Patent Publication Number: US-6982556-B2

Title: System and method for classifying defects in and identifying process problems for an electrical circuit

Description:
This application is a continuation-in-part of application Ser. No. 10/455,359 filed Jun. 6, 2003, in contents in which are incorporated in reference herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to testing electrical circuits, and more particularly to a system and method for detecting and classifying defects in an electrical circuit during or after a manufacturing process. The present invention is also a system and method for identifying one or more process problems that caused the defects detected during a test. 
   2. Description of the Related Art 
   Because of their small size and superior performance, thin-film-transistor (TFT) arrays have evolved as a preferred technology for a variety of applications including but not limited to flat-panel LCD displays and imaging and sensing systems used in consumer electronics. 
   During the manufacturing process, defects may develop which, is left unaddressed, may diminish the performance of the array. These defects include electrical shorts between the gate and common lines connecting the transistors and their associated storage elements. The need to test for defects becomes more important as the number of transistors in the array increases. This may be attributable to several factors. One is that the probability of a short developing tends to vary linearly with the length of the gate and common lines. The number of these lines and their proximity to one another also plays a role in increasing the chances of a short occurring. For example, in a double-gate or double-common-line configuration, the gap between the gate and common lines is likely to be narrower than in single-gate-line and single-common-line layouts. The likelihood of a short developing consequently increases. 
   Once a short has been located in a TFT array, it can be repaired by cutting the short. Existing methods for locating shorts and other defects in transistor arrays, however, have proven to be inaccurate. This is especially true of shorts between the gate and common lines of the array, as this type of defect does not give off a distinctive signal at the affected pixel location which can be detected by existing methods. As a result, the defect may never be located or at best may only be detected to lie within a certain general area which includes other pixel elements that are properly functioning. Because of this imprecision, the defect may not be able to be corrected because it cannot be located with any degree of accuracy. In a worst case, an attempt to eliminate the defect may result in destroying a properly functioning portion of the array, thereby compounding the problem and in many cases tendering the transistor array unusable for all intents and purposes. 
   In view of the foregoing considerations, it is apparent that there is a need for a system and method for, first, detecting the existence of a defect in a thin-film-transistor array and, second, accurately detecting a location of the defect so that corrective action may be taken without disturbing other portions of the array that are properly functioning. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to improve the accuracy and efficiency of testing of electronic circuits including ones containing transistor arrays. 
   Another object of the present invention is to provide a system and method for accurately detecting defects in a transistor array including but not limited to a thin film transistor array. 
   Another object of the present invention is to provide a system and method for determining a type of defect in a transistor array. 
   Another object of the present invention is to provide a system and method for precisely determining a location of a defect in a transistor array during a testing procedure. 
   Another object of the present invention is to provide a system and method for classifying defects in a circuit under test and then to identify one or more problems that occurred in a manufacturing process that caused or likely caused the defects. 
   Another object of the present invention is to provide a system and method as described above which accurately performs defect classification and process problem identification while taking noise and other external influences into consideration. 
   These and other objects and advantages of the present invention are achieved by providing a method for detecting a defect in a transistor array which in accordance with one embodiment includes applying a test signal to the array, monitoring pixel voltage along a gate line of the array, and detecting a defect associated with the gate line based on a variation in the pixel voltage during the monitoring step. The defect may be a short between the gate line and a common line of the array. The gate line and common line may be associated with a same pixel element or different pixel elements. The method further includes detecting a location of the defect based on a rate of change in the variation of the pixel voltage along the gate line. The rate of change may be measured in any one of a variety of ways. For example, the rate of change may be measured as a sudden increase or decrease of the pixel voltage or as a change in slope of a pixel voltage profile. Alternatively, the location of the defect may corresponds to the pixel voltage hitting a minimum or maximum value as determined by a set of profile curves plotted by a signal analyzer connected to the transistor array. The transistor array may be a TFT array or another type of circuit which includes an array of transistors connected, for example, in a matrix pattern. 
   In accordance with another embodiment, the present invention is a system for detecting a defect in a transistor array. The system includes a signal generator for applying a test signal to the array and a detector for detecting a defect in the array based on a variation in pixel voltage along an array gate line. The defect may be a short between the gate line and a common line of the array. The gate line and common line may be associated with a same pixel element or different pixel elements. The detector further detects a location of the defect based on a rate of change in the variation of the pixel voltage along the gate line. The rate of change may be measured in any one of a variety of ways. For example, the rate of change may be measured as a sudden increase or decrease of the pixel voltage or as a change in slope of a pixel voltage profile. Further, the location of the defect may correspond to the pixel voltage hitting a minimum or maximum value as determined by a set of profile curves plotted by a signal analyzer connected to the transistor array. 
   In accordance with another embodiment, the present invention is a signal analyzer for testing a TFT array. The signal analyzer includes at least one electrode for inputting a test signal into the TFT array and a processor which monitors a variation in pixel voltage along a gate line of the array and detects a defect associated with the gate line based on the pixel voltage variation. The signal analyzer may detect any of the types of defects previously mentioned, using one or more of the previously mentioned techniques. 
   The present invention is also a system and method for performing circuit defect analysis and process problem identification. One embodiment of the method includes applying a test signal to a circuit, obtaining a signal generated in response to the test signal, comparing the response signal to reference information, classifying a defect in the circuit based on a result of the comparing step, and identifying a problem in a manufacturing process which caused the defect based on the classification. The reference information may include one or more signal profiles corresponding to predefined types of defects that can occur during the manufacturing process. The signal profiles are preferably generated based on past test data taken over a period of time. If desired, the profiles may be processed into a statistical representation of their corresponding defect types. 
   Defect classification is preferably performed by determining whether the response signal falls within one or more of the signal profiles. If a clear correspondence to one profile exists, then the circuit is identified as including the predefined defect type corresponding to that signal profile. If the response signal falls within two or more signal profiles, then probabilities may be determined for each profile. The defect may then be classified as corresponding to the defect type whose signal profile has the highest probability. The probabilities may be computed mathematically or logically using any one of a variety of techniques. During the comparison step, a determination may be made as to whether the response signal falls within predetermined signal zones assigned to each signal profile. The defect may then be classified based on whether the response signal falls within any one of those zones. If the profiles in adjacent zones overlay, a dividing line between the zone may be adjusted to ensure that an equal error distribution exists for the profiles in those zones. 
   Process problem identification is preferably performed by comparing the classified defect to stored information. This information may include a table linking the predefined types of defects to one or more process problems. By looking up the classified defect in the table, a determination can be made as to what problem occurred during the manufacturing process that caused the defect. The problem identification can then serve as feedback information for purposes of adjusting the process to eliminate the problem. In one exemplary application, the method of the present invention classifies defects and identifies process problems for TFT arrays of the type used, for example, in a display panel. In this case, the response signals correspond to pixel voltages detected in response to the input of test signals. 
   An embodiment of the system of the present invention for performing defect analysis includes a signal generator which applies a test signal to a circuit, a detector which obtains a signal generated in response to the test signal, and a processor which compares the response signal to reference information, classifies a defect in the circuit based on a result of the comparison, and identifies a problem in a manufacturing process which caused the defect based on the classification. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is a diagram showing a portion of a thin-film-transistor array that includes elements for controlling the illumination of four corresponding pixel locations in a flat-panel LCD display screen and two types of gate-to-common short defects in TFT array, and FIG.  1 ( b ) is a diagram showing the pixel layout in different process steps of the TFT array of FIG.  1 ( a ). 
