Patent Publication Number: US-10761434-B2

Title: Substrate holding apparatus, exposure apparatus, and article manufacturing method

Description:
BACKGROUND 
     Field of the Disclosure 
     The present disclosure relates to a substrate holding apparatus, an exposure apparatus, and an article manufacturing method. 
     Description of the Related Art 
     An exposure apparatus is used when manufacturing devices such as semiconductor devices and liquid crystal display elements by using photolithographic technology. An exposure apparatus projects a mask pattern on a substrate via a projection optical system to transfer the pattern. 
     A device manufacturing process includes a process of applying resist to a substrate, a process of transferring a pattern onto the substrate by exposing the resist to light, and a process of developing the substrate with the pattern transferred thereon. Generally, the process of applying resist to a substrate and the process of developing the substrate with a pattern transferred thereon are performed by a coater developer, which is different from an exposure apparatus for transferring a pattern onto a substrate by exposing the resist to light. 
     In the device manufacturing process, a device is manufactured while delivering a substrate between different apparatuses in this way. Each apparatus is provided with a substrate holding mechanism for holding a substrate by absorption to deliver the substrate. It is common for the substrate holding mechanism to include a base for holding a substrate and a substrate lifting mechanism such as a lift pin for elevating and lowering the substrate. 
     A gap is produced between the base and the substrate lifting mechanism which are disposed below the substrate. Accordingly, on the substrate, there arise regions where the base and the substrate lifting mechanism are disposed and regions where none of them are disposed. 
     Nowadays, it is common to manufacture devices by using a transparent substrate through which exposure light transmits. When exposing a transparent substrate to light, the exposure light transmitting the substrate is reflected by a base or a substrate lifting mechanism, and the resist applied to the substrate is exposed to the reflected light. When the gap provided on the base causes differences in the light quantity radiated onto the resist, exposure non-uniformity arises on the substrate. 
     As a technique for reducing the above-described exposure non-uniformity, Chinese Patent Application Publication No. 105045048 discusses a configuration for limiting the reflection of exposure light by providing an antireflection member in the gap below the substrate. Chinese Patent Application Publication No. 105045048 discusses a problem that the resist on the substrate is exposed to the exposure light entering the gap below the substrate as reflected light, and the exposure amount becomes excessive in the region on the substrate positioned above the gap. To reduce the exposure amount in the region on the substrate positioned above the gap, a substrate holding apparatus discussed in Chinese Patent Application Publication No. 105045048 includes an antireflection member in the gap. 
     Meanwhile, the inventors of the present application found that the exposure light entering the gap below the substrate advances to the lower portion of the substrate holding apparatus and is attenuated and that most part of the exposure light does not reach the resist on the substrate. 
     SUMMARY 
     According to an aspect of the present invention, a substrate holding apparatus includes a base provided with a gap and configured to hold a substrate, and a reflection member disposed in the gap and configured to reflect light transmitting through the substrate towards the substrate, wherein a reflectance of the reflection member to the light that has transmitted the substrate is higher than a reflectance of the base to the light that has transmitted the substrate. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an exposure apparatus. 
         FIGS. 2A and 2B  are schematic diagrams illustrating a substrate holding mechanism. 
         FIG. 3  is a diagram illustrating a base included in a substrate holding mechanism according to a first exemplary embodiment. 
         FIG. 4  is a diagram illustrating a base configuring a substrate holding mechanism according to a second exemplary embodiment. 
         FIG. 5  is a diagram illustrating a base configuring a substrate holding mechanism according to a third exemplary embodiment. 
         FIGS. 6A, 6B, and 6C  are diagrams illustrating a base configuring a substrate holding mechanism according to a fourth exemplary embodiment. 
