Abstract:
One embodiment disclosed relates to an apparatus for detecting defects in substrates. An irradiation source is configured to generate an incident beam, and a lens system configured to focus the incident beam onto a target substrate so as to cause emission of electrons. A multiple-bin detector is configured to detect the emitted electrons, and each bin of the detector detects the emitted electrons within a range of energies. A processing system configured to process signals from the multiple-bin detector. Other embodiments are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to charged-particle apparatus for inspection and/or review and/or metrology of semiconductor wafers and other substrates. 
   2. Description of the Background Art 
   The detected signal in electron microscopes is typically a sum of secondary electrons and backscattered electrons. In some systems, the secondary electrons and backscattered electrons are separated and sent to different detectors. In some systems, the secondary electrons are subject to a threshold energy filter, with lower energies being discarded and higher energies being counted. 
   The use of such a threshold energy filter with a secondary electron detector has proved to be useful in enhancing the sensitivity to certain defect types in wafer inspection. This is because the secondary electrons from a wafer have energy modulation due to wafer features. 
     FIG. 1  is a flow chart depicting a conventional method  100  of detecting electrons using threshold energy filtering in a defect detection system. A target area on a wafer (or other substrate of interest) is irradiated  102  with an incident electron (charged-particle) beam. Due to the irradiation of the incident beam onto the substrate, secondary electrons and/or backscattered electrons are generated  104 . Prior to detection of the secondary and/or backscattered electrons, a threshold energy filter is applied  106 . The threshold energy filtering prevents those electrons with energies below a threshold energy level from being detected. Only the secondary and/or backscattered electrons with energies above the threshold energy level are detected and analyzed  108 . 
     FIG. 2  schematically depicts an example conventional scanning electron microscope (SEM) system  200 . The SEM system  200  includes a multiplexer control system  250  arranged to receive a plurality of multiplexer control signals and to output a plurality of image control signals to an electron beam generator subsystem (including, for example, an electron source  202 , a suppressor  204 , an extractor  206 , an electrostatic lens  208 , a gun valve  210 , an upper quadrupole  211 , a lower quadrupole  212 , an aperture  213 , a Faraday cup  214 , a Wien filter  218 , a magnetic lens  219 , and a lower octopole  220 ). 
   The electron beam generator subsystem is arranged to receive the plurality of image control signals and to generate an electron beam  203  that is directed substantially toward an area of interest on the specimen  222 . The SEM system  200  also includes a detector subsystem arranged to detect charged particles  205  emitted from the specimen  222  to allow generation of an image from the detected charged particles, which particles may include secondary electrons and/or backscattered electrons. 
   The detector subsystem may include an energy filter and ground mesh  223 , and detector  228 . The energy filter and ground mesh  223  may be arranged to select between secondary and backscattered electrons. If a high negative potential is applied to the energy filter  223 , it is likely that backscattered electrons will only reach the detector  228  since backscattered electrons typically have a much higher energy value than the secondary electrons. 
   Once the electrons are detected by the detector  228 , an image generator (for example, including analog-to-digital circuit  234  for converting the detected signal into a digital signal, a de-multiplexer circuit  236  for separating the setup phase data and image phase data, setup frame buffer  238 , image frame buffer  240 , and CPU  242  for processing the image frame data, among other components) is arranged to receive the detected signal  232  and generate and/or store an image data. For example, successive image frame data may be averaged together to create the image. Alternatively, the setup frame data may be utilized to generate an image. 
   The SEM system  200  may be implemented so as to include a multiplexer control system  250  in a form suitable for multiplexing SEM operating parameters. The multiplexer control system  250  may include a plurality of multiplexer control blocks. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flow chart depicting a conventional method of detecting electrons using threshold energy filtering in a defect detection system. 
       FIG. 2  schematically depicts an example conventional scanning electron microscope (SEM) system. 
       FIG. 3A  is a schematic diagram of a dual bin electron detector for use within a defect detection or metrology system in accordance with an embodiment of the invention. 
       FIG. 3B  is a schematic diagram of another dual bin electron detector for use within a defect detection or metrology system in accordance with an embodiment of the invention. 
       FIG. 4A  is a schematic diagram of a multiple-bin electron detector (an energy spectrometer) having an electrostatic dispersive element for use within a defect detection or metrology system in accordance with an embodiment of the invention. 
       FIG. 4B  is a schematic diagram of a multiple-bin electron detector (an energy spectrometer) having a magnetic dispersive element for use within a defect detection or metrology system in accordance with an embodiment of the invention. 
       FIG. 5A  is a schematic diagram of a spectrometer for use within a defect detection or metrology system in accordance with an embodiment of the invention. 
       FIG. 5B  is a schematic diagram of a second multi-bin detector for use within a defect detection or metrology system in accordance with an embodiment of the invention. 
       FIG. 5C  is a schematic diagram of a second multi-bin detector for use within a defect detection or metrology system in accordance with an embodiment of the invention. 
