Abstract:
One embodiment relates to an electron beam apparatus. The apparatus includes a mechanism for moving a substrate relative to the electron beam column at a controlled speed. A probe beam gun is configured to generate a probe beam through the column, and a pre-charging beam gun configured to generate a pre-charging beam through the column. Control circuitry configured to pre-scan a scan line with the pre-charging beam at least once and to subsequently sense scan the scan line with the probe beam at least once. The control circuitry is further configured so that there is a prescribed delay time between said pre-scanning and said sense scanning of the scan line. In another embodiment, a single electron beam and a deflection system configured to deflect the electron beam into pre-scans and sense scans. Other embodiments and features are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to electron beam apparatus used for automated inspection, or review, or metrology. 
   2. Description of the Background Art 
   It is often desirable to detect certain types of electrical defects in semiconductor wafers that have leakage due to electrical capacitance and resistance. Conventionally, such electrical defects are detected during electron beam (e-beam) inspection in one of two techniques. 
   In a first conventional technique, an electron probe beam is directed onto the substrate without pre-charging the substrate. The secondary electron signal is detected and analyzed in an attempt to determine the state of the interface. 
   In a second conventional technique, the substrate is pre-charged by flooding a relatively large area with an electron beam for an initial time period on the order of minutes (typically, a few minutes). The flooding is configured to provide a) a controlled voltage state for inspection uniformity across the spatial extent of one wafer or multiple wafers and/or b) a proper voltage state so that a subsequent probe step may actually detect the electrical defect of interest. After the flooding step, an electron probe is directed onto the substrate. Generally, there is some delay between the flooding step and the probe step which may be on the order of seconds (typically, a several seconds). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an electron beam apparatus with a probe beam gun and a flood beam gun in accordance with an embodiment of the invention. 
       FIG. 2  is a timing diagram for operating the probe beam gun and the flood beam gun of  FIG. 1  in accordance with an embodiment of the invention. 
       FIG. 3  is a flow chart showing a method of operating the pre-charge and probe beams per  FIG. 2  in accordance with an embodiment of the invention. 
       FIG. 4  is a schematic diagram of an electron beam apparatus with a single beam deflected for probe and flooding scans in accordance with an embodiment of the invention. 
       FIG. 5  is a schematic diagram depicting a scheme for real-time pre-charge and sense scanning in accordance with an embodiment of the invention. 
       FIG. 6  is a schematic diagram depicting another scheme for real-time pre-charge and sense scanning in accordance with an embodiment of the invention. 
   

   SUMMARY 
   One embodiment relates to an electron beam apparatus. The apparatus includes a mechanism for moving a substrate relative to the electron beam column at a controlled speed. A probe beam gun is configured to generate a probe beam through the column, and a pre-charging beam gun configured to generate a pre-charging beam through the column. Control circuitry configured to pre-scan a scan line with the pre-charging beam at least once and to subsequently sense scan the scan line with the probe beam at least once. The control circuitry is further configured so that there is a prescribed delay time between said pre-scanning and said sense scanning of the scan line. 
   In another embodiment, a single electron beam and a deflection system configured to deflect the electron beam into pre-scans and sense scans. 
   Another embodiment pertains to a method of automated electron beam inspection or review. A substrate is moved relative to an electron beam column at a controlled speed, and at least one electron beam is generated through the column. A scan line is pre-scanned at least once and subsequently sense scanned at least once. There is a prescribed delay time between said pre-scanning and said sense scanning of the scan line. 
   Other embodiments are also disclosed. 
   DETAILED DESCRIPTION 
   Drawbacks of the Conventional Flooding Technique 
   The above-discussed conventional flooding technique suffers from substantial drawbacks and disadvantages. 
   One drawback is that, when detecting electrical defects in semiconductor devices with small time constants (for example, less than one microsecond), the charging done by the flooding step will decay away before the probe step is performed. Examples of such devices may include active semiconductor diodes, MOS capacitors with thin dielectrics, and other devices. In other words, the conventional flooding step where a large area of a wafer is flooding in the order of minutes may be ineffective in controlling the surface voltage of the electrical element to be probed because the delay between the flooding and probe steps is too long. 
   Another drawback is that, in order to detect certain electrical defects, it is useful for the probe scan at each point on the wafer to follow the flooding step at that point after a small time delay, where the length of the time delay is prescribed by the particular electrical circuitry being probed. 
   For example, consider an electrical element having a capacitance with a controlled leakage and an electrical defect which manifests itself as a larger leakage in that element. The conventional flooding step may establish a voltage on the capacitor by charging it. In this case, a non-defective element with low leaking should hold sufficient charge and have a different voltage than a defective element where the charge leaks away quickly. However, if the delay between the flooding step and the probe step is too long, then the pre-charge will fully leak away from both a non-defective and a defective element. So, the voltage of both defective and non-defective elements would be the same, and the defective element would not be detected. On the other hand, if the delay between the flooding step and the probe step is too short, then hardly any charge will leak away no matter whether the element was defective or not. So, the voltage of both defective and non-defective elements would again be the same, and the defective element would again not be detected. In this case, applicants have determined that it is desirable to control the time between the flood scan and the probe scan to be in a range near the time constant of the leakage. 
