Abstract:
One embodiment disclosed relates to an electron source for generating an electron beam. The electron source includes an electron emitter having a tip from which an electron beam is extracted. The electron further includes a non-planar extractor with an extractor opening and a built-in beam-limiting aperture. The extractor opening is larger than the beam-limiting aperture, and central axes of both the extractor opening and the beam-limiting aperture are aligned with the tip along a beam axis. Another embodiment relates to a method of generating an electron beam using an electron source having a non-planar extractor. Another embodiment relates to an array of electron sources for generating an array of electron beams. The array of electron sources includes an array of electron emitters and an array of non-planar extractor structures. Other embodiments, aspects and features are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electron beam apparatus and methods of using same. 
     2. Description of the Background Art 
     Electron beam apparatus include scanning electron microscope (SEM) instruments, such as those used in automated inspection and review of manufactured substrates, electron beam lithography systems, and other apparatus that use electron beam technology. Such electron beam apparatus generally generate one or more beams of electrons using an electron source or an array of electron sources. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional diagram of an electron source with a conventional planar extractor. 
         FIG. 2  is a cross-sectional diagram of an electron source with a non-planar extractor having a built-in beam-limiting aperture in accordance with an embodiment of the invention. 
         FIG. 3  is a cross-sectional diagram of an electron source with a “volcano-shaped” extractor having a built-in beam-limiting aperture in accordance with another embodiment of the invention. 
         FIG. 4  is a planar diagram of a one-dimensional array of electron sources in accordance with an embodiment of the invention. 
         FIG. 5  is a planar diagram of a two-dimensional array of electron sources in accordance with an embodiment of the invention. 
     
    
    
     SUMMARY 
     One embodiment disclosed relates to an electron source for generating an electron beam. The electron source includes an electron emitter having a tip from which an electron beam is extracted. The electron source further includes a non-planar extractor with an extractor opening and a built-in beam-limiting aperture. The extractor opening is larger than the beam-limiting aperture, and central axes of both the extractor opening and the beam-limiting aperture are aligned with the tip along a beam axis. 
     Another embodiment relates to a method of generating an electron beam using an electron source having a non-planar extractor. The electron beam is travels through both an extractor opening and a beam-limiting aperture of the non-planar extractor 
     Another embodiment relates to an array of electron sources for generating an array of electron beams. The array of electron sources includes an array of electron emitters and an array of non-planar extractor structures. 
     Other embodiments, aspects and features are also disclosed. 
     DETAILED DESCRIPTION 
       FIG. 1  is a cross-sectional diagram of an electron source  100  with a conventional planar extractor  104 . The electron source  102  is radially symmetric around the beam axis of the electron beam  108 , such that the cross-sectional diagram is through a diameter of the electron source  100 . 
     As shown in  FIG. 1 , the source  100  includes an emitter  102  that has a tip  103  from which an electron beam  108  is emitted. The electron beam  108  is transmitted through an extractor opening  118  of the planar extractor  104 . As further shown, the planar extractor  104  has a uniform thickness  122 . There is an emitter-extractor gap  112  between a plane of the emitter  102  and the planar extractor  104 . 
     After passing through the extractor opening  130 , the electron beam  108  may be focused by the pole piece  106  of a magnetic lens. As depicted in  FIG. 1 , there is a pole piece-extractor gap  132  between the planar extractor  104  and the pole piece  106 . 
     Applicants have determined a few disadvantages of the structure of the conventional electron source  100 . First, the relatively large trapped volume in the structure results in a relatively poor vacuum in the emitter region. In addition, angular beam filtering, which is typically necessary for most sources, has to occur in a separate module (not shown) which is further down in the column. 
       FIG. 2  is a cross-sectional diagram of an electron source  200  with a non-planar extractor  204  having a built-in beam-limiting aperture  230  in accordance with an embodiment of the invention. The electron source  202  is radially symmetric around the beam axis of the electron beam  208 , such that the cross-sectional diagram is through a diameter of the electron source  200 . 
     As shown in  FIG. 2 , the source  200  includes an emitter  202  that has a tip  203  from which an electron beam  208  is emitted. The tip  203  of the emitter  202  may be formed, for example, using tungsten (W), tungsten with a layer of zirconium oxide (ZrO/W), or carbon nanotubes. 
     The electron beam  208  is transmitted through a larger cylindrical extractor opening  218  and through a smaller cylindrical beam-limiting aperture  230  of the non-planar extractor  204 , where a central axis of both the extractor opening  218  and the beam-limiting aperture  230  are aligned with the tip  203  of the emitter  202 . The non-planar extractor  204  may be formed, for example, using gold or another conductive non-magnetic metal or alloy. Use of a magnetic metal or alloy for the non-planar extractor  204  may also be possible. 
     As further shown, the beam-limiting aperture  230  of the non-planar extractor  204  has an aperture thickness  231  at the beam-limiting aperture  230  which is substantially less than an outer thickness  222  at an outer radius  223 . There is an outer gap  212  between a plane of the emitter  202  and the outer radius  223  of the non-planar extractor  204 . 
     The transition between the outer radius  223  and the extractor opening  218  may include a rounded or radiused edge  226 . The transition between the extractor opening  218  and the beam-limiting aperture  230  may include an inner sloped surface  228  of the non-planar extractor  204 . 
