Projection aligner

[0001] The present invention relates to a projection aligner for projecting a mask pattern formed on a mask onto an object to be exposed to transfer the mask pattern to the object.

[0002] Projection aligners have been used to form wiring patterns of PCBs (Printed Circuit Boards), for example. A typical projection aligner is provided with a high-power light source such as an ultra-high-pressure mercury-vapor lamp which is provided to emit a light beam toward a mask, and a projecting optical system to project the light beam passed through the mask so that a substrate of the PCB is exposed to the beam passed through the mask and an image of the mask pattern is transferred.

[0003] The PCB usually includes through holes for electrically connecting wiring patterns formed on different surfaces thereof. Therefore, the projection aligner have to adjust the positions of the mask and the substrate such that the mask pattern is transferred to the substrate at a correct location relative to the through holes to ensure electrical connection between the wiring patterns.

[0004] The substrate of the PCB, however, expands/contracts due to, for example, temperature variation and surface preparation such as mechanical polishing. Such expansion/contraction of the substrate causes displacement of the through holes and, in turn, positional mismatch between the through holes and the mask pattern transferred to the substrate.

[0005] In order to prevent the above mentioned positional mismatch, the projection aligner adjusts the size of the mask pattern image formed on the substrate to the that of the substrate by changing the magnification of the projecting optical system.

[0006] However, the expansion ratio of the substrate is often not uniform, but varies with directions. Normally, the expansion ratio is larger in the longitudinal direction of the substrate than that in the transversal direction. One of the reason of the above is that the mechanical polishing of the substrate is performed by passing the substrate through a polishing device along its longitudinal direction, which causes the substrate to expand more in the longitudinal direction thereof than in the transversal direction.

[0007] Since the expansion ratio of the substrate varies with direction, it has been difficult to adjust the size of the mask pattern image to that of the expanded/contracted substrate to prevent positional mismatch between the through holes and the pattern formed on the substrate.

[0008] The present invention is advantageous in that a projection aligner is provided that is capable of appropriately adjusting the size of the image of the mask pattern projected to an object to be exposed to the size of the object even if the object has expanded/contracted with different ratios in different directions.

[0009] According to an aspect of the invention, the projection aligner includes a projection optical system that forms the image of the mask pattern onto the object, a expansion ratio determiner that measures lengths of the object in first and second directions and determines first and second expansion ratios based on those lengths. The first and second expansion ratios are defined as expansion ratios of the object in the first and second directions, respectively. The projection aligner further includes a magnification controller that adjusts the magnification of the projection optical system to a value between the first and second expansion ratios.

[0010] Since the magnification of the projection optical system is adjusted to a value between the first and second expansion ratios, the size of the image of the mask pattern projected on the object does not differ much from that of the object in both the first and second direction even if the object is expanded/contracted with different ratios in the first and second directions.

[0011] In some cases, the object is provided with at least two first marks arranged thereon along the first direction and at least two second marks arranged thereon along the second direction. In such cases, the expansion ratio determiner can determine the length in the first direction based on a distance between the first marks and the length in the second direction based on a distance between the second marks.

[0012] The expansion ratio determiner may include a camera arranged to capture image of the object, and an image processor adapted to determine the lengths of the object in the first and second directions from the image captured by the camera. In this case, the image processor may utilize the marks mentioned above for determining the lengths of the object. Alternatively, the image processor may utilize alignment marks of the object for determining the lengths of the object.

[0013] Optionally, the first and second directions may be respectively selected to be parallel to longitudinal and transversal directions of the object, especially when the object is a substrate of a printed wiring board since the expansion ratio of such kind of object often differs for a relatively large amount between the longitudinal and transversal direction thereof.

[0014] According to another aspect of the invention, the projection aligner includes, a projection optical system that forms the image of the mask pattern onto the object, a length determiner that measures lengths of the mask and the object in first and second directions, and a magnification controller that adjusts the magnification of the projection optical system to a value between first and second size ratios which are ratios of the length of the object to the length of the mask in the first and second directions, respectively.