       FIG. 2  is a diagram showing an equivalent circuit for the elements at each point of intersection of the thin-film-transistor array of FIG.  1 . 
       FIG. 3  is a flow diagram showing steps included in a method for detecting the existence of a short between a gate line and common line of a TFT array in accordance with one embodiment of the present invention. 
     FIGS.  4 ( a ) and  4 ( b ) are graphs showing exemplary test signal patterns that may be applied to a TFT array for purposes of detecting shorts between gate and common lines in accordance with the present invention. 
       FIG. 5  is a diagram showing a profile of signal voltages (including positive pixel voltages Vp) generated in response to the test signal pattern in FIG.  4 ( a ) along a gate line when a gate-to-self-common short is present. This profile may provide a basis for locating defects in a TFT array in accordance with the present invention. 
       FIG. 6  is a diagram showing a profile of signal voltages (including negative pixel voltages Vp) generated in response to the test signal pattern in FIG.  4 ( b ) along a gate line when a gate-to-self-common short is present. This profile may provide another basis for locating defects in a TFT array in accordance with the present invention. 
       FIG. 7  is a diagram showing a profile of signal voltages generated from the positive and negative pixel voltages in  FIGS. 5 and 6 . This profile may be used as another basis for detecting defects in a TFT array in accordance with the present invention. 
       FIG. 8  is a diagram showing a profile of signal voltages (including positive pixel voltages Vp) generated in response to the test signal pattern in FIG.  4 ( a ) along a gate line when a gate-to-adjacent-common short is present. This profile may provide a basis for locating defects in a TFT array in accordance with the present invention. 
       FIG. 9  is a diagram showing a profile of signal voltages (including negative pixel voltages Vp) generated in response to the test signal pattern in FIG.  4 ( b ) along a gate line when a gate-to-adjacent-common short is present. This profile may provide another basis for locating defects in a TFT array in accordance with the present invention. 
       FIG. 10  is a diagram showing a profile of signal voltages generated from the positive and negative pixel voltages in  FIGS. 8 and 9 . This profile may be used as another basis for detecting defects in a TFT array in accordance with the present invention. 
       FIG. 11  shows a tester for detecting defects in a TFT array in accordance with one embodiment of the present invention. 
       FIG. 12  is a flow diagram showing steps included in one embodiment of a method for classifying defects and identifying corresponding process problems during a product defect analysis. 
       FIG. 13  shows one type of defect histogram that may be used in accordance with the present invention, wherein the histogram was obtained using an ideal distribution of defect signals for defects d 1 , d 2 , and dn and where Vd 1 , Vd 2 , and Vdn are representative defect signals in respective signal zones for d 1 , d 2 , and dn. 
       FIG. 14  shows another type of defect histogram obtaining under actual measurement conditions for defects d 1 , d 2 , and dn where Vd 1 , Vd 2 , and Vdn correspond to defect signals in respective signal zones for d 1 , d 2 , and dn. 
       FIG. 15  is a diagram showing a portion of a TFT array used in a display panel which may be tested in accordance with the method of the present invention. 
       FIG. 16  is a flow diagram showing steps included in a process for manufacturing a TFT array as shown in FIG.  15 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention relates to a system and method for detecting a defect in an electronic circuit containing an array of transistors, and then accurately determining a location of the defect so that corrective action may be taken without disturbing other portions of the circuit that are properly functioning. The system and method are particularly well suited to detecting shorts that form between signal-carrying lines during the manufacturing process. The signal-carrying lines include but are not limited to gate lines and common lines, however the detection of defects of in other portions of the circuit is also possible. For example, the present invention may be implemented to detect at least the following types of opens and shorts: gate line open, common line open, local drain electrode open, local source electrode open, local gate electrode open, local gate-drain short, local gate-source short, local drain-source short, ITO pixel electrode-gate line short, ITO pixel electrode-data line short, Cst short through the insulator between ITO pixel electrode and common line metal, a pinhole in a gate insulator, a gate-to-data line short, and data line-to-common line short. 
   Additional defects which are detectable by the present invention in circuits having transistor arrays include local semiconductor island missing, local contact layer (such as n+ layer) absence, damaged Cst electrode, data-data line short, local n+ layer short, ITO-ITO short over data line, ITO-ITO short over gate line, a partial ITO pixel electrode absence, partial overlap between data line and ITO pixel electrode without short, and a partial overlap between gate line and ITO pixel electrode without short. 
   The present invention is ideally suited for use in detecting the existence and then determining with pinpoint accuracy the location of shorts in a TFT array used in a display such as a flat-panel LCD display. The invention, however, is not intended to be limited to this specific transistor-array application. On the contrary, the system and method of the present invention may advantageously be used to determine the existence and location of defects in TFT arrays used in virtually any other application. For convenience purposes, the remaining portion of this disclosure addresses the application of a TFT array in a display panel. 
   FIG.  1 ( a ) is a diagram showing a portion of a exemplary thin-film-transistor array that includes elements for controlling the illumination of four corresponding pixel locations in a flat-panel LCD display screen and two types of the gate to common short defects in TFT array. In order to better understand FIG.  1 ( a ), reference may be made to FIG.  1 ( b ) which shows the pixel layout in different process steps of the TFT array of FIG.  1 ( a ). 
   The array includes a plurality of gate lines  1  and data lines  2  arranged in the form of a matrix. Each point of intersection between these lines include a storage element  3  connected to a switching transistor  4 . The storage element includes a capacitor which stores a voltage value that activates an associated liquid crystal material, which is added in the cell process, when the transistor is switched off. The liquid crystal material is sandwitched between an ITO pixel electrode and another ITO electrode on the opposing glass which is placed against TFT array glass in the cell assembly process. The gate lines control switching of the transistors and the data lines provide image signal data. The array also includes a plurality of common lines  6  situated parallel to the gate lines and connected to the storage capacitor of each pixel along respective rows of the array. The common lines function to provide the reference electric potential for the storage capacitor. Reference numeral  7  corresponds to a metal pattern connecting the source electrode of TFT and ITO pixel electrode via a contact open area illustrated in FIG.  1 ( b ). 
   The array shown in FIG.  1 ( a ) has what is commonly referred to as a double common-line layout, since each row of pixels has double-common lines connected to the storage capacitors and each gate line is placed between self- and adjacent-common lines. While the system and method of the present invention are ideally suited to detecting the existence and location of errors in a TFT array of this type, those skilled in the art can appreciate that the invention may just as easily be applied to TFT arrays having other configurations including but not limited to the layouts of single-common line, single-gate line, and double-gate line where each row of pixels has double-gate lines connected to the TFT gate electrodes and each common line is placed between upper and lower self-gate lines. 
     FIG. 2  is an equivalent circuit diagram showing elements included at each point of intersection of the array. In this diagram, the storage capacitor is labeled Cst and the transistor TFT. For illustrative purposes, the pixel voltage V p  is shown as corresponding to the storage capacitor voltage. In operation, when a gate signal switches the TFT on, Cst is charged to the image signal voltage present on the data signal line at that time. Liquid crystal material controls the amount of light passing through the ITO electrodes and operation of liquid crystal material is controlled by the voltage applied across the ITO electrodes. After the TFT is turned off, the voltage across the ITO electrodes can be maintained until a next turn-on time with help from Cst for charge holding. Each gate line controls the turning on and off of all the TFTs connected to it and a scanning signal is applied to one gate line at a time sequentially. 
   As previously explained, during the manufacturing process it is possible for defects to form in the TFT array. One defect that is particularly troublesome is a short between the gate and common lines. At least two types of shorts are possible. One short can form between a common line and gate line of the same pixel element. This type of short is illustratively shown by metal residue  8  in FIG.  1 ( a ) and may be referred to as a gate to self-common line short. Another short can form between the gate line of one pixel element and the common line of another pixel element. This type of short is illustratively shown by metal residue  9  in FIG.  1 ( a ) and may be referred to as a gate to adjacent-common line short. 
