Abstract:
A system and method for verifying defects in electrical circuit patterns including supplying a plurality of like electrical circuit patterns to a defect verification assembly after identification of candidate defects at an automated inspection assembly; verifying selected candidate defects as being one of: an actual defect, other than an actual defect; and marking a candidate defect in response to a recurrence of a given candidate defect at substantially corresponding locations on at least two electrical circuit patterns.

Description:
[0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/550,061, entitled “Verification of non-Recurring Defects in Pattern Inspection” filed on Mar. 5, 2004, the disclosure of which is incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to systems and methods for the automated optical inspection of patterns such as electrical circuits, and more particularly to systems and methods for reducing the quantity of candidate defects detected during inspection.  
       BACKGROUND OF THE INVENTION  
       [0003]     The fabrication of electrical circuits, such as printed circuit boards, interconnect devices and flat panel displays, typically includes an automated optical inspection operation. Automated optical inspection typically identifies a collection of candidate defects, including both actual defects and false defects. Following automated optical inspection, a panel containing electrical circuit patterns is supplied to a verification station whereat candidate defects are evaluated and then identified as being either a actual defect or a false defect. Where possible, actual defects are repaired.  
         [0004]     False defects include, for example, both random false defects and recurring false defects. Random false defects include, for example, dust and oxidation. Recurring false defects include, for example, geometric pattern anomalies that are repeated throughout a batch of patterns to be inspected. Although the recurring false defects deviate from an ideal pattern to be detected as a defect, they are not sufficiently deviant to be categorized as an actual defect requiring subsequent repair.  
         [0005]     Many candidate defects are quickly categorized during manual defect verification as false defects, for example geometric deviations in shape. Other candidate defects require additional time during manual verification in order to determine whether they are a random defect or a recurring defect. Some random defects, for example short circuits, require even more additional time to effect repair of the defect.  
       SUMMARY OF INVENTION  
       [0006]     The present invention seeks to provide an improved methodology for verifying candidate defects in patterns, for example patterns comprising portions of electrical circuits disposed on a substrate panel.  
         [0007]     A general aspect of the present invention relates to systems and methods for evaluating candidate defects that avoid evaluating recurring false defects during a defect verification operation.  
         [0008]     Another general aspect of the present invention relates to at least one of recording a location of a false defect and recording a time interval for evaluating a candidate defect. The recorded information is utilized during the verification of candidate defects on subsequent patterns to avoid unnecessary evaluation of recurring false defects.  
         [0009]     Another general aspect of the present invention relates to a methodology for fabricating electrical circuits in which a portion of an electrical circuit is formed on a substrate and the substrate is automatically optically inspected. Candidate defects are evaluated as to whether they are an actual defect or a false defect. The evaluation operation avoids evaluating those false defects that recur in a plurality of electrical circuit substrates.  
         [0010]     In accordance with an embodiment of the present invention there is thus provided a method for verifying defects in electrical circuit patterns including supplying a plurality of like electrical circuit patterns to a defect verification assembly after inspection of the electrical circuit patterns at an automated inspection assembly to detect candidate defects; indicating locations of candidate defect; and evaluating at least some indicated locations at the defect verification assembly to determine whether a candidate defect is an actual defect; and avoiding evaluating at least one recurring false defect. This methodology is employed as part of an inspection operation during the fabrication of printed circuit boards and other electrical circuits.  
