Abstract:
A method for detecting defects in devices that are fabricated in repetitive patterns upon the surface of a substrate by the, repetitive utilization of masks and similar devices. A mask flaw will become manifest in a series of defective devices as the mask is successively utilized. The detection of repetitive defects is undertaken by determining the electrical resistance of devices in a group, such as a column, fabricated upon the wafer surface, where the repetitive defect will occur multiple times. The mean electrical resistance of the group is determined and a percent deviation of each device from the mean is then determined. The percent deviation of all of the devices in the group are multiplied together to create a multiplied percent deviation number and the multiplied percent deviation number is then compared with a figure of merit value to make a determination of whether defective devices exist within the group.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to methods for detecting defects in devices fabricated on wafer substrates, and more particularly to a defect detection method for devices that are fabricated utilizing repetitive fabrication techniques such as are conducted with photolithographic masks. 
   2. Description of the Prior Art 
   Devices such as recording heads are typically fabricated in large numbers upon the surface of a wafer substrate, and following fabrication the wafers are sliced and diced to ultimately yield individual recording head devices. During the fabrication process, photolithographic techniques are employed which utilize a plurality of masks to shield and expose desired areas of the wafer surface for processing, as is well known to those skilled in the art. Such photolithographic masks are used in a repetitive manner to successively expose adjacent areas across the surface of the disk. Therefore, where a defect exists in an element of such a mask, the defect will repetitively inserted into particular devices on the substrate surface as the mask is successively utilized. The use of such a flawed mask, therefore creates a repetitive pattern of defective devices across the surface of the wafer. 
   Device testing for defects is conducted in the prior art by testing the electrical resistance of devices after they have been fabricated. In this regard, a resistance test range is developed and devices that exhibit a resistance within the range are deemed acceptable, while devices that exhibit a resistance beyond the range are deemed defective. A particular problem with this type of resistance testing is encountered in the testing of devices such as recording heads for tape drives. Such recording heads include a plurality of sensors disposed adjacent one another, and significantly, the resistance of each adjacent sensor typically differs from each other sensor. One reason for the resistance differences between the sensors is that the electrical lead lines of adjacent sensors differ in length. As a result, even though the sensors of a tape head might be virtually identical, the electrical resistance of the sensors will differ to a significant degree owing to the differing electrical lead line lengths of the sensor. As a result, a simple electrical resistance test requires a rather large range in order to not have good sensors be designated as defective because the electrical resistance of such sensors is beyond the acceptable test range. On the other hand, where the acceptable resistance test range is made large, defective sensors may be included within the acceptable test range. A defect range of 5 to 7% has been common in the prior art, where further analysis reveals that many of the devices initially identified as being defective are actually not defective. There is therefore a need for a test method that more accurately determines whether acceptable or defective devices exist on a wafer substrate. 
   SUMMARY OF THE INVENTION 
   The testing method of the present invention is particularly adapted to the testing of devices that are fabricated in repetitive patterns upon the surface of a said substrate. Such devices are fabricated by the successive, repetitive utilization of masks and similar devices such as a stepper lens upon such substrate surfaces. Where a mask includes a flaw in a particular element, the flaw will become manifest in a series of defective devices as the mask is successively utilized across the surface. Similarly, an aberration in a stepper lens will create a series of defects. 
   The detection of repetitive defects is undertaken by determining the value of a parameter such as the electrical resistance of devices fabricated upon the wafer surface. Other measurable parameters would include line width, plating thickness and the like, though the detailed description of the present invention will reference the electrical resistance by way of example. Devices are selected in a group, such as a column of devices upon the wafer surface, where the repetitive defect will occur multiple times. In the defect testing method, the electrical resistance of each of the devices in the group is determined. The mean electrical resistance of the group is determined and the electrical resistance of each device is compared to the mean to determine a percent deviation of each device from the mean. Thereafter, the percent deviation of all of the devices in the group are multiplied together to create a multiplied percent deviation number that is reflective of the deviation of the electrical resistance of the devices within the group from the mean. The multiplied percent deviation number is then compared with a figure of merit value to make a determination of whether defective devices exist within the group. Where the multiplied percent deviation number is less than the figure of merit, it is taken that none of the devices within the group is defective, and where the multiplied percent deviation number is greater than the figure of merit, it is indicative that defective devices created as a result of a repetitive pattern defect exist within the group. 
   It is an advantage of the method for detecting defects of the present invention that repetitive pattern defects are detected. 
   It is another advantage of method for detecting defects of the present invention that the defects are detected at the wafer fabrication level of the devices. 
