Abstract:
An electron beam source for use in an electron gun. The electron beam source includes an emitter terminating in a tip. The emitter is configured to generate an electron beam. The electron beam source further includes a suppressor electrode laterally surrounding the emitter such that the tip of the emitter protrudes through the suppressor electrode and an extractor electrode disposed adjacent the tip of the emitter. The extractor electrode comprises a magnetic disk whose magnetic field is aligned with an axis of the electron beam.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of U.S. provisional patent application Ser. No. 60/715,973, filed Sep. 10, 2005, which is herein incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention generally relate to electron guns (sources), and more particularly, electron guns that may be used, for instance, in electron beam lithography or electron microscopy. 
   2. Description of the Related Art 
   Electron beam columns are well known for use, for instance, in electron beam lithography for imaging a pattern onto a substrate typically coated with a resist sensitive to electron beams. Subsequent development of the exposed resist defines a pattern in the resist which later can be used as a pattern for etching or other processes. Electron beam columns are also used in electron microscopy for imaging surfaces and thin samples. Conventional electron beam columns for electron microscopy and lithography are well known and typically include an electron gun having an electron emitter for producing an electron beam. The beam from the gun may be used to produce a scanning probe or illuminate a sample or an aperture using a series of electron beam lenses, which may be magnetic or electrostatic. 
   Electron beam columns generally include a source of electrons, such as a Schottky emission gun or a field emission gun, which typically includes an emitter (cathode), an electrostatic pre-accelerator lens that focuses the electron beam and a series of lenses that refocuses and images the source aperture or sample onto the target. 
   It has generally been difficult to obtain very high beam currents focused into a small spot using Schottky electron sources. Although the brightness of the emitter is high in such sources, the angular intensity of the electron beam emerging from the emitter region is limited by the properties of the emitter itself. Consequently, a rather large aperture angle must be used in the electron gun, which makes spherical and chromatic aberration in the gun lens a major factor in limiting the small spot size that can be achieved, which is generally referred to as the smallest cross-section diameter of the beam. 
   One approach to reduce aberrations in the gun lens is to use a magnetic lens as the focus element. Using this approach, the emitter tip and the extraction region are immersed in a magnetic field, which results in a significant increase in the operating solid angle of emission compared to all-electrostatic systems. However, one disadvantage of this design is that the lens coil and its cooling fluid may float at near the tip potential, which requires a more complicated high voltage power supply and cable. Further, the mechanical design is a large departure from conventional Schottky or field emission designs, which adds further complication to the approach. 
   Other attempts to reduce aberrations in the gun lens have been made. However, those attempts have proven to be difficult since the size and focal length of standard electrostatic lenses are limited by the large stand-off distance required in high voltage systems. 
   Therefore, a need exists in the art for a new electron beam source for an electron gun with minimal aberrations. 
   SUMMARY OF THE INVENTION 
   Various embodiments of the invention are generally directed to an electron beam source for use in an electron gun. The electron beam source includes an emitter terminating in a tip. The emitter is configured to generate an electron beam. The electron beam source further includes a suppressor electrode laterally surrounding the emitter such that the tip of the emitter protrudes through the suppressor electrode and an extractor electrode disposed adjacent the tip of the emitter. The extractor electrode comprises a magnetic disk whose magnetic field is aligned with an axis of the electron beam. 
   Various embodiments of the invention are also generally directed to an electron beam source for use in an electron gun. The electron beam source includes an emitter terminating in a tip. The emitter is configured to generate an electron beam. The electron beam source further includes a suppressor electrode laterally surrounding the emitter such that the tip of the emitter protrudes through the suppressor electrode and an extractor electrode disposed adjacent the tip of the emitter. The extractor electrode comprises an extraction support and a magnetic disk disposed on the extraction support. The magnetic disk is a permanent magnet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  illustrates a side cross sectional view of a portion of an electron gun in accordance with one or more embodiments of the invention. 
       FIG. 2  illustrates a side cross sectional view of a portion of an electron gun in accordance with another embodiment of the invention. 
       FIG. 3  illustrates a top view and a cross sectional view of a magnetic disk in accordance with one or more embodiments of the invention. 
       FIG. 4  illustrates a plot of the magnetic field for the magnetic disk along the beam axis (axial flux density) in accordance with one or more embodiments of the invention. 
       FIG. 5  illustrates the effect of a shunt on the axial flux density between the magnetic disk and the focus electrode in accordance with one or more embodiments of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a side cross sectional view of a portion of an electron gun  10  in accordance with one or more embodiments of the invention. The remainder of the electron gun  10  is not shown. The electron gun  10  may be a field emission or Schottky emission gun. Details of such a device are described in L. Swanson and G. Schwind, “A Review of The ZrO/W Schottky Cathode”, Handbook of Charged Particle Optics editor Jon Orloff, CRC Press LLC, New York, (1997), which is incorporated herein by reference. The electron gun  10  includes an emitter (cathode)  14 , which is configured to generate an electron beam. The emitter  14  may be an oriented single crystal tungsten structure with a sharp point (approximately 1 micrometer radius) and mounted on a hair pin filament (not shown). The emitter  14  may be surrounded by a negatively biased suppressor electrode  16 , which may be a conductive structure that prevents thermionically emitted electrons from leaving the emitter  14  anywhere but near its tip. The pointed tip of the emitter  14  protrudes slightly from the suppressor electrode  16  and faces an extractor electrode  24 , which defines an upper aperture  29 . The extractor electrode  24  may be biased positively with respect to the emitter  14  and defines a lower aperture  28  below the upper aperture  29  to shape the electron beam entering the downstream gun lens (not shown). 
