Abstract:
One embodiment relates to a method for run-time correction of defect locations on a substrate during defect review. The substrate is loaded into a stage of a review apparatus, and coordinates for the defect locations on the substrate is received. The defect locations are grouped, and at least one local reference site in proximity to each group of defect locations is determined. The local reference site(s) is (are) used to determine a positional offset for the defect locations in each group. Another embodiment relates to an apparatus for reviewing defect locations on a substrate which provides for run-time correction of the defect locations. Other embodiments and features are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to specimen inspection and review systems. 
         [0003]    2. Description of the Background Art 
         [0004]    Automated inspection and review systems are important in process control and yield management for the semiconductor and related microelectronics industries. Such systems include electron beam (e-beam) based systems and other types of systems. 
         [0005]    As defect locations in a substrate are detected by an inspection tool, the coordinates of these defect locations are stored. Subsequently, a review tool, such as a high-magnification scanning electron microscope (SEM), may be used for more detailed viewing and analysis of the defect locations. 
         [0006]    The substrate is aligned on the stage of the review microscope, and a coordinate transformation is established between the review microscope and the defect location coordinates. Thereafter, the transformed coordinates are used by the review tool to find or “re-locate” the defect locations, such that they may be viewed and analyzed in greater detail. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram depicting an electron beam apparatus which may be utilized to practice the method disclosed herein in accordance with an embodiment of the invention. 
           [0008]      FIG. 2  is a schematic diagram showing an alignment mark on a substrate and a defect located by an inspection system. 
           [0009]      FIG. 3  is a schematic diagram illustrating a shift in position of a defect location between inspection and review in accordance with an embodiment of the invention. 
           [0010]      FIG. 4  is a flow chart of a process for run-time correction of a defect location during defect review in accordance with an embodiment of the invention. 
       
    
    
     SUMMARY 
       [0011]    One embodiment relates to a method for run-time correction of defect locations on a substrate during defect review. The substrate is loaded into a stage of a review apparatus, and coordinates for the defect locations on the substrate is received. The defect locations are grouped, and a local reference site in proximity to each group of defect locations is determined. The local reference site is used to determine a positional offset for the defect locations in each group. 
         [0012]    Another embodiment relates to an apparatus for reviewing defect locations on a substrate which provides for run-time correction of the defect locations. The apparatus includes a source configured to generate an electron beam, deflectors configured to scan the electron beam over a field of view, electron lenses configured to focus the electron beam onto a surface of a substrate being reviewed, and a detector configured to detect electrons from the substrate. The apparatus further includes electronic control and image analysis circuitry configured to retrieve a list of defect locations on the substrate, determine a local reference site in proximity to each defect location, and use the local reference site to determine a positional offset for the defect location. 
         [0013]    Other embodiments and features are also disclosed. 
       DETAILED DESCRIPTION 
       [0014]    In prior inspection and review systems, a coordination transformation is established between the review microscope and the defect coordinates from the inspection system. The substrate is aligned on the stage of the review microscope, and the coordinate transformation is used to re-locate the defects by the review system. 
         [0015]    However, applicant has determined that the coordinate transformation used by prior inspection and review systems does not always provide for precise and accurate “re-location” of the defects by the review system. In particular, for fields of view less than one micron, positioning errors frequently result in the actual defect location being outside the field of view of the review system. 
         [0016]    The present application discloses methods and apparatus for correcting positioning errors caused by thermal effects and other imperfections of the positioning system of the review scanning electron microscope (SEM). These methods and apparatus are surprisingly efficient, accurate and robust in providing run-time correction of these positioning errors. 
         [0017]    The thermal effects that introduce positioning errors may include, for example, heat transfer from the stage to the substrate being reviewed. Such heat transfer may occur over time, such that the positional errors which are induced by the heating are time dependent. The other imperfections may include, for example, minute spatial variations or modulations (waves) in the stage where the wafer is loaded and the positioning is controlled. Such spatial variations or modulations may introduce further positioning errors and complicates the task of re-locating the defects for review. 
