Abstract:
An apparatus includes an illumination system for illuminating a pattern formed on a mask with exposure light, a projection optical system for focusing and projecting an image of the mask pattern on a photosensitive substrate, and a stop member located on or near a plane having a Fourier transform relationship with a pattern surface of the mask. The illumination system has a variable optical member for setting variable the size of the exposure light passing through the plane having the Fourier transform relationship with the pattern surface of the mask. The stop member shields or reduces a portion near the central portion of the exposure light.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a projection exposure apparatus used in the manufacture of semiconductor integrated circuits or liquid crystal displays. 
     2. Related Background Art 
     In a conventional projection exposure apparatus, an attempt has been made to change the incident angle range (so-called σ value=(numerical aperture of illumination light)/(numerical aperture of projection optical system)) of illumination light on a reticle pattern so as to increase the resolving power. A change in σ value is generally realized by arranging a light-shielding member (stop) near a Fourier transform plane for a reticle pattern in an illumination optical system. A method (annular illumination method) of shielding or reducing a central portion of illumination light for setting an incident angle range on a Fourier transform plane can also be realized by adding a light-shielding member (light-reducing member). 
     A phase-shifting method has recently been proposed, and a phase shift reticle for realizing this method has become popular. For this reason, an illumination system having a small σ value suitable for a phase-shifting method is required in a projection exposure apparatus. The annular illumination method has also become popular, and an appropriate annular illumination system is required. In either method, a higher resolving power and a larger focal depth can be obtained than those of a conventional exposure method. However, in the change method using the conventional light-shielding member as described above, a loss in illumination light amount upon partial light-shielding of the illumination light using the light-shielding member becomes large. The illumination intensity on a photosensitive substrate (wafer) is decreased to undesirably prolong the exposure time. The processing capacity per unit time in the projection exposure apparatus is decreased, resulting in a practical problem. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a projection exposure apparatus capable of changing the σ value and reducing illuminance in an illumination system capable of performing annular illumination. 
     According to the present invention, a variable optical member (6-10) for changing the size (beam diameter) of exposure light within a plane (12) having a Fourier transform relationship with a pattern surface of a mask (20) in an illumination system (1-19) for illuminating the mask (reticle 20) with exposure light is provided. A stop member (13; 13A; 13B; 131-134) for shielding or reducing the central portion of the exposure beam is arranged on or near the Fourier transform plane (12). A secondary source image is generally formed on the Fourier transform plane within the illumination system. The variable optical member serves to change the size of the secondary source image. 
     In the illumination system of the projection exposure apparatus, generally, illumination light emitted from a light source such as a mercury lamp is transmitted through an optical system to form a secondary source, and the secondary source image is formed on a reticle pattern. The radiation angle range of light incident on the reticle is determined by the size of the secondary source and a synthetic focal length of the optical system from the secondary source to the reticle. In a conventional arrangement, a light-shielding member (σ stop) is arranged at this secondary source to limit the size of the secondary source, thereby limiting the incident angle range of the light incident on the reticle. 