   As another example consider an electrical element having a capacitance with a leakage and an electrical defect which manifests itself as a larger or smaller capacitance. The conventional flooding step may establish a voltage on the capacitor by charging it. In this case, a non-defective element having a correct capacitance should leak away only a certain amount of its charge and so have a different voltage than a defective element where the capacitance is larger or smaller. However, if the delay between the flooding step and the probe step is too long, then the pre-charge on both non-defective and defective elements may leak away, and the differential in capacitance between the non-defective and defective elements would not be sensed. In this case, applicants have determined that it is desirable to control the time between the flood scan and the probe scan to be less than the time constant of the leakage of the capacitor. 
   Pre-Charge and Sense Scanning with Prescribed Delays 
     FIG. 1  is a schematic diagram of an electron beam apparatus  100  with a probe beam gun  102  and a flood beam gun  106  in accordance with an embodiment of the invention. The probe beam gun  102  and the flood beam gun  104  may each include an electron source and gun lenses so as to generate, respectively, a probe electron beam  104  and a “flood” or pre-charge electron beam  108 . The probe beam  104  and the pre-charge beam  108  may be transformed and controllably deflected by an electron column  101 . 
   The apparatus  100  may be configured such that the beams pass a focus electrode  110  and a Wehnelt electrode  112  before impinging upon the wafer  114  or other substrate under automated inspection or review. The voltages on these electrodes may be set so as to focus the beams ( 104  and  108 ) onto beam spots on the substrate. In accordance with an embodiment of the invention, the beam spots may be separated by a known distance  116 . 
   The beam spots may be scanned over a line (or area) of the substrate  114  by controlling beam scan deflectors in the column  101 . A detection system (not illustrated) is utilized to detect secondary and/or backscattered electron signals. 
   In accordance with an embodiment of the invention, the wafer or other substrate  114  is set to move (be translated) relative to the column at a known constant speed in one direction. The motion is such that a line on the substrate which is perpendicular to the direction of motion passes first under the flood beam  108  and subsequently passes under the probe beam  104 . 
   In accordance with an embodiment of the invention, the focus plate or “quick focus” lens  110  may be adjusted to controllably focus and de-focus the beams onto the surface. If the substrate is alternately pre-charge (flood) scanned and sense (probe) scanned, the focus electrode  110  may be switched alternatively between a first voltage during the pre-charge scanning step and a second voltage during the sense scanning step. The first voltage may be set so as to de-focus the flood beam to help establish the local charging around the points where the beam hits the substrate in a more beneficial way, while the second voltage may be set so as to more tightly focus the probe beam in a way that is more beneficial for collecting well-defined image data. 
   In accordance with an embodiment of the invention, the Wehnelt or field control plate  112  may be used to controllably establish the electrostatic field on the surface of the substrate. If the substrate is alternately pre-charge (flood) scanned and sense (probe) scanned, the Wehnelt or field control plate  112  may be switched alternatively between a first voltage during the pre-charge scanning step and a second voltage during the sense scanning step. The first voltage may be set so as to facilitate the attainment of a desired surface charge state during the pre-charge scanning, while the second voltage may be set so as to facilitate the attainment of a high signal-to-noise ratio during the sense scanning. 
     FIG. 2  is a timing diagram for operating the probe beam gun and the flood beam gun of  FIG. 1  in accordance with an embodiment of the invention. First, as shown in  FIG. 1 , the substrate sample  114  is continuously moved under the column such that the substrate first goes under the flood (pre-charge) beam and then goes under the probe beam. 
   As shown in  FIG. 2 , the flood (pre-charge) beam may be in an ON state while the probe beam is in an OFF state, and the flood beam may be in an OFF state while the probe beam is in an ON state. In other words, the pre-charge beam and the probe beam may be turned on at different times and sequentially while maintaining a constant duty cycle. In the example illustrated in  FIG. 2 , the duty cycle is one half (½), but other specific duty cycles may be utilized depending, for example, on the amount of pre-charging required for the specific electronic devices. 
   In this embodiment, the separation  116  of the pre-charge and probe beams, together with the speed of the substrate, determines the delay time between the pre-charge scan and the probe scan. Hence, for a given prescribed delay time, the beam separation and/or the substrate speed may be adjusted so as to achieve that prescribed delay time. 
     FIG. 3  is a flow chart showing a method  300  of operating the pre-charge and probe beams per  FIG. 2  in accordance with an embodiment of the invention. Again, as shown in  FIG. 1 , the substrate sample  114  is continuously moved under the column such that the substrate first goes under the flood (pre-charge) beam and then goes under the probe beam. The speed of the substrate movement and/or the beam separation may be set  302  so as to achieve a prescribed delay so as to detect particular electrical defects on the substrate. 
