Abstract:
A stage apparatus including a stage capable of moving an object, first and second measurement units adapted to measure a displacement of the stage in a predetermined direction based on a variation of an optical path length of measurement light, which are arranged so as to have an overlap area to simultaneously measure a stage position while the stage is being moved, a switching unit to switch measurement by the first measurement unit to measurement by the second measurement unit by delivering a measurement value from the first measurement unit to the second measurement unit in the overlap area, and a correction unit, in the stage position upon switching by the switching unit, to correct a wavelength variation of the measurement light based on the measurement value delivered from the first measurement unit to the second measurement unit.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a position measuring technique for a stage apparatus for holding an object by moving the object, and, more particularly, to a position measuring technique for a stage apparatus placed in a semiconductor exposure apparatus. 
   BACKGROUND OF THE INVENTION 
   An exposure apparatus, utilized in the fabrication of semiconductor devices, liquid crystal display devices, and the like, has a stage apparatus to move an original plate, such as a mask or a reticle, or an exposed substrate, such as a semiconductor wafer or a glass substrate. 
   In this stage apparatus, generally, a laser interferometer and a reflecting mirror are used for stage position measurement. 
   Japanese Patent Application Laid-Open No. 2002-319541 discloses an apparatus having plural laser interferometers for stage position measurement. The position measurement is performed by selectively using the plural laser interferometers. 
   On the other hand, in the laser interferometer, a laser wavelength changes due to slight variations of air pressure, temperature and humidity, and causes a measurement error. Accordingly, the exposure apparatus is placed in a chamber for environmental control. At the same time, regarding the remaining variations, the wavelength is corrected with measurement values from an air pressure gauge, a temperature gauge, a humidity indicator, and the like. 
   When the position measurement is performed by selectively using plural laser interferometers, it is necessary to prevent occurrence of displacement before and after changing of an interferometer. For this purpose, it is necessary to provide overlapping sections where plural laser interferometers are able to measure and to inherit a prior measurement value before changing of the interferometer. 
   The wavelength correction described above is performed with an optical path length of a laser interferometer, the base point being determined upon being reset. In a stage apparatus in which the interferometer is not changed, wavelength correction is performed after the start of the apparatus, unless the apparatus is stopped due to an error, or the like. However, in a case wherein the interferometer is changed or interrupted, i.e., when a laser interferometer, which was light-shielded and disabled and has been brought online, the laser interferometer is reset each time. In the above case, as an XYZ position varies, the optical path length as a base point of the wavelength correction also varies. Without consideration of the amount of variation, a measurement error causes a displacement. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in consideration of the above problems, and has as its object to reduce a stage displacement caused upon changing of a measuring unit, such as a laser interferometer. 
   According to one aspect of the present invention, there is provided a stage apparatus comprising a stage capable of moving an object, first and second measurement units adapted to measure a displacement of the stage in a predetermined direction based on a variation of an optical path length of measurement light, being arranged so as to have an overlap area to simultaneously measure a stage position while the stage is being moved, a switching unit adapted to switch measurement by the first measurement unit to measurement by the second measurement unit by delivering a measurement value from the first measurement unit to the second measurement unit in the overlap area, and a correction unit adapted, in the stage position upon switching by the switching unit, to correct a wavelength variation of the measurement light based on the measurement value delivered from the first measurement unit to the second measurement unit. 
   According to another aspect of the present invention, there is provided a position measurement method for a stage apparatus having a stage capable of moving an object, and first and second measurement units adapted to measure a displacement of the stage in a predetermined direction based on a variation of an optical path length of measurement light, being arranged so as to have an overlap area to simultaneously measure a stage position while the stage is being moved, the method comprising a switching step of switching measurement by the first measurement unit to measurement by the second measurement unit by delivering a measurement value from the first measurement unit to the second measurement unit in the overlap area, and a correction step of, in the stage position upon switching at the switching step, correcting a wavelength variation of the measurement light based on the measurement value delivered from the first measurement unit to the second measurement unit. 
   According to the present invention, the wavelength correction of the laser interferometer before measurement unit changing can be inherited, and a displacement due to the changing of interferometer can be reduced. 
   Other features and advantages of the present invention will be apparent from the following descriptions taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a schematic front view of an exposure apparatus according to an embodiment of the present invention; 
       FIG. 2  is a schematic perspective view showing a case wherein measurement by selective use of interferometers according to the embodiment is applied to a Z-direction; 
       FIGS. 3A to 3D  are explanatory views showing a wavelength correction method according to the embodiment; 
       FIG. 4  is a block diagram showing a wavelength correction controller according to the embodiment; 
       FIG. 5  is a flowchart showing a device fabrication method; and 
       FIG. 6  is a flowchart showing a wafer process. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Hereinbelow, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
   Note that the following embodiment is an example of implementation of the present invention, and can be appropriately corrected or changed in accordance with the construction and various conditions of an apparatus to which the present invention is applied. 
