Method of and apparatus for detecting plane position

Claims

What is claimed is:

1. A plane positioning apparatus comprising:

a projection system for projecting a beam to a given portion on a substrate surface;

a light receiving system for receiving a reflective beam from said substrate surface to output a photoelectric signal corresponding to variation of the light receiving position;

a calculating circuit to output a deviation signal corresponding to a deviation amount of said substrate surface from a predetermined fiducial plane in accordance with said photoelectric signal;

a substrate shifting system for shifting and setting said substrate to a given position in a direction perpendicular to said fiducial plane in accordance with said deviation signal;

a detecting device for detecting level variation of said deviation signal generated at a time of displacing said substrate surface and said fiducial plane interrelatedly within a range having substantially a linear relationship with said deviation amount;

a calculating device for calculating a value of inclination of level variation characteristics at a point where said substrate surface and said fiducial plane are substantially matched in accordance with said level variation characteristics; and

a correcting device for correcting an allowable range set for said level variation characteristics in accordance with said calculated value of inclination in order to control said substrate shifting system.

2. A plane positioning apparatus of claim 1, wherein

said correcting device includes an offset generating member for giving an offset corresponding to a deviation between a target plane where said substrate surface is to be set and said substrate surface to at least one of said light receiving system, said calculating circuit, and said substrate shifting system.

3. A plane positioning apparatus comprising:

a projecting system for projecting beams to a plurality of given portions on a substrate surface;

a light receiving system including a plurality of photoelectric elements to receive individually reflective beams respectively from a plurality of given portions on said substrate surface and output photoelectric signals in accordance with variation of light receiving positions;

a calculating circuit for outputting individually deviation signals in accordance with each of deviation amounts with respect to a fiducial plane predetermined at a plurality of given portions on said substrate surface based on each of said plural photoelectric signals;

a shifting system for shifting said substrate in a direction perpendicular to said fiducial plane or a direction inclined to said fiducial plane in accordance with at least one of said deviation signals;

a detecting device for individually detecting level variation characteristics of each deviation signal generated at a time of displacing each of a plurality of given portions on said substrate surface and said fiducial plane in a direction perpendicular to said fiducial plane interrelatedly within a range having substantially a linearly relationship with said deviation amounts;

a calculating device for calculating each of values of inclination of said level variation characteristics at a point where each of a plurality of given portions on said substrate surface is substantially matched with said fiducial plane; and

a correcting device for individually correcting an allowable level variation range set for each of said level variation characteristics in accordance with a calculated value of inclination for each of said plurality of given portions in order to make uniform an allowable range of deviation values for each of said plurality of given portions.

4. A plane positioning apparatus comprising:

a shifting system for holding a substrate to be positioned substantially in parallel to a given fiducial plane and shifting the substrate in a direction perpendicular to said fiducial plane;

a multipoint measuring system having a plurality of measuring points set on a surface of said substrate to measure positional deviation amounts from said fiducial plane at said measuring points with respect to a direction perpendicular to said fiducial plane and output deviation signals corresponding to said deviation amounts at said measuring points;

a controller for controlling said shifting system in accordance with at least one of the deviation signals output at said measuring points;

a detecting device for detecting a ratio of level variation of said deviation signals with respect to variation of said deviation amounts for each of said measuring points; and

a setting device for individually setting an allowable range of the level variation of said deviation signals at said measuring points in accordance with a detected rate of change per measuring point so as to make an allowable deviation amount from said fiducial plane substantially equal at each of the measuring points on said substrate when said substrate is positioned by said shifting system.

5. A plane positioning apparatus of claim 4, wherein

a pattern image of a mask substrate is provided for a projection exposure apparatus for imaging on a photosensitive substrate by projection through an optical projection system, and

said shifting system sets a position of either one of said mask substrate and said photosensitive substrate with regard to a plane perpendicular to an optical axis of said optical projection system as said fiducial plane.

6. A plane positioning apparatus of claim 5, wherein

said projection exposure apparatus has a Z stage to hold said photosensitive substrate and cause it to be shifted in the direction of the optical axis of said optical projection system, and a fiducial board is arranged on said Z stage with the fiducial marks being formed thereon to measure the inclination or curvature of the projected image of said optical projection system with respect to said fiducial plane when said mask substrate is installed.

