Abstract:
A method and apparatus for aligning a charged particle beam with an aperture includes providing a hollow beam aperture means adapted for shaping a charged particle beam into a hollow charged particle beam. Then direct the charged particle beam through the aperture. Provide deflection coils for deflecting the charged particle beam relative to the aperture. Vary the current to the alignment deflection coils while measuring the aperture electrical current generated by charged particles reaching the hollow beam aperture as a function of the current to the alignment deflection coils. Then adjust the current in the alignment deflection coils based on the aperture electrical current to center the charged particle beam on the hollow beam aperture. Preferably, separate hollow beam and peripheral beam apertures with associated sensing and current are used to center the beam on respective ones of the apertures.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to particle imaging systems and more particularly to methods and apparatus for alignment of charged particle beam projection lithography systems.  
           [0003]    2. Description of Related Art  
           [0004]    A problem with using charged particle projection optics has been the adverse effects of space charge upon beam projection optics. To ameliorate such effects by enhancing charged particle projection optics, hollow beam annular illumination technology is being developed because hollow beam annular illumination technology can suppress space charge effects dramatically. Hollow beam annular illumination with its reduced space charge problems, permits high throughput charged particle lithography equipment to be produced, because with a hollow beam even a projection system which employs a high beam current can produce well focussed images.  
           [0005]    U.S. Pat. No. 5,821,542 of Golladay for “Particle Beam Imaging System Having Hollow Beam Illumination” states that “throughput is significantly lower for e-beam exposure systems than for photoexposure systems, thus making e-beam tools too costly for general production.” The Golladay patent states further “Higher throughput in e-beam lithography systems can presently be achieved by increasing the e-beam current, but only with an unacceptable degradation in resolution. The degradation in resolution can be attributed to interactions between electrons within the electron beam. The natural repulsion between electrons, due to having charges of the same polarity, causes a number of deleterious effects which limit resolution at the workpiece . . . . ” To overcome the above stated problem the Golladay patent describes a charged particle beam imaging system in which an annular aperture comprises a central circular area which is substantially non-transmissive to a beam of charged particles and a first ring shaped area which is substantially transmissive to the beam of charged particles surrounded by a second ring-shaped area which is substantially non-transmissive to the beam.  
           [0006]    U.S. Pat. No. 5,834,783 of Muraki et al. for “Electron Beam Exposure Apparatus and Method and Device Manufacturing Method” and U.S. Pat. No. 5,973,332 of Muraki et al. for “Electron Beam Exposure Method, and Device Manufacturing Method Using Same” describe an E-beam exposure apparatus including a “hollow beam forming stop . . . whose central portion is shielded . . . .” Muraki et al. states “Since the space charge effect of hollow electron beam (hollow cylindrical beam) is smaller than that of a nonhollow electron beam (e.g. a Gaussian beam), the electron beam can be brought to focus on the wafer to form a source image free from any blur on the wafer . . . the electron density at the peripheral portion becomes higher than that at the central portion.” 
           [0007]    As is discussed below in more detail, even with the above described advantages of hollow beam systems, there are problems with alignment of the charged particle beam with the apertures which define the configuration of the hollow beam.  
         SUMMARY OF THE INVENTION  
         [0008]    A method and apparatus for aligning a charged particle beam with an aperture includes providing a hollow beam aperture means adapted for shaping a charged particle beam into a hollow charged particle beam. Then direct the charged particle beam through the aperture. Provide deflection coils for deflecting the charged particle beam relative to the aperture. Vary the current to the alignment deflection coils while measuring the aperture electrical current generated by charged particles reaching the hollow beam aperture as a function of the current to the alignment deflection coils. Then adjust the current in the alignment deflection coils based on the aperture electrical current measured to center the charged particle beam on the hollow beam aperture. Preferably, separate hollow beam and peripheral beam apertures with associated sensing and current are used to center the beam on respective ones of the apertures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The foregoing and other aspects and advantages of this invention are explained and described below with reference to the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1A is a top view of a cylindrically shaped composite peripheral and hollow beam aperture for an E-beam projection system for shaping an E-beam in accordance with this invention.  
         [0011]    [0011]FIG. 1B is a sectional view taken along line  1 B- 1 B′ in FIG. 1A of the composite peripheral and hollow beam aperture of FIG. 1A.  
         [0012]    [0012]FIG. 1C is a perspective sectional view, taken along line  1 B- 1 B′ in FIG. 1A, of the composite peripheral and hollow beam aperture of FIG. 1A, showing an E-beam directed onto and through the aperture.  
         [0013]    [0013]FIGS. 2A and 2B are similar views to those of the annular aperture in FIGS. 1A and 1B which show an example of a misaligned annular aperture with an alignment fabrication error.  
         [0014]    [0014]FIG. 3A shows a perspective view similar to FIG. 1C of an E-beam exposure system with the E-beam directed onto a composite aperture with a hollow beam directed onto a reticle.  
         [0015]    [0015]FIG. 3B shows a beam intensity distribution of a collimated E-beam entering the composite aperture of FIG. 3A.  
         [0016]    [0016]FIG. 3C shows a beam intensity distribution of scan of a collimated hollow E-beam leaving the composite aperture of FIG. 3A.  
         [0017]    [0017]FIG. 4 is a graph showing an example of monitored aperture current amplitude of the hollow E-beam method of FIG. 3A which is less than the exemplary result desired.  
         [0018]    [0018]FIGS. 5A and 5B show alternate dual aperture embodiments of several improved methods and apparatus, in accordance with this invention, which are described below.  