         [0018]    Applicant has determined that the prior methods involving global translation, scaling and angular corrections cannot correct for positional errors caused by thermal expansion during the defect review after the wafer alignment, nor can they correct for the aforementioned spatial variations or modulations in the stage. Experiments by the applicant have shown that the methods and apparatus disclosed herein advantageously correct such positional errors with submicron precision and accuracy. 
         [0019]      FIG. 1  is a schematic diagram depicting an electron beam apparatus which may be utilized to practice the methods disclosed herein in accordance with an embodiment of the invention. The electron beam apparatus comprises, among other components, an electron gun or source  101 , condenser lenses (not depicted), beam deflectors  105 , electron lenses  103 , a detection system  108 , image capture circuitry  110 , and an image analysis system  112 . 
         [0020]    In  FIG. 1 , the 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 (or other substrate)  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. 
         [0021]    A detector  108  for detecting secondary or scattered electrons is also depicted. For example, the stage  106  may be electrically grounded, and a positive voltage may be applied to the detector  108  so as to attract the scattered electrons. Image capture circuitry  110  may be configured with buffers to capture image frames from the detector  108 . The image capture circuitry  110  may also be configured to process the image frames, for example, by accumulating or averaging frames. 
         [0022]    An electronic control and image analysis circuitry  112  may be configured to control the apparatus so as to obtain, process, and analyze the image data. The electronic control and image analysis circuitry  112  may comprise, for example, apparatus control circuitry  113 , one or more processor(s)  114 , data storage  115 , and memory  116 . These various components may be communicatively interconnected (for example, by way of a bus communication system). The electronic control and image analysis circuitry  112  may interface with a display, such as a monitor screen, and user input devices, such as a keyboard and mouse. 
         [0023]    In accordance with an embodiment of the invention, the data storage  115  may be configured to store a list of defect locations (detected by an inspection system), SEM image data, and other data, and the memory  116  may be configured with processor-executable code so as to perform various steps of the techniques disclosed in the present application. 
         [0024]      FIG. 2  is a schematic diagram showing an alignment mark  202  on a substrate and a defect  204  located by an inspection system. The alignment mark  202  is shown on a grid where each square represents a size of a pixel of the review imaging system. As seen, the alignment mark  202  may be an L-shaped mark which has a line width of about two pixels. The defect  204  is depicted within a circle which has a radius given by the defect location accuracy (DLA) size plus the defect size. The DLA size represents the defect location accuracy caused by the imperfections of the position system of the inspection or review tools. 
         [0025]      FIG. 3  is a schematic diagram illustrating a shift in position of a defect location between inspection and review in accordance with an embodiment of the invention. In  FIG. 3 , the dotted lines  302  represent the die grids printed on the wafer. The filled (shaded) L-shaped marks  304  are patterns or alignment marks that repeat periodically on the wafer or reticle. The lighter cross symbol  306  represents the defect location reported by the inspection system. 
         [0026]    During review run-time, however, the actual defect location shifts due to a thermal effect and other imperfections in the position system. The thermal effect may be due to heat transfer between the stage and the substrate and so may be time dependent. 
         [0027]    An example of the shifted position of an L-shaped mark is shown by the unfilled (white) L-shaped mark  308 , and the corresponding shifted position of the defect is shown by the darker cross symbol  310 . A first arrow  312  shows a first position change vector for the L-shaped mark, and the second arrow  314  shows a second position change vector for the defect. 
         [0028]    Note that the change vectors at different positions on a substrate may generally be different from each other. This is due to the thermal effect effectively expanding (or contracting) the substrate. Such an expansion (or contraction) results in position changes that depend on the location on the substrate. However, in accordance with an embodiment of the invention, by grouping defects by their proximity to alignment marks, the change vectors of defects within the group at run time may be effectively approximated by the change vector of the local (i.e. closest) alignment mark at run time. 
         [0029]      FIG. 4  is a flow chart of a process  400  for run-time correction of a defect location during defect review in accordance with an embodiment of the invention. This process  400  may be performed after an inspection system detects defects in a substrate. The inspection system generally reports locations of the detected defects relative to predetermined alignment marks at reference sites on the substrate. 
