Patent Publication Number: US-2023143407-A1

Title: Exposure apparatus and exposure method, and device manufacturing method

Description:
This is a Continuation of application Ser. No. 17/563,419, filed Dec. 28, 2021, which is a Divisional of application Ser. No. 17/082,402, filed Oct. 28, 2020, which is a Divisional of application Ser. No. 16/522,716, filed Jul. 26, 2019, which is a Divisional of application Ser. No. 15/848,615, filed Dec. 20, 2017, which is a Divisional of application Ser. No. 15/585,866, filed May 3, 2017, which is a Divisional of application Ser. No. 14/432,522, filed Jun. 10, 2015, which is a 371 National Stage of International Application No. PCT/JP2013/076820, filed Oct. 2, 2013, which claims priority to Japanese Application No. 2012-219952, filed Oct. 2, 2012. The disclosures of the prior applications are hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to exposure apparatuses and exposure methods, and device manufacturing methods, and more particularly to an exposure apparatus and an exposure method used in a lithography process for manufacturing microdevices (electronic devices) such as a semiconductor device, and a device manufacturing method which uses the exposure apparatus or the exposure method. 
     BACKGROUND ART 
     Conventionally, in a lithography process to manufacture electronic devices (microdevices) such as semiconductor devices (integrated circuits and the like) and liquid crystal display devices, a projection exposure apparatus of a step-and-repeat method (a so-called stepper), a projection exposure apparatus of a step-and-scan method (so-called scanning stepper (also called a scanner)) and the like are mainly used. 
     In this type of exposure apparatus, along with finer device patterns due to higher integration of semiconductor devices, requirements for high overlay accuracy (alignment accuracy) are increasing. Therefore, requirements are increasing for higher accuracy on position measurement of substrates such as a wafer or a glass plate and the like on which a pattern is formed. 
     As a device for satisfying such requirements, for example, in PTL 1, an exposure apparatus is proposed equipped with a position measurement system that uses a plurality of encoder type sensors (encoder heads) which are mounted on a substrate table. In this exposure apparatus, the encoder heads measure a position of the substrate table by irradiating measurement beams on a scale which is placed facing the substrate table and receiving return beams from the scale. 
     However, as a premise for the exposure apparatus according to PTL 1 equipped with the position measurement system to achieve exposure with high precision, grating pitch and grating shape of the grating that the scale has are “not to vary at all” for over a long period of time. Further, even if the grid varies, there are no means for monitoring the variation except for monitoring the variation based on exposure results. 
     However, when considering that a permissible value of a positioning error required in a wafer stage of the current exposure apparatus is at a nm level, it is difficult to think that the grid will not vary over a long time when viewed at a nm level. 
     Further, transition from an age of a 300 mm wafer to an age of a 450 mm wafer is near at hand and when it comes to an exposure apparatus handling 450 mm wafers, it is considered that while the wafer stage increases in size, the permissible value of the positioning error will become tighter than the present (or around the same as the current level). Using the position measurement system according to PTL 1 described above in the exposure apparatus handling 450 mm wafers without any changes is considered to be difficult, when considering a further increase in the size of the scale (grating) that accompanies larger wafers. 
     A similar problem also occurs in an exposure apparatus disclosed in, for example, PTL 2 and the like that is equipped with an encoder system. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] U.S. Patent Application Publication No. 
     [PTL 2] U.S. Patent Application Publication No. 2008/0088843 
     SUMMARY OF INVENTION 
     Solution to Problem 
     The present invention was made under the circumstances described above, and according to its first aspect, there is provided a first exposure apparatus which exposes an object with an energy beam and forms a pattern on the object, the apparatus comprising: a movable body which is movable along a plane including a first axis and a second axis that are orthogonal to each other holding the object, a position measurement system which has a plurality of heads placed at one of the movable body and an outside of the movable body that irradiates a measurement beam from a part of the plurality of heads onto a measurement surface placed on an other of the movable body and the outside of the movable body, receives a return beam from the measurement surface, and measures positional information of the movable body, and a control system which drives the movable body based on the positional information, wherein a one-dimensional or a two-dimensional grating is formed on the measurement surface, the plurality of heads includes a first head group placed within a predetermined plane facing the measurement surface that includes at least three heads having a measurement direction in at least one direction of two directions within the predetermined plane and a direction orthogonal to the predetermined plane, and of the at least three heads, at least two heads are placed on a same straight line within the predetermined plane, and at least one head remaining is placed at a position different from the straight line within the predetermined plane, and the at least one head is a reference head used for acquiring the positional information, and the at least two heads are measurement heads used for measuring a difference of measurement values in the measurement direction with respect to measurement values of the reference head. 
     According to this apparatus, the movable body is driven by the control system, based on positional information obtained by the position measurement system. Further, difference of measurement values in the measurement direction is measured for at least one reference head and the remaining at least two measurement heads, respectively, which are used for obtaining the positional information belonging to the first head group. This measurement of the difference, by being performed concurrently with the movement of the movable body, allows variation quantity of the grid in the measurement direction of the measurement surface to be monitored, based on data of the difference. 
     According to a second aspect of the present invention, there is provided a second exposure apparatus that exposes an object with an energy beam and forms a pattern on the object, the apparatus comprising: a movable body which holds the object and is movable along a plane including a first axis and a second axis which are orthogonal to each other; a position measurement system which has a plurality of heads placed at one of the movable body and outside of the movable body that irradiates a measurement beam from a part of the plurality of heads on a measurement surface placed at an other of the movable body and the outside of the movable body, receives a return beam from the measurement surface, and measures positional information of the movable body; and a control system which drives the movable body based on the positional information, wherein one of a one-dimensional and a two-dimensional grating is formed on the measurement surface, and the plurality of heads are placed within a predetermined plane facing the measurement surface and includes a head group including at least two heads whose measurement directions are in at least one direction of at least a first direction and a second direction within the predetermined plane and a direction orthogonal to the predetermined plane, and two of the at least two heads are placed apart in a direction intersecting each axis which are an axis along the first direction and an axis along the second direction within the predetermined plane, and one head of the two heads is a reference head used for acquiring the positional information, and a remaining one head of the two heads is a measurement head used for measuring a difference of measurement values in the measurement direction with respect to measurement values of the reference head. 
     According to a third aspect of the present invention, there is provided a device manufacturing method comprising: forming a pattern on an object using one of the first and the second exposure apparatus described above; and developing the object on which the pattern is formed. 
     According to a fourth aspect of the present invention, there is provided a first exposure method in which an object is exposed with an energy beam and a pattern is formed on the object, the method comprising: irradiating a measurement beam from a part of a plurality of heads placed on one of a movable body which moves holding the object along a plane including a first axis and a second axis orthogonal to each other and an outside of the movable body on a measurement surface placed on an other of the movable body and the outside of the movable body on which a one-dimensional or a two-dimensional grating is formed, receiving a return beam from the measurement surface, and measuring positional information of the movable body; and driving the movable body based on the positional information that has been obtained, wherein, the plurality of heads includes a first head group, placed within a predetermined plane facing the measurement surface, the heads including at least three heads whose measurement directions are in at least two directions within the predetermined plane and at least one direction orthogonal to the predetermined plane, and of the at least three heads, at least two heads are placed on a same straight line within the predetermined plane, and at least a remaining head is placed within the predetermined plane at a position different from the straight line, and in the driving, the movable body is driven within the predetermined plane, and concurrently with this driving, difference data of measurement values in the measurement direction of different heads belonging to the first head group is taken in, and grid errors in the measurement direction of the measurement surface are calibrated based on the difference data taken in, the difference data including difference data of measurement values in the measurement direction, of measurement values of a reference head which is the at least one head used for measuring the positional information and measurement values of each measurement head of the at least two heads, belonging to the first head group. 
     According to this method, the movable body is driven based on the positional information that has been obtained, and concurrently with moving the movable body within the predetermined plane, difference data in the measurement direction of different heads belonging to the first group including difference data of measurement values in the measurement direction of at least one reference head and the remaining at least two measurement heads, respectively, which are used for obtaining the positional information belonging to the first head group are taken in, and based on the difference data which has been taken in, calibration is performed of grid errors in the measurement direction of the measurement surface. 
     According to a fifth aspect of the present invention, there is provided a second exposure method in which an object is exposed with an energy beam and a pattern is formed on the object, the method comprising: irradiating a measurement beam from a part of a plurality of heads placed on one of a movable body which moves holding the object along a plane including a first axis and a second axis orthogonal to each other and an outside of the movable body on a measurement surface placed on an other of the movable body and the outside of the movable body on which a one-dimensional or a two-dimensional grating is formed, receiving a return beam from the measurement surface, and measuring positional information of the movable body; and driving the movable body based on the positional information that has been obtained, wherein, the plurality of heads includes a head group including at least two heads, placed within a predetermined plane facing the measurement surface, whose measurement directions are in at least one direction of at least two directions within the predetermined plane and a direction orthogonal to the predetermined plane, and two heads of the at least two heads are placed apart in a direction intersecting each axis which are an axis along the first direction within the predetermined plane and an axis along the second direction, and in the driving, the movable body is driven within the predetermined plane, and concurrently with this driving, difference data of measurement values in the measurement direction of different heads belonging to the head group is taken in, and based on the difference data taken in, grid errors in the measurement direction of the measurement surface are calibrated, the difference data including difference data of measurement values in the measurement direction of measurement values of a reference head used for measuring the positional information which is one head of the two heads belonging to the head group and measurement values of one head remaining of the two heads. 
     According to a sixth aspect of the present invention, there is provided a device manufacturing method, comprising: forming a pattern on the object by one of the first and the second exposure methods described above; and developing the object on which the pattern is formed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a view that schematically shows a structure of an exposure apparatus related to an embodiment. 
         FIG.  2    is a view that shows a structure of an encoder system placed around a projection optical system. 
         FIG.  3    is a view that shows a structure of an encoder system placed around an alignment system. 
         FIG.  4    is a view for explaining an example of a placement of three heads belonging to a first head group placed at a corner section of a first quadrant on a wafer table upper surface. 
         FIG.  5    is an enlarged view of a wafer stage which is partially fragmented. 
         FIG.  6    is a view that shows a placement of encoder heads on the wafer stage. 
         FIG.  7    is a view that shows a placement and a measurement direction of the first head group placed at the corner section of the first quadrant on the wafer table upper surface. 
         FIGS.  8 A to  8 C  are views that show a placement and a measurement direction of a third head group placed at a corner section of a third quadrant on the wafer table upper surface, a second head group placed at a corner section of a second quadrant on the wafer table upper surface, and a fourth head group placed at a corner section of a fourth quadrant on the wafer table upper surface, respectively. 
         FIG.  9    is a block diagram that shows a main structure of a control system related to stage control of the exposure apparatus in  FIG.  1   . 
         FIG.  10    is a view that shows a relation between a placement of encoder heads and scale plates and measurement areas of an encoder system. 
         FIGS.  11 A and  11 B  are views that show other placement examples of heads belonging to each head group. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, an embodiment of the present invention will be described, based on  FIGS.  1  to  10   . 