         [0011]     In accordance with another embodiment of the present invention there is thus provided a system for inspecting electrical circuit for defects including at least one automated inspection facility operative to automatically inspect electrical circuit substrates for defects and to indicate locations of candidate defects thereon; a recurring defect marker operative to receive indications of candidate defects for a plurality of electrical circuits, and to mark candidate defect locations that recur in a plurality of electrical circuits; and at least one verification facility receiving said indications of candidate defect not marked as being a recurring candidate defect. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]     The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:  
         [0013]      FIG. 1  is a simplified block diagram illustrating a system for inspecting electrical circuit substrates for defects in accordance with an embodiment of the present invention;  
         [0014]      FIG. 2  is a simplified flow diagram of a methodology for verifying defects in accordance with an embodiment of the present invention;  
         [0015]      FIG. 3  is a simplified flow diagram of a methodology for generating a false defect mask employed in the methodology seen in  FIG. 2 ; and  
         [0016]      FIG. 4  is a simplified pictorial diagram illustrating operation of the methodology seen in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]     Reference is made to  FIG. 1 , which is a simplified block diagram illustrating a system  10  for inspecting electrical circuit substrates  12  for defects, in accordance with an embodiment of the present invention. System  10  includes at least one automated inspection facility having, for example, at least one automated optical inspection (AOI) system  14 , and a verification facility  16  having at least one verification station. A verification facility may include, for example, at least one of a manually operated verification station and a verification station with an automatic verification functionality. Electrical circuit substrates  12  include, for example, metallic members deposited on a substrate surface using any suitable electrical circuit fabrication process. As used herein, the term electrical circuit refers to any suitable electrical circuit, or portion of an electrical circuit, including, without limitation, printed circuit boards, ball grid array substrates, multi-chip modules, integrated circuits, flat panel displays and other suitable patterned substrates.  
         [0018]     AOI  14  is operative, for example during electrical circuit fabrication, to acquire images of electrical circuit substrates  12  and to inspect the images to identify candidate defects in an electrical circuit pattern deposited, for example, on a surface thereof. For each electrical circuit inspected, a candidate defect file  15  indicating respective locations of candidate defects on substrates  12  is output by an AOI systems  14 . Candidate defect files  15  typically indicate both actual defects and defects which in actuality are misdetections, namely a non-defect incorrectly determined to be a defect.  
         [0019]     A verification station  16  receives an AOI inspected electrical circuit  18  along with a corresponding candidate defect file. The verification station  16  provides a defect verification functionality in which candidate defects are evaluated and verified, for example as being either an actual defect or a misdetection.  
         [0020]     In  FIG. 1 , it is seen that system  10  includes several AOI systems  14  and several verification stations  16 , all handling inspection and defect verification for a plurality of in-fabrication electrical circuit substrates  12 . Candidate defect files  15  are passed to verification station  16  through a defect server  20 , which is operative to correlate between a given electrical circuit substrate  12  and its corresponding candidate defect file  15 . It is noted, however, that system  10  may include as few as one AOI  14  passing candidate defect files  15  directly to one verification station  16 , in which case a defect server  20  would be obviated.  
         [0021]     In general, the functionality of AOI systems is well known and several suitable AOI systems are readily commercially available. Suitable AOI systems include, for example without limitation, the InFinex™, Inspire™, Spiron™, V-300™, and DISCOVERY™ AOI systems all of which are available from Orbotech Ltd. of Yavne, Israel.  
         [0022]     The functionality of defect verification systems is also well known and suitable systems are also readily commercially available. Suitable verification facilities include, for example, the VRS™ family of verification stations commercially available from Orbotech Ltd. of Yavne, Israel. In accordance with an embodiment of the invention, a defect verification facility  16  includes a microscope and an automatically movable stage. The stage automatically places locations of candidate defect on a substrate beneath the microscope for defect verification by a human operator. Verification systems employing automatic, that is computerized without human intervention, defect verification may also be employed alone, or in addition to human operator based verification. A suitable automatic verification station is described in U.S. patent application Ser. No. 10/793,224, entitled, “System and Method for Inspecting Electrical Circuits Utilizing Reflective and Fluorescent Imagery”, filed Mar. 5, 2004, the disclosure of which is incorporated herein in its entirety, assigned to Orbotech Ltd., of Yavne, Israel.  
         [0023]     In accordance with an embodiment of the present invention, candidate defect files  15  are updated prior to verification to mark least some defects as having a high probability of correspondence to a recurring misdetection. Thus, for example, recurring non-random candidate defects meeting a predetermined criteria classifying them as being misdetections, are suitably marked. Verification is not performed on candidate defects suitably marked as corresponding to a recurring misdetection. Marking may be actually marking candidate defects where further verification is not to be performed. Alternatively marking may be marking candidate defects where further verification is to be performed, and not marking those candidate defects where further verification is to be avoided.  
