optical
 system because of the small numerical aperture. In a case where the
 length-to-width ratio of the fine pattern is widely different from 1:1,
 for example, and where the pattern is sufficiently long in the longer
 dimension, so that the dependence of the image intensity on the numerical
 aperture in the longer dimension is small, if an amount of light exposure
 with which the width (line width) in the shorter dimension of the pattern
 is correctly formed is applied, over-exposure occurs in the longer
 dimension of the pattern. If a positive resist is used in such a case, the
 pattern length in the longer dimension becomes undesirably shortened.
 It is difficult to correct the change in pattern length by adjusting the
 length in the longer dimension of the original-plate pattern used for
 exposure. The reason for this is as follows. The amount of correction for
 the change in pattern length varies according to the periodic structure in
 the longer dimension of the original-plate pattern. Therefore, it is
 necessary to determine an amount of correction by executing an extensive
 case sorting operation. Accordingly, it is actually difficult to correct
 the change in pattern length by this method.
 In the conventional method in which a fine periodic pattern is split into a
 plurality of separate patterns, and multiple exposure is carried out for
 the separate patterns, no particular consideration is given to the
 difference in dimensional errors between the longer and shorter dimensions
 of the pattern.
 SUMMARY OF THE INVENTION
 In view of the above-described circumstances, an object of the present
 invention is to provide an exposure method whereby an image of a pattern
 whose length and width are different from each other can be transferred
 onto a substrate by exposure with high dimensional control accuracy.
 Another object of the present invention is to provide a device producing
 method whereby a pattern whose length and width are different from each
 other can be formed on a substrate with high dimensional control accuracy
 by using the above-described exposure method.
 The present invention provides an exposure method in which an image of a
 mask pattern whose length and width are different from each other is
 transferred onto a predetermined substrate under application of
 predetermined exposure light or a predetermined charged particle beam. The
 method includes the steps of: generating a first pattern which limits a
 configuration in the shorter dimension of the mask pattern, and a second
 pattern which limits a configuration in the longer dimension of the mask
 pattern; projecting an image of one of the first pattern and the second
 pattern onto the substrate to transfer the image thereonto; and projecting
 an image of the other of the first pattern and the second pattern onto the
 substrate over the image of the one of the first pattern and the second
 pattern to transfer the image of the other pattern onto the substrate.
 The first and second patterns may be set such that the first pattern is
 wider than the mask pattern in the longer dimension of the mask pattern,
 and the second pattern has at least one opening pattern element disposed
 at an end in the longer dimension of the mask pattern. If the second
 pattern has at least two opening pattern elements, the distance between
 the respective outer sides of the opening pattern elements is not shorter
 than the length of the mask pattern, and the distance between the
 respective inner sides of the opening pattern elements is shorter than the
 length of the first pattern.
 The substrate may be coated with a positive photosensitive material.
 In addition, the present invention provides a device producing method in
 which a device pattern whose length and width are different from each
 other is formed on a predetermined substrate by using a mask having a mask
 pattern formed thereon. The method includes the steps of: generating a
 first pattern which limits a configuration in the shorter dimension of the
 mask pattern, and a second pattern which limits a configuration in the
 longer dimension of the mask pattern: coating the substrate with a
 photosensitive material; projecting an image of one of the first pattern
 and the second pattern onto the substrate under application of
 predetermined exposure light or a predetermined charged particle beam to
 transfer the image onto the substrate; projecting an image of the other of
 the first pattern and the second pattern onto the substrate over the image
 of the one of the first pattern and the second pattern under application
 of predetermined exposure light or a predetermined charged particle beam
 to transfer the image of the other pattern onto the substrate; and
 developing the substrate.