       FIG.  1    shows a schematic structure of an exposure apparatus  100  related to the embodiment. Exposure apparatus  100  is a projection exposure apparatus of a step-and-scan method, or a so-called scanner. As it will be described later on, a projection optical system PL is provided in the present embodiment, and in the description below, the description will be made with a direction parallel to an optical axis AX of projection optical system PL described as a Z-axis direction, a direction in which a reticle and a wafer are relatively scanned within a plane orthogonal to the Z-axis direction described as a Y-axis direction, a direction orthogonal to the Z-axis and the Y-axis described as an X-axis direction, and rotation (tilt) directions around the X-axis, the Y-axis and the Z-axis described as a θx direction, a θy direction and a θz direction, respectively. 
     Exposure apparatus  100  is equipped with an illumination system  10 , a reticle stage RST which holds a reticle R, a projection unit PU, a wafer stage device  50  including wafer stages WST 1 , WST 2  on which a wafer W is mounted, a control system for these parts and the like. 
     Illumination system  10  includes a light source, and an illumination optical system which has an illuminance equalizing optical system including an optical integrator and the like, a reticle blind and the like (each of which are not shown), as is disclosed in, for example, U.S. Patent Application Publication No. 2003/0025890 and the like. Illumination system  10  illuminates a slit shaped illumination area IAR on reticle R set (limited) by the reticle blind (masking system) with illumination light (exposure light) IL at an almost equal illuminance. Here, as illumination light IL, an ArF excimer laser beam (wavelength 193 nm) is used as an example. 
     On reticle stage RST, reticle R which has a circuit pattern and the like is formed on its pattern surface (a lower surface in  FIG.  1   ) is fixed, for example, by vacuum chucking. Reticle stage RST is finely drivable within an XY plane and is drivable in a scanning direction (the Y-axis direction which is a direction orthogonal to the page surface in  FIG.  1   ) at a predetermined scanning velocity by a reticle stage driving system  11  (not shown in  FIG.  1   , refer to  FIG.  9   ) that includes, for example, a linear motor and the like. 
     Positional information (including information on position in the θz direction (θz rotation quantity)) of reticle stage RST within the XY plane (movement plane) is constantly detected by a reticle laser interferometer (hereinafter referred to as a “reticle interferometer”)  16  shown in  FIG.  1    that irradiates a measurement beam on a movable mirror  15  (in practice, a Y movable mirror (or a retroreflector) having a reflection surface orthogonal to the Y-axis direction and an X movable mirror having a reflection surface orthogonal to the X-axis direction are provided), at a resolution of, for example, around 0.25 nm. Incidentally, to measure positional information of reticle R at least in directions of three degrees of freedom, instead of, or in combination with reticle interferometer  16 , an encoder system as is disclosed in, for example, U.S. Patent Application Publication No. 2007/0288121 and the like can be used. Projection unit PU is placed below reticle stage RST in  FIG.  1    (on the −Z side), and is held by a main frame (metrology frame) which structures a part of a body which is not shown. Projection unit PU has a barrel  40 , and projection optical system PL consisting of a plurality of optical elements held by barrel  40 . As projection optical system PL, for example, a dioptric system is used consisting of a plurality of optical elements (lens elements) arranged along an optical axis AX parallel to the Z-axis direction. Projection optical system PL, for example, is double telecentric, and has a predetermined projection magnification (such as, for example, ¼ times, ⅕ times or ⅛ times). Therefore, when illumination area IAR is illuminated by illumination light IL from illumination system  10 , by illumination light IL which has passed reticle R which is placed so that its pattern surface almost coincides with a first plane (an object plane) of projection optical system PL, a reduced image of a circuit pattern of reticle R within illumination area IAR (a reduced image of a part of the circuit pattern) is formed via projection optical system PL, on an area (exposure area) IA conjugate with illumination area IAR on wafer W which has its surface coated with a resist (a sensitive agent) that is placed on a second plane (image plane) side of projection optical system PL. Then, by synchronous driving of reticle stage RST and wafer stage WST 1  or WST 2 , scanning exposure of one shot area (divided area) on wafer W is performed and the pattern of reticle R is transferred on the shot area, with reticle R being relatively moved in the scanning direction (the Y-axis direction) with respect to illumination area IAR (illumination light IL) while wafer W is relatively moved in the scanning direction (the Y-axis direction) with respect to exposure area IA (illumination light IL). That is, in the present embodiment, the pattern of reticle R is generated on wafer W by illumination system  10  and projection optical system PL, and the pattern is formed on wafer W by exposing a sensitive layer (a resist layer) on wafer W by illumination light IL. 
     Incidentally, the main frame can either be of a conventional gate type, or a suspension support type as is disclosed in, for example, U.S. Patent Application Publication No. 2008/0068568 and the like. 
     In the periphery of a −Z side edge of barrel  40 , a scale plate  21  is placed parallel to the XY plane, for example, at a height substantially flush with the lower end surface of barrel  40 . Scale plate  21  in the present embodiment as is shown in  FIG.  2    is structured, for example, from four L-shaped sections (components)  21   1 ,  21   2 ,  21   3  and  21   4 , and in the center, for example, a rectangular-shaped opening  21   a  is formed into which the −Z side end of barrel  40  is inserted. Here, the width of scale plate  21  in the X-axis direction and in the Y-axis direction are a and b, respectively, and the width of opening  21   a  in the X-axis direction and in the Y-axis direction are ai and bi, respectively. 
     At a position spaced apart from scale plate  21  in the +X direction, as is shown in  FIG.  1   , a scale plate  22  is placed substantially flush with scale plate  21 . Scale plate  22 , as is shown in  FIG.  3   , is also structured, for example, from four L-shaped sections (components)  22   1 ,  22   2 ,  22   3  and  22   4 , and in the center, for example, a rectangular-shaped opening  22   a  is formed into which the −Z side end of an alignment system ALG which will be described later is inserted. The width of scale plate  22  is a in the X-axis direction and is b in the Y-axis direction, respectively, and the width of opening  22   a  is ai in the X-axis direction and is bi in the Y-axis direction, respectively. Incidentally, in the present embodiment, while the width of scale plates  21 ,  22  and the width of openings  21   a,    22   a  were the same in the X-axis and the Y-axis directions, respectively, the width does not necessarily have to be the same, and the width can be different in at least one of the X-axis and the Y-axis directions. 
     In the present embodiment, scale plates  21 ,  22  are supported in a suspended manner by the main frame (metrology frame) which is not shown supporting projection unit PU and alignment system ALG. On a lower surface (−Z side surface) of scale plates  21 ,  22 , a reflection type two-dimensional grating RG (refer to  FIGS.  2 ,  3  and  5   ) is formed, consisting of a grating that has a predetermined pitch of, for example, 1 μm having a periodic direction in a −45 degrees direction (a −135 degrees direction when the Y-axis serves as a reference) with the X-axis serving as a reference, and a grating that has a predetermined pitch of, for example, 1 μm having a periodic direction in a 45 degrees direction (a −45 degrees direction when the Y-axis serves as a reference) with the X-axis serving as a reference. However, due to the structure of two-dimensional grating RG and encoder heads to be described later on, a non-effective area of a width t is included in each of the vicinity of the outer periphery of each of sections  21   1  to  21   4 ,  22   1  to  22   4  structuring scale plates  21 ,  22 . Two-dimensional grating RG of scale plates  21 ,  22  covers at least a movement range of wafer stages WST 1 , WST 2  at the time of exposure operation and at the time of alignment (measurement), respectively. 
     Wafer stage device  50 , as is shown in  FIG.  1   , is equipped with a stage base  12  supported substantially horizontal on a floor surface by a plurality of (for example, three or four) vibration-proof mechanisms (omitted in drawings), wafer stages WST 1 , WST 2  placed on stage base  12 , a wafer stage driving system  27  (only a part of the system is shown in  FIG.  1   , refer to  FIG.  9   ) for driving wafer stages WST 1 , WST 2 , and a measurement system and the like which measures a position of wafer stages WST 1 , WST 2 . The measurement system is equipped with encoder systems  70 ,  71  and a wafer laser interferometer system (hereinafter shortly referred to as wafer interferometer system)  18  and the like shown in  FIG.  9   . Incidentally, encoder systems  70 ,  71  and wafer interferometer system  18  will be described furthermore later in the description. However, in the present embodiment, wafer interferometer system  18  does not necessarily have to be provided. 
     Stage base  12 , as is shown in  FIG.  1   , is made up of a member that has a plate-like outer shape, and its upper surface is finished to a high flatness and serves as a guide surface when wafer stages WST 1 , WST 2  move. Inside stage base  12 , a coil unit is housed that includes a plurality of coils  14 a placed in a shape of a matrix with an XY two-dimensional direction serving as a row direction and a column direction. 
     Incidentally, another base member can be provided to support stage base  12  in a levitated manner separately so that stage base  12  functions as a counter mass (reaction force canceller) which moves according to the momentum conservation law by an action of a reaction force of the driving force of wafer stages WST 1 , WST 2 . 
     Wafer stage WST 1 , as is shown in  FIG.  1   , has a stage main section  91 , and a wafer table WTB 1  which is placed above stage main section  91  and is supported in a non-contact manner with respect to stage main section  91  by a Z tilt driving mechanism  32   a  (not shown in  FIG.  1   , refer to  FIG.  9   ). Similarly, wafer stage WST 2 , as is shown in  FIG.  1   , has stage main section  91 , and a wafer table WTB 2  which is placed above stage main section  91  and is supported in a non-contact manner with respect to stage main section  91  by a Z tilt driving mechanism  32   b  (not shown in  FIG.  1   , refer to  FIG.  9   ). 
     In the present embodiment, because wafer stage WST 2  and wafer stage WST 1  are structured similarly, hereinafter, wafer stage WST 1  will be discussed and described representatively. 
     Wafer table WTB 1  is supported in a non-contact manner by Z tilt driving mechanism  32   a  by adjusting a balance of an upward force (repulsive force) such as an electromagnetic force and a downward force (a gravitational force) including self-weight at three points, and is finely driven at least in directions of three degrees of freedom which are the Z-axis direction, the θx direction, and the θy direction. A slider section  91   a  is provided at a bottom section of stage main section  91 . Slider section  91   a  has a magnet unit consisting of a plurality of magnets having an XY two-dimensional arrangement within the XY plane, a housing which houses the magnet unit, and a plurality of air bearings provided at a periphery of a bottom surface of the housing. The magnet unit structures a planar motor  30  along with the coil unit previously described that performs an electromagnetic force (Lorentz force) drive as is disclosed in, for example, U.S. Pat. No. 5,196,745 and the like. Incidentally, as planar motor  30 , the motor is not limited to a planar motor of the Lorentz force driving method, and a planar motor of a variable magneto-resistance driving method can also be used. 
     Wafer stage WST 1  is supported in a levitated manner by a predetermined clearance (clearance/spacing/void (gap)/spatial distance) on stage base, for example, via a clearance of around several μm by the plurality of air bearings described above, and is driven in the X-axis direction, the Y-axis direction and the θz direction by planar motor  30 . Accordingly, wafer table WTB 1  (wafer W) can be driven in directions of six degrees of freedom (the X-axis direction, the Y-axis direction, the Z-axis direction, the θx direction, θy direction and the θz direction (hereinafter shortly described as X, Y, Z, θx, θy, and θz)) with respect to stage base  12 . 
     Magnitude and direction of the electric current supplied to each coil  14 a structuring the coil unit is controlled by a main controller  20 . In the present embodiment, as is shown in  FIG.  9   , wafer stage driving system  27  is structured including a pair of planar motors  30  having a common stator (coil unit) which drives each of wafer stages WST 1  and WST 2 , and Z tilt driving mechanisms  32   a,    32   b  that wafer stages WST 1  and WST 2  are equipped with, respectively. Incidentally, planar motor  30  is not limited to a motor of a moving magnet method, and can also be a motor of a moving coil method. Further, as planar motor  30 , a planar motor of a magnetic levitation method can also be used. In this case, the air bearings previously described do not have to be provided. Further, wafer stage WST 1  can be driven in directions of six degrees of freedom by planar motor  30 . Further, wafer table WTB 1  can be finely driven in at least one direction of the X-axis direction, the Y-axis direction, and the Oz direction. That is, wafer stage WST 1  can be structured by a coarse/fine movement stage. 
     On wafer table WTB 1 , wafer W is mounted via a wafer holder which is not shown, and is fixed by a chuck mechanism which is not shown, for example, by vacuum chucking (or electrostatic suction). Although it is omitted in the drawings, on wafer table WTB 1 , one or a plurality of reference mark members is provided on which a plurality of reference marks such as a pair of first reference marks and a second reference mark are formed that are detected by a pair of reticle alignment systems  13 A,  13 B and alignment system ALG, respectively. 
     Encoder systems  70 ,  71  obtain (measure) positional information of wafer tables WTB 1 , WTB 2  in directions of six degrees of freedom (X, Y, Z, θx, θy, θz) in a movement area at the time of exposure including an area directly below projection optical system PL and a movement area at the time of measurement including an area directly below alignment system ALG, respectively. Here, a structure and the like of encoder systems  70 ,  71  will be described in detail. Incidentally, movement area at the time of exposure (a first movement area) is an area in which the wafer stage is moved during an exposure operation within an exposure station (a first area) where exposure of the wafer is performed via projection optical system PL, and the exposure operation, for example, is not only exposure of all shot areas on the wafer onto which the pattern should be transferred, but also includes preparatory operations for the exposure (for example, detection of reference marks previously described) and the like. Movement area at the time of measurement (a second movement area) is an area in which the wafer stage is moved during a measurement operation within a measurement station (a second area) where measurement of the positional information is performed by detection of alignment marks of the wafer by alignment system ALG, and the measurement operation, for example, is not only detection of a plurality of alignment marks on the wafer, but also includes detection of reference marks by alignment system ALG (furthermore, measurement of positional information (height difference information) of the wafer in the Z-axis direction) and the like. 