         [0024]     There are several measures by which a given candidate defect can be identified as a recurring misdetection. Suitable measures include, for example, at least one of the combination of a location of the candidate defect among substrates in a batch of substrates to be inspected, for example to ensure that the defect is not random, a time interval that is required to evaluate whether a given defect is real or false (typically actual defects require a longer time interval because they are also repaired, an operation that requires time), and a type of defect. In accordance with an embodiment of the invention, a type of defect is supplied by AOI  14 .  
         [0025]     In system  10  illustrated in  FIG. 1 , a verification result, generated at verification stations  16  as part of a defect verification file  22 , is supplied to a marked candidate defect server (MCDS)  24 . MCDS  24  is in communication with a marker data generator  26  operative to generate, and optionally store, a marked non-defect data file  28  containing marked candidate defects. In an embodiment of the invention, a recurring defect is a defect that meets at least one of several criteria, for example, a type of defect, a location of similar defect on several electrical circuit substrates  12 , and a time interval required to evaluate the defect at a verification station  16 .  
         [0026]     In accordance with an embodiment of the invention, a marked non-defect data file  28  is employed to update candidate defect files  15  to generate updated candidate defect files  30 . Updated candidate defect files  30  include, for example, all candidate defects output by an AOI system  14  with respect to a given electrical circuit  12 , but in which recurring false defects are marked so that they are skipped at verification stations  16 . The remaining candidate defects on inspected electrical circuits  18  are verified at verification facilities  16  using the updated candidate defect files  30 , thereby saving time that would otherwise be necessary to verify recurring misdetections.  
         [0027]     Reference is now made to  FIG. 2  which is a simplified flow diagram of a methodology  40  for verifying defects in accordance with an embodiment of the present invention. The methodology of  FIG. 2  is implemented, for example, on system  10  of  FIG. 1 .  
         [0028]     Methodology  40  is in effect a learning system in which the locations of some candidate defects, for example recurring false defects, are learned during the inspection of a batch of successive electrical circuits  12 . Whenever a new recurring defect is identified, the marked non-defect data file  28  is updated. This data file is then used to update candidate defect files from subsequently inspected electrical circuits  12 , to avoid performing a verification operation on a marked type of misdetection at a marked location on subsequently inspected electrical circuits.  
         [0029]     Methodology  40  commences with the verification of candidate defects on a new batch of panels, or substrates. The verification of a batch of substrates typically takes place after all of the substrates in a batch of substrates have been inspected by AOI systems  14 . Optionally, verification may start prior to the completion of an automated inspection operation on all of the substrates in a batch.  
         [0030]     Prior to the defect verification of a substrate, recurring misdetection data is employed to revise candidate defect files to avoid verifying certain types of misdetections. The recurring misdetection data is used, for example, to mark candidate defect location for which candidate defects on previously inspected electrical circuits have at least one of the following characteristics: the candidate defect is of a predetermined type, is at a predetermined location, has been verified as a non defect in less than a threshold time interval. A suitable combination of characteristics is known ahead of time, to a high degree of certainty, as being be indicative of misdetection by an AOI system. The marking of certain candidate defects saves time and verification resources by avoiding verifying defects which are known in advance to be misdetections. The remaining candidate defects are verified on a first panel, typically one at a time in sequence. In accordance with an embodiment of the invention, verification proceeds while avoiding recurring false defects.  
         [0031]     The verification of defects on a substrate proceeds in a looped sequence. Each actual defect that is found on a substrate is repaired, if repairable. It is noted that some types of false defects, for example recurring misdetections, are avoided entirely such that they are skipped and not evaluated.  