7. A projection exposure apparatus comprising:

an optical projection system to project and image a mask pattern in a given imaging surface;

an XY stage to hold a photosensitive substrate substantially in parallel with said imaging surface and to be shifted two dimensionally in a plane parallel to said imaging surface;

a Z stage to shift said photosensitive substrate in a direction of an optical axis of said optical projection system;

a first focus detecting system having measuring points at plural positions predetermined in a projection field of said optical projection system to detect a positional deviation of a surface of said photosensitive substrate in the direction of the optical axis at each of said measuring points;

a fiducial board arranged on a part of said Z stage with a light emitting portion being formed thereon to emit light with a given configuration;

a second focus detecting system for detecting a positional relationship of said fiducial board in the direction of the optical axis with said imaging surface in such a manner that when said light emitting portion is positioned at an arbitrary point in said projection field, rays of light reflected by said mask return to said fiducial board through said optical projection system, among those rays of light from said light emitting portion to reach said mask through said optical projection system, and are photoelectrically detected by shifting said Z stage;

a specifying device for specifying coordinate positions of said XY stage so that said light emitting portion is positioned in turn in each of the measuring points or positions close thereto by said first focus detecting system; and

a calibrating system for calibrating detection errors by said first focus detecting system individually or averagely at each of said measuring points in accordance with each positional relationship detected by operating said second focus detecting system at each of the specified coordinate position.

8. A method for detecting plane positions comprising:

measuring a positional deviation amount between a given fiducial plane and an inspecting surface in a direction perpendicular to said fiducial plane at each of plural measuring points set on the inspecting surface;

classifying said plural measuring points into at least two groups in accordance with a surface condition within said inspecting surface, at the same time determining weighting coefficients to be given to said deviation amounts per group; and

detecting a position of said inspecting surface in a direction perpendicular to said fiducial plane by giving said determined weighting coefficients to said deviation amounts, at the same time averaging a sum of deviation amounts subsequent to said giving of weighting coefficients.

9. An apparatus for detecting plane positions comprising:

a projection system for projecting a pattern light on an inspecting surface;

a light receiving system for receiving said pattern light reflected by said inspecting surface to output signals corresponding to the pattern light;

an input device for inputting information regarding position of said inspecting surface to be measured;

a selecting device for dividing a pattern image into a plurality of portions to selectively detect at least two portions of said pattern image in accordance with the information from said input device;

a first position detecting system for output ting a first detection signal corresponding to a height position of said inspecting surface for each of said pattern image portions;

a weighting device to give a weighting coefficient to weight each of said first detection signals; and

a second position detecting system for averaging a sum of said weighted signals by a sum of the weighting coefficients so that a second detection signal corresponding to the height of said inspecting surface is outputted.

10. An apparatus for detecting plane positions of claim 9, wherein

said weighting device includes a determining device for determining at least one of said weighting coefficients to be different from other weighting coefficients.

11. An apparatus for detecting plane positions comprising:

a projection system for projecting a pattern light on an inspecting surface;

a light receiving system for receiving said pattern light reflected by said inspecting surface to output signals corresponding to a pattern light receiving position;

an input device for inputting information regarding a position of said inspecting surface to be measured;

a selecting device for dividing a pattern image into a plurality of portions to selectively detect at least two portions of said pattern image in accordance with the information from said input device;

a first position detecting system for outputting a first detection signal corresponding to a height position of said inspecting surface for each of said portions of said pattern image;

a weighting device for classifying said pattern image portions into predetermined groups to give a weighting coefficient to weight each of signals obtained by averaging a sum of said first detection signals by a number of said pattern image portions in a group for each of said groups; and

a second position detecting system for averaging a sum of said weighted signals by a sum of the weighting coefficients so that a second detection signal corresponding to the height of said inspecting surface is outputted.

12. An apparatus for detecting plane positions comprising:

a multipoint measuring system having a plurality of measuring points set on an inspecting surface to measure positional deviation amounts from a fiducial plane at each of said plurality of measuring points with respect to a direction perpendicular to the fiducial plane, and output deviation signals corresponding to said deviation amounts at the measuring points;

a determining device for classifying said measuring points at least into two groups, at the same time determining weighting coefficients to weight said deviation signals for each of said groups; and

a position detecting system for giving said determined weighting coefficients to said deviation signals, and averaging a sum of the weighted signals so that a detection signal corresponding to the height of said inspecting surface is outputted.

