Abstract:
One embodiment relates to a method of inspecting an array of cells on a substrate. A reference image is generated using a cell image that was previously determined to be defect free. A reference contour image which includes contours of the reference image is also generated. The reference contour image is used to detect defects in the array of cells on the substrate. Another embodiment relates to a system for detecting defects in an array on a substrate. Other embodiments, aspects and features are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present application claims the benefit of U.S. provisional patent application No. 61/757,807, filed Jan. 29, 2013, entitled “Ebeam Array Inspection (Detection and Classification) of Patterned Defects in Semiconductor Wafers Using Golden Cell Contours,” the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to inspection and review of substrates, such as, for example, semiconductor wafers and reticles for lithography. 
         [0004]    2. Description of the Background Art 
         [0005]    In a conventional electron beam (e-beam) inspection instrument, a manufactured substrate (such as a silicon wafer or a reticle) is scanned with a focused beam of electrons which results in the emission of secondary electrons from the substrate surface. The emitted electrons are detected, and the detection data is typically converted into images of the surface of the specimen. These images are then analyzed numerically to detect abnormalities (referred to as defects) in the manufactured substrate. The detected defects may be subsequently reviewed by further imaging. 
         [0006]    The detected defects may also be classified, either manually or automatically, into different classes or categories. The classification of a defect may be used to determine its cause such that appropriate adjustments may be made in the manufacturing process so as to improve its yield. 
         [0007]    It is highly desirable to improve methods and apparatus for the detection and classification of defects imaged by electron beam instrumentation. 
       SUMMARY 
       [0008]    One embodiment relates to a method of inspecting an array of cells on a substrate. A reference image is generated using a cell image that was previously determined to be defect free. A reference contour image which includes contours of the reference image is also generated. The reference contour image is used to detect defects in the array of cells on the substrate. 
         [0009]    Another embodiment relates to a system for detecting defects in an array on a substrate. A source generates an incident electron beam, and a scanning system for controllably deflecting the incident electron beam to scan the incident electron beam over a target region on the substrate such that secondary electrons are emitted therefrom. A detection system detects the secondary electrons and generates an image data frame of the target region. A control and processing system with control code is programmed to generate a reference image using a cell image that was previously determined to be defect free, generate a reference contour image which includes contours of the reference image, and use the reference contour image to detect defects in the array of cells on the substrate. 
         [0010]    Other embodiments, aspects and features are also disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic diagram of an e-beam apparatus in accordance with an embodiment of the invention. 
           [0012]      FIG. 2  is a flow diagram of a method for array defect detection and classification in accordance with an embodiment of the invention. 
           [0013]      FIG. 3  is a flow diagram of a golden cell set-up and definition procedure in accordance with an embodiment of the invention. 
           [0014]      FIG. 4  is a flow diagram of an exemplary pitch computation procedure in accordance with an embodiment of the invention. 
           [0015]      FIG. 5A  depicts example cell location selections for a two-dimensional cell array in accordance with an embodiment of the invention. 
           [0016]      FIG. 5B  depicts example cell location selections for a one-dimensional cell array in accordance with an embodiment of the invention. 
           [0017]      FIG. 6  is a flow diagram of an exemplary procedure for high-resolution golden cell image acquisition and position adjustment in accordance with an embodiment of the invention. 
           [0018]      FIG. 7  is a flow diagram of an exemplary procedure for golden cell contour extraction and context ROI definition in accordance with an embodiment of the invention. 
           [0019]      FIG. 8  depicts example previous-layer, top-layer and context ROI polygons within a stitched golden cell in accordance with an embodiment of the invention. 
           [0020]      FIG. 9  is a flow diagram of an exemplary procedure for automated defect detection using golden cell contours in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    A conventional method of defect detection in an array region of a wafer relies on cell-to-cell (C2C) comparison or cell-to-reference-cell comparison (see, for example, U.S. Pat. No. 7,869,643). However, in electron beam (ebeam) inspection, it is possible to have very small pixel sizes which causes an insufficient number of cells to be available in a given frame to perform cell-to-cell comparison. 