[0015] It is well known that the mask also expands due to temperature variation, for example. The projection aligner configures as above, however, can determine the size ratio between the mask and the object accurately even if the mask has expanded since it also measures the size of the mask. As a result, the projection aligner can form the image of the mask pattern on the object in an appropriate size irrespectively of the expansion of the mask.

[0016] Optionally, the length determiner may include first and second cameras arranged to capture images of the mask and the object, respectively, and an image processor adapted to determine the lengths of the mask and the object in the first and second directions from the images captured by the first and second cameras.

[0017] Further optionally, the image processor may determine the lengths based on alignment marks formed to each of the mask and the object at the vicinity of each corner thereof.

[0018] According to-another aspect of the invention, there is provided a projection aligner for transferring an image of a mask pattern of a mask which includes a plurality of local mask areas onto an object which includes a plurality of local object areas. Each local mask area includes a portion of the mask pattern that is to be transferred onto different one of the plurality of local object areas. The projection aligner includes a projection optical system that forms the image of the mask pattern onto the object, and a length determiner that measures lengths of each of the local mask areas and the local object areas in both first and second directions. The projection aligner further includes a size ratio determiner that determines first and second size ratios, which are ratios of the length of the local object area to the length of the local mask area in the first and second directions, respectively, for each of the object local areas. Further, the projection aligner includes a magnification controller that adjusts the magnification of the projection optical system to a value between first and second mean size ratios. The first means size ratios is a mean value of the first size ratios, and, the second means size ratios is a mean value of the second size ratios.

[0019] In the projection aligner arranged as above, the size ratios of the object to the mask is determine by taking into account local variations in the expansion of the object. Accordingly, the projection aligner can adjust the size of the mask pattern image so that the mask pattern image is formed on correct location on the object irrespective the local variation of the expansion of the object.

[0020] Optionally, The projection aligner may include a plurality of first cameras each arranged to capture images of different one of the local mask areas, a plurality of second cameras each arranged to capture images of different one of the local object areas, and an image processor adapted to determine the lengths of each of the local mask areas and the local object area in both first and second directions from the images captured by the first cameras and the second cameras.

[0021] Further optionally, each of the local mask areas and the local object areas may be defined as a rectangular area and has an alignment mark in the vicinity of each corner thereof, and the image processor may determine the lengths based on the distance between the marks.

[0022] FIG. 1 schematically shows a configuration of a projection aligner according to an embodiment of the invention;

[0023] FIG. 2 schematically shows a configuration of a substrate height detecting unit of the projection aligner shown in FIG. 1;.

[0024] FIG. 3 schematically shows a side view of the projection aligner of FIG. 1;

[0025] FIG. 4 schematically shows a top view of a mask 4 of the projection aligner of FIG. 1;

[0026] FIG. 5 schematically shows a top view of a substrate B to be exposed by the projection aligner of FIG. 1;

[0027] FIG. 6 schematically shows a concept of a projection aligner having a plurality of mask-position detectors and a plurality of object-position detectors according to an embodiment of the invention;

[0028] FIG. 7 schematically shows light rays passing through a lens unit and reflected by a roof mirror in the projection aligner of FIG. 1;

[0029] FIG. 8 schematically shows light rays traveling from the mask towards the substrate in the projection aligner of FIG. 1;

[0030] FIG. 9 schematically shows the light beams projected onto the substrate in the projection aligner of FIG. 1 in which projection optical systems are adjusted to enlarge the images projected onto the substrate;

[0031] FIGS. 10 and 11 schematically shows the light rays traveling from the mask to the substrate in the projection aligner of FIG. 1 before and after image location adjustment is achieved, respectively; and

[0032] FIG. 12 schematically shows the light beams projected from the light sources onto the substrate in the projection aligner of FIG. 1 in which the image location adjustment is achieved.

[0033] Hereinafter, a projection aligner according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0034] FIG. 1 schematically shows a configuration of a projection aligner 1 according to an embodiment of the invention. The projection aligner 1 has a plurality of light sources 2, a mask 4, a substrate holder 8 and a plurality of projecting optical systems. The substrate holder 8 carries a substrate B as an object to be exposed. The substrate holder 8 and the mask 4 are driven to move synchronously in the same direction for scanning.