     In wafer tables WTB 1 , WTB 2 , as is shown in the planar view in  FIGS.  2  and  3   , with a center of the upper surface serving as an origin point (coincides with the center of wafer W), a first encoder head group  61   1 , a second encoder head group  61   2 , a third encoder head group  61   3 , and a fourth encoder head group  61   4  are placed in a part of each corner of a first quadrant, a second quadrant, a third quadrant and a fourth quadrant. Incidentally, in the following description, the encoder head group will be shortly described as a head group. The first head group  61   1  includes three encoder heads (hereinafter appropriately referred to as heads)  60   1 ,  60   1a ,  60   1b  placed in the corner section at the +X side and the +Y side on the upper surface of wafer tables WTB 1 , WTB 2 . The three heads  60   1 ,  60   1a ,  60   1b  in the present embodiment are placed at each apex position of a right triangle. To be more specific, head  60   1  is provided at the vicinity of the corner (apex) at the +X side and the +Y side on the upper surface of wafer tables WTB 1 , WTB 2 . Head  60   1a , as is shown enlarged in  FIG.  4   , is placed at a point shifted by Δx in a −X direction from an installation position of head  60   1 . Further, head  60   1b  is placed at a point shifted by Δy in a −Y direction from an installation position of head  60   1 . 
     Referring back to  FIG.  2    (or  FIG.  3   ), the second head group  61   2  includes three heads  60   2 ,  60   2a ,  60   2b  placed in the corner section at the −X side and the +Y side on the upper surface of wafer tables WTB 1 , WTB 2 . The three heads  60   2 ,  60   2a ,  60   2b  in the present embodiment are provided in an arrangement symmetrical to the three heads  60   1 ,  60   1a ,  60   1b  on wafer tables WTB 1 , WTB 2 , with respect to a straight line (center line) parallel to the Y-axis that passes through the center of the upper surface (the origin point described above). The third head group  61   3  includes three heads  60   3 ,  60   3a ,  60   3b  placed in the corner section at the −X side and the −Y side on the upper side of wafer tables WTB 1 , WTB 2 . The three heads  60   3 ,  60   3a ,  60   3b  in the present embodiment are provided in an arrangement symmetrical (point symmetric) to the three heads  60   1 ,  60   1a ,  60   1b  on wafer tables WTB 1 , WTB 2 , with respect to the center of the upper surface. 
     The fourth head group  61   4  includes three heads  60   4 ,  60   4a ,  60   4b  placed in the corner section at the +X side and the −Y side on the upper surface of wafer table WTB 1 , WTB 2 . The three heads  60   4 ,  60   4a ,  60   4b  in the present embodiment are provided in an arrangement symmetrical to the three heads  60   1 ,  60   1a ,  60   1b  on wafer tables WTB 1 , WTB 2 , with respect to a straight line (center line) parallel to the X-axis that passes through the center of the upper surface. 
     As is shown in  FIG.  2   , a separation distance between heads  60   1  and  60   2  in the X-axis direction and the separation distance between heads  60   3  and  60   4  in the X-axis direction are equally A. Further, a separation distance between heads  60   1 ,  60   4  in the Y-axis direction and the separation distance between heads  60   2  and  60   3  in the Y-axis direction are equally B. These separation distances A, B are larger than widths ai, bi of opening  21   a  of scale plate  21 . In a precise sense, A≥ai+2t, B≥bi+2t, taking into consideration width t of the non-effective area previously described. Each of head  60   1  to  60   4 ,  60   1a  to  60   4a , and  60   1b  to  60   4b , as is shown representatively taking up head  60   1  in  FIG.  5   , are each housed inside holes of a predetermined depth in the Z-axis direction formed in wafer tables WTB 1 , WTB 2 . 
     Heads  60   1 ,  60   1a ,  60   1b  belonging to the first head group  61   1 , as is shown in  FIGS.  6  and  7   , are two-dimensional heads whose measurement directions are in a direction of 135 degrees (−45 degrees) with the X-axis as a reference and in the Z-axis direction. Similarly, heads  60   3 ,  60   3a ,  60   3b  belonging to the third head group  61   3 , as is shown in  FIGS.  6  and  8 A , are two-dimensional heads whose measurement directions are in the direction of 135 degrees (−45 degrees) with the X-axis as a reference and in the Z-axis direction. 
     Heads  60   2 ,  60   2a ,  60   2b  belonging to the second head group  61   2 , as is shown in  FIGS.  6  and  8 B , are two-dimensional heads whose measurement directions are in a direction of 45 degrees (−135 degrees) with the X-axis as a reference and in the Z-axis direction. Similarly, heads  60   4 ,  60   4a ,  60   4b  belonging to the fourth head group  61   4 , as is shown in  FIGS.  6  and  8 C , are two-dimensional heads whose measurement directions are in the direction of 45 degrees (−135 degrees) with the X-axis as a reference and in the Z-axis direction. 
     Heads ( 60   1 ,  60   1a ,  60   1b ) belonging to the first head group  61   1 , heads ( 60   2 ,  60   2a ,  60   2b ) belonging to the second head group  61   2 , heads ( 60   3 ,  60   3a ,  60   3b ) belonging to the third head group  61   3 , and heads ( 60   4 ,  60   4a ,  60   4b ) belonging to the fourth head group  61   4 , as is obvious from  FIGS.  2  and  5   , respectively measure positional information of wafer tables WTB 1 , WTB 2  (wafer stages WST 1 , WST 2 ) in each measurement direction by respectively irradiating a measurement beam (refer to reference code MB in  FIG.  5   ) on two-dimensional grating RG formed on the surface of sections  21   1 ,  21   2 ,  21   3 ,  21   4  of scale plate  21 , or sections  22   1 ,  22   2 ,  22   3 ,  22   4  of scale plate  22  opposing the heads, and receiving reflection/diffraction beams from two-dimensional grating RG. Here, as each of the heads  60   1 ,  60   1a ,  60   1b ,  60   2 ,  60   2a ,  60   2b ,  60   3 ,  60   3a ,  60   3b ,  60   4 ,  60   4a  and  60   4b , a sensor head having a structure similar to a displacement measurement sensor head disclosed in, for example, U.S. Pat. No. 7,561,280 can be used. 
     In the respective heads  60   1 ,  60   1a ,  60   1b ,  60   2 ,  60   2a ,  60   2b ,  60   3 ,  60   3a ,  60   3b ,  60   4 ,  60   4a  and  60   4b  structured in the manner described above, influence of air fluctuation can be almost ignored because the optical path length of the measurement beam in the air is extremely short. However, in the present embodiment, the light source and the photodetector are provided external to each head, or to be more specific, provided inside (or outside) stage main section  91 , and only the optical system is provided inside each head. Then, the light source, the photodetector and the optical system are optically connected via an optical fiber which is not shown. To improve positioning accuracy of wafer table WTB 1  (or WTB 2 ), aerial transmission of laser beams and the like can be performed between stage main section  91  and wafer table WTB 1  (or WTB 2 ). 
     When wafer stages WST 1 , WST 2  are positioned within the movement area at the time of exposure previously described, heads  60   1 ,  60   1a ,  60   1b  belonging to the first head group  61   1  structure two-dimensional encoders  70   1 ,  70   1a ,  70   1b , and  71   1 ,  71   1a ,  71   1b  (refer to  FIG.  9   ) that irradiate measurement beams (measurement light) on (section  21   1  of) scale plate  21 , receive diffraction beams from a grating formed on a surface (lower surface) of scale plate  21  having a periodic direction in a direction of 135 degrees with the X-axis serving as a reference, that is, a direction of −45 degrees with the X-axis serving as a reference (hereinafter appropriately referred to as a −45 degrees direction or an α direction), and measure the position of wafer tables WTB 1 , WTB 2  in the −45 degrees direction (α direction) and the Z-axis direction. When wafer stages WST 1 , WST 2  are positioned within the movement area at the time of exposure previously described, heads  60   2 ,  60   2a ,  60   2b  belonging to the second head group  61   2  structure two-dimensional encoder  70   2 ,  70   2a ,  70   2b , and  71   2 ,  71   2a ,  71   2b  (refer to  FIG.  9   ) that irradiate measurement beams (measurement light) on (section  21   2  of) scale plate  21 , receive diffraction beams from a grating formed on a surface (lower surface) of scale plate  21  having a periodic direction in a direction of −135 degrees with the X-axis serving as a reference, that is, a direction of 45 degrees with the X-axis serving as a reference (hereinafter appropriately referred to as a 45 degrees direction or a (3 direction), and measure the position of wafer tables WTB 1 , WTB 2  in the 45 degrees direction (β direction) and the Z-axis direction. 
     When wafer stages WST 1 , WST 2  are positioned within the movement area at the time of exposure previously described, heads  60   3 ,  60   3a ,  60   3b  belonging to the third head group  61   3  structure two-dimensional encoders  70   3 ,  70   3a ,  70   3b , and  71   3 ,  71   3a ,  71   3b  (refer to  FIG.  9   ) that irradiate measurement beams (measurement light) on (section  21   3  of) scale plate  21 , receive diffraction beams from a grating formed on a surface (lower surface) of scale plate  21  having a periodic direction in the −45 degrees direction (α direction), and measure the position of wafer tables WTB 1 , WTB 2  in the −45 degrees direction (α direction) and the Z-axis direction. 
     When wafer stages WST 1 , WST 2  are positioned within the movement area at the time of exposure previously described, heads  60   4 ,  60   4a ,  60   4b  belonging to the fourth head group  61   4  structure two-dimensional encoders  70   4 ,  70   4a ,  70   4b , and  71   4 ,  71   4a ,  71   4b  (refer to  FIG.  9   ) that irradiate measurement beams (measurement light) on (section  21   4  of) scale plate  21 , receive diffraction beams from a grating formed on a surface (lower surface) of scale plate  21  having a periodic direction in the 45 degrees direction (β direction), and measure the position of wafer tables WTB 1 , WTB 2  in the 45 degrees direction (β direction) and the Z-axis direction. 
     Further, when wafer stages WST 1 , WST 2  are positioned within the movement area at the time of measurement previously described, heads  60   1 ,  60   1a ,  60   1b  belonging to the first head group  61   1  structure two-dimensional encoders  70   1 ,  70   1a ,  70   1b , and  71   1 ,  71   1a ,  71   1b  (refer to  FIG.  9   ) that irradiate measurement beams (measurement light) on (section  22   1  of) scale plate  22 , receive diffraction beams from a grating formed on a surface (lower surface) of scale plate  22  having a periodic direction in the −45 degrees direction (α direction), and measure the position of wafer tables WTB 1 , WTB 2  in the −45 degrees direction (α direction) and the Z-axis direction. 
     When wafer stages WST 1 , WST 2  are positioned within the movement area at the time of measurement previously described, heads  60   2 ,  60   2a ,  60   2b  belonging to the second head group  61   2  structure two-dimensional encoders  70   2 ,  70   2a ,  70   2b , and  71   2 ,  71   2a ,  71   2b  (refer to  FIG.  9   ) that irradiate measurement beams (measurement light) on (section  22   2  of) scale plate  22 , receive diffraction beams from a grating formed on a surface (lower surface) of scale plate  22  having a periodic direction in the 45 degrees direction (β direction), and measure the position of wafer tables WTB 1 , WTB 2  in the 45 degrees direction (β direction) and the Z-axis direction. 
     When wafer stages WST 1 , WST 2  are positioned within the movement area at the time of measurement previously described, heads  60   3 ,  60   3a ,  60   3b  belonging to the third head group  61   3  structure two-dimensional encoders  70   3 ,  70   3a ,  70   3b , and  71   3 ,  71   3a ,  71   3b  (refer to  FIG.  9   ) that irradiate measurement beams (measurement light) on (section  22   3  of) scale plate  22 , receive diffraction beams from a grating formed on a surface (lower surface) of scale plate  22  having a periodic direction in the −45 degrees direction (α direction), and measure the position of wafer tables WTB 1 , WTB 2  in the −45 degrees direction (α direction) and the Z-axis direction. 
     When wafer stages WST 1 , WST 2  are positioned within the movement area at the time of measurement previously described, heads  60   4 ,  60   4a ,  60   4b  belonging to the fourth head group  61   4  structure two-dimensional encoders  70   4 ,  70   4a ,  70   4b , and  71   4 ,  71   4a ,  71   4b  (refer to  FIG.  9   ) that irradiate measurement beams (measurement light) on (section  22   4  of) scale plate  22 , receive diffraction beams from a grating formed on a surface (lower surface) of scale plate  22  having a periodic direction in the 45 degrees direction (β direction), and measure the position of wafer tables WTB 1 , WTB 2  in the 45 degrees direction (β direction) and the Z-axis direction. 
     As it can be seen from the description above, in the present embodiment, regardless of irradiating the measurement beam (measurement light) on either one of scale plates  21 ,  22 , or in other words, regardless of the area where wafer stages WST 1 , WST 2  are located either within the movement area at the time of exposure or the movement area at the time of measurement previously described, heads  60   1 ,  60   1a ,  60   1b ,  60   2 ,  60   2a ,  60   2b ,  60   3 ,  60   3a ,  60   3b ,  60   4 ,  60   4a  and  60   4b  on wafer stage WST 1  structure two-dimensional encoders  70   1 ,  70   1a ,  70   1b ,  70   2 ,  70   2a ,  70   2b ,  70   3 ,  70   3a ,  70   3b ,  70   4 ,  70   4a ,  70   4b , respectively, along with the scale plates on which the measurement beam (measurement light) is irradiated, and heads  60   1 ,  60   1a ,  60   1b ,  60   2 ,  60   2a ,  60   2b ,  60   3 ,  60   3a ,  60   3b ,  60   4 ,  60   4a  and  60   4b  on wafer stage WST 2  structure two-dimensional encoders  71   1 ,  71   1a ,  71   1b ,  71   2 ,  71   2a ,  71   2b ,  71   3 ,  71   3a ,  71   3b ,  71   4 ,  71   4a ,  71   4b , respectively, along with the scale plates on which the measurement beam (measurement light) is irradiated. 
     Measurement values of the two-dimensional encoders (hereinafter shortly referred to as an encoder as appropriate)  70   1 ,  70   1a ,  70   1b ,  70   2 ,  70   2a ,  70   2b ,  70   3 ,  70   3a ,  70   3b ,  70   4 ,  70   4a ,  70   4b ,  71   1 ,  71   1a ,  71   1b ,  71   2 ,  71   2a ,  71   2b ,  71   3 ,  71   3a ,  71   3b ,  71   4 ,  71   4a ,  71   4b  are each supplied to main controller  20  (refer to  FIG.  9   ). Of encoders  70   1  to  70   4  or encoders  71   1  to  71   4 , main controller  20  obtains positional information of wafer tables WTB 1 , WTB 2  in directions of six degrees of freedom (X, Y, Z, θx, θy, θz) within the movement area at the time of exposure including the area directly below projection optical system PL, based on the measurement values of at least three encoders (that is, at least three encoders that output effective measurement values) that face lower surface of (sections  21   1  to  21   4  which structure) scale plate  21  on which two-dimensional grating RG is formed. 
     Main controller  20  performs calibration (to be described later on) of a grid (grid error) of a coordinate system at the time of exposure that corresponds to a variation of two-dimensional grating RG of scale plate  21  while wafer tables WTB 1 , WTB 2  move within the movement area at the time of exposure including, for example, when exposure is being performed, using the measurement values of all of the heads in at least the three groups to which the heads of at least the three encoders belong used for measuring the positional information of wafer tables WTB 1 , WTB 2  in directions of six degrees of freedom (X, Y, Z, θx, θy, θz). 