   In a first state  304 , the pre-charge beam  108  may be scanned over a line or swath (including a plurality of scan lines) on the moving substrate. Once that scan is completed, the pre-charge beam may be stopped from impinging upon the moving substrate, and beam control parameters (for example, voltages on the focus and/or Wehnelt electrodes) may be switched  306  in preparation for the probe scanning. The stoppage may be performed, for example, by blocking the pre-charge beam or by deflecting the pre-charge beam away from the direction of the substrate. 
   In a second state  308 , the probe beam  104  may be scanned over the same line or swath (including a plurality of scan lines) on the moving substrate. Once that scan is completed, the probe beam may be stopped from impinging upon the moving substrate, and beam control parameters (for example, voltages on the focus and/or Wehnelt electrodes) may be switched  310  in preparation for the pre-charge scanning. 
   Thereafter, the method  300  may loop back  312  and repeat for the next line or swath. 
   As discussed above, because the substrate speed and beam separation are accurately known or controlled, a prescribed time delay between the pre-charging and the scanning may be set  302 . Hence, the above-discussed embodiment may be advantageously utilized so as to achieve a prescribed delay which, for example, is in a range near a time constant of controlled leakage from a non-defective element so as to detect a larger leakage, or is less than a time constant of leakage from a non-defective capacitor on the substrate so as to detect a larger or smaller capacitance. 
     FIG. 4  is a schematic diagram of an electron beam apparatus  400  with a single beam  404  deflected for probe and flooding scans in accordance with an embodiment of the invention. The beam gun  402  may include an electron source and gun lenses so as to generate the incident beam  404 . The beam  404  may be transformed and controllably deflected by an electron column  401 . 
   The apparatus  400  may be configured such that the beam  404  passes a focus electrode  410  and a Wehnelt electrode  412  before impinging upon the wafer  414  or other substrate under automated inspection or review. The voltages on these electrodes may be set so as to focus the beam  404  onto a beam spot on the substrate. A detection system (not illustrated) is utilized to detect secondary and/or backscattered electron signals. 
   In accordance with an embodiment of the invention, the wafer or other substrate  414  is set to move (be translated) relative to the column at a known constant speed in one direction. 
   The beam spot may be deflected and scanned over the substrate  414  by controlling beam scan deflectors in the column  401 . As discussed further below in relation to  FIGS. 5 and 6 , there are various schemes for deflecting the single beam  404  to achieve pre-charge  406  and probe  408  scans with pre-scribed delays therebetween. 
     FIG. 5  is a schematic diagram depicting a scheme for real-time pre-charge and sense scanning in accordance with an embodiment of the invention. Although the wafer or other substrate typically moves relative to a stationary column,  FIG. 5  depicts a column  401  moving relative to the wafer  414  for purposes of explanation. Instead of the wafer  414  moving left relative to the column  401  in  FIG. 4 , the column  401  is shown moving to the right relative to the wafer  414  in  FIG. 5 . In other words, moving left to right at the top of  FIG. 5  corresponds to the passage of time as the substrate  414  moves under the column  401 . 
     FIG. 5  depicts the pre-charging and image capture of three image frames  502 . For each frame  502 , the beam  404  is first controllably deflected such that a series of pre-scans or pre-charge scans  406  is performed over the substrate area imaged in that frame. Next, the beam  404  is controllably deflected such that a series of probe scans  408  is performed over the substrate area imaged in that frame. 
   Four scan lines per frame are shown for purposes of illustration in  FIG. 5 , but in general each frame may have K scan lines per frame, where K is a configurable number. Furthermore, while  FIG. 5  shows a specific instance where each scan line is pre-scanned once and probe scanned once, more generally each scan line may be pre-scanned M times and scanned N times, where M and N are configurable numbers. 
     FIG. 6  is a schematic diagram depicting another scheme for real-time pre-charge and sense scanning in accordance with an embodiment of the invention. Like  FIG. 5 ,  FIG. 6  depicts a column  401  moving relative to the wafer  414  for purposes of explanation. In other words, moving left to right at the top of  FIG. 6  corresponds to the passage of time as the substrate  414  moves under the column  401 . 
     FIG. 6  depicts the pre-charging and image capture for a series of lines. For each line, the beam  404  is first controllably deflected such that a pre-scan or pre-charge scan  406  is performed over that line. Next, the beam  404  is controllably deflected such that a probe or sense scan  408  is performed over a previously pre-scanned line. 
   The pre-scribed time period between a particular line being pre-scanned and sense scanned is determined by the “K Line Shift”  602  depicted in  FIG. 6 , along with the speed of the relative movement between the column  401  and substrate  414 . 
   In the example illustrated in  FIG. 6 , the K Line shift corresponds to three and a half “duty cycles” of the pre-charge/sense scanning cycle. More generally, the K Line shift and/or the substrate translation speed may be set so as to achieve a prescribed delay time between the pre-charge and sense scanning of a line. 
   While  FIG. 6  shows a specific instance where each scan line is pre-scanned once and probe scanned once, more generally each scan line may be pre-scanned M times and scanned N times, where M and N are configurable numbers. 
   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.