[0035] In the following description, a direction in which the mask 4 and the substrate holder 8 move is referred to as an X-axis direction. Further, a Y-axis is defined, which is on a plane parallel to the mask 4 and perpendicular to the X-axis, and a Z-axis is defined as a direction of light beams emitted from the light sources 2 and incident on the substrate B. According to the embodiment, the light beams are perpendicularly incident on the surface of the substrate B.

[0036] Each of the projecting optical systems corresponds to different one of the light sources 2. Each projecting optical system includes a collimating lens 3, a mirror 5, a lens unit 6, and a roof mirror 7 that are arranged to project a portion of a mask pattern of the mask 4 onto the substrate B using the light beam emitted from the corresponding light source 2. The projecting optical systems are arranged such that the light beams impinge on the mask in two rows in a staggered configuration which extends in the y-axis direction and such that the whole mask pattern can be transferred onto the substrate B by a single scan (i.e., only by moving the substrate B and the mask 4 in one-way). Note that the mirror 5, lens unit 6 and the roof mirror 7 of adjacent projecting optical systems are arranged in opposite direction so that they do not interfere to each other.

[0037] The wavelength and output power of the light source 2 are determined such that the photosensitive material applied on the substrate B is sensitive to the light. An example of such a light source 2 is an ultrahigh-pressure mercury-vapor lamp. Each of the light beams emitted from-the light sources 2 irradiate a strip of an area on the mask 4, through the collimating lenses 3. The light beams transmitted through the mask 4 are reflected by the mirrors 5.

[0038] The mirror 5 includes two reflection planes, i.e., first and second plane mirrors 5a and 5b. The mirror 5 is arranged such that the first plane mirror 5a deflects the light beam that has passed through the mask 4 toward the lens unit 6 and such that the second plane mirror 5b deflects the light beam coming from the lens unit 6 toward the substrate B.

[0039] In the present embodiment, the mirror 5 is formed in a triangular prism whose cross section on an X-Z plane is a right-angled isosceles triangle. The mirror 5 is arranged such that a normal to each of the first and second plane mirrors 5a and 5b forms 45 degrees with respect to the X-axis, and a ridge line formed by the first and second plane mirrors 5a and 5b extends in the Y-axis direction.

[0040] The first plane mirror 5a reflects the light beam transmitted through the mask 4 to proceed in the X-axis direction so that the light beam is incident on the lens unit 6. The light beam passed through the lens unit 6 is reflected by the roof prism 7 and is incident on the lens unit 6 again. The second plane mirror 5b reflects the light beam emerging from the lens unit 6 to proceed in the Z-axis direction so that the light beam is incident on the substrate B. Thus, the light beam passes through the lens unit 6 twice and forms an image of the mask pattern on the substrate B.

[0041] The lens unit 6 includes a plurality of lens elements arranged in the X-axis direction, and has a positive power as a whole.

[0042] The roof mirror 7 has a pair of mirror surfaces that are inwardly directed to form 90 degrees in the X-Y plane. The light beam emerged from the lens unit 6 is reflected by the roof mirror 7, returns to the lens unit 6 in a direction in parallel with the incident direction in the XY-plane. The roof mirror 7 is positioned near a focal point of the lens unit 6. With this arrangement, an erect image of the pattern of the mask 4 is formed on the substrate B. A right angle prism that internally reflects the light beam by surfaces forming the right angle can be used instead of the roof mirror 7.

[0043] The projection aligner 1 is also provided with a mask-driving mechanism 14 and an object-driving mechanism 18 for synchronously moving the mask 4 and the substrate holder 8, respectively, in the x-axis direction. A mirror driving mechanism 15 is also provided for each mirror 5 for positioning the mirror 5 in both x-axis and z-axis direction. Further, a roof mirror driving mechanism 17 is provided for each roof mirror 7 for positioning the roof mirror 7 in both x-axis and y-axis directions.

