Abstract:
A background reduction system may include, but is not limited to: a charged particle source configured to generate a charged-particle beam; a louvered structure including one or more apertures configured to selectively transmit charged particles according to their angle of incidence; and a charged-particle detector configured to receive charged particles selectively transmitted by the louvered structure.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates to reducing background noise in particle detectors by angular transmission filtering with a louver. 
       BACKGROUND 
       [0002]    When a charged particle strikes a surface, the particle may be scattered back (possibly losing some energy to the surface); induce the emission of a secondary particle (either ion or electron), or cause the release of a photon from the surface. From these outgoing (hereafter secondary) particles, characteristics of the surface can be determined, for example physical structure or material composition. The secondary particles can also, in turn, strike other surfaces inside an experiment, leading to tertiary emission and so forth. Since in many measurements, only the first surface is of interest, emission from other surfaces constitutes an unwanted background to the measurement. 
         [0003]    As a specific example, the composition of a surface can be determined by studying Auger emitted electrons. Auger electrons can be observed by looking at the energy spectrum of the particles leaving the surface struck by a primary beam. The energy spectrum is measured by an energy dispersive analyzer: for example a hemispherical or cylindrical mirror analyzers or magnetic sector, to name a few. These devices observe a window of energies and exit angles from the surface. However, electrons from other surfaces outside or sometime inside the analyzer can be generated and reach the detector plane of the analyzer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a cross-sectional diagram depicting components of a background reduction system. 
           [0005]      FIG. 2  is a cross-sectional diagram depicting a louver. 
           [0006]      FIG. 3  is a plot of transmittance window of an example louver for background reduction. The figure overlays the aperture transmission window with that of the louver. 
           [0007]      FIG. 4  is a cross-sectional diagram depicting a louvered portion of a background reduction system including apertures having a first angle of acceptance and apertures having a second angle of acceptance. 
           [0008]      FIG. 5A  shows a top view of a louvered portion of a background reduction system including substantially linear apertures. 
           [0009]      FIG. 5B  shows a top view of a louvered portion of a background reduction system including substantially arcuate apertures. 
       
    
    
     SUMMARY 
       [0010]    A charged-particle scanning system may include, but is not limited to: a charged particle source configured to generate a charged-particle beam; a louvered structure including one or more apertures configured to selectively transmit charged particles according to their angle of incidence; and a charged-particle detector configured to receive charged particles selectively transmitted by the louvered structure. 
       DETAILED DESCRIPTION 
       [0011]    During particle detection operations such as those performed in electron scattering detection processes for semiconductor wafer certification it may be the case that secondary and tertiary scattering outside or inside the detector may constitute a significant contribution to an overall detected signal, thereby necessitating long integration times and substantial back-end processing of detected signals to reduce the signal-to-noise ratio to allowable levels. The below described systems and methods provide various mechanisms for reducing the level of background emissions presented to the detector of particle detection system. 
         [0012]    In  FIG. 1 , a cross-sectional diagram depicting components of a scanning detection system  100  which includes an energy analyzer  101  is illustrated. As shown, a charged-particle beam  102  (e.g. an electron beam) originates from an charged-particle source  103  (e.g. an electron gun) and travels down an optical axis and through an objective lens  104  to become focused upon the surface of a target substrate  105 . 
         [0013]    The energy analyzer  101  is positioned to detect secondary charged particles emitted from the target substrate  105  due to the impingement of the charged-particle beam  102  on the target substrate  105 . 
         [0014]    Charged-particle trajectories  106  are depicted for secondary charged particles emitted from the target substrate  105  following impingement of the charged-particle beam  102  on the target substrate  105 . As indicated, the secondary charged particles whose trajectories  106  are within a certain range of polar angles θ may pass through an entrance aperture  107 . 
         [0015]    The trajectories  106  of the charged particles may be deflected away from an electrode  108 . The deflected charged particles may impinge upon a detector  109 . 
         [0016]    In a particular example the detector  109  may be a position-sensitive detector  109 . Higher-energy charged particles travel farther and impinge upon the detector  109  at positions farther away from a z-axis defined by the charged-particle beam  102 . For purposes of illustration,  FIG. 1  depicts the trajectories of charged particles with various initial polar angles θ (e.g. about 30 degrees) but with one of three example energy levels. The charged particles at the lower energy level land at a closer radial position  110  along the detector  109 . The charged particles at the middle energy level land at a middle radial position  111  along the detector  109 . Finally, the charged particles at the higher energy level land at a farther radial position  112  along the detector  109 . The detector  109  may be configured to detect such positions (e.g. through a matrix of detector cells) to provide position-dependent data to a back-end processing device for analysis. The trajectories  106  of the charged particles may be such that the charged particles take a substantially uninterrupted path (e.g. do not reflect off of any surface) between the target substrate  105  and the detector  109 . 
