Abstract:
One embodiment disclosed relates to a scanning electron beam apparatus. The apparatus includes an electron beam column, a scanning system, and a detection system. Circuitry in the apparatus is configured to store detected pixel data from each scan into one of the multiple frame buffers. A multi-frame data processor is configured to analyze the pixel data available in the multiple frame buffers. Another embodiment disclosed relates to a scanning electron beam apparatus having a data processor is configured to process the image data with a filter function having a filter strength, store results of the processing, and repeat the processing and the storing using various filter strengths. The results of the processing may comprise a critical dimension measurement at each filter strength.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to electron beam apparatus and methods of using same. 
   2. Description of the Background Art 
   In scanning electron microscopy, a beam of electrons is scanned over a specimen, and the resulting electrons that are returned from the specimen surface are used to create an image of the specimen surface. In a typical system, the beam makes multiple scan passes over a specific area and pixel data from the multiple scans are accumulated or integrated (in effect, added together per pixel) to reduce noise in the resultant image. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a scanning electron beam apparatus in accordance with an embodiment of the invention. 
       FIG. 2  is a schematic diagram of a multi-frame processor of a scanning electron beam apparatus in accordance with an embodiment of the invention. 
       FIG. 3  is a flow chart depicting a conventional method of operation for a scanning electron microscope. 
       FIG. 4  is a flow chart depicting a method of operation for a scanning electron beam apparatus in accordance with an embodiment of the invention. 
       FIG. 5  is a graph showing pixel intensity versus frame number in accordance with an embodiment of the invention. 
       FIG. 6A  depicts an image of averaged pixel data. 
       FIG. 6B  depicts an image of pixel intercept data in accordance with an embodiment of the invention. 
       FIG. 7  depicts an image of pixel slope data in accordance with an embodiment of the invention. 
       FIG. 8  is a cross-sectional diagram illustrating a feature with footings for purposes of discussion. 
       FIG. 9  is a graph showing relative intensity versus pixel number for a line scan across a critical dimension feature with a footing. 
       FIG. 10  is a graph showing derivatives of the line scan of  FIG. 8  with minimal and heavy filtering. 
       FIG. 11  is a graph showing critical dimension versus filter strength in accordance with an embodiment of the invention. 
   

   SUMMARY 
   One embodiment of the invention pertains to a scanning electron beam apparatus. The apparatus includes an electron beam column, a scanning system, and a detection system. Circuitry in the apparatus is configured to store detected pixel data from each scan into one of the multiple frame buffers. A multi-frame data processor is configured to analyze the pixel data available in the multiple frame buffers. 
   Another embodiment disclosed relates to a scanning electron beam apparatus having a data processor configured to process the image data with a filter function having a filter strength, store results of the processing, and repeat the processing and the storing using various filter strengths. The results of the processing may comprise a critical dimension measurement at each filter strength. 
   DETAILED DESCRIPTION 
     FIG. 1  is a schematic diagram (cross-sectional view) of a scanning electron beam apparatus  10  in accordance with an embodiment of the invention. As shown in  FIG. 1 , an electron beam source  12  at the top of a column produces an electron beam  34 . One implementation that could be used includes an electron gun  36  that consists of a thermal field emitter (TFE) with the electrons accelerated by a surface field generated by power supply  32 . Alternative electron gun embodiments could be employed. The electrons emitted by electron gun  36  are then, within beam source  12 , directed through electrodes  38  and gun lens  39  (each also controlled by power supply  32 ) to form electron beam  34  that enters focusing column and lens assembly  14  to be directed to specimen  20 . 
   In focusing column and lens assembly  14 , the electron beam  34  may pass through an aperture  41  that limits the beam current and forms what is labeled electron beam  34 ′ in  FIG. 1 . Electron beam  34 ′ then passes through objective lens  42 , including magnetic coils  43  and pole pieces  44 , that generate a strong magnetic field. That magnetic field is used to focus beam  34 ′ to form electron beam  18  with a small spot size directed at the specimen  20 . Additionally, the location of electron beam  18  is controlled with scan plates  45 , located within the magnetic field created by coils  43  and pole pieces  44 . The scan plates  45  are powered by raster generator  48  to direct beam  18  in both the x and y directions across specimen  20  by signals on lines  46  and  47 , respectively. 
   As the beam  34 ′ passes through the magnetic field of objective lens  42  and the scan plates  45 , it is focused into beam  18  and directed onto specimen  20 . The specimen  20  is typically biased to a selected potential by a second power supply  52  to create a decelerating field for the primary electrons of beam  18  as they approach specimen  20 . The result is that the “landing energy” of those electrons as they reach specimen  20  may be much lower than the energy with which they are provided by electron gun  36  and with which they travel through column and lens assembly  14 . 
   Secondary and backscatter (together “scattered”) electrons  28  are released as a result of the interaction of electron beam  18  with specimen  20  and are directed back toward lens  42 . As the scattered electrons  28  are released, they spiral through lens  42  as a result of the magnetic field, and then travel toward a detector  55  as they leave the field within lens  42 . The electron signal received by detector  55  is then collected by collector plate  56  which in-turn generates a signal that may be amplified by an amplifier  58  before being provided to a multiple-frame processor  59 . Other input signals to the multi-frame processor  59  include signals x and y from raster generator  48  on lines  46  and  47 , respectively. Additionally, electron beam source  12 , focusing column and lens assembly  14 , and specimen  20  are all contained within a vacuum chamber  23 . 
     FIG. 2  is a schematic diagram of a multi-frame processor  59  of a scanning electron beam apparatus  10  in accordance with an embodiment of the invention. An amplified detected signal is received from the amplifier  58  to a digitizer  202 . The digitizer  202  converts the analog signal to digitized pixel data. Each frame of the digitized pixel data is provided to and stored in one of the multiple frame buffers  204 . The data in the multiple frame buffers  204  are accessible by the multiple frame image processor  206 . The multiple frame image processor  206  processes and analyzes the multiple frame data as described further below in relation to  FIGS. 4 through 7 . 