 According to another aspect of the present invention, there is provided an
 exposure method in which an image of a mask pattern having a plurality of
 sides is transferred onto a substrate under application of predetermined
 exposure light or a predetermined charged particle beam. The method
 includes the steps of: generating a first pattern which defines a
 configuration of at least one side of the mask pattern, the first pattern
 having end portions of a desired configuration which are positioned at
 both ends of the at least one side; generating a second pattern which
 defines a configuration of one or two other sides of the mask pattern
 which connect with the at least one side, wherein an area which extends
 from a point of intersection between the side defined by the first pattern
 and another side defined by the second pattern to one end portion of the
 first pattern lies in an area which divides the second pattern in a state
 where the first pattern and the second pattern are matched to each other;
 projecting an image of one of the first pattern and the second pattern
 onto the substrate to transfer the image thereonto; and projecting an
 image of the other of the first pattern and the second pattern onto the
 substrate over the image of the one of the first pattern and the second
 pattern to transfer the image of the other pattern onto the substrate.
 The side of the mask pattern which is defined by the first pattern may
 intersect another side defined by the second pattern at an angle not
 larger than 90 degrees.
 According to another aspect of the present invention, there is provided an
 exposure method in which an image of a mask pattern including an
 intersection where two sides intersect each other is transferred onto a
 substrate under application of predetermined exposure light or a
 predetermined charged particle beam. The method includes the steps of:
 generating a first pattern which defines a configuration of one side of
 the mask pattern that forms the intersection, the first pattern having end
 portions of a desired configuration which are positioned at both ends of
 the one side; generating a second pattern which defines a configuration of
 the other side of the mask pattern that forms the intersection, wherein an
 area which extends from a point of intersection of the two sides to one
 end portion of the first pattern lies in an area which divides the second
 pattern in a state where the first pattern and the second pattern are
 matched to each other; projecting an image of one of the first pattern and
 the second pattern onto the substrate to transfer the image thereonto; and
 projecting an image of the other of the first pattern and the second
 pattern onto the substrate over the image of the one of the first pattern
 and the second pattern to transfer the image of the other pattern onto the
 substrate.
 The side of the mask pattern which is defined by the first pattern may
 intersect the side of the mask pattern which is defined by the second
 pattern at an angle not larger than 90 degrees.
 According to another aspect of the present invention, there is provided a
 method of forming a device pattern on a substrate. The method includes the
 steps of: transferring a first pattern onto the substrate, the first
 pattern being longer than the device pattern in regard to the longer
 dimension of the device pattern; exposing a part of the first pattern
 transferred onto the substrate to define a size in regard to the longer
 dimension; and developing the substrate.
 In the above-described method, a second pattern may be transferred over at
 least a part of the first pattern transferred onto the substrate to define
 a size in the longer dimension.
 According to another aspect of the present invention, there is provided a
 photomask used in an exposure apparatus. The photomask includes a first
 pattern longer than a device pattern to be formed on a substrate in regard
 to the longer dimension of the device pattern, and a second pattern formed
 in a second area different from a first area where the first pattern is
 formed. The second pattern includes a transmitting portion which is
 overlaid on a part of the first pattern on the substrate. The first
 pattern and the second pattern are each transferred onto the identical
 area on the substrate by the exposure apparatus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 An embodiment of the present invention will be described below with
 reference to the accompanying drawings.
 FIG. 2 shows a projection exposure apparatus used in this embodiment.
 Referring to FIG. 2, an exposure light source 1 is a KrF excimer laser
 light source which emits pulsed laser light of wavelength 248 nanometers
 as exposure light IL. The exposure light IL is reflected by an optical
 path-bending mirror 4. Thereafter, the exposure light IL passes via a
 first lens 8A, an optical path-bending mirror 9 and a second lens 8B and
 enters a fly's-eye lens 10. The first lens 8A and the second lens 8B
 constitute a beam shaping optical system. The beam shaping optical system
 shapes the sectional configuration of the exposure light IL in conformity
 to the entrance surface of the fly's-eye lens 10. It should be noted that
 other excimer laser light, e.g. ArF excimer laser light, or the i-line
 (wavelength: 365 nanometers) from a mercury-vapor lamp can also be used as
 exposure light.
