Abstract:
One embodiment relates to an electron beam apparatus. The apparatus includes a source for generating an incident electron beam, an electron lens for focusing the incident electron beam so that the beam impinges upon a substrate surface and interacts with surface material so as to cause secondary emission of scattered electrons, and a detector configured to detect the scattered electrons. The apparatus further includes an advantageous device configured to trap the scattered electrons which are emitted at sharp angles relative to the sample surface plane of the substrate surface. Other embodiments are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of provisional patent application No. 60/711,856 filed Aug. 26, 2005, entitled “Sharp Scattering Angle Trap for Electron Beam Apparatus,” by inventors Yehiel Gotkis et al., which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electron beam apparatus. 
     2. Description of the Background Art 
     Electron beam apparatus include, for example, scanning electron microscopes (SEMs), electron beam inspection/review tools, and electron beam metrology tools. The image quality of an electron beam apparatus is often undesirably compromised or reduced by various factors. 
     It is highly desirable to improve the quality of images obtained using electron beam apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustrative diagram depicting a configuration in a conventional electron beam apparatus. 
         FIG. 2  shows example scattered electron trajectories in the conventional electron beam apparatus. 
         FIG. 3  is an illustrative diagram depicting a configuration in an electron beam apparatus with a sharp scattering angle trap in accordance with an embodiment of the invention. 
         FIG. 4  schematically shows example scattered electron trajectories in the electron beam apparatus with a sharp scattering angle trap in accordance with an embodiment of the invention. 
         FIG. 5  shows an example trajectory of a sharp scattering angle electron in a conventional apparatus without a sharp scattering angle trap. 
         FIG. 6  shows electron micrographs of a surface structure taken without a sharp scattering angle trap and with a sharp scattering angle trap. 
         FIG. 7  shows electron micrographs of an edge structure taken without a sharp scattering angle trap and with a sharp scattering angle trap. 
         FIG. 8  shows electron micrographs of an area with particle defects taken without a sharp scattering angle trap and with a sharp scattering angle trap. 
         FIG. 9  depicts a plan (top) view of a configuration with a single electron trap in accordance with an embodiment of the invention. 
         FIG. 10  depicts a plan (top) view of a configuration with multiple electron traps in accordance with an embodiment of the invention. 
         FIG. 11  is an illustrative diagram depicting a configuration with an upper detector and a ring-shaped trap in accordance with an embodiment of the invention. 
     
    
    
     SUMMARY 
     One embodiment relates to an electron beam apparatus. The apparatus includes a source for generating an incident electron beam, an electron lens for focusing the incident electron beam so that the beam impinges upon a substrate surface and interacts with surface material so as to cause emission of scattered electrons, and a detector configured to detect the scattered electrons. The apparatus further includes an advantageous device configured to trap the scattered electrons which are emitted at sharp angles relative to the substrate surface plane. 
     Another embodiment relates to a method of imaging a surface area. An incident beam is generated, and the incident beam is focused so that the beam impinges upon a substrate surface and interacts with surface material so as to cause emission of scattered electrons. The scattered electrons are detected. Advantageously, there is trapping of the scattered electrons which are emitted at sharp angles relative to the substrate surface plane. 
     Other embodiments are also disclosed. 
     DETAILED DESCRIPTION 
     The present application discloses a technique to substantially improve image quality using an electron beam apparatus. As disclosed herein, the technique utilizes a mechanism to trap or screen electrons scattered at sharp (relatively to the sample surface plane) scattering angles. Unexpectedly and surprisingly, electron micrographs obtained using the technique disclosed herein appears to have substantially reduced adverse surface imaging effects. 
       FIG. 1  is a schematic diagram depicting a configuration in a conventional electron beam apparatus. Electron beam apparatus typically comprise, among other components, an electron gun or source  101 , condenser lenses (not depicted), beam deflectors  105 , electron lenses  103 , and a detection system  108 . 
     In  FIG. 1 , an electron source  101  generates an incident electron beam  102 . One or more electron lens  103  focuses the beam  102  so that it impinges upon a surface of a semiconductor wafer  104 . The wafer  104  is shown as being held in a stage  106 . Deflectors  105  may be used to scan the beam  102  over the area being imaged. 
     A detector  108  for detecting secondary or scattered electrons is also depicted. As shown, the stage  106  may be electrically grounded, and a positive voltage +V detector  may be applied to the detector  108  so as to attract the scattered electrons. 
       FIG. 2  schematically shows example scattered electron trajectories  202  in the conventional electron beam apparatus. In such a conventional electron beam apparatus, the probing electron beam  102  interacts with the surface material of the object under observation (for example, the surface of the wafer  104 ), and the resulting scattered electrons  202  are the information carriers about the structure under observation. 
       FIG. 3  is a schematic diagram depicting a configuration in an electron beam apparatus with a sharp scattering angle trap in accordance with an embodiment of the invention. As shown, the sharp scattering angle trap may be implemented as a trap electrode  302  to which a positive voltage +V trap  is applied relative to the stage potential. Preferably, the trap electrode  302  is positioned very closely to the surface of the substrate under observation. The trap electrode  302  may include a semi-circular or otherwise shaped portion  304  at the end near the area under observation. This portion  304  may be used to increase the trapping effectiveness of the electrode  302 . 
