1.  Field of the Invention
The present invention relates to an exposure method and an apparatus for being used to transfer a pattern such as a mask pattern to a photosensitive substrate in the photolithography step for producing, for example, semiconductor elements, image pickup elements (for example, CCD), liquid crystal display elements, or thin film magnetic heads.  Especially, the present invention is preferably used for a projection exposure apparatus of the scanning exposure type such as the step-and-scan system. 
2.  Description of the Related Art
For example, when the semiconductor element is produced, the reduction projection type exposure apparatus (stepper) of the step-and-repeat system (full field exposure type) has been hitherto used in many cases as an exposure apparatus for transferring a pattern on a reticle as a mask to respective shot areas on a wafer applied with photoresist.  Recently, on the other hand, in order to respond to such a request that a large area circuit pattern is transferred highly accurately without exerting excessive load on the projection optical system, attention is paid to the projection exposure apparatus of the so-called step-and-scan system for successively transferring an image of a pattern on a reticle to respective shot areas on a wafer by synchronously moving the reticle and the wafer with respect to a projection optical system in a state in which a part of the pattern on the reticle is projected onto the wafer via the projection optical system. 
In the case of the projection exposure apparatus of the scanning exposure type such as the step-and-scan system, it is also necessary that the exposure is performed in a state in which the surface of the wafer is adjusted or conformed (focused) with respect to the image plane of the projection optical system in the same manner as in the full field exposure type.  However, in the case of the scanning exposure type, the exposure is performed while moving the surface of the wafer with respect to the exposure area defined by the projection optical system.  Therefore, when any difference in level or the like exists on the surface of the wafer, it is desirable to previously detect the difference in level or the like.  Accordingly, the following pre-reading control technique as been developed for the scanning exposure type, for example, as disclosed in Japanese Patent Application Laid-Open No.  6-283403, the content of which is incorporated hereinto by reference.  That is, the focus position (position in the optical axis direction of the projection optical system) of the surface of the wafer is detected (pre-read) in a pre-reading area disposed on the front side in the scanning direction with respect to the exposure area.  The focus position and the angle of inclination of the surface of the wafer are controlled on the basis of the result of the detection in accordance with the autofocus system and the autoleveling system. 
As described above, in the conventional scanning exposure type projection exposure apparatus, the surface of the moving wafer can be focused within a predetermined allowable range with respect to the image plane in the exposure area of the projection optical system by using the pre-reading control technique.  In the conventional projection exposure apparatus, for example, the system, in which a movable stage is driven in a contact manner along a guide having good straightness (hereinafter referred to as xe2x80x9ccontact type guide systemxe2x80x9d), has been principally adopted for the wafer stage for driving the wafer. 
On the contrary, recently, the necessity to move the wafer at a high speed is increased in order to improve the throughput in the exposure step.  Therefore, those dominantly used is the stage apparatus based on the non-contact guide system including a system in which a movable stage is arranged with an air guide (air bearing) along a guide having good flatness at least partially, and the movable stage is driven in a non-contact manner by using a linear motor, and a system in which a movable stage is supported in accordance with a magnetically floating system over a guide surface having good flatness at least partially, and the movable stage is driven in a non-contact manner by using a linear motor.  The stage apparatus based on the non-contact guide system has the following many advantages.  That is, for example, the stage apparatus is scarcely affected by foreign matters on the guide surface, and it is excellent in maintenance performance.  Further, the vibration from a base board as a guide in the horizontal direction is hardly transmitted to the movable stage. 
However, the stage apparatus based on the non-contact guide system tends to suffer from the existence of waviness having a relatively large pitch and a relatively large amplitude along the guide surface, as compared with the contact type guide system.  It has been feared that the surface of the wafer as the exposure objective is defocused, even when the focusing is performed in accordance with the autofocus system and the autoleveling system by using the pre-reading control technique.  That is, in the case of the conventional pre-reading control, the control is made assuming that the flatness of the running surface of the wafer stage is satisfactory from the pre-reading position for the focus position on the wafer surface to the exposure position.  Therefore, it has been feared that if the flatness of the running surface is unsatisfactory, the defocus amount remains corresponding to the product of the angle of inclination of the running surface and the distance from the pre-reading position to the exposure position. 
Recently, in order to respond to finer circuit patterns as the transfer objective, the numerical aperture of the projection optical system is further increased, and the depth of focus of the projected image becomes small in accordance therewith.  Therefore, a situation arises, in which it is impossible to neglect the defocus amount based on the angle of inclination of the running surface of the wafer stage as described above. 