 An aperture stop plate 11 of an illumination system is rotatably disposed
 at the exit surface of the fly's-eye lens 10. The aperture stop plate 11
 has four different aperture stops 13A, 13B, 13C and 13D formed around its
 axis of rotation. The aperture stop 13A is a circular aperture stop for
 ordinary illumination. The aperture stop 13B consists essentially of a
 plurality of decentered small apertures so as to be used for modified
 illumination. The aperture stop 13C is an annular zone-shaped aperture
 stop for annular zone illumination. The aperture stop 13D is a small
 circular aperture stop for a small coherence factor (a value). The desired
 illumination system aperture stop can be placed at the exit surface of the
 fly's-eye lens 10 by driving the aperture stop plate 11 with a driving
 motor 12.
 A part of exposure light IL passing through the aperture stop at the exit
 surface of the fly's-eye lens 10 is reflected by a beam splitter 14 and
 passes through a condenser lens 15 to enter an integrator sensor 16 which
 is formed from a photoelectric detector. A main control system (not shown)
 including a computer can indirectly monitor the illuminance (pulse energy)
 of exposure light IL at the surface of a wafer W and the integral of light
 exposure on the surface of the wafer W from the detected signal from the
 integrator sensor 16. Exposure light IL passing through the beam splitter
 14 passes via a first relay lens 17A, a variable field stop (reticle
 blind) 18, a second relay lens 17B, an optical path-bending mirror 19 and
 a condenser lens 20 to illuminate a pattern surface of a reticle R. The
 plane where the variable field stop 18 is placed and the pattern surface
 of the reticle R are conjugate with each other. The aperture configuration
 of the variable field stop 18 defines an illumination area on the pattern
 surface of the reticle R. Under the application of exposure light IL, an
 image of a pattern in the illumination area on the reticle R is projected
 onto a photoresist-coated wafer W through a projection optical system PL
 at a predetermined projection magnification .beta.(.beta. is, for example,
 1/4+L or 1/5+L ).
 Let us take a Z-axis in a direction parallel to an optical axis AX of the
 projection optical system PL, and X-and Y-axes in a plane perpendicular to
 the Z-axis so as to form an orthogonal coordinate system. The reticle R is
 held on a reticle stage 21 which effects positioning in the X and Y
 directions and also in the direction of rotation. The wafer W is held by a
 vacuum on a wafer holder 22. The wafer holder 22 is secured to the surface
 of a wafer stage 23. The wafer stage 23 corrects the position of the wafer
 W in the Z direction and the tilt angle thereof to bring the surface of
 the wafer W into focus to the image plane of the projection optical system
 PL. The wafer stage 23 also performs stepping and positioning of the wafer
 W in the X and Y directions. The position of the reticle stage 21 (reticle
 R) and the position of the wafer stage 23 (wafer W) are measured with high
 accuracy by respective laser interferometers (not shown). The operations
 of the reticle stage 21 and the wafer stage 23 are controlled by the main
 control system (not shown) on the basis of the result of the measurement.
 During exposure, upon completion of the exposure for a certain shot area on
 the wafer W, the wafer stage 23 is stepped to move the subsequent shot
 area to the exposure field, and exposure is carried out for this shot
 area. This operation is repeated by the step-and-repeat method to effect
 exposure for each shot area on the wafer W. During the exposure process,
 the amount of light exposure for each shot area is controlled on the basis
 of the detected signal from the integrator sensor 16.
 A fiducial mark member 24 is secured to the surface of the wafer stage 23
 in the vicinity of the wafer holder 22. The fiducial mark member 24 is a
 glass substrate having fiducial marks 25A and 25B formed thereon. The
 fiducial marks 25A and 25B are formed, for example, in a cross shape from
 a chromium film or the like. An image processing type alignment sensor 28
 (disclosed, for example, in U.S. Pat. No. 5,493,403) is installed on a
 side of the projection optical system PL to detect the position of a wafer
 mark attached to each shot area on the wafer W. The fiducial mark member
 24 further has a fiducial mark (not shown) for the alignment sensor 28.
