Abstract:
A transfer method includes a step of disposing a mask and a substrate with a projection optical system interposed therebetween in planes substantially perpendicular to the optical axis thereof; and a step of moving the image plane of the projection optical system and the substrate relative to each other along the optical axis in accordance with a variation in the positional relationship between the mask and the projection optical system along the optical axis while the mask and the substrate are moved perpendicular to the optical axis of the projection optical system, in order to transfer a pattern on the mask onto the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of application Ser. No. 08/309,745 filed Sep. 21, 1994, now U.S. Pat. No. 5,796,467. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to projection exposure method and apparatus of the step and scan type in which for example, a mask and a photosensitive substrate are scanned in synchronism with each other relative to an illuminated area of rectangular shape or arcuate shape, whereby a pattern on the mask is exposed onto the photosensitive substrate. 
     2. Related Background Art 
     When manufacturing a semiconductive element, a liquid crystal display element, a thin film magnetic head or the like by the use of the photolithography technique, use has heretofore been made of a projection exposure apparatus in which the image of a pattern formed on a photo-mask or a reticle (hereinafter generically referred to as the “reticle”) is projected through a projection optical system onto a wafer having photoresist or the like applied thereto or a photosensitive substrate such as a glass plate. The recent tendency is toward making a chip pattern on a semiconductive element large. Accordingly, in the projection exposure apparatus, it is required to expose a pattern of a larger area (make the area larger) on the photosensitive substrate. 
     In order to meet such a requirement for a larger area, a projection exposure apparatus of the step and scan type has been developed in recent years. In the projection exposure apparatus of this type, each shot area on a photosensitive substrate is stepping-moved to an exposure starting position, whereafter for example, relative to a rectangular, arcuate or hexagonal illuminated area (which is called a “slit-like illuminated area”), a reticle and the photosensitive substrate are scanned in synchronism with each other. Then, patterns on the reticle of larger area than the slit-like illuminated area are successively exposed onto respective shot areas. 
     Heretofore, in both of a projection exposure apparatus of the above-described step and scan type, and an exposure apparatus of the step and repeat type which is the non-scan type, it has been usual that a focusing mechanism for a reticle pattern disposed on the object plane of a projection optical system and the exposed surface of a photosensitive substrate disposed on the image plane (imaging plane) of the projection optical system is provided only on the photosensitive substrate side. This focusing mechanism is comprised of a detecting sensor for the exposed surface of the photosensitive substrate, and a correcting mechanism for correcting the position of the exposed surface of the photosensitive substrate in the direction of the optical axis of the projection optical system (focus position) and correcting the inclination angle of the surface relative to a plane perpendicular to the optical axis. The detecting sensor detects the focus position of the exposed surface and the inclination angle of the exposed surface. The correcting mechanism corrects the position and inclination of a stage supporting the photosensitive substrate on the basis of the result of the detection by the detecting sensor. 
     In the projection exposure apparatus of the step and scan type, there are cases where with the scanning of the reticle, the positional relationship between the pattern on the reticle and the projection optical system varies in the direction of the optical axis of the projection optical system. Therefore, the position and inclination of the imaging plane of the projection optical system are also varied. These variations in the imaging plane result in focus deviation. 
     For example, suppose a 6-inch reticle as the reticle and let it be assumed that the reticle supporting surface of a reticle stage (usually the reticle has its pattern surface side vacuum-chucked and fixed) is upwardly or downwardly inclined by 1 second with respect to the scanning direction. When at this time, the 6-inch reticle is scanned from end to end, the distance between the pattern surface of the reticle and the projection optical system varies by 0.74 μm. To convert this into an amount of variation in the position of the image plane, it can be multiplied by the square of the projection magnification. Supposing a reduction projection optical system having a projection magnification of  {fraction (1/2+L )}, the amount of variation in the image plane reaches  0.18 μm. This amount cannot be neglected, particularly when the margin of the depth of focus is small as in the exposure of a pattern of sub-micron order by the projection optical system. 