     FIG. 3  shows steps included in a method for detecting the existence of a short between a gate line and common line of a TFT array in accordance with one embodiment of the present invention. These steps may be equally applied to both types of shorts mentioned above. The method includes as an initial step applying a test signal to the array. (Block  10 ). The test signal may be applied to one or more lines of the array. Signal patterns are applied to the gate, data, and common lines in sequence for each row of the array or in multiple rows at the same time. The test voltages in the pattern are set to allow signals indicative of the existence of defect along the gate lines to produce a distinctive pattern which may be identified and measured by a detector. 
   A second step of the method includes monitoring the pixel voltages along each gate line as the test signals are applied. (Block  20 ). When no defect exists along the gate lines, the pixel voltages are expected to produce a certain signal profile, depending on the magnitude and frequency of the test signals applied. For example, the pixel voltage profile monitored along the gate lines may have a constant value when no gate-to-common-line short exists. On the other hand, when such a defect exists a different profile may be identified and detected. For example, as will be discussed in greater detail below the profile of the pixel voltages monitored along a gate line under test may follow a predictable variation. 
   A third step of the method includes detecting a defect associated with the gate line based on the variation in pixel voltage detected during the monitoring step. (Block  30 ). For example, under certain circumstances and test voltage patterns, the pixel voltage may vary linearly starting from a feeding end of the line. When this occurs, a high degree of probability exists that a defect exists along the gate line under test. As previously mentioned, the gate line and common line may be connected to a same pixel element in which case one profile variation is produced. If the gate and common lines are connected to different pixel elements, a different profile variation may be produced. The specific variation detected provides a basis for determining, for example, not only that a defect exists along the gate line but also what specific type of defect exists, e.g., a gate line to self-common line short or a gate line to adjacent-common line short. 
   A fourth step of the method includes determining a location of the defect along the gate line under test. (Block  40 ). The location of the defect may be determined by further analyzing the pixel voltage profile of the affected gate line. For example, in one implementation the pixel voltage profile may continue to vary linearly along the gate line up to a point where the defect exists. At this point, a detectable change in the profile may occur, e.g., the profile may change slope or the rate of variation may change. Alternatively, it may be determined that the profile hit a maximum or minimum value depending, for example, on whether the test signals applied correspond to a positive or negative pixel voltage. Because a close correspondence exists between the profile and points along the gate line being tested, the location of defects may be detected with precision based on detectable profile variations. 
   A fifth step includes a correcting or otherwise eliminating the defect. (Block  50 ). If the defect is a short, this step may include cutting the short with any one of a variety of known cutting tools. Other known methods for correcting defects may also be employed. 
   The method of the present invention may be modified in various ways. For example, in an alternative embodiment the pixel voltage profile may be measured indirectly and some of the signal patterns in FIGS.  4 ( a ) and ( b ) are modified accordingly. In some TFT array test equipment used in manufacturing line, the pixel voltage is measured through some medium such as optical modulator or electron beam to detect the defect location. In some other TFT array test equipment used in manufacturing line, the defect location is detected by sensing the amount of charge stored in the storage capacitor after charging operation. In order to apply the present invention to this charge sensing technology, the pixel voltage profile described in the present invention needs to be obtained through the information obtained from the charge sensing operation. One of the methods to obtain the pixel voltage profile through the charge sensing technique is to switch the data line into some reference electric potential at Vp measurement time at FIG.  4  and scan one gate line at a time so that the charge flow to the reference electric potential is measured. The amount of charge flow then reflects the pixel voltage on storage capacitor. Scanning the gate line can be done by raising the Vg and Vcom signals by the same magnitude at the same time in case of FIG.  4 ( a ). This way of scanning drives the TFTs and storage capacitors on the gate line in equal condition even if there is a short between the gate and common lines. 
   Specific examples of the method of the present invention will now be discussed. These examples are provided merely for purposes of illustrating how the invention may be applied in various exemplary contents and therefore are not intended to limit the invention in any way. In discussing these examples, reference may be made to the aforementioned drawings. 
   As previously discussed,  FIG. 1  shows two types of gate-to-common line shorts in a TFT array having a double common-line layout. For both types of shorts, it may be assumed that all the gate lines are connected together by gate shorting bat. The shorting bat connects the metal lines of the same signal together electrically by a low-resistance metal bar-shaped pattern in the perimeter of TFT array area. The shorting bars are currently used for the gate and data lines by many TFT-CLD manufacturers to decrease the number of test probes to the gate and data lines or reduce the damage due to ESD (electro-static discharge) problem. The shorting bars are eventually removed in a later process. 
   It may also be assumed that all the common lines are connected together because they are normally connected by the TFT panel layout without shorting bar. In one variation, even and odd gate lines may be respectively connected by even and odd gate shorting bars and similar methodology as explained herein may be applied. When the shorting bar is not used, pixel charging and discharging are performed row by row and similar methodology may also be applied.  FIG. 2  shows an equivalent circuit for one cross point in the TFT array, where TFT and Cst indicate the thin film transistor and storage capacitor for each pixel respectively. 
   FIGS.  4 ( a ) and  4 ( b ) show test signal patterns that may be applied to the TFT array for purposes of detecting, short defects between the gate and common lines. More specifically, FIG.  4 ( a ) shows a test signal pattern that may be applied for a positive pixel voltage and FIG.  4 ( b ) shows a test signal pattern that may be applied for a negative pixel voltage. As the legends show, the thick and thin lines in these graphs represent signal patterns that may be applied to the data and gate shorting bars respectively and the dashed line represents the signal pattern applied to the common signal pad. These shorting bars are intentionally formed in the TFT array for purposes of testing or reducing ESD damage and are preferably laid out along a perimeter of TFT array area. To locate defects, test signals may be provided to multiple signal lines through the shorting bars. (These shorting bars are in contrast to the short defects that the invention is applied to detect and repair. These defects are ones which accidently form within the TFT array area, for example, as a result of an anomaly during manufacturing.) 
   When no gate-to-common line short exists, the potential voltage is constant along the gate line because one end of the gate line is electrically almost floating. On the other hand, when a gate-to-common short exists the potential voltage along the gate line is not constant but rather varies linearly from the feeding end (e.g., where the gate line is connected to a gate shorting bar) to the short point and then stays constant from the short point to the floating end of the gate line. The slope of linear variation of the gate potential voltage may be determined by simple calculation based on Ohmic Law, where the resistance values such as the resistance per unit length of gate line and the resistance per unit length of the common line are used. In order to locate the short point, it is preferable for the pixel voltage (Vp) to be closely influenced by the potential voltage on the gate line. This may be achieved, in one way, by initially charging Cst to a known voltage and then recharging Cst into the voltage whose value is limited by the gate voltage. 
   It is further noted that when a gate-to-common line short exists, the potential voltage may also not be constant along the common line. Also, its change of amplitude with time may not be constant along the common line. This variance will affect the value of Vp along the common line, which therefore may provide a further basis for detecting defects in the TFT array. This may be explained in greater detail as follows. 
   As previously noted, the pixel voltage profile along the gate line is another expression for the pixel voltage profile along the self-common line. And, the pixel voltage profile along the adjacent-common line is another expression for the pixel voltage profile along the adjacent-gate line. The pixel voltage V p  corresponds to the storage capacitor voltage and the common line transmits the reference electric potential for the storage capacitor. The gate line transmits the gate signal to the gate electrodes of all the TFTs connected to the gate line. Thus, the variance of V p  along the common line may provide a further basis for detecting gate-to-common line short defects. 
   In the examples which follow, the common lines are assumed to be connected to the common signal pattern at both ends, as shown in FIG.  2 . Also, in these examples the following voltages will be used as test pattern signals and signals which are monitored to determine the existence and location of a gate-to-common line short in the array:
         Vp=pixel voltages measured along a gate line under test   Vgh=high value of gate signal   Vgm=middle value of gate signal   Vgl=low value of gate signal   Vdh=high value of data signal   Vdl=low value of data signal   Vnn=start and end values of data signal for positive Vp   Vpp=end value of data signal for negative Vp   Vcl=low value of common signal   Vch=high value of common signal       