         [0019]    [0019]FIG. 5C shows additional details of the embodiment of FIG. 5A.  
         [0020]    [0020]FIG. 5D is a graph which shows an example of a bell shaped curve of current in upper E-beam of FIGS. 5A and 5C.  
         [0021]    [0021]FIG. 5E is a graph which shows an example of the narrowed aperture current profile of the intermediate E-beam monitored inside the hollow cylinder in the lower portion of the upper aperture of FIGS. 5A and 5C.  
         [0022]    [0022]FIG. 5F is a graph that shows the hollow E-beam profile of the lower aperture current of E-beam monitored in the lower aperture in FIGS. 5A and 5C.  
         [0023]    [0023]FIG. 6 shows a graph of “aperture current amplitude” of current received by the peripheral aperture current monitor of FIGS.  5 A/ 5 C as the E-beam is scanned across the upper, peripheral aperture with a minimum value when the E-beam is centered over the upper, peripheral aperture.  
         [0024]    [0024]FIG. 7 shows a graph of “aperture current amplitude” of current received by an aperture current monitor of FIGS.  5 A/ 5 C as an intermediate E-beam is scanned across a hollow beam aperture, with a very clear maximum value when the E-beam is centered over the hollow beam aperture.  
         [0025]    [0025]FIG. 8A shows an aperture current monitor which includes an amplifier and an A/D converter that provide an output signal to a computer system seen in FIG. 8B.  
         [0026]    [0026]FIG. 8B shows a computer system for controlling adjustment of E-beam alignment and centering onto the apertures by varying the current to the aligners by varying of the current through the respective apertures in FIGS.  3 A,  5 A- 5 C as adjusted by varying the excitation current of the aligners in accordance with the algorithm in FIG. 10A.  
         [0027]    [0027]FIG. 9A shows a schematic block diagram of a process for manufacture of a semiconductor chip adapted to employing the apparatus and the method of this invention.  
         [0028]    [0028]FIG. 9B shows a flow chart of lithography steps of FIG. 9A which are dominant steps in the wafer processing steps of this invention.  
         [0029]    [0029]FIG. 10A shows an algorithm for controlling E-beam alignment and centering onto the an aperture which is performed by varying the current to the aligners, by varying of the current through an aperture in FIGS. 3A, 5A,  5 B and  5 C as adjusted by varying the excitation current of the aligners under control the computer system shown in FIG. 8B.  
         [0030]    [0030]FIG. 10B shows another related algorithm which is employed with the embodiments of FIGS.  5 A- 5 C where there is a lower aperture onto which the E-beam needs to be centered. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    FIGS.  1 A- 1 C show various views of a composite peripheral and hollow electron beam annular aperture  10  for shaping an E-beam  24  into a hollow E-beam  96 / 97  for illumination (E-beam exposure) of a target comprising workpiece  60 , shown in FIG. 3A which is shown as a reticle.  
         [0032]    [0032]FIG. 1A is a top view of the cylindrically-shaped, electrically-conductive-metallic, composite peripheral-and-hollow E-beam aperture  10  connected to an electrical circuit through electrical output line  50 .  
         [0033]    [0033]FIG. 1B is a sectional view taken along line  1 B- 1 B′ in FIG. 1A through the central axis of the composite peripheral and hollow E-beam aperture  10 .  
         [0034]    [0034]FIG. 1C is a perspective sectional view of the composite peripheral and hollow E-beam aperture  10  taken along line  1 B- 1 B′ in FIG. 1A with a collimated E-beam  24  projected directed onto the top  14  thereof. The collimated E-beam  24  has passed from a conventional E-beam source (not shown for convenience of illustration) through a first crossover and through a first lens L 1  which collimates the collimated E-beam  24 .  
         [0035]    The composite aperture  10  acts as a mask blocking both the periphery of the collimated beam  24  and the center of collimated E-beam  24  thereby shaping/patterning the collimated E-beam  24  into a hollow E-beam  96  which passes through the composite aperture  10  and out of the bottom thereof and through lens L 2  which focuses collimated E-beam  24  as an E-beam  97  onto a spot  58  on a workpiece  60  shown in FIG. 3A.  
         [0036]    Composite aperture  10  comprises an electrically-conductive-metallic shell  11  comprising a hollow metallic cylinder  12  open on the bottom and covered by a metallic top  14  which has a coaxial, circular, central hole  14 ′ therethrough. Aperture  10  also includes an electrically-conductive-metallic center pole  16  that is suspended coaxially with and inside cylinder  12  and top  14 .  
         [0037]    The upper end of center pole  16  is located inside central hole  14 ? aligned coaxially therewith. An annular passageway for the hollow E-beam  96  is provided by the combination of the center pole  16  and the wall of central hole  14 ′ since the center pole  16  is aligned coaxially with the cylinder  12  and the central hole  14 ?.  
         [0038]    The hollow E-beam  96  passes between the walls of central hole  14 ′ in top  14  and the pole  16 . Inside the upper side-walls of the cylinder  12 , below the top  14  and above a set of radial struts  18  is an upper space  13  through which the hollow E-beam  96  passes.  
         [0039]    After hollow E-beam  96  passes through the upper space  13  it reaches a set of several openings  17  between the struts  18  and the E-beam passes through openings  17  into an lower space  19 . In other words openings  17  provide interconnections between the upper space  13  and the lower space  19  inside the lower side-walls of the cylinder  12  allowing the hollow E-beam  96  to pass therethrough.  