 The reticle R also has alignment marks 31A and 31B formed on the pattern
 surface thereof in a positional relationship obtained by changing the
 positional relationship between the fiducial marks 25A and 25B at the
 magnification of projection from the wafer to the reticle. Image
 processing type reticle alignment microscopes 27A and 27B are installed
 above the alignment marks 31A and 31B to effect alignment through mirrors
 26A and so forth.
 To effect alignment of the reticle R, for example, the fiducial marks 25A
 and 25B are illuminated from the bottom thereof by illuminating light of
 the same wavelength as that of exposure light in a state where the center
 between the fiducial marks 25A and 25B is set substantially in the center
 of the exposure field of the projection optical system PL. The images of
 the fiducial marks 25A and 25B are formed in the vicinities of the
 alignment marks 31A and 31B. The reticle alignment microscope 27A detects
 the amount of displacement of the alignment mark 31A relative to the image
 of the fiducial mark 25A. The reticle alignment microscope 27B detects the
 amount of displacement of the alignment mark 31B relative to the image of
 the fiducial mark 25B. The reticle stage 21 is positioned so as to correct
 the amounts of displacement, thereby effecting alignment of the reticle R
 relative to the wafer stage 23. At this time, the distance (base line
 quantity) from the detection center of the alignment sensor 28 to the
 center of the pattern image of the reticle R is calculated by observing
 the corresponding fiducial mark with the alignment sensor 28. To perform
 overlay exposure to the wafer W, the wafer stage 23 is driven on the basis
 of a position determined by correcting the result of the detection by the
 alignment sensor 28 by the base line quantity, thereby enabling the
 pattern image of the reticle R to be transferred onto each shot area on
 the wafer W with high overlay accuracy. It should be noted that one
 example of the measurement of the base line is disclosed, for example, in
 U.S. Pat. No. 5,243,195.
 The following is a description of an example of an operation carried out to
 form a predetermined circuit pattern of a semiconductor device in each
 shot area on the wafer W by using the projection exposure apparatus
 according to this embodiment.
 FIG. 3(b) shows a circuit pattern to be formed in each shot area on the
 wafer in this embodiment. Referring to FIG. 3(b), a periodic circuit
 pattern 33 is formed on the wafer. The circuit pattern 33 has pattern
 elements 34 made of a metal film with a predetermined thickness. The
 pattern elements 34 are elongated in the Y direction and arrayed at a
 predetermined pitch in the X direction. The ratio (duty ratio) of the
 width in the X direction of each pattern element 34, which constitutes the
 circuit pattern 33, to the distance in the X direction between each pair
 of adjacent pattern elements 34 is 1:1. In this case, the pattern elements
 34 have a length-to-width ratio which is not 1:1; the length in the Y
 direction of each pattern element 34 is longer than the width in the X
 direction. Let us assume that a reticle pattern 33R shown in FIG. 3(a) is
 an original-plate pattern obtained when the circuit pattern 33 is enlarged
 at the projection magnification of the projection optical system PL in
 FIG. 2 from the wafer to the reticle.