     SUMMARY OF THE INVENTION 
     In view of the above-noted point, it is an object of the present invention to provide a projection exposure method which, when the pattern of a mask is to be projection-exposed onto a photosensitive substrate by the step and scan system, can always expose a good image onto the photosensitive substrate even if with the scanning of the mask, the positional relationship of the mask to a projection optical system varies. It is also an object of the present invention to provide a projection exposure apparatus which can carry out such a projection exposure method. 
     The present invention, as shown, for example, in FIGS. 1 and 2 of the accompanying drawings, provides a transfer method including: 
     a step of disposing a mask (R) having a pattern to be transferred formed thereon and a substrate (W) with a projection optical system PL interposed therebetween in planes substantially perpendicular to the optical axis thereof; and 
     a step of adjusting the positional relationship between the image plane of the projection optical system (PL) and the substrate (W) in conformity with a variation in the positional relationship between the mask (R) and the projection optical system (PL) in the direction of the optical axis of the projection optical system (PL) so that the image plane of the projection optical system (PL) and the predetermined surface of the substrate (W) may coincide with each other while the mask (R) and the substrate (W) are moved perpendicular to the optical axis of the projection optical system (PL) in order to transfer a pattern on the mask (R) onto the substrate (W). 
     Also, a projection exposure apparatus of the present invention, as shown, for example, in FIGS. 1 and 2, includes: 
     (a) a projection optical system (PL) for projecting the image of a pattern formed on a mask (R) onto a photosensitive substrate (W); 
     (b) synchronous scanning means ( 3 ,  11 ,  14 ,  16 ,  20 ) for scanning the mask (R) and the photosensitive substrate (W) in synchronism with each other perpendicular to the optical axis of the projection optical system (PL); 
     (c) a memory device ( 17 ) for storing therein information regarding a variation in the position of the image plane of the projection optical system (PL) when the mask (R) is being scanned in the scanning by the synchronous scanning means ( 3 ,  11 ,  14 ,  16 ,  20 ); and 
     (d) an adjusting device ( 10 ) for adjusting the positional relationship between the photosensitive substrate (W) and the image plane of the projection optical system (PL) based on the information stored in the memory device ( 17 ), so that the image plane of the projection optical system (PL) and the predetermined surface of the photosensitive substrate (W) may coincide with each other while the mask (R) and the photosensitive substrate (W) are scanned in synchronism with each other by the synchronous scanning means ( 3 ,  11 ,  14 ,  16 ,  20 ) to thereby expose the pattern of the mask (R) onto the photosensitive substrate (W). 
     According to the present invention, the information regarding a variation in the positional relationship between the mask (R) and the projection optical system (PL) in the direction of the optical axis of the projection optical system (PL) while the mask (R) is moved in the predetermined direction perpendicular to the optical axis of the projection optical system (PL) is stored in the memory device ( 17 ). When the pattern on the mask (R) is to be transferred onto the substrate (W), the position and inclination of the photosensitive substrate (W) is corrected based on the information stored in the memory device ( 17 ). This leads to the advantage that even when the positional relationship between the mask and the projection optical system varies during scanning, the mask pattern can be exposed onto the photosensitive substrate in a good state. Also, even if the mounting surface for the pattern surface of the mask (R) and the running surface of the mask stage ( 3 ) are not parallel to each other due to manufacturing error or the like of the mask stage ( 3 ), the mask pattern can always be exposed onto the photosensitive substrate with good focus accuracy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a construction diagram showing a projection exposure apparatus according to an embodiment of the present invention. 
     FIG. 2 shows an example of a case where with scanning, the positional relationship between a reticle and a projection optical system varies within a slit-like illuminated area  1 . 
     FIGS. 3A,  3 B and  3 C show examples of a function indicative of the positional relationship of the reticle and the projection optical system to the scanning position of the reticle. 
     FIGS. 4A and 4B show an example of the operation of correcting the focus position of a wafer in conformity with the scanning position of the reticle. 