   Given these voltages, when no defect exists in the TFT array, all the storage capacitors are charged to a predetermined voltage (e.g., Vdh) during a time period T 3  in FIG.  4 ( a ) as long as Vgh is higher than Vdh by at least Vth, which is a threshold voltage of TFT. If TFTs remain in the off state during time periods T 4  and T 5  (discussed in greater detail below), the pixel voltages neatly remain at Vdh until the TFT-array tester measures them. In FIG.  4 ( b ), when there is no defect in TFT array all the storage capacitors are charged to Vdl during time period T 3  as long as Vgm is higher than Vdl by at least a predetermined threshold voltage Vth. If TFTs remain in the off state during time periods T 4  and T 5 , the pixel voltages nearly remain at Vdl until the TFT-array tester measures them. For illustrative purposes, it may be assumed in the following examples that Vgh=25, Vgm=−15, Vgl=−25, Vdh=20, Vdl=−20, Vnn=−20, Vpp=25, Vcl=−25, Vch=20, and the resistance per unit length of gate line=2* the resistance per unit length of common line. 
   Detection of Gate-to-Self-Common-Line Short 
   The method of the present invention may be adapted to detect shorts between a common line and a gate line connected to a same pixel element. A short of this type is illustratively shown by reference numeral  8  in FIG.  1 ( a ). Examples of test patterns which may be applied to the array to detect this type of short and their corresponding signal profiles are discussed below. 
   Analysis of Signal Patterns for Positive Pixel Voltages 
   Referring to  FIG. 5 , analysis of pixel voltages Vp along a gate line under test with a gate-to-self-common short is preferably performed in a step-by-step sequence using the signal patterns for positive pixel voltage set forth in FIG.  4 ( a ). At time T 1 , the potential voltages along the gate line and self-common line are shown by Vg(T 1 ) and Vcom(T 1 ) respectively. The pixel voltage Vp is charged to Vnn as denoted by Vp(T 1 ). 
   At the beginning of time period T 2 , the self-common signal drops from 0 to Vcl and Vcom drops from Vcom(T 1 ) to Vcom(T 2 - 3 ). This causes Vp to drop from Vp(t 1 ) to Vp(T 2 ( 0 )) but Vp charges to Vp(T 2 ) during the T 2  period because the TFT is turned on as long as Vp is lower than a predetermined amount (Vg−Vth). When Vp charges to (Vg−Vth), the TFT is turned off and Vp becomes saturated at (Vg−Vth). Since the data signal is now Vnn, the highest level of Vp(T 2 ) is limited to Vnn. 
   At the beginning of time period T 3 , the data signal becomes Vdh and Vp(T 3 ) becomes limited by (Vg (T 3 )−Vth) or Vdh whichever is lower. If Vdh is lower than (Vg (T 3 )−Vth) toward the feeding end, then Vp (T 3 ) becomes saturated by Vdh and a slope change occurs toward the feeding end. 
   At the beginning of time period T 4 , the gate signal drops from Vgh to Vgl and Vcom drops from Vcom(T 2 - 3 ) to Vcom (T 4 - 5 ). This causes Vp to drop from Vp(T 3 ) to Vp(T 4 - 5 ). Vg (T 4 - 5 ) makes Vp stay at Vp (T 4 - 5 ). If 10 volts is used as the Vp criteria (Vpass) above which the pixels are reported as good ones, then there will be multiple pixels reported as bad ones along the gate line with a short defect as shown in FIG.  5 . 
   Using conventional techniques, this defect will at best be reported as a line defect even though the source of the line defect is a short defect between the gate and common lines at a specific location. Typically, the line defect is reported with the gate and data line numbers of two end points, but the location of actual short defect is not given. As can be seen from Vp(T 4 - 5 ) in  FIG. 5 , the method of the present invention generates a final pixel voltage profile along the gate line with a short defect, which can be relied on for pinpointing with accuracy the location of the short defect by finding the lowest pixel voltage or the cross point of two lines of Vp having different slope. 
   Analysis of Signal Patterns for Negative Pixel Voltages 
   Referring to  FIG. 6 , analysis of pixel voltages Vp along the gate line with a gate to self-common short is performed in a step-by-step sequence using the signal patterns for negative pixel voltage in FIG.  4 ( b ). In this example, the values at times T 1  and T 2  may be considered negligible or at least not substantially affecting the end result of the analysis. Accordingly, the discussion will begin at time T 3 . 
   During time period T 3 , the potential voltages along the gate lien and self-common line are shown by Vg(T 3 ) and Vcom(T 3 ) respectively. The Vp is charged to Vdl as denoted by Vp(T 3 ). It is also noted that during time period T 3 , the short defect between the gate and self-common lines makes the gate signal more positive compared to the case where no such defect exists. This is because the gate line at Vgm is shorted to the self-common line at Vch, which is higher than Vgm. A higher gate signal makes the TFTs on the gate line turn on with lower on-resistance and the storage capacitors on the gate line charge to Vdl more quickly than those on other gate lines without short defect. 
   At the beginning time period T 4 , the common signal drops from Vgm to Vgl and Vcom drops from Vcom(T 3 ) to Vcom(T 4 - 5 ). This causes Vp to drop from Vp(T 3 ) to Vp(T 4 ( 0 )). Also during this time period, Vp begins to approach Vp(T 3 ) once agains since the data signal is at Vdl and Vg has a potential profile of Vg (T 4 - 5 ). 
   At the beginning time period T 5 , the data signal becomes Vpp and Vp charges to Vp(T 5 ) because Vp becomes saturated at (Vg (T 5 )−Vth) before reaching Vpp. If −10 volts is used as the Vp criteria (Vpass) below which the pixels are reported as good ones, then there will be multiple pixels reported as bad ones along the gate line with a short as shown in FIG.  6 . 
   Using conventional methods, this defect would at best be reported as a line defect even though the source of the line defect is a short defect between the gate and common lines at a specific location. However, as can be seen from Vp(T 5 ) in  FIG. 6 , the method of the present invention generates a final pixel voltage profile along the gate line with a short, which can be relied on to locate the short defect by finding the end of low-constant pixel voltage where Vp starts to decrease toward the feeding end. 
   Analysis of Pixel Patterns for both Positive and Negative Pixel Voltages 
   If Vp (T 4 - 5 ) of  FIG. 5  obtained from the test pattern of FIG.  4 ( a ) is subtracted by Vp(T 5 ) of  FIG. 6  obtained from the test pattern of FIG.  4 ( b ), this result may be used to generate a final pixel voltage profile along the gate line with a short defect, as shown in FIG.  7 . If 20 volts is used as the Vp criteria (Vpass) above which the pixels are reported as good ones, then there will be multiple pixels reported as bad ones along the gate line with a short as shown in FIG.  7 . Using conventional methods, this defect at best is reported as a line defect even though the source of the line defect is a short between the gate and common lines at certain location. As in  FIG. 5 , using the present invention the short defect can be located with pinpoint accuracy by finding the lowest pixel voltage or the cross point of two lines of Vp having different slope. 
   Detection of Gate-to-Adjacent-Common-Line Short 
   The method of the present invention may be adapted to detect shorts between a common line and a gate line connected to different pixel elements. A short of this type is illustratively shown by reference numeral  9  in FIG.  1 ( a ). Examples of test patterns which may be applied to the array to detect this type of short and their corresponding signal profiles are discussed below. 
   Analysis of Signal Patterns for Positive Pixel Voltages 
   Referring to  FIG. 8 , analysis for pixel voltages Vp along the gate line with a short to adjacent-common line and for pixel voltages Vp_aj along the adjacent-common line with a short to adjacent gate line is performed in a step-by-step sequence using the signal patterns for positive pixel voltage in FIG.  4 ( a ). 
   At time T 1 , the potential voltages along the gate line and adjacent-common line are shown by Vg(T 1 ) and Vcom_aj (T 1 ) respectively. The pixel voltage Vp of gate line is charged to Vnn as denoted by Vp(T 1 ). The pixel voltage Vp of the adjacent-common line is charged to Vnn as denoted by Vp_aj (T 1 ). 
   At the beginning time period T 2 , the common signal drops from 0 to Vgl and Vcom of adjacent-common line drops from Vcom_aj(T 1 ) to Vcom_aj(T 2 - 3 ). This causes Vp_aj to drop from Vp_aj(T 1 ) to Vp_aj(T 2 ( 0 )). The Vcom of gate line with a short drops from 0 to Vgl and this causes Vp to drop from Vp(T 1 ) to Vp(T 2 ( 0 )). Also, during the time period T 2 , Vp charges to Vp(T 2 ) because the TFT is turned on as long as Vp is lower than a predetermined voltage (Vg−Vth), where Vth is a threshold voltage of TFT. When Vp charges to (Vg−Vth), the TFT is turned off and Vp becomes saturated at (Vg−Vth). Since the data signal is now Vnn, the highest level of Vp(T 2 ) is limited to Vnn. The gate signal for TFTs on the adjacent-common line is now at Vgh and Vp_aj (T 2 ) reaches Vnn. 
   At the beginning time period T 3 , the data signal becomes Vdh and Vp(T 3 ) becomes limited by (Vg (T 3 )−Vth) or Vdh, whichever is lower. If Vdh is lower than (Vg (T 3 )−Vth) toward the feeding end, then Vp(T 3 ) becomes saturated by Vdh and experiences a slope change toward the feeding end. The gate signal for TFTs on the adjacent-common line, however, is still at Vgh and Vp_aj (T 3 ) reaches Vdh. 
   At the beginning time period T 4 , the gate signal drops from Vgh to Vgl and Vcom_aj drops from Vcom_aj (T 2 - 3 ) to Vcom_aj (T 4 - 5 ). This causes Vp_aj to drop from Vp_aj (T 3 ) to Vp_aj (T 4 - 5 ). During time periods T 4  and T 5 , Vp_aj stays at Vp_aj (T 4 - 5 ) since the gate signal for TFTs on the adjacent-common line with the short defect is at Vgl. Vg (T 4 - 5 ) makes Vp stay at Vp (T 3 - 5 ) since the gate signal is low enough to turn off all the TFTs on gate line with short defect. If 10 volts is used as the Vp criteria (Vpass) above which the pixels are reported as good ones, then there will be multiple pixels reported as bad ones along the gate line with a short defect as shown in FIG.  8 . 
   Using conventional methods, this defect at best is reported as a line defect even though the source of the line defect is a short between the gate and common lines at a specific location. Typically, the line defect is reported with the gate and data line numbers of two end points, but the location of the actual short defect is not given. As can be seen from Vp (T 3 - 5 ) in  FIG. 8 , the present invention generates a final pixel voltage profile along the gate line with a short, from which the short defect can be located by finding the end of the low-constant pixel voltage where Vp starts to increase toward the feeding end. 
   Analysis of Signal Patterns for Negative Pixel Voltages 
   Referring to  FIG. 9 , analysis for pixel voltage, Vp, along the gate line with a short defect to adjacent-common line and for pixel voltage, Vp_aj, along the adjacent-common line with a short defect to adjacent gate line is done in step by step sequence using the signal patterns for negative pixel voltage in FIG.  2 ( b ). In this example, the values at times T 1  and T 2  may be considered negligible or at least not substantially affecting the end result of the analysis. Accordingly, the discussion will begin at time T 3 . 
   At time T 3 , the potential voltages along the gate line and adjacent-common line are shown by Vg (T 3 ) and Vcom_aj (T 3 ) respectively. The Vp and Vp_aj are charged to Vdl as denoted by Vp (T 3 ) and Vp_aj (T 3 ) respectively. It is also noted that during time period T 3 , the short defect between the gate and adjacent-common lines makes the gate signal more positive compared to the case where no such defect exists because the gate line at Vgm is shorted to the adjacent-common line at Vth, which is higher than Vgm. A higher gate signal makes the TFTs on the gate line turn on with lower on-resistance and the storage capacitors on the gate line charge to Vdl more quickly than those on other gate lines without short defect. The TFTs on the adjacent-common line with the short defect receives normal gate signal and the storage capacitors on the same line charge to Vdl. 
