Abstract:
A liquid immersion photolithography system includes an exposure system that exposes a substrate with electromagnetic radiation, and also includes an optical system that images the electromagnetic radiation on the substrate. A liquid is between the optical system and the substrate. The projection optical system is positioned below the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a Continuation of application Ser. No. 10/831,300, filed Apr. 26, 2004, titled IMMERSION PHOTOLITHOGRAPHY SYSTEM AND METHOD USING INVERTED WAFER-PROJECTION OPTICS INTERFACE, which is a Continuation of application Ser. No. 10/607,170, filed Jun. 27, 2003, titled IMMERSION PHOTOLITHOGRAPHY SYSTEM AND METHOD USING INVERTED WAFER-PROJECTION OPTICS INTERFACE, each of which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to liquid immersion photolithography, and more particularly, to a method and system for confining liquid flow in an immersion photolithographic system.  
         [0004]     2. Description of the Related Art  
         [0005]     Optical lithography, using lens systems and catadioptric systems, is used extensively in the semiconductor manufacturing industry for the printing of circuit patterns. To date, the gap between a final lens element and a semiconductor wafer surface has been filled with gas, usually air or nitrogen. This gaseous gap works well particularly when the wafer is scanned under the optics during exposure and there is relative movement between the wafer and the lens system during the image transfer.  
         [0006]     The practical limits of optical lithography assume that the medium through which imaging is occurring is air. This practical limit is defined by the equation Λ=λ/4·n·NA, where 8 is the wavelength of incident light, NA is numerical aperture of the projection optical system, and n is the index of refraction of the medium (where 4 is used instead of 2 due to the use of off axis illumination). The gas interface between the final lens element and the wafer surface limits the maximum resolution of the optical system to a numerical aperture of &lt;1.0. If the gas space between the final lens element and the wafer surface can be filled with a refractive material, such as oil or water, then the numerical aperture, and hence the resolution capability, of the system can be significantly increased, corresponding to the index of refraction n.  
         [0007]     Thus, by introducing a liquid between a last lens element of the projection optical system and a wafer being imaged, the refractive index changes, thereby enabling enhanced resolution with a lower effective wavelength of the light source. Immersion lithography effectively lowers a 157 nm light source to a 115 nm wavelength (for example, for n=1.365), enabling the printing of critical layers with the same photolithographic tools that the industry is accustomed to using today.  
         [0008]     Similarly, immersion lithography can push 193 nm lithography down to, for example, 145 nm (for n=1.33). 435 nm, 405 nm, 365 nm, 248 nm, 193 nm and 157 nm tools can all be used to achieve effectively better resolution and “extend” the usable wavelengths. Also, large amounts of CaF 2 , hard pellicles, a nitrogen purge, etc.—can be avoided. Also, depth of focus can be increased by the use of liquid immersion, which may be useful, for example, for LCD panel manufacturing.  
         [0009]     However, despite the promise of immersion photolithography, a number of problems remain, which have so far precluded commercialization of immersion photolithographic systems. One problem of existing immersion photolithographic systems involves the difficulties of confining the liquid that is used in an interface between the projection optical system and the wafer being exposed. In conventional systems, liquid is injected between the projection optical system and the wafer. Fairly complex systems have been proposed in order to maintain the confinement of the liquid.  
         [0010]     An additional problem exists where the scanning motion of the wafer is such that the wafer is moved away from the exposure area, resulting in a spilling of the liquid. Such spillage is also a problem even when the wafer is present under the projection optical system due to the inherent viscosity properties of the liquid.  
         [0011]     Accordingly, what is needed is a simple system and method for confining the liquid between the projection optical system and the wafer.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention is directed to an immersion photolithography system and method using an inverted wafer-projection optics interface that substantially obviates one or more of the problems and disadvantages of the related art.  
         [0013]     There is provided a liquid immersion photolithography system including an exposure system that exposes a substrate with electromagnetic radiation, and also includes a projection optical system that focuses the electromagnetic radiation on the substrate. A liquid supply system provides a liquid between the projection optical system and the substrate. The projection optical system is positioned below the substrate.  
         [0014]     In another aspect there is provided a liquid immersion photolithography system that includes an exposure system that exposes a substrate with electromagnetic radiation, and also includes a projection optical system that focuses the electromagnetic radiation on the substrate. A means for providing a liquid is between the projection optical system and the substrate. The projection optical system is positioned below the substrate. A meniscus is formed between the projection optical system and the wafer.  
