Patent Number: 
Section: 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 (Csph) that is proportional to the integral of the magnetic field flux density first derivative squared (dB/dz)2dz. This is characterized by the following formula: Csphxe2x88x92∫(dB/dz)2dz. 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. 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 xe2x88x92160 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 Z1A 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 Z1B to the apertured scatter filter 425 along the optical axis 405. Lens 415 defines an opening DA that is 120 mm. The Internal length GA of lens 415 is also 120 mm. Lens 420 defines an opening DB that is 30 mm. The internal length GB of lens 420 is also 30 mm. First magnetic clamp 416 defines an opening DCA that is 40 mm. The first magnetic clamp 416 is a distance SCA (80 mm) in a direction parallel to the optical axis. Second magnetic clamp 421 defines an opening DCB that is 10 mm. The second magnetic clamp 421 is a distance SCB (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""s Electron Beam Software Ltd. of London, England. The comparative results are summarized in the following table. 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.