   It is a further advantage of the method for detecting defects of the present invention that device fabrication costs and time are reduced because defects are detected at the wafer fabrication level. 
   It is yet another advantage of the defect detection method of the present invention that defects in tape recording heads are more accurately determined. 
   It is yet a further advantage of the method for detecting defects of the present invention that defects in individual sensors of tape recording heads can be identified, such that the recording head having such defective sensors can be identified earlier in the fabrication process. 
   These and other features and advantages of the present invention will no doubt become apparent to those skilled in the art upon reading the following detailed description which makes reference to the several figures of the drawing. 

   
     IN THE DRAWINGS 
     The following drawings are not made to scale as an actual device, and are provided for illustration of the invention described herein. 
       FIG. 1  is a diagram depicting the use of a fabrication mask upon the surface of a wafer substrate during device fabrication; 
       FIG. 2  is an enlarged view of a wafer surface having a plurality of devices fabricated thereon to the successive repetitive use of a fabrication mask; 
       FIG. 3  is an enlarged view of a plurality of tape recording head devices, each including a plurality of sensors; 
       FIG. 4  is a graph depicting the electrical resistance values of sensors of a recording head such as are depicted in  FIG. 3 ; 
       FIG. 5  is a representation of the tape recording head depicted in  FIG. 3 , where the electrical resistance value for the sensors are set forth. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Recording heads for tape drives and hard disk drives, as well as many other types of devices, are fabricated in large numbers upon the surface of waver substrates, and  FIG. 1  is a generalized depiction of a fabrication step during the manufacturing of such devices. As depicted in  FIG. 1 , a plurality of devices are simultaneously fabricated in rows and columns upon the surface  14  of a wafer substrate  10 . An exemplary mask  16 , as is utilized in the many photolithographic process steps to define the various elements of the devices, is shown above the wafer substrate  10 . The exemplary mask has rows  1 ,  2 ,  3 ,  4  and columns  1 ,  2 ,  3 ,  4 ,  5 ,  6 . In such photolithographic process steps, the mask  16  is used successively at different locations across the wafer surface to fabricate the elements of the devices. As is well known to those skilled in the art, many different such masks (not depicted) are utilized in the many process steps that are conducted to create the many devices upon the wafer surface. 
   As can be seen in  FIG. 1 , the mask includes a plurality of discrete mask elements  20  for the plurality of devices that are arranged in rows and columns, and it is intended that each element throughout each row and throughout each column be identical. However, flaws will often occur in such masks at one or more of the mask element locations. As a result, such flaws get repetitively created in the devices as the mask  16  is repetitively utilized across the surface of the wafer. Therefore, for instance, if a flaw were to exist in the mask element  24  located at row  1 , column  2 , each time the mask is repetitively utilized across the wafer surface, the device on the substrate surface that is then located at row  1 , column  2 , will be created with the flawed mask element. 
     FIG. 2  is a closer view of a wafer surface showing the repetitive mask application on the wafer surface. While the devices upon a wafer surface will be arranged in many rows and many columns,  FIG. 2  depicts a portion of a wafer surface including a plurality of rows and columns. For purposes of explanation, the devices depicted on the wafer surface of  FIG. 2 , were created in part, utilizing a mask having four rows and six columns (such as is depicted in  FIG. 1 ), and the outline borders of the mask are depicted in the darker black rectangles. It can be seen that the mask has been repetitively applied upon the wafer surface. 
   With regard to  FIG. 2  and the application of the mask to the substrate devices, a first row of the mask is utilized to create devices in the six columns designated as  10 ,  11 ,  12 ,  13 ,  14  and  15 . A second row of the mask is used to create devices  20 ,  21 ,  22 ,  23 ,  24  and  25 ; a third row of the mask is used to create devices  30 ,  31 ,  32 ,  33 ,  34  and  35 , and the fourth row is used to create devices  40 ,  41 ,  42 ,  43 ,  44  and  45 . The fifth, sixth, seventh and eighth rows involve devices that were created by a repositioning of the mask that was utilized to create the first four rows. Therefore the devices in row five are designated as  10   a ,  11   a ,  12   a ,  13   a ,  14   a  and  15   a ; the devices in row six are designated as  20   a ,  21   a ,  22   a ,  23   a ,  24   a  and  25   a ; and the devices in row seven are designated as  30   a ,  31   a ,  32   a ,  33   a ,  34   a  and  35   a  and the devices in row eight are designated as  40   a ,  41   a ,  42   a ,  43   a ,  44   a  and  45   a . Similarly with regard to rows nine, ten, eleven and twelve, which involve successive repositioning of the mask, the devices in row nine are designated as  10   b ,  11   b ,  12   b ,  13   b ,  14   b  and  15   b ; the devices in row ten are designated as  20   b ,  21   b ,  22   b ,  23   b ,  24   b  and  25   b ; and the devices in row eleven are designated as  30   b ,  31   b ,  32   b ,  33   b ,  34   b  and  35   b , and the devices in row twelve are designated as  40   b ,  41   b ,  42   b ,  43   b ,  44   b  and  45   b.    