         [0022]    In such cases, the user will generally be forced to inspect the array region using cells in adjacent dies as reference cells or previously-determined defect-free cell images (so-called “golden” cell images) for the reference cells. In the former case, there is a disadvantageous loss of sensitivity and throughput due to the need to use cells in adjacent dies. In the latter case, there is a disadvantageous loss of sensitivity since the golden (previously-determined defect-free) cell images may have been obtained with very different image quality in comparison to the test image. 
         [0023]    Even if there is a sufficient number of cells in a given frame to perform C2C comparison, the sensitivity of using another cell in the frame (or a golden cell) for the C2C comparison may be somewhat limited and may be improved by the use of methods and apparatus using “golden” cell contours as disclosed herein. 
         [0024]    Methods and apparatus for contour-based defect detection and classification using an e-beam apparatus are disclosed herein. These methods and apparatus may be employed to substantially improve the sensitivity of defect detection when compared against a conventional approaches. 
         [0025]    In particular, array defect detection using golden cell contours as disclosed herein has higher sensitivity than previous approaches, such as using an adjacent cell or a golden cell as a reference. The previous approaches also have a higher noise floor due to process variation across cells or dies. However, the contour-based defect detection technique disclosed herein uses contour-based segmentation and local differencing to substantially reduce the noise floor and thereby substantially enhance sensitivity. 
         [0026]      FIG. 1  provides a cross-sectional diagram of the e-beam inspection apparatus  100  which may be utilized in accordance with an embodiment of the invention. As shown in  FIG. 1 , a source  101  generates an incident electron beam (primary electron beam)  102 . The incident electron beam  102  passes through a Wien filter  104 . The Wien filter  104  is an optical element configured to generate electrical and magnetic fields which cross each other. Scanning deflectors  106  and focusing electron lenses  107  are utilized. The scanning deflectors  106  are utilized to scan the e-beam across the surface of the target substrate  110 . The target substrate  110  may be, for example, a patterned substrate, such as an integrated circuit being manufactured or a reticle for lithography. 
         [0027]    The focusing electron lenses  107  are utilized to focus the incident electron beam  102  into a beam spot on the surface of the wafer or other substrate sample  110 . In accordance with one embodiment, the focusing lenses  107  may operate by generating electric and/or magnetic fields. 
         [0028]    As a result of the scanning of the incident electron beam  102 , secondary electrons are emitted or scattered from the surface of the target substrate  110  (which may be, for example, a semiconductor wafer or a reticle). The target substrate  110  may be held by a movable stage  111 . The secondary electrons are then extracted from the target substrate  110  by exposure to the electromagnetic field of the objective (final) lens  108 . The electromagnetic field acts to confine the emitted electrons to within a relatively small distance from the incident electron beam optic axis and to accelerate these electrons up into the column. In this way, a secondary e-beam  112  is formed from the secondary electrons. 
         [0029]    The Wien filter  104  deflects the secondary e-beam  112  from the optic axis of the incident electron beam  102  to a detection axis (the optic axis for the detection system  114  of the apparatus). This serves to separate the scattered e-beam  112  from the incident electron beam  102 . The detection system  114  detects the secondary e-beam  112  and generates data signals that may be utilized to create images of the surface of the target substrate. 
         [0030]    An instrument control and data processing (control/processing) system  150  may include one or more processors (i.e. microprocessors or microcontrollers)  152 , data storage (including, for example, hard disk drive storage and memory chips)  154 , a user interface  157  and a display system  158 . The data storage  154  may store or hold computer-readable program code (instructions)  155  and data  156 , and the processor  152  may execute the program code  155  and process the data  156 . The user interface  157  may receive user inputs. The display system  158  may be configured to display image data and other information to a user. 