         FIG. 7  is a schematic diagram illustrating an occurrence mechanism of exposure non-uniformity. 
         FIG. 8  is a diagram illustrating a relation between a wavelength of light and a transmittance of a photosensitive material. 
         FIG. 9  is a diagram illustrating a configuration for reducing exposure non-uniformity. 
         FIG. 10  is a schematic diagram illustrating a mechanism for reducing exposure non-uniformity. 
         FIG. 11  is a diagram illustrating a relation between a position and a shape factor k of a reflection member. 
         FIGS. 12A and 12B  are diagrams illustrating a diffuse reflectance and a regular reflectance on the upper surface of the base, respectively. 
         FIGS. 13A and 13B  are diagrams illustrating a diffuse reflectance and a regular reflectance on the upper surface of a substrate lifting mechanism, respectively. 
         FIGS. 14A and 14B  are diagrams illustrating a diffuse reflectance and a regular reflectance on the upper surface of a reflection member, respectively. 
         FIG. 15  is a diagram illustrating details of a substrate holding mechanism. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below with reference to the accompanying drawings. A substrate holding apparatus according to the present invention is suitable to hold a transparent substrate such as a sapphire substrate and a glass substrate. The sapphire substrate is used as a substrate, for example, for Light Emitting Diode (LED) elements. The glass substrate is used as a substrate, for example, for a liquid crystal panel. 
       FIG. 1  is a schematic view illustrating a configuration of an exposure apparatus  1  according to an aspect of an exemplary embodiment. The exposure apparatus  1  is used to form a pattern on a substrate. The exposure apparatus  1  includes a light source apparatus  100  including a light source  110 , an illumination optical system  200  for illuminating an original plate  310 , and an original plate stage  300  for holding the original plate  310 . The exposure apparatus  1  further includes a projection optical system  400  for projecting an image of a pattern of the original plate  310  onto a substrate  510  held on a substrate holding mechanism  500  (substrate stage or substrate holding apparatus). 
     A high-pressure mercury lamp or excimer laser is used as the light source  110 . As general exposure light, a g-line (with a wavelength of about 436 nm) and near-ultraviolet light with a wavelength region of 100 to 400 nm are used. For example, the g-line and an i-line (with a wavelength of about 365 nm) of an ultra-high-pressure mercury lamp, KrF excimer laser light (with a wavelength of about 248 nm), ArF excimer laser light (with a wavelength of about 193 nm), and F2 laser light (with a wavelength of about 157 nm) are used. 
     Light emitted from the light source  110  is guided to the original plate  310  via an optical system  210  included in the illumination optical system  200 . The projection optical system  400  including an optical system  410  and an aperture diaphragm  420  projects the pattern of the original plate  310  onto the substrate  510  with a predetermined projection magnification. The substrate  510  is applied with a photosensitive material (resist) sensitive to light with a specific wavelength. When the image of the pattern of the original plate  310  is projected onto the resist, a latent image pattern is formed on the resist. 
     The substrate  510  is held by the substrate holding mechanism  500  as a substrate holding apparatus. The substrate holding mechanism  500  holds the substrate  510  through vacuum suction or electrostatic suction using a suction pad (not illustrated). A configuration of the substrate holding mechanism  500  will be described in detail below. 
     According to the present exemplary embodiment, the exposure apparatus  1  is a scanning exposure apparatus (scanner) for transferring the pattern of the original plate  310  onto the substrate  510  while synchronously scanning the original plate  310  and the substrate  510  in the scanning direction. In the following descriptions, the vertical direction is defined as the Z-axis direction, the scanning direction of the substrate  510  in a plane perpendicular to the Z-axis direction is defined as the Y-axis direction, and the non-scanning direction as the direction perpendicular to the Y-axis and the Z-axis directions is defined as the X-axis direction. 
     The light quantity (exposure amount) projected on the substrate  510  by the projection optical system  400  is an important factor for determining a line width of the pattern. Exposing the resist on the substrate  510  to light with a suitable exposure amount enables forming a high-accuracy pattern. 