     FIGS. 5A and 5B show an example of the operation of correcting the inclination of the wafer in conformity with the scanning position of the reticle. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will hereinafter be described with reference to the drawings. The present embodiment uses as a projection optical system a refraction type (lens projection type) projection optical system comprising a refracting lens. 
     FIG. 1 shows a projection exposure apparatus according to the present embodiment. In FIG. 1, exposure light IL from an illuminating system (not shown) provided with an exposure light source is applied to a slit-like illuminated area  1  on the pattern surface (lower surface) of a reticle R with substantially uniform illuminance. The reticle R is held on the upper surface of a reticle holder  2  by a vacuum chuck, and the reticle holder  2  is fixed on a reticle stage  3  which is placed on a reticle guide  4  for sliding movement in X-direction by an air guide or the like. 
     During scanning exposure, the reticle stage  3  is scanned on the reticle guide  4  in X-direction or −X-direction at a predetermined speed by a linear motor system or the like. The scanning direction of the reticle R is ±X-directions. 
     A movable mirror  5  is fixed to one end of the reticle stage  3  in X-direction. A reticle interferometer monitoring portion  13  applies a measuring laser beam LBR to the movable mirror  5  and detects the reflected light thereof. It normally measures the position of the reticle stage  3  in X-direction. The result of the measurement by the reticle interferometer monitoring portion  13  is supplied to a main controlling system  20 , which controls the movement speed and moved position, of the reticle stage  3  through a reticle stage driving portion  14 . 
     The image of a pattern within the illuminated area  1  on the reticle R is formed in a slit-like exposure area  6  on a wafer W through a projection optical system PL. The Z-axis is taken in parallelism to the optical axis of the projection optical system PL. The wafer W is held on a wafer holder  8  by a vacuum chuck, the wafer holder  8  is placed on a Z levelling stage (including a driving portion)  10  as a correcting mechanism for the exposed surface of the wafer W, and the Z levelling stage  10  is placed on an XY stage  11 . The Z levelling stage  10  is comprised of a Z stage for roughly positioning the wafer W in Z-direction parallel to the optical axis of the projection optical system PL, and a levelling table for adjusting, for example, the amount of expansion and contraction of three actuators to thereby finely adjust the position (focus position) and angle of inclination of the wafer with respect to Z-direction. Also, the XY stage  11  has the functions of positioning the wafer W placed on the Z levelling stage  10  in two-dimensional plane (XY plane) perpendicular to the optical axis of the projection optical system, and scanning the wafer W in ±X-directions. 
     A movable mirror  12  is fixed on the Z levelling stage  10 . A wafer interferometer monitoring portion  15  applies a measuring laser beam LBw to the movable mirror  12  and detects the reflected light thereof. It normally measures the coordinates of the Z levelling stage  10  (wafer W) on the XY plane. The result of the measurement is supplied to the main controlling system  20 , which thus controls the movement of the XY stage  11  through a wafer stage driving portion  16 . Further, the main controlling system  20  controls the movement of the Z levelling stage  10 . Also, during scanning exposure, the main controlling system  20  controls the movement of the XY stage  11  and scans the wafer W in X-direction (or −X-direction) at a predetermined speed. When the projection magnification of the projection optical system PL is β, the main controlling system  20  causes the wafer W to be scanned in X-direction at a speed V WO  (=β·V RO ) in synchronism with causing the reticle R to be scanned in −X-direction at a speed V RO . Conversely, when the reticle R is caused to be scanned in X-direction, the wafer W is caused to be scanned in −X-direction. Also, the main controlling system  20  generally controls the operation of the entire apparatus, in addition to controlling the movement of the reticle stage  3 , the Z levelling stage  10  and the XY stage  11 . 