   Further, the present invention is also achieved by supplying a storage medium (or recording medium) holding software program code to realize a stage position measurement method and a wavelength correction method, a device fabrication method, and the like, to be described later to a system or apparatus, then reading the program code from the storage medium by a computer (or CPU or MPU) of the system or apparatus, and executing the program. 
     FIG. 1  is a schematic front view of an exposure apparatus according to a first embodiment of the present invention. 
   In  FIG. 1 , reference numeral  32  denotes an illumination unit to illuminate a reticle as an original plate;  33 , a reticle having a pattern to be transferred;  34 , a projection lens (projection optical system) to project the pattern formed on the reticle  33  onto a wafer as a substrate;  35 , a lens barrel support member to support the lens  34 ;  36 , an active mount for a main body (lens barrel support member) to support the lens barrel support member  35  to suppress vibration and to insulate vibration from a floor; and  38 , an alignment base on which the main body active mount  36  and an active mount  37  for a stage are placed. 
   Numerals  21  and  22  denote fixed mirrors for Z-measurement fixed to the lens barrel support member  35 ; and  39 , a movable mirror for Z-measurement having two reflecting surfaces, integrated with an X mirror  29 , to be described later. 
   Numeral  31  denotes an X stage movable in an X-direction;  40 , a Y stage movable in a Y-direction with respect to the X stage  31 ; and  41 , a stage base to support the Y stage  40  and the X stage  31 . The stage active mount  37  suppresses vibration of the stage base  41  caused by movement of the stages and insulates vibration from the floor. Note that the X stage  31  and the Y stage  40  are supported with the stage base  41  in a non-contact state by hydrostatic bearings (not shown). 
   Numeral  42  denotes an X linear motor for driving to move the X stage  31  in the X-direction. In the X linear motor  42 , a movable member is provided on the X stage  31  and a fixed member is provided on the stage base  41 . Note that the fixed member of the X linear motor  42  may be supported on the stage base  41  in a non-contact state by a hydrostatic bearing (not shown), or may be fixed on the stage base  41 . Further, a Y linear motor (not shown) for driving moves the Y stage  40  in the Y-direction. The Y linear motor, having a movable member provided on the Y stage  40  and a fixed member provided on the X stage  31 , generates a driving force in the Y-direction between the X stage  31  and the Y stage  40 . 
   Numeral  23  denotes a laser interferometer to measure a relative position between the lens barrel support member  35  and a top stage  27  for a substrate in the X-direction and to measure the attitude of the top stage  27 . Further, a laser interferometer  24  (not shown) is used for measurement in the X-direction and measurement of the attitude of the top stage  27 . 
   Numeral  25  denotes a laser interferometer on the X stage  31  to measure a distance between the lens barrel support member  35  and the movable mirror  39  on the top stage  27  and to calculate the position of the top stage  27  in the Z-direction. 
   Note that the top stage  27  is mounted on the Y stage  40 , and is slightly moved by an actuator (not shown) with respect to the Y stage  40 . Further, the top stage  27  is measurable by Z displacement sensors  43  provided in the Y stage  40 . The Z displacement sensors  43  are sensors, such as linear encoders or electrostatic capacitance sensors, provided in addition to the Z-interferometer  25 . The Z displacement sensors  43  measure displacements of the top stage  27  with respect to the Y stage  40  in three positions (the third position is not shown), thereby measuring displacements of the top stage  27  in the Z-direction and an inclination (tilt) direction. 
   Numeral  26  denotes a wafer chuck (substrate holder) to hold a semiconductor substrate (wafer) (not shown) coated with a photosensitive material as a subject of pattern projection. The top stage  27  is a θZ-stage to align the wafer chuck  26  in the Z-, θ-. ωX- and ωY-directions. 
   Numeral  44  denotes an air pressure gauge to measure an air pressure in the apparatus;  45 , a wavelength correction unit to measure the amount of variations of wavelengths of laser interferometers. The wavelength correction unit  45  detects wavelength variations by continuously measuring the inside of the lens barrel having a fixed optical path length using a laser interferometer. These units are employed for wavelength correction of the laser interferometers in the apparatus. 