13. An apparatus for detecting plane positions comprising:

a projection system for projecting a pattern light on an inspecting surface;

a light receiving system for receiving said pattern light reflected by said inspecting surface to output signals corresponding to a pattern light receiving portion;

an input device for inputting information regarding a position of said inspecting surface to be measured;

a first selecting device for dividing said pattern image into a plurality of portions to selectively detect at least two portions of said pattern image in accordance with the information from said input device;

a position detecting system for outputting detection signals corresponding to height positions of said inspecting surface for each of said pattern image portions;

a second selecting device for selecting at least two of said detection signals corresponding to local areas having substantially the same height positions on said inspecting surface; and

an inclination detecting system for detecting inclination of said inspecting surface from a difference between at least two detection signals selected by said second selection device.

14. An apparatus for detecting plane position of claim 13, wherein

said second selecting device selects at least two detection signals at least at each of two local areas on said inspecting surface, and

said inclination detecting system includes a calculating circuit to obtain said difference per each local area and sets an inclination of said inspecting surface to minimize a sum of the signal difference per each local area.

15. An apparatus for detecting plane positions comprising:

a multipoint detecting system having plural measuring points in a projection field of an optical projection system and detecting a deviation amount of a surface of a photosensitive substrate exposed with a mask pattern through the optical projection system in a direction of an optical axis of said optical projection system at each of said measuring points;

a stage capable of mounting said photosensitive substrate thereon, and movable in the direction of the optical axis of said optical projection system;

a first position detecting system for detecting first detection signals corresponding to deviation amounts of said substrate surface from an imaging plane of said optical projection system at each of at least two measuring points among said plural measuring points;

a weighting device for giving a weighting coefficient to weight each of said first detection signals; and

a second position detecting system for averaging a sum of the weighted signals by a sum of the weighting coefficients so that a second detection signal corresponding to a deviation amount of a predetermined plane of said substrate from said imaging plane is outputted.

16. An apparatus according to claim 15,

wherein said first position detecting system has a circuit for selecting at least two of the plural measuring points in accordance with a surface condition of said photosensitive substrate.

17. An apparatus for detecting plane positions comprising:

a multipoint detecting system having plural measuring points in a projection field of an optical projection system and detecting a deviation amount of a surface of a photosensitive substrate exposed with a mask pattern through the optical projection system in a direction of an optical axis of said optical projection system at each of said measuring points;

a stage capable of mounting said photosensitive substrate thereon, and movable in the direction of the optical axis of said optical projection system;

a first position detecting system for detecting first detection signals corresponding to deviation mounts of said substrate surface from an imaging plane of said optical projection system at each of at least two measuring points among said plural measuring points;

a weighting device for classifying said at least two measuring points into predetermined groups to give a weighting coefficient to weight each of signals obtainable by averaging a sum of said first detection signals by a number of said measuring points in a group for each of said groups; and

a second position detecting system for averaging a sum of the weighted signals by a sum of the weighting coefficients so that a second detection signal corresponding to a deviation amount of a predetermined plane of said substrate from said imaging plane is outputted.

18. An apparatus for detecting plane positions comprising:

a multipoint detecting system having plural measuring points in a projection field of an optical projection system and detecting a deviation amount of a surface of a photosensitive substrate exposed with a mask pattern through the optical projection system in a direction of an optical axis of said optical projection system at each of said measuring points;

a position detecting system for detecting detection signals corresponding to height positions of said substrate surface at each of at least two measuring points among said plural measuring points;

a selecting device for selecting at least two detection signals corresponding to said substrate surface having substantially the same height positions among said detection signals; and

an inclination detecting system for detecting inclination of said substrate surface in accordance with a difference between at least two detection signals selected by said selecting device.

19. An apparatus according to claim 18, further comprising:

a system for inclining said substrate in accordance with the detected inclination so that said surface of said substrate is made substantially parallel to an imaging plane of said projection optical system.

20. An apparatus according to claim 19,

wherein said inclining system comprises a leveling stage capable of holding and inclining said substrate, and a system for driving said leveling stage so that said surface of said substrate is made substantially parallel to said imaging plane of said projection optical system.