     In the present embodiment, an optical type focus position detecting system for auto focusing (hereinafter referred to as the “AF sensor”) is used as a sensor for detecting the state of the exposed surface of the wafer W. The AF sensor is comprised of a light transmitting system  7   a  and a light receiving system  7   b , and detection light AL from the light transmitting system  7   a  is projected as a slit pattern image onto the exposed surface of the wafer W within the exposure area  6  obliquely with respect to the optical axis of the projection optical system PL. The reflected light from this slit pattern image re-forms a slit pattern image on the slit portion of a vibration slit plate in the light receiving system  7   b , and the detection light AL passed through the slit of the vibration slit plate is photoelectrically converted by a photoelectric converter. This photoelectrically converted signal is synchronously detected in relation to a driving signal for the vibration slit plate, whereby there is obtained a focus signal linearly varying within a predetermined range in conformity with the position of the exposed surface of the wafer W in the direction of the optical axis. This focus signal is supplied to the main controlling system  20 . In this case, calibration is effected so that the focus signal may become e.g. 0 in a state in which the exposed surface of the wafer W coincides with the imaging plane of the projection optical system PL. 
     The detailed construction of the AF sensor is disclosed, for example, in commonly assigned U.S. Pat. No. 4,650,983. Also, in addition to the AF sensor  7   a ,  7   b , use may be made, in combination, of a levelling sensor for detecting the angle of inclination of the exposed surface of the wafer W in the slit-like exposure area  6  by a collimator system. Also, to enhance the in-focus controlling property in scanning exposure, use may be made of a pilot reading detection system for detecting the focus position at multiple measuring points within an area including an area on the side in the scanning direction with respect to the exposure area  6 . This system is shown in commonly assigned U.S. patent application Ser. No. 172,098 (filed Dec. 23, 1993). 
     Also, a fiducial mark member  9  having its surface set at the same level as the exposed surface of the wafer W is fixed on the Z levelling stage  10 . A slit pattern is formed on the surface of the fiducial mark member  9 . This fiducial mark member  9  is moved into the exposure area  6  of the projection optical system PL and the slit pattern on the fiducial mark member  9  is illuminated with illuminating light of the same wavelength as the illuminating light IL for exposure, from within the fiducial mark member  9 . The light passed through this slit pattern forms a slit pattern image on the pattern surface of the reticle R through the projection optical system PL, the reflected light from the pattern surface is returned to the slit pattern on the surface of the fiducial mark member  9  through the projection optical system PL, and the thus returned reflected light is directed to a photoelectric converter within the fiducial mark member  9  through a beam splitter or the like. 
     If in this case, the pattern surface of the reticle R in the illuminated area  1  and the exposed surface of the wafer W in the exposure area  6  are in in-focus state, the quantity of light returned to the slit pattern becomes maximum and therefore, by monitoring the output signal of the photoelectric converter, it is possible to monitor the in-focus state. Since the focus position of the exposed surface of the wafer W relative to the focus position detecting system can be monitored by the above-described AF sensor  7   a ,  7   b , the position of the pattern of the reticle R in the direction of the optical axis can be monitored by detecting the focus position of the exposed surface of the wafer W relative to the focus position detecting system when the output signal of the photoelectric converter within the fiducial mark member  9  becomes maximum. This detecting system is shown in commonly assigned U.S. patent application Ser. No. 993,460 (filed Dec. 15, 1992). 
     Preferably, design is made such that an operator can input various kinds of information to the main controlling system  20  through input/output devices  18 . The information to be inputted includes, for example, the offset information of the focus signal of the AF sensor found by test print or the like. Also, a lens controlling portion  19  is connected to the main controlling system  20 . The lens controlling portion  19 , as disclosed in U.S. Pat. Nos. 4,687,322, 4,780,747, etc., calculates any variation in the position of the imaging plane of the projection optical system PL caused by a variation in atmospheric pressure or exposure light absorbed by the projection optical system PL, and supplies the result of this calculation to the main controlling system  20 . A lens driving portion  21  can move at least one lens in the projection optical system PL in the direction of the optical axis of the projection optical system PL (Z-direction). It can also incline said lens with respect to a plane perpendicular to the optical axis. The main controlling system  20  can control the imaging characteristic of the projection optical system PL through the lens driving portion  21  to thereby bring about a desired state. 