         [0040]    Then hollow E-beam  96  passes from lower space  19  out of cylinder  12  and through the lens L 2  which focuses the collimated E-beam  24  into a converging hollow E-beam  97  that is focused onto a very small spot  58  on a workpiece (reticle)  60 .  
         [0041]    As indicated above, inside cylinder  12 , there are the several, radially-disposed, conductive metallic struts  18 , which are electrically and mechanically connected to cylinder  12 . Struts  18  are provided to support the center pole  16  and to conduct electrons which are collected thereby towards the electrical output line  50 . The struts  18  are secured to the inner wall of the lower end of cylinder  12  (well below the annular top  14 ) and they are firmly connected to the center pole  16  both mechanically and electrically. As can be seen in FIG. 1C, there are openings  17 , between (aside from) the struts  18 , through which only the hollow E-beam  96 , which (as stated above) is a portion of collimated E-beam  24 , can pass.  
         [0042]    To summarize, the composite aperture  10  includes an annular, upper aperture  15  formed between the center pole  16  and the side walls of the central hole  14 ′ in top  14 . The hollow E-beam  96  passes through upper aperture  15 ; while the peripheral (outer) portion  89  of collimated E-beam  24  strikes the top  14  of shell  11  and is masked thereby and the electrons striking the top  14  cause some electrical current to flow through electrical output line  50 . The central beam  88  which is the central portion of collimated E-beam  24 , i.e. the interior portion thereof, strikes the center pole  16  and is masked thereby and adds some more electrons to the electrical current flowing through the electrical output line  50 .  
         [0043]    [0043]FIG. 3A shows a perspective view similar to FIG. 1C of an upper portion of an E-beam exposure system  9  with an E-beam  21  divergent from an upper crossover  86  directed through lens L 1  which produces a collimated E-beam  24  consisting of electrons travelling in parallel towards the surface of the top  14  of the shell  11  of the composite aperture  10 . The hollow E-beam  96  which passes out of the composite aperture  10  is directed therefrom onto a point  58  on a workpiece (reticle)  60 . Ideally, the collimated E-beam  24  is supposed to be centered on the top  14  and the center pole  16  of composite annular aperture  10 .  
         [0044]    Since the location (alignment) of the center pole  16  determines the shape and location of the hollow E-beam  96 , it is crucial to align a charged particle beam  21  directed onto the top surface of the composite aperture  10  with a high degree of accuracy. As shown in FIG. 3A, when the hollow E-beam  96  leaves the composite aperture  10 , it is focussed down by lens L 2  onto the point  58  on the workpiece (reticle)  60 .  
         [0045]    A problem that arises is that the E-beam  21  may not be accurately centered on the composite aperture  10 , which is a significant fact because of the extremely tight tolerances of submicron devices which means that accuracy of machining in forming and assembling the components of the annular aperture  10  is crucial. I have found that the problem of alignment of E-beam  21  is attributable to machining and assembly errors that affect the location of upper aperture  15  and center pole  16  in the composite aperture  10 .  
         [0046]    [0046]FIGS. 2A and 2B are similar views to those of composite aperture  10  in FIGS. 1A and 1B, which include phantom lines which show an example of a misaligned annular aperture  10 ′ due to an alignment fabrication error in the positioning of the center pole  16 . FIG. 2A shows a modified view of FIG. 1A that illustrates misalignment of the center pole  16 ′ (in phantom) as contrasted to the properly aligned center pole  16 . FIG. 2B is a sectional view modified based on FIG. 1B of a composite peripheral and hollow E-beam aperture  10 ′ showing the center pole  16 ′ (in phantom) and the properly aligned center pole  16 .  
         [0047]    [0047]FIGS. 2A and 2B illustrate the contrast between a misaligned center pole  16 ′ (dotted lines) in the annular aperture  15 , and a ideally aligned center pole  16  in the upper aperture  15 . Cross-section lines have been omitted for clarity of illustration. In FIGS. 2A and 2B, in solid lines the center pole  16  is shown in the ideal position in which it is coaxial with the composite aperture  10 . In contrast, a misaligned center pole  16 ′, which is shown in phantom, will cause the hollow E-beam to have an asymmetric current distribution which makes it impossible to obtain a symmetrical E-beam current distribution.  
         [0048]    Proper alignment of the center pole  16  is crucial to achievement of the result which is obtained when the charged particle beam  36 , which is directed onto the composite aperture  10 , is aligned with a specified degree of accuracy, since the center of the E-beam  36  must coincide with the central axis of the composite aperture  10  which is greatly affected by the geometry of the center pole  16 / 16 ′ or other deviations from specifications. If the accuracy of alignment of the E-beam with the composite aperture  10  is not sufficient, the expected advantages of the hollow E-beam illumination method which is supposed to work against the problems caused by space charge effects will not be obtained or will be greatly suppressed.  
         [0049]    Any asymmetry of the annular E-beam shape (more exactly saying, “beam current distribution”) produces larger magnitudes of aberrations due to space charge effects than we would expect from an ideally shaped annular E-beam. Also, highly controlled E-beam positioning stability onto the composite aperture  10  is important. If proper alignment does not exist, then the beam current of the E-beam will vary with time, and a serious dose error will result.  
       First Embodiment of the Invention  
       [0050]    Aperture Current Monitor  
         [0051]    Referring again to FIG. 3A, to solve the above problem, I have designed a control method and a control system employing an aperture current monitor  52  which monitors current received by the composite aperture  10 , including the metallic shell  11  and the center pole  16 , via electrical output line  50  which connects the cylinder  14  to the aperture current monitor  52 .  