       FIG. 6  is a hypothetical example energy distribution of scattered electrons for purposes of discussion. 
       FIG. 7  is a flow chart depicting a method of detecting electrons using an energy spectrometer in a defect detection or metrology system in accordance with an embodiment of the invention. 
   

   SUMMARY 
   One embodiment of the invention pertains to an apparatus for detecting defects in substrates. An irradiation source is configured to generate an incident beam, and a lens system configured to focus the incident beam onto a target substrate so as to cause emission of electrons. A multiple-bin detector is configured to detect the emitted electrons, and each bin of the detector detects the emitted electrons within a range of energies. A processing system configured to process signals from the multiple-bin detector. 
   Another embodiment pertains to a method of imaging defects in substrates. An incident beam of irradiation is generated and focused onto a target substrate so as to cause emission of electrons. The emitted electrons are detected in multiple energy bins, wherein each energy bin counts the emitted electrons within a range of energies. The signals from the multiple energy bins are processed. 
   Other embodiments are also disclosed. 
   DETAILED DESCRIPTION 
   As discussed above, threshold energy filters are sometimes used with secondary electron detectors to enhance sensitivity to certain wafer defect types. However, the use of such threshold energy filters includes some shortcomings and disadvantages. 
   First, such threshold energy filters discard (do not count) lower energy electrons, and by doing so lose potentially useful information. Second, the wafer charge level often changes as a function of position on the wafer. If so, then the effective cutoff energy for the secondary electrons shifts, reducing the efficacy of the energy filtering, unless the energy threshold is dynamically adapted. 
   The present application discloses the use of an energy spectrometer in electron beam inspection, review and/or metrology tools. The use of such an energy spectrometer, instead of a threshold energy filter, comprises an enhancement to the conventional systems. 
   Previously, energy spectrometers have been provided in various research microscopes, but they have not been utilized for the purposes disclosed herein in production inspection or metrology tools. However, as disclosed herein, an energy spectrometer with two or more “bins” (rather than a threshold energy filter) may be advantageously utilized in conjunction with an electron detector in a defect detection (or metrology) system. 
   For example, the signals from the various energy bins in the spectrometer may be used to derive a measurement of the charge state of a substrate or wafer. Hence, the energy spectrometer may be used to implement adaptive thresholding, where the threshold energy is made to dynamically vary to take into account changes in wafer or charge characteristics. As another example, multiple energy thresholds may be implemented using an energy spectrometer. In addition, post-processing techniques may be applied to the data obtained from the energy spectrometer so as to select the best or superior threshold arrangements. 
     FIG. 3A  is a schematic diagram of a dual-bin electron detector  300  for use within a defect detection or metrology system in accordance with an embodiment of the invention. The secondary and/or backscattered electrons  205  emitted from the substrate enter into the detector  300 . In this embodiment, a grounded mesh  304  and a high voltage mesh  306  are utilized to separate the emitted electrons  205  depending on their energies. 
   The grounded mesh  304  is electrically grounded, while a negative high voltage level (−HV) from a high voltage source  305  is applied to the high voltage mesh  306 . Configured in this way, the meshes effectively separate those emitted electrons with higher energies from those with lower energies. The higher-energy electrons  307  have sufficient energy to pass the high voltage mesh  306  and so reach and are detected by a higher-energy detector  308 . The lower-energy electrons  309  do not have sufficient energy to pass the high voltage mesh  306  and so are collected and detected by a lower-energy detector  310 . 
     FIG. 3B  is a schematic diagram of another dual-bin electron detector  350  for use within a defect detection system in accordance with an embodiment of the invention. In this embodiment, a magnetic field  352  is configured in a region in the detector  350  such that the electrons  205  entering the detector  350  have their trajectories bent towards the energy-filtering mechanism ( 304  and  306 ) and the higher-energy detector  308 . The lower energy electrons  309  without sufficient energy to pass the energy-filtering mechanism ( 304  and  306 ) have their trajectories bent by the magnetic field  352  towards the lower-energy detector  310 . 
     FIG. 4A  is a schematic diagram of an energy spectrometer  400  having an electrostatic dispersive element for use within a defect detection or metrology system in accordance with an embodiment of the invention. In this embodiment, the electrons  205  emitted from the substrate enter an energy dispersive element in the spectrometer  400 . The energy dispersive element may be implemented as an electrostatic dispersive element, a magnetic dispersive element, or a combined electric/magnetic dispersive element. In  FIG. 4A , an electrostatic dispersive sector is shown, including an upper electrode part  402  with a negative voltage applied thereto and a lower electrode part  404  with a positive voltage applied thereto. The upper  402  and lower  404  electrodes are configured with a bend such that those electrons with a higher energy (and hence a higher speed) have their trajectories bent less than those electrons having a lower energy (and hence a lower speed). This results in a dispersion of the electrons. The dispersed electrons may be detected by a segmented detector  406  so as to form an energy spectrum of the detected electrons. 