[0044] The projection aligner 1 includes a mask-position detector 24 that includes an illuminator for illuminating the mask 4 and a CCD camera for capturing the entire image of the mask 4 illuminated by the illuminator, and an object-position detector 28 that includes an illuminator for illuminating the substrate B and a CCD camera for capturing the entire image of the substrate B illuminated by the illuminator. The wavelength and light amount of the illuminators are determined to be ones to which the photosensitive material applied on the substrate B is not sensitive.

[0045] Both the mask 4 and substrate B have alignment marks near each corner thereof. A controller 10 specifies the positions of those alignment marks in the image captured by the CCD cameras and determines the longitudinal and transversal sizes of the substrate and the mask from those positions. The controller 10 further determines the expansion ratio of the image of the mask pattern to be transferred onto the substrate B. Note that, each of the mask-position detector 24 and the object-position detector 28 may include a plurality of cameras each arranged to capture a small area around different one of the alignment marks to allow determination of the position of each of the alignment marks, and in turn the determination of the expansion ratio, in high accuracy.

[0046] The projection aligner 1 further includes a substrate height detecting unit 38 for detecting the position of the photosensitive surface of the substrate B in the z-axis direction.

[0047] FIG. 2 schematically shows the configuration of the substrate height detecting unit 38. The substrate height detecting unit 38 includes a laser source 38a, a photo-detector 38b, and two converging lenses 38c and 38d.

[0048] The laser source 38a emits a laser beam LB toward the photosensitive surface of the substrate B at an predetermined incident angle of . The wavelength and power of the laser beam LB is selected so that the laser beam LB does not expose the photosensitive material applied on the substrate B. One of the converging lens 38d is placed in front of the laser source 38a to form a beam spot on the substrate B.

[0049] The photo-detector 38b is arranged to receive the laser beam LB reflected at the substrate B. A one dimensional position sensitive detector may be utilized as the photo-detector 38b, which includes an elongated light receiving surface and being able to detect the position of the light incident thereon.

[0050] The other converging lens 38c is placed in front of the photo-detector 38b to form an image of the beam spot reflected at the substrate B on the light receiving surface of the photo-detector 38b.

[0051] The photo-detector 38b and the converging lens 38c are arranged so that the image of the beam spot is formed at the center of the light receiving surface of the photo-detector 38b when the photosensitive surface of the substrate B is located at a distance BH₀ from the substrate holder in the z axis direction.

[0052] In the substrate height detecting unit 38 configured as above, the position where the laser beam LB is reflected on the substrate B, and in turn the position where the beam spot is formed on the photo-detector 38b, displaces if the height of the substrate B or the position of the photosensitive surface of the substrate B in the z-axis direction changes.

[0053] The displacement in the z-axis direction of the photosensitive surface of the substrate B and the displacement of the beam spot formed on the photo-detector 38b are proportional to each other. Thus, the height BH of the photosensitive surface of the substrate B from the substrate holder 8 can be derived from the following equation:

[0054] where, L_D represents the displacement of the beam spot on the photo-detector 38b from the center thereon, and represents the magnification of the image formed on the photo-detector 38b by the converging lens 38c which is generally equal to the ratio of length of the optical path between the photo-detector 38b and the converging lens 38c, ₂, to that between the converging lens 38c and the photosensitive surface of the substrate B, ₁, that is ₂/₁.

[0055] Note that a database may be provided to the projection aligner, which includes data on the relation between BH and L_D that is prepared experimentally, so that the height of the photosensitive surface of the substrate B can be determined based on the data of that database instead of utilizing equation (1).

[0056] Hereinafter, the operation of the projection aligner 1 shown in FIG. 1 will be described.

[0057] First, the projection aligner 1 adjusts the focus of the projecting optical system to form a clear image of the mask pattern on the photosensitive surface of the substrate B. The focusing of the projecting optical system is achieved by the following procedure.

[0058] First, the controller 10 determines the height BH of the substrate B based on the output of the substrate height detecting unit 38 and equation (1). Then, the controller 10 calculates the sum of the optical path length from the mask 4 to the lens unit 6 and that from the lens unit 6 to the photosensitive surface of the substrate B, which will be referred to hereinafter as a total optical pass length D_L, based on the height of the substrate, BH, and the position of the mirror 5.

