Abstract:
A lithographic system includes a source of a laser beam; a beamsplitter dividing the laser beam into a plurality of beams; and a plurality of reflecting surfaces that forms interference fringes on a substrate using the plurality of beams. Resolution of the lithographic system is adjustable by adjusting angular orientation of the reflecting surfaces. The beamsplitter is movable along the optical path to adjust the resolution. The reflecting surfaces may be facets of a prism. Each reflecting surface corresponds to a particular beamsplitter position along the optical path, and/or to a particular resolution. The beamsplitter includes a linear grating or a checkerboard grating. The beams are N-way symmetric. A numerical aperture of the system is adjustable by moving the beamsplitter along the optical path. A liquid can be between the substrate and the prism.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a divisional of U.S. patent application Ser. No. 10/927,309, filed Aug. 27, 2004, which is incorporated by reference herein in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to lithographic systems, and more particularly to interferometric lithographic systems with adjustable resolution. 
   2. Related Art 
   In interferometric lithographic systems, a laser beam is sent into a beam splitter, the beam splitter divides the laser beam, and then the laser beam is recombined at a substrate that is being exposed, to form a pattern. Typically the patterns being formed involve lines, or rulings, which are used to test such components as photoresist, etc. 
   Thus, in an interferometric lithographic system, two laser beams are coherently matched, to form fringes at the substrate plane. The fringe pattern exposes the photoresist, which forms a type of a grating pattern. Different interferometric lithographic systems have different ways of generating the two laser beams that will ultimately produce the interference fringes at the substrate. 
   In conventional interferometric lithographic systems, in order to change the resolution of the system, it is generally necessary to change a number of parameters of the optical system. Typically, this involves replacing some elements of the optics modules. This procedure can be time consuming. Additionally, replacing optical elements or components frequently requires realignment of the components, further increasing the time that the procedure requires. 
   For example, one of the parameters that needs to change in order to change the resolution of the system is the angle at which the laser beams strike the substrate. This may be viewed as analogous to changing the numerical aperture of the optical system (although, since lenses are not involved in formation of the image in interferometric systems, the numerical aperture at issue is more of a derivative concept). 
   Other parameters that may need to be changed involve how the beams are separated, and the alignment of various optical components needed to produce the interference fringes. Because of the relatively small coherence length of the laser, there is generally little room for the optical designer to work with, in making sure that the two laser beams actually form the required fringes. In other words, the alignments have to be exact, which is often very difficult to achieve in practical systems, particularly where optical components have to be swapped in and out of the system. 
   Having changeable optical elements drives the system complexity and cost upward. Each time a user has to change a beam splitter or a optical module, there is risk of losing critical alignments. 
   Accordingly, there is a need in the art for a system with adjustable resolution and without the alignment complexities of the conventional interferometric lithographic systems. 
   SUMMARY OF THE INVENTION 
   The present invention relates to an adjustable resolution interferometric lithography system that substantially obviates one or more of the disadvantages of the related art. 
   More particularly, in an exemplary embodiment of the present invention, a lithographic system includes a source of a laser beam; a beamsplitter dividing the laser beam into a plurality of beams; and a plurality of reflecting surfaces that forms interference fringes on a substrate using the plurality of beams. Resolution of the lithographic system is adjustable by adjusting angular orientation of the reflecting surfaces. The beamsplitter is movable along the optical path to adjust the resolution. The reflecting surfaces may be facets of a prism. Each reflecting surface corresponds to a particular beamsplitter position along the optical path, and/or to a particular resolution. The beamsplitter comprises a linear grating or a checkerboard grating. The beams are two-way symmetric, four-way symmetric, or N-way symmetric. The resolution is adjustable to any of, for example, 90 nm, 65 nm, 45 nm and 35 nm. A numerical aperture of the system is adjustable by moving the beamsplitter along the optical path. The laser beam includes any of 193 nm light, 248 nm light, and 157 nm light. A liquid can be between the substrate and the prism. 
   Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, 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. 
   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 FIGURES 
     The accompanying drawings, which are included to provide a further understanding 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: 
       FIG. 1  shows an exemplary interferometric lithographic system of the present invention. 
       FIG. 2  illustrates an alternative embodiment of the present invention, where mirrors are used. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
   In one embodiment, the present invention achieves variable resolution of an interferometric lithographic system through the use of a multifaceted pyramidal prism and a movable beam splitter, such as shown in, for example,  FIG. 1 . As shown in  FIG. 1 , a prism  101  is mounted in an optical path that has an optical axis  102 . The optical axis  102  may be thought of as the Z axis of the system. A beam splitter  103  is positioned further upstream in the optical path. The prism  101  is mounted above a wafer (or substrate)  104 , and usually a liquid (not shown in the figure) is circulated between the prism  101  and the wafer  104 , if the system is an immersion lithographic system. It should be noted that the present invention is not limited to immersion lithographic systems, but may also be used with air or other gas between the prism  101  and the wafer  104 . 
   Also shown in  FIG. 1  is a laser beam  106 , which is split by the beam splitter  103  into two or more beams. Shown in  FIG. 1  is the case of two-way symmetry, in other words, the beam  106  is split into two beams, for example, when the beam splitter  103  is in position A, the beam  106  is split into two beams  107 A. When the beam splitter is in position B, the laser beam  106  is split into two beams  107 B, and so forth. The beams  107 A- 107 D, upon traversing the beam splitter  103  (in whichever position A-D it is located), then enter the prism  101 , and internally reflect off of one of the surfaces  105 A- 105 D. The beams then form interference fringes on the wafer  104 . 
   The beam splitter  103  is movable in the Z axis, for example, by being mounted on a Z axis stage (not shown). Each position of the beam splitter  103  (positions A-D) correspond to the beams being reflected off of the appropriate facet  105  of the prism  101 . Note that although four positions of the beam splitter  103  are shown in  FIG. 1 , corresponding to resolutions of 90 nm, 65 nm, 45 nm and 35 nm, for a 193 nm input laser beam  106 , the invention is not limited to these particular resolutions, or to that particular laser beam wavelength. Other wavelengths are possible, such as 365 nm, 248 nm or 157 nm. Also, more or fewer prism facets  105  may be used, to correspond to more or fewer Z axis positions of the beam splitter  103 . Actual resolution is defined by beamsplitter  103  angle position, and prism facet  105  angle. 
   Note further that to improve resolution, generally the wavelength of the laser beam  106  needs to be decreased. Additionally, the index of refraction of the fluid between the wafer  104  and the prism  101  may be changed. Water has an index of refraction of approximately 1.43 (at 193 nm), but other media can have higher index of refraction, thereby allowing to improve the resolution further. Moreover, salts or the like can be added to the water, to increase its index of refraction further. 
   Although the embodiment shown in  FIG. 1  is an example of a two-way symmetric system, the invention is not limited to this embodiment. For example, a four-way symmetric lithographic system can also be designed, with the prism  101  having a square cross section (when viewed upwards into the prism  101 ), and the beam splitter  106  generating four laser beams that collectively form an interference pattern on the wafer  104 . 
   Any number of beam splitters may be used as a beam splitter  103 . The most common type of beam splitter in this application is a grating. Typical pitch of the grating in this case is approximately 0.5-2 microns, although the invention is not limited to any particular grating dimension. 
   The advantage of using a prism such as shown in  FIG. 1  is that there are no alignment difficulties, once the prism  101  is manufactured. Upon placement of the prism  101  in the optical system, the only adjustment is the Z axis adjustment of the beam splitter  103 , which is a relatively straightforward operation, if the beam splitter  103  is placed on a Z axis stage. 
   The prism  101  may be generalized to have N-way symmetry. For example, there may be circumstances when a six-way symmetric interferometric lithographic system may be used. Note also that the number N need not be an even number, in other words, three-way, or five-way symmetries are also possible (corresponding to triangular, or pentagonal cross-sections of the prism  101 ). 