21. An apparatus according to claim 18,

wherein said position detecting system has a circuit for selecting at least two of the plural measuring points in accordance with a surface condition of said photosensitive substrate.

22. A projection exposure apparatus comprising:

a projection optical system for imaging and projecting a pattern of a mask onto a photosensitive substrate;

a substrate stage holding said photosensitive substrate thereon and inclinable with respect to an imaging plane of said projection optical system;

an inclination detecting system for detecting inclination of a surface of said photosensitive substrate independently of said projection optical system;

a pattern board provided on said substrate stage and having a predetermined fiducial pattern;

an illuminating system for applying illuminating light of substantially the same wavelength as that of illuminating light for exposure from an underside of said pattern board to said fiducial pattern;

a photoelectric detector for receiving light created from said fiducial pattern and applied to a pattern surface of said mask through said projection optical system and reflected by said pattern surface and returned through said projection optical system and said fiducial pattern;

a detecting system for detecting a focus position of said projection optical system based on a detection signal output from said photoelectric detector when said pattern board and the imaging plane of said projection optical system are moved relative to each other in a direction of an optical axis of said projection optical system;

a calculating device for finding the imaging plane of said projection optical system based on a focus position at each of a plurality of different positions in a projection field of said projection optical system;

a controller for controlling inclination of said substrate stage in conformity with the found imaging plane so that the imaging plane of said projection optical system and a surface of said pattern board may substantially coincide with each other; and

a calibrating system for calibrating said inclination detecting system so that said inclination detecting system may detect the surface of said pattern board as a fiducial surface when the imaging plane of said projection optical system and the surface of said pattern board are made substantially coincident with each other by said controller.

23. A projection exposure apparatus comprising:

a projection optical system for imaging and projecting a pattern of a mask onto a photosensitive substrate;

a substrate stage holding said photosensitive substrate thereon and movable in a direction of an optical axis of said projection optical system and a direction perpendicular to the direction of said optical axis;

a pattern board provided on said substrate stage and having a predetermined fiducial pattern;

an illuminating system for applying illuminating light of substantially the same wavelength as that of illuminating light for exposure from an underside of said pattern board to said fiducial pattern;

a photoelectric detector for receiving light created from said fiducial pattern and applied to a pattern surface of said mask through said projection optical system and reflected by said pattern surface and returned through said projection optical system and said fiducial pattern;

a measuring system for measuring position of a particular mark on said mask in a plane perpendicular to the optical axis of said projection optical system based on a detection signal output from said photoelectric detector when the light created from said fiducial pattern is applied to said particular mark; and

a detecting system for detecting a focus position at a predetermined point in a projection field of said projection optical system based on a detection signal output from said photoelectric detector when said pattern board and an imaging plane of said projection optical system are moved relative to each other in the direction of the optical axis of said projection optical system.

24. A projection exposure apparatus comprising:

a projection optical system for projecting a pattern on a mask onto a substrate;

a system for measuring a position or a positional deviation of a substrate surface along an optical axis of said projection optical system, at each of plural measuring points in a projection field of said projection optical system; and

a detecting system for weighting and averaging the measured positions or positional deviations so as to detect a single deviation between an imaging plane of said projection optical system and the substrate surface, along a direction of the optical axis.

25. An apparatus according to claim 24,

wherein said detecting system has a device for selecting at least two of the measured positions or positional deviations in accordance with a surface condition of said substrate, and a device for respectively weighting each of the selected positions or positional deviations, said detecting system performing processing to average the weighted positions or positional deviations.

26. An apparatus according to claim 24,

wherein said detecting system has a device for selecting at least two of the measured positions or positional deviations in accordance with a surface condition of said substrate, and a device for grouping each of the at least two of the measured positions or positional deviations and for respectively weighting average positions or positional deviations obtained by averaging a sum of the at least two of the measured positions or positional deviations with a number of data at each group, said detecting system performing processing to average the weighted positions or positional deviations.

27. An apparatus according to claim 24, further comprising:

a driving system for relatively moving the imaging plane of said projection optical system and said substrate along the direction of the optical axis in accordance with a detected single deviation so that the imaging plane and the substrate surface are substantially coincident with each other.

28. An apparatus according to claim 27,

wherein said driving system comprises a stage capable of holding said substrate and moving together therewith along the direction of the optical axis, and a member for driving said stage so that the imaging plane and the substrate surface are substantially coincident with each other.