     To the contrary, according to the present invention, an optical system for forming a secondary source is a variable optical system (zoom system) to change the size of the secondary source. The incident angle range of the illumination light on the reticle can be set variable without causing a loss in light amount. According to the present invention, it is possible to add a conventionally used light-shielding member. In the variable optical system (zoom system) described above, the size of the secondary source can be proportionally enlarged or reduced, but the shape of the secondary source cannot be changed. Therefore, a light-shielding member may be added. In particular, when the central portion (near the optical axis of the illumination system) of the secondary source is shielded or reduced to obtain annular illumination, the resolving power and the focal depth in the projection optical system can be increased. In this case, since the peripheral portion of the illumination beam (secondary source) does not require the conventional light-shielding member (σ stop), the loss in illumination light amount can be smaller than that in the conventional case. That is, although a loss in light amount upon formation of annular illumination occurs, even if the annular ratio is set to be 0.5 ((numerical aperture of light-shielding portion)/(numerical aperture of outer diameter of illumination light)), the loss is only 25%. To the contrary, when the σ value is set variable using the conventional σ stop and is changed from 0.7 to 0.3, the loss in light amount is about 80%. In addition, a loss also occurs upon formation of annular illumination. According to the present invention, since the outer diameter of the illumination beam distribution on the Fourier transform plane is defined by the variable optical member (zoom system), the loss in light amount by limitation of the σ value (outer diameter) rarely occurs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing an arrangement of a projection exposure apparatus according to an embodiment of the present invention; 
     FIGS. 2A and 2B are views showing the shapes of stops for annular illumination; and 
     FIG. 3 is a view showing a stop exchange mechanism for setting annular illumination conditions variable. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an embodiment of the present invention. Exposure illumination light emitted from a mercury lamp 1 serving as a light source is focused by an elliptical mirror 2 and is transmitted through a variable optical system (zoom optical system) having optical members 7 to 10 through mirrors 3 and 5 and a relay lens 4. The light is converted as an almost parallel beam which reaches a fly-eye lens 11. The zoom optical system (7 to 10) is arranged such that the optical members 7, 8, 9, and 10 are movable in the light axis direction in a support member 6 through holders 7a, 8a, 9a, and 10a. The optical members 7, 8, 9, and 10 are moved to constitute a zoom system. The size (diameter) of the beam incident on an incident surface 11a of the fly-eye lens (optical integrator) 11 is set variable by a variable magnification of the zoom system while the almost parallel state of the beam is maintained. A driver DRV1 independently drives the holders 7a, 8a, 9a, and 10a in accordance with commands from a main control system MCS. 
     An exit surface 11b of the fly-eye lens 11 corresponds to an optical Fourier transform plane 12 of a reticle 20. A secondary source is formed on the plane 12 by the fly-eye lens 11. At this time, the size (diameter) of the secondary source almost corresponds (almost proportional) to the diameter of the illumination beam on the incident surface 11a of the fly-eye lens 11. By a variable magnification operation using the optical members 7, 8, 9, and 10 of the zoom system, the size (the substantial number of elements through which the illumination beam in the fly-eye lens 11 passes) of the secondary source can be set variable. 
     The illumination light emitted from the fly-eye lens 11 is guided through mirrors 14 and 18, relay lenses 15 and 17, and a condenser lens 19 to illuminate the reticle 20 with a uniform illuminance distribution. The pattern on the reticle 20 is exposed and transferred to a wafer 22 through a projection optical system 21. A field stop (reticle blind) 16 for limiting the illumination area on the reticle 20 is also provided in FIG. 1. Light rays represented by broken lines in FIG. 1 represent a beam from a secondary source as one point on the optical axis of the fly-eye lens 11. 
     A stop 13 exchangeable by a driver DRV2 is arranged near the exit surface 11b of the fly-eye lens 11. The stop 13 is a stop for shielding the central portion of the illumination beam passing through the Fourier transform plane 12 to realize annular illumination. The shapes of the stop 13 are exemplified in FIGS. 2A and 2B. Referring to FIG. 2A, a stop 13A is constituted by an annular (ring-like) outer frame 13a 0 , a circular central light-shielding portion 13a 1 , and a holder 13a 2 . Note that the illumination beam (variable by the zoom optical system) in FIG. 2A is present within a circle Ia represented by a broken circle. That is, the outer frame 13a 0  is sufficiently larger than an illumination beam Ia. Even if the illumination beam Ia becomes large by the optical elements 7, 8, 9, and 10 of the zoom system, the beam is not shielded by the outer frame 13a 0 . On the other hand, a stop 13B shown in FIG. 2B is constituted by an outer frame 13b 0 , a central light-shielding portion 13b 1 , and a holder 13b 2 . The inner diameter of the outer frame 13b 0  is smaller than the diameter of an illumination beam Ib passing through the stop 13B, thereby light-shielding the peripheral portion of the illumination beam. 