         [0052]    [0052]FIG. 8A shows the aperture current monitor  52  that including amplifier  55  and an A/D converter  56  that provide an output signal on line  53  to the Central Processing Unit (CPU) of a computer system  260  seen in FIG. 8B.  
         [0053]    Adjustment of Alignment  
         [0054]    Associated with the aperture  10  there are alignment coils (upper aligners)  22  which control the E-beam position and the angle of the collimated E-beam  24 . The upper aligners  22  make it possible to align and center the collimated E-beam  24  accurately onto the aperture  10  below it.  
         [0055]    Referring to the algorithm of FIG. 10A, in accordance with this invention, E-beam alignment and centering onto the composite aperture  10  is done by varying the current on lines  73  to the aligners  22  in FIG. 3A and lines  74 / 74 ′ in FIGS. 5A and 5C to vary the current through the composite aperture  10  in FIG. 3A or the upper aperture  120 / 210  in FIGS.  5 A/ 5 B as adjusted by varying the excitation current of the aligners  22  or upper aligners  162 / 243  respectively under control of the computer system  260  shown in FIG. 8B in accordance with the algorithm in FIG. 10A in step  401  thereof.  
         [0056]    The aperture current monitors  52  of FIG. 3A (which is illustrative of monitor  130  of FIG. 5A, and monitor  252  of FIG. 5B) are shown in detail in FIG. 8A. Monitor  52  amplifies the signal on line  50  with amplifier  55  and converts the signal in an Analog-to-Digital (A/D) converter  56 . The output of A/D converter  56  is supplied on line  53  (which is one of several digital signal input lines  53 ,  131 ,  151 ,  231  and  253 ) to the CPU  61  of computer system  260  in FIG. 8A. The CPU  61  monitors the composite aperture current on line  53  and after performing the algorithm of FIG. 10A, CPU  61  sends a digital signal on output line  68  to a digital-to-analog (D/A) converter  70  which feeds an analog control signal output to amplifiers  72  which in this case supply an output voltage on line  73  to adjust the current through the upper aligners  22  as required by the computer system  260 .  
         [0057]    The computer system  260  employs the algorithm shown in FIG. 10A to determine the value of the adjusted current in the upper aligners  22 . The CPU continues to perform the algorithm of FIG. 10A until the E-beam  24  is properly aligned as indicated by the voltage on line  53  in FIG. 3A.  
         [0058]    [0058]FIG. 10B shows another related algorithm which is employed with the embodiments of FIGS.  5 A- 5 C where there is a lower aperture  110 / 220  onto which the E-beam  194 / 294  needs to be centered.  
         [0059]    Referring to FIG. 8B in conjunction with FIG. 3A, the line  53  from FIG. 3A is connected to one of the IN terminals of the CPU  61  of the process control computer control system  260 . As is the usual configuration of a computer system, the CPU  61  is connected to a display monitor  62  and a keyboard  63  as well as a random access memory (RAM)  65  and a Direct Access Storage Device (DASD) which in this case is shown as a disk drive  64  for storing data for the process control computer system  260 . A printer  78  is also connected to the CPU. The computer system  260  provides output signals on OUT line  68  to a D/A converter  70  which supplies signals via line  71  to amplifiers  72  which are connected by lines  73  to the upper aligners  22  in FIG. 3A. The CPU  61  generates a scan of the collimated E-beam  24  from left-to-right or right-to-left which produces the curve  87 ′ seen in FIG. 3B which is displayed on the computer display monitor  62  in FIG. 8B. When the collimated E-beam  24  is centered on the composite aperture  10  the curve  87 ′ reaches a maximum.  
         [0060]    [0060]FIG. 3C shows a curve  96 ′ resulting from passage of collimated E-beam  24  through the composite aperture.  
         [0061]    The flow chart of the computer program which is shown in FIG. 10A provides for adjustment of the current in the upper aligners in response to the output of the aperture current monitor  52 .  
         [0062]    In step  400 , the program starts.  
         [0063]    Then in step  401  the program generates an incremental digital change which slightly varies the excitation of the upper aperture aligners  22 .  
         [0064]    In step  402 , the program receives the digital value of the aperture current measured by the aperture current monitor  52 , while the E-beam position is held constant at the top crossover  21 .  
         [0065]    In step  403 , the CPU tests to determine whether the aperture current indicates that the beam is centered on the upper aperture  15 .  
         [0066]    If NO, (the beam is not centered on the upper aperture  15 ) then the program loops back on line  404  to step  401 .  
         [0067]    If YES, (the beam is centered on the upper aperture  15 ) then the program ENDs in step  405 .  
         [0068]    [0068]FIG. 10B is a flow chart of a computer program which provides for adjustment of the current in the lower aligners in response to the output of the aperture current monitor  152 / 230 .  
         [0069]    In step  406 , the program starts.  
         [0070]    Then in step  407  the program generates an incremental digital change which slightly varies the excitation of the lower aperture aligners  143 / 262 .  
         [0071]    In step  408 , the program receives the digital value of the aperture current measured by the lower aperture current monitor  152 / 230 , while the E-beam position is held constant at the lower crossover  192 / 292 .  
         [0072]    In step  409 , the CPU tests to determine whether the aperture current indicates that the beam is centered on the lower aperture  152 / 230 .  
         [0073]    If NO, (the beam is not centered on the lower aperture) then the program loops back on line  410  to step  407 .  