         [0032]     Upon completion of the evaluation of a defect, a verification operator signals to advance to the next defect to be evaluated, for example by pressing a “go” button. In accordance with an embodiment of the invention, at the time of signaling the system to advance to evaluate the next location whereat a candidate defect is located, the time interval required to evaluate the defect as being an actual defect or misdetection is recorded. This can be done automatically using, for example, an internal system timer. The type of defect, for example provided by the AOI system inspecting the defect, may also be recorded.  
         [0033]     Evaluation of candidate defects on a panel progresses by looping back to verification of a next candidate defect until the last candidate defect on a panel is evaluated and verified. Once all of the candidate defects in an updated candidate defect file have been evaluated and verified, for example as being an actual defect or misdetection, and if possible repaired, the defect type mask is updated with any new defects that are to be avoided in subsequently verified substrates. At the beginning of a batch, it is likely for there to be a larger quantity of misdetections that are evaluated. As verification progresses through a batch, new misdetections, such as candidate defects which to a high degree of certainty are recurring misdetections, are added to the non-defect data file and their verification is avoided in subsequent verification.  
         [0034]     Reference is now made to  FIG. 3  is a simplified flow diagram of a methodology for generating a false defect mask  50  employed in the methodology  40  of  FIG. 2 , in accordance with an embodiment of the invention. Methodology  50  of  FIG. 3  relates to generating a defect mask for filtering out recurring misdetections. In accordance with an embodiment of the invention employing methodology  50 , a recurring misdetections is a candidate defect that recurs at the same location on different electrical circuit substrates  12 . In accordance with an embodiment of the invention, a recurring false alarm defect is characterized by a recurring defect having at least one of: a recurring location and a recurring time interval for inspection which does not exceed a maximum threshold time interval. Optionally, the defect may also be characterized by type.  
         [0035]     Methodology  50  seen in  FIG. 3  commences with evaluating a new verified defect file  22 , for example supplied by a verification facility  16  ( FIG. 1 ). If the verification file is new, it is read. Then for each candidate defect which was verified as being a non-defect, that is to say a false alarm defect, its location, defect type, time interval for verification, and whether it recurred on a parameterized number of previous substrates  12  are all considered. For example, if a given type of candidate defect, for example any defect which is not merely a spec of dust, is found to be a misdetection that recurs at generally the same location on at least n substrate panels (where n is a parameterized number of substrates, for example 3 substrates), then it is deemed to be a recurring misdetection. In accordance with an embodiment of the invention, further criteria may be added, for example that the time interval for evaluating the defect did not exceed a maximum evaluation time interval.  
         [0036]     It is noted that the repair of a real defect typically requires at least a minimum time interval which is much longer than the time interval required merely to evaluate a candidate defect and dismiss it as being a misdetection. Thus, the additional requirement that the actual evaluation time interval needs to be less than a given evaluation time interval further ensures that a recurring defect is indeed a misdetection that does not require repair.  
         [0037]     Newly identified recurring misdetections are added to a non-defect data file for the present job or batch. Candidate defect files coming from the AOI  14  ( FIG. 1 ) are updated using non-defect data to ensure that recurring misdetections are not considered during defect verification of subsequent electrical circuit substrates.  
         [0038]     Reference is now made to  FIG. 4 , which is a simplified pictorial diagram illustrating operation of the methodology seen in  FIG. 2 . An image reference  60  and several acquired images  62 ,  64  and  66  are depicted. Reference  60  depicts a reference pattern, for example a portion of an electrical circuit, supplied for example by a CAM reference generator such as is available from Frontline Solutions of Yavne, Israel. Each of the acquired images represents an actual pattern, such as an electrical circuit, to be inspected. The acquired images  62 ,  64  and  66  are acquired, for example, using a suitable AOI system  14  ( FIG. 1 ) and are then evaluated using reference  60 , using for example any commercially available AOI system, in order to ascertain the presence of candidate defects. Candidate defects are indicated in  FIG. 4  by roman numerals I-VII.  