[0059] The focusing of the projecting optical system is achieved when the photosensitive surface of the substrate B is placed at a location optically conjugate to the mask 4 with respect to the lens unit 6, that is, when the total optical pass length D_L is twice as long as the focal length f of the lens unit 6. The controller 10 determines whether the substrate B is at a location optically conjugate to the mask 4 or not by subtracting the double of the focusing length f of the lens unit 6 from the total optical pass length D_L. If the length difference D_L obtained as a result of the subtraction above is not zero, then the controller 10 adjust the focusing of the projecting optical system by operating the mirror driving mechanism 15 to move the mirror 5 in the x-axis direction.

[0060] FIG. 3 schematically shows the side view of the projection aligner 1 of FIG. 1 observed from the y-axis direction. Note that, in FIG. 3, only one of the projection optical system is shown and the lens unit 6 and the roof mirror 7 are indicated as a single lens and a single plane mirror, respectively, for the purpose of clarity only.

[0061] In the projection aligner 1 according to the present embodiment, the total optical pass length D_L can be changed by moving the mirror 5 in the x-axis direction. As may be understood from FIG. 3, if the mirror 5 is moved for a distance of D_L/2 in the x-axis direction, both of the optical path from the mask 4 to first plane mirror 5a and the optical path from the second plane mirror 5b to the substrate B changes in length for D_L/2 since the first and second plane mirrors 5a and 5b of the mirror 5 are inclined against the x-axis at an angle of 45 degree. As a result, the total optical path length D_L changes for D_L, that is, increases D_L when the mirror 5 is moved in the direction away from the lens unit 6 and decreases D_L if moved toward the lens unit 6.

[0062] Accordingly, if D_L>0, the controller 10 moves the mirror 5 for a distance of D_L/2 toward the lens unit 6, and if D_L<0, in a direction away from the lens unit 6. By moving the mirror 5 as above, the total optical path length D_L becomes as long as two times of the focal length f of the lens unit 6 and, as a result, the image of the mask pattern is formed on the substrate with vivid clarity.

[0063] After the focusing of the projecting optical system, the projection aligner 1 determines the size ratio of the substrate B to the mask 4 and adjust the magnification of the projecting optical system, or the expansion ratio of the image of the mask pattern transferred onto the substrate B, in accordance with the size ratio obtained.

[0064] The size ratio of the substrate B to the mask 4 is determined based on the distances between alignment marks formed on the substrate B and the mask 4.

[0065] FIG. 4 schematically shows a top view of the mask 4. The mask 4 has a rectangular shape and is held in the projection aligner 1 such that each side thereof is parallel to either the x-axis or the y-axis. The mask pattern is formed at a middle area of the mask 4 indicated by reference numeral 4a and will be referred to as a mask pattern area 4a in this specification. The mask pattern area 4a is surrounded by an area 4b to which no pattern is formed.

[0066] The mask 4 is provided with alignment marks M1a, M1b, M1c, and M1d. The alignment marks M1a, M1b, M1c, and Mid are formed at each corner of a virtual rectangular on the mask which is shown in broken line in FIG. 4. The virtual rectangular encloses the whole mask pattern area 4a and is defined by sides parallel to the sides of the mask 4.

[0067] The controller 10 operates the camera of the mask-position detector 24 to capture the image of the whole mask 4 and determines the lengths of the mask 4 in the x-axis direction (the direction the light beams are scanned over the mask) and in the y-axis direction (the direction perpendicular to the direction the light beams are scanned) from the distances between the marks M1a, M1b, M1c and M1d in the image obtained. More specifically, the controller 10 calculates the average of the distance between the marks M1a and M1b and the distance between the marks M1c and M1d as the length of the mask 4 in the x-axis direction, l_1x. Similarly, the controller calculates the average of the distance between the marks M1b and M1c and the distance between the marks M1a and M1d as the length of the mask 4 in the y-axis direction, l_1y.

[0068] FIG. 5 schematically shows a top view of the substrate B. Similar to the mask 4, the substrate B normally has an elongated rectangular shape and is held by the substrate holder 8 such that its sides are parallel to the x axis (the longitudinal direction of the substrate B) and the y-axis (the transversal direction of the substrate B). The middle portion of the substrate B is a pattern area B1 onto which the mask pattern is to be transferred.