     A memory  17  is connected to the main controlling system  20 , and in the memory  17 , there is stored for example, the positional relationship between the reticle R and the projection optical system PL resulting from the scanning. The relationship is pre-measured by the use of the fiducial mark member  9  and stored as a function information of the position X R  of the reticle R in the scanning direction. The positional relationship between the reticle R and the projection optical system PL, as shown, for example, in FIGS. 3A to  3 C, is decomposed into three components, i.e., a position Z R  in Z-direction (FIG.  3 A), the angle of inclination θ R  of the reticle R in XZ plane parallel to the plane of the drawing sheet of FIG. 1 (FIG. 3B) and the angle of inclination φ R  of the reticle R in YZ plane perpendicular to the plane of the drawing sheet of FIG. 1 (FIG. 3C) and is pre-stored in the memory  17 . It will now be described how to find the function information indicative of this positional relationship. 
     FIG. 2 shows the manner in which the reticle R is scanned relative to the illuminated area  1 . Also, FIG. 2 shows a case where the reticle scanning plane (the running surface of the reticle stage  3 ) and the pattern surface of the reticle R are not parallel to each other, as an example of the case where the positional relationship between the projection optical system PL and the reticle R in the illuminated area  1  varies with the scanning of the reticle. In this case, it is seen that in a state in which the reticle R is at the rightmost position P 1 , a state in which the reticle R is at the central position P 2  and a state in which the reticle R is at the leftmost position P 3 , the level Z R  of the pattern surface of the reticle R relative to the projection optical system PL varies within the slit-like illuminated area  1 . The main controlling system  20  moves the reticle R by a predetermined step amount from the position P 1  and also measures the position of the pattern surface of the reticle R within the illuminated area  1  in the Z-direction at a plurality of predetermined points (e.g. four points) by the use of the fiducial mark member  9  shown in FIG.  1 . In this manner, the main controlling system takes data regarding the positional relationship between the projection optical system PL and the reticle R in the illuminated area  1  while moving the reticle R, whereby there are finally obtained such functions as shown in FIGS. 3A,  3 B and  3 C. 
     Description will now be made of the operation of the present embodiment when the pattern of the reticle R is exposed onto the wafer W by the scan system. The main controlling system  20  first starts the synchronized scanning of the reticle R and the wafer W. Thereafter, the main scanning system  20  multiplies the amount of variation of the pattern surface of the reticle R determined from the function information in the memory  17  by the square of the projection magnification of the projection optical system PL to thereby calculate the amount of variation of the imaging plane of the projection optical system each time the scanning position X R  of the reticle R varies by the predetermined step amount. The main controlling system  20  adds the foreseen fluctuation value of the image plane supplied from the lens controlling portion  19  to the amount of variation of the imaging plane to thereby find the total value of the amount of variation of the imaging plane of the projection optical system PL. Then, it supplies the Z levelling stage  10  with such control information that the exposed surface of the wafer W coincides with the imaging plane determined by said total value. Accordingly, the exposed surface of the wafer W is sequentially controlled so as to coincide with the true imaging plane of the projection optical system PL in conformity with the scanning position of the reticle R. 
     Consider, for example, a case where only the focus position of the wafer W is adjusted when the pattern surface of the reticle R is not parallel to the running plane of the reticle stage. In this case, it is to be understood that the reticle R and the wafer W are in an in-focus state (the angle of inclination being neglected) when as shown in FIG. 4A, the reticle R is at a position P 2  and the wafer W is at a position Q 2 . When the reticle R is at a position P 1  and the pattern surface is close to the projection optical system PL side, the exposed surface of the wafer W is lowered as indicated at a position Q 1 . Conversely, when as shown in FIG. 4B, the reticle R is at a position P 3  and the pattern surface is far from the projection optical system PL, the exposed surface of the wafer W is elevated as indicated at a position Q 3 . Thereby, the pattern image of the reticle R is always exposed onto the wafer W with high resolution. 