   Typical dimensions of the prism  101  is approximately 100 millimeters long by about 25 millimeters at the bottom face, by about 50 millimeters at the top face, although it will be understood that the invention is not limited to these particular dimensions. Generally, the exact dimensions of the prism, the pitch of the grating of the beam splitter  103 , and the dimensions and orientations of the prism facets  105  depend on the resolution desired, the laser beam wavelength, the index of refraction of the medium between the prism  101 , distances between components and the wafer  104 , and other alignment issues. 
   It will also be appreciated that the present invention allows the adjustment of both the resolution and the equivalent numerical aperture. In many (though not in all) circumstances, these adjustments are dependent upon each other, although there may be circumstances where the equivalent numerical aperture and the resolution may be independently adjustable. 
     FIG. 2  illustrates an alternative embodiment of the present invention, where mirrors  202  are used instead of the prism  101 . The mirrors  202  may be either swapped in and out for different beam splitter  103  positions, or may be rotated and moved to adjust the angles. The effect of the mirrors  202  is essentially the same as the effect of the prism facets  105  in  FIG. 1 . It is currently believed by the inventors that the use of the mirrors  202 , such as shown in  FIG. 2 , is more difficult, since it requires a considerably greater number of independent alignment adjustments. 
   In particular, with reference to  FIG. 2 , the path length of each diffracted beam relative to every other diffracted beam has to be precisely matched. This makes the adjustment of the mirrors  202  in both the angular degree of freedom and the spatial degrees of freedom very critical. 
   At present, it is believed that for a 193 nm laser beam, the optical system such as shown in  FIG. 1  has a resolution limit of approximately 34 nanometers (using water as an immersion medium). The primary limiting factor is the internal reflection angle within the prism  101  (at the bottom face), and the angles that can be achieved. Generally, to improve the resolution further, immersion media with a higher index of refraction needs to be used, or a laser beam with a smaller wavelength, or both. With those factors in mind, it is believed that a resolution of about 20 nanometers may be achieved with this approach. 
   The grating is typically a chrome-on-glass type grating, although the invention is not limited to this particular type of grating, or to the use of a grating generally. For example, a phase shifted etched glass beam splitter may be used. To generate a four-way symmetric set of beams, a checkerboard type grating needs to be used. 
   Note that the dimensions shown in the figure (90 nm, 65 nm, 45 nm and 35 nm) refer to the smallest feature size that can be imaged on the substrate  104 , but the invention is not limited to specific dimensions. 
   The resolution is therefore changed by moving the upper unit beam splitter  103  in the Z-axis so that the beams  107  reflect off the required prism facet  105  in the prism  101 . The resolution is controlled primarily by the angle on the final interfering beams, and hence by the facet angle on the reflecting surface in the prism  101 . In the design shown in  FIG. 1 , there are four preset resolutions: 90 nm, 65 nm, 45 nm, and 38 nm. These resolutions can be changed by replacing the multi-faceted prism  101  with a different one with different predefined facet angles. Change in resolution can be made under computer control by driving a motorized Z-axis with the beam splitter  103 . The design is flexible in terms of the number of different facet angles that can be predefined. In the example shown there are four sets of facets ( 105 A- 105 D). The facets  105 A- 105 D can be based on two-fold, four-fold, or even six-fold symmetry. The actual symmetry requirement is defined by the beam splitter  103  design. 
   The totally reflecting facets  105  can be coated with thin films so that the final reflected wafer polarization is the same as the incident/input polarization, or to generate any preferred polarization state. Likewise, other surfaces could be coated to preserve desired polarization states. 
   Utilizing the design described herein allows for the least complex optical path, cuts costs, maintains alignment with resolution changes, and allows computer control of resolution. 
   Having thus described a preferred embodiment of a system and method, it should be apparent to those skilled in the art that certain advantages of the described method and apparatus have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.