[0069] The substrate B is provided with alignment marks M2a, M2b, M2c and M2d of which the positional relations, especially the distances between them, are the same as that of the alignment marks M1a, M1b, M1c and M1d of the mask 4 if the substrate B is not expanded or contracted from its original size.

[0070] The controller 10 operates the object-position detector 28 to capture the image of the whole substrate B and determines the lengths l_2x and l_2y of the substrate B in the x-axis and the y-axis directions, respectively, from the positions of the marks M2a, M2b, M2c and M2d in the captured image in a similar manner to that described above in relation with the mask 4.

[0071] Next, the controller 10 adjusts the magnification of the projection optical system, Mgn, to a value between a first size ratio SR1 and a second size ratio SR2, where the first size ratio SR1 is the length ratio of the substrate B to the mask 4 in x-axis direction, i.e., SR1l_2x/l_1x, and the second size ratio SR2 that in y-axis direction, i.e., SR2l_2y/l_1y.

[0072] For example, the controller 10 adjusts the magnification Mgn to a value derived from the following equation:

[0073] Alternatively, the controller 10 may adjust the magnification Mgn of the projecting optical system to one of the followings:

[0074] where m, n are arbitrary positive real numbers.

[0075] It should be noted that, in some embodiments of the invention, the projection aligner 1 is configured, as schematically shown in FIG. 6, to include a plurality of the mask-position detectors 24 and a plurality of the object-position detectors 28. Each of the mask-position detectors 24 are arranged to capture a different one of a plurality of small local areas (4a₁, 4a₂) defined on the mask 4, and each of the object-position detectors 28 are arranged to capture a different one of a plurality of small local areas (B1_a, B1_b) defined on the substrate B. Each of the local areas defined on the mask and the object are provided with four alignment marks. In the projection aligner 1 configured as above, the controller 10 determines the first and second size ratios SR1 and SR2 for each of the local areas defined on the substrate B. Then, the projection aligner 1 adjusts the magnification Mgn of the projecting optical system to a value between an average of the first size ratios, SR1_m, and an average of the second size ratios, SR2_m, which are defined as SR1_ml_2xm/l_1xm and SR2_ml_2ym/l_1ym, where l_1xm and l_1ym respectively represent the mean value of l_1x and l_1y of the local areas defined on the mask 4, and l_2xm and l_2ym respectively represent the mean value of l_2x and l_2y of the local areas defined on the substrate B. In other words, the controller 10 may calculate one of the equations (2) through (6) by replacing l_1x, l_1y, l_2x and l_2y with l_1xm, l_1ym, l_2xm and l_2ym, respectively, and adjust the magnification of the projecting optical system to the value Mgn obtained as a result of the calculation.

[0076] The adjustment of the magnification of the projecting optical system is achieved by moving the roof mirror 7 and the mirror 5 in the x-axis direction and z-axis direction, respectively.

[0077] FIG. 7 schematically shows light rays passing through the lens unit 6 and reflected by the roof mirror 7 observed from the z-axis direction, and FIG. 8 schematically shows the light rays traveling from the mask 4 towards the substrate B observed from the y-axis direction. Note that, in both FIGS. 7 and 8, the lens unit 6 and the roof mirror 7 are represented as a single plane for simplification of the drawings.

[0078] In FIG. 7, the parallel light rays traveling from the mask 4 toward the lens unit 6 are indicated by chain double-dashed lines. If the roof mirror 7 reflects these light rays at the focal point of the lens unit 7 (see the plane 7a in FIG. 7), then the light rays that have passed through the lens unit 6 again become parallel to the optical axis of the lens unit 6. However, if the roof mirror 7 is moved for a distance L₁ along the x-axis in a direction away from the lens unit 6 (see the plane 7b in FIG. 7), which corresponds to shifting the exit pupil of the lens unit 6 in the same direction for a distance 2L₁, the light rays reflected by the roof mirror 7 declines against the optical axis of the lens unit 6 after passing therethrough (see the broken lines).