     Also, consider a case where the inclination of the wafer W is adjusted when the running plane of the reticle stage is not parallel to XY plane. Here, consider only the inclination of the reticle R in XZ plane. In this case, it is to be understood that when the reticle R is at the position P 2  and the wafer W is at the position Q 2 , the reticle R and the wafer W are in in-focus state. When the reticle R is at the position P 1  and the pattern surface is inclined downwardly (in −Z-direction) along its scanning direction (−X-direction), the exposed surface of the wafer W is likewise inclined downwardly (in −Z-direction) along its scanning direction (+X-direction) as indicated at the position Q 1 . Also, when as shown in FIG. 5B, the reticle R is at the position P 3  and the pattern surface is inclined upwardly (in +Z-direction) along its scanning direction (−X-direction), the exposed surface of the wafer W is likewise inclined upwardly (in +Z-direction) along its scanning direction (+X-direction). Thereby, the pattern image of the reticle R is always exposed onto the wafer W with high resolution. 
     In the present embodiment, the main controlling system  20  adjusts the level and inclination of the exposed surface of the wafer W in conformity with the variation in the imaging plane of the projection optical system PL, but a particular lens in the projection optical system PL can be moved in the direction of the optical axis through the lens driving portion  21  to thereby move the imaging plane of the projection optical system in the direction of the optical axis. Also, it is possible to incline a particular lens or shift a particular lens in a direction perpendicular to the optical axis to thereby incline the imaging plane. By driving the lens as described above, the imaging plane of the projection optical system PL and the exposed surface of the wafer W can be brought into coincidence with each other. This may also be effected by combining the movement of a lens in the projection optical system and the adjustment of the position of the exposed surface of the wafer W. 
     Finally, description will be made of other examples of the method of finding the function information shown in FIGS. 3A,  3 B and  3 C. 
     A first method is a method using wafer exposure (test print). In FIG. 1, a wafer W having a very good degree of flatness is first placed on the wafer holder  8 . The reticle R is then fixed at a certain position (X R ) in the scanning direction, and by a method similar to that in the case of the step and repeat system, the pattern on the reticle R as exposed onto the wafer W in a matrix-like form. The height and inclination of the image plane within the slit-like exposure area  6  are found from the data of the projected image of the thus exposed wafer W. The position X R  of the reticle R is then changed a little from the initial position X R1  to take further data as described above, until the functions as shown in FIGS. 3A,  3 B and  3 C are finally obtained. 
     A second method is a method which does not require wafer exposure, and uses a focus detecting mechanism of the through-the-lens type not shown in FIG.  1 . For example, the spatial image of the pattern of the reticle R is measured by a detector having its light receiving surface set at the same level as the exposed surface of the wafer W on the Z levelling stage  10 , and the position at which the image is sharpest can be considered to be the in-focus position. By effecting such measurement while changing the position X R  of the reticle R in the scanning direction, there can be finally obtained the function information as shown in FIGS. 3A,  3 B and  3 C. A focusing mechanism of the aforementioned type is disclosed in detail in U.S. Pat. No. 4,711,569. 
     The position of the image plane of the projection optical system can also be found by the use of a detecting system for applying a slit pattern image from the fiducial mark member  9  to the reticle R, and photoelectrically detecting the light transmitted through the reticle R (see U.S. Pat. No. 4,952,815), or a detecting system using a grating plate (see U.S. Pat. No. 4,808,807), and the function information as shown in FIGS. 3A,  3 B and  3 C can be obtained. 
     The function information obtained by one of the methods as described above is stored, for example, from the input/output devices  18  into the memory  17  through the main controlling system  20 . Also, the function information need not be continuous with respect to the position X R  of the reticle R in the scanning direction as shown in FIGS. 3A,  3 B and  3 C, but may be of a discrete table type. 
     While the above embodiment has been described with respect to a case where a refracting system is used as the projection optical system, the present invention is equally applicable to a case where a reflection type projection optical system or a catadioptric type projection optical system is used as the projection optical system. Thus, the present invention is not restricted to the above-described embodiment, but may assume various constructions without departing from the basic principles illustrated herein.