   In accordance with one embodiment of the invention, the extractor electrode  24  includes a magnetic disk  100  disposed on an extraction support  150 , which may be made from a non magnetic material. The magnetic disk  100  may be a permanent magnet made from materials such as samarium cobalt, neodymium iron boron and the like. The magnetic disk  100  is ring shaped (toroidal) having an opening  110  for allowing the electron beam to pass therethrough. In one embodiment, the top surface of the magnetic disk  100  is about 1 mm apart from the tip of the emitter  14 . The magnet disk  100  is disposed such that the axis of the opening  110  is aligned with the beam axis. In this manner, the magnetic disk  100  acts as a fixed focal length lens. The magnetic disk  100  may be encased in a stainless steel sheath for increasing structural rigidity or reducing contamination, e.g., outgassing or particulates. The electron gun  10  may further include a focus electrode  25 , such as an electrostatic lens, to further focus the electron beam coming out of the extractor electrode  24 . In this manner, the magnetic disk  100  has a fixed focal length, while the focus electrode  25  has a variable focal length (by varying the voltage). 
     FIG. 2  illustrates a side cross sectional view of a portion of an electron gun  210  in accordance with another embodiment of the invention. The electron gun  210  has an extractor electrode  224 , which includes an extraction aperture disk  220  disposed on a magnetic disk  200 , both of which are supported by an extraction support  250 . The extraction aperture disk  220  is configured to protect the magnetic disk  200  from being bombarded by the electron beam. The extraction aperture disk  220  may be made from non magnetic material, such as molybdenum, stainless steel, titanium and the like. The magnetic disk  200  may be a permanent magnet made from materials such as samarium cobalt, neodymium iron boron, and the like. The magnetic disk  200  is ring shaped (toroidal) having an opening  230  for allowing the electron beam to pass therethrough. The rest of the components of the electron gun  210 , e.g., an emitter  214  and a suppressor electrode  216 , are substantially the same as the components of the electron gun  10 . Accordingly, other details of various components of the electron gun  210  are provided with reference to the electron gun  10  described above. 
     FIG. 3  illustrates a top view and a cross sectional view of a magnetic disk  300  in accordance with one or more embodiments of the invention. In one embodiment, the magnetic disk  300  has an inner diameter (ID) of about 1 mm, an outer diameter (OD) of about 5 mm, a thickness (L) of about 1.025 mm, a taper bore angle (α) of about 0 degrees, and a saturation magnetization (M S ) of about 875 emu/cm 3 , which has been selected to match samarium cobalt type 32 HS. With such geometry, the magnetic disk  300  may be used to reduce the spherical aberration coefficient from about 19.8 mm (without magnetic disk) to about 2.9 mm (with magnetic disk). In this manner, embodiments of the invention may be used to reduce the spherical aberration coefficient of a conventional 50 kV electron gun by a factor of about 6. The spherical aberration coefficient may be further reduced to less than about 2.5 mm by increasing the OD to about 10 mm and decreasing the ID to about 0.5 mm. The spherical aberration coefficient may also be reduced by moving the magnetic disk closer to the emitter  14 . In addition, the magnetic field of the magnetic disk may collimate the electron beam, thereby increasing the effective angular intensity of the beam current. 
   In one embodiment, the magnetic field of the magnetic disk  100 ,  200 ,  300  is aligned with the beam axis. The magnetic field may be calculated everywhere in space, using a charge density method, such as one described in “Field Computation By Moment Methods” by Roger F. Harrington, Wiley-IEEE Press (1993). The magnetic field along the beam axis (axial flux density) may then be extracted to a file, which may be used as an input to an electron optical simulation program ABER by Munro&#39;s Electron Beam Software Ltd., headquartered in London, England. The optical properties and aberrations of the lenses are then computed. Such aberrations include spherical aberration, chromatic aberration, distortion, astigmatism, coma, and field curvature. The geometric parameters, i.e., ID, OD, L and α, of the magnetic disk as well as the location of the magnet disk may be varied to affect the optical properties and aberrations. 
     FIG. 4  illustrates a plot of the magnetic field for the magnetic disk along the beam axis (axial flux density)  400  in accordance with one or more embodiments of the invention. Notably, the axial flux density  400  is greatest at or substantially near the location of the magnetic disk. 
   It has been assumed that the axial flux density between the magnetic disk  100  and the focus electrode  25  would increase aberrations. Accordingly, a high permeability shunt may be added to the electron gun to reduce the axial flux density between the magnetic disk  100  and the focus electrode  25 . The shunt may be disposed as part of the extraction support  150  or the suppressor electrode  16 .  FIG. 5  illustrates the effect of a shunt  500  on the axial flux density between the magnetic disk  100  and the focus electrode  25 . The solid line represents the axial flux density  510  for the electron gun with the shunt  500 , while the dotted line represents the axial flux density  520  for the electron gun without the shunt  500 . Notably, axial flux density  510  between the magnetic disk  100  and the focus electrode  25  is significantly reduced to substantially zero. However, the spherical aberration coefficient may be higher for the electron gun with a shunt than for the electron gun without a shunt. On the other hand, placing a shunt as part of the suppressor electrode  16  may cause the axial flux density to extend farther into the extraction region, which may reduce aberrations. 
   In addition to adding a shunt to the electron gun, the thickness (L) of the magnetic disk may be reduced to reduce the magnitude of the axial flux density between the magnetic disk  100  and the focus electrode  25 . Further, the magnetic disk  100  may be disposed closer to the emitter  14  to reduce the spherical aberration coefficient. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.