     For example, when repetitively forming the same pattern in the pattern forming region on the substrate  510 , it is desirable to expose the resist to light so that exposure non-uniformity does not occur in the entire region of the pattern forming region. Non-uniformity of the exposure amount projected on the substrate  510  can be reduced by devising the configurations of the illumination optical system  200  and the projection optical system  400 . However, non-uniformity of the light quantity incident on the resist may occur if the resist on the substrate  510  is irradiated with what is called flare light. 
     Of the light transmitting the original plate  310 , light that transmits the optical system  410  and the opening of the aperture diaphragm  420  included in the projection optical system  400  radiated onto the resist on the substrate  510  is referred to as regular light, and light other than the regular light is referred to as flare light. 
     &lt;Occurrence of Exposure Non-Uniformity&gt; 
     Causes of the occurrence of exposure non-uniformity in the exposure apparatus will be described below with reference to  FIGS. 2A to 7 .  FIG. 2A  illustrates a state where the substrate  510  is held by the substrate holding mechanism  500 .  FIG. 2B  illustrates a state where the substrate  510  is elevated in the Z-axis direction by a substrate lifting mechanism  522 . The substrate  510  is conveyed by a substrate conveyance apparatus such as a conveyance robot (not illustrated) in a state of being separated from the substrate holding mechanism  500  by the substrate lifting mechanism  522 . 
     Referring to  FIG. 2A , the substrate holding mechanism  500  vacuum suctions the substrate  510  by using a suction pad (not illustrated) positioned between a base  521  and the substrate  510 . The suction pad is disposed on the base  521 . The method for holding the substrate  510  is not limited to vacuum suction. For example, the substrate  510  may be held through electrostatic suction. When exposing the substrate  510  to light, the substrate  510  is held by the base  521 , as illustrated in  FIG. 2A . 
     The substrate lifting mechanism  522  is movable in the Z-axis direction. When the substrate  510  is separated from the base  521 , as illustrated in  FIG. 2B , the substrate lifting mechanism  522  moves in the Z-axis direction to upwardly elevate the substrate  510  in the Z-axis direction. 
     The substrate lifting mechanism  522  includes lifting portions  522 B which vertically move in the Z-axis direction, and contact portions  522 C which come in contact with the substrate  510 . The contact portions  522 C have upper surfaces  522 A. Since the contact portions  522 C contact the substrate  510 , the material of the contact portions  522 C is determined in consideration that the contact portions  522 C hardly cause damage to the substrate  510  or wear out the substrate  510  by the contact. Generally, the contact portions  522 C are made of a resin material. 
     As illustrated in  FIG. 2A , in a state where the substrate  510  is held by the base  521 , a small gap is provided between the substrate  510  and the substrate lifting mechanism  522 . This gap prevents the substrate  510  from interfering with the substrate lifting mechanism  522 , and reduces a risk that the position of the substrate  510  changes in the Z-axis direction. 
     A positional relation between the base  521  and the substrate lifting mechanism  522  will be described below with reference to  FIGS. 3 to 6C .  FIG. 3  illustrates an example in which the base  521  configuring the substrate holding mechanism  500  is divided into eight substrate holding portions in the XY plane with gaps provided between the adjoining substrate holding portions. Substrate holding portions  521   a ,  521   b ,  521   c , and  521   d  are disposed on the positive side in the Y-axis direction, and substrate lifting portions  522   a ,  522   b , and  522   c  are disposed in the gaps between the substrate holding portions  521   a  and  521   b , between the substrate holding portions  521   b  and  521   c , and between the substrate holding portions  521   c  and  521   d , respectively. Further, substrate holding portions  521   e ,  521   f ,  521   g , and  521   h  are disposed on the negative side in the Y-axis direction, and substrate lifting portions  522   d ,  522   e , and  522   f  are disposed in the gaps between the substrate holding portions  521   e  and  521   f , between the substrate holding portions  521   f  and  521   g , and between the substrate holding portions  521   g  and  521   h , respectively.  FIG. 3  illustrates an example in which lift bars are disposed as substrate lifting portions. 