         [0017]    As described above, it may be the case that background emissions may constitute a significant contribution to an overall detected signal, thereby necessitating long integration times and substantial back-end processing of detected signals to reduce the signal-to-noise ration to allowable levels. For example, as shown in  FIG. 1 , background emissions  113  (e.g. charged particles not emanating from the impingement of the charged-particle beam  102  on the target substrate  105 , charged particles emanating from the impingement of the charged-particle beam  102  on the target substrate  105  that reflect off an intervening surface between the target substrate  105  and the detector  109 , and the like) may be present within the energy analyzer  101 . 
         [0018]    In order to reduce the amount of background emissions  113  which reach the detector  109 , the system  100  may include a louver structure  114  which may be configured to allow only particles having trajectories  106  within a defined angle of acceptance relative to the louver structure  114  to reach the detector  109 . 
         [0019]    For example, as shown in  FIG. 2 , the louver structure  114  may be a substantially planar structure. The louver structure  114  may include one or more apertures  201  through a main body portion  202  of the louver structure  114  thereby allowing particles having varying trajectories  106  to pass through the louver structure  114 . The apertures  201  may be defined by one or more louver bars  203 . The size and spacing of the apertures  201  and/or louver bars  203  may be configured to specify a desired angle of acceptance for such particles. For example, as shown in  FIG. 2 , the apertures  201  may be configured parallel an angle of incidence α (e.g. from about 10 to 50 degrees) associated with a desired trajectory  106 . The apertures  201  of the louver structure  114  may have an apparent width W 1  (e.g. from about 0.010 to 0.015 inches or, more particularly, about 0.013 inches) while louver bars  203  of the louver structure  114  may have an apparent width W 2  (e.g. from about 0.002 to about 0.006 inches and, more particularly, about 0.004 inches) such that particles having trajectories  106  between an angle β and an angle γ (e.g. trajectories  106 ′ and 106″) may be transmitted by the louver structure  114  while particles having trajectories less than β or greater than γ will be reflected and/or absorbed by the louver structure  114 . In a specific example, an entrance band for the apertures  201  may be specified where α is approximately 30 degrees relative to the surface of the louver structure  114  and β and γ are +/−7.5 degrees, respectively, about α. Such an entrance band may result in a particle transmission ratio of approximately 53% (as detailed in  FIG. 3 ). 
         [0020]    In another example, the angles of acceptance of the apertures  201  of the louver structure  114  may vary across the louver structure  114  in order to accept particles of differing trajectories  106  at various portions of the louver structure  114 . For example, as shown in  FIG. 4 , a first portion  114 A of the louver structure  114  may have apertures  201  sized such that particles having trajectories  106  between an angle β and an angle γ (with respect to the surface of the louver structure  114 ) may pass through the louver structure  114  while particles having trajectories less than β or greater than γ (e.g. background emissions  113 ) will be reflected by the louver structure  114 . Further, a second portion  114 B of the louver structure  114  may have apertures  201 ′ configured such that particles having trajectories  106 ′ between an angle β′ and an angle γ′ (where β′ or γ′ of portion  114 B is different than β or γ, respectively, of portion  114 A) may pass through the louver structure  114  while particles having trajectories less than β′ or greater than γ′ (e.g. background emissions  113 ) will be reflected and/or absorbed by the louver structure  114 . 
         [0021]    In another example, as shown in  FIG. 5A , the apertures  201  may be substantially linear in shape and be disposed across the louver structure  114  in a substantially parallel manner. In another example, as shown in  FIG. 6B , the apertures  201  at least partially arcuate in shape (e.g. in a semi-circular shape with respect to the z-axis defined by the charged-particle beam  102 ) and disposed across the louver structure  114  in a substantially parallel manner. 
         [0022]    The louver structure  114  may be constructed from any material. Specifically, the louver structure  114  may be constructed from aluminum, stainless steel, titanium, and the like. Additionally, the louver structure  114  may further include one or more particle-absorbing coatings (e.g. a carbon sputter coating). 
         [0023]    It will be noted that the above described background reduction methodologies may be applied to any number of particle detectors. For example, the methodologies may be applied to hyperbolic-field and magnetic bend energy analyzers. 
         [0024]    The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. Specific dimensions and geometries will vary and depend on each implementation. 
         [0025]    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 surface analysis of any material (including possibly biological samples). 
         [0026]    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. 
         [0027]    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.