         [0030]    Per block  402 , an initial procedure may be performed. This initial procedure may include sorting and then grouping the defects. For example, the defects may be sorted and grouped by their locations on the substrate so as to provide for efficient review of the defects by the review system. For example, the sorting and grouping of defects may provide a sequence of groups, and sequence of defects within each group, which may be imaged in an efficient procedure. For example, the sequences may be determined so as to minimize the time required to move the field of view of the review system to the various defect locations. In addition, an optimum reference site may be chosen for each group. For example, the reference site chosen may correspond to an alignment mark which is closest proximity-wise to the locations (or average location) of the defects in the group. 
         [0031]    Per block  404 , a first group (for example, having a group number of one) may be selected. In this implementation, the defects in this first group are imaged first (see blocks  406  through  416 ). Thereafter, the group number may be incremented  418  so as to select a next group and image the defects in that group, until the last group is selected. In this implementation, the last group may have group number N g . 
         [0032]    For each selected group, the steps shown in blocks  406  through  416  may be performed. Per block  406 , the scanning electron microscopes (SEM) apparatus of the review system may illuminate (with electrons) and focus within its field of view (“go to”) a reference site (or multiple reference sites in sequence) for the selected group. For example, in the illustration of  FIG. 3 , the imaging apparatus may illuminate and focus on the alignment mark  304  which may be the closest in proximity to the defects in the selected group. (If multiple reference sites are used for the selected group, then multiple such alignment marks may be illuminated and focused sequentially.) In one implementation, a relatively large initial field of view may be utilized to roughly locate the run-time position of the alignment mark. Subsequently, a smaller field of view may be utilized for greater precision in locating the mark. 
         [0033]    Per block  408 , the imaging apparatus of the review system obtains and stores (“grabs”) the run-time SEM image of the reference site(s) for the group. This run-time image during review is then analyzed to calculate a measure of the shift or offset of the alignment mark(s) at the reference site. In other words, the location of the alignment mark(s) in the run-time image may be determined, and the two-dimensional offset vector O between the run-time location(s) and the location(s) from the inspection data may be determined. For example, the position shift of the alignment pattern may be obtained by comparing the image data of the pattern acquired by the review system during run-time against the image data from the inspection system. The offset vector O may be referred to as the local offset error for the group of defects under review. In the case where multiple references sites are used, the offset vector O may be a mean or median vector of the shift or offset of the alignment marks at the reference sites. 
         [0034]    In this implementation, the number of defects in a selected group is denoted by N d . Per block  410 , a first defect in the selected group (for example, having a defect number of one) may be selected and its location retrieved from the stored inspection data. Let P represent the vector or two-dimensional location of the selected defect from the inspection data. 
         [0035]    Per block  412 , a nominal or approximate two-dimensional (or vector) position P′ is calculated for the selected defect under review. As indicated, the two-dimensional position P′=P+O. In other words, the nominal position vector P′ is calculated by adding the local offset vector O to the two-dimensional location P of the selected defect from the inspection data. 
         [0036]    Per block  414 , the SEM apparatus of the review system may illuminate (with electrons) and focus within its field of view (“go to”) the site on the substrate at the nominal position vector P′ and obtain and store (“grab”) an SEM image from that site. For example, in the illustration of  FIG. 3 , the imaging apparatus may go to the location indicated by the darker cross mark  310  and obtain and store an SEM image from that site. 
         [0037]    Per block  416 , the process  400  may then increment the defect number to select a next defect in the group and loop back to perform blocks  412  and  414  for the selected defect. This may continue until SEM images are obtained for all defects in the selected group. 
         [0038]    Per block  418 , the process  400  may then increment the group number to select a next group and loop back to block  406 . This may continue until all the groups have been processed. 
         [0039]    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 in an automatic inspection and review system and applied to the inspection and review of wafers, optical masks, X-ray masks, electron-beam-proximity masks and stencil masks and similar substrates in a production environment. 
         [0040]    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. 
         [0041]    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.