Abstract:
The column surrounding an electron or ion beam is shielded with a second shield which is outside the column and isolated from the column, being connected to chassis ground at a location remote from the column. Also, wiring into the column is double shielded with the shields connected to ground at the end remote from the column and not at the column itself.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to electron and ion beam columns, and, more particularly, to shielding of such columns from electromagnetic (EMI, RFI) interference. 
     2. Description of Related Art 
     Electron and ion beam columns use electrical lenses and scan coils, together with mechanical shields, to focus the electrons or ions in the column. In the areas of the column which the beam passes that are not in close proximity to a lense or a coil, the beam is subject to external electrical interference such as EMI, RFI, etc. Also subject to interference are the column control signals. 
     In addition, the control of these beams is becoming increasingly more stringent as geometries of integrated circuits are becoming smaller. For example, one use for an ion beam is to open a vertical connection in an integrated circuit by removing a vertical conductive region. With IC geometries used today and prior art ion beam systems, the beam must be accurate to within about 0.2 micrometers (μm) and must hold this accuracy during the entire operation which takes over 20 minutes. Typically, electron/ion beam system manufacturers do not guarantee image drift for as long as 20 minutes. 
     Therefore, it can be appreciated that preventing unwanted electrical noise from interfering with an electron or ion beam is highly desirable. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a double shield for an electron or ion beam has a metallic column enclosing a major portion of the beam and forming at least a portion of a vacuum retaining structure. The metallic column provides a first shield for the beam. A second shield is provided for the beam separate from the metallic column, the second shield and the metallic shield forming a double shield for the beam over a major portion of the metallic column. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross sectional drawing, partial schematic diagram, of an electron or ion beam column according to the present invention; and 
     FIG. 2 is a drawing of column control and high voltage cables shown in FIG.  1 . 
     It will be appreciated that for purposes of clarity and where deemed appropriate, reference numerals have been repeated in the figures to indicate corresponding features. Also, the relative size of various objects in the drawings has in some cases been distorted to more clearly show the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an ion beam  10  is shown inside column  12  and is shielded with a second shield  14  according to the present invention. Beam  10  is produced by source  16  which can be a source of electrons or a source of ions. Beam  10  is directed by extractor cap and acceptance aperture  18  through first lens  20  (commonly referred to as lens  1 ), to beam defining aperture  22 . As it continues, beam  10  is shaped by lens  2 , steering quadrupole  24 , and in normal operation passes through second lens  26  (commonly referred to as lens  2 ) and deflection plates  28  onto workpiece  30 . Beam blanking plates  32  and blanking aperture  34  provide a means for diverting beam  10  away from workpiece  30 . 
     Also shown in FIG. 1 is a top plate  40  which is a portion of the chassis of the vacuum system which supports column  12  and shield  14 . Partially shown is cover plate  42  which provides the upper seal for the vacuum. 
     Numerous connections, both mechanical and electrical, must be made through column  12 . Coupling  50  provides connection to a mechanical vacuum pump (not shown), and coupling  52  provides connection to ion pump  54 . Other mechanical links such as feedthroughs  56 ,  58  and others not shown in FIG. 1 are necessary to position elements inside column  12 . Beam  10  is generated and controlled by high voltages (voltages&gt;100 volts) and column control signals which pass through column  12  such as octapole connector  60  shown with octupole cable  62  attached. Other connections are through lens cable connector  64  shown with lens cable  66  attached, through high voltage cables  68 , and through other connections not shown in FIG.  1 . 
     Column  12  has a ground connection  70  located near source  16  which in prior art systems is connected at all times to chassis ground by ground wire  72 . However, with the present invention this connection is generally only used when the source is being conditioned prior to normal use of the source. The conditioning of the source involves heating the source and raising the voltage to the source in gradual steps so that peaks in the source (which disappear as the source becomes liquid) will not cause arching in the column. During this conditioning, which includes the gradual increasing of the source voltage to over 10 KV, ground connection  70  is connected to chassis ground. After the source has been conditioned, ground connection  70  is removed to avoid ground loops through column  12 . 
     FIG. 1 is not drawn to scale in that beam  10  and its associated lenses, shields, etc. are smaller in relation to the inside of column  12  than shown. Much of the inside of column  10  is taken up with cabling to the various devices in column  10 . 
     Although column  12  is made of steel and is electrically grounded to the chassis of the vacuum system through top plate  40 , electrical fields external and internal to column  12  and ground currents through column  12  affect beam  10  creating aberrations in the beam. To help overcome these problems, shield  14  is placed around column  12  to form a second shield for column  12 . Shield  14  is placed between column  12  and any other electrical equipment. For example, ion pump  54  is external to shield  14 . 
     Shield  14 , as presently used, is made of nickel plated cloth tape. However, one of the various high magnetic permeability materials, such as Mumetal, would provide improved magnetic shielding by better shunting electromagnetic fields (EMI, RFI) around beam  10 . Advantageously, shield  14  is isolated by insulators  74  from top plate  40 , but has a ground connection  76  remote from top plate  40  through wire  78  which is connected to the vacuum system chassis where the high voltages are generated. Thus, ground connection  76  is wired to a node remote from top plate  40 . With this grounding connection, much of the electrical noise interference is intercepted before it reaches column  12  and there is not a current ground path through column  12 . 
     Turning now to FIG. 2, a column control cable  62  is shown. Cable  62  has several column controlling conductors which are shielded from each other by a first shield  80 , and the conductors are further shielded by second shield  82  forming a double shield for the conductors. Both shields  80  and  82  are connected to ground only at a node remote from column  12  such as the column control generator, and are left unconnected at column  12 . Thus, ground loops through column  12  are further reduced. All of the high voltage and column controlling connections to column  12 , which may have single or multiple isolated control or high voltage conductors, have their shields similarly connected. 
     In addition to the double shielding described above, other methods used to reduce column aberrations are removing the mechanical pump from the vacuum system chassis and isolating its mechanical vibrations from column  12 , increasing as much as possible the physical distance of such electrical devices as ion pump  54 , the ion pump controller (not shown), and the servo pump vacuum meter (not shown) from column  12 . Moreover, all high voltage cables are separated from each other and other cables as much as possible. 
     Using the shielding described herein beam aberrations have been decreased which provides better focusing of the beam to enable concentrating the beam to an area as small as  10  nanometers in diameter. This reduced spot size also allows the time for ion milling operations to be reduced. For example a milling operation which previously took over 20 minutes can be accomplished with the present invention in about 6 minutes thus reducing the problems associated with image drift. In addition, the useful life of source  16  has increased with the present invention. 
     Although the invention has been described in part by making detailed reference to certain specific embodiments, such detail is intended to be, and will be understood to be, instructional rather than restrictive. It will be appreciated by those skilled in the art that many variations may be made on the structure and mode of operation without departing from the spirit and scope of the invention as disclosed in the teachings contained herein. For example the cable double shielding could be extended into column  12  such that the shields would extend up to the individual components inside column  12 .