       FIG. 4  illustrates a state where the base  521  is divided into substrate holding portions  521   a ′,  521   b ′,  521   c ′, and  521   d ′. The substrate holding portions  521   a ′,  521   b ′,  521   c ′, and  521   d ′ are provided with substrate lifting portions  522   a ′,  522   b ′,  522   c ′, and  522   d ′, respectively. A substrate lifting portion  522   e ′ is disposed in the gap between the substrate holding portions  521   a ′ and  521   b ′. A substrate lifting portion  522   f  is disposed in the gap between substrate holding portions  521   c ′ and  521   d ′. Each of these substrate lifting portions illustrated in  FIG. 4  is configured by a lift bar. 
     Referring to  FIG. 5 , the base  521  is divided into substrate holding portions  521   a ″,  521   b ″,  521   c ″,  521   d ″,  521   e ″, and  521   f ′. Each of these substrate holding portions is provided with through-holes. In each through-hole, a lift pin is provided as a substrate lifting mechanism  522  which moves in the Z-axis direction inside the through-hole. 
     Referring to  FIG. 6A , the base  521  is composed of a base plate  521 A and a plurality of substrate holding portions  521 B disposed on the base plate  521 A. Gaps are provided between the adjoining substrate holding portions  521 B. By disposing substrate conveyance portions  527  with the substrate  510  mounted thereon in the gaps, the substrate conveyance portions  527  are overlaid on the base  521 , and the substrate  510  is held by the base  521 . The substrate conveyance portions  527  are driven when necessary in the X-axis, the Y-axis, and the Z-axis directions by a substrate conveyance apparatus  528 . The substrate  510  can be elevated and lowered in the Z-axis direction by elevating and lowering the substrate conveyance portions  527  in the Z-axis direction in a state where the substrate  510  is held. 
       FIG. 6B  illustrates a state where the substrate conveyance portions  527  are overlaid on the base  521 .  FIG. 6C  illustrates a state where the substrate conveyance portions  527  are overlaid on the base  521  when viewed from the Y-axis direction. It can be seen that the substrate conveyance portions  527  are disposed in the gaps between the adjoining substrate holding portions  521 B. 
     As described above, in the example illustrated in  FIGS. 6A, 6B, and 6C , the substrate  510  is elevated and lowered by the substrate conveyance portions  527  instead of the substrate lifting mechanism  522 . As described above, the substrate lifting mechanism  522  may be of the lift bar type illustrated in  FIGS. 3 and 4  or of the lift pin type illustrated in  FIG. 5 . The substrate conveyance portions  527  may have a function of a substrate lifting mechanism, as illustrated in  FIGS. 6A, 6B, and 6C . In any case, the substrate lifting portions configuring the substrate lifting mechanism  522  are disposed in the gaps provided in the base  521 . As described above with reference to  FIGS. 3 to 6C , gaps are provided between the adjoining substrate holding portions  521 B and between a substrate holding portion  521 B and a substrate lifting portion. Exposure light passing through these gaps causes exposure non-uniformity. 
     An occurrence mechanism of exposure non-uniformity will be described below with reference to  FIG. 7 .  FIG. 7  illustrates a state where the resist  511  applied on the substrate  510  is irradiated with exposure light. A light ray  10  entering the resist  511  from the space above the base  521  transmits the resist  511  and the substrate  510  and is reflected by the upper surface  521 A of the base  521 . A light ray  12  entering the resist  511  from the space above the substrate lifting mechanism  522  transmits the resist  511  and the substrate  510  and is reflected by the upper portion of the substrate lifting mechanism  522 . Meanwhile, a light ray  11  entering the resist  511  from above the gap between the base  521  and the substrate lifting mechanism  522  transmits the resist  511  and the substrate  510 . 
     The exposure light entering the resist  511  is partly absorbed by the resist  511 . The absorptance and transmittance of light differ depending on the wavelength of the exposure light and the optical characteristics of the resist  511 . The transmittance of the exposure light to the resist  511  can be calculated as follows. 
     When light is assumed to be a one-dimensional plane wave which propagates in the Z direction, the amplitude E(Z, t) of the plane wave at time t is
 