 Referring to FIG. 3(a), the reticle pattern 33R has five rectangular
 light-blocking pattern elements 34R in a light-transmitting area as a
 background. The light-blocking pattern elements 34R each have a width a in
 the X direction and a length L1 (L1&gt;a) in the Y direction and are arrayed
 in the X direction at regular intervals b, i.e. at a pitch (a+b). The
 width LX1 in the X direction of the reticle pattern 33R is (5a+4b). In the
 following, let us express the size of each portion of the reticle pattern
 33R by the size of an image thereof as projected onto the wafer. In this
 embodiment, the width a of each of the five light-blocking pattern
 elements 34R is 180 nanometers; the interval b is 180 nanometers; the
 width LX1 is 1620 nanometers; and the length L1 in the Y direction of the
 light-blocking pattern elements 34R is 1800 nanometers. In this case, each
 light-blocking pattern element 34R constituting the reticle pattern 33R
 has a length-to-width ratio which is not 1:1. That is, for the
 light-blocking pattern elements 34R, the X direction is the shorter
 dimension, and the Y direction is the longer dimension. In this
 embodiment, the reticle pattern 33R is split into a first pattern 36 which
 defines the configuration in the shorter dimension, as shown in FIG. 4(a),
 and a second pattern 38 which defines the configuration in the longer
 dimension, as shown in FIG. 4(b). As shown in FIG. 2, the reticle R is
 split into two pattern areas 32A and 32B in the X direction. The first
 pattern 36 is formed in the -X side pattern area 32A, and the second
 pattern 38 is formed in the +X side pattern area 32B.
 Referring to FIG. 4(a), the first pattern 36 has five. rectangular
 light-blocking pattern elements 37 formed in a light-transmitting area as
 a background. The light-blocking pattern elements 37 each have a width a
 in the X direction and a length L2 in the Y direction and are arrayed in
 the X direction at regular intervals b. That is, the light-blocking
 pattern elements 37 are equal to the light-blocking pattern elements 34R
 of the reticle pattern 33R in terms of the width a and interval b in the
 shorter dimension. The length L2 in the longer dimension of the
 light-blocking pattern elements 37 is 2000 nanometers, for which the
 following condition holds:
EQU L2&gt;L1 (1)
 In FIG. 4(b), the second pattern 38 has two rectangular opening pattern
 elements 39A and 39B formed in a light-blocking area as a background. The
 opening pattern elements 39A and 39B each have a width LX2 (&gt;LX1) in the X
 direction and a width c in the Y direction and are spaced apart from each
 other in the Y direction by a distance L3. In this case, the width LX2 of
 the opening pattern elements 39A and 39B is 2000 nanometers, and the width
 c is 200 nanometers. The distance L3 is 1800 nanometers. That is, in
 combination with Eq.(1), the following conditions hold:
EQU L2&gt;L3=L1 (2)
EQU L3+2c&gt;L2 (3)
 In actual practice, the distance L3 may be set within the following range:
EQU L2&gt;L3.gtoreq.L1 (4)
 The width c in the Y direction of the opening pattern elements 39A and 39B
 of the second pattern 38 may be set so as to satisfy the following
 condition within the range defined by Eq.(3):
EQU c.ltoreq.5.multidot.a (5)
 Next, an example of an operation carried out to form a circuit pattern 33
 by using the above-described first and second patterns 36 and 38 will be
 described with reference to the flowchart of FIG. 1. In this case, the
 reticle R in FIG. 2 has the first pattern 36 (FIG. 4(a)) and the second
 pattern 38 (FIG. 4(b)) formed in the pattern areas 32A and 32B,
 respectively.
 At step 101 in FIG. 1, a metal film is evaporated onto a wafer to be
 exposed (hereinafter referred to as "wafer W"). At step 102, the metal
 film on the wafer W is coated with a positive photoresist. Thereafter, at
 step 103, the wafer W is placed on the wafer holder 22 of the projection
 exposure apparatus in FIG. 2. At this time, prealignment is performed on
 the basis of the outer dimensions, for example. Then, as has already been
 stated above, alignment of the pattern areas 32A and 32B of the reticle R
 relative to the wafer stage 23 is performed by using the fiducial mark
 member 24 and the reticle alignment microscopes 27A and 27B. Thereafter,
 to carry out overlay exposure, the position of a wafer mark attached to a
 predetermined shot area on the wafer W is detected through the alignment
 sensor 28, and the array coordinates of each shot area are calculated on
 the basis of the result of the detection.