29. An apparatus according to claim 24, further comprising:

a stage capable of holding said substrate and moving together therewith along the direction of the optical axis; and

a controller for controlling the stage to allow said substrate surface and said imaging plane to be matched in accordance with said single deviation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for detecting plane positions of a substrate (a mask, reticle, semiconductor wafer, glass plate, and others which are used in the process of fabricating semiconductor devices or liquid crystal devices, for example). More particularly, the invention relates to an apparatus for positioning the surface of a substrate with respect to a given fiducial plane, a method of detecting plane positions preferably usable for a projection exposure apparatus (stepper, for example) which transfers the image of circuit patterns to a photosensitive substrate, for example, and an apparatus therefor.

2. Related Background Art

A plane detecting apparatus has hitherto been used widely in setting up proximity gaps, focusing, leveling, and the like, and often incorporated in an aligner which transfers to a given area on a photosensitive substrate the patterns on a mask or reticle (hereinafter referred to collectively as reticle) in the process, particularly in the lithography process, of fabricating semiconductor devices, liquid crystal devices, or the like. Especially in a stepper, when the reticle patterns are projected onto a photosensitive substrate (a wafer or a glass plate with photoresist being coated thereon) for exposure through an optical projection system having a high resolution, it is prerequisite to conduct an operation to match the surface of the photosensitive substrate exactly with the imaging plane of the reticle patterns, that is, a focusing adjustment.

In recent years, the focal depth of projection optical systems has increasingly become narrower and now it is only possible to obtain a focal depth of approximately .+-.0.7 .mu.m even with i rays having its wavelength of 365 nm as an exposure illuminating light. In addition, the projection field of the projection optical systems tends to have increasingly become greater year after year. Therefore, it becomes difficult to design and manufacture a projection system with which to obtain an extremely great focal depth over a wide exposure field (an angle of 22 mm, for example) entirely. In this respect, there have been proposed several methods whereby to expand the focal depth.

However, in order to attain a desirable focusing over a wide exposure field entirely, it is necessary to secure the flatness of an area on the photosensitive substrate locally within such an exposure field as well as the flatness of the imaging plane in any case. In other words, both the so-called curvature and inclination of the field should desirably be arranged. While the curvature and inclination of the field depend largely on the optical performance of the projection optical system itself, the flatness and parallelism of a reticle may cause them in some cases. On the other hand, the local area of the photosensitive substrate, that is, the flatness per stepper area (shot area) differs depending on the photosensitive substrate to be used, but it is possible to establish the parallelism between the surface of the shot area on the photosensitive substrate and the imaging plane by inclining the holder which holds the photosensitive substrate just minutely.

As a method to adjust focusing including a consideration given to the inclination of the surface of each one of shot areas on the photosensitive substrate such as described above, techniques are known as disclosed in U.S. Pat. No. 4,558,949, U.S. Pat. No. 4,383,757, and others, for example. Particularly, in U.S. Pat. No. 4,383,757, there is disclosed a technique wherein the spots of light beams are projected onto four points on a photosensitive substrate through a projection optical system in order to photoelectrically detect the spotted images by the reflective rays of light for the focusing adjustment as well as the calibration of the inclination (leveling) of the photosensitive substrate.

However, according to the two conventional techniques disclosed as above, the detection is made only to determine the displacement of the surface of the photosensitive substrate in the axial direction of the projection from a fiducial plane which is hypothetically set up (where it is matched with the imaging plane as much as possible). It is not a method to detect the displacement between the imaging plane and the surface of the photosensitive substrate directly. As a result, if the hypothetically established fiducial plane serving as the base on which to measure the positional displacement of the photosensitive substrate in the axial direction should be displaced from the imaging plane of the projection optical system due to drift or the like, such a displaced portion becomes a residual focus offset at the time of stepping the patterns to the photosensitive substrate.