     FIG. 2  shows the arrangement of an interferometer system to measure the position or displacement of the top stage  27  by the laser interferometers  23  and  24  and a Z-measurement laser interferometer  25 . In  FIG. 2 , the wafer chuck  26  holds a wafer (not shown). The top stage  27 , which supports the wafer chuck  26 , is moved by a long stroke in the X- and Y-directions, and moved in the X-, ωX-, ωY- and θ-rotational directions by a short stroke, with a guide and an actuator (both not shown). 
   Numeral  28  denotes a Y mirror attached to the top stage  27 . The X mirror  29  is also attached to the top stage  27 . Numeral  30  denotes a Z mirror integrally provided with an upper surface of the X mirror  29 . The Y mirror  28  is arranged with its reflecting surface vertical to the Y-direction, the X mirror  29 , with its reflecting surface vertical to the X-direction; and Z mirror  30 , with its reflecting surface parallel to an XY-plane. 
   Numerals  23   a  to  23   c  denote Y interferometers for measurement in the Y-direction, which respectively input a laser beam parallel to the Y-direction in a predetermined position of the reflecting surface of the Y mirror  28 , and detect displacement information along a beam incidence direction (Y-direction) from reflected light. Numerals  24   a  and  24   b  denote X interferometers for measurement in the X-direction, which respectively input a laser beam parallel to the X-direction in a predetermined position of the reflecting surface of the X mirror  29 , and detect displacement information along a beam incidence direction (X-direction) from reflected light. 
   The interferometers  23  and  24  are respectively fixed with a support member (not shown) as a measurement reference. For example, the interferometers  23  and  24  are fixed with the lens barrel support member as a structure integral with the projection optical system  34 . 
   Numerals  25   a  and  25   b  denote Z interferometers for measurement in the Z-direction, mounted on the X stage  31 , respectively arranged to output a beam vertically to the XY-plane, otherwise, arranged such that the beam is refracted to be vertical to the XY-plane with optical devices, such as mirrors. The interferometers  25   a  and  25   b  respectively output the beam to the reflecting surface of the Z mirror  30  parallel to the ZY-plane attached to the top stage  27 , via the mirrors  21  and  22  attached to the lens barrel support member  35 , thereby to measure the position of the top stage  27  in the Z-direction. 
   The first mirror  21  and the second mirror  22  guide output light from the Z interferometers  25  to the Z mirror  30 . The mirrors  21  and  22  are fixed with their reflecting surfaces at an acute angle to the lens barrel  35  as a measurement reference or measurement light from the Z interferometer  25 . The first mirror  21  and the second mirror  22  are long mirrors along a stroke direction (X-direction) of the movable member (Z stage  31 ) where the Z interferometer  25  is arranged. 
   In the exposure apparatus as shown in  FIG. 1 , a lens barrel is at the center of the lens barrel support member  35 . Further, the wafer chuck  26  is mounted on the top stage  27 . In this structure, a layout is limited not to block a measuring optical path from the Z interferometer  25 . Further, in a case wherein the top stage  27  has a long-stroke movable range, to perform the Z-directional measurement in the entire stroke range, the mirrors  21 ,  22  and  30  are also moved by a long stroke. However, such elongated mirrors cause the following trouble. 
   (1) It is difficult to realize a high-accuracy flatness in terms of process and attachment. 
   (2) Even if the above flatness can be realized, the cost is high. 
   (3) The reduction of the characteristic value of the reflecting mirror degrades the control band. 
   Accordingly, in the present embodiment, the position of the top stage  27  can be measured while switching is made between two Z interferometers  25   a  and  25   b  in correspondence with an X-coordinate of the top stage  27 , thereby the measuring can be made by avoiding an obstacle to block measuring light as well as the lens barrel. The switching is made with delivery of a measurement value from a previously-used interferometer to the succeeding interferometer by a controller (not shown). Upon switching, the stage is positioned in a measurement area where measurement by the two interferometer systems overlap each other. The overlap measurement area is designed in consideration of a switching period such that switching can be performed even when the stage is being moved. 
   The Z-directional position of the top stage  27  is measured by integrating a displacement amount obtained by the laser interferometer  25   a  or  25   b  into an initial position of the top stage  27  stored in the above-described controller. In a laser interferometer, its laser wavelength changes due to slight variations of air pressure, temperature and humidity, and causes a measurement error. Accordingly, the wavelength is corrected by the controller using measurement values from an air pressure gauge, a temperature gauge, a humidity indicator, and the like. 
   Next, a wavelength correction method in a case wherein the interferometer switching is performed will be described with reference to  FIGS. 3A to 3D  to  FIG. 6 . In the present embodiment, the correction is made using an air pressure gauge. However, the method is also applicable to the cases of a temperature gauge, a humidity indicator, and the like. 