 E ( Z,t )= E   0  exp[ i ( kZ−ωt )]   (1)
 
where k denotes the wave number and w denotes the frequency. Using the complex index of refraction N, the frequency ω is represented as
 
                   ω   =     kc   N             (   2   )               
Then, the amplitude E(Z, t) can be represented by:
 
                           E   ⁡     (     Z   ,   t     )       =       E   0     ⁢     exp   ⁡     [     i   ⁡     (           N   ⁢           ⁢   ω     c     ⁢   Z     -     ω   ⁢           ⁢   t       )       ]                     =       E   0     ⁢     exp   ⁡     [     i   ⁢     {         (     n   +   ki     )     ⁢     ω   c     ⁢   Z     -     ω   ⁢           ⁢   t       }       ]                     =       E   0     ⁢     exp   ⁡     [       -       k   ⁢           ⁢   ω     c       ⁢   Z     ]       ×     exp   ⁡     [     i   ⁢           ⁢     ω   ⁡     (         n   c     ⁢   Z     -   t     )         ]                       (   3   )               
Since ω=2πc/λ,
 
     
       
         
           
             
               
                 
                   
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     The light energy I(Z, t) can be obtained based on the norm of the square of the amplitude E(Z, t) or the square of the norm of the amplitude E(Z, t). Therefore, the light energy I(Z, t) is 
     
       
         
           
             
               
                 
                   
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                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Based on the formula 5, a transmittance T of light is calculated as follows: 
     
       
         
           
             
               
                 
                   T 
                   = 
                   
                     
                       
                         I 
                         ⁡ 
                         
                           ( 
                           d 
                           ) 
                         
                       
                       
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                           d 
                         