         [0074]    If YES, (the beam is centered on the lower aperture  110 / 220  then the program ENDs in step  411 .  
         [0075]    [0075]FIG. 4 is a graph showing an example of monitored aperture current amplitude of the hollow E-beam method of FIG. 3A which is less than the exemplary result desired. As shown in FIG. 4, the peak of the graph expected in monitored aperture current is not very high beneath the arrow, i.e. the contrast between the peak beneath the arrow is not always sufficient to distinguish between the peaks and the valleys with sufficient certainty. Accordingly, I have discovered that since it is difficult to distinguish between optimum alignment and the peripheral alignment values to the left and the right that an alternate embodiment would be desirable to make it less difficult to align the upper E-beam  24  onto the upper aperture  15  with an optimum degree of accuracy by generating curves which are more easily interpreted by the computer system  260  of FIG. 8B than the curve shown in FIG. 4.  
         [0076]    Thus, FIGS. 5A and 5B show alternate dual aperture embodiments of several improved methods and apparatus, in accordance with this invention, which are described below. FIG. 5C shows additional details of the embodiment of FIG. 5A.  
       Dual Aperture Embodiments of the Invention  
       [0077]    [0077]FIGS. 5A and 5C are very similar in that a peripheral aperture  120  is located on top and a hollow beam aperture  110  with a center pole  116  is located on the bottom. In FIG. 5B, the reverse configuration is employed with a hollow beam aperture  210  with a center pole  216  is located on the top and a peripheral aperture  220  located on the bottom.  
         [0078]    The E-beam projection system  90  of FIGS. 5A and 5C in accordance with this invention consists of the peripheral aperture  120  (on top) which shapes the upper E-beam  187  into a narrower intermediate beam  190 / 194  and the hollow beam aperture  110  (on the bottom), which shapes the intermediate E-beam  190 / 194  into a hollow lower E-beam  196 / 197  which hits the reticle  160  at a focal point  198 . For convenience of illustration and initial description of this embodiment, FIG. 5A is less detailed than FIG. 5C, in that it omits the graphs of waveforms which are described later with reference to FIG. 5C.  
         [0079]    The upper, peripheral aperture  120  blocks/masks (i.e. defines) the outer periphery  188  of the upper E-beam  187 . The lower, hollow beam aperture  110  blocks/masks (i.e. defines) the inner portion  195  of the intermediate E-beam  190 / 194 , thereby producing the hollow lower E-beam  197  which is focussed by lens L 4  onto a spot  198  on the workpiece (reticle)  160  which is below the lens L 4 . In the apparatus shown in FIGS.  5 A/ 5 C, the upper, peripheral aperture  120  determines the outer periphery (sets the outer limit) of the upper defined E-beam  190 ; while the lower aperture  110  determines the inner periphery (sets the inner limit) of the lower defined hollow E-beam  196 .  
         [0080]    On the other hand, referring to FIG. 5B, the opposite order of arrangement of the two apertures  210 / 220  also provides a hollow E-beam  290 / 296  which has the same beam shape. The description which follows immediately below applies specifically to FIG. 5A, but, generally it also applies to FIG. 5B with corresponding reversals in the sequence of the elements and the effects thereof as described below.  
         [0081]    Referring again to FIGS. 5A and 5C, before each aperture  110 / 120 , there is a set of aligners  162 / 143  which are alignment coils. Each set of aligners  162 / 143  consists of at least two successive alignment coils. The first set of aligners  162  controls the position and the angle of the upper E-beam  187 . The second set of aligners  143  controls the position and the angle of the intermediate E-beam  194 . Thus the two sets of aligners  162 / 143  make it possible for the upper E-beam  187  and the intermediate E-beam  194  to be aligned accurately onto the apertures  120 / 110  below them.  
         [0082]    [0082]FIG. 5A is a perspective sectional view of a system  90  similar to system  9  of FIG. 3A with two stacked apertures, instead of one, aligned along a single E-beam column. An E-beam  210  generated by a conventional E-beam source (not shown for convenience of illustration) is projected towards an upper crossover  186  where it diverges and then passes through a collimating lens L 1  which projects a collimated E-beam  187  consisting of electrons travelling in parallel towards the surface of the top  124  of the peripheral aperture  120  on the top of the E-beam column.  
         [0083]    Below the peripheral aperture  120  is a hollow E-beam aperture  110 . The two stacked apertures  120 / 110  substitute for the composite aperture  10  of FIG. 3A. The peripheral aperture  120  acts as a mask blocking the periphery of the collimated beam  187 . The hollow beam aperture  110  blocks the center beam  195  of collimated E-beam  194  thereby shaping/patterning the collimated E-beam  194  into a hollow E-beam  196  which passes out of the bottom of the hollow beam aperture  110  and through lens L 4  which focuses collimated E-beam  196  as an E-beam  197  onto a spot  198  on workpiece (reticle)  160 .  
         [0084]    The peripheral aperture  120  comprises an electrically-conductive-metallic shell  121  comprising a hollow metallic cylinder  122  open on the bottom and covered by a metallic top  124 . The top  124  has a coaxial, circular, central hole  126  therethrough. The collimated E-beam  187  passes from the peripheral aperture  120  forming a narrower collimated E-beam  190  which is projected through lens L 2 , forming converging beam  191  which passes through crossover  192  as beam  193  that passes through lens L 3  as collimated beam  194  which is projected onto the top of the hollow beam aperture  110 .  
