Abstract:
Disclosed is an apparatus for electron beam inspection of a specimen with improved potential throughput. The apparatus includes an immersion objective lens focusing the primary electrons into a beam that impinges onto a spot on the specimen. Also disclosed is a method for automatic electron beam inspection of a specimen. The method includes producing a magnetic field towards the specimen that reduces aberration towards an outer portion of the multiple pixel imaging region.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to specimen inspection. More particularly, the present invention relates to e-beam inspection systems. 
   2. Description of the Background Art 
   An example of an electron beam (e-beam) inspection system is shown in  FIG. 1  for purposes of background explanation. The secondary electron emission microscope (SEEM) system of  FIG. 1  is a projection type system, where a large spot of electrons rather than a small one is formed at the surface of the specimen, and the secondary electrons from this spot are imaged onto a two-dimensional detector. Typically, the specimen may comprise a semiconductor wafer having integrated circuit related structures formed on its surface. Alternatively, the specimen may be another type of sample. 
   The system of  FIG. 1  is described in U.S. Pat. No. 5,973,323, entitled “Apparatus and Method for Secondary Electron Emission Microscope,” inventors Adler et al., and assigned at issuance to KLA-Tencor Corporation of San Jose, Calif. The disclosure of U.S. Pat. No. 5,973,323 is hereby incorporated by reference. As described in that patent,  FIG. 1  shows the basic configuration for the Secondary Electron Emission Microscopy (SEEM) apparatus. An electron gun source  10  emits a beam  11  of primary electrons e 1  along path  12 . The electron beam  11  is collimated by electron lens  13  and continues along path  12 . Magnetic beam separator  14  then bends the collimated electron beam  11  to be incident along electron optical axis OA normal to the surface to be inspected. Objective electron lens  15  focuses the primary electrons, e 1 , into a beam having a spot size typically in the range 1-10 mm and an incident energy on the order of 1 keV on specimen  9 . 
   Primary electrons e 1  incident on the specimen  9  produce secondary electrons e 2  which travel back along the axis OA perpendicular to the inspection surface to objective electron lens  15 , where they are re-collimated. Magnetic beam separator  14  bends the electrons to travel along image path  16 . The electron beam along image path  16  is focused by projection electron lens  17  to image plane  18 , where there is an electron detector  19 , which is a camera or preferably a time delay integrating (TDI) electron detector. The operation of an analogous TDI optical detector is disclosed in U.S. Pat. No. 4,877,326, entitled “Method and Apparatus for Optical Inspection of Substrates,” inventors Chadwick et al., and assigned at issuance to KLA Instruments Corporation. The disclosure of U.S. Pat. No. 4,877,326 is incorporated herein by reference. The image information may be processed directly from a ‘back thin’ TDI electron detector  19 , or the electron beam may be converted into a light beam and detected with an optional optical system  20  and a TDI optical detector. 
     FIG. 2  shows parallel imaging in the Secondary Electron Emission Microscopy inspection technique. Beam  54  is produced from an electron gun source, and beam  54  has a width “W,” typically about one to two millimeters, at the surface of sample  55 . Sample  55  has the characteristic dimension D, which is much greater than the width W of the electron beam. In SEEM, the width of the electron beam  54  is much larger than in secondary electron microscopy (SEM), but it may still be necessary to move the sample  55  with respect to the beam to scan the sample  55 . However, in the preferred embodiment, SEEM requires only mechanical movement of the stage of the sample  55  with respect to beam  54 , and not an electron beam deflection system for electromagnetically steering beam  41 . The SEEM inspection system can operate much faster than the SEM inspection system because SEEM images thousands or millions of pixels in parallel. 
     FIG. 2  further shows a magnified view of the imaging portion of the beam  54  on the sample  55  to illustrate the parallel, multiple pixel imaging region  56  within beam  54 . A rectangular detector array region  56  occupies a central portion of the beam  54  and defines the imaging aperture. (The detector array is either of the time delay integrating (TDI) or non-integrating type.) The detector array  56  may, for example, image between about 500 thousand and one million pixels in parallel. 
   Despite advances in e-beam inspection, such as SEEM described above, further improvement may be made. For example, it is typically desirable to increase the throughput of an inspection system (the rate at which specimens may be inspected by the system). Factors that limit the throughput of an e-beam inspection system include the usable size and intensity of the beam at the specimen plane. Generally, the larger the usable size of the beam and the higher the usable intensity of the beam, the higher the potential throughput. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the basic configuration for the Secondary Electron Emission Microscopy apparatus. 
       FIG. 2  shows parallel imaging in the Secondary Electron Emission Microscopy inspection technique. 
       FIG. 3  illustrates a cross-section of an electrostatic objective lens used in an e-beam inspection system. 
       FIG. 4  illustrates a cross-section of a conventional magnetic objective lens used in an e-beam inspection system. 
       FIG. 5  illustrates an immersion objective lens used in an e-beam inspection system in accordance with an embodiment of the invention. 
       FIG. 6  depicts a simulation of electron trajectories for a system with a conventional electrostatic objective lens. 
       FIG. 7  depicts a simulation of electron trajectories for a system with an immersion objective lens in accordance with an embodiment of the invention. 
   

   SUMMARY 
   The present invention provides an apparatus and method for electron beam inspection of a specimen with improved potential throughput. The apparatus includes an immersion objective lens focusing the primary electrons onto an area of the specimen while producing a magnetic field towards the specimen. The method includes producing a magnetic field towards the specimen that reduces aberration towards an outer portion of the multiple pixel imaging region. 