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                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     The light transmitting the resist  511  with the transmittance T as calculated based on the formula (6) transmits the substrate  510  and reaches the upper portions of the base  521  and the substrate lifting mechanism  522 .  FIG. 8  illustrates a relation between the wavelength of light and the transmittance at a specific resist  511 . In an exposure apparatus using a mercury lamp, the i-line (with a wavelength of about 365 nm), an K-line (with a wavelength of about 405 nm), the g-line (with a wavelength of about 436 nm), etc. are used as exposure light.  FIG. 8  illustrates a transmittance Ti for the i-line, a transmittance Th for the h-line, and a transmittance Tg for the g-line. It can be seen that the value of the transmittance differs for each wavelength. 
     The light reaching the upper portions of the base  521  and the substrate lifting mechanism  522  is reflected by a reflection surface through regular reflection and diffuse reflection. Regular reflection refers to reflection with which the reflection angle is determined by the angle of the light incident on the reflection surface. In regular reflection, the incident angle and reflection angle are generally equal. Diffuse reflection refers to reflection not dependent on the incident angle of the light incident on the reflection surface. In diffuse reflection, the intensity of the light reflected at an angle θ with respect to the vertical line from the reflection surface depends on cos θ. Diffuse reflection is also referred to as Lambert reflection. 
     The light reflected by the upper surfaces of the base  521  and the substrate lifting mechanism  522  through regular reflection or diffuse reflection transmits the substrate  510  and then enters the resist  511  as flare light. Meanwhile, a light ray  11  illustrated in  FIG. 7  enters the gap between the base  521  and the substrate lifting mechanism  522 . Most of the light ray  11  is attenuated without entering the resist  511  again. 
     Accordingly, on the substrate  510 , there arises a region where much flare light is generated and a region where flare light is hardly generated. As described above, flare light is generated depending on the reflection characteristics of the upper surfaces of the base  521  and the substrate lifting mechanism  522 , and therefore is difficult to be sufficiently reduced. Since a different light quantity of flare light is incident on the resist  511  for each region on the substrate  510 , exposure non-uniformity will accordingly occur. 
     &lt;Method for Reducing Exposure Non-Uniformity&gt; 
     A method for reducing exposure non-uniformity will be described below with reference to  FIGS. 9 to 11 . Referring to  FIG. 9 , the above-described exposure non-uniformity is reduced by disposing reflection members  523  in the gaps between the base  521  and the substrate lifting mechanism  522 . Configurations other than the reflection members  523  illustrated in  FIG. 9  are similar to the configurations illustrated in  FIGS. 2A and 2B , and redundant descriptions thereof will be omitted. The reflection members  523  are made of a resin material such as acrylics and Teflon® or a metal material such as aluminum. A metal material having undergone surface processing such as plating can also be used. 
     It is desirable to dispose the reflection members  523  at the same height as the upper surface of the base  521  or the substrate lifting mechanism  522 . However, since the gap between the upper surface of the base  521  and the upper surface of the substrate lifting mechanism  522  is generally very small, it is difficult to dispose the reflection members  523  in this gap. Therefore, the reflection members  523  are disposed in regions below contact portions  522 C of the substrate lifting mechanism  522 . 
     A mechanism in which exposure non-uniformity is reduced will be described below with reference to  FIG. 10 . Optical paths of the light rays  10  and  12  have been described above with reference to  FIG. 7 , and redundant descriptions thereof will be omitted. The light ray  11  transmits through the resist  511  and the substrate  510 , reaches the upper surface  523 A of the reflection member  523 , and then is reflected toward the substrate by the upper surface  523 A. 
     The upper surface  523 A of the reflection member  523  is a diffuse reflection surface. Since light rays  41  out of reflected light have a small reflection angle, the light rays  41  pass through the gap between the base  521  and the substrate lifting mechanism  522 , transmit the substrate  510 , and then reach the resist  511 . Meanwhile, light rays  42  out of reflected light have a large reflection angle with respect to the vertical line from the upper surface  523 A. Therefore, the light rays  42  are radiated onto the side faces of the base  521  and the substrate lifting mechanism  522  and almost all the light rays  42  are absorbed. As a result, almost none of the light rays  42  reaches the resist  511 . 
     Since the reflection member  523  is disposed in the gap between the base  521  and the substrate lifting mechanism  522 , as described above, flare light will also enter the resist  511  positioned at the upper portion of the gap. This configuration reduces the difference between the light quantity which is reflected by the base  521  and reaches a first resist region above the base  521  and the light quantity which is reflected by the reflection member  523  and reaches a second resist region above the reflection member  523 . This configuration also reduces the difference of the light quantity which is reflected by the substrate lifting mechanism  522  and reaches a third resist region above the substrate lifting mechanism  522  and the light quantity which is reflected by the reflection member  523  and reaches the second resist region above the reflection member  523 . 