         [0031]    The control/processing system  150  may be connected to, and may be used to control, various components of the e-beam column so as to implement methods or procedures disclosed herein. For example, the movement of the stage  111 , and the scanning by the deflectors  106  may be controlled by computer-readable program code  155  executed by the control/processing system  150 . 
         [0032]    In addition, the control/processing system  150  may also process the electron image data from the detection system  114 . In particular, the computer-readable program code  155  in the control/processing system  150  may be used to implement procedures relating to the contour-based array inspection of patterned defects as described herein. 
         [0033]    Furthermore, in accordance with an embodiment of the invention, the control/processing system  150  may, optionally, interface with a design server  170 . The design server  170  may store pre-OPC (optical proximity correction) design patterns  172  and post-OPC design patterns  174 . During an inspection run, the design server  170  may provide said data in design clips in real-time at the request of the control/processing system  150  of the e-beam inspection apparatus  100 . 
         [0034]    For embodiments where no design server is available, a image editing tool may be used by the user to manually draw the golden contours (i.e. contours in the previously-determined defect-free images). The image editing tool may display an input SEM image to guide the user in drawing the golden contours. 
         [0035]      FIG. 2  is a flow diagram of a method  200  for array defect detection and classification in accordance with an embodiment of the invention. As shown, the method  200  may include a golden cell set-up and definition procedure  300  and a procedure  900  for defect detection using a golden cell contour. Both of these procedures are described further below. 
         [0036]    Note that various modes of inspection may be provided by the system, including array and non-array modes. In one implementation, a user may select an array mode of inspection. In addition, under the array mode, multiple detection techniques may be provided by the system. A user may select a contour-based detection technique. Furthermore, under the contour-based technique, multiple types of reference cells may be used. In accordance with an embodiment of the invention, a user may specify the reference type as a “golden cell” (i.e. a known defect-free cell). 
         [0037]      FIG. 3  is a flow diagram of a golden cell set-up and definition procedure  300  in accordance with an embodiment of the invention. User input steps in this procedure (and elsewhere in this specification) may be implemented using a display screen and user interface (such as a mouse, touchscreen or trackball, for example). 
         [0038]    As shown, a pitch computation procedure  400  may be performed. An exemplary pitch computation procedure  400  is described below in relation to  FIG. 4 . 
         [0039]    Next, a procedure  600  for high-resolution golden cell image acquisition ans position adjustment may be performed. An exemplary such procedure  600  is described below in relation to  FIG. 6 . 
         [0040]    Subsequently, a golden cell contour extraction and context region of interest (ROI) definition procedure  700  may be performed. An exemplary such procedure  700  is described below in relation to  FIG. 7 . 
         [0041]      FIG. 4  is a flow diagram of an exemplary pitch computation procedure  400  in accordance with an embodiment of the invention. In the method  200  of  FIG. 2 , this procedure  400  may be performed as part of the golden cell set-up and definition procedure  300  of  FIG. 3 . User input steps in this procedure may be implemented using a display screen and user interface (such as a mouse, touchscreen or trackball, for example). 
         [0042]    In step  401 , the system may provide a large field of view (FoV) image of the array. The large FoV image shall be longer than one cell in each dimension. 
         [0043]    In step  402 , the user defines a unit cell and an array cell block within the large FoV image. The array cell block may be two-dimensional (with multiple cells arranged in a two-dimensional array) or one-dimensional (with multiple cells arranged in a one-dimensional array). In one implementation, step  402  may be implemented with steps  404  and  406 . 
         [0044]    Per step  404 , the user may select the top left (Cell Loc 1 ) and bottom right (Cell Loc 2 ) (i.e. opposite corners) of a unit cell at one corner of the array cell block. In one implementation, the defined unit cell may be the cell in the upper left corner of the array cell block. Per step  406 , the user may select the bottom right (Cell Loc 3 ) of the array cell block (i.e. the bottom right of the cell in the bottom right of the array cell block). 