     FIGS. 3A to 3D  schematically show the Z interferometer system. Numerals  50   a  and  50   b  denote laser interferometers for measurement in the Z-direction, which measure the position of a Z mirror (not shown) attached on the top stage  27 . The laser interferometers  50   a  and  50   b  correspond to the interferometers  25   a  and  25   b  in  FIG. 2 . In this example, the mirrors  21   a  and  21   b ,  22   a  and  22   b  on the optical path are not shown for the sake of convenience. When the apparatus is started, the top stage  27  performs a mechanical butting against the Y stage  40  with the actuator provided between the top stage and the Y stage. Then, in a butting position (origin position in  FIGS. 3A to 3D ), the interferometer  50   b  is reset (the value is set to “0”), and, at the same time, the value of an air pressure gauge  44  is sampled, and the wavelength correction unit  45  is reset. The resetting is made so as to perform wavelength correction with the reset time of the interferometer  50   b  (time t=t 0 ) as a base point. In this example, the optical path length to the top stage  27  upon reset of the interferometer  50   b  (fixed value assured as a mechanical design value) is Zi(t 0 ), and a wavelength correction coefficient obtained from an air pressure value indicated by the air pressure gauge  44  is Wi(t 0 ). Further, a variation amount, of the wavelength correction coefficient obtained from an output from the wavelength correction unit  45  at time t, with the time t 0  as a base point, is W(t). In the present embodiment, the absolute value Wi(t 0 ) of the wavelength correction coefficient when t=t 0  holds is obtained from the air pressure value from the air pressure gauge  44 , and a relative value W(t) from the time point is obtained from the output from the wavelength correction unit  45 . The absolute value of the wavelength correction coefficient at time t can be calculated by adding W(t) to Wi(t 0 ). Wavelength correction coefficients at respective time points may be obtained only using the air pressure gauge without the wavelength correction unit. In such a case, the correction accuracy can be improved by using an air pressure gauge with a short sampling period for higher level realtime measurement. 
     FIG. 3B  schematically shows the top stage  27  at time (t=t 1 ) driven from the status in  FIG. 3A  in the Z-direction by P(t 1 ) in accordance with an output value from the interferometer  50   b . At this time, a position correction amount ΔP(t 1 ) is expressed as follows.
 Δ P ( t 1)= k 1* Zi ( t 0)+ k 2* P ( t 1) k 1= W ( t 1), k 2= Wi ( t 0)+ W ( t 1)  (1) 
The value k 1  is a relative value of the wavelength correction coefficient from time t 0  to t 1 , and the value k 2  is an absolute value of the wavelength correction coefficient at t 1 .
 
   Accordingly, the position P(t 1 )′ of the top stage  27  after the wavelength correction is expressed as follows.
 
 P ( t 1)′= P ( t 1)+Δ P ( t 1)  (2)
 
   These calculations are performed by a wavelength correction controller  1  as shown in  FIG. 4 . A stage driver  2  performs driving in accordance with a reference (target value). The position of the stage (current value) is measured by an interferometer  3 , and outputted to the wavelength correction controller  1 . The wavelength correction controller  1  performs a wavelength correction calculation based on the output from an air pressure gauge  4  and a wavelength correction unit  5 , and feeds back a corrected current value to the stage driver  2 . 
     FIG. 3C  schematically shows the top stage  27  at a time (t=t 2 ) driven from the status in  FIG. 3B  in the X-direction, and interferometer switching is performed. The Z position P(t 2 ) may be different from that shown in  FIG. 5 . At this time, as the interferometer  50   a  is enabled to perform measurement, as well as the interferometer  50   b , a value P(t 2 ) of the interferometer  50   b  is preset in the interferometer  50   a , and the Z-positional measurement and control using the interferometer  50   b  are switched to those using the interferometer  50   a . Further, the wavelength correction controller  1  holds wavelength correction coefficient variation amount W(t 2 ) from time t 0  to t 2 . 
     FIG. 3D  schematically shows the top stage  27  at time (t=t 3 ) driven to a position P(t 3 ) after the interferometer switching. At this time, a position correction amount ΔP(t 3 ) and a corrected position P(t 3 )′ are expressed as follows.
