Abstract:
An off-line partial wafer scanner system is disclosed that resolves partial wafermap related issues that result holt-lot in semiconductor assembly. The system eliminates the need to teach locator die at die attach (die bonder) machine. The reaching of partial map locator die is done in die interface areas instead of at the die bonder machine. This resolves the die quantity discrepancies issue, die bonder operator error and missing locator die information due to error made at the die bonder. An image-processing algorithm is employed to achieve reliable off-line locator die teaching system. In partial wafermap processing, the locator die is recognized by utilizing a vision system. A production operator records the wafer identification, and quarter or halves the wafer using a saw machine. The production operator scans the first quarter Q 1  or first half H 1  with a scanner and determines the locator die location as well as exact quantity of dies. The production operator uploads to a wafermap server the locator die location as well as exact quantity of dies. When dies are mounted from a partial wafer the production operator downloads from the wafermap server the locator die coordinates to a die mounting machine and the dies from any of the partial wafers are mounted using the downloaded locator die coordinates.

Description:
FIELD OF INVENTION 
   This invention relates to partial-wafer wafermap process utilizing a vision system and more particularly to automatic recognition of locator die in a partial wafer. 
   BACKGROUND OF INVENTION 
   Partial wafermap process is a process where a single silicon wafer is cut into two or more pieces for the reason of smaller lot quantity. The wafers are usually halved or quartered. Before the implementation of inkless wafer processing, an ink dot placed on a die by a prober in a wafer fab identifies a bad die. No ink dot on the die represents a good die. When the inked wafer is sent over to an assembly and test site, the wafer could be halved or quartered to accommodate smaller lot quantity. This is when the term “partial lot” is used. Any part of the halved or quartered wafer (normally termed as H 1 , H 2  for halved wafer, and Q 1 , Q 2 , Q 3 , Q 4  for quartered wafer) could be processed on any die mounting machine without requiring any relationship between the partial wafers. 
   A wafermap (or inkless system) eliminates the inking process at the prober in the wafer-fab. Replacing the ink for marking the bad dies from the good ones, a map file is created for each of the probed wafer that indicate the location of good and bad dies based on known position termed as reference die. When one particular wafer is halved or quartered, one half or one quarter of the wafer has a reference die. There is a need for a new reference die for each of the partial wafers. 
   Prior art methods are described in U.S. Pat. No. 6,216,055 of Balamurugan et al. Entitled “Partial Semiconductor wafer processing” and U.S. Pat. No. 6,174,788 of Balamurugan entitled “Partial Semiconductor Wafer Processing With Multiple Cuts of Random Sizes.” These patent are incorporated herein by reference. The prior art teaches a method of providing a locator die teach by the production operator on die mounting machine (pick and place equipment). 
   The current process does the following steps:
         1. Lot started   2. Send wafer to partial production operator (PO).   3. PO will record wafer identification, and quarter or half wafer using saw machine.   4. Q 1  for first quarter or H 1  for first half will be issued to the line, and Q 2 , Q 3 , Q 4  or H 2  will be kept in the die bank.   5. Die mounting machine PO will do a locator die teach on the die mounting machine.       

   In step 4 above, the die exact quantity of the partial wafer is not known. Due to this, a quantity estimate will have to be done by the partial operator. This will cause quantity discrepancies at the die bonder causing yield losses and other lot processing problems. 
   In step 5 above is the step that needs to be taken by the machine operator in order to reach the locator die position for each H 1  or Q 1  wafer so that the locator die coordinate is recorded for the subsequent processing of Q 2 , Q 3 , Q 4  or H 2  or H 2  parts. 
     FIG. 1  illustrates the flow of partial map process at the die bonder in the prior art. The steps are:
         1. Load partial wafer to die bonder.   2. Download wafermap data for whole wafer.   3. Display the whole wafer in die bonder monitor.   4. Move the wafer table to first die pickup position.   5. Move display cursor to first die pickup position.   6. Teach two limit dies in X direction.   7. Teach two limit dies in the Y direction.   8. Using limit die coordinates information remove other partial wafer dies from map.   9. Start die mount.       
   The above process also requires dependencies of the system on all die bonder machine operators. Any error made in this process will prevent further processing of H 2  or Q 2 , Q 3  or Q 4  part due to lack of locator die. 
   The current method is error prone, and is limited. It is highly desirable to provide a method where there is no need for teaching of the locator die at the die mounting machine. 
   SUMMARY OF INVENTION 
   In accordance with one embodiment of the present invention an automatic recognition of the locator die in a partial-wafer wafermap processing is achieved utilizing a vision system. 
   In accordance with an embodiment of the present invention a new method of partial-wafer wafermap processing is provided that eliminates the teaching of the locator die in the die bonder that comprises the steps scanning the wafer and determining the locator die location as well as exact quantity of dies; uploading to a wafermap server the locator die location as well as exact quantity of dies; and mounting the dies from any of the parts at a die mounting machine using the downloaded locator die coordinates. 
   In accordance with another embodiment of the present invention an improved apparatus used to scan the wafer is provided that holds the wafer against the scanner without touching the scanner glass by a vacuum inlet. 
   In accordance with another embodiment of the present invention an improved vision processing method to recognize the locator die is provided including recognizing the wafer diameter, recognizing quarter or half wafer from the scanned images, verifying reference die validity and recognizing the locator die coordinates. 