 After one pattern area 32A on the reticle R has been set in the
 illumination area by the variable field stop 18, the aperture stop 13C for
 annular zone illumination is set at the exit surface of the fly's-eye lens
 10 by rotating the aperture stop plate 11. In this case, it is assumed
 that the numerical aperture NA of the projection optical system PL is
 0.60, and that the outer diameter of the annular zone-shaped aperture stop
 13C is 0.80 in terms of the a value, and the inner diameter of the
 aperture stop 13C is 2/3+L of the outer diameter. Thereafter, the
 exposure light source 1 is driven to emit light for each shot area on the
 wafer W to transfer thereonto an image of the first pattern 36 in the
 pattern area 32A, which defines the configuration in the shorter
 dimension, while controlling the amount of light exposure on the basis of
 the detected signal from the integrator sensor 16. The correct exposure
 for this exposure process can be predetermined by test printing or the
 like.
 At step 104, after the other pattern area 32B has been set in the
 illumination area by the variable field stop 18, the aperture stop 13A for
 ordinary illumination is set at the exit surface of the fly's-eye lens 10
 by rotating the aperture stop plate 11. In this case, it is assumed that
 the numerical aperture NA of the projection optical system PL is 0.60, and
 the a value of the aperture stop 13A is 0.80. Thereafter, the exposure
 light source 1 is driven to emit light for each shot area on the wafer W
 to transfer thereonto an image of the second pattern 38 in the pattern
 area 32B, which defines the configuration in the longer dimension, while
 controlling the amount of light exposure on the basis of the detected
 signal from the integrator sensor 16. In this case, alignment of each shot
 area on the wafer W is performed such that the center of the image of the
 first pattern 36 and the center of the image of the second pattern 38
 coincide with each other.
 Thereafter, at step 105, the photoresist on the wafer W is developed. Thus,
 in each shot area on the wafer W, regions corresponding to the images of
 the light-blocking pattern elements 34R of the reticle pattern 33R in FIG.
 3(a) are left in the form of projecting resist pattern elements. At step
 106, the metal film on the wafer W is etched using the resist pattern as a
 mask. Thereafter, the resist pattern is removed. Consequently, a circuit
 pattern 33 as shown in FIG. 3(b) is formed in each shot area on the wafer
 W. Thereafter, the wafer W is moved to the subsequent process for forming
 a circuit pattern in the subsequent layer.
 As a specific example, an exposure process was carried out by setting the
 amount of light exposure for transferring the image of the first pattern
 36 at step 103 such that the width in the X direction of each projecting
 resist pattern element (the image of each light-blocking pattern element
 37) left after the development would be a, i.e. 180 nanometers, and
 further setting the amount of light exposure for transferring the image of
 second pattern 38 at step 104 to the same value as in the exposure for the
 first pattern 36. As a result, the error of the length in the Y direction
 of the resist pattern after the development relative to the design value
 (L1) was not more than 50 nanometers.
 In contrast, when exposure was carried out with an amount of light exposure
 set such that the width in the X direction of each resist pattern element
 after the development would be a (180 nanometers) by using the reticle
 pattern 33R itself, which is shown in FIG. 3(a), the error of the length
 in the Y direction of the resist pattern relative to the design value was
 250 nanometers. In other words, with the exposure method according to this
 embodiment, the error of the length in the longer dimension of the resist
 pattern after the development is reduced to 1/5+L or less. Thus, the
 length of each pattern element 34 of the finally formed circuit pattern 33
 can be controlled with improved accuracy.
 Although in the foregoing embodiment a periodic reticle pattern 33R as
 shown in FIG. 3(a) is split into the first pattern 36 and the second
 pattern 38 and exposure is carried out for each of the first and second
 patterns 36 and 38, it should be noted that the present invention is also
 applicable to a case where the pattern to be exposed is an isolated
 pattern. That is, in a case where the reticle pattern to be exposed is,
 for example, a single light-blocking pattern element 34R as shown in FIG.