     FIG. 3  is a flow chart depicting a conventional method  300  of operation for a scanning electron microscope. In this method  300 , an area being imaged or analyzed is scanned  302  with an electron beam. Data from detected secondary and/or backscattered electrons for each pixel in the area is accumulated  304  in a single data “bin”. A set of such bins forms a single frame buffer, with one bin per pixel of the frame. The area is typically scanned multiple times (i.e. with multiple passes) so as to reduce noise. As such, if the number of scans is not complete, then the process loops back to repeat steps  302  and  304 . When the number of scans of the area have been completed, then the accumulated data in the single frame buffer is analyzed  306 . The accumulation of the data for each pixel in a bin effectively averages the detected data from the multiple passes. 
     FIG. 4  is a flow chart depicting a method  400  of operation for a scanning electron beam apparatus in accordance with an embodiment of the invention. Like the conventional method  300 , an area being imaged or analyzed is scanned  302  multiple times with an electron beam. However, data from detected secondary and/or backscattered electrons for each pixel is not accumulated in a single data bin. Rather, for each scan, the detected data for pixels in each scanned frame is stored in a separate frame buffer. If the number of scans is not complete, then the process loops back to repeat steps  302  and  402 . When the number of scans of the area have been completed, then the pixel data available in the multiple frame buffers is analyzed  404 . In this method  400 , because the pixel data is stored separately  402  for each pass (instead of being accumulated), dynamic information is advantageously preserved for later analysis  404 . 
   In alternate embodiments, there need not be a one-to-one correspondence between scans and frame buffers. For example, every two successive scans may be accumulated in a separate frame buffer, resulting in a two-to-one ratio between scans and frame buffers. More generally, every N successive scans may be accumulated in a separate frame buffer, resulting in an N-to-1 ratio. These embodiments also preserve dynamic information, but less than in the embodiment with the one-to-one ratio. 
     FIG. 5  is a graph showing pixel intensity versus frame number in accordance with an embodiment of the invention. Here, the intensity of a single pixel is shown to change with successive scans (i.e. increasing frame number). In this particular instance, the pixel intensity generally increases with frame number for the range shown. 
   An average intensity level  502  for this pixel over the scans is shown. Using such average intensity levels for each pixel, “pixel average” image data may be obtained. An example of such an image is shown in  FIG. 6A . This average image is analogous to the conventional images obtained by scanning electron microscopes that accumulate the data from multiple scans in a single frame buffer. 
   A function is fit to the pixel intensity versus frame number data. In this particular embodiment, a linear fit is performed. As shown in  FIG. 5 , the linear fit provides a y-axis intercept (i.e. frame “0” intercept)  504  and a slope  506 . 
   Using the intercept values for each pixel, “pixel intercept” image data may be obtained. An example of such an image is shown in  FIG. 6B  (from the same raw data as used to generate  FIG. 6A ). This intercept image may be considered to show a “true image” of the area in that it theoretically corresponds to image data generated without charging effects caused by the beam scan. 
   Using the slope values for each pixel, “pixel slope” image data may be obtained. An example of such an image is shown in  FIG. 7  (from the same raw data as used to generate  FIGS. 6A and 6B ). This slope image may be considered to depict charging characteristics of the area caused by the beam scans. 
     FIG. 8  is a cross-sectional diagram illustrating a feature  804  with footings  806  for purposes of discussion. The feature  804  may be formed on a substrate  802  during a semiconductor manufacturing process. The left  806 -L and right  806 -R footings or shoulders may be undesired artifacts of the process forming the feature  804 . While the example feature illustrated corresponds to a line formed on a substrate, in other examples the feature may be a via or contact hole or other types of features. 
     FIG. 9  is a graph showing relative intensity versus pixel number for a line scan across a critical dimension feature with footings on both sides. In this example, the main peaks  902  correspond to the left  902 -L and right  902 -R edges of the feature, and the side peaks or shoulders  904  in the data correspond to the left  904 L and right  904 -R footings or shoulders of the feature. 
     FIG. 10  is a graph showing derivatives of the line scan of  FIG. 8  with minimal and heavy filtering. As shown in  FIG. 10 , with minimal filtering the shoulders or side peaks are still visible in the derivative data. However, with heavy filtering, the shoulders or side peaks disappear. (Note that the heavy filtering data were inverted to allow superposition of the two data sets.) 
     FIG. 11  is a graph showing critical dimension (in nanometers) versus filter strength (in arbitrary units) in accordance with an embodiment of the invention. At higher filter strengths, the critical dimension (CD) measurement is shown as a single data point per filter strength  1102  because the higher filter strengths causes smoothing over of the data such that shoulders or footings are no longer visible. At lower filter strengths, two CD data points are shown per filter strength. The lower CD data points  1104  correspond to CD measurements using the dominant peaks caused by the main edges of the feature. The upper CD data points  1106  correspond to CD measurements using the secondary or shoulder peaks caused by the footings or shoulders of the feature. 
   This technique provides CD measurement data at multiple filter strengths. This enables the generation of the depicted CD curve, including the splitting of the curve at lower filter strengths. 
   As shown in  FIG. 11 , this new processing technique advantageously enables the accurate determination of both a “dominant” or “main edges only” CD  1105  from the feature and a “footing visible” or “footing included” CD  1107 . In this instance, the footing-included CD  1107  is larger than the dominant CD  1105 . In another example, where a non-clean contact hole or via has footings on the bottom of it, the footing-included CD may be smaller than the dominant CD. 
   The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. 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.