     As a result, the light quantity distribution of flare light radiated onto the resist  511  can be made uniform to a certain extent. Referring to  FIG. 9 , the first resist region is a region  511 A, the second resist region is a region  511 B, and the third resist region is a region  511 C. 
     However, as described above, not all of the exposure light entering the reflection member  523  reaches the resist  511 . Therefore, additional devisal is required to further reduce exposure non-uniformity. 
     The present exemplary embodiment achieves further reduction of exposure non-uniformity by suitably setting the reflection characteristics, such as reflectance, on each reflection surface of the base  521 , the substrate lifting mechanism  522 , and the reflection member  523 . A method for determining the reflection characteristics will be described below. 
     As described above, the reflection characteristics of incidence light change with the shape of the reflection surface. If the reflection surface has a shape factor k (0&lt;k&lt;1) and the substance configuring the reflection surface has a reflectance R, the light quantity which reaches the resist  511  after being reflected by the reflection surface can be represented by a factor kR. The shape factor k has parameters such as the position of the reflection member  523  in the Z-axis direction and a width w of the gap between the base  521  and the substrate lifting mechanism  522 , and is determined through light analysis and optical simulation. 
       FIG. 11  illustrates a relation between the position of the reflection member  523  in the Z-axis direction and the shape factor k. The horizontal axis illustrated in  FIG. 11  indicates a distance d from the upper surface  521 A of the base  521  to the upper surface  523 A of the reflection member  523  in the Z-axis direction. When the distance d=0, the height of the upper surface  521 A of the base  521  and the height of the upper surface  523 A of the reflection member  523  are equal.  FIG. 11  illustrates that the shape factor k changes with the width w of the gap between the base  521  and the substrate lifting mechanism  522 .  FIG. 11  also illustrates that the shape factor k increases with increasing width w (because w1&gt;w2) and that the shape factor k decreases with increasing distance of the position of the reflection member  523  from the upper surface  521 A of the base  521 . 
     The regular reflectance is defined by the percentage of the ratio of the light quantity of the regular reflection light to the light quantity of the light entering the reflection surface. The diffuse reflectance is defined by the percentage of the ratio of the light quantity of the diffuse reflection light in all directions, excluding the regular reflection light, to the light quantity of the light entering the diffuse reflection surface. The shape factor k for the regular reflection light and the shape factor k for the diffuse reflection light can be respectively defined. 
     The light quantity of flare light depends on a coefficient TkR as a product of the above-described factor kR and the transmittance T of the resist  511 . Ideally, equalizing the factor kR of each of the base  521 , the substrate lifting mechanism  522 , and the reflection member  523  enables equalizing the coefficient TkR for each region on the resist  511 . This makes it possible to reduce non-uniformity of the exposure amount for each region on the resist  511  resulting in reduced exposure non-uniformity. 
     In terms of a regular reflectance R, a diffuse reflectance R′, a shape factor k for the regular reflectance, and a shape factor for the diffuse reflectance, the shape factor k of the reflection member  523  is adjusted to satisfy at least either one of the following formulas (7-1) and (7-2). 
                         ∑   λ     ⁢       T   λ     ⁡     (         k   S   ′     ⁢     R     S   ⁢           ⁢   ¨   ⁢           ⁢   λ     ′       +       k   S     ⁢     R     S   ⁢           ⁢   ¨   ⁢           ⁢   λ           )         -       ∑   λ     ⁢       T   λ     ⁡     (         k   C   ′     ⁢     R     λ¨   ⁢           ⁢   C     ′       +       k   C     ⁢     R     λ¨   ⁢           ⁢   C           )           &lt;          1   ⁢   %                  (     7   ⁢     -     ⁢   1     )                     ∑   λ     ⁢       T   λ     ⁡     (         k   S   ′     ⁢     R     S   ⁢           ⁢   ¨   ⁢           ⁢   λ     ′       +       k   S     ⁢     R     S   ⁢           ⁢   ¨   ⁢           ⁢   λ           )         -       ∑   λ     ⁢       T   λ     ⁡     (         k   l   ′     ⁢     R     λ¨   ⁢           ⁢   l     ′       +       k   l     ⁢     R     λ¨   ⁢           ⁢   l           )           &lt;          1   ⁢   %                  (     7   ⁢     -     ⁢   2     )               
Referring to the formulas (7-1) and (7-2), subscripts S, C, and l indicate parameters of the reflection member  523 , the base  521 , and the substrate lifting mechanism  522 , respectively. A subscript λ indicates a parameter dependent on the wavelength. The formulas (7-1) and (7-2) indicate that, when the exposure light contains a plurality of wavelength components, the values relating to these parameters are summed up for each wavelength.
 