In this respect, therefore, there are methods to reduce such a residual focus offset as disclosed in U.S. Pat. No. 4,650,983 and U.S. Pat. No. 4,629,313, for example. In the U.S. Pat. No. 4,650,983, a fiducial pattern is arranged on a stage (holder) to stack a photosensitive substrate thereon, and this fiducial pattern is inversely projected on a specific pattern on a reticle through a projection optical system. Then, the height of the stage is adjusted so that the contrast of the image of the fiducial pattern to be formed on the specific pattern becomes the greatest. Subsequently, a focus detection system (inclined incident type) for the focus adjustment of the photosensitive substrate is calibrated so that the surface where the fiducial pattern is formed can be detected as the best focus plane (the optimal imaging plane). Also, in the U.S. Pat. No. 4,629,313, a fiducial pattern on a stage serves as a slit type photosensor, and the best focus plane is specified by detecting the contrast of the pattern images formed by a projection optical system when the slit pattern on a reticle is projected.

Also, as another method, there is known a technique as disclosed in U.S. Pat. No. 4,952,815 wherein a luminescent mark which emits slit rays of light is arranged on a stage, and the image of this luminescent mark is inversely projected onto a specific mark on a reticle. Then, the stage is shifted in the XY directions, and the best focus plane is detected by photoelectrically detecting the light which is transmitted above the reticle when the reticle mark is scanned with the image of the luminescent mark.

Also, as a method which is developed from the inclined incident type focus detection system disclosed in the U.S. Pat. No. 4,558,949, there is known a multipoint inclined incident type focus detection system as disclosed in U.S. Pat. No. 5,118,957, for example, wherein a pinhole image is projected by an inclined incident system onto each of plural points (five points, for example) in a shot area on a photosensitive substrate, respectively, without any intervention of projection optical systems, and each of the reflective images therefrom is collectively received by a two dimensional position detecting device (CCD). This method disclosed as a prior art is the so-called multipoint AF system of an inclined incident type whereby to implement both a highly precise focus detection and inclination detection. However, there is no disclosure or suggestion in this prior art at all as to the calibration of the focus offset with respect to the best focus plane at the time of stepping.

In each of the above-mentioned prior arts, any one of the systems to detect the positional displacement (focus displacement) in the axial direction of the projection light on the surface of a photosensitive substrate at the time of actual pattern alignment is simply to detect only the positional displacement of the photosensitive substrate in the axial direction and not to detect any focusing state of the reticle patterns and the photosensitive substrate directly. Ideally, therefore, calibration is conducted occasionally in consideration of drift and the like on the apparatus. Furthermore, with the flatness and parallelism of the photosensitive substrate taken into consideration, the multipoint AF system is superior because it performs the focus detections at plural points in a shot area individually and almost simultaneously.

Nevertheless, when a multipoint AF system is employed and a calibration is needed for such an AF system, it is found difficult to apply any of the conventional methods of calibration as it is. In other words, for the conventional calibration method, it is always necessary to detect a specific pattern on the reticle by some detection system. Accordingly, it is required to inscribe such a specific pattern in the circumference of the circuit pattern area or in the street line area of the target reticle at all times. As a result, the plural positions of the measuring points in the projection field determined by the multipoint AF system are completely different from the position of the specific pattern on the reticle which is to be detected at the time of calibration. Naturally, there is a problem that any accurate calibration is possible by the calibration as it is due to the aberration of the projection optical system, warping of reticle, or the like unless such a difference in the measuring position is taken into account. Besides the multipoint AF systems, the same problem is encountered in using a fixed point AF system in which the focus measurement point on the photosensitive substrate for the inclined incident method is set in the center of its projection field, that is, only one point in the center of the shot area.

Here, in FIG. 34, there is illustrated the conventional structure of a focus detecting system (plane position detecting system) of an inclined incident type in which the plane position of one shot area on a photosensitive substrate is detected as an amount of deviation with respect to the fiducial plane (the imaging plane of a projection optical system, for example). The focus detecting system shown in FIG. 34 is equivalent to the fixed AF system which has been described above. In FIG. 34, a projection optical system PL projects the reticle pattern to be imaged on a wafer W. The pattern image of the reticle is formed in the optimal imaging plane (best focus plane) which is perpendicular to the optical axis AX in a state where the contrast becomes greatest. A Z stage 20 with the wafer W stacked thereon is minutely moved on an XY stage 21 in the axial direction AX (in the direction Z) to enable the surface of a specific shot area on the wafer W to be matched with the best focus plane.