   As is further depicted in  FIG. 2 , the mask is successively repositioned to the right side of the devices described immediately above such that a row to the right of the first row of the mask, as described above, includes devices designated as  10   c ,  11   c ,  12   c ,  13   c , etc., and a row therebelow includes devices designated as  20   c ,  21   c ,  22   c , etc., and the devices in the next row are designated as  30   c ,  31   c ,  32   c , etc. and the devices in the next row include  40   c ,  41   c ,  42   c , etc. Similarly, with regard to rows therebelow, they include devices designated as  10   d ,  11   d ,  12   d ,  13   d , etc., and the devices in the row therebelow are designated as  20   d ,  21   d ,  22   d , etc. It is therefore to be understood that the mask  16  is repositioned successively in the creation of successive sets of devices. 
   It can now be understood that where a particular mask element contains a flaw, the devices that are created utilizing the mask element will be likewise flawed. For instance, if the mask element at the position of row  1 , column  2  contains a flaw, then the devices  11 ,  11   a ,  11   b ,  11   c ,  11   d , etc., will each be flawed due to the repetitive use of the mask which contains the flaw in that position. However, devices such as  10 ,  10   a ,  10   b ,  10   c ,  10   d , etc.,  12 ,  12   a ,  12   b , etc. will not flawed as the mask element for these devices was not flawed. Another fabrication device that can cause a repetitive series of defects is a stepper lens that has an aberration, and the present invention can be applied in the detection of defects from such a defective device. 
   Recording heads that are fabricated utilizing the photolithographic process steps, and the like, are generally tested for flaws by an electrical resistance test. In this test, the electrical resistance of each device is determined, and devices having a significant deviation in resistance are identified as being defective. 
   A particular problem exists in performing such resistance tests for tape drive heads and similar devices due to the nature of such heads. Particularly, a tape drive head is generally fabricated with a plurality of sensor devices arrayed side by side, and  FIG. 3  depicts several tape heads as fabricated upon a wafer substrate. As depicted in  FIG. 3 , an exemplary tape head  100  includes a plurality of sensor devices  101 – 107 , where a typical tape head may have perhaps 18 such sensors. Tape head  200  includes sensors  201 – 207 , tape head  300  includes sensors  301 – 307 , and tape head  400  includes sensors  401 – 408 . Tape head  500  is a successive mask application of tape head  100  and includes sensors  101   a – 107   a . This repetitive pattern for the sensors is carried out across the surface of the wafer. Each sensor is electrically connected to two contact pads by two electrical leads. Significantly, due to the geometry of the size and spacing of the contact pads, and the size and spacing of the sensors, the length of the electrical leads for each sensor device differs. For instance, the electrical leads  110  and  112  for sensor  101  are of different length than the electrical leads  116  and  120  for sensor  106 . 
   In the prior art tape head fabrication and testing process, following the completion of the wafer level fabrication of the heads, the wafer is sliced into rows for further fabrication steps. Ultimately, following row level fabrication, quality testing is performed by a resistance check in which two test probes are connected to the contact pads of each sensor, such as  122  and  124  for device  101 , or  126  and  128  for sensor  106 , and the resistance of each of the sensor devices is obtained. Where the test resistance is within a pre-selected range, the sensor is determined to be acceptable, and sensor with a resistance beyond the range are determined to be defective. Where one sensor is determined to be bad, the entire tape head device  100  is bad. A problem with this prior art resistance testing, to which the present invention is directed, is that two sensors, such as sensors  101  and  10   c , may be substantially identical and not defective. Yet each will have a different electrical resistance value simply due to the fact that the electrical leads to contact pads  122  and  124 , and  126  and  128 , respectively, are of different lengths for the two devices, as has been described above. Generally due to the fact that each sensor, such as  101 ,  104  and  106 , have different electrical lead lengths, the test resistance of each device will differ, and  FIG. 4  is a graphical depiction of the test resistances of a plurality of sensors of a tape head, where the resistance of sensor  104  (and likewise sensor  104   a , etc.) is seen to be high, indicating a defect may exist in it. Owing to the variation in resistances, and the need to have a suitable resistance test range, approximately 5 to 7% of devices have previously been rejected where further testing revealed that many of them were not defective. They simply had electrical resistances that were beyond the test range due to variations in electrical lead lengths. 