         [0085]    The hollow beam aperture  110  comprises an electrically-conductive-metallic shell  111  comprising a hollow metallic cylinder  112  open on the bottom and covered by a metallic top  114 . Hollow beam aperture  110  includes a top  114  surface which has a coaxial, circular, central hole  115  therethrough. The hollow beam aperture  110  also includes an electrically-conductive-metallic center pole  116  which is suspended coaxially with aperture  110  inside the cylinder  112  and top  114 . The upper end of center pole  116  is preferably located inside the central hole  115  aligned coaxially therewith. The central beam  195  which is the central portion of beam  194  is blocked by center pole  116 . An annular passageway is provided for the outer portion  196  of beam  194  which comprises a hollow E-beam. Hollow E-beam  196  is shaped by the center pole  116  in lower aperture  110  and the wall of central hole  126  in the upper, peripheral aperture  120  since the center pole  116  is aligned coaxially with the cylinders  122  and  112  and the central holes  126  and  115 .  
         [0086]    The hollow E-beam  196  passes between the walls of central hole  115  in top  114  and the pole  116 . An upper space  113  is defined (inside aperture  110 ) by the upper side-walls of the cylinder  112 , below the top  114  and above a set of radial struts  118  through which the hollow E-beam  196  passes. After the hollow E-beam  196  passes through the upper space  113  it reaches a set of several openings  117  between the struts  118 . Openings  117  (indicated between the struts  118  by phantom lines) connect the upper space  113  to a lower space  119  inside the lower side-walls of the cylinder  112  allowing the hollow E-beam  196  to pass therethrough.  
         [0087]    Then, the hollow E-beam  196  passes from lower space  119  out of cylinder  112  and through the lens L 4  which focuses the collimated E-beam  124  into a converging hollow E-beam  197  which is focused onto a very small spot  198  on a workpiece (reticle)  160 .  
         [0088]    As indicated above, inside cylinder  112 , there are the several, radially-disposed, conductive metallic struts  118 , which are electrically and mechanically connected to cylinder  112 . Struts  118  are provided to support the center pole  116  and to conduct electrons which are collected thereby towards the electrical output line  150 . The struts  118  are secured to the inner wall of the lower end of cylinder  112  (well below the annular top  114 ) and they are firmly connected to the center pole  116  both mechanically and electrically. The openings  117 , between (aside from) the struts  118  permit only the hollow E-beam  196 , which (as stated above) is a portion of collimated E-beam  194  to pass therethrough.  
         [0089]    As stated above, the central beam  195 , i.e. the interior portion, of collimated E-beam  194  from lens L 3 , strikes the center pole  116  and is masked thereby. The central beam  195  provides a current of electrons which flow through the electrical output line  150  to the hollow beam aperture current monitor  152 . Monitor  152  supplies an electrical output current to line  153  to the CPU in FIG. 8B.  
         [0090]    To summarize, the peripheral aperture  120  includes upper opening  126  and the peripheral (outer) portion  188  of collimated E-beam  187  strikes the top  124  of shell  121  and is masked thereby. The electrons striking top  124  cause some electrical current to flow through electrical output line  128 , and the narrower, collimated E-beam  190  passes out of peripheral aperture  120 .  
         [0091]    The hollow beam aperture  110  includes an opening  115  between the center pole  116  and the side walls of the central hole in top  114 . The hollow E-beam  196  passes through the hole  115  in hollow beam aperture  110  and out below aperture  110  to lens L 4 .  
         [0092]    Ideally, the collimated E-beam  187  is supposed to be centered on the top  124  of the shell  121  and lower, along the column, the collimated E-beam  194  is supposed to be centered on the top  114  of the shell  111 .  
         [0093]    Since the location (alignment) of the center pole  116  determines the shape and location of the hollow E-beam  196 , it is crucial to align a charged particle beam  194  directed onto the top surface of the hollow beam aperture  110  with a high degree of accuracy. Ideally, the collimated E-beam  194  is supposed to be centered on the top  114  of the shell III and the center pole  116  of the hollow beam aperture  110 . As shown a in FIG. 5C, when the hollow E-beam  196  leaves the hollow beam aperture  110 , it is focussed down by lens L 4  onto the point  198  on the workpiece (reticle)  160 .  
         [0094]    A problem that arises is that the E-beam  194  may not be accurately centered on the hollow beam aperture  110 , which is a significant fact because of the extremely tight tolerances of submicron devices which means that accuracy of machining in forming and assembling the components of the hollow beam aperture  110  is crucial. I have found that the problem of alignment of E-beams  194  is attributable to machining and assembly errors which affect the location of the upper aperture  115  and the center pole  116  in the hollow beam aperture  110 .  
         [0095]    The hollow E-beam  196  which passes out of hollow beam aperture  112  is directed therefrom onto a point  198  on a workpiece (reticle)  160 .  
         [0096]    [0096]FIG. 5B is a perspective sectional view similar to FIG. 5A of a system  90 ′ which also includes two stacked apertures aligned along a single E-beam column. On the top of the E-beam column, there is a hollow E-beam aperture  210  and therebelow is a peripheral E-beam aperture  220 . The two stacked apertures  210 / 220  substitute for the apertures  120 / 110  of FIG. 5A.  
         [0097]    In FIG. 5B, an E-beam  310  diverging from an upper crossover  286  passes through a collimating lens L 1  thereby projecting a collimated E-beam  287  consisting of electrons travelling in parallel towards the surface of the top of an electrically-conductive-metallic center pole  216 .  