     Similarly, of encoders  70   1  to  70   4  or encoders  71   1  to  71   4 , main controller  20  obtains positional information of wafer tables WTB 1 , WTB 2  in directions of six degrees of freedom (X, Y, Z, θx, θy, θz) within the movement area at the time of measurement including the area directly below alignment system ALG, based on the measurement values of at least three encoders (that is, at least three encoders that output effective measurement values) that face the lower surface of (sections  22   1  to  22   4  which structure) scale plate  22  on which two-dimensional grating RG is formed. 
     Main controller  20  can perform calibration (to be described later on) of a grid (grid error) of a coordinate system at the time of measurement that corresponds to a variation of two-dimensional grating RG of scale plate  22  while wafer tables WTB 1 , WTB 2  move within the movement area at the time of measurement, including, for example, when alignment is being performed, using the measurement values of all of the heads in at least the three groups to which the heads of at least the three encoders belong used for measuring the positional information of wafer tables WTB 1 , WTB 2  in directions of six degrees of freedom (X, Y, Z, θx, θy, θz). 
     Further, in exposure apparatus  100  of the present embodiment, the position of wafer stages WST 1 , WST 2  (wafer tables WTB 1 , WTB 2 ) can be measured by wafer interferometer system  18  (refer to  FIG.  9   ), independently from encoder systems  70 ,  71 . The measurement values of wafer interferometer system  18  are used secondarily, for example, as a backup and the like at the time of output abnormalities of encoder systems  70 ,  71 . Incidentally, details on wafer interferometer system  18  are omitted. 
     Alignment system ALG, as is shown in  FIG.  1   , is an alignment system of an off-axis method that is placed in a predetermined spacing on the +X side of projection optical system PL. In the present embodiment, as alignment system ALG, as an example, an FIA (Field Image Alignment) system is used which is a type of an alignment sensor of an image-processing method that measures a mark position by illuminating a mark with a broadband (wideband) light such as a halogen lamp and the like, and performing image-processing on this mark image. Imaging signals from alignment system ALG are supplied to main controller  20  (refer to  FIG.  9   ), via an alignment signal processing system which is not shown. 
     Incidentally, alignment system ALG is not limited to the FIA system, and for example, it is a matter of course that an alignment sensor which performs detection by irradiating a coherent detection light on a mark and detecting scattered light or diffracted light generated from the mark, or by making two diffracted lights (for example, diffracted light of the same order or diffracted light diffracting in the same direction) generated from the mark interfere with each other can be used alone, or appropriately combined. As alignment system ALG, an alignment system that has a plurality of detection areas as in the one disclosed in, for example, U.S. Patent Application Publication No. 2008/0088843 and the like can be employed. 
     Other than this, in exposure apparatus  100  of the present embodiment, a multi-point focal point detection system of an oblique incidence method (hereinafter, shortly described as multi-point AF system) AF (not shown in  FIG.  1   , refer to  FIG.  9   ) is provided placed at the measurement station along with alignment system ALG and having a structure similar to the one disclosed in, for example, U.S. Pat. No. 5,448,332 and the like. At least a part of a measurement operation by multi-point AF system AF is performed concurrently with the mark detection operation by alignment system ALG, and positional information of the wafer table is measured by the encoder system previously described during the measurement operation. Detection signals of multi-point AF system AF is supplied to main controller  20  (refer to  FIG.  9   ), via an AF signal processing system (not shown). Main controller  20  detects positional information (height difference information/unevenness information) of the wafer W surface in the Z-axis direction, based on the detection signals of multi-point AF system AF and the measurement information of the encoder system previously described, and performs a so-called focus/leveling control of wafer W during scanning exposure in the exposure operation based on the information detected beforehand and the measurement information of the encoder system previously described (positional information in the Z-axis, the θx and the θy directions). Incidentally, the multi-point AF system can be provided at the vicinity of projection unit PU within exposure station, and focus/leveling control of wafer W may be performed driving the wafer table while measuring the positional information (unevenness information) of the wafer surface at the time of the exposure operation. 
     In exposure apparatus  100 , furthermore, a pair of reticle alignment systems  13 A,  13 B (not shown in  FIG.  1   , refer to  FIG.  9   ) of a TTR (Through The Reticle) method which uses light of the exposure wavelength as is disclosed in, for example, U.S. Pat. No. 5,646,413 and the like, is provided above reticle R. Detection signals of reticle alignment systems  13 A,  13 B are supplied to main controller  20 , via an alignment signal processing system which is not shown. Incidentally, instead of the reticle alignment system, reticle alignment can be performed using the aerial image measuring instrument which is not shown provided on wafer stage WST. 
       FIG.  9    shows a block diagram which is partly omitted of a control system related to stage control of exposure apparatus  100 . This control system is structured centering on main controller  20 . Main controller  20  includes a so-called microcomputer (or a workstation) consisting of a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory) and the like that has overall control of the entire device. 
     In exposure apparatus  100  structured in the manner described above, when manufacturing a device, one of wafer stages WST 1 , WST 2  on which the wafer is loaded is moved within the measurement station (movement area at the time of measurement) by main controller  20 , and wafer measurement operation is performed by alignment system ALG and multi-point AF system. That is, to wafer W held by one of wafer stages WST 1 , WST 2  within the movement area at the time of measurement, mark detection using alignment system ALG, or a so-called wafer alignment (such as Enhanced Global Alignment (EGA) disclosed in, for example, U.S. Pat. No. 4,780,617 and the like) and measurement of surface information (height difference/unevenness information) of the wafer using the multi-point AF system are performed. On this operation, by encoder system  70  (encoders  70   1  to  70   4 ) or encoder system  71  (encoders  71   1 to  71   4 ), positional information of wafer tables WTB 1 , WTB 2  in directions of six degrees of freedom (X, Y, Z, θx, θy, θz) is obtained (measured). Incidentally, before beginning or after completing the wafer alignment, position of the second reference mark on the reference mark member provided on one of wafer stages WST 1 , WST 2  is measured by main controller  20  using alignment system ALG. Then, array coordinates of a plurality of shot areas on wafer W calculated as results of wafer alignment are replaced with array coordinates using the second reference mark as a reference. 
     After the measurement operation such as wafer alignment, one of the wafer stages (WST 1  or WST 2 ) moves to the movement area at the time of exposure, and by main controller  20 , reticle alignment and the like to obtain a positional relation between a projection center of the reticle pattern and the center of the pair of the first reference marks is performed, using reticle alignment systems  13 A,  13 B, reference mark members (not shown) on the wafer table (WTB 1  or WTB 2 ) and the like in a procedure similar to a normal scanning stepper. 
     Then, by main controller  20 , based on the results of the reticle alignment and the array coordinates of the plurality of shot areas using the second reference marks obtained as a result of the wafer alignment as a reference, exposure operation by the step-and-scan method is performed, and the pattern of reticle R is transferred, respectively, onto a plurality of shot areas on wafer W. The exposure operation by the step-and-scan method is performed by alternately repeating a scanning exposure operation in which synchronous movement of reticle stage RST and wafer stage WST 1  or WST 2  is performed, and a movement (stepping) operation between shots in which wafer stage WST 1  or WST 2  is moved to an acceleration starting position for exposure of a shot area. At the time of the exposure operation, positional information in directions of six degrees of freedom (X, Y, Z, θx, θy, θz) of one of the wafer tables (WTB 1  or 
     WTB 2 ) is obtained (measured) by encoder system  70  (encoders  70   1  to  70   4 ) or encoder system  71  (encoders  71   1  to  71   4 ) . 
     Further, exposure apparatus  100  of the present embodiment is equipped with the two wafer stages WST 1 , WST 2 . Therefore, concurrently with performing exposure by the step-and-scan method on the wafer loaded on one of wafer stages, for example, wafer stage WST 1 , a concurrent processing operation is performed in which wafer alignment and the like is performed on the wafer mounted on the other wafer stage WST 2 . 
     In exposure apparatus  100  of the present embodiment, as is previously described, main controller  20  obtains (measures) positional information of wafer table WTB 1  in directions of six degrees of freedom (X, Y, Z, θx, θy, θz) using encoder system  70  (refer to  FIG.  9   ) in both the movement area at the time of exposure and the movement area at the time of measurement. Further, main controller  20  obtains (measures) positional information of wafer table WTB 2  in directions of six degrees of freedom (X, Y, Z, θx, θy, θz) using encoder system  71  (refer to  FIG.  9   ) in both the movement area at the time of exposure and the movement area at the time of measurement. 
     Here, a principle and the like of position measurement in directions of three degrees of freedom (the X-axis direction, the Y-axis direction and the Oz direction, also shortened to (X, Y, θz))) within the XY plane by encoder systems  70 ,  71  will be further described. Here, measurement values of encoder heads  60   1  to  60   4  or encoders  70   1  to  70   4  mean measurement values in a measurement direction which is not in the Z-axis direction of encoder heads  60   1  to  60   4  or encoders  70   1  to  70   4 . 
     In the present embodiment, by employing the structure and placement of encoder heads  60   1  to  60   4  and scale plate  21  previously described, within the movement area at the time of exposure at least three of encoder heads  60   1  to  60   4  constantly faces (the corresponding sections  21   1  to  21   4  of) scale plate  21 . 
       FIG.  10    shows a relation between a placement of encoder heads  60   1  to  60   4  and each section  21   1  to  21   4  of scale plate  21  on wafer stage WST 1  and measurement areas A 0  to A 4  of encoder system  70 .  FIG.  10    shows only heads  60   1 ,  60   2 ,  60   3 , and  60   4  which are used for measuring positional information in directions of six degrees of freedom of wafer table WTB 1 . Incidentally, because wafer stage WST 2  is structured similarly to wafer stage WST 1 , the description here will be made only for wafer stage WST 1 . 
     In the case the center of wafer table WTB 1  (coincides with the center of the wafer) is positioned within a first area A 1  which is within the movement area at the time of exposure that is also an area located on the +X side and on the +Y side with respect to exposure center (the center of exposure area IA) P (an area within the first quadrant (however, excluding an area A 0 ) whose origin point is exposure center P), heads  60   4 ,  60   1 ,  60   2  on wafer stage WST 1  face sections  21   4 ,  21   1 ,  21   2  of scale plate  21 , respectively. In the first area A 1 , effective measurement values are sent to main controller  20  from heads  60   4 ,  60   1 ,  60   2  (encoders  70   4 ,  70   1 ,  70   2 ). Incidentally, the position of wafer stages WST 1 , WST 2  in the description below means the position in the center of the wafer stage (coincides with the center of wafer table and the center of the wafer, respectively). That is, instead of using the expression, the position in the center of wafer stages WST 1 , WST 2 , the expression used will be, the position of wafer stages WST 1 , WST 2 . 
     Similarly, in the case wafer stage WST 1  is positioned within a second area A 2  which is within the movement area at the time of exposure that is also an area located on the −X side and on the +Y side with respect to exposure center P (an area within the second quadrant (however, excluding area A 0 ) whose origin point is exposure center P) , heads  60   1 ,  60   2 ,  60   3  face sections  21   1 ,  21   2 ,  21   3  of scale plate  21 , respectively. In the case wafer stage WST 1  is positioned within a third area A 3  which is within the movement area at the time of exposure that is also an area located on the −X side and on the −Y side with respect to exposure center P (an area within the third quadrant (however, excluding area A 0 ) whose origin point is exposure center P), heads  60   2 ,  60   3 ,  60   4  face sections  21   2 ,  21   3 ,  21   4  of scale plate  21 , respectively. In the case wafer stage WST 1  is positioned within a fourth area A 4  which is within the movement area at the time of exposure that is also an area located on the +X side and the −Y side with respect to exposure center P (an area within the fourth quadrant (however, excluding area A 0 ) whose origin point is exposure center P) , heads  60   3 ,  60   4 ,  60   1  face sections  21   3 ,  21   4 ,  21   1  of scale plate  21 , respectively. 
     In the present embodiment, as is shown in  FIG.  10   , in the case wafer stage WST 1  is positioned within a cross-shaped area A 0  (an area that passes through exposure center P including an area of a width A-ai-2t having a longitudinal direction in the Y-axis direction and an area of a width B-bi-2t having a longitudinal direction in the X-axis direction (hereinafter called a zero th  area)), the four heads  60   1  to  60   4  on wafer stage WST 1  face (the corresponding sections  21   1  to  21   4 ) of scale plate  21 . Accordingly, within zero th  area A 0 , effective measurement values are sent from heads  60   1  to  60   4  (encoders  70   1  to  70   4 ) to main controller  20 . Incidentally, in the present embodiment, adding to the above conditions (A≥ai+2t, B≥bi+2t), conditions A≥ai+W+2t, B≥bi+L+2t can be added, taking into consideration the size (W, L) of the shot area on the wafer where the pattern is formed. Here, W is the width in the X-axis direction and L is the length in the Y-axis direction of the shot area, respectively. W equals a distance of the scanning exposure section and L equals a stepping distance in the X-axis direction, respectively. 
     Main controller  20 , based on the measurement values of heads  60   1  to  60   4  (encoders  70   1  to  70   4 ) , calculates a position (X, Y, θz) within the XY plane of wafer stage WST 1 . Here, the measurement values of encoders  70   1  to  70   4  (expressed as C 1  to C or  respectively) are dependent on the position (X, Y, θz) of wafer stage WST 1 , as in the following formulas (1) to (4). 
         C   1 =−(cos θ z +sin θ z ) X/√ 2+(cos θ z −sin θ z ) Y/√ 2+√2 p  sin θ z    (1)
 