   
     DESCRIPTION OF DRAWING 
       FIG. 1  illustrates the flow of partial map process at the die bonder in the prior art. 
       FIG. 2  illustrates an inkless wafer before quartering/halving process. 
       FIG. 3  illustrates an inkless wafer after halving process. 
       FIG. 4  illustrates the coordinates of the locator die in the wafer half with the reference die and the relative location of the auxiliary reference die in the other wafer half 
       FIG. 5  illustrates an inkless wafer after a quartering process. 
       FIG. 6  illustrates the coordinates of the locator die in the wafer quarter with the reference die and the relative location of the auxiliary references die in the other wafer quarters. 
       FIG. 7  illustrates a new partial wafermap procedure that eliminates the teaching of the locator die in the die bonder according to one embodiment of the present invention. 
       FIG. 8  there is illustrated a block diagram of the system according to one embodiment of the present invention. 
       FIG. 9  illustrates the image of a quarter wafer after binary operation. 
       FIG. 10  illustrates a window set in the image of a quarter wafer. 
       FIG. 11  is a plot of linear distance vs. pixel value to show the profile of the cross section of a reference die. 
       FIG. 12  is plot of linear distance vs. pixel value to show the pixel profile of a horizontal cross section to determine the location of a locator die. 
       FIG. 13  is plot of linear distance vs. pixel value to show the pixel profile of a vertical cross section to determine the location of a locator die. 
       FIG. 14  illustrates a side view of the scanning apparatus according to one embodiment of the present invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENT 
   Referring to  FIG. 2  there is illustrated a full inkless wafer before quartering/halving process. The wafer flat is at the bottom of the drawing and the reference die  60  is on the lower right adjacent the mirror die as marked. This is the reference position in the full wafer wafermap processing. In the mounting sequence, the pick and place equipment moves from the reference die to the first pick die position at the top upper right where there is a full die. The pick up sequence goes from right to left along the top row to the last full die in the row and then down one row and then from left to right on the second row. At the end of the second row the pick up drops down one row and goes from right to left again. The process is repeated till the last good die is picked. The location of good and bad dies is based on the known reference die position. 
   When one particular wafer is halved or quartered, a new reference die needs to be identified for each of the partial wafers.  FIG. 3  illustrates an inkless wafer after halving process.  FIG. 3  there is shown the saw separation between the two haves by the space and the position of the locator die. From the co-ordinate position of the locator die the reference die location is determined for half H 1  and the auxiliary reference die position is known for half H 2 . This is illustrated in  FIG. 4  with a solid line through the wafer representing the cut. If locator die coordinate is (x 1 , y 1 ) then auxiliary reference die (x 2 ,y 2 ) is x 2 =x 1 +1 and y 2 =y 1 . 
     FIG. 5  illustrates an inkless wafer after quartering process. In  FIG. 5  there is shown the saw separation by the space between the quarters and the position of the locator die. From the co-ordinate position of the locator die the reference die location is determined. The locator die position is shown in  FIG. 5  for the quartered wafer. The locator die is in the same quarter as the reference die but near the center of the wafer at the junction of the vertical and horizontal cut as illustrated at x 1 , y 1 . From the coordinate position of the locator die location is determined the auxiliary reference die position for the other quarters Q 2 , Q 3  and Q 4 . See FIG.  6 . The auxiliary reference die for the second quarter Q 2  is at x 2 , y 2  where x 2 =x 1 +1 and y 2 =y 1 . The auxiliary reference die for the third quarter Q 3  is at x 3 , y 3  where x 3 =x 1 +1 and y 3 =y 1 +1. The auxiliary reference die for the fourth quarter Q 4  is at x 4 , y 4  where x 4 =x 1  and y 4 =y 1 +1. For additional discussion see above cited U.S. Pat. Nos. 6,216,055 and 6,174,788 incorporated herein by reference.
         The new process takes the following steps illustrated in FIG.  7 .   Step 1 the Lot is started.   Step 2 the wafer is sent to partial process production operator (PO).   Step 3 the production operator (PO) will record the wafer identification, and quarters or halves the wafer using the saw machine.   Step 4 the first quarter Q 1  or first half H 1  is scanned and locator die location as well as exact quantity of dies will be uploaded to a wafermap server (WISH)  15 .   Step 5 any of the parts Q 1 , Q 2 , Q 3 , Q 4  or H 1  or H 2  can be issued to a die mounting machine and the remaining sent to the die bank. The wafermap server (WISH) outputs to the die mounting machine the locator die coordinates