     The light amount distribution on the incident surface 11a of the fly-eye lens 11 in FIG. 1 is almost flat (uniform) at its central portion, but is abruptly declined at its peripheral portion. For this reason, when the illumination beam passing through the zoom system is directly used, the illuminance evening effect of the fly-eye lens 11 is weakened, and illuminance uniformity on the reticle 20 is degraded. In order to prevent this, the stop 13B shown in FIG. 2B is preferably used. When the peripheral portion of the illumination beam is shielded, the illumination light amount is decreased accordingly. If the light amount has a higher priority than the illuminance uniformity, the aperture 13A in FIG. 2A is preferably used. 
     FIG. 3 shows an arrangement in which the stop 13 is exchangeable. A light-shielding holding member 130 has a turret structure, and four stops 131, 132, 133, and 134 can be exchanged by rotating the holding member 130 by the driver DRV2, so that the stops are selectively inserted into or removed from an illumination light path Ic. The four stops have different shapes. The shape of each stop is not limited to shield the central portion of the illumination beam but may include an outer diameter stop (σ stop). These stops are exchangeably used in accordance with the magnifications of the zoom optical system. Even if the beam diameter is changed by the zoom optical system, the stop may not be exchanged. The exchange and change described above may be set when an operator inputs data to the main controller MCS of the exposure apparatus through an input unit (e.g., a keyboard) IPM. Alternatively, information (pattern information or illumination condition information) such as a bar code may be added to the reticle 20 in advance, and the exposure apparatus may read this information to automatically set the illumination conditions. 
     If the input unit IPM is a bar code reader, the main controller MCS supplies drive commands to the drivers DRV1 and DRV2 in accordance with information read by the input unit IPM, and the distribution of the exposure beam within the Fourier transform plane 12 is set in an optimal state. More specifically, in this embodiment, the outer diameter of the exposure beam on the Fourier transform plane 12 is defined by the optical members 7 to 10 of the zoom optical system. At the same time, the inner diameter of the exposure beam is defined by the stop 13. The loss in light amount upon formation of annular illumination can be minimized. To change the annular ratio, the stop 13 is not exchanged, and at least one of the optical members 7 to 10 of the zoom optical system is driven to change only the outer diameter. Alternatively, the optical members 7 to 10 of the zoom optical system are not driven and the stop 13 is exchanged to change only the inner diameter; or the stop 13 is exchanged and at least one of the optical members 7 to 10 of the zoom optical system is driven to change both the inner and outer diameters. If the reticle 20 is constituted by a phase shift reticle, the stop 13 is removed from the illumination optical path by the driver DRV2, and at the same time, at least one of the optical members 7 to 10 of the zoom optical system is driven by the driver DRV1, thereby setting the σ value to fall within the range of about 0.1 to 0.4. 
     In the above embodiment, the optical members 7 to 10 constituting the zoom optical system are arranged between the relay lens 4 (mirror 5) and the fly-eye lens 11 to set the outer diameter (i.e., the coherence factor σ value of the illumination system) of the exposure beam within the Fourier transform plane 12 variable. However, the zoom optical system suitably applied to the present invention is not limited to the above arrangement. At least one optical member located between the light source 1 and the Fourier transform plane 12 on which the exit surface 11a of the fly-eye lens 11 is located need only be constituted by a variable system. For example, the fly-eye lens 11 may be constituted by a variable magnification system. 
     The transmittances of the stops shown in FIGS. 2A, 2B, and 3 need not be zero, and the stops may be made of a material for reducing the light amount. The shape of the central light-shielding portion need not be circular, but may be a rectangular. Since the secondary source image is formed at almost the central portion of each of a plurality of lens elements constituting the fly-eye lens 11, the edge of the central light-shielding portion preferably conforms to the arrangement of the lens elements.