Now, in order to detect the height position of the surface of the wafer W, that is, the deviation amount of the shot area surface in the direction Z with respect to the best focus plane, there are provided a light projector LSU, an imaging lens system L.sub.1 for the light projector, an imaging lens system L.sub.2 for a light receiver, and the light receiver RVU as the focus detection system of an inclined incident type. The light projector LSU projects imaging beams onto the surface of the wafer W in the diagonal direction through the lens system L.sub.1, and the light receiver RVU receives the positively reflective beams from the wafer W through the lens system L.sub.2. Then, the light receiver RVU outputs to a focus error detecting circuit FD the photoelectric signals which vary in accordance with the positions where the reflective beams are received. Usually, the imaging beams from the light projector LSU are projected to the vicinity of the position where the optical axis AX of the projection optical system PL exists, and the arrangement is made so that the surface of the wafer W (a local plane where the beams from the light projector LSU are being projected, to be exact) is allowed to match the best focus plane when the positively reflected beams are received to match the detection center of the light receiver RVU.

Also, the error detecting circuit FD calculates the level deviation signal proportional to the positional deviation amount of the wafer surface in the direction Z with respect to the best focus plane on the basis of the signal from the light receiver RVU, and transmits it to a driving portion (hereinafter referred to as Z-DRV) 18 of the Z stage 20. The Z-DRV 18 servo controls the Z stage 20 in order to make the level of the deviation signal a predetermined target value (zero or a given value). Here, in this case, there is provided in the Z-DRV 18, a circuit to detect the difference between the deviation signal level and the target value and determine whether such a difference is within an allowable range or not. This circuit is needed to define the allowable range significantly narrow against the target value thereby to make it possible to implement a stabilized servo control at a high speed.

In the apparatus shown in FIG. 34, the level variation of the deviation signals output from the focus error detecting circuit FD is proportional to the positional variation of the surface of the wafer W in the direction Z in the vicinal range of the best focus plane. Its proportional constant corresponds to the detection sensitivity for plane positions by the focus detection system of the inclined incident type shown in FIG. 34, and given the positional deviation amount of the wafer in the direction Z as .DELTA.Z (.mu.m) and the amount of the level variation of the deviation signal as .DELTA.V in terms of a voltage at that time, the detection sensitivity (inclination) can be defined as .DELTA.V/.DELTA.Z. The greater the value .DELTA.V/.DELTA.Z, the higher becomes the detection sensitivity. Thus, the response of the Z stage 20 is enhanced. On the other hand, however, the stability of the servo system may be affected. Also, a problem is encountered that if the detection sensitivity is lowered on the contrary, the accuracy of the servo system pursuance is reduced.

In a servo system of the kind, the response, stability, and pursuance accuracy are optimally set, but the detection sensitivity itself is not necessarily uniform among focus detection systems when a plurality of aligners are examined. In other words, it often depends on an individual apparatus. Therefore, even if the allowable range against a target value at the time of a servo control is set equal in terms of the deviation signal level, the amplitude of the actual allowable range for the positional deviation in the direction Z may differ per apparatus due to difference in detection sensitivity (inclination and rate of change). Consequently, even when the result of exposure is desirable after the focus adjustment by a certain aligner, there may occur a problem that the conditions obtainable from the focusing parameters at that time are not reproduced as they are in another aligner. This presents a serious problem in terms of the accuracy management in fabricating devices in the manufacturing lines using a number of aligners.

The description has been made of a fixed point AF system so far, but when the foregoing multipoint AF system is employed, it is necessary to output deviation signals by the focus error detecting circuit for each of many numbers of the measuring points which are set in the shot area. Therefore, unless the detection sensitivity (.DELTA.V/.DELTA.Z) is uniform at each of the measuring points, the allowable positional deviation range for the servo system control at each of the measuring points becomes different. A problem is likewise encountered that the advantages obtainable in using the multipoint AF system are reduced by half.

Also, when a step exists in one shot area, it is conceivable to detect the height positions at the upper and lower portions of such a step using a multipoint AF system and set up the surface of the shot area to the fiducial plane in accordance with an averaged value of the respective results of the detections. However, it is impossible for this method to detect the height positions at one point of the plural measuring points, or if the detection results are questionable (the detection errors are great), there is encountered a problem that it is impossible to obtain the height position of the entire shot area exactly. Now, it is obvious that the focal depth of the projection optical system has increasingly become smaller year after year and that the ste