       FIG. 4  shows example scattered electron trajectories in the electron beam apparatus with a sharp scattering angle trap in accordance with an embodiment of the invention. As shown in  FIG. 4 , scattered electrons  202  are generally attracted to and detected by the detector  108 . However, those electrons  402  which are scattered at sharp angles into the vicinity of the trap electrode  302  are attracted to and captured by the trap electrode  302 . In particular, the relative positive voltage attracts sharp-angle scattered electrons  402  to the end portion  304  of the trap electrode  302 . 
     Applicants believe that such electrons  402  scattered at sharp angles relative to the surface plane often are generated within the uppermost material layers of the substrate  104 . In other words, applicants believe that scattering events occurring within the uppermost material layers frequently result in sharp-angle scattered electrons  402  which travel at trajectories that are relatively near to the plane of the target surface. 
     A schematic trajectory  502  of such a sharp-angle scattered electron in a conventional apparatus is depicted in  FIG. 5 . Because there is no sharp scattering angle trap in the configuration depicted in  FIG. 5 , the detector  108  eventually attracts and detects the sharp-angle scattered electrons. Applicants believe that these sharp-angle scattered electrons produce surface associated and frequently undesirable imaging effects. 
     Scanning electron micrographs obtained by the applicants using a sharp scattering angle trap show substantial reduction of adverse surface imaging effects. For example,  FIG. 6  shows scanning electron micrographs of a surface structure taken without a sharp scattering angle trap and with a sharp scattering angle trap. The upper electron micrograph  600  of  FIG. 6  was taken without a sharp scattering angle trap and shows overly bright areas  602  which are obscured by a substantially higher intensity of detected electrons. Applicants believe that the higher intensity in these areas  602  is due to the upper surface layers having been intentionally modified. 
     In contrast, the lower electron micrograph  610  of  FIG. 6  was taken with a sharp scattering angle trap being activated. This lower micrograph  610  is of the same area as the upper micrograph  600 . Remarkably, the previously overly bright areas  612  are no longer obscured. Instead, substantial detail in these areas  612  is now visible. Advantageously, the improved visibility of the surface details provides for improved defect detection and/or review using an inspection or review tool. 
     Another example is shown in  FIG. 7 .  FIG. 7  shows electron micrographs of an edge structure taken without a sharp scattering angle trap and with a sharp scattering angle trap. The upper electron micrograph  700  of  FIG. 7  was taken without a sharp scattering angle trap and shows substantial edge over-contrast. The edge over-contrast results in a relatively thick bright horizontal line  702  which masks the feature edge, making any edge associated measurements unreliable. Applicants believe that this edge over-contrast is due to a relatively higher proportion of electrons being scattered at sharp scattering angles in the vicinity of the edge. 
     In contrast, the lower electron micrograph  710  of  FIG. 7  was taken with a sharp scattering angle trap being activated. This lower micrograph  710  is of the same area as the upper micrograph  700 . Remarkably, the over-contrast at the edge  712  appears to have disappeared, such that the edge  712  is much better pronounced in the lower micrograph  710 . Advantageously, more accurate edge profile characterization and associated measurements of critical dimensions may be made with such better-defined edges using a critical dimension scanning electron microscope (CD-SEM). 
     A third example is shown in  FIG. 8 .  FIG. 8  shows electron micrographs of an area with particle defects taken without a sharp scattering angle trap and with a sharp scattering angle trap. The upper electron micrograph  800  of  FIG. 8  was taken without a sharp scattering angle trap and shows darker regions  802  around the particles obscuring detail in the image. Applicants believe that the darker regions are due to the charging of particles on the surface of the substrate. 
     In contrast, the lower electron micrograph  810  of  FIG. 8  was taken with a sharp scattering angle trap being activated. This lower micrograph  810  is of the same area as the upper micrograph  800 . Remarkably, the darker regions  802  due to particle charging have disappeared, such that the particles  812  and other surface details are much better defined in the lower micrograph  810 . Advantageously, such more accurate surface details provides for improved defect detection and/or review using an inspection or review tool. 
       FIG. 9  depicts a plan (top) view of a configuration with a single trap electrode  302  in accordance with an embodiment of the invention. As shown in  FIG. 9 , the trap electrode  302  is preferably positioned across from the secondary electron detector  108 . In other words, the trap electrode  302  and the detector  108  are on opposite sides of the area being imaged. 
       FIG. 10  depicts a plan (top) view of a configuration with multiple trap electrodes  302  in accordance with an embodiment of the invention. In the particular example shown, there are two trap electrodes  302 , each electrode  302  being positioned opposite to the secondary electron detector  108 . Of course, more than two electrodes and more than a single detector may be used in other implementations. 
       FIG. 11  is an illustrative diagram depicting a configuration with an upper detector  1102  and a ring-shaped trap electrode  1104  in accordance with an embodiment of the invention. The ring-shaped trap electrode  1104  is configured circumferentially around the area being imaged. As in the other embodiments, the ring-shaped trap electrode  1104  has a positive voltage applied thereto relative to the stage potential so as to attract the sharp-angle scattered electrons. Meanwhile, other electrons (which are not sharp-angle scattered) are still able to make their way up the column to the upper detector  1102 . 
     The above discussion includes theoretical reasons for the improved imaging observed in accordance with embodiments of the present invention. Although the theoretical reasons are believed to explain the improved imaging observed, it is not intended that the present invention necessarily be limited by such theory. 
     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.