         [0045]    In step  407 , the system may determine exact size of the unit cell and compute the pitch in each dimension of the array cell block. This determination may be based on the user inputs for Cell Loc 1 , Cell Loc 2 , and Cell Loc 3 . 
         [0046]      FIG. 5A  depicts example cell location selections for a two-dimensional cell array in accordance with an embodiment of the invention. As shown, Cell Loc 1  and Cell Loc  2  define the opposite corners of a unit cell in the upper left of the array cell block. Cell Loc  3  defines the bottom right corner of the cell in the bottom right of the array cell block. From Cell Loc  1 , Cell Loc  2 , and Cell Loc  3 , the horizontal and vertical pitches may be computed for the cells in the two-dimensional cell array. 
         [0047]      FIG. 5B  depicts example cell location selections for a one-dimensional cell array in accordance with an embodiment of the invention. As shown, Cell Loc 1  and Cell Loc  2  define the opposite corners of a unit cell at the left of the array cell block. Cell Loc  3  defines the bottom right corner of the cell at the right of the array cell block. From Cell Loc  1 , Cell Loc  2 , and Cell Loc  3 , the horizontal pitch may be computed for the cells in the one-dimensional cell array. 
         [0048]      FIG. 6  is a flow diagram of a procedure  600  for high-resolution golden cell image acquisition and position adjustment in accordance with an embodiment of the invention. In the method  300  of  FIG. 3 , this procedure  600  may be performed after the pitch computation procedure  400  of  FIG. 4 . User input steps in this procedure may be implemented using a display screen and user interface (such as a mouse, touchscreen or trackball, for example). 
         [0049]    Per block  610 , the system may acquire a golden (i.e. previously-determined to be defect-free or “known good”) cell image at high resolution. As shown, the golden cell image acquisition may include several steps in an exemplary implementation. In step  611 , the system may compute the number of field of views (FoVs) needed to acquire the golden cell image at a high resolution setting of the system. The computation of the number of FoVs may be based on the cell size, the positional uncertainty due to the stage, and the FoV (or pixel size). In step  612 , the multiple FoVs are acquired, each FoV covering part of the unit cell and overlapping adjacent FoVs. In addition, a lower-resolution (i.e. larger FoV) image covering the entire unit cell may be acquired per step  615 , and the low-resolution image may be up-sampled per step  616 . The up-sampling increases a number of pixels in the image so that the lower-resolution image may be compared pixel for pixel against the high-resolution images acquired in step  612 . Per block  618 , the high-resolution images may be aligned using the up-sampled lower resolution image as a guide to determine the alignment offsets. The high-resolution images may then be stitched based on the alignment offsets to generate the golden cell image at high resolution. 
         [0050]    Per block  620 , the system may adjust a position of the golden cell image. As shown, the position adjustment may be performed using several steps in an exemplary implementation. In a first step  622 , the system may tile and stitch golden cells to generate a tiled-stitched golden multiple-cell image. This step may be performed by tiling copies of the high-resolution golden cell image and stitching together the tiled copies to create the tiled-stitched golden multiple-cell image. In other words, the high-resolution golden cell image is replicated and then stitched to create a high-resolution multiple-cell image. Subsequently, in a second step  624 , the system may overlay a rectangle over the tiled-stitched golden multiple-cell image. The rectangle may be sized according to the array pitch(es) previously determined. In a third step  626 , the user may then adjust the position of the rectangle to specify a corner (for example, the top-left corner) of a single stitched golden cell within the tiled-stitched golden multiple-cell image. Finally, in a fourth step  628 , the user may click on a button labeled “clip golden cell” to select a single golden cell based on the user-positioned rectangle. In other words, a smaller frame of pixels encompassing a single golden cell is selected at the desired position is obtained. 
         [0051]      FIG. 7  is a flow diagram of a procedure  700  for golden cell contour extraction and context ROI definition in accordance with an embodiment of the invention. This procedure  700  may be performed after the procedure  600  of  FIG. 6 . User input steps in this procedure may be implemented using a display screen and user interface (such as a mouse, touchscreen or trackball, for example). 