     After being angularly limited by the beam-limiting aperture  230 , the electron beam  208  may be focused by the pole piece  206  of a magnetic gun lens. As depicted in  FIG. 2 , there is a pole piece-extractor gap  232  between the non-planar extractor  204  and the pole piece  206 . 
     As further depicted in  FIG. 2 , a differential pumping system  240  may be configured so as to pump the volume between the emitter  202  and the extractor  204  to a higher vacuum (lower pressure) while the volume on the other side of the extractor  204  is pumped to a lower vacuum. Such differential pumping is more effective with the non-planar extractor  204  in comparison to the conventional planar extractor  104 . 
     Advantageously, the structure of the electron source  200  in  FIG. 2  also provides a beam-limiting aperture  230  to filter the angular trajectories of the electrons in the beam in close proximity to the emitter  202 . In addition, the structure of the electron source  200  in  FIG. 2  may be implemented with an extractor geometry that is separate from the emitter to allow for modular construction for easy replacement of components such as the emitter  102 . Furthermore, the structure of the electron source  200  in  FIG. 2  allows for high vacuum in the vicinity of the tip  203  of the emitter  202 . 
       FIG. 3  is a cross-sectional diagram of an electron source  300  with a non-planar “volcano-shaped” extractor  304  having a built-in beam-limiting aperture  330  in accordance with another embodiment of the invention. The electron source  302  is radially symmetric around the beam axis of the electron beam  308 , such that the cross-sectional diagram is through a diameter of the electron source  300 . 
     As shown in  FIG. 3 , the source  300  includes an emitter  302  that has a tip  303  from which an electron beam  308  is emitted. The tip  303  of the emitter  302  may be formed, for example, using tungsten (W), tungsten with a layer of zirconium oxide (ZrO/W), or carbon nanotubes. 
     The electron beam  308  is transmitted through a larger cylindrical extractor opening  318  and through a smaller cylindrical beam-limiting aperture  330  of the volcano-shaped extractor  304 , where a central axis of both the extractor opening  318  and the beam-limiting aperture  330  are aligned with the tip  303  of the emitter  302 . The volcano-shaped extractor  304  may be formed, for example, using gold or another conductive non-magnetic metal or alloy. Use of a magnetic metal or alloy for the volcano-shaped extractor  304  may also be possible. 
     As further shown, the beam-limiting aperture  330  of the volcano-shaped extractor  304  has an aperture thickness  331  at the beam-limiting aperture  330  which is substantially less than an outer thickness  322  at an outer radius  323 . There is an outer gap  312  between a plane of the emitter  302  and the outer radius  323  of the volcano-shaped extractor  304 . 
     The transition between the outer radius  323  and the extractor opening  318  may include an outer sloped surface  324  followed by a rounded or radiused edge  326 . The transition between the extractor opening  318  and the beam-limiting aperture  330  may include an inner sloped surface  328  of the extractor  304 . 
     In accordance with an embodiment of the invention, there may be a minimum gap  314  between a circle formed at an outer radial extent of the rounded or radiused edge  326  and the plane of the emitter  302 . A high-field region  316  is formed within a cylindrical volume defined by the minimum gap  314  which includes the tip  303  of the emitter  302  therein. 
     After being angularly-limited by the beam-limiting aperture  330 , the electron beam  308  may be focused by the pole piece  306  of a magnetic lens. As depicted in  FIG. 3 , there is a pole piece-extractor gap  332  between the volcano-shaped extractor  304  and the pole piece  306 . 
     As further depicted in  FIG. 3 , a differential pumping system  340  may be configured so as to pump the volume between the emitter  302  and the extractor  304  to a higher vacuum (lower pressure) while the volume on the other side of the extractor  304  is pumped to a lower vacuum. Such differential pumping is more effective with the non-planar extractor  304  in comparison to the conventional planar extractor  104 . 
     Advantageously, the structure of the volcano-shaped electron source  300  in  FIG. 3  provides a substantially enhanced local extraction field for a small emitter or for each emitter in a MEMS (micro electro mechanical system) based emitter array. In addition, the structure of the electron source  300  in  FIG. 3  allows for high vacuum in the vicinity of the tip  303  of the emitter  302 . 
     The structure of the electron source  300  in  FIG. 3  also provides a beam-limiting aperture  330  to filter the angular trajectories of the electrons in the beam in close proximity to the emitter  302 . In addition, the structure of the electron source  300  in  FIG. 3  may be implemented with an extractor geometry that is separate from the emitter to allow for modular construction for easy replacement of components such as the emitter  302 . 
       FIG. 4  is a planar diagram of a one-dimensional array  400  of electron sources  402  in accordance with an embodiment of the invention. In one implementation, the array  400  of electron sources  402  may be formed using micro-electrical mechanical systems (MEMS) technology. Each electron source  402  in the array  400  may comprise an electron source  200  or  300  as described above. 
       FIG. 5  is a planar diagram of a two-dimensional array of electron sources in accordance with an embodiment of the invention. In one implementation, the array  500  of electron sources  502  may be formed using MEMS technology. Each electron source  502  in the array  500  may comprise an electron source  200  or  300  as described above. 
     The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. In the above description, numerous specific details are given to provide a thorough understanding of embodiments of the invention. However, the above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the invention. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.