   At the beginning time period T 4 , the common signal drops from Vgm to Vgl and the Vcom_aj drops from Vcom_aj (T 3 ) to Vcom_aj (T 4 - 5 ). This causes Vp_aj to drop from Vp_aj (T 3 ) to Vp_aj (T 4 - 5 ). During time period T 4 , Vp_aj stays at Vp_aj (T 4 - 5 ) since the gate signal at Vgl turns off the TFTs on the adjacent-common line and Vp stays at Vp (T 3 - 4 ) since the data signal is at Vdl. 
   At the beginning time period T 5 , the data signal becomes Vpp and Vp charges to Vp (T 5 ) because Vp becomes saturated at (Vg (T 5 )−Vth) before reaching Vpp. If −10 volts is used as the Vp criteria (Vpass) below which the pixels are reported as good ones, then there will be multiple pixels reported as bad ones along the gate line with a short defect as shown in FIG.  9 . 
   Using conventional methods, this defect at best is reported as a line defect even though the source of the line defect is a short between the gate and common lines at a specific location. However, as can be seen from Vp (T 5 ) in  FIG. 9 , the present invention generates a final pixel voltage profile that can be used to precisely locate the short, by finding the end of low-constant pixel voltage where Vp starts to decrease toward the feeding end. 
   Analysis of Signal Patterns for Both Positive and Negative Pixel Voltages 
   If Vp(T 3 - 5 ) of  FIG. 8  obtained from the test pattern of FIG.  4 ( a ) is subtracted by Vp(T 5 ) of  FIG. 9  obtained from the test pattern of FIG.  4 ( b ), this result may be used to generate a final pixel voltage profile along the gate line with a short defect, as shown in FIG.  10 . If 20 volts is used as the Vp criteria (Vpass) above which the pixels are reported as good ones, then there will be multiple pixels reported as bad ones along the gate line with a short defect. 
   Using conventional methods, this defect at best is reported as a line defect even though the source of the line defect is a short between the gate and common lines at a specific location. As in  FIG. 8 , the short can be located defect by finding the end of the low-constant pixel voltage where Vp starts to increase toward the feeding end. Also, as can be seen from Vp_aj (T 4 - 5 ) in  FIGS. 8 and 9 , the Vp_aj profile along the shorted adjacent-common line does not provide clearly distinctive feature to locate the gate to adjacent-common short defect, although it can generate some partial line defects depending on the value of Vpass. 
   Other test methodologies may be combined with the method of the present invention to improve the detection accuracy of defects in a TFT array. In this regard, it is noted that it may be considered ideal for one test methodology to detect all types of defects with very high accuracy. As previously described indicated, the present invention can detect the presence and location of gate-to-common-line short defects. For other types of defects, the present invention can detect their location with varying accuracy depending on the type, location, and/or severity of the defects. Thus, it is possible to develop new test methodology to improve the defect detection accuracy for some of the defects. It is also possible to combine the present invention with new methodologies to make use of both methods as long as they can work together, preferably without canceling each other&#39;s benefits. For example, in the case of short defects between signal lines, the presence and type of these defects can be identified by performing a preliminary test to check for leakage current between them. A more specific test method may then be used, if necessary, based on the result of the preliminary leakage test. 
     FIG. 11  shows a tester  70  for detecting defects in a TFT array  100  in accordance with one embodiment of the present invention. The tester includes a signal generator  80  and a processor/detector  90 . The signal generator generates for input a test signal into the TFT array. This test signal may correspond to any one or more of the test signal patterns shown in FIGS.  4 ( a ) and  4 ( b ). The processor/detector monitors voltages produced in the TFT array as a result of the test signals and generates one or more of the previously mentioned pixel voltage profiles for purposes of detecting the presence and location of gate-to-common line shorts and/or other defects in the TFT array. The tester may also perform any of the other steps of the methods of the present invention described herein. 
   Manufacturing Process Problem Identification 
   The present invention also relates to various embodiments of a system and method which classify a defect in an electrical circuit and then identify at least one reason why the defect occurred. The cause is preferably one relating to an anomaly in a manufacturing process used to fabricate the circuit. Once this anomaly (or process problem) is identified, corrective action may be taken to reduce the likelihood of the defect occurring again in subsequently made circuits. The system and method are ideally suited to the analysis of circuits including but not limited to TFT-arrays for LCD displays, printed circuit boards (PCBs), printed board assemblies (PBAs), integrated circuits (IC), or any of the other types of circuits described herein. Moreover, the type(s) of defects to be classified include any of those previously described as well as ones described below, and the same is true of the manufacturing process problems which heretofore have been discussed and ones will be explained in the discussion which follows. 
   The Inventor has recognized that when a defect is detected and/or located during a manufacturing process of an electrical circuit, access to statistical information showing a distribution of one or more process problems that likely caused the defect would be highly desirable. The present invention provides access to this information based on process problem data collected over time. Using this data, process problems can be monitored in different units of time (e.g., weekly or monthly) and corrective action may be taken to improve the accuracy and efficiency of the manufacturing process. 
   In accordance with at least one embodiment, the system and method of the present invention uses the collected data as a statistical benchmark to classify defects detected in the circuit and identify the problems that occurred during the manufacturing process that caused, or likely caused, those defects. The collected data is preferably updated to improve problem and defect detection. This may involve modifying one or more process steps and/or parameters to reduce the likelihood of the defect occurring in subsequently made circuits. 
     FIG. 12  is a flow diagram showing steps included in a method for performing circuit defect analysis in accordance with one embodiment of the present invention. An initial step of the method includes applying a test signal to a circuit at some point during the manufacturing process. (Block  200 ). This may include some intermediate point or after the circuit has been completely formed, or both. The test signal may correspond to any of the test signals previously discussed herein, such as the ones depicted in FIGS.  4 ( a ) or  4 ( b ). 
   A second step includes obtaining a signal generated in response to the test signal. (Block  210 ). This signal may be obtained, for example, using a signal probe or any other type of test equipment detector. The test location for obtaining the signal may be selected to acquire an accurate representation of the portion of the circuit under test, taking into consideration, for example, noise and other external influences that may degrade the quality of the signal. 
   A third step includes comparing the signal output from the circuit in response to the test signal to reference information. (Block  220 ). The reference information may take any one of a variety of forms. For example, the reference information may include a signal profile (e.g., a signal curve) corresponding to a predefined type of defect that possibly may occur during the manufacturing process. Preferably, a plurality of signal profiles corresponding to different types of predefined defects are included. The signal profiles may be generated from previous tests of circuits that were detected as having the predefined defects. Preferably, the test data is processed to provide a more accurate statistical basis from which defect detection may be realized. Alternatively, the signal profiles may be generated based on a statistical representation of normal (e.g., non-defective) signal data, an example of which corresponds to a mean of signal values obtained for a non-defective circuit. Signal profiles and/or reference information of other types may be used if desired for purposes of performing defect detection. 
     FIG. 13  is a graph showing a defect histogram that was obtained using the reference information in accordance with the present invention. The histogram includes three ideal defect signals Vd 1 , Vd 2 , and Vdn, each corresponding to a different type of predefined circuit defect. In ideal conditions, each defect has its own unique defect signal, e.g., a distinctively discrete signal defect signal value when it is tested and a defect such as d 1  is supposed to give the same defect signal of Vd 1  at every test. Then, every defect detected can be identified to certain predefined defect type, such as listed above, by matching the defect signal to one of the unique defect signals of the predefined defect types. In the histogram, the occurring frequency of defects belonging to the specific defect type increases by one for each matching. 
   In actual measurements of the circuit under test (e.g., a TFT-array) for defect detection, the defect signal may have some noise due to non-ideal factors such as the noise of the measurement equipment. This noise may cause the defect signal to deviate from its ideal value corresponding to the predefined defect type. Thus, it is preferably to go through a process to determine which predefined defect type the detected defect belongs to. One way is to find one of the predefined defects that has the closest defect signal to that of the detected defect. If the detected defect has a defect signal just falling half way between two representative defect signals of two predefined defect types, then each of those two predefined defect types has an increment of one half in their frequency count. In other words, each defect type has its own defect signal zone a predetermined amount (e.g., half way) up to its neighboring representative defect signal, as shown in the FIG.  13 . Preferably, these zones are situated so that the zones do not overlap. In the histogram graph, the signal profiles show, for example, that defect signal Vd 2  occurred more times during the period of time the data was collected than either of the other two defect signals Vd 1  and Vdn. 
     FIG. 14  is a graph showing a distribution of defect signal profiles that occur under more realistic conditions where, in addition to measurement noise, other degrading influences such as the spreading of the defect signal itself are taken into consideration. Even if there is no measurement noise, many defects show spread distribution of a defect signal because of the varying degree of severity of the defect and the different amounts of signal delay depending on the defect location. While the graph in either of  FIGS. 13 and 14  may be used in practicing the present invention, the signal profiles shown in  FIG. 14  may be preferably because a more accurate result is likely to be obtained. 
   In the graph of  FIG. 14 , each signal profile is illustratively shown as corresponding to a statistical curve developed based on test data taken over a predetermined period of time or on estimated data based on statistical calculation. Each curve has an associated standard deviation σ, mean value V, and probability value N which defines the range within which voltage values corresponding to a respective one of the predefined circuit defects have appeared and thus are likely to appear again in future tests. As in the first graph, each profile curve is set within a separate defect signal zone. However, as reflected by signal profiles for defect types d 2  and dn, it is possible for the signal profiles to overlap because of noise and/or other influences. 
   The reference information used in accordance with the present invention may be stored in a memory, database, or other storage system or medium included within or coupled to the processing system performing the defect analysis. This information is preferably stored in statistical form and may be subsequently modified based on the results obtained for each test to produce a more accurate model for defect signal classification and manufacturing process problem detection, to be described in greater detail below. 