         [0015]     In another aspect there is provided a method of exposing a substrate including positioning a projection optical system below the substrate, projecting electromagnetic radiation onto the substrate using a projection optical system, and delivering a liquid between the projection optical system and the substrate.  
         [0016]     Additional features and advantages of the invention will be set forth in the description that follows. Yet further features and advantages will be apparent to a person skilled in the art based on the description set forth herein or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
         [0017]     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGS.  
       [0018]     The accompanying drawings, which are included to provide a further understanding of the exemplary embodiments of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:  
         [0019]      FIG. 1  shows a cross-sectional view of a liquid immersion photolithography system according to one embodiment of the present invention.  
         [0020]      FIG. 2  shows an isometric view of the system of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0022]     The present invention allows a space between a final lens element of a projection optical system and a wafer surface to be filled with a liquid. It allows a significant increase in the effective numerical aperture of the optical system. The volume of liquid is contained and held in position using a combination of pressure control on the liquid and gravity. The projection optical system (exposure system) is inverted compared to conventional systems currently in use. In other words, conventional systems expose downward or to the side, while the projection optical system of the present invention exposes upwards. The wafer is exposed with its resist-coated surface down, and the resist is in contact with a liquid meniscus. During wafer scanning, the meniscus traverses the resist-coated surface of the wafer.  
         [0023]     The present invention allows the gap-filling liquid to be held in place even while the edge of the wafer is passed over the optics. The housing of the projection optical system, with the liquid, can be scanned off the edge of the wafer and rescanned onto the wafer while maintaining the liquid interface.  
         [0024]     Catch basins around the housing catch and contain any liquid displaced. The liquid meniscus is controlled by liquid pressure. This interface is thus easily compatible with many types of liquid.  
         [0025]      FIG. 1  illustrates an embodiment of a liquid immersion photolithographic system according to the present invention. As shown in  FIG. 1 , a projection optical system  100  is placed below a wafer  101 . The wafer  101  includes resist-coated wafer surface  106 . The projection optical system  100  includes a plurality of lens elements  102 A,  102 B, etc. The lens elements  102 A,  102 B are mounted within a housing  103 . The top of the housing  103  includes an opening  110  for projecting an image onto the wafer  101 . The top of the housing  103  is shown as horizontal in  FIG. 1 , although that need not necessarily be the case.  
         [0026]     The region between the top of the housing  103  and the lens  102 A (designated  107  in  FIG. 1 ) is pressure controlled, and is sealed from the rest of the projection optics  100  by a liquid seal  104 . The region  107  is filled with a liquid, normally under pressure from a liquid source (not shown in  FIG. 1 ) so as to counterbalance the force of gravity. During exposure, the liquid forms a meniscus  108 , as shown in  FIG. 1 . Catch basins  105  are used to remove any stray liquid, which may occur as the wafer  101  is scanned along a horizontal axis. It will be appreciated that more or fewer catch basins (compared to what is shown in  FIG. 1 ) may be used. The catch basins  105  may also be annular around the housing  103 .  
         [0027]     Note that in the present invention, gravity is allowed to do the work of confining the liquid. The meniscus  108  is essentially controlled by gravity, while the wafer  101  is scanned. Furthermore, when the wafer  101  moves beyond the projection optics  100 , the liquid will not readily spill over the edge of the wafer  101 , unlike in conventional immersion photolithographic systems.  
         [0028]     A liquid enclosing collar system (i.e., the catch basin  105 ) is attached to the end of the lithographic systems lens. As noted above, the projection optical system  100  exposes the image upwards onto the underside of the wafer  101  (i.e., wafer surface  106 ). The wafer  101  is resist coated, and the wafer surface  106  to be imaged is the lower surface. The top of the housing  103  provides a liquid interface between the final lens element  102 A and the wafer surface  106  of the wafer  101  on which the projection optical system  100  is focused. The opening  110  in the top of the housing  103  allows the light beam from the projection optics  100  to be imaged on the wafer surface  106 . It also allows intimate contact between the liquid and the wafer surface  106 . It is important to ensure that the enclosed region  107  remains full of liquid, despite the top of the housing  103  being open to the wafer surface  106  and despite the wafer  101  potentially moving in an unrestricted manner above the projection optical system  100 . The liquid is held in place by control of the pressure exerted on the liquid through a recirculation system (i.e., a liquid supply system, not shown in the figures). The pressure is controlled to balance gravity and maintain the meniscus  108  across the opening  110  when the wafer  101  is not present. When the wafer  101  is slid over the projection optical system  100 , the pressure is increased to allow the liquid to “push out” of the aperture and contact the wafer surface  106 . When the liquid interface slides over the edge of the wafer  101  due to the motion of the wafer  101  relative to the projection optics  100 , the pressure on the liquid is adjusted to “pull back” the liquid from the wafer surface  106  into the region  107 .  