   DETAILED DESCRIPTION 
   As described above, the usable size and intensity of the beam at the specimen plane may limit the throughput of an e-beam inspection system. Hence, in order to increase the potential throughput of the system, it is desirable to increase the usable beam size and/or intensity. 
   The present invention uses an immersion objective lens to raise the achievable throughput of e-beam inspection systems. One embodiment of the present invention improves the potential throughput of an SEEM system. The immersion objective lens replaces previously used electrostatic or conventional magnetic objective lenses. 
     FIG. 3  illustrates a cross-section of an electrostatic objective lens  302  used in an e-beam inspection system  300 . Shown in  FIG. 3  are the specimen  9  (corresponding to specimen  9  in  FIG. 1 ) and the electrostatic objective lens  302  (corresponding to the objective lens  15  in FIG.  1 ). The electrostatic objective lens  302  comprises electrodes  304  that produce an electrostatic field that focuses the e-beam onto an appropriate spot area of the specimen  9 . 
   These previously used electrostatic objective lenses  302  are disadvantageous in that they may cause large aberrations for large imaged areas towards the outer portions of the e-beam. This results in a smaller usable beam size. In addition, electrostatic objective lenses  302  are relatively difficult to mechanically design and implement due to potential electrical arcing between electrodes  304 . 
     FIG. 4  illustrates a cross-section of a conventional magnetic objective lens  402  used in an e-beam inspection system  400 . Shown in  FIG. 3  are the specimen  9  and the conventional magnetic objective lens  402  (corresponding to the objective lens  15  in FIG.  1 ). The conventional magnetic objective lens  402  comprises a current driven electromagnet  404  that produces a magnetic field. The magnetic field is primarily produced from the pole pieces  406  of the electromagnetic structure  404 . In conventional magnetic objective lenses  402 , the gap between the pole pieces  406  faces the optical axis OA. The magnetic field produced by the electromagnetic structure  404  focuses the e-beam onto an appropriate spot area of the specimen  9 . 
   These previously used conventional magnetic objective lenses  402  cause substantial aberrations for large imaged areas towards the outer portions of the e-beam (although these are typically less than those of a comparable electrostatic objective lens). The aberrations are at least in part due to the divergent action of the acceleration field on the secondary electrons coming from the specimen  9 . This divergence of secondary electrons when using a conventional electrostatic lens  302  or a conventional magnetic lens  402  is described further below in relation to FIG.  6 . 
     FIG. 5  illustrates an immersion objective lens  502  used in an e-beam inspection system  500  in accordance with an embodiment of the invention. An immersion objective lens  502  differs from a conventional magnetic objective lens  402  in that the electromagnetic structure  504  has a gap between pole pieces  506  that faces the specimen  9  (instead of facing the optical axis). 
   The immersion objective lens may comprise a current driven electromagnet  504  that produces a magnetic field. The magnetic field is primarily produced from the pole pieces  506  on the bottom portion of the electromagnetic structure  504 . The gap between the pole pieces  506  faces the specimen  9 . The magnetic field produced by the electromagnetic structure  504  not only focuses the e-beam onto an appropriate spot area of the specimen  9 , but it also immerses the specimen  9  in a magnetic field. In a preferred embodiment of the invention, the electromagnetic structure  504  is axially symmetric about the optical axis, so that the specimen  9  is immersed in a magnetic field that is also axially symmetric about the optical axis. 
   Advantageously, using an immersion objective lens  502  in the e-beam inspection system  500  reduces the aberration problems that effectively limit the usable spot size of the electron beam. This is because the magnetic field at the specimen  9  reduces the divergence of secondary electrons traveling from the specimen  9 . This reduced divergence of secondary electrons when using a immersion objective lens  502  is described further below in relation to FIG.  7 . 
     FIG. 6  depicts a simulation of electron trajectories for a system with a conventional electrostatic objective lens. As shown in  FIG. 6 , the secondary electrons that are farther away from the optical axis tend to deviate from its intended trajectories, resulting in increased blur and displacement of the beam. 
   The dots in the diagram represent simulated electrons. One can see that the simulated electrons hit close to the grid intersections (the intended trajectory locations) towards the center of the field (near the optic axis) and are more spread out towards the edge of the field. The closed curves represent regions where the simulated electrons are statistically likely to hit. The regions are tighter around the grid intersections towards the center of the field and are more spread out towards the edge of the field. 
     FIG. 7  depicts a simulation of electron trajectories for a system with an immersion objective lens in accordance with an embodiment of the invention. As shown in  FIG. 7 , the secondary electrons that are farther away from the optical axis deviate significantly less than the secondary electrons in  FIG. 6 , resulting in reduced blur and displacement. This is due to the use of the immersion objective lens  502 . The immersion objective lens  502  “immerses” the specimen  9  in an axially symmetric magnetic field. The axially symmetric magnetic field causes the secondary electrons to have a spiral component to their motion, and this spiraling lessens the divergence as the secondary electrons travel from the specimen  9 . 
   Again, the dots in the diagram represent simulated electrons, and the closed curves represent regions where the simulated electrons are statistically likely to hit. Here, the simulated electrons hit close to the grid intersections (the intended trajectory locations) throughout the field, and the regions are tighter around the grid intersections throughout the field. 
   The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. Specific dimensions, geometries, and lens currents of the immersion objective lens will vary and depend on each implementation. 
   The above-described invention may be used in an automatic inspection system and applied to the inspection of wafers, X-ray masks and similar substrates in a production environment. While it is expected that the predominant use of the invention will be for the inspection of wafers, optical masks, X-ray masks, electron-beam-proximity masks and stencil masks, the techniques disclosed here may be applicable to the high speed electron beam imaging of any material (including perhaps biological samples). 
   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.