Taking the foregoing points into consideration, a first object of the present invention is to provide an exposure method which makes it possible to highly accurately conform a surface of a substrate as an exposure objective to an image plane even when the substrate as the exposure objective is relatively moved with respect to an optical system such as a projection optical system, for example, when the exposure is performed in accordance with the scanning exposure system. 
A second object of the present invention is to provide an exposure method which makes it possible to highly accurately conform a surface of a substrate to an image plane even when the angle of inclination of a running surface of a stage for moving the substrate is gently changed when the substrate as an exposure objective is relatively moved with respect to a projection optical system, for example, when the exposure is performed in accordance with the scanning exposure system. 
A third object of the present invention is to provide an exposure apparatus in which the exposure method as described above can be used, and a high-performance device which is produced by using the exposure method as described above. 
A first exposure method according to the present invention lies in an exposure method for forming a predetermined image on a substrate by using an optical system which radiates an exposure light beam onto the substrate and a substrate stage which relatively moves the substrate with respect to the optical system, wherein a surface of the substrate and an image plane of the optical system are set to be in a predetermined positional relationship in at least a part of an area in an exposure area defined by the optical system, the method comprising the steps of: measuring a height of the surface of the substrate at a measuring point disposed in front of the part of the area in the relative movement direction; and setting the positional relationship to perform exposure on the basis of information about the measured height of the surface of the substrate and information about an inclination angle of a running surface of the substrate stage. 
According to the first exposure method of the present invention as described above, even when the running surface of the substrate stage is inclined, the surface of the substrate can be highly accurately conformed to the image plane of the optical system by correcting the height of the surface of the substrate on the basis of the measured angle of inclination of the running surface of the substrate stage. 
In another aspect, a second exposure method according to the present invention lies in an exposure method for forming a predetermined image on a substrate using an optical system which radiates an exposure light beam onto the substrate and a substrate stage which relatively moves the substrate with respect to the optical system, the method comprising the steps of: measuring at least one of a rolling amount of the substrate stage, a pitching amount of the substrate stage, and a displacement amount of the substrate stage in an optical axis direction of the optical system at a plurality of positions within a movement stroke of the substrate stage using a fiducial member having good flatness arranged on the substrate stage, and storing a result of measurement; and correcting at least one of a running direction of the substrate stage in and a height of a surface of the substrate on the basis of the result of measurement during exposure for the substrate. 
According to the second exposure method as described above, the distribution of the rolling amount and the pitching amount of the substrate stage is previously measured.  By doing so, it is possible to determine the variation values of the rolling amount and the pitching amount of the substrate stage brought about by the torsion of the substrate stage or the like.  Accordingly, for example, it is possible to reduce the influence of the error in the leveling control of the substrate stage performed during the exposure by using the variation value. 
In still another aspect, a third exposure method according to the present invention lies in an exposure method for forming a predetermined image on a substrate using an optical system which radiates an exposure light beam onto the substrate and a substrate stage which relatively moves the substrate with respect to the optical system, wherein the substrate and an image plane of the optical system are set to be in a predetermined positional relationship in at least a part of an area in an exposure area defined by the optical system, the method comprising the steps of: measuring a height position of the substrate at a first measuring point which arrives at an image-forming area on the substrate prior to the part of the area in the relative movement direction; measuring a height of the substrate at a second measuring point which arrives at the image-forming area on the substrate prior to the first measuring point in the relative movement direction; and moving the substrate to a height position within a detectable range of a unit for measuring the height position of the substrate at the first measuring point, on the basis of a measured value obtained at the second measuring point. 
According to the third exposure method as described above, for example, the height of the substrate is roughly detected with a wide detection range at the second measuring point, and thus the height of the substrate can be derived for example, into a narrow detection range with a high resolution at the first measuring point.  Therefore, even when the difference in level is large on the surface of the substrate, it is possible to conform the surface of the substrate to the image plane highly accurately. 
In still another aspect, a first exposure apparatus according to the present invention lies in an exposure apparatus which comprises an optical system which radiates an exposure light beam onto a substrate, and a substrate stage which relatively moves the substrate with respect to the optical system, wherein a surface of the substrate and an image plane of the optical system are set to be in a predetermined positional relationship in at least a part of an area in an exposure area defined by the optical system, in order to form a predetermined image on the substrate, the exposure apparatus including: a focus position-measuring unit which measures a height of the surface of the substrate at a measuring point disposed in front of the part of the area in the relative movement direction; and a focusing stage which conforms the surface of the substrate to the image plane of the optical system in the exposure area of the optical system on the basis of a value measured by the focus position-measuring unit and information about an inclination angle of a running surface of the substrate stage. 
