Abstract:
An apparatus for projection lithography is disclosed. The apparatus has at least one magnetic doublet lens. An aperture scatter filter is interposed between the two lenses of the magnetic doublet lens. The aperture scatter filter is in the back focal plane of the magnetic doublet lens system, or in an equivalent conjugate plane thereof. The apparatus also has two magnetic clamps interposed between the two lenses in the magnetic doublet lens. The clamps are positioned and configured to prevent substantial overlap of the magnetic lens fields. The magnetic clamps are positioned so that the magnetic fields from the lenses in the magnetic doublet lens do not extend to the aperture scatter filter.

Description:
PRIORITY APPLICATION INFORMATION 
     This application claims priority of Provisional Patent Application Serial No. 60/158,268 filed Oct. 7, 1999 and entitled “Electron Beam Imaging Apparatus.” 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention is directed to a lithographic process for device fabrication in which charged particle energy is used to delineate a pattern in an energy sensitive material. The pattern is delineated by projecting the charged particle energy onto a patterned mask, thereby projecting an image of the mask onto the energy sensitive material. 
     2. Art Background 
     In device processing, an energy sensitive material, denominated a resist, is coated on a substrate such as a semiconductor wafer (e.g., a silicon wafer), a ferroelectric wafer, an insulating wafer, (e.g. a sapphire wafer), a chromium layer supported by a substrate, or a substrate having a combination of such materials. An image of a pattern is introduced into the resist by subjecting the resist to patterned radiation. The image is then developed to produce a patterned resist using expedients such as a solution-based developer or a plasma etch to remove one of either the exposed portion or the unexposed portion of the resist. The developed pattern is then used in subsequent processing, e.g. a mask to process, i.e. etch, the underlying layer. The resist is then removed. For many devices, subsequent layers are formed and the process is repeated to form overlying patterns in the device. 
     In recent years, lithographic processes in which a charged particle beam is used to delineate a pattern in an energy sensitive resist material have been developed Such processes provide high resolution and high throughput. One such process is the SCALPEL® (scattering with angular limitation projection electron beam lithography) process. The SCALPEL® process is described in U.S. Pat. No. 5,260,151 which is hereby incorporated by reference. 
     Referring to FIG. 1, a doublet lens system  15  is used in the lithography tool for the SCALPEL® process. A first lens system (not shown) is used to direct and focus incident radiation  10  from the radiation source (not shown) onto the mask  20 . The mask  20  is used to pattern particle beam  10 . The entire mask  20  is not illuminated at once. Mask  20 , as shown, consists of a membrane  13 , which is transparent to the particle beams incident thereon, and blocking regions  14 . 
     The developed image of the mask pattern is defined by blocking regions  14 , which scatter the particle beams  10  incident thereon. Unblocked illumination, illustrated as beams  12 , is transmitted through the membrane regions  13 . Blocked illumination, illustrated as beams  11  is caused to converge by means of a first electromagnetic/electrostatic projector lens  30  in lens system  15 . Filter  19  is an aperture scatter filter. The aperture scatter filter  19  is designed so that the unscattered radiation (beams  12 ) passes through the aperture  21  therein. The scattered radiation  11  is blocked by the aperture scatter filter  19 , which is located in the mutual focal plane of the lenses  30  and  31 . 
     Second projector lens  31  of lens system  15  is of such configuration and so powered as to bring the unscattered beams  12  into an approximately parallel relationship. The action of the lens  31  is sufficient to direct beams  12  into orthogonal incidence onto wafer  24 . 
     Lens system  15  consists of two lenses. Consequently, the lens system is referred to as a doublet electromagnetic lens arrangement. Such a doublet electromagnetic lens arrangement is described in Waskiewicz, W., et al., “Electron-optics method for High-Throughput in a SCALPEL system: preliminary analysis,” Microelectronic  Engineering , Vol. 41/42, pp. 215-218 (1998). The doublet electromagnetic lens system described in Waskiewicz et al. provides telecentric reduction imaging from the mask to the wafer. Such an arrangement uses two lenses of similar construction. The lenses are laid out sequentially and separated by a distance equal to the sum of their two focal lengths. Referring again to FIG. 1, the object (i.e. the mask  20 ) is located in the back focal plane of the first projector lens  30  of lens system  15 . An image of the mask  20  is formed at the front focal plane (i.e. the layer of energy sensitive material  23  on wafer  24 ) of the second projector lens  31  of second lens system  15 . The magnification provided by the lens system is determined by the ratio of the focal length of lens  30  to the focal length of lens  31 . The bore (D) to gap (S) ratio for both lenses are identical and the excitations (NI) are set equal but opposite. 