         [0039]     A candidate defect file  72 ,  74  and  76  is associated with each a corresponding acquired images  62 ,  64  and  66  respectively. Each of the defect files  72 ,  74  and  76  includes several information fields characterizing a defect: an X coordinate of a defect location, a Y coordinate of a defect location, a type of defect at the location and a time required to evaluate the defect. It is noted that the data structure of candidate defect files  72 ,  74  and  76  and the recurring defect mask  80  may be any suitable data structure for use in association with pattern inspection and verification systems, and the data structure show is merely exemplary. The file structure of the defect file seen in  FIG. 4  is highly simplified for the purposes clarity and to facilitate teaching of the invention.  
         [0040]     A recurring defect mask  80  is generated from an evaluation of candidate defect files  72 ,  74  and  76 . The recurring defect mask is applied to filter out recurring defects during the defect verification of defects on subsequently inspected electrical circuits, as described with reference to  FIGS. 1 and 2 . A recurring non-defect data file  80  includes several information fields characterizing a recurring misdetection to be masked: an X coordinate of a defect location, a Y coordinate of a defect location, a type of defect at the location and an acceptable radius covering an area in which defects of the same (or similar) type will not be considered in defect verification to be performed on subsequently inspected electrical circuits. It is noted that the data structure of recurring defect mask  80  may be any suitable data structure for use in association with pattern inspection and verification systems. The file structure of the recurring defect mask seen in  FIG. 4  is highly simplified for the purposes clarity and to facilitate teaching of the invention.  
         [0041]     Four candidate defects I, II, III and IV are seen in candidate defect file  72 , associated with acquired image  62 . Four candidate defects II, III, V and VI are seen in candidate defect file  74 , associated with acquired image  64 . Three candidate defects II, III and VII are seen in candidate defect file  76 , associated with acquired image  66 .  
         [0042]     Candidate defects I, IV, VI and VII are each characterized, for example by AOI  14  in  FIG. 1 , as “spot” defects. A spot defect could be for example dust. During a defect verification operation each of the candidate defects I, IV, VI and VII occur in each of images  62 ,  64  and  66  at a different location. Each candidate defects I, IV, VI and VII required 2 seconds to evaluate and determine that it is a misdetection. Because the candidate defects I, IV, VI are randomly located and non-repeating, these defects are not included in recurring defect mask  80 .  
         [0043]     Candidate defect II is characterized in  FIG. 1  as a “line width” defect which in each of acquired images  62 ,  64  and  66  requires 4 seconds to verify. Although candidate defect II deviates from the reference sufficiently to be marked as a candidate defect, the difference, which is a relatively small change in width of a conductor  82 , would not cause an impediment to functioning of the electrical circuit and typically would not require repair. Candidate defect II is therefore considered a misdetection. Furthermore, because candidate defect II recurs at the same location in each of acquired images  62 ,  64  and  66 , and because the time required to evaluate the defect is less than a given time threshold, candidate defect II is characterized as a recurring defect and is included in recurring defect mask  80 .  
         [0044]     Candidate defect III is characterized in  FIG. 1  as a “short defect” which appears in each of acquired images  62 ,  64  and  66 . Candidate defect requires 35 seconds to verify. The relatively lengthy time interval for verifying candidate defect II results because typically candidate defect III is repaired, for example by removing the excess conductor with a scalpel. Although candidate defect III recurs at the same location in each of acquired images  62 ,  64  and  66 , because the time required to evaluate and repair the defect exceeds a threshold time interval, candidate defect III is an actual defect that is not included in recurring defect mask  80 .  
         [0045]     Candidate defect V is characterized in  FIG. 4  as a line width defect which appears only in acquired images  64 . Candidate defect requires 17 seconds to verify. The relatively lengthy time interval for verifying candidate defect II results because a decision needs to be made whether the defect would indeed impair functioning of the electrical circuit. Although, no attempt may be made to repair candidate defect V, it would not be included in recurring defect mask  80  because the candidate defect occurs only in acquired image  64 , and does not recur elsewhere at the same location.  
         [0046]     It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the present invention includes modifications and variations thereof which would occur to a person of skill in the art upon reading the foregoing description and which are not in the prior art.