         [0098]    The upper hollow beam aperture  210  comprises an electrically-conductive-metallic shell  211  comprising a hollow metallic cylinder  212  open on the bottom and covered by a metallic top  214 . The top  214  surface has a coaxial, circular, central, hole  215  therethrough. The hollow beam aperture  210  also includes an electrically-conductive-metallic center pole  216  which is suspended coaxially with aperture  210  by struts  218  inside the cylinder  212  and top  214 .  
         [0099]    The upper end of center pole  216  is preferably located inside the central hole  215  aligned coaxially with cylinder  212  and central hole  215 . Center pole  216  blocks the central portion  288  of collimated E-beam  287  while the hollow E-beam  290  (the outer portion  290  of beam  287 ) passes through an annular hole/passageway provided by the central hole  215  and the center pole  216  in the hollow beam aperture  210 . In summary, the hollow E-beam  290  was shaped by the center pole  216  in the hollow beam aperture  210 .  
         [0100]    Since the location (alignment) of the center pole  216  determines the shape and location of the hollow E-beam  290 , it is crucial to align a charged particle beam  287  directed onto the top surface of the hollow beam aperture  215  with a high degree of accuracy. Ideally, the collimated E-beam  287  is supposed to be centered on the top  214  of the shell  211  and the center pole  216  of the hollow beam aperture  215 .  
         [0101]    As indicated above, inside cylinder  212 , there are the several, radially-disposed, conductive metallic struts  218 , which are electrically and mechanically connected to cylinder  212 . Struts  218  are provided to support the center pole  216  and to conduct electrons which are collected thereby towards the electrical output line  250 . The struts  218  are secured to the inner wall of the lower end of cylinder  212  (well below the annular top  214 ) and they are firmly connected to the center pole  216  both mechanically and electrically. The openings  217 , between (aside from) the struts  218  permit only the hollow E-beam  290 , which (as stated above) is a portion of collimated E-beam  287  to pass therethrough towards lenses L 2 , L 3  and peripheral shell  221 .  
         [0102]    The collimated, hollow E-beam  290  passes from the hollow beam aperture  210  as a collimated hollow E-beam which is projected through lens L 2 , forming converging hollow E-beam  291  which passes through crossover  292  as E-beam  293  that passes through lens L 3  as hollow, collimated E-beam  294  which is projected onto the top of the peripheral aperture  220 .  
         [0103]    The peripheral aperture  220  comprises an electrically-conductive-metallic shell  221  comprising a hollow metallic cylinder  222  defining a hollow space  225  that is open on the bottom and covered by a metallic top  224 . The top  224  has a coaxial, circular, central hole  226  therethrough.  
         [0104]    The peripheral aperture  220  acts as a mask blocking the periphery of the collimated hollow E-beam  294 . The peripheral aperture  220  blocks the periphery of collimated E-beam  294  thereby shaping/patterning the collimated E-beam  294  into a narrower hollow E-beam  296 . In other words, the central portion  296  of the hollow E-beam  296  passes between the walls of central hole  226  in top  224 , while the peripheral portions of E-beam  296  are blocked by the metallic top  224 .  
         [0105]    The narrower hollow E-beam  296  passes out of the bottom of the peripheral aperture  220  and through lens L 4  which focuses collimated E-beam  296  into a converging hollow E-beam  297  that is focused onto a very small a spot  298  on a workpiece (reticle)  260 .  
         [0106]    As stated above, the peripheral portion  295  (the outer portion) of collimated E-beam  294  from lens L 3  strikes the  224  and is masked thereby. The peripheral beam  295  provides a current of electrons which flow through the electrical output line  228  to the hollow beam aperture current monitor  230 . Monitor  230  supplies an electrical output current to line  231  to the CPU in FIG. 8B.  
         [0107]    To summarize, the lower, peripheral aperture  220  includes an upper opening  226  and the peripheral (outer) portion  295  of collimated E-beam  294  strikes the top  224  of shell  221  and is masked thereby and the electrons striking the top  224  cause some electrical current to flow through electrical output line  228 , and the inner E-beam  296  portion of collimated E-beam  294  passes through the lower, peripheral aperture  220 .  
         [0108]    The lower aperture  220  includes an opening  225  between the side walls of the peripheral shell  221 . The hollow E-beam  296  passes through the lower aperture  220 . Ideally, the collimated E-beam  296  is supposed to be centered on the top  124  of the shell  121  and lower, along the column, the collimated E-beam  194  is supposed to be centered on the top  224  of the shell  221 .  
         [0109]    As shown in FIG. 5B, when the hollow E-beam  296  leaves the hollow beam aperture  220 , it is focussed down by lens L 4  as beam  297  onto the point  298  on the workpiece (reticle)  260 . The problem that arises is that E-beam  294  may not be accurately centered on hollow beam aperture  220 , which is a significant fact because of the extremely tight tolerances of submicron devices which means that accuracy of machining in forming and assembling the components of the hollow beam aperture  210  is crucial.  
         [0110]    I have found that the problem of alignment of E-beams  194  is attributable to machining and assembly errors which affect the location of the upper aperture  215  and center pole  216  in hollow beam aperture  210 . The hollow E-beam  196  which passes out of the hollow beam aperture  210  is directed therefrom through peripheral aperture  220  onto the point  298  on the workpiece (reticle)  260 .  