         C   2 =−(cos θ z −sinθ z ) X/√ 2−(cos θ z +sin θ z ) Y/√ 2+√2 p  sin θ z    (2)
 
         C   3 =(cos θ z +sin θ z ) X/√ 2−(cos θ z −sin θ z ) Y/√ 2+√2 p  sin θ z    (3)
 
       C 4 =(cos θ z −sin θ z ) X/√ 2+(cos θ z +sin θ z ) Y/√ 2+√2 p  sin θ z    (4)
 
     However, p, as is shown in  FIG.  6   , is the distance in the X-axis and the Y-axis direction of each of the heads  60   1  to  60   4  from the center of wafer table WTB 1  (WTB 2 ). 
     Main controller  20  calculates the position (X, Y, θz) within the XY plane of wafer stage WST 1  by specifying three heads (encoders) that face scale plate  21  corresponding to areas A 0  to A 4  in which wafer stage WST 1  is positioned, forming simultaneous equations by choosing formulas that the measurement values of these heads follow from formulas (1) to (4) above, and solving the simultaneous equations using the measurement values of the three heads (encoders). For example, in the case wafer stage WST 1  is positioned within the first area A 1 , main controller  20  forms simultaneous equations from formulas (1), (2) and (4) that the measurement values of heads  60   2 ,  60   2 ,  60   4  (encoders  70   1 ,  70   2 ,  70   4 ) follow, and solves the simultaneous equations by substituting the measurement values of each head into the left side of formulas (1), (2), and (4), respectively. 
     Incidentally, in the case wafer stage WST 1  is positioned within zero th  area A 0 , main controller  20  can select three arbitrary heads from heads  60   1  to  60   4  (encoder  70   1  to  70   4 ). For example, after wafer stage WST 1  has moved to the zero th  area from the first area, heads  60   1 ,  60   2 ,  60   4  (encoders  70   1 ,  70   2 ,  70   4 ) corresponding to the first area should be selected. 
     Main controller  20  drives (performs position control of) wafer stage WST 1  within the movement area at the time of exposure, based on results of the calculation (X, Y, θz) described above. 
     In the case wafer stage WST 1  is positioned within the movement area at the time of measurement, main controller  20  measures positional information in directions of three degrees of freedom (X, Y, θz), using encoder system  70  (encoders  70   2  to  70   4 ). Here, the measurement principle and the like are the same as the case previously described when wafer stage WST 1  is positioned within the movement area at the time of exposure, except for the point that exposure center P is replaced with the detection center of alignment system ALG and (sections  21   1  to  21   4  of) scale plate  21  is replaced with (sections  22   1  to  22   4  of) scale plate  22 . 
     Furthermore, main controller  20  uses three heads of heads  60   1  to  60   4  facing scale plates  21 ,  22  while switching to three heads with at least one different, according to the position of wafer stages WST 1 , WST 2 . Here, on switching the encoder heads, a joint processing is performed to assure continuity of the measurement values of the position of the wafer stage, as is disclosed in, for example, U.S. Patent Application Publication No. 2008/0094592 and the like. Further, in the present embodiment, in a method similar to the method disclosed in, for example, U.S. Patent Application Publication No. 2011/0053061, switching between heads  60   1  to  60   4  at the time of exposure operation by the step-and-scan method and joint processing are performed. 
     Next, the principle and the like for position measurement in directions of three degrees of freedom (Z, θx, θy) by encoder systems  70 ,  71  will be described furthermore. Here, measurement values of encoder heads  60   1  to  60   4  or encoders  70   1  to  70   4  mean measurement values in the Z-axis direction of encoder heads  60   1  to  60   4  or encoders  70   1  to  70   4 . 
     In the present embodiment, by employing the structure and the placement of encoder heads  60   1  to  60   4  and scale plate  21  previously described, within the movement area at the time of exposure, at least three of encoder heads  60   1  to  60   4  face (the corresponding sections  21   1  to  21   4  of) scale plate  21 , corresponding to areas A 0  to A 4  in which wafer stage WST 1  (WST 2 ) is positioned. Effective measurement values from the heads (encoders) facing scale plate  21  is sent to main controller  20 . 
     Main controller  20  calculates a position (Z, θx, θy) of wafer stages WST 1  (WST 2 ), based on the measurement values of encoders  70   1  to  70   4 . Here, measurement values in the Z-axis direction (expressed as D 1  to D 4 , respectively, to distinguish them from measurement values C 1  to C 4  in the measurement direction which is not the Z-axis direction previously described, that is, an uniaxial direction within the XY plane) of encoders  70   1  to  70   4  are dependent on the position (Z, θx, θy) of wafer stage WST 1  (WST 2 ), as in the following formulas (5) to (8). 
         D   1   =−p  tan θ y+p  tan θ x+Z    (5)
 