     When designed properly, the doublet lens not only substantially eliminates the rotation introduced into the image by an individual lens in the doublet, but also eliminates rotation-related aberrations in the image. These aberrations are primarily chromatic aberrations. Removing these aberrations provides the lowest total image blur. Doublet lens systems are described in Heritage, M. B., “Electron-projection microfabrication system,”  J. Vac. Sci. Technol ., Vol. 12, No. 6, pp. 1135-1140 (1975), which is hereby incorporated by reference. 
     In the classic magnetic doublet design, the first and second lenses are separated along their common optical axis to ensure that there is a space between the lenses that is field-free. The field-free space is a space that is not affected by the magnetic field generated by the lenses. Typically, both lenses have a common focal length (F) within this field-free space. Such an arrangement is illustrated in FIG.  2 . FIG. 2 illustrates the magnetic flux as a function of distance along the optical axis relative to the position of magnetic lenses  30  and  31 . In the region between lenses  30  and  31  the magnetic flux is zero. This is the desired field-free space. 
     However, in certain applications, design constraints do not permit the spacing between the first and second lenses that provides for a field-free space. In the lithography tool for the SCALPEL® process, for example, the mutual focal plane of lenses  30  and  31  is at the apertured scatter filter  19 . Furthermore, in order to increase the speed at which the image is written (and thereby to achieve the desired throughput from the tool) the electron beam scans about the optical axis. In order to control the off-axis aberrations, e.g. astigmatism, that result from off-axis scanning, the bore of the doublet lens is increased while the axial separation between the two lenses either remains the same or is shortened to control space-charge blur. Consequently, the magnetic fields of the doublet lens overlap. This problem is illustrated in FIG.  3 . In FIG. 3, the magnetic flux of each lens is affected by this overlap. This is observed with reference to dashed line  50  in FIG.  3 . Observe that, due to the proximity between lenses  30  and  31 , the flux as a function of axial position for lens  31  on one side of line  50  is not a mirror image of flux as a function of axial position on the other side of line  50 . Thus, the desired axial magnetic field symmetry for lens  31  (and for lens  30 ) in FIG. 3 is not preserved. 
     This overlap causes field distortion. Also, the apertured scatter filter  19  is immersed in the magnetic field of lenses  30  and  31 . Since this field overlap compounds aberrations and total blur growth and also causes projection magnification changes. Consequently, a solution to the magnetic field overlap of the projection lens doublet that is compatible with the SCALPEL® tool design is sought. 
     SUMMARY OF INVENTION 
     The present invention is directed to a magnetic doublet lens system in which the spacing between the two lenses is such that their magnetic fields overlap. The magnetic doublet lens system is equipped with magnetic clamps that effect substantial separation of the magnetic fields. The magnetic clamps are made of a ferromagnetic material. The present invention is also directed to an apparatus for electron beam lithography that has a magnetic doublet lens system that is equipped with magnetic clamps to effect substantial separation of the magnetic fields between the two lenses. Substantial separation, in the context of the present invention, is sufficient separation of ensure that the magnetic field of one lens in the doublet lens system is not adversely affected by the magnetic field of the other lens in the magnetic doublet lens system. Adverse affects are doublet compound image aberrations, total blur growth and projection magnification changes attributable to magnetic field overlap. However, one skilled in the art will appreciate that the magnetic clamps have a configuration and placement that preserves the symmetry of the magnetic doublet lens and the common focal plane of the two lenses in the magnetic doublet lens system. For example, if the magnetic doublet lens is designed for 4:1 image demagnification (i.e., image reduction), then the magnetic clamps are designed to preserve this relationship. An example of a suitable clamp design for such a magnetic doublet lens is one in which the dimensional relationship (i.e. for cylindrical clamps the ratio of height and diameter) is also 4:1. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an electron beam lithography tool. 
     FIG. 2 illustrates the magnetic flux of a magnetic doublet lens arrangement in which the lenses are separated by a field-free space. 
     FIG. 3 illustrates the magnetic flux of a magnetic doublet lens arrangement in which the magnetic flux from each lens overlaps. 
     FIG. 4 is a schematic cross-section of a doublet lens with magnetic clamps that effect separation of the magnetic fields associated with the two individual lenses in the doublet. 
     FIG. 5 is a schematic cross-section of an alternative embodiment of a doublet lens with magnetic clamps in which magnetic clamps are affixed at both ends of the doublet lens. 
     FIG. 6 is a schematic cross-section of a doublet lens system of the present invention that illustrates the separation of the magnetic fields from the two lenses in the lens system that is provided by the magnetic clamps. 
     FIG. 7 is a schematic cross-section of a doublet lens system of present invention that provides for 4:1 demagnification of the image in the beam transmitted through the lens. 
    