 3(a), exposure should be carried out by splitting the light-blocking
 pattern element 34R into a first light-blocking pattern element 37 which
 defines the shorter dimension as shown in FIG. 4(a) and a second pattern
 38 which defines the longer dimension as shown in FIG. 4(b). In this case,
 the width in the X direction of the second pattern 38 may be shorter than
 in the foregoing embodiment.
 The present invention is applicable not only to an exposure process using a
 reticle pattern which is periodic in only one direction, but also to an
 exposure process for transferring an image of a reticle pattern which is
 periodic in two mutually intersecting directions as shown in FIG. 5(a).
 FIG. 5(a) shows a reticle pattern 40R which is periodic in both the X and Y
 directions. Referring to FIG. 5(a), the reticle pattern 40R has five rows
 of pattern elements arrayed in the X direction at a predetermined pitch.
 Each row has rectangular light-blocking pattern elements 41AR and 41BR
 which are elongated in the Y direction and arrayed in the Y direction at
 an interval d. The second and fourth rows of pattern elements are
 displaced in the Y direction by a half pitch relative to the other rows of
 pattern elements. The light-blocking pattern elements 41AR and 41BR each
 have a length-to-width ratio which is not 1:1. That is, the length in the
 Y direction is longer than the width in the X direction.
 To transfer the image of the reticle pattern 40R, the reticle pattern 40R
 is split into a first pattern 42 which defines the configuration in the
 shorter dimension as shown in FIG. 5(b), and a second pattern 44 which
 defines the configuration in the longer dimension as shown in FIG. 5(c).
 The first pattern 42 has light-blocking pattern elements 43A which are
 elongated in the Y direction and arrayed in the X direction at a
 predetermined pitch. The light-blocking pattern elements 43A each have the
 same width in the X direction as the light-blocking pattern elements 41AR
 and 41BR. The length in the Y direction of each light-blocking pattern
 element 43A is set longer than the overall length of the corresponding row
 of light-blocking pattern elements in the reticle pattern 40R. The second
 pattern 44 has opening pattern elements 45A, 45B and 45C formed in a
 light-blocking film as a background. The opening pattern elements 45A and
 45C are disposed at respective positions corresponding to the longitudinal
 ends of the first row of light-blocking pattern elements 41AR and 41BR of
 the reticle pattern 40R. The opening pattern element 45B is disposed at a
 position corresponding to the border between the light-blocking pattern
 elements 41AR and 41BR. The opening pattern elements 45A, 45B and 45C each
 have a length e in the Y direction. Similarly, the same opening pattern
 elements are formed at respective positions corresponding to the ends and
 border in each of the other rows of light-blocking pattern elements of the
 reticle pattern 40R. The width in the X direction of each of the opening
 pattern elements 45A, 45B and 45C is wider than the width of the
 light-blocking pattern elements 41AR and BR. The length e in the Y
 direction of the opening pattern elements 45A, 45B and 45C is set equal to
 the distance d between the light-blocking pattern elements 41AR and 41BR.
 In this case also, an image of the reticle pattern 40R in FIG. 5(a) which
 is reduced at a predetermined magnification can be transferred with high
 line-width control accuracy in both the X and Y directions by optimizing
 the exposure conditions for the first pattern 42 and the exposure
 conditions for the second pattern 44.
 Although in the foregoing embodiment the first and second patterns 36 and
 38 are formed in respective pattern areas on the same reticle, it should
 be noted that the first and second patterns 36 and 38 may be formed on
 different reticles, respectively. The present invention is applicable not
 only to an exposure process carried out by a projection exposure apparatus
 using a projection optical system PL which comprises a refracting system
 as in the foregoing embodiments, but also to an exposure process carried
 out by a projection exposure apparatus using a projection optical system
 which comprises a reflecting system using a mirror or a catadioptric
 (reflective refracting) optical system, or a step-and-scan type projection
 exposure apparatus (e.g. U.S. Pat. Nos. 5,194,893, 5,473,410 and
 5,591,958).