   The present invention provides a test method that is more accurate.  FIG. 5  is a representation of the wafer surface of  FIG. 3  in which the test resistance of each sensor has been taken at the wafer fabrication level, and is written into the sensor location. Significantly, the resistance testing is performed on the wafer fabrication level, and the resistance of sensors in both columns and rows is available for analysis. A wafer may have thousands of tape head type devices, and it is desirable that a test method be developed that rapidly determines good devices as well as points towards bad ones. The present invention includes a method that allows for the rapid analysis of resistance test data. 
   Initially, it is significant to note that the resistance of each sensor in a tape head row will be expected to differ due in part to the differing electrical lead lengths of the sensors; while the resistance of sensors in the same column of the tape head devices can be expected to be quite similar as the lead lengths are substantially identical for sensors in the same column. 
   An initial step in the test method of the present invention is to determine the resistance of each sensor, as is shown in  FIG. 5 . Then, the resistance of each sensor in a column is added together to create a total resistance for the column. The total resistance value for the column is then divided by the number of sensors in the column to create a mean resistance for the sensors in the column. This is done for each column of sensors. Next, the deviation of each individual sensor&#39;s resistance in the column from the column mean is determined. This deviation is then divided by the mean and multiplied by 100 to create a percent deviation, which will most typically be a number greater than one. This percent deviation calculation is done for each sensor in each column. Thereafter, all of the percent deviation values for all of the sensors in a column are multiplied together. This yields a large multiplied percent deviation number that is representative of the variation of the resistance of all of the sensors in the column from the mean resistance of the sensors in the column. 
   In a column where all of the sensor devices are good, the deviation of the resistances of each of the good sensors from the mean will generally be small, and the multiplied percent deviation number will be of a relatively small value. In contrast thereto, in a column where a plurality of the sensors is defective, such as sensors  104 ,  104   a  (of tape head  500 ),  104   b , etc., due to a mask defect and the repetitive usage of the mask, as described above, the multiplied percent deviation value for such a column having a plurality of defective devices will tend to be significantly larger than the column where there are good sensors. 
   Through the application of this resistance determination method for deriving a multiplied percent deviation number a value, termed herein a figure of merit, is determined. This figure of merit depends at least in part upon the number of devices within a column and therefore cannot be arbitrarily set. It is determined through conducting this resistance determination and mathematical analysis upon one or more wafers, and is then applied to other similarly configured wafers. Where the multiplied percent deviation number is greater than the figure of merit, it is an indication that tape heads on the wafer having a sensor within the suspect column must be further examined. In the example described herein, the further examination would reveal that tape heads including sensors  104 ,  104   a ,  104   b ,  104   c , etc. are problematical, while tape heads including sensors  204 ,  204   a  (of tape head  200 ),  204   b , etc. and  304 ,  304   a  (of tape head  300 ),  304   b , etc. and  404 ,  404   a  (of tape head  400 ),  404   b , etc. are not defective. 
   As will be clear to those skilled in the art after reading the preceding specification, the test method of the present invention is suitable for the detection of defects in devices such as tape heads, where the defects are due to the repetitive nature of the fabrication process. That is, while the present invention has been particularly described with regard to a defect in a mask that is utilized during the device fabrication process, other defects that are repetitive in nature, such as are created by a stepper lens with an aberration, are subject to the same test procedure of the present invention. 
   Additionally, while the test method has been particularly described with reference to the electrical resistance of each device, other repetitive device parameters may also be utilized, and the mean value and percent deviation values of the parameter may be utilized in the test method. Such other parameters include the line width of the devices and the plating thickness of the devices, which may be measured for each device, and the measured values utilized in the same manner as the electrical resistance measured values are utilized in the test method described herein. 
   A significant feature of the present invention is that it detects defects at an earlier stage in the fabrication process then has heretofore been accomplished in the prior art. Specifically, the present invention involves defect detection at the wafer level of fabrication, where the prior art test procedure involves defect detection at the row level of device fabrication. As a result of earlier defect detection, expensive fabrication steps and fabrication time are avoided for those devices that are identified on the wafer level as being defective. 
   While the present invention has been shown and described with regard to certain preferred embodiments, it is to be understood that modifications in form and detail will no doubt be developed by those skilled in the art upon reviewing this disclosure. It is therefore intended that the following claims cover all such alterations and modifications that nevertheless include the true spirit and scope of the inventive features of the present invention.