According to the first exposure apparatus of the present invention as described above, it is possible to carry out the first exposure method of the present invention. 
In still another aspect, a second exposure apparatus according to the present invention lies in an exposure apparatus which comprises an optical system which radiates an exposure light beam onto a substrate, and a substrate stage which relatively moves the substrate with respect to the optical system, and which forms a predetermined image on the substrate, the exposure apparatus including: a measuring unit with a fiducial member having good flatness arranged on the substrate stage, which measures at least one of a rolling amount of the substrate stage, a pitching amount of the substrate stage, and a displacement amount of the substrate stage in an optical axis direction of the optical system at a plurality of positions within a movement stroke of the substrate stage; and a focusing stage which stores a result of measurement performed by the measuring unit and which corrects at least one of a running direction of the substrate stage and a height of a surface of the substrate on the basis of the result of measurement during exposure for the substrate. 
According to the second exposure apparatus of the present invention as described above, it is possible to carry out the second exposure method of the present invention. 
In still another aspect, a third exposure apparatus according to the present invention lies in an exposure apparatus comprising an optical system which radiates an exposure light beam onto a substrate, and a substrate stage which relatively moves the substrate with respect to the optical system, wherein a surface of the substrate and an image plane of the optical system are set to be in a predetermined positional relationship in at least a part of an area in an exposure area defined by the optical system, in order to form a predetermined image on the substrate, the exposure apparatus including: a first focus position-measuring unit which measures a height position of a surface of the substrate at a first measuring point disposed in front of the part of the area in the relative movement direction; a second focus position-measuring unit which measures a height position of the substrate at a second measuring point set in the vicinity of the first measuring point; and a focusing stage which allows a height of the surface of the substrate to be included within a detectable range of the first focus position-measuring unit on the basis of a result of measurement performed by the second focus position-measuring unit. 
According to the third exposure apparatus as described above, for example, a detection system, which has extremely fine (high) detection resolution (accuracy) although the detectable range (detection range) is narrow, is used for the first focus position-detecting unit.  As a result, when the angle of inclination of the running surface of the substrate stage is partially changed to a great extent, the first measuring point on the substrate tends to be deviated from the detectable range of the first focus position-detecting unit.  When the first measuring point is deviated from the detectable range as described above, for example, the height of the substrate is roughly measured in the vicinity of the first measuring point by using, for example, the second focus position-detecting unit which has a wide detectable range although the detecting resolution is not so fine.  Based on a result of the measurement, the height (focus position) in the vicinity of the first measuring point is conformed to the image plane of the optical system (PL).  Thus, the height of the first measuring point is included in the detectable range of the first focus position-detecting unit.  Therefore, the first or third exposure method of the present invention can be carried out with ease. 
It is desirable that the second measuring point (84A, 84B) is arranged in front of the first measuring point (81A to 81E) in the relative movement direction.  When the second measuring point is disposed on the front side as described above, for example, the surface of the substrate can be conformed to the image plane before starting the scanning exposure when the scanning exposure is performed. 
It is desirable that a detection range of the second focus position-measuring unit is wider than a detection range of the first focus position-measuring unit.  Accordingly, even when the height of the substrate greatly varies at the edge of the substrate, the height of the substrate can be detected by using the second focus position-detecting unit. 
It is desirable that a plurality of second measuring points (84A, 84B) are provided.  The angle of inclination of the substrate can be also roughly corrected on the basis of the result of measurement performed at the plurality of measuring points. 
It is desirable that the focusing stage controls (performs max-min control) a position of the substrate in an optical axis direction of the optical system on the basis of an intermediate value between a maximum value and a minimum value of the height position of the substrate measured at the first measuring point and the second measuring point (desirably at five points or more), and it controls (performs autoleveling control) an angle of inclination of the substrate by correcting an error brought about when the position concerning the optical axis direction is controlled (subjected to autofocus control), by using a value of the height position of the substrate.  According to the simulation, the residual error of the defocus amount is decreased as a whole with respect to waviness at various wavelengths under this condition. 
In still another aspect, a device according to the present invention lies in a device in which a predetermined pattern is formed, wherein the exposure method or the exposure apparatus of the present invention is used to transfer the image of the pattern onto a substrate (W).  Accordingly, the surface of the substrate can be conformed to the image plane of the projection optical system highly accurately.  Thus, it is possible to produce the high-performance device. 
The predetermined pattern in the exposure method or the exposure apparatus of the present invention described above is exemplified by a pattern for transfer formed on a mask.