         D   2   =p  tan θ y+p  tan θ x+Z    (6)
 
         D   3   =p  tan θ y−p  tan θ x+Z    (7)
 
         D   4   =−p  tan θ y−p  tan θ x+Z    (8)
 
     However, p is a distance (refer to  FIG.  6   ) of heads  60   1  to  60   4  in the X-axis and the Y-axis direction, from the center of wafer table WTB 1  (WTB 2 ). 
     Main controller  20  calculates the position(Z, θx, θy) of wafer stage WST 1  (WST 2 ) by choosing formulas that the measurement values of three heads (encoders) follow from formulas (5) to (8) corresponding to areas A 0  to A 4  in which wafer stage WST 1  (WST 2 ) is positioned, and by substituting the measurement values of the three heads (encoders) into simultaneous equations structured from the three chosen formulas and solving the simultaneous equations. For example, in the case wafer stage WST 1  (or WST 2 ) is positioned within the first area A 1 , main controller  20  forms simultaneous equations from formulas (5), (6), and (8) that the measurement values of heads  60   1 ,  60   2 ,  60   4  (encoders  70   1 ,  70   2 ,  70   4 ) (or heads  60   1 ,  60   2 ,  60   4  (encoders  71   1 ,  71   2 ,  71   4 ) follow, and solves the equations by substituting the measurement values into the left side of formulas (5), (6), and (8), respectively. 
     Incidentally, in the case wafer stage WST 1  (or WST 2 ) is positioned within zero th  area A 0 , three arbitrary heads can be selected from heads  60   1  to  60   4  (encoders  70   1  to  70   4 ) (or heads  60   1  to  60   4  (encoders  71   1  to  71   4 )), and the simultaneous equations formed from the equations that the measurement values of the chosen three heads follow can be used. 
     Main controller  20  performs focus/leveling control on wafer stage WST 1  (or WST 2 ) within the movement area at the time of exposure, based on the calculation results (Z, θx, θy) above and the height difference information (focus mapping data) previously described. 
     In the case wafer stage WST 1  (or WST 2 ) is positioned within the movement area at the time of measurement, main controller  20  measures positional information in directions of three degrees of freedom (Z, θx, θy), using encoder system  70  or  71 . Here, the measurement principle and the like is similar to the case when wafer stage WST 1  is positioned within the movement area at the time of exposure, except for the point that the exposure center is replaced with the detection center of alignment system ALG and that (sections  21   1  to  21   4  of) scale plate  21  is replaced with (sections  22   1  to  22   4  of) scale plate  22 . Main controller  20  performs focus/leveling control on wafer stage WST 1  or WST 2 , based on measurement values of encoder system  70  or  71 . Incidentally, in the movement area at the time of measurement (measurement station), focus/leveling does not necessarily have to be performed. That is, acquisition of mark position and height difference information (focus mapping data) should be performed, and by subtracting the Z tilt at the time of acquisition (at the time of measurement) of the wafer stage height difference information from the height difference information, height difference information of a reference surface of the wafer stage, such as for example, the upper surface serving as a reference, should be obtained. Then, at the time of exposure, focus/leveling becomes possible, based on this height difference information and positional information in directions of three degrees of freedom (Z, θx, θy) (of the reference surface) of the wafer stage. 
     Furthermore, main controller  20  uses three heads of heads  60   1  to  60   4  facing scale plates  21 ,  22  while switching to three heads with at least one different, according to the position of wafer stages WST 1 , WST 2 . Here, on switching the encoder heads, the joint processing as is previously described is performed to secure continuity of the measurement values of the position of wafer stage WST 1  (or WST 2 ). 
     Next, correction (calibration) of grid variation quantity of the coordinate system at the time of exposure in exposure apparatus  100  related to the present embodiment is described, which is performed while the series of sequences described above is performed. Here, the case will be described when wafer stage WST 1  moves in the movement area at the time of exposure. 
     This correction (calibration) of the grid variation quantity is performed by main controller  20 , concurrently with the position control of wafer table WTB 1  in directions of six degrees of freedom which is performed based on the measurement values of the three encoders chosen from encoders  70   1  to  70   4  as is previously described. 
     Main controller  20 , for example, takes in the measurement values in an a direction of heads  60   1 ,  60   1a , (encoders  70   1 ,  70   1a ) and sequentially integrates difference data which is expressed in formula (9) below, that is, deviation Δα/δx corresponding to the X position of the grid in the α direction (α grid), each time wafer stage WST 1  moves by Δx in the X-axis direction in a state, for example, where heads  60   1 ,  60   1a ,  60   1b  belonging to the first head group  61   1  are facing the corresponding section  21   1  of scale plate  21 , for example, during exposure and the like. This allows a discrete distribution of the α grid variation quantity in the X-axis direction to be obtained. 
       Δα/δ x=ζ   1  ( x−Δx, y )−ζ 1  ( x, y )   (9)
 
     Further, main controller  20 , for example, takes in the measurement values in the α direction of heads  60   1 ,  60   1b , (encoders  70   1 ,  70   1b ) and sequentially integrates difference data which is expressed in formula (10) below, that is, deviation Δα/δy corresponding to the Y position of the α grid, each time wafer table WTB 1  moves by Δy in the Y-axis direction in a state, for example, where heads  60   1 ,  60   1a ,  60   1b  belonging to the first head group  61   1  are facing the corresponding section  21   1  of scale plate  21 , for example, during exposure and the like. This allows a discrete distribution of the α grid variation quantity in the Y-axis direction to be obtained. 
       Δα/δ y=ζ   1  ( x, y−Δy )−ζ 1  ( x, y )   (10)
 
     Main controller  20  can obtain an a correction map for correcting a drift (α grid variation) generated in the first quadrant section (the first section  21   1  of scale plate  21 ) of two-dimensional grating RG expressed as a function ζ 1  (x, y), from the discrete distribution of the a grid variation quantity in the X-axis direction described above and the discrete distribution of the a grid variation quantity in the Y-axis direction. 
     Main controller  20  takes in measurement values in the a direction, for example, of heads  60   3 ,  60   3a  (encoders  70   3 ,  70   3a ) and sequentially integrates difference data similar to formula (9), each time wafer stage WST 1  moves by Δx in the X-axis direction in a state, for example, where heads  60   3 ,  60   3a ,  60   3b  belonging to the third head group  61   3  face the corresponding section  21   3  of scale plate  21 , for example, during exposure and the like, along with taking in measurement values in the α direction, for example, of heads  60   3 ,  60   3b  ( 70   3 ,  70   3b ) and sequentially integrates difference data similar to formula (10), each time wafer stage WST 1  moves by Δy in the Y-axis direction. Then, main controller  20  obtains the α correction map for correcting the drift (α grid variation) generated in the third quadrant section (the third section  21   3  of scale plate  21 ) of two-dimensional grating RG expressed as a function (ζ 3  (x, y)), from the discrete distribution of the α grid variation quantity in the X-axis direction and the discrete distribution of the α grid variation quantity in the Y-axis direction that are obtained from the integration of the difference data described above. 
     Main controller  20  takes in measurement values in a β direction, for example, of heads  60   2 ,  60   2   a  (encoders  70   2 ,  70   2a ) and sequentially integrates difference data expressed in formula (11) below, each time wafer stage WST 1  moves by Δx in the X-axis direction in a state, for example, where heads  60   2 ,  60   2a ,  60   2b  belonging to the second head group  61   2  are facing the corresponding section  21   2  of scale plate  21 , for example, during exposure and the like, along with taking in measurement values in the β direction, for example, of heads  60   2 ,  60   2b  ( 70   2 ,  70   2b ) and sequentially integrates difference data expressed in formula (12) below, each time wafer stage WST 1  moves by Δy in the Y-axis direction. Then, main controller  20  obtains a β correction map for correcting a drift (β grid variation) generated in the second quadrant section (the second section  21   2  of scale plate  21 ) of two-dimensional grating RG expressed as a function ζ 2  (x, y), from the discrete distribution of the βgrid variation quantity in the X-axis direction and the discrete distribution of the β grid variation quantity in the Y-axis direction that are obtained from the integration of the difference data described above. 
       Δβ/δ x=ζ   2  ( x−Δx, y )−ζ 2  ( x, y )   (11)
 
       Δβ/δ y=ζ   2  ( x, y−Δy )−ζ 2  ( x, y )   (12)
 
     Main controller  20  takes in measurement values in the β direction, for example, of heads  60   4 ,  60   4a  (encoders  70   4 ,  70   4a ) and sequentially integrates difference data similar to formula (11), each time wafer stage WST 1  moves by Δx in the X-axis direction in a state, for example, where heads  60   4 ,  60   4a ,  60   4b  belonging to the fourth head group  61   4  are facing the corresponding section  21   4  of scale plate  21 , for example, during exposure and the like, along with taking in measurement values in the β direction, for example, of heads  60   4 ,  60   4b  ( 70   4 ,  70   4b ) and sequentially integrates difference data similar to formula (12), each time wafer stage WST 1  moves by Δy in the Y-axis direction. Then, main controller  20  obtains the β correction map for correcting a drift (β grid variation) generated in the fourth quadrant section (the fourth section  21   4  of scale plate  21 ) of two-dimensional grating RG expressed as a function (ζ 4  (x, y)), from the discrete distribution of the βgrid variation quantity in the X-axis direction and the discrete distribution of the β grid variation quantity in the Y-axis direction that are obtained from the integration of the difference data described above. 
     Main controller  20  also makes a Z correction map in a similar manner as the a correction map and the  0  correction map described above, for example, during exposure and the like. 
     That is, main controller  20 , takes in measurement values in the Z-axis direction, for example, of heads  60   1 ,  60   1a  (encoders  70   1 ,  70   1a ) and sequentially integrates the difference data expressed in formula (13) below, each time wafer stage WST 1  moves by Δx in the X-axis direction in a state, for example, where heads  60   1 ,  60   1a ,  60   1b  belonging to the first head group  61   1  are facing the corresponding section  21   1 , for example, during exposure and the like, along with taking in measurement values in the Z-axis direction, for example, of heads  60   1 ,  60   1b  ( 70   1 ,  70   1b ) and sequentially integrates difference data expressed in formula (14) below, each time wafer stage WST 1  moves by Δy in the Y-axis direction. Then, main controller  20  obtains the Z correction map for correcting a Z grid variation (drift) generated in the first quadrant section (the first section  21   1  of scale plate  21 ) of two-dimensional grating RG expressed as a function η 1  (x, y), from the discrete distribution of the Z grid variation quantity in the X-axis direction and the discrete distribution of the Z grid variation quantity in the Y-axis direction that are obtained from the integration of the difference data described above. 
       Δ Z/δx=η   1  ( x−Δx, y )−η 1  ( x, y )   (13)
 