         [0029]     The top of the housing  103  near the aperture  110 , shown in  FIG. 1 , may be specially contoured and surface finished to control the shape and properties of the interface liquid. For example, the surface of the top of the housing  103  may be made hydrophobic. The catch basins  105  surrounding the top of the housing  103  restrain the liquid that overflows or leaks from the top of the housing  103 . This liquid can be filtered, temperature controlled and recycled back into the region  107 .  
         [0030]     Conditioning of the wafer surface  106  and the top of the housing  103  can further improve the performance. In the case of the liquid being water, the surfaces can be made hydrophobic. The gap (distance) between the wafer surface  106  and the top of the housing  103  is optimized by the dynamics of wafer exposure. While the system is designed for dynamic exposure of wafers in a scanning system, it also can be used in a step-and-scan type exposure system.  
         [0031]     In typical dry exposure systems, the gap between the lens  102 A and the wafer  101  is on the order of 3-4 millimeters. In the present invention, the dimension of the gap between the housing  103  and the wafer  101  may be made as low as 50 microns, although larger or smaller dimensions, for example, up to half a millimeter for the gap between the housing  103  and the wafer  101 , may also be used (nominally, 100 microns are expected to be in the typical range, although ranges of 50-150 microns, 40-200 microns, or even up to 1 mm, and even in some cases greater than 1 mm, may be possible). It should be noted that water is the preferred liquid for 193 nanometer lithography, which is relatively lossless at 193 nm. For 157 nanometer lithography, losses within the liquid are a concern, which tends to require smaller gaps between the lens  102 A and the wafer  101 . In other words, the lens  102 A would move closer to the wafer  101  (down to about 1 mm or so). In the case of 157 nm lithography, the gap between the housing  103  and the wafer  101  may be down to 50 microns or less.  
         [0032]     It will also be appreciated that in the present invention, the liquid may be removed completely, in the event that exposure of the wafer  101  calls for a dry exposure. For dry exposure, the optics needs to be adjusted accordingly (e.g., focus, spherical abberation, reduction in the numerical aperture, etc.)  
         [0033]     As noted above, for 193 nm imaging, the liquid is preferably water (e.g., de-ionized water), although other liquids, for example, cyclo-octane, Krytox® (Foemblin oil) and perfluoropolyelher fluids, may be used.  
         [0034]      FIG. 2  illustrates an isometric view of the liquid immersion photolithographic system of  FIG. 1 . In  FIG. 2 , common elements with  FIG. 1  have been labeled identically. (Note that in this simulated figure, the wafer  101  appears transparent.)  
         [0035]     Placing of the projection optical system  100  below the wafer  101 , rather than above it, permits taking advantage of gravity to form a meniscus  108  such that the confinement of the liquid is substantially simplified. This removes the need for complicated confinement systems, fairly complex liquid recirculation and pumping mechanisms, etc. It also considerably simplifies the effects of any stray liquid that can be simply captured using the catch basins  105 .  
         [0036]     As an alternative, it is possible to have “fountainhead” effect, where the liquid is expelled from the housing  103  towards the wafer  101 , achieving a similar effect as that of the meniscus, and then flows in the catch basins for recycling.  
         [0037]     The present invention results in a number of benefits to a liquid immersion photolithographic system. Confinement of the liquid is simplified.  
         [0038]     Spillage is reduced or eliminated entirely. The system may be used both as a wet exposure system (with the liquid), and as a dry exposure system (without the liquid, with optics adjustes), as appropriate. All of these benefits allow the use of existing photolithographic tools and familiar wavelengths to define much smaller features on a semiconductor surface.  
       CONCLUSION  
       [0039]     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention.  
         [0040]     The present invention has been described above with the aid of functional building blocks and method steps illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks and method steps have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Also, the order of method steps may be rearranged. Any such alternate boundaries are thus within the scope and spirit of the claimed invention. One skilled in the art will recognize that these functional building blocks can be implemented by discrete components, application specific integrated circuits, processors executing appropriate software and the like or any combination thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.