Abstract:
Space charges of electrons which repel one another near masks in electron beam exposure machines are reduced by bleeding gas into the vicinity of the apertures in the mask and pumping the gas out from the direction electron travel.

Description:
[0001]    This is a continuation of Ser. No. 09/247,177 filed Jan. 2, 1998, which is incorporated herin by reference, as if fully recited herein. 
     
    
     
         [0002]    FIELD OF THE INVENTION  
           [0003]    This invention relates to high resolution electron beam exposure machines such as the type used for submicron lithography, and particularly to space charge neutralization of electrons therein.  
         BACKGROUND OF THE INVENTION  
         [0004]    Electron beam exposure machines for submicron lithography, such as for writing on optical or x-ray mask surfaces, or directly on semiconductor surfaces, pass electrons through apertures of a mask. Such devices are limited by the space charges of electrons which repel one another.  
         SUMMARY OF THE INVENTION  
         [0005]    An embodiment of the invention involves bleeding gas into the vicinity of the apertures in the mask and pumping the gas out from the direction electron travel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a schematic representation of an electron beam exposure system embodying features of the invention.  
         [0007]    [0007]FIGS. 2 and 3 are schematic cross sectional representations of a membrane having apertures and its vicinity for use in the embodiment of FIG. 1.  
         [0008]    [0008]FIG. 4 is a schematic representation of a beam exposure system. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0009]    In the electron-beam exposure system of FIG. 1, an electron source ESI and a first electron-beam deflector-lens system DL 1  direct a beam EB 1  of electrons about an axis A toward a crossover or focus plane CP 1 . The sources ES 1  and arrangement DL 1  together form a structure that includes an electron gun and lenses.  
         [0010]    A mask or membrane MA 1  at the crossover plane CP 1  contains an image which blocks portions of the beam EB 1  and allows other portions of the beam to pass through so as to change the beam EB 1  into an electron image beam. A second electron-beam deflector-lens system DL 2  deflects and demagnifies the beam IB 1  emerging from the mask MA 1  and focuses the beams on a writing surface WS 1  of a workpiece WP 1 .  
         [0011]    The writing surface WS 1  is the top surface of an electron-sensitive layer supported on a substrate in the workpiece WP 1 . A conventional x-y movable table TA 1  controls the position of the workpiece WP 1  relative to the beam.  
         [0012]    Expressly imaging electrons over the surface of the writing surface WS 1  on the electron sensitive layer in a high-speed manner makes it possible to create integrated-circuit masks or to write directly on a resist-coated wafer to fabricate extremely small and precise low-cost integrated circuits.  
         [0013]    The deflector-lens systems DL 1  and DL 2  form an electron-beam column, i.e. beams EB 1  and IB 1 . The paths of the electrons emitted by the source ES 1  successively diverge, converge, diverge again, and converge again as the electrons travel downstream along a longitudinal axis toward the writing surface WS 1 . A vacuum enclosure EN 1  envelops the entire system and a pump PU 1  evacuates the interior of the enclosure EN 1  to a high vacuum.  
         [0014]    A pipe PI 1  leads gas from a gas supply GS 1  to a location just above the mask MA 1 . The gas passes through openings in the mask MA 1  and is drawn out by the action of the pump PU 1 . To direct the gas flow downwardly and prevent upward gas flow, a shroud SH 1  separates the interior of the enclosure EN 1  into upper and lower parts. Separate vacuum lines VL 1  and VL 2  from the pump PU 1  evacuate the part of the enclosure interior above the shroud SH 1  and below the shroud. A control CL 1  controls the operation of deflector lens systems DL 1  and DL 2  as well as the source ES 1  and the table TA 1 .  
         [0015]    [0015]FIG. 2 illustrates a cross-section of the mask or membrane MA 1 . In one embodiment, apertures AP 1  in the mask MA 1  form the openings that make the image and have tapered edges TE 1  and lie in the path of electrons coming from the source ES 1 . The pipe PI 1  bleeds gas in above the mask MA 1  and the line VL 1  from the pump PU 1  pumps the gas out from below the mask MA 1  at a high rate. This results in a gas pressure which is relatively high only above and in the openings or apertures AP 1  and dissipates quickly down the column. Various pressures above the mask may be used and one-tenth atmosphere is one example. This produces the pressure mainly just above the mask MA 1 . In the embodiment of FIG. 2 the tapered edges TE 1  of the apertures AP 1  increase the sizes of the apertures downwardly. In the embodiment of FIG. 3, the apertures AP 1  have edges TE 2  which taper to decrease the sizes of the apertures downwardly.  
         [0016]    In another embodiment, pipes feed the gas in horizontally and pump it out fast from below so that gas pressure is high only in the apertures and dissipates quickly down the column.  