         [0052]    Per block  720  (which may be performed after block  620  of  FIG. 6 ), the user may select a technique for extracting golden cell contours from the selected golden cell. As indicated, multiple technique options may be provided, such as: A) a smoothed design technique; B) a semi-automated contour extraction technique; and C) a smoothed manually-drawn polygons technique. The smoothed design technique may involve smoothing pre-OPC design data of the top and bottom layers. The semi-automated contour extraction technique may be a watershed technique. The semi-automated contour extraction technique may involve a user specifying seed rectangles or polygons for top and bottom layers to be used for the contour extraction. In one implementation, polygons of the design may be used as the seeds for extracting the contours. The smoothed manually-drawn polygons technique may involve a user manually drawing polygons using an image editing tool to create top and bottom layer contours which may then be smoothed. 
         [0053]    Per block  730 , the system may then extract golden cell contours using the selected technique. A contour may be defined and stored as a list of points or locations. The golden cell contours may then be overlaid on top of the golden cell image in the display to the user. 
         [0054]    Per block  740 , the user may then draw polygons to specify context regions of interest (ROIs). The context ROIs are to be used for subsequent binning and nuisance filtering. 
         [0055]    Per block  750 , the system may save the golden cell contour and context ROIs in an inspection recipe. The inspection recipe may be subsequently used to inspect an area of a substrate with the patterned array of cells. The golden cell contour and/or context ROIs may be used, for example, to create a mask image to provide user-specified segmentation for defect detection, nuisance filtering, and binning and/or classification. 
         [0056]      FIG. 8  depicts example bottom-layer, top-layer and context ROI polygons within a stitched golden cell in accordance with an embodiment of the invention. Shown in  FIG. 8  is an outline of a tiled-stitched golden multiple-cell image, and a selected stitched golden cell  820  therein. Also depicted are bottom layer (i.e. previous layer) polygons  830  and top layer (i.e. subsequent layer) polygons  840 . In addition, example context ROI polygons  850  are depicted. 
         [0057]      FIG. 9  is a flow diagram of a procedure  900  for automated defect detection (i.e. automated inspection) using golden cell contours in accordance with an embodiment of the invention. This procedure  900  may be performed after the procedure  300  of  FIG. 3 . 
         [0058]    Per block  910 , the extracted golden cell contour may be tiled to create a corresponding 2×2 tiled golden cell contour reference image. Preferably, some additional margin is added to create a reference image that is slightly larger than 2×2 cells so as to account for stage uncertainty. 
         [0059]    Per block  920 , a test image may then be rectified and aligned to the golden cell contour reference image. The test image is an image of an area to be inspected for defects by comparison against the golden cell contour reference image. 
         [0060]    Per block  930 , a contour-based defect detection procedure may be executed using the test image and the golden cell contour reference image. As depicted in  FIG. 9 , this procedure may involve several steps. In a first step  932 , contour-based segmentation may be performed based on the contours of the golden cell contour reference image. In other words, the test image may be segmented or separated into regions based on the contours of the golden cell contour reference image. In one implementation, corners and/or contours at various distances from the edges of the golden cell contours may be segmented out to improve defect detection sensitivity. In a second step  934 , local contrast may be performed at each image pixel in the test image. The local difference image may then be segmented into image regions (or image segments) using the contour-based segmentation. In a third step  936 , auto-thresholding may be performed on the segmented difference image to generate a defect candidate pixel list. In a fourth step  938 , defect merging (i.e. merging of candidate defect pixels) and attribute-based nuisance filtering may be performed to generate a defect blob list. 
         [0061]    Per block  940 , the context ROIs may be used to perform further nuisance filtering to generate a final defect list. Per block  950 , the context ROIs are used to assign bin codes to defects in the final defect list. 
         [0062]    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. 
         [0063]    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.