     Exposure non-uniformity can be preferably reduced by adjusting the shape factor k and the reflectances R and R so that the formulas (inequalities) (7-1) and (7-2) are satisfied. According to the verification by the inventors of the present application, it has been found that, if the reflection member  523  according to the present invention is not used, the numerical values of the left-hand sides of the inequalities (7-1) and (7-2) become about 1.5 to 2.0%. 
     Although, in the present exemplary embodiment, the reflection member  523  is disposed in the gap between the base  521  and the substrate lifting mechanism  522 , the reflection member  523  may be disposed in the gap between the adjoining substrate holding portions  521 B, as illustrated in  FIGS. 3 to 6C . It is because even if the substrate lifting mechanism  522  is not disposed between the adjoining substrate holding portions  521 B, a problem similar to the above-described problem may occur if a gap is produced between the adjoining substrate holding portions  521 B. 
     Subsequently, specific examples of materials configuring the base  521 , the substrate lifting mechanism  522 , and the reflection member  523  will be described. The base  521  is made of a black ceramic material. The substrate lifting mechanism  522  is made of a black resin material. The reflection member  523  is made of Teflon®. 
       FIGS. 12A and 12B  illustrate the diffuse reflectance and the regular reflectance on the upper surface  521 A of the base  521  made of a black ceramic material, respectively.  FIGS. 13A and 13B  illustrate the diffuse reflectance and the regular reflectance on the upper surface  522 A of the substrate lifting mechanism  522  made of a black resin material, respectively.  FIGS. 14A and 14B  illustrate the diffuse reflectance and the regular reflectance on the upper surface  523 A of the reflection member  523  made of Teflon®, respectively. 
     As described above, the upper surface of the reflection member  523  is disposed below the upper surface of the base  521  and the upper surface of the substrate lifting mechanism  522 . Thus, the shape factors ks and ks′ of the reflection member  523  are smaller than the shape factors k of the base  521  and the substrate lifting mechanism  522 . Accordingly, the difference between the factor kR of the reflection member  523  and the factor kR of the base  521  and the substrate lifting mechanism  522  is reduced by using a material having a comparatively high reflectance as the material of the reflection member  523 . This enables reducing exposure non-uniformity. 
     For example, with a projection exposure apparatus in which the wavelength region of exposure light ranges from 350 to 450 nm, it is desirable that the reflectance changes by a small amount on the upper surfaces of the base  521 , the substrate lifting mechanism  522 , and the reflection member  523  in the wavelength region. If the reflectance largely changes, the exposure amount distribution may possibly largely change between when the i-line (with a wavelength of about 365 nm) is used as exposure light and when the g-line (with a wavelength of about 436 nm) is used as exposure light. This indicates that the magnitude of exposure non-uniformity changes with the wavelength of exposure light, which is not desirable from the viewpoint of reduction of exposure non-uniformity. 
     The change in reflectance on the upper surface of each member can be decreased by using materials having a small change in reflectance in the above-described wavelength region as the materials of the base  521 , the substrate lifting mechanism  522 , and the reflection member  523 . 
     The materials configuring the base  521 , the substrate lifting mechanism  522 , and the reflection member  523  are not limited to the materials illustrated in  FIGS. 12A to 14B . For example, the base  521  may be made of aluminum having undergone black alumite processing or a metal material having undergone Diamond-Like Carbon (DLC) coating. The substrate lifting mechanism  522  and the reflection member  523  may be made of a resin material such as acrylics or a metal material having undergone various types of coating. 
     &lt;Position Adjustment for Reflection Member&gt; 
     It is desirable to provide a position adjustment mechanism for adjusting the position of the reflection member  523  in the Z-axis direction. The reflectances of the base  521 , the substrate lifting mechanism  522 , and the reflection member  523  are mainly determined by the physical property value of materials. However, the reflectances may vary depending on manufacturing errors. The reflectances may also vary depending on the characteristics of the resist  511 , the wavelength of exposure light, and exposure process. 
     The exposure non-uniformity which may be caused by the variation in these reflectances can be reduced by driving the reflection member  523  in the Z-axis direction. Since the shape factor k changes by driving the reflection member  523  in the Z-axis direction, exposure non-uniformity can be reduced by suitably varying the values of the left-hand sides of the above-described inequalities (7-1) and (7-2). 
     Next, the substrate holding apparatus including a position adjustment mechanism will be described with reference to  FIG. 15 .  FIG. 15  illustrates a part of the configuration illustrated in  FIG. 9 , i.e., the configuration of a position adjustment mechanism  524  for the reflection members  523 . As illustrated in  FIG. 15 , the reflection members  523  are disposed separately from the base  521  and the substrate lifting mechanism  522 . The reflection members  523  are driven by the position adjustment mechanism  524  and are movable in the Z-axis direction. 
     The base  521  and the position adjustment mechanism  524  are attached to the upper portion of a top plate  525  on a movable stage (not illustrated), which is movable in the X-axis and the Y-axis directions. A lifting portion  522 B configuring the substrate lifting mechanism  522  penetrates through the top plate  525 , and is driven in the Z-axis direction along a guide  526  by an actuator (not illustrated). The position adjustment mechanism  524  is driven in the Z-axis direction by an actuator (not illustrated) provided on the top plate  525 . 
     Other Modifications 
     The exemplary embodiments have been described above centering on the examples in which the substrate holding apparatus according to the present invention is applied to a scanner as the exposure apparatus  1 . In addition, the substrate holding apparatus of the present invention is also applicable to a stepper for projecting the pattern of the fixed original plate  310  onto the substrate  510 . 
     Articles Manufacturing Method 
     There is described a method for manufacturing an article (e.g., a semiconductor integrated circuit element and a liquid crystal display element) by using the above-described exposure apparatus. The article manufacturing method includes a process of forming a pattern by radiating exposure light onto a substrate held by using the substrate holding apparatus according to the exemplary embodiments, and a process of processing the substrate with the pattern formed thereon (development and etching). Exposure non-uniformity can be effectively reduced by using the substrate holding apparatus according to the present invention. As a result, the accuracy for forming a pattern on a substrate can be improved. 
     In comparison with conventional methods, the present article manufacturing method is advantageous in at least one of the performance, quality, productivity, and production cost of articles. The above-described exposure apparatus can offer such articles as high-definition devices (e.g., semiconductor integrated circuit elements and liquid crystal display elements). 
     While the present invention has specifically been described based on the above-described exemplary embodiments, the present invention is not limited thereto, and can be modified in diverse ways within the ambit of the appended claims. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2018-103896, filed May 30, 2018, and Japanese Patent Application No. 2019-048237, filed Mar. 15, 2019, which are hereby incorporated by reference herein in their entirety.