       Δ Z/δy=η   1  ( x, y−Δy )−η 1  ( x, y )   (14)
 
     Main controller  20  takes in and performs integration of the difference data in a manner similar to the description above, each time wafer stage WST 1  moves by Δx and moves by Δy, also for heads belonging to the second head group, the third head group and the fourth head group in a state facing scale plate  21 , for example, during exposure and the like, and obtains the Z correction map, along with reproducing functions (η 2  (x, y), η 3  (x, y), η 4  (x, y) , respectively) that express the drift shape (Z grid variation). 
     Main controller  20  repeatedly performs the difference measurement described above, concurrently with the position measurement in directions of six degrees of freedom of wafer table WTB described above, and performs an update of grid errors of the coordinate system of encoder system  70 . Hereinafter, this update of grid errors will also be called refreshing of the coordinate system of encoder system  70 . 
     As is previously described, main controller  20  uses three of heads  60   1  to  60   4  facing scale plate  21 , switching to three with at least one different according to the position of wafer stage WST 1 , for example, at the time of exposure. That is, main controller  20  drives wafer table WTB 1  based on the positional information of wafer table WTB 1  obtained by three of heads  60   1  to  60   4  (three of encoders  70   1  to  70   4 ), along with switching at least one of the three heads used for calculating the positional information of wafer table WTB 1  according to the position of wafer table WTB 1  to a head belonging to another head group which is not used for calculating the positional information of wafer table WTB 1 . Main controller  20 , with this switching, switches the head group subject to taking in the difference data for calibration of the grid errors of two-dimensional grating RG of scale plate  21  described above to another head group. In the present embodiment, simultaneously with the switching of the head used for position measurement of wafer table WTB, switching of redundant heads used for difference measurement previously described is performed. 
     Main controller  20  performs refreshing of the coordinate system of encoder system  71  similarly to the description above in the case wafer stage WST 2  is moving in the movement area at the time of exposure, including the time of exposure. 
     Now, during exposure, because movement of wafer stages WST 1 , WST 2  is performed according to a shot map and wafer stages WST 1 , WST 2  pass only limited areas, difference data that can be acquired is little. Therefore, main controller  20 , on the refreshing of the coordinate systems of encoder systems  70 ,  71  described above, corrects in a real-time manner a primary component which is a low-order component of grid variation from the integration of difference data, such as, for example, only scaling (α, β grids), and bending (Z grid), as is previously described. 
     Then, the integration value of the difference data is monitored, and in the case variation quantity (correction quantity) of the low-order component (primary component) becomes larger than a first quantity defined beforehand, a more elaborate correction is to be performed. Here, a more elaborate correction means, for example, to perform acquisition of difference data previously described by moving wafer stages WST 1 , WST 2  in almost the entire area of their effective strokes to acquire more difference data, and based on the more acquired difference data, to perform grid variation quantity (error) correction similar to the one previously described, or variation quantity correction of α, β and Z grids subject to at least a secondary component which is performed in a method similar to the following description in a wider range of two-dimensional grating RG of scale plate  21 . 
     In the case variation quantity (correction quantity) of the low-order component (primary component) is larger than a second amount defined beforehand which is larger than the first quantity, main controller  20  gives notice to an operator, for example, by display and the like, informing that correction subject to components of a higher order is required. In response to this notice, in the case correction of a higher order is instructed from the operator, main controller  20  loads a reference wafer (a resist is coated on its surface) having a plurality of reference marks arranged as is designed formed on wafer table WTB 1  or WTB 2 , and loads a measurement reticle having a plurality of marks placed in a predetermined positional relation on reticle stage RST. Then, exposure is performed, for example, in a step-and-repeat method (or a step-and-scan method). After the exposure has been completed, main controller  20  carries the reference wafer after exposure, for example, to a coater/developer which is in-line connected to exposure apparatus  100 , and with this gives instructions for development. Then, when receiving notice that development of the reference wafer has been completed from the coater/developer, main controller  20  loads the reference wafer after development again on wafer table WTB 1  or WTB 2 , and sequentially detects a position with respect to the reference mark corresponding to the mark consisting of the resist image formed on the reference wafer, for example, with alignment system ALG. Then, based on the detection results, correction of the variation quantity of α, β and Z grids subject to higher-order components is performed. 
     Other than this, a setting can be employed, in which, for example, the operator sets a threshold value (for example, a third amount defined beforehand which is larger than the second quantity) for the variation quantity (correction quantity) of the low-order component (primary component) previously described, and main controller  20  monitors whether the variation quantity of the low-order component exceeds the threshold value or not, and in the case the variation quantity (correction quantity) exceeds the threshold value, a notice is to be sent to the operator informing that grid maintenance is required. That is, the variation quantity (correction quantity) of the low-order component previously described can be used as a monitor index to judge whether or not grid maintenance is required. 
     As is described in detail so far, according to exposure apparatus  100  related to the present embodiment, for example, when wafer stage WST 1  (or WST 2 ) is located in the movement area at the time of exposure such as during exposure, wafer table WTB 1  or WTB 2  (wafer stage 
     WST 1  or WST 2 ) is driven by main controller  20 , based on positional information in directions of six degrees of freedom obtained using three of encoders  70   1  to  70   4  of encoder system  70  (or three of encoders  71   1  to  71   4  of encoder system  71 ). And concurrently with this driving of wafer table WTB 1  or WTB 2  (wafer stage WST 1  or WST 2 ), by main controller  20 , in the head group facing scale plate  21  of the first head group  61   1 , the second head group  61   2 , the third head group  61   3  and the fourth head group  61   4 , difference data of measurement values in measurement directions (α direction and Z direction, or β direction and Z direction) of one head  60   i  serving as a reference belonging to each group and two heads  60   ia  and  60   ib  (i=1 to 4), respectively, are taken in, and based on the difference data which are taken in, it becomes possible to monitor variation quantity of grids in measurement directions (α direction and Z direction, or β direction and Z direction) for sections corresponding to four sections  21   1  to  21   4  of two-dimensional grating RG formed on the lower surface of scale plate  21 , respectively. Further, by main controller  20 , calibration (correction) of the grid errors (especially the low-order component), that is, refreshing of the coordinate systems of encoder systems  70 ,  71  is performed in a real-time manner. Therefore, according to exposure apparatus  100 , by encoder system  70  or  71  with two-dimensional grating RG formed on the lower surface of scale plate  21  serving as a measurement surface, position in direction of six degrees of freedom within the movement area at the time of exposure of wafer stage WST 1  or WST 2  can be measured and controlled with good accuracy for over a long period, which in turn allows the pattern of reticle R to be transferred with good accuracy on the plurality of shot areas on wafer W. 
     In exposure apparatus  100  related to the present embodiment, by main controller  20 , refreshing of the coordinate systems of encoder systems  70 ,  71  is executed in a real-time manner on exposure and the like. Therefore, even if in the case scale plate  21  (two-dimensional grating RG) increases furthermore in size along with wafer stages WST 1 , WST 2  to cope with 450 mm wafers, it becomes possible to measure the position in directions of six degrees of freedom within the movement area at the time of exposure of wafer table WTB 1  or WTB 2  with good accuracy for over a long period. 
     Further, in exposure apparatus  100 , main controller  20  can execute refreshing of the coordinate systems of encoder systems  70 ,  71  in a real-time manner, while wafer stage WST 1  or WST 2  is moving within the movement area at the time of measurement such as during wafer alignment measurement (mark detection by alignment system ALG) and at the time of detection of positional information in the Z-axis direction (height difference information/unevenness information) of the wafer W surface which is performed concurrently with the wafer alignment. In such a case, accuracy of wafer alignment such as EGA and focus/leveling control accuracy of wafer W during scanning exposure can be maintained at high precision for over a long period. 
     Furthermore, in exposure apparatus  100  related to the present embodiment, placement spacing A, B of heads  60   1  to  60   4  are each decided to be larger than the sum of widths ai, bi of the openings of scale plates  21 ,  22  and sizes W, L of the shot areas, respectively. This allows positional information of wafer stages WST 1 , WST 2  to be measured without switching heads  60   1  to  60   4  during scanning (constant velocity) drive of wafer stages WST 1 , WST 2  that hold the wafer for exposure of the wafer. Accordingly, the pattern can be formed on the wafer with good precision, and overlay accuracy can be maintained with high precision, especially from exposure of a second layer (second layer) onward. 
     Incidentally, in the description of the embodiment above, while the four heads  60   1 ,  60   2 ,  60   3 ,  60   4  for position measurement of the wafer table provided at the four corners of the wafer table upper surface satisfy the placement conditions described above, as is obvious from the placement of heads  60   ia ,  60   ib  and  60   i  belonging to each head group  61   i  (i=1 to 4) (refer to  FIG.  2   ), heads  60   1a ,  60   2a ,  60   3a ,  60   4a , and heads  60   1b ,  60   2b ,  60   3b ,  60   4b  also satisfy placement conditions similar to the conditions described above. 
     Incidentally, in the embodiment above, while the case has been described where scale plates  21 ,  22  are each structured from four sections and four head groups  61   1  to  61   4  corresponding to this are provided on wafer stages WST 1 , WST 2 , the present invention is not limited to this, and scale plates  21 ,  22  can be structured from a single member. In this case, two-dimensional grating RG can be a single two-dimensional grating having a large area. In such a case, when movement strokes of wafer stages WST 1 , WST 2  are sufficiently large, in addition to two heads (for example, two of heads  60   2  to  60   4 ) that allow position measurement of wafer stages WST 1 , WST 2  in a predetermined degrees of freedom, such as for example, six degrees of freedom along with head  60   1 , only two redundant heads  60   1a ,  60   1b  belonging to the first head group  61   1  can be provided. 
     Further, the placement of the heads belonging to each head group in the description of the embodiment above is a mere example. For example, as a placement of the heads belonging to the first head group, a placement can be employed as is shown in, for example,  FIG.  11 A . In this case, on refreshing of the coordinate system, refreshing of the coordinate system can be performed in a similar manner as in the embodiment above by supposing that the function expressing the drift shape (variation of α, β, Z grids) previously described is not function x, y, but is function α, β. 
     Or, as a placement of the heads belonging to the first head group  61   1 , for example, a placement can be employed as is shown in  FIG.  11 B . In this case, measurement directions of heads  60   1 ,  60   1a ,  60   1b , are in 2 directions, which are the X-axis direction and the Z-axis direction. Accordingly, in the first section of scale plate  21  where the heads belonging to the first head group  61   1  faces, a one-dimensional or a two-dimensional grating is provided having a periodic direction at least in the X-axis direction. In the case the placement shown in  FIG.  11 A  or  FIG.  11 B  is employed as the placement of the heads belonging to the first head group, as the placement for the heads belonging to each of the second head group, the third head group, and the fourth head group, a placement point-symmetric to the placement shown in  FIG.  11 A  or  FIG.  11 B  with respect to the wafer table center or line-symmetric to the placement shown in  FIG.  11 A  or  FIG.  11 B  with respect to a straight line parallel to the X-axis or the Y-axis that passes through the center (however, the measurement direction within the XY plane of a head belonging to at least one group of the second head group, the third head group, and the fourth head group is orthogonal to the measurement direction of the head belonging to the first head group) is employed. 
     Incidentally, while the case has been described where three heads belong to each head group, the embodiment is not limited to this, and each head group may have two heads. For example, in the case at least one direction of the X-axis direction and the Y-axis direction is included in the measurement direction of the two heads, these two heads are preferably placed apart in directions intersecting the X-axis and the Y-axis, for example, similarly to heads  60   1a ,  60   1b  in  FIG.  11 B . Further, in the case at least one direction of the a direction and the β direction previously described is included in the measurement direction of the two heads, these two heads are preferably placed apart in the X-axis direction (or the Y-axis direction) intersecting the a direction and the β direction, for example, similarly to heads  60   1a ,  60   1b  in  FIG.  11 A . 