         [0017]    Although electrons passing through the openings or apertures of the mask MA 1  are usually spaced from each other far enough vertically to prevent mutual repulsion, statistically there are enough instances when the electrons pass simultaneously, or nearly simultaneously, past the exit of the same apertures to repel one another. This creates a dispersion which detrimentally affects the resolution of the image being formed. In the aforementioned embodiments, the gases passing through the apertures provide a space charged shield between the electrodes and prevent their mutually r pulsion. That is, the gas molecules shield electrons from each other.  
         [0018]    The apertures AP 1  may have diverse sizes but may be vary from submicron diameters to three or four microns and substantially. Electrons passing through small apertures AP 1  may be separated from each other vertically by mean free distances such as 57 microns. However, these are only mean distances and statistically electrons passing through apertures AP 1  may be adjacent each other. Electrons, of course are small in size relative to such apertures AP 1 . Gases with molecular sizes of one nanometer passing through such apertures AP 1  may have a horizontal mean free path of one micron as compared to the 10 horizontal micron mean free path of electrons.  
         [0019]    The presences of the gas molecules in the exits of the apertures shield electrons simultaneously occurring within the apertures, or almost simultaneously occurring within the apertures, from each other.  
         [0020]    Example of gases used are helium, neon, argon, krypton, xenon, hydrogen, nitrogen, oxygen, chlorine, mercury gas, sodium gas, cesium gas, and sulphur hexaflouride (SP 6 ).  
         [0021]    The invention overcomes substantial portions of the repulsion of electrons from each other as they leave the mask apertures. The molecules shield the electrons from each other. It will be recognized that the structures disclosed with respect to FIGS.  1  to  3  are just examples.  
         [0022]    [0022]FIG. 4 is a diagram of an electron beam exposure system corresponding to that shown in FIG. 1 in which the beam from the electron source passes through small apertures in the mask and result in separate beamlets. In the electron-beam exposure system of FIG. 4, a field emission electron gun  10  and a first lens  14  constitute an electron source  17  which directs a beam  15  of electrons about an axis A toward a beam limiting aperture  20  of 200 μm. A second lens  24  with an auxiliary blanker converges the resulting electron beam  15  and directs it toward a crossover or focus plane  27  and between selector deflection plates  30 .  
         [0023]    Apertures  32  in an apertured mask or membrane  34  at the crossover plane  27  divides the beam  15  into individual beamlets according to a profile selected by the selector deflection plates  30 . A set of secondary deflection plates  37 , 4 μm deflection plates  40 , and secondary deflection plates  44  as well as 512 μm deflection coils  47  deflect the beamlets that now form the beam  15  in a manner described with respect to FIGS.  2  to  4 . The two sets of deflection plates  37  and  44  work together as a unit. A third lens assembly  50  demagnifies the beams emerging from the mask  34  and, with a fast focus correction coil  52 , focuses the beams on the writing surface  54  of a workpiece  57 .  
         [0024]    The writing surface  54  is the top surface of an electron-sensitive layer supported on a substrate in the workpiece  57 . A conventional x-y movable table  60  controls the position of the workpiece  57  relative to the beam. A control  90  controls the operation of the members  10 ,  14 ,  17 ,  24 ,  20 ,  37 ,  40 ,  44 ,  50 ,  47 ,  52 , and  60 .  
         [0025]    Selectively imaging selected fixed groups of electron spots over the surface of the writing surface  54  on the electron sensitive layer in a high-speed manner makes it possible to create integrated-circuit masks or to write directly on a resist-coated wafer to fabricate extremely small and precise low-cost integrated circuits.  
         [0026]    The members  10  to  52  constitute an electron-beam column having highly accurate high-speed deflection capabilities. As shown in FIG. 4, the paths of the electrons emitted by this source successively diverge, converge, diverge again, and converge again as the electrons travel downstream along a longitudinal axis toward the writing surface  54 .  
         [0027]    For use, a vacuum enclosure  64  envelops the entire system composed of member  10  to  52  and a pump  67  evacuates the interior of the enclosure  64  to a high vacuum. For bleeding gas into the vicinity of the apertures  32  in the mask  34 , a pipe  70  leads gas from a gas supply  74  to a location just above the mask  34 . The gas passes through the apertures  32  and is drawn out by the action of the pump  67 . To direct the gas flow downwardly and prevent upward gas flow, a shroud  77  separates the interior of the enclosure  64  into upper and lower parts. Separate vacuum lines  80  and  84  from the pump  67  evacuate the part of the enclosure interior above the shroud  70  and below the shroud.  
         [0028]    The embodiments shown in FIGS. 2 and 3 are substantially the same for FIG. 4 as for FIG. 1. The mask or membrane  34  corresponds to the mask or membrane MA 1 , the apertures  32  to the apertures AP 1 , the pipe  70  to the pipe PI 1 , and the shroud  77  to the shroud SH 1 .  
         [0029]    While embodiments of the invention have been described in detail, it will be evident that the invention may be embodied otherwise.