 To form a device pattern in each of a plurality of divided areas (shot
 areas) on a wafer, the first and second patterns 36 and 38 may be
 alternately transferred for each divided area. Alternatively, the
 arrangement may be such that the first pattern 36 is first transferred
 onto each of the plurality of divided areas, and thereafter, the second
 pattern 38 is transferred onto each of the divided areas over the first
 pattern. The latter method has an advantage over the former method in
 throughput because it needs to change the illumination conditions (to
 drive the aperture stop plate 11) only once in the exposure process for
 one wafer. To minimize the amount of movement of the wafer stage 23 to
 thereby improve the throughput furthermore, it is preferable in the latter
 method that the transfer of the second pattern 38 should be started with
 the last divided area in the sequence of transferring the first pattern
 36. In other words, it is preferable that the exposure sequence of the
 divided areas on the wafer for the first pattern 36 and that for the
 second pattern 38 should be reverse to each other.
 Illuminating light for exposure that is used in exposure apparatuses to
 which the present invention is applicable are not necessarily limited to
 emission lines (e.g. g-line or i-line) emitted from mercury-vapor lamps,
 KrF excimer laser light and ArF excimer laser light. It is also possible
 to use F.sub.2 laser light (wavelength: 157 nanometers) or harmonics from
 a YAG laser.
 The present invention is also applicable to a projection exposure apparatus
 using as illuminating light for exposure EUV (Extreme Ultra Violet) light
 having an oscillation spectrum in a wavelength region of from 5 to 15
 nanometers (soft X-ray region), for example. It should be noted that the
 projection exposure apparatus using EUV light has a reduction projection
 optical system which defines the shape of an illumination area on a
 reflective mask as an arcuate slit-like shape. The reduction projection
 optical system consists essentially of a plurality of reflecting optical
 elements (mirrors). The reflective mask and a wafer are synchronously
 moved with a speed ratio corresponding to the magnification of the
 reduction projection optical system, thereby transferring a pattern formed
 on the reflective mask onto the wafer. It should be noted that the present
 invention is also applicable to a proximity type X-ray exposure apparatus
 using hard X-rays (wavelength: 1 nanometer, for example) as illuminating
 light for exposure.
 The present invention is also applicable to an exposure process carried out
 by an exposure apparatus such as an electron beam exposure apparatus which
 transfers a mask pattern onto a substrate coated with an electron beam
 resist or the like by using a charged particle beam. Thus, the present
 invention is not necessarily limited to the above-described embodiments
 but may adopt various arrangements without departing from the gist of the
 present invention.
 According to the exposure method of the present invention, a mask pattern
 is exposed by splitting it into a first pattern which controls the
 configuration in the shorter dimension and a second pattern which controls
 the configuration in the longer dimension. Therefore, an image of a
 pattern whose length and width are different from each other can be
 transferred onto a substrate by exposure with high dimensional control
 accuracy in both the shorter and longer dimensions.
 The first and second patterns may be set such that the first pattern is
 wider than the mask pattern in the longer dimension of the mask pattern,
 and the second pattern has one or a plurality of opening pattern elements
 arrayed in the longer dimension of the mask pattern, and that if there are
 at least two opening pattern elements, the spacing therebetween is not
 shorter than the length of the mask pattern and is shorter than the length
 of the first pattern. In this case, it is possible to improve the control
 accuracy for the length in the longer dimension of the pattern image
 transferred.
 In a case where a substrate to be exposed is coated with a positive
 photosensitive material, it is possible to gradually increase the
 dimensional accuracy of the pattern image transferred in two exposure
 operations. In such a case, the present invention is particularly
 effective.
 According to the device producing method of the present invention, a
 pattern whose length and width are different from each other can be formed
 on a substrate with high dimensional control accuracy by using the
 exposure method according to the present invention.