 Δ P ( t 3)= k 1* Zi ( t 2)+ k 2*( P ( t 3)− P ( t 2)) k 1= W ( t 3)− W ( t 2), k 2= Wi ( t 0)+ W ( t 3), Zi ( t 2)= Zi ( t 0)− P ( t 2), P ( t 3)′= P ( t 3)+Δ P ( t 3) 
The value k 1  is a relative value of the wavelength correction coefficient from time t 2  to time t 3 , i.e., the time upon switching to the interferometer  50  to time t 3 . As the switching to the interferometer  50   a  means the resetting of the interferometer  50   a , which has been previously disabled, the value is equivalent to that of the interferometer  50   b  at time t 0 . Accordingly, the relative value k 1  with time t 2  as a base point is necessary. Further, the value k 2  is an absolute value of the wavelength correction coefficient at time t 3 . The value Zi(t 2 ) is the optical path length at time t 2 , and the right side second term (P(t 3 )−P(t 2 )) is a driving amount from the time t 2 .
 
   After time t 3 , when switching from the interferometer  50   a  to the interferometer  50   b  occurs, the wavelength correction may be performed in accordance with the above method. Thus, the wavelength correction can be correctly inherited upon interferometer switching. 
   For example, when switching from the interferometer  50   a  to the interferometer  50   b  has occurred at time t=t 4 , then the correction expression is as follows.
 
Δ P ( t 5)= k 1* Zi ( t 4)+ k 2*( P ( t 5)− P ( t 4)) k 1= W ( t 5)− W ( t 4), k 2= Wi ( t 0)+ W ( t 5), Zi ( t 4)= Zi ( t 0)− P ( t 4), P ( t 5)′= P ( t 5)+Δ P ( t 5)
 
   Note that the mechanical butting, as shown in  FIG. 3A , is performed when the interferometer  50   b  is operative. However, the butting may be performed when the interferometer  50   a  is inoperative. 
   Further, in the present embodiment, the X and Y positions upon switching are not defined. However, the X and Y positions may be any positions as long as they are within the overlap measurable range for the interferometers  50   a  and  50   b . Further, the X and Y positions may be fixed positions. Note that in a case wherein switching is performed when the top stage  27  is driven at a high speed, it is preferable that the positions are set in consideration of switching time by resetting of the interferometer (or preset), calculation processing, and the like. 
   In the present embodiment, the interferometer for Z-directional measurement is mounted on the X stage. However, the interferometer may be mounted on the Y stage. Further, in the present embodiment, only one or two interferometer systems are provided on the top stage  27 . However, the present invention is not limited to these numbers of interferometer systems, but three or more interferometer systems may be provided. In a case wherein measurement is performed in three positions, rotational information of the top stage  27  in the tilt direction (ωX and ωY), as well as the Z-directional displacement with the lens barrel support member  35  as a reference, can be obtained. 
   In the present embodiment, the present invention is applied to a wafer stage. However, the invention is also applicable to a reticle stage. 
   Further, the present invention is applicable to any system to perform interferometer changing in any direction, as well as in the Z-directional measurement. 
   [Device Fabrication Method] 
   Next, a semiconductor device fabrication process utilizing the above exposure apparatus will be described.  FIG. 5  shows a general semiconductor device fabrication flow. At step S 1  (circuit designing), a circuit pattern of the semiconductor device is designed. At step S 2  (mask fabrication), a mask is fabricated based on the designed circuit pattern. 
   On the other hand, at step S 3  (wafer fabrication), a wafer is fabricated by using a material such as silicon. At step S 4  (wafer process), called a preprocess, an actual circuit is formed on the wafer by the above-described exposure apparatus by a lithography technique using the above mask and wafer. At the next step, step S 5  (fabrication), called a postprocess, a semiconductor chip is fabricated by using the wafer carrying the circuit formed at step S 4 . Step S 5  includes an assembly process (dicing and bonding), a packaging process (chip encapsulation), and the like. At step S 6  (inspection), inspections, such as an operation check, a durability test, and the like, are performed on the semiconductor device formed at step S 5 . The semiconductor device is completed through these processes, and is shipped at step S 7 . 
   The above wafer process at step S 4  includes the following steps ( FIG. 6 ), i.e., an oxidation step at which the surface of the wafer is oxidized, a CVD step at which an insulating film is formed on the surface of the wafer, an electrode formation step at which electrodes are formed by vapor deposition on the wafer, an ion implantation step at which ions are injected into the wafer, a resist processing step at which the wafer is coated with a photoresist, an exposure step at which the mask circuit pattern is exposure-printed on the wafer after the resist processing by the above-described exposure apparatus, a development step at which the wafer exposed at the exposure step is developed, an etching step at which portions other than the developed resist are removed, and a resist stripping step at which the resist, which is unnecessary after the completion of etching, is removed. These steps are repeated, to form multiple layers of circuit patterns on the wafer. 
   As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof, except as defined in the claims. 
   CLAIM OF PRIORITY 
   This application claims priority from Japanese Patent Application No. 2004-304343, filed on Oct. 19, 2004, which is hereby incorporated by reference herein.