    
     DETAILED DESCRIPTION 
     In the present invention, magnetic clamps are inserted between the individual lenses in a magnetic doublet lens system. In the embodiment of the present invention wherein the magnetic doublet lens system with the magnetic claims is inserted into an electron beam lithography tool, an apertured scatter filter is inserted in an essentially field-free space between the two lenses, wherein the essentially field-free space is provided by the magnetic clamps. 
     The essentially field-free space is obtained by using the magnetic clamps to effect substantial separation of the magnetic fields of the two lenses. By substantially separating the magnetic fields, doublet compound aberrations, total blur growth and projection magnification changes attributable to magnetic field overlap are avoided. 
     For example, magnetic lenses have a spherical aberration co-efficient (C sph ) that is proportional to the integral of the magnetic field flux density first derivative squared (dB/dz) 2 dz. This is characterized by the following formula: 
     
       
         C sph   − ∫(dB/dz) 2 dz. 
       
     
     Other aberrations and distortions depend on the field distribution B(z) in the same way. Any distortion in the magnetic field is likely to add aberrations and distortions into the final image. In the present invention, magnetic clamps are designed to prevent distortions in the magnetic field that are caused by overlap of the magnetic fields in the magnetic doublet lens system. However, the magnetic clamps are also designed and placed to preserve the symmetry of the magnetic doublet lens. As one skilled in the art is aware, symmetry is required to maintain beam rotation and related anisotropic aberrations within the limits required for acceptable imaging. 
     A schematic of one embodiment of the present invention is illustrated in FIG.  4 . FIG. 4 illustrates a cross-section of a magnetic lens doublet system  100 . The magnetic lens doublet system has a first lens  110  and a second lens  120 . Lens  110  is equipped with magnetic clamp  111 . Lens  120  is equipped with magnetic clamp  121 . Magnetic clamps  111  and  121  are a ferromagnetic material, such as soft iron or ferrite. The size, configuration and location of the magnetic clamps are determined by a number of factors. The first factor is that the magnetic clamps prevent the fields from lenses  110  and  120  from substantially penetrating into the region  125  between the magnetic clamps. In the embodiment of the present invention wherein the lens system is placed in an electron beam lithography tool, the apertured scatter filter  130  is placed in region  125 . The second factor is that the magnetic clamps must be configured so as not to interfere with the radiation transmitted through the lens system. The third factor is that the magnetic clamps must be sized to preserve the symmetry of the doublet. That relationship is reflected by symmetry of the doublet about the common focal plane of the lens. As previously noted, the desired symmetry of the axial magnetic field of a lens is not preserved when the magnetic fields of the two lenses in the magnetic doublet lens system overlap. Also, if the magnetic doublet lens system provides for a  4 : 1  image reduction, the magnetic lenses must have a size and a placement along the lens system focal length that preserves that relationship. 
     Another embodiment of the present invention is illustrated in FIG.  5 . In this embodiment, each lens,  210  and  220  of magnetic doublet lens  200  has two magnetic clamps. Lens  210  is equipped with lenses  211  and  212 . Lens  220  is equipped with clamps  221  and  222 . As in the previous embodiment, an apertured scatter filter  230  is placed in the field-free space  225  between lens  210  and lens  220 . 
     The doublet of the projection lens system of the present invention is described with reference to FIG.  6 . The lens  310  of doublet lens  300  generates a field  315  (drawn as a series of lines). The field  315  is contained by magnetic clamp  311 . Similarly, the lens  320  of doublet lens  300  generates a field  325  (drawn as a series of lines). The field  325  is contained by magnetic clamp  321 . As illustrated in FIG. 6, the magnetic field lines  315  and  325  do not extend into the space  330  that contains the apertured scatter filter  335 . Lens  310  is connected to magnetic clamp  311  via connector  339 . Lens  320  is connected to magnetic clamp  321  via connector  340 . Connectors  339  and  340  are a magnetic material such as ferrite or soft iron. 
     EXAMPLE 