   A fourth step includes classifying a defect in the circuit based on the comparison performed between the response signal generated by the input test signal and the reference information. (Block  230 ). This step may be performed by determining whether the response signal falls within any of the signal zones included in the stored profile signal distribution. For example, a response signal falling within the signal zone, or profile curve, corresponding to Vd 1  may be classified as corresponding to the predefined defect corresponding to that zone. 
   A different approach to classification may be taken when the signal profiles in adjacent zones overlap, or more specifically when it is determined that the response signal falls within the signal profiles of two predefined types of defects. This apparent conflict may be resolved in a number of ways. One way involves computing probability values indicating the likelihood that the circuit defect is one of the predefined defects corresponding to the signal profiles. The defect is then classified as the predefined defect having the higher probability. A number of techniques for computing these probability values may be taken. 
   One technique involves taking mathematical approaches to conflict resolution. One approach is based on Equation 4 and another approach is based on Equations 5-10. These equations are discussed in greater detail below. 
   Another technique involves taking a logical approach to conflict resolution. According to this approach, a rule-based system stores data and other forms of information indicating combinations of defects that are likely to occur together. The rules of this system, for example, may indicate that a first type of predefined defect usually does not occur unless a second predefined type of defect also exists. These rules may form the basis for resolving the conflicting case of when the response signal falls within the signal profiles of adjacent signal zones. For example, when such a conflict arises between Vd 1  and Vdn in  FIG. 14 , the rule-based system may determine whether a response signal obtained in the same or a separate test corresponds to another predefined defect, which usually occurs with the defect corresponding to Vd 1 . If another predefined defect does not exist, it may be concluded that Vd 1  has a lower probability of corresponding to the circuit defect than Vdn. The circuit defect may therefore be classified as the Vdn defect. Conversely, if the other defect does exist, then Vd 1  may be considered to have a higher probability than Vdn and the defect may be classified accordingly. 
   Another technique is a variation of the previous method, where the probabilities of the conflicting profiles are assigned based on the absence or detection of one or more manufacturing process problems known to be associated with the defect corresponding to the response signal. In this case, the same or separate tests may be performed to determining whether the one or more manufacturing process problems exist. 
   Another technique involves redefining the signal zones for the overlapping signal profiles. This may be accomplished by adjusting the position of the dividing line between the two zones based on the intersection of the two signal profile curves. This is illustratively shown in  FIG. 14 , where the dividing line D 2n  separating defect zone d 2  and defect zone dn is adjusted based on the intersection of the two corresponding profiles. 
   Another techniques involves redefining the signal zones for the overlapping signal profiles based on a desired error distribution. This may be accomplished, for example, by adjusting the position of the dividing line between the two zones to ensure that the error distribution between the profiles is at least substantially equal. Coincidentally, this equal error distribution is shown in  FIG. 14 , where the position of dividing line D 2n  is adjusted so that regions A and B have equal areas. 
   A fifth step includes identifying at least one manufacturing process problem that caused, or likely caused, the classified defect. (Block  240 ). This may be accomplished by accessing information that links a list of predefined defect classifications with a plurality of manufacturing process problems. This information may, for example, be stored in a memory, a database system, or a rules- or knowledge-based system. The information is preferably derived from test data compiled over a predetermined period of time, which data indicates that certain predefined defects were caused by one or more specific manufacturing process problems. If desired, the data may also provide an indication of what stage during the manufacturing process the defect may have occurred. An example of information compiled from test data of this type is set forth in Table 1 provided below. The fifth step may therefore be implemented by finding the classified defect in the stored list of predefined defect classifications and then acknowledging one or more of the process problems linked to that defect. 
   An optional sixth step includes adjusting the process to prevent the problem from occurring during the manufacture of subsequent circuits. (Block  250 ). For example, if the process problem identified in the fifth step was the existence of a foreign particle on an IC substrate before gate insulator film deposition, an adjustment may be made in the form of cleaning the substrate before performing a subsequent gate insulator film deposition process or increasing the frequency of cleaning the inside surface of the gate insulator film deposition vacuum chamber. 
   One possible application of the method of the present invention is the identification of process problems which caused defects in the manufacture of a TFT array. An exemplary embodiment of the method of the present invention adapted to perform this application will now be discussed. 
   Classification of TFT-Array Defects and Identification of Associated Manufacturing Process Problems 
   As previously described, TFT-arrays are typically used in LCD display panels. In order to ensure their proper operation, the arrays should be tested before being sold. Testing is preferably performed by the manufacturer using equipment which drives the array with a pattern of electrical signals. During this testing process, the storage capacitor of each pixel experiences electrical charging and discharging operations. A sensor measures these voltages and then they are compared to predetermined target voltages which non-defective pixels would exhibit. If a pixel has a defect, its corresponding pixel voltage will be different from a target pixel voltage. Thus, when a difference exists between the measured voltage and this normal voltage, the pixel under test may be considered to have a defect. 
   During the manufacturing process, many types of defects may form in a TFT array. These defects include but are not limited to data line open, gate line open, common line open, local drain electrode open, local source electrode open, local gate electrode open, local gate-drain short, local gate-source short, local drain-source short, ITO pixel electrode-gate line short, ITO pixel electrode-data line short, Cst (storage capacitor) short through the insulator, pinhole in gate insulator, Gate-data line short, data-common line short, local semiconductor island missing, local contact layer (such as n+ layer) absence, damaged Cst electrode, data-data line short, local n+ layer short, ITO-ITO short over data line, ITO-ITO short over gate line, ITO pixel electrode absence, overlap between data line and ITO pixel electrode, and overlap between gate line and ITO pixel electrode. 
   In accordance with this exemplary embodiment of the present invention, it may be assumed that each type of defect that can occur in a TFT array has its own unique defect signal profile. A list of predefined defect types may therefore be developed and stored in association with their unique signal profiles for purposes of defect classification. During testing, a pixel voltage corresponding to a defective pixel may be identified as corresponding to one of the predefined defect types, by matching the pixel voltage to one of the unique signal profiles. 
   As previously indicated,  FIG. 13  shows an example of a defect histogram based on an ideal distribution of defect signals Vd 1 , Vd 2 , and Vdn for predefined defect types d 1 , d 2 , and dn and their defect signal zones due to measurement noise. The vertical axis in this graph is labeled “number of defects collected over time” because the graph of  FIG. 13  is a histogram, which, for example, may be generated for currently running tests indicating a total number of times the defect signals for defect types d 1 , d 2  and dn have occurred during production tests over a period of time. For ideal distribution of the defect signals, previous tests or circuit simulation give Vd 1 , Vd 2  and Vdn. 
   Actual defective pixel voltages are influenced by their defect severity and location, in addition to the noise generated from the test equipment. Varying degrees of defect severity and signal delay due to different defect location cause the pixel voltages produced from defective pixels to further deviate from their ideal form. Defect signals for defects d 1 , d 2 , and dn that are produced from those additional non-ideal factors and noise may be shown, for example, in FIG.  14 . Because all those combined non-ideal factors can skew the defect signals to values greater or less than the ideal values of Vd 1 , Vd 2 , and Vdn as shown in  FIG. 13 , the signal profiles of the defect signals in  FIG. 14  are shown as statistical curves with predetermined standard deviations. 
   Because of the occurrence of the non-ideal factors during the testing process, ambiguities are introduced into the system. These ambiguities can corrupt the defect classification process if left unabated. The present invention may take a variety of approaches to accurately classify defective pixel voltages produced by defects in an array which contains non-ideal factors. One approach involves identifying which defect signal profile (e.g., d 1 , d 2 , dn) most closely matches the defect signal of a defective pixel. The predefined defect type corresponding to this signal profile may then be used as a basis for classifying the detected defect. This approach, however, may not be optimal when the defect signal of detected defective pixel falls within the signal profiles of two predefined types of defects. 
   When the defect signal of a detected pixel voltage falls within the defect signal profile of two or more predefined types of defects, the present invention may employ probabilistic techniques to identify the most likely defect type. One such technique is based on the recognition that for some mask and process designs, the probability of one type of defect occurring may be very low if one or more other types process problems are not found to simultaneously exist. In such a case, the low-probability defect type may be excluded from the list of defect classifications, favoring instead another closely matching defect type with higher probability. 
   Determination of Parameter Values. As the display size of a TFT-array panel increases, the test equipment may take multiple measurements to test the panel. This is because the measurement sensor of the array tester cannot cover the entire panel size. Only one portion of the panel may be tested at a time. Thus, for large displays multiple measurements are performed for one TFT-array panel and a stepping motion is required between the sensor and the panel. Because of environmental changes and other influences, the detected pixel voltage distribution may change with every new measurement after stepping. The defect signals for each predefined defect type may therefore be expected to change from panel-to-panel and from step-to-step. In accordance with the present invention, these values may be periodically adjusted, e.g., with every measurement. 
   One way to properly adjust the representative defect signal of each predefined defect type is to use a measured mean value of normal pixel voltages. For example, if the representative defect signal of defect type d 1  is Vd 1 i at initial mean value of Vmi_d 1 , then an adjusted defect signal for a new measurement with new mean value of Vmn_d 1  can be obtained by Equation 1.
 