[0079] As may be understood from FIG. 8, if the light rays traveling toward the substrate B are not parallel to the optical axis of the lens unit 6, the size of the image projected onto the substrate B can be enlarged/reduced by varying the optical path length from the lens unit 6 to the photosensitive surface of the substrate B.

[0080] In the projection aligner 1 according to the present embodiment, the above-mentioned optical path length is changed by moving the mirror 5 in the z axis direction. This method is advantageous since the total optical path length D_L does not change with the movement of the mirror 5 and therefore the image of the mask pattern is always clearly formed on the substrate B irrespective the scaling factor of the image. It should be also noted that the location where the optical axis of the projecting optical system impinges on the substrate B does not displaces with the movement of the mirror 5 in the z-axis direction.

[0081] If the roof mirror 7 is moved to a position of which the distance from the lens unit 6 is longer than the focal length of the lens unit 6, then the image on the substrate B can be enlarged by moving the mirror 5 toward the substrate B to decrease the length of the optical path from the lens unit 6 to the substrate B, and vice versa. In contrast, if the roof mirror 7 is located between the lens unit 6 and its focal point, then the image on the substrate B can be enlarged by moving the mirror 5 away from substrate B to increase the length of the optical path between the lens unit and the substrate B, and vice versa.

[0082] The displacement L₁ of the roof mirror 7 in the x-axis direction from the focal point of the lens unit 6 and the displacement D₁ of the mirror 5 in the z-axis direction from the location at where the optical path length from the lens unit 6 to the substrate B is the same as the focal length of the lens unit 6 should satisfy the following relation to adjust the magnification of the projection optical system to Mgn:

[0083] Thus, the controller 10 positions the roof mirror 7 and the mirror 5 so that the equation (7) is satisfied.

[0084] FIG. 9 schematically shows the light beams projected from the light sources 2 onto the substrate B in the projection aligner 1 in which the projection optical systems are adjusted to enlarge the images projected onto the substrate B. Note that the collimating lenses 3, the mirrors 5, the lens units 6 and the roof mirrors 7 are omitted in FIG. 8 for the simplification of the drawing.

[0085] As described above, the projection optical system in the projection aligner 1 of the present embodiment is able to enlarge/reduce the image projected onto the substrate B by shifting the roof mirror 7 and the mirror 5 in x-axis and z-axis directions, respectively. However, when the plurality of the projection optical systems enlarge or reduce the images, the images on the substrate B overlap to each other or gaps appear between the images.

[0086] Since such overlapping of the images and gaps between the images inhibit correct transfer of the mask pattern onto the substrate B, the locations of the images projected onto the substrate B are adjusted in the y-axis direction so that such overlapping or gaps do not occur. In the projection aligner 1 according to the present embodiment, the above-mentioned adjustment, which will be referred hereinafter as image location adjustment, is achieved by moving the roof mirror 7 in y-axis direction as well as moving the mirror 5 in z-axis direction.

[0087] FIGS. 10 and 11 schematically shows the light rays traveling from the mask 4 to the substrate B. In particular, FIG. 10 shows the light rays in the projection aligner 1 in which the image location adjustment is not yet performed. FIG. 11 shows the light rays in the projection aligner 1 in which the image location adjustment is performed. In FIG. 11, the chain double-dashed lines represent the light rays traveling toward the roof mirror 7 and the broken lines the light rays traveling toward the substrate B after being reflected by the roof mirror 7. Note that the lens unit 6 is represented as a single plane and the mirror 5 is omitted in both FIGS. 10 and 11 for simplifying the drawings.

[0088] In FIG. 10, the mask 4 and the photosensitive surface of the substrate B are both located at a distance from the lens unit 6 equal to the focal length thereof. The roof mirror 7 is located at the focal point O_M of the lens unit 6. In FIG. 10, the light rays that travel from the mask 4 toward the lens unit 6 enter the lens unit 6 in parallel to the optical axis thereof. After passing through the lens unit 6, the light rays are reflected by the roof mirror 7, pass through the lens unit 6 again, and travel in parallel to the optical axis of the lens unit 6. Accordingly, the location on the substrate B where the image is projected by those light rays does not change even if the optical path length from the lens unit 6 to the substrate B is varied by moving the mirror 5 (which is omitted in FIG. 9).