     FIG. 4B  is a schematic diagram of an energy spectrometer  410  having a magnetic dispersive element  412  for use within a defect detection or metrology system in accordance with an embodiment of the invention. The magnetic dispersive element  412  may be configured, for example with a magnetic dipole arrangement, such that those electrons with a higher energy (and hence a higher speed) have their trajectories bent less than those electrons having a lower energy (and hence a lower speed). 
     FIG. 5A  is a schematic diagram of a multi-bin detector  500  for use within a defect detection or metrology system in accordance with an embodiment of the invention. In this embodiment, the detector  500  has a plurality of electrostatic depressors  502 . The first electrostatic depressor  502 A has a first negative high voltage level (−HVA) applied thereto. The second electrostatic depressor  502 B has a second negative high voltage level (−HVB) applied thereto. The third electrostatic depressor  502 C has a third negative high voltage level (−HVC) applied thereto. While three such depressors  502  are shown in  FIG. 5 , other implementations may include more or fewer such depressors  502  (and hence more or fewer energy bins). The second voltage level (−HVB) is more negative than the first voltage level (−HVA). The third voltage level (−HVC) is more negative than the second voltage level (−HVB). And so on, if there are more depressors  502 . 
   The electrons  205  emitted from the substrate enter the detector  500 . Those electrons  504 A with insufficient energy to pass the first electrostatic depressor  502 A are deflected into a first detector  506 A which detects electrons for a first energy bin. Those electrons  504 B with insufficient energy to pass the second electrostatic depressor  502 B are deflected into a second detector  506 B which detects electrons for a second energy bin. Those electrons  504 C with insufficient energy to pass the third electrostatic depressor  502 C are deflected into a third detector  506 C which detects electrons for a third energy bin. The electrons  504 D with sufficient energy to pass the last electrostatic depressor  502 C are detected by a last detector  506 D. 
     FIG. 5B  is a schematic (cross-sectional) diagram of a second multi-bin detector  510  for use within a defect detection or metrology system in accordance with an embodiment of the invention. In this embodiment, the detector  510  is configured with flat annular coaxial detectors  512 . 
     FIG. 5C  is a schematic (cross-sectional) diagram of a third multi-bin detector  520  for use within a defect detection or metrology system in accordance with an embodiment of the invention. In this embodiment, the detector  520  is configured with conical annular coaxial detectors  522 . 
     FIG. 6  is a hypothetical example energy distribution  600  of scattered electrons for purposes of discussion. In this hypothetical example, the semiconductor wafer (or substrate) includes a main surface portion and vias (or contact holes) that go to a lower level beneath the surface. In this example, those secondary electrons emitted from the wafer surface are distributed around a first peak  602  at lower energies than those secondary electrons emitted from the vias which are distributed around a second peak  604  at higher energies. 
   In  FIG. 6 , a dynamically adjustable threshold level  606  is also shown. For example, this threshold level  606  may be utilized to form an image with electrons from the surface (by using those electrons with energies below the threshold level  606  to form the image), or to form an image with electrons from the vias (by using those electrons with energies above the threshold level  606  to form the image). 
   Several energy bins (each with a different energy range) are also shown in  FIG. 6 . Intensity data of electrons collected in these bins may be processed, for example, to dynamically adjust the threshold level  606 . For example, a measure of the average energy of the electrons from the surface  602  may be determined by keeping track of the intensity in Bin B relative to the intensity in Bin C of  FIG. 6 . Shifts in the average energy of electrons from the surface may be used to deduce shifts in surface charge. In accordance with one embodiment of the invention, the threshold level  606  may be dynamically shifted in correspondence with the shifts in the average energy of electrons from the surface. 
     FIG. 7  is a flow chart depicting a method  700  of detecting electrons using an energy spectrometer in a defect detection system in accordance with an embodiment of the invention. As in the conventional method  100  discussed above, a target area on a wafer (or other substrate of interest) is irradiated  102  with an incident electron (charged-particle) beam. Due the irradiation of the incident beam onto the substrate, secondary electrons and/or backscattered electrons are generated  104 . 
   However, instead of applying  106  a threshold energy filter, an energy spectrometer is used to detect  702  secondary and/or backscattered electrons in multiple bins. Examples of such multiple energy bin detectors are discussed above in relation to  FIGS. 3A ,  3 B,  4 , and  5 . The multiple bin detected data is processed  704  and may be used, for example, to dynamically adjust  706  an energy threshold value. Image data may be selected  708  based on this dynamically-adjusted energy threshold so as to advantageously form images that enhance defect detection. 
   The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. The above-described invention may be used, for example, in an automatic inspection or review system and applied to the inspection or review of wafers, optical masks, X-ray masks, electron-beam-proximity masks and stencil masks and similar substrates in a production environment. 
   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.