         [0111]    E-beam alignment and centering onto each aperture  210 / 220  is accomplished by monitoring current through the respective apertures with the monitors  130 / 152  which are shown in FIGS. 5A and 5C. Similarly, referring again to the embodiment in FIG. 5B, E-beam alignment and centering onto each aperture  210 / 220  is accomplished by monitoring current through the respective apertures with the monitors  252 / 230 .  
       Monitoring of Aperture Current  
       [0112]    Monitoring of aperture current will be discussed with reference to FIGS. 5A, 5C and FIGS.  5 D- 5 F.  
         [0113]    [0113]FIG. 5D is a graph which shows an example of the bell shaped curve  187 ′ of current in upper E-beam  187 .  
         [0114]    [0114]FIG. 5E is a graph which shows an example of the narrowed aperture current profile  190 ′ of the intermediate E-beam  190  monitored inside the hollow cylinder  121  in the lower portion of the upper, peripheral aperture  120  of FIGS. 5A and 5C.  
         [0115]    [0115]FIG. 5F is a graph which shows the hollow E-beam profile  196 ′ of the lower aperture current of E-beam  196  monitored in the lower aperture  110  in FIG. 5C. By controlling the deflection of the upper E-beam  187  to “minimize” the monitored current intercepted by the upper, peripheral aperture  120  and at the same time by controlling the deflection of the intermediate E-beam  194  to “maximize” monitored current intercepted by the lower aperture  110 , we can align and center the beams  187 / 194  onto apertures  120 / 110 .  
         [0116]    An important goal in this invention, which is that a portion of the charged particle beam should hit the center pole  116  of the second aperture  110 , but should not hit the top  114  of the metallic shell  111 , i.e. the peripheral portion of the lower, hollow beam aperture  110 . If that goal is met, we get a very good peak in monitored aperture current (good contrast) in each of the apertures  120 / 110 . We can independently control the beam position on apertures  120 / 110 , i.e. we can control alignment of the inner periphery and the outer periphery of the E-beam  187 / 194  by using the corresponding set of aligners  162  or  143  with certain high accuracy.  
         [0117]    Referring to FIG. 8B in conjunction with FIGS.  5 A/ 5 C lines  131 / 153  from FIG. 5A are connected to IN terminals of the CPU  61  of the process control computer control system  60 . The computer system  260  provides output signals on OUT lines  74 / 74 ′ to a D/A converter  70  which supplies signals via line  71  to amplifiers  72  which are connected by lines  73  to the upper aligners  22  in FIG. 3A.  
         [0118]    [0118]FIG. 6 shows a graph of “aperture current amplitude” of current received by the peripheral aperture current monitor  130  of FIGS.  5 A/ 5 C as the E-beam  187  is scanned across the upper, peripheral aperture  120  with a minimum value when the E-beam  187  is centered over the aperture  120 .  
         [0119]    [0119]FIG. 7 shows a graph of “aperture current amplitude” at the second aperture current monitor  152  of FIGS.  5 A/ 5 C as intermediate E-beam  194  is scanned from across the second, hollow beam aperture  110 , with a very clear maximum value when the E-beam  194  is centered over aperture  110 .  
         [0120]    [0120]FIG. 9A shows a block diagram of a process for producing a semiconductor chip adapted to employing the apparatus and method of this invention. The semiconductor fabrication method of FIG. 9A comprises mainly a wafer production step P 10  (or wafer preparation step) which produces a finished wafer in step P 11 , a mask production step P 20  (or mask preparation step) which produces a finished reticle, mask in step P 21 , a wafer processing step P 12 , an assembly step P 40  yielding a chip P 41  and an inspection step P 42 . Each step comprises several substeps as will be well understood by those skilled in the art. Among these main steps, the wafer processing step P 12  is a most important step to achieve the specified finest pattern width and registration limit. In this step, the designed circuit patterns are stacked successively on the wafer from step P 11  and many operative semiconductor chips like memory devices are formed on the wafer from step P 11 .  
         [0121]    The wafer processing steps P 12  comprises a step of thin film formation wherein a dielectric layer for insulation is formed or a metallic layer for lead lines and for electrodes is formed. An oxidization step can be employed to oxidize the thin film or the wafer substrate. A lithography step P 31  involves use of the reticle/mask P 21  to form a photoresist or other resist pattern to process the thin film or wafer substrate selectively, a selected set of process steps P 32  including etching the thin film or wafer substrate and implanting ions or impurities into the thin film or wafer substrate using the resist pattern from step P 31  as a mask. There is the conventional resist stripping step to remove the resist from the wafer and chip inspection step. As indicated at P 34 , the wafer processing steps P 30  are repeated as many times as necessary to make a semiconductor chip be operable as designed, as will be understood by those skilled in the art.  
         [0122]    [0122]FIG. 9B shows a flow chart of lithography steps P 31  of FIG. 9A which are dominant steps in the wafer processing steps P 12 /P 30  adapted for employment with the method and apparatus of this invention. Lithography steps P 31  comprise a resist-coat step P 311  in which the wafer substrate is coated with resist on circuit elements formed in a previous steps. An exposure step P 312  then exposes the wafer coated with resist through the reticle/mask of step P 21  employing a deflector in accordance with this invention. A resist development step P 313  follows for developing the resist exposed in exposure step P 312  followed by a resist annealing step P 314  performed to enhance durability of the resist pattern produced in step P 313 .  
         [0123]    While this invention has been described in terms of the above specific embodiment(s), those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims, i.e. that changes can be made in form and detail, without departing from the spirit and scope of the invention. Accordingly all such changes come within the purview of the present invention and the invention encompasses the subject matter of the claims which follow.