[0089] If the roof mirror 7 is moved, as shown in FIG. 11, in the y-axis direction for a distance L₂ from the focal point O_M (or from the optical axis of the lens unit 6), the position of the exit pupil of the lens unit 6 moves for 2L₂ from the focal point OM in the same direction. As a result, the light rays reflected by the roof mirror 7 inclines against the optical axis of the lens unit 6 after passing therethrough.

[0090] Accordingly, if the optical path length from the lens unit 6 to the substrate B is changed by shifting the mirror 5 in the z-axis direction, the location on the substrate B where the image is formed displaces in the y-axis direction. The displacement of the image in the y-axis direction Y is related to displacement of the mirror 5 in the z-axis direction D₂, or the amount of change of the optical length from the lens unit 6 to the substrate B, and the displacement of the roof mirror 7 in y-axis direction L₂ by the following equation:

[0091] Note that D₂ in equation (8) should be equal to D₁ of equation (7) since both D₁ and D₂ represent the displacement of the mirror 5 in the z-axis direction.

[0092] The displacement Y for the image projected by the a-th projecting optical system from the most left or right one in FIG. 11 is determined from the following equation:

[0093] where n_L is the total number of the projecting optical systems included in the projection aligner 1, and constant number W is the length in the y-axis direction of the unmagnified image projected onto the substrate B by one projecting optical system.

[0094] In the projection aligner 1 according to the present embodiment, the controller 10 determines the displacement L₂ of the roof mirror 7 in the y-axis direction so that the equations (8) and (9) are satisfied.

[0095] FIG. 12 schematically shows the light beams projected from the light sources 2 onto the substrate B in the projection aligner 1 in which the image location adjustment is performed. Note that the collimating lenses 3, the mirrors 5, the lens units 6 and the roof mirrors 7 are omitted in FIG. 12 for simplifying the drawing.

[0096] As shown in FIG. 12, the location of the image formed by the projection optical system at the center does not shifts in the y-axis direction, while the image formed by the n-th projection optical system counted from the one at the center shifts for a distance W(Mgn1)n. As a result, the images projected onto the substrate B do not overlaps to each other and no gaps appear between the images. Thus, the image location adjustment of the projection aligner 1 allows correct transfer of the mask pattern onto the substrate B.

[0097] After the magnifications of the projecting optical systems are adjusted and the image location adjustment is performed as above, the controller 10 operates the mask-driving mechanism 14 and the object-driving mechanism 15 to synchronously move the mask 4 and the substrate B in the x-axis direction to scan the light beams L from the light sources 2 across the mask 4 and the substrate B. The controller 10 moves the mask 4 at a predetermine velocity V_M and the substrate B at a velocity V_BMgnV_M. In addition, the controller 10 moves the mask 4 and the substrate B such that the image at the center of the mask pattern area 4a is transferred on the center of the pattern area B1 of the substrate B.

[0098] By operating the projection aligner 1 according to the present embodiment of the invention as above, the mask pattern of the mask 4 is transferred onto the substrate B without having significant displacement between the transferred pattern and the through holes formed to the substrate B.

[0099] It should be noted that the embodiment of the invention described above may be modified in various ways. For example, the substrate height detecting unit 38 may also be arranged such that the laser beam LB emitted from the laser source 38a impinges on the photosensitive surface of the substrate B in the vicinity of where one of the light beams from the light sources 2 strikes the substrate B. In this case, the controller 10 may monitor the output of the substrate height detecting unit 38 during the exposure of the substrate B, and control the position of the mirrors 5 in the x-axis direction so that the total optical path length D_L satisfies the condition D_L2f substantially all the time during the exposure. With this, the projection aligner 1 becomes able to correctly transfer the mask pattern on a substrate even if the substrate has uneven thickness.

[0100] The present disclosure relates to the subject matter contained in Japanese Patent Application No. P2001-394058, filed on Dec. 26, 2001, which is expressly incorporated herein by reference in its entirety.