     Or, four or more heads may belong to each head group. Also in this case, a placement of the heads is employed in which all heads belonging to each head group are not positioned on the same straight line. In this case, on the refreshing of the coordinate system at the time of exposure previously described, difference data of measurement values in a common measurement direction of the head serving as a reference and of each of the remaining heads belonging to each head group that are used for measurement of positional information in directions of six degrees of freedom of the wafer table can be acquired, or difference data in the common measurement direction of all heads different from one another belonging to each head group can be acquired. The point is, any placement can be employed as long as difference data in the common measurement direction of the heads different from one another belonging to each head group, including the difference data of the measurement values in the common measurement direction of the head serving as a reference and of at least two heads of each of the remaining heads belonging to each head group that are used for the measurement of positional information belonging to each head group are acquired, and based on the difference data which has been acquired, variation quantity of the grid in the common measurement direction of scale plate  21  (two-dimensional grating RG) is monitored and the grid error can be calibrated. 
     In the case four or more heads belong to each head group, at the time of refreshing of the coordinate system, because more difference data can be acquired in the X-axis or the Y-axis direction at one time, calibration of grid variation quantity (grid error) targeting on components of a predetermined order which is a second order or more can be performed real time. And, in this case, as a mode of “a more detailed grid error correction” previously described, calibration of grid variation quantity (grid error) targeting on components of a higher order than the predetermined order can be performed similarly to the previous description, for example, using a reference wafer and the like. 
     Incidentally, in the embodiment described above, while the case has been described where only the measurement value of one head serving as a reference of the three heads belonging to each head group is used for calculating the position of wafer tables WTB 1 , WTB 2  in directions of six degrees of freedom, the embodiment is not limited to this, and measurement values of at least two of the three heads belonging to each head group can be used to calculate the position of wafer tables WTB 1 , WTB 2  in directions of six degrees of freedom. For example, an average of the measurement values of the three heads belonging to each head group can be used to calculate the position of wafer tables WTB 1 , WTB 2  in directions of six degrees of freedom. In such a case, by an averaging effect, position measurement of a higher accuracy becomes possible. 
     Further, in the embodiment above, while the case has been described where a two-dimensional head having a measurement direction in two directions which are one direction within the XY plane and the Z-axis direction was used as each head, and grid error calibration (correction) of the coordinate system at the time of exposure is performed in the two directions, the embodiment is not limited to this. For example, the grid error calibration (correction) of the coordinate system at the time of exposure can be performed in one direction of the one direction within the XY plane and the Z-axis direction. Further, as each head, a three-dimensional head having measurement directions in two directions orthogonal within the XY plane and the Z-axis direction can be used. Or, for example, as heads  60   ia , 60   ib , a one-dimensional head or a two-dimensional head  60   i  head  60   i  consisting of a two-dimensional head or a three-dimensional head that has at least one common measurement direction of the two or three measurement directions of head  60   i , can be used. In the case of using a one-dimensional head whose measurement direction is in an uniaxial direction within the XY plane, combined with this, a surface position sensor of a non-encoder method whose measurement direction is in the Z-axis direction can be employed. 
     Incidentally, in the embodiment described above, while the example was described where two-dimensional gratings RG were formed scale on the lower surfaces of sections  21   1  to  21   4 ,  22   1  to  22   4  of plates  21 ,  22 , respectively, the embodiment is not limited to this, and the embodiment described above can also be applied in the case a one-dimensional grating is formed whose periodic direction is only the measurement direction of the corresponding encoder heads  60   1  to  60   4  (the uniaxial direction within the XY plane). 
     Further, in the embodiment described above, while the exposure apparatus was described equipped with an encoder system having heads mounted on wafer stages WST 1 , WST 2  and scale plates  21 ,  22  (two-dimensional gratings RG) placed external to wafer stages WST 1 , WST 2 , the embodiment is not limited to this, and can also be applied to an exposure apparatus equipped with a type of encoder system that has a plurality of heads provided external to the wafer stage, such as for example, above (or below), and a measurement surface such as a grating and the like provided facing this, on the upper surface (or lower surface) of the wafer stage, such as the exposure apparatus and the like disclosed in, for example, U.S. Patent Application Publication No. 2008/0088843 and the like. 
     Incidentally, in the embodiment described above, while the case has been described where the exposure apparatus was a scanning stepper, the embodiment is not limited to this, and the embodiment described above can also be applied to a stationary type exposure apparatus such as a stepper. In the case of a stepper and the like, by measuring the position of the stage on which an object subject to exposure is mounted with the encoder, generation of position measurement error caused by air fluctuation can be suppressed to almost zero different from the case when measuring the position of the stage by an interferometer, and based on measurement values of the encoder, positioning of the stage can be performed with high precision, and transfer of the reticle pattern onto the wafer can consequently be performed with high precision. Further, the embodiment above can also be applied to a projection exposure apparatus by a step-and-stitch method in which a shot area and a shot area are synthesized. 
     Further, in the embodiment described above, while the example was given in the case exposure apparatus  100  is a twin stage type exposure apparatus equipped with two wafer stages, the embodiment is not limited to this, and as is disclosed in, for example, U.S. Patent Application Publication No. 2007/0211235 and U.S. Patent Application Publication No. 2007/0127006 and the like, the embodiment described above can also be applied to an exposure apparatus which is equipped with a measurement stage including a measurement member (such as, for example, a reference mark, and/or a sensor) separate from the wafer stage, or a single stage exposure apparatus which is equipped with one wafer stage. 
     Further, the exposure apparatus in the embodiment described above can be a liquid immersion type as is disclosed in, for example, PCT International Publication No. 99/49504, U.S. Patent Application Publication No. 2005/0259234 and the like. 
     Further, the projection optical system in the exposure apparatus in the embodiment described above is not limited only to a reduction system, and can either be an equal magnifying or a magnifying system, and projection optical system PL is not limited only to a refractive system, and can either be a reflection system or a catadioptric system, and the projection image can either be an inverted image or an erected image. 
     Further, illumination light IL is not limited to an ArF excimer laser beam (wavelength  193 nm), and can be ultraviolet light such as a KrF excimer laser beam (wavelength 248 nm), or a vacuum-ultraviolet light such as an F2 excimer laser beam (wavelength 157 nm). As is disclosed in, for example, U.S. Pat. No. 7,023,610, as the vacuum-ultraviolet light, a harmonic wave can be used which is obtained by amplifying a single wavelength laser beam in an infrared or visible range oscillated from a DFB semiconductor laser or a fiber laser, with a fiber amplifier doped with, for example, erbium (or both erbium and ytterbium) and performing wavelength conversion into an ultraviolet light using a non-linear optical crystal. 
     Further, in the embodiment described above, while a transmissive mask (reticle) on which a predetermined light shielding pattern (or a phase pattern or a light attenuation pattern) was formed on the transmissive substrate, was used, instead of this reticle, as is disclosed in, for example, U.S. Pat. No. 6,778,257, an electron mask (also called a variable shaped mask, an active mask, or an image generator, and includes, for example, a DMD (Digital Micro-mirror Device) which is a kind of a non-emission type image display device (spatial light modulator) and the like) which forms a light-transmitting pattern, a reflection pattern, or an emission pattern, based on electronic data of the pattern to be exposed can also be used. In the case of using such a variable shaped mask, because the stage on which the wafer or the glass plate and the like is mounted is scanned with respect to the variable shaped mask, by measuring the position of the stage by using the encoder, an effect equivalent to the embodiment described above can be obtained. 
     Further, the embodiment described above can also be applied to an exposure apparatus (lithography system) that forms a line-and-space pattern on wafer W by forming an interference fringe on wafer W, as is disclosed in PCT International Publication No. 2001/035168. 
     Furthermore, the embodiment described above can also be applied to an exposure apparatus which synthesizes two reticle patterns on a wafer via the projection optical system and performs double exposure almost simultaneously on a shot area on the wafer by performing scanning exposure once, as is disclosed in, for example, U.S. Pat. No. 6,611,316. 
     Further, the object on which the pattern should be formed (the object subject to exposure on which the energy beam is irradiated) in the embodiment above is not limited to the wafer, and may be other objects such as a glass plate, a ceramic substrate, a film member, or a mask blank. 
     Furthermore, the usage of the exposure apparatus is not limited to the exposure apparatus for manufacturing semiconductors and can be widely applied, for example, to an exposure apparatus for liquid crystals that transfers a liquid crystal display devices pattern onto a square-shaped glass plate, an exposure apparatus for manufacturing organic ELs, thin film magnetic head, an imaging element (such as a CCD), a micromachine and a DNA chip and the like. Further, the embodiment described above can also be applied to an exposure apparatus that transfers a circuit pattern onto a glass substrate or a silicon wafer for manufacturing not only microdevices such as semiconductor devices but also a reticle or a mask that is used in an optical exposure apparatus, and EUV exposure apparatus, an X-ray exposure apparatus, an electron beam exposure apparatus and the like. 
     Electronic devices such as semiconductor devices are manufactured through steps such as; a step for performing function/performance design of a device, a step for making a reticle based on the design step, a step for making a wafer from a silicon material, a lithography step for transferring a pattern of a mask (reticle) onto the wafer by the exposure apparatus (pattern formation apparatus) and the exposure method related to the embodiment previously described, a development step for developing the wafer which has been exposed, an etching step for removing by the etching an exposed member of an area other than the area where the resist remains, a resist removing step for removing the resist that is no longer necessary since etching has been completed, a device assembly step (including a dicing process, a bonding process, and a package process), and an inspection step. In this case, in the lithography step, because the device pattern is formed on the wafer using the exposure apparatus of the embodiment described above in the exposure method previously described, a highly integrated device can be manufactured with good productivity. 
     Further, the exposure apparatus (pattern formation apparatus) of the embodiment described above is manufactured by assembling various kinds of subsystems that include the respective constituents which are recited in the claims of the present application so as to keep predetermined mechanical accuracy, electrical accuracy and optical accuracy. In order to secure these various kinds of accuracy, before and after the assembly, adjustment to achieve the optical accuracy for various optical systems, adjustment to achieve the mechanical accuracy for various mechanical systems, and adjustment to achieve the electrical accuracy for various electric systems are performed. A process of assembling various subsystems into the exposure apparatus includes mechanical connection, wiring connection of electric circuits, piping connection of pressure circuits, and the like among various types of subsystems. Needless to say, an assembly process of individual subsystem is performed before the process of assembling the various subsystems into the exposure apparatus. When the process of assembling the various subsystems into the exposure apparatus is completed, a total adjustment is performed and various kinds of accuracy as the entire exposure apparatus are secured. Incidentally, the making of the exposure apparatus is preferably performed in a clean room where the temperature, the degree of cleanliness and the like are controlled. 
     Incidentally, the disclosures of the PCT International Publications, the U.S. Patent Application Publications and the U.S. Patents that are cited in the description so far related to exposure apparatuses and the like are each incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     As described so far, the exposure apparatus and the exposure method of the present invention is suitable for exposing an object. Further, the device manufacturing method of the present invention is suitable for manufacturing electronic devices such as semiconductor devices or liquid crystal display devices.