     The following example is described with reference to FIG.  7 . FIG. 7 is a schematic of a magnetic doublet lens system placed in an electron beam lithography tool. The tool  400  has an optical axis  405 . The magnetic doublet lens system  410  is placed between the mask plane  411  and the image plane  412 . The magnetic doublet lens system  410  has a first lens  415  and a second lens  420 . Both lenses  415  and  420  have wound cores and soft iron bodies. First lens  415  is coupled to a first magnetic clamp  416 . Second lens  420  is coupled to a second magnetic clamp  421 . The clamps are the same material as the body of the lens (soft iron). An apertured scatter filter  425  is placed between the first magnetic clamp  416  and the second magnetic clamp  421 . 
     The lens system  410  is configured to demagnify an image of the mask  411 . The degree of demagnification is 0.25 (i.e., an image reduction of 4:1). The demagnified image is transmitted into an energy sensitive material on a wafer in image plane  412 . The distance between the mask plane  411  and the apertured scatter filter is 320 mm. The distance between the image plane  412  and the apertured scatter filter  425  is 80 mm. 
     The lens system  410  is centered about the optical axis  405 . Using the position of the apertured scatter filter  425  on the optical axis  405  as the zero reference point, the focal length of the first lens is −160 mm. The focal length of the second lens is 40 mm. The focal length of lens  415  is illustrated by the distance from the point Z 1A  to the apertured scatter filter  425  along the optical axis  405 . The focal length of lens  420  is illustrated by the distance from the point Z 1B  to the apertured scatter filter  425  along the optical axis  405 . 
     Lens  415  defines an opening D A  that is 120 mm. The Internal length G A  of lens  415  is also 120 mm. Lens  420  defines an opening D B  that is 30 mm. The internal length G B  of lens  420  is also 30 mm. 
     First magnetic clamp  416  defines an opening D CA  that is 40 mm. The first magnetic clamp  416  is a distance S CA  (80 mm) in a direction parallel to the optical axis. Second magnetic clamp  421  defines an opening D CB  that is 10 mm. The second magnetic clamp  421  is a distance S CB  (20 mm) from lens  420  in a direction parallel to the optical axis. Thus the 4:1 image reduction is achieved by a 4:1 relationship between the first lens  415  and the second lens  420 . 
     The performance of the above described lens system was modeled. The performance of a system without the first and second magnetic clamps  416  and  421  (but otherwise identical) was also modeled. The performance of the two systems was then compared. The modeling was performed using second-order finite element modeling software from Munro&#39;s Electron Beam Software Ltd. of London, England. 
     The comparative results are summarized in the following table. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Without Clamps 
                 With Clamps 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Lens Excitation (AT) 
                 2112.71 
                 2103.06 
               
               
                   
                 Magnification 
                 0.2509 
                 0.2501 
               
               
                   
                 Rotation Angle (mrad) 
                 −0.946 
                 −0.0262 
               
               
                   
                 Landing Angle (mrad) 
                 0.12 
                 0.12 
               
               
                   
                 Beam Blur at center (nm) 
                 19.3 
                 19.3 
               
               
                   
                 Beam Blur at corner (nm) 
                 33.0 
                 32.6 
               
               
                   
                   
               
             
          
         
       
     
     The comparison provided in Table 1 demonstrates the benefits of magnetic clamps. Specifically, the system without clamps had a much lower rotation angle in the region in which the apertured scatter filter was located compared to the system without clamps. This demonstrates that the field effects in the apertured scatter filter region were much lower in the system without clamps compared to the system with clamps. Furthermore, this improvement was obtained without an adverse effect on magnification, landing angle or beam blur. Also, as demonstrated by the reduction in lens excitation for the lens system with clamps, the lens system of the present invention is more efficient than a lens system without such clamps. 
     Although the present invention has been described in terms of numerous examples, one skilled in the art will appreciate that numerous other embodiments are within the scope of the following claims. Consequently, the preceding examples should not be construed as limiting the present invention in any way, except in a manner that is consistent with the following claims.