 Vd   1   n=Vd   1   i*Vmn   —   d   1 / Vmi   —   d   1    (1) 
 
This equation can be used because it can be assumed that the change in voltage measurement is linear to normal pixel voltage and the defect pixel voltage, and this makes the defect signal also change linearly. The mean value of normal pixel voltages are normally available from TFT-array test equipment at every new measurement.
 
   The value of Vd 1 i may also change at different pixel locations because of varying amounts of signal delay depending on the pixel location. In these circumstances, the value of Vd 1 i at each pixel location may be adjusted through a combination of computer simulation techniques performed for the array and a small number of defect signal measurements. If a signal profile for one of the predefined defect types cannot be adjusted in a manner which distinguishes it from the signal profiles of other predefined defect types, then it may be discarded. 
   Defect Classification with Distribution of Defect Signal. In general, the defect signal of a predefined defect type may have a statistical distribution as shown in FIG.  14  and can be represented by normal distribution function as follows,
 
Θ di=N   di   exp[− ( v−V   di ) 2 /(2σ di   2 )]/√{square root over (2πσ di   2 )}  (2) 
 
where Θdi represents the distribution function of defect signal of predefined defect type di at specific measurement, v is a variable of defect signal, V di  is a mean value and σ di  is a standard deviation of normal distribution function for predefined defect signals of type di, and N di  is the probability of any arbitrary defect to be originated from the predefined defect type di.
 
   Thus, one can expect 
                 ∑     i   =   1     k     ⁢           ⁢     N   di       =   1           (   3   )             
 
where k denotes the total number of predefined defect types.
 
   The value of N di  may be determined by objectively considering how probable it is that each predefined defect type can take place. If every predefined defect type has equal probability, then one can obtain from Equation (3) that every N d  is equal to 1/k. The value of σ di  can depend on the process variation related to the predefined defect type and the noise of the measurement system. 
   The defect signal zones for the predefined defect types should then be redefined by adjusting the dividing lines to give the same amount of error for two neighboring signal profile distributions. In  FIG. 14 , the dividing line D 2n  between the defect signal zone for d 2  and dn is determined so that the area under the tailed distribution of d 2  to the right of D 2n  is equal to the area under the tailed distribution of dn to the left of D 2n . Applying this concept to Eq. (2), one obtains following expression 
           ∫     v   =   D2n     ∞     ⁢       {       N   d2     ⁢           ⁢       exp   ⁡     [       -       (     v   -     V   d2       )     2       /     (     2   ⁢           ⁢     σ   d2   2       )       ]       /       2   ⁢           ⁢   π   ⁢           ⁢     σ   d2   2             }     ⁢           ⁢     ⅆ   v         =       ∫     v   =     -   ∞       D2n     ⁢       {       N   dn     ⁢           ⁢       exp   ⁡     [       -       (     v   -     V   dn       )     2       /     (     2   ⁢           ⁢     σ   dn   2       )       ]       /       2   ⁢           ⁢   π   ⁢           ⁢     σ   dn   2             }     ⁢           ⁢     ⅆ   v             
 
   Setting the dividing line in this manner produces the same amount of error for purposes of classifying a detected voltage of a defective pixel within the signal zones for two neighboring predefined defect types. As a result, this error is cancelled out when the information of defect classification is collected for many data. 
   Another way to classify each defect between two neighboring predefined defect types is to use Bayes&#39; Theorem and the probability that each defect originated from a predefined defect type d 1 , when the defect signal Vd falls between Vd 1  and Vd 2 . This is given by Equation (5) as follows:
 
 P ( D   1   |E )= P ( D   1 )* P ( E|D   1 )/{ P ( D   1 )* P ( E|D   1 )+ P ( D   2 )* P ( E|D   2 )}  (5) 
 
where P(D 1 |E) is the probability that a detected pixel voltage Vd between Vd 1  and Vd 2  corresponds to predefined defect type d 1 , P(D 1 ) is the probability that any defect corresponds to predefined defect type d 1 , P(E|D 1 ) is the probability of defect occurring as a member of d 1 , P(D 2 ) is the probability of any defect corresponding to predefined defect type d 2 , and P(E|D 2 ) is the probability of a defect occurring as a member of d 2 .
 
   If Equations (2) and (5) are compared, the following equations can be obtained
 
 P ( D   1 )* P ( E|D   1 )=α N   d1   exp[− ( Vd−V   d1 ) 2 /(2σ d1   2 )]/√{square root over (2πσ d1   2 )}  (6) 
 
 P ( D   2 )* P ( E|D   2 )=α N   d2   exp[− ( Vd−V   d2 ) 2 /(2σ d2   2 )]/√{square root over (2πσ d2   2 )}  (7) 
 
where α is a proportional constant.
 
   From Equations (5), (6), and (7), the following equation may be obtained:
 
 P ( D   1   |E )=[ N   d1   exp[− ( Vd−V   d1 ) 2 /(2σ d1   2 )]/√{square root over (2πσ d1   2 )}]/{
 
 N   d1   exp[− ( Vd−V   d1 ) 2 /(2σ d1   2 )]/√{square root over (2πσ d1   2 )}+ N   d2   exp[− ( Vd−V   d2 ) 2 /(2σ d2   2 )]/√{square root over (2πσ d2   2 )}}  (8) 
 
   If the defect signal profiles for some of the predefined defect types have broad statistical distributions, then statistical distributions for other predefined defect types beyond the two neighboring distributions should be taken into consideration. This may be accomplished by generalizing Equations (5) and (8) as 
               P   ⁢           ⁢     (       D   1     |   E     )       =     P   ⁢           ⁢     (     D   1     )     *   P   ⁢           ⁢       (     E   |     D   1       )     /       ∑     j   =   1     k     ⁢           ⁢     {     P   ⁢           ⁢     (     D   j     )     *   P   ⁢           ⁢     (     E   |     D   j       )       }                   (   9   )                 P   ⁢           ⁢     (       D   1     |   E     )       =       [       N   d1     ⁢           ⁢       exp   ⁡     [       -       (     Vd   -     V   d1       )     2       /     (     2   ⁢           ⁢     σ   d1   2       )       ]       /       2   ⁢           ⁢   π   ⁢           ⁢     σ   d1   2             ]     /       ∑     j   =   1     k     ⁢           ⁢     {       N   dj     ⁢           ⁢       exp   ⁡     [       -       (     Vd   -     V   dj       )     2       /     (     2   ⁢           ⁢     σ   dj   2       )       ]       /       2   ⁢           ⁢   π   ⁢           ⁢     σ   dj   2             }                 (   10   )             
 
   Defect Classifications with Exceptional Defects. Some defect types reveal their classification in a clearer way than other types. For example, defect types such as line open or line short defects show the defect classification as soon as they are detected. An ITO-ITO short defect can be classified when detected defect signals for two adjacent pixels show very close values, because these defects have practically the same pixel voltage. For these exceptional defects, defect classification is rather straightforward and precedes the procedure explained in previous sections. 
   Defect Classification with Input from Array Repair. The defects in a TFT-array panel, detected by TFT-array test equipment, can be reviewed by an operator under a microscope. During this examination, the operator first may try to identify the defect and then repair the defect according to the result of the visual defect identification. Thus, based on the visual defect identification, the operator of TFT-array repair equipment can add more valuable information to the effort of defect classification. For example, using the defect classification described in previous sections, one can provide multiple choices with priority for the cause of the defect and help the operator of TFT-array repair equipment choose one cause of the defect out of the multiple choices. Then, one can use the operator&#39;s choice as a final decision in defect classification. 
   Conversion of Defect Classification into Process Problems. Once a detected pixel voltage has been classified as corresponding to one of the plurality of predefined defect types, a determination is made as to what anomaly in the manufacturing process may have been the cause of the defect. This determination may be made based on data collected from previous tests linking the predefined types of defects to specific process problems. Linking this information requires a thorough understanding of the mask design and manufacturing process for TFT-arrays. This may be accomplished manually based on the expertise of an engineer or automatically, for example, through the use of a rules-based system. Table 1 shows an example of how various predefined defect type classifications may be converted into process problems. 
   
     
       
         
             
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               Defect 
               Process Problem Description 
             
          
         
         
             
             
             
          
             
               Classification 
               Major Process Area 
               Detailed Description 
             
             
                 
             
             
               Data line open 
               Source/Drain line 
               Foreign particle on the substrate 
             
             
                 
               patterning 
               before S/D metal film deposition 
             
             
               Gate line open 
               Gate line patterning 
               Foreign particle on the substrate 
             
             
                 
                 
               before gate metal film deposition 
             
             
               Common line 
               Gate line patterning 
               Foreign particle on the substrate 
             
             
               open 
                 
               before gate metal film deposition 
             
             
               Local drain 
               Source/Drain line 
               Foreign particle on the substrate 
             
             
               electrode open 
               patterning 
               before S/D metal film deposition 
             
             
               Local source 
               Source/Drain line 
               Foreign particle on the substrate 
             
             
               electrode open 
               patterning 
               before S/D metal film deposition 
             
             
               Local gate 
               Gate line patterning 
               Foreign particle on the substrate 
             
             
               electrode open 
                 
               before gate metal film deposition 
             
             
               Local gate - 
               Cleaning after gate 
               Foreign particle on the substrate 
             
             
               drain short 
               line patterning and 
               before gate insulator film 
             
             
                 
               before gate insulator 
               deposition 
             
             
                 
               film deposition 
             
             
               Local gate - 
               Cleaning after gate 
               Foreign particle on the substrate 
             
             
               source short 
               line patterning and 
               before gate insulator film 
             
             
                 
               before gate insulator 
               deposition 
             
             
                 
               film deposition 
             
             
               Local drain - 
               Source/Drain line 
               Foreign particle on the PR 
             
             
               source short 
               patterning 
               (photoresist) coated substrate 
             
             
                 
                 
               before exposure for S/D line 
             
             
                 
                 
               patterning 
             
             
               ITO pixel 
               Cleaning after gate 
               Foreign particle on the substrate 
             
             
               electrode - 
               line patterning and 
               before gate insulator film 
             
             
               gate line short 
               before gate insulator 
               deposition and on the PR coated 
             
             
                 
               film deposition 
               substrate before exposure for 
             
             
                 
               and ITO patterning 
               ITO patterning 
             
             
               ITO pixel 
               Cleaning after data 
               Foreign particle on the substrate 
             
             
               electrode - 
               line patterning and 
               before passivation insulator film 
             
             
               data line short 
               before passivation 
               deposition and on the PR coated 
             
             
                 
               insulator film 
               substrate before exposure for 
             
             
                 
               deposition and ITO 
               ITO patterning 
             
             
                 
               patterning 
             
             
               Cst short through 
               Cleaning after gate 
               Foreign particle on the substrate 
             
             
               the insulator 
               line patterning and 
               before gate insulator film 
             
             
                 
               before gate insulator 
               deposition 
             
             
                 
               film deposition 
             
             
               Pinhole in gate 
               Cleaning after gate 
               Foreign particle on the substrate 
             
             
               insulator 
               line patterning and 
               before gate insulator film 
             
             
                 
               before gate insulator 
               deposition 
             
             
                 
               film deposition 
             
             
               Gate - data line 
               Cleaning after gate 
               Foreign particle on the substrate 
             
             
               short 
               line patterning and 
               before gate insulator film 
             
             
                 
               before gate insulator 
               deposition 
             
             
                 
               film deposition 
             
             
               Data - common 
               Cleaning after gate 
               Foreign particle on the substrate 
             
             
               line short 
               line patterning and 
               before gate insulator film 
             
             
                 
               before gate insulator 
               deposition 
             
             
                 
               film deposition 
             
             
               Local 
               Semiconductor 
               Foreign particle on the substrate 
             
             
               semiconductor 
               island patterning 
               before semiconductor film 
             
             
               island missing 
                 
               deposition 
             
             
               Local contact 
               Contact layer 
               Foreign particle on the substrate 
             
             
               layer absence 
               patterning 
               before contact layer film 
             
             
                 
                 
               deposition 
             
             
               Damaged Cst 
               Either gate line 
               Foreign particle on the substrate 
             
             
               electrode 
               patterning or ITO 
               before gate metal or ITO film 
             
             
                 
               patterning 
               deposition 
             
             
               Data - data 
               Source/Drain line 
               Foreign particle on the PR 
             
             
               line short 
               patterning 
               coated substrate before exposure 
             
             
                 
                 
               for S/D line patterning 
             
             
               Local n+ 
               Etching of n+ 
               Foreign particle on the substrate 
             
             
               layer short 
               layer 
               before n+ layer etching 
             
             
               ITO - ITO 
               ITO patterning 
               Foreign particle on the PR 
             
             
               short over 
                 
               coated substrate before exposure 
             
             
               data line 
                 
               for ITO patterning 
             
             
               ITO - ITO 
               ITO patterning 
               Foreign particle on the PR 
             
             
               short over 
                 
               coated substrate before exposure 
             
             
               gate line 
                 
               for ITO patterning 
             
             
               ITO pixel 
               ITO patterning 
               Foreign particle on the substrate 
             
             
               electrode absence 
                 
               before ITO film deposition 
             
             
               Overlap between 
               Either data line 
               Foreign particle on the PR 
             
             
               data line and 
               patterning or 
               coated substrate before exposure 
             
             
               ITO pixel 
               ITO patterning 
               for data line patterning or ITO 
             
             
               electrode 
                 
               patterning 
             
             
               Overlap between 
               Either gate line 
               Foreign particle on the PR 
             
             
               gate line and 
               patterning or 
               coated substrate before exposure 
             
             
               ITO pixel 
               ITO patterning 
               for gate line patterning or ITO 
             
             
               electrode 
                 
               patterning 
             
             
                 
             
          
         
       
     
   
   As seen in the Table 1, some of the defect classifications share the same process problems and defect type classifications such as ITO pixel electrode-gate line short and ITO pixel electrode-data line short are related to multiple process problems. The information from Table 1 was derived for a 5-mask design and common Cst structure such as shown in FIGS.  1 ( a ) and  15 . 
     FIG. 16  is a flowing diagram showing steps included in a process for manufacturing the TFT-array structure show in FIG.  15 . An initial step includes depositing and patterning gate and common lines using a first mask. (Block  300 ). A gate insulator layer is then deposited (Block  310 ), followed by the deposition and patterning of semiconductor and contact layers using a second mask (Block  320 ). Next, data lines are deposited and patterned using a third mask. (Block  330 ), the contact layers are etched using the data lines as an etch-blocking layer (Block  340 ), and a passivation insulator is deposited (Block  350 ). Using a fourth mask, a via area is opened (Block  360 ) and then an ITO pixel electrode is deposited and etched (Block  370 ). Through the method of the present invention, defects detected in the TFT array may be classified. Then, based on this classification, for example, through the use of Table 1, the cause of each defect may then be identified as corresponding to one or more problems that occurred during the various stages of the manufacturing process. The existence of these defects may then be fed back to an operator or control system and appropriate action may be taken to prevent the defect from arising in subsequently manufactured arrays, e.g., a foreign particle which caused an ITO-ITO short over a gate line may be removed. 
   A system for classifying defects in a circuit under test and then determining one or more process problems which caused the defect may correspond to the tester shown in FIG.  11 . In this system, signal generator  80  inputs test signals and processor/detector  90  detects signals generated at predetermined locations within the circuit. The processor then performs steps analogous to those included in the method of the present invention, for example, under control of a computer program to perform defect classification and process problem identification. 
   Other modifications and variations to the invention will be apparent to those skilled in the art from the foregoing disclosure. Thus, while only certain embodiments of the invention have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention.