Abstract:
A multi-beam e-beam system employs a set of independently controllable (for blanking and deflection) subsystems placed in a solenoid field, each system having a demagnifying lens comprising at least one passive pole piece, so that the final image demagnifies imperfections in the upstream electron beam. Upper and lower sections of the system employ the focusing effect of the solenoid field to form an image at a shaping aperture and a demagnified image of the beam at the shaping aperture on the workpiece. Small focus corrections due to magnetic lens field non-uniformity and/or target height variations, are accomplished with an electrostatic unipotental lens built into the pole pieces and target voltage variations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     U.S. patent application Ser. No. 10/132,896, assigned to the assignee hereof and filed concurrently herewith discloses types of lenses useful in the practice of the subject invention and is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention is multi-beam electron beam lithography. 
     BACKGROUND OF THE INVENTION 
     It is generally accepted in the mask industry that single beam electron beam mask writers will not be able to deliver the current density at high resolution to achieve exposure speeds required for products below 100 nm GR (Ground Rules). The usable current in probe forming systems is limited by stochastic Coulomb interactions, primarily at apertures or beam crossovers, which translates into loss of resolution. By contrast, multibeam systems suffer much less from this problem since the total current delivered to the target is spread over many beams, in most cases, each with its own apertures and crossovers. 
     Multibeam systems proposed to date have problems with manufacturing feasibility primarily because of unattainable stability and uniformity requirements placed on the electron sources, i.e. field emitters and photocathodes. Some of these systems employ multibeams through part of the optics column but share the same crossover, which does not improve the electron interaction problem. Their stability requirements are further magnified since they typically use 1 to 1 imaging of the source at the target. 
     A uniform magnetic field (solenoid field) oriented along the electron beam axis is the simplest of electron lenses and has been employed in various electron beam systems. Electrons radiating from a point object execute, by virtue of their transverse velocity component, one cyclotron orbit in the transverse plane, returning to the optic axis. Thus, an image is formed with unity magnification. A major advantage of the solenoid lens is that there is no prescribed optic axis, hence a shift (deflection) of the beam by a transverse field will cause the beam to shift position, but maintain the same focal plane. A major disadvantage of these lenses is that they produce no demagnification of the object, so that defects in the source (reticle, shaping aperture) are reproduced in the image at the same scale. 
     The restriction of lenses formed by solenoid fields in the prior art to a one-to-one object to image ratio imposes severe limitations on the image quality. It is well known that the conventional object to image ratio of 4:1 in optical steppers is more “forgiving”, than a 1:1 demagnification ratio. 
     Such a 1:1 magnification ratio in a multiple-beam system is illustrated in U.S. Pat. Nos. 6,175,122, 5,981,962 and 5,962,859, which show a plurality of shaped-beam systems, contained within the same solenoid field. In such a system, imperfections in the aperture result in the same imperfections in the image, thus limiting resolution. 
     SUMMARY OF THE INVENTION 
     The invention relates to a multi-beam lithography system in which a set of electron beam sub-systems having a substantial demagnification are immersed in a solenoid field and operate in parallel. 
     A feature of the invention is the use of a single solenoid field common to several electron beam sub-systems. 
     Another feature of the invention is the use of magnetic lenses having substantial demagnification, so that imperfections in the object are reduced in the image by the demagnification factor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an embodiment of the invention in cross section. 
     FIG. 2 shows a top view of an embodiment of the invention. 
     FIG. 3 shows a top view of an alternative embodiment of the invention. 
     FIGS. 4A,  4 B and  4 C show a cross section of a portion of the invention, an associated beam trace and an associated plot of field strength. 
     FIGS. 5A, and  5 B show a cross section of another embodiment of the invention and an associated plot of field strength. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown in cross section an embodiment of the invention having three independently controlled variable shape electron beams apparatus immersed in a common solenoid field. These subsystems (subsystems in the overall multi-beam system) are positioned in close proximity (on the order of 20 mm center-to-center) and simultaneously expose an array of stitched subfields that together expose the full field pattern, illustratively an integrated circuit. The imaging system employs high demagnification of the object, thus suppressing flaws in the source (i.e. the surface of a shaping aperture). 
     The overall solenoid magnetic field is provided by coils  12 - 1 ,  12 - 2  and  12 - 3  in sections  100 ,  200  and  300 , respectively. A capped cylinder of any high permeability magnetic material  6 , encloses the coils, except for a gap  65  at the bottom for insertion of a workpiece, such as a resist coated substrate for glass mask production, reticle for projection lithography systems or wafer for direct write integrated circuit exposure. The cap also shields the electron beam from undesirable stray magnetic field influences. 
     In section  100  of each subsystem, electron gun  105 , illustratively a conventional cathode of LaB 6  crystal that can be controlled and served individually to provide high stability and uniformity, generates the subsystem beam. Electrons emitted from gun  105  are accelerated to anode  107 . A first shaping aperture  109  in plate  110  permits the passage of an electron beam having a square cross section, illustratively 175 μm on a side. Electrostatic deflection plates  112  and  114  deflect the square beam over second shaping aperture  210  in plate  205  to position the square beam from the first shaping aperture appropriately with respect to aperture  210 . As is described in U.S. Pat. No. 4,945,246 for a single beam system, each column generates a shaped beam having a shape that may be a vertical line, a horizontal line or a rectangle of desired shape by deflecting the square beam from aperture  109  such that only a beam of the desired shape passes through aperture  210 . A solenoid field generated in Section  100  by coil  12 - 1  focuses an image of the beam emerging from aperture  109  at the plane of aperture  210 . The number of ampere turns in Section  100  is selected in conjunction with the accelerating voltage of the beam to provide a beam focus in the desired transverse plane. Optionally, a plate  205  of the same magnetically permeable material as enclosure  6  separates the first and second sections of the system. An aperture  206  in plate  205  is oversized to permit the beam to strike shaping aperture  210  without striking plate  205 . 
     In Section  200 , a demagnifying lens constructed according to the teachings of the referenced copending patent forms a demagnified image of the beam emerging from aperture  210  near the bottom of Section  200 . The lenses will be referred to as “passive” since they are not energized by current within the lens, but achieve a focus by affecting the external solenoid field. High permeability plates  205  and  255  separate the magnetic fields in the three sections, reducing the load on the drivers that power the separate solenoid coils and providing the ability to have several different strength solenoids stacked one on top of each other and thus vary the focal length of each section independently. Illustratively, the coil  12 - 2  for Section  200  is energized with about 2,000 ampere-turns, compound lenses  230  have a magnification of 0.0114, the current density in a beam is about 100A/cm 2 , and the system generates beam “flashes” having a duration of approximately 50 ns, depending on resist sensitivity, beam energy and current density. 
     The alignment, shaping, blanking and deflection of each beam is accomplished in each subsystem with electrostatic fields to prevent coupling between adjacent beams and to assure that the focal planes in the solenoid field are not affected. Proper alignment of the beam can be assured throughout the column by superimposing offset voltages on the aforementioned shaping, blanking and deflection elements, as is standard in the art. Conventional electronic circuits for supplying DC voltage, driving the coils and the electrostatic deflectors are shown schematically by box  175  in the lower right of the Figure. Minor refocussing to compensate for demagnification lens field variations and target height changes can be accomplished by introducing a weak Unipotential (Einzel) lens (most easily by utilizing the magnetic lens pole pieces as ground elements and placing a biassed aperture between them), and/or applying a bias voltage to the substrate  60 . 
     At the bottom of the Figure, Section  300  contains deflector plates  312  and  314  that position each beam at a desired location on workpiece  60 . In this Figure, workpiece  60  moves in and out of the plane of the drawing on a conventional stage shown schematically as box  66 . 
     A preferred embodiment of the invention having demagnifying lens  230 , having upper pole tips  232 , lower pole tips  234  and high permeability block  236  produced demagnification greater than 80×, while maintaining spherical and chromatic aberration below 4 mm. Other parameters are: a beam voltage of 10 keV, magnification in the upper lens  232  of 0.133, magnification in the lower lens  234  of 0.086, giving a total demagnification of 87 with C SI =3.25 mm, C C =3.83 mm. The excitation of solenoid  12 - 2  was 2000 ampere-turns. The diameter of magnetic intermediate piece  236  was nominally the same as the electrostatic deflectors and 20 mm. The diameter of the bore through the pole pieces was 4 mm and the lens gaps were 4 mm for the upper and lower sections, with 7 mrad aperture half angle at the second demagnification lens image plane. The aperture in the plane of plate  210  is the object and the plane of plate  255  is the approximate location of the second image plane in this case. At the intermediate image plane  212 , there is a limiting aperture that defines the final semi-angle at the target and also minimizes the isotropic off axis aberrations. 
     Illustratively, the approximate length of the first section was 200 mm, the second 150 mm and the third section 150 mm (to target) for a total system length of approximately 550 mm. 
     Passive pole pieces  230  in the second section are supported by non-magnetic materials. The deflectors in the first and third sections are supported by non-magnetic, non-conducting materials not shown in the drawing for simplicity. 
     Referring now to FIG. 2, there is shown a top view of some alternative embodiments of the invention. Illustratively, the system is to write a pattern directly on a wafer or on a mask, which is later to be used in a stepper to expose integrated circuit patterns. For a typical 4× stepper, the area to be exposed extends 3 cm by 4 cm, so that the e-beam system mask must cover an area of 12 cm by 16 cm. Boxes  501 - 506  in the group of two rows denoted by bracket  500  represent schematically e-beam subsystems constructed according to the invention that have an illustrative deflection range of +/−11 mm in the x and y directions. The e-beam systems in the two rows denoted are separated horizontally by 20 mm (e.g. the center of subsystem  503  is 40 mm from the center of subsystem  501  and the x-position of subsystem  502  is located midway between them), so that there is an overlap region of 2 mm in the x-direction. The first and second rows are displaced for convenience in displaying the Figure. Preferably, the two rows are placed with a distance between centers of 20 mm, denoted by bracket  508 , and therefore have the same overlap of 2 mm in the y-direction. 
     Confining our attention for the moment to the group of a single row divided into even and odd sub-rows together labelled  500 , a preferred method of operation is to transport the workpiece mechanically (on stage  66  in FIG. 1) vertically downward in FIG.  2 . Initially, the systems  502 ,  504 ,  506  in the even subrow write a pattern in a first horizontal strip extending 2 cm in the y-direction and subsystems  501 ,  503  and  505  do not write. Next, the stage is moved by 2 cm in the y-direction, so that the spaces not covered by the even subrow (systems  502 - 506 ) in the first step are now covered by systems  501 - 505  in the odd subrow. The remainder of the first horizontal strip is then written by systems  501 - 505  while simultaneously systems  502 - 506  write the next horizontal strip. It will be evident to those skilled in the art that nine iterations will write the desired 16 cm in the y-direction. In the first step, subsystems  501 ,  503  and  505  do not write and in the ninth step, subsystems  502 ,  504  and  506  do not write. 
     Alternatively, additional rows denoted with brackets  510 - 570  could be provided, so that the groups collectively cover the area to be written. In that case, just two steps will write out the area. On the first step, subsystems  501 ,  503  and  505  do not write and on the second step, subsystems  572 ,  574  and  576  do not write. 
     FIG. 3 illustrates an alternative embodiment, in which the systems  501 ′,  502 ′,  506 ′ are all in the same single row. In that case, a single step will write a horizontal strip 2 cm along the y-direction. In this embodiment also, additional rows can be provided to reduce the number of steps. When the groups  500 ′,  510 ′, . . .  570 ′ (eight groups) of rows collectively cover the chip, the entire chip can be written in a single step. 
     In this discussion, it has been assumed that the spacing between groups (rows) is related to the coverage of a group along the y-axis so that one step will bring the top of the nth group to the bottom of the (n+1)th group (not counting overlap). This is not required in general, and the spacing could be made greater, so that it takes k steps to bring the top of the nth group to the bottom of the (n+1)th group, whether an individual group is a single row, as in group  500 ′ of FIG. 3, or staggered rows, as in group  500  of FIG.  2 . This approach would reduce the complexity of the hardware and require a longer time to write the entire pattern. 
     Referring to FIG. 4A, a simplified passive lens for use in the invention modifies the magnetic field lines to form a demagnification lens. Illustratively, the material is Ferrite™, a ceramic with high magnetic permeability, available from the Ceramic Magnetics company. As is conventional, coils  10  and  12 , forming a solenoid field and pole piece  30  have cylindrical symmetry. The axial solenoid field  20  is modified by pole piece  30  to have a very strong peak in the pole piece gap (also referred to as the lens gap)  33  with negative side lobes (relative to the uniform solenoid field). Pole piece  30  has flat top and bottom surfaces  34  and two pole tips  32 , having outer surfaces that make an acute angle with respect to the solenoid axis  101 . In general, the closer the outer surfaces of pole tips  32  are to the vertical, the sharper the peak in magnetic field trace  130  in FIG.  4 C and the deeper the dips in field strength  132  and  134 . Preferably, the pole tip surfaces have an angle of less than 45° with respect to the geometric axis. This pole piece configuration has been shown to easily provide demagnification in the 10× range (shown in beam trace  120  in FIG.  2 B), with spherical and chromatic aberration coefficients below 3 mm. 
     Those skilled in the art will appreciate that the unexpectedly low value for the spherical aberration results from the ability of these lenses to create the dips in magnetic field strength  132  and  134 , which have no counterparts in a conventional lenses driven by coils contained within the pole pieces. 
     To achieve even higher demagnification, two or more of these lenses can be used in the same solenoid field, illustrated in FIGS. 5A and 5B. There, pole pieces  32  are the same as those in FIG.  4 A. Segments  34  of the poles are not used in this illustration, but could be added to further strengthen the lens field in the pole piece gap, and thereby increase the demagnification. An optional permeable member  36 , of the same permeable material, merges with lower pole piece  32  of the upper pair and with the upper pole piece  32  of the lower pair, so that a single piece of material  36  conducts the field lines from the upper gap to the lower gap. A single piece eliminates problems with misalignment between the pieces, but is not required. So long as the three pieces abut and carry the field lines, separate pieces can be used. Filling the region between the two lenses with high permeability magnetic material produces a field free region that can be used for separation and demagnification purposes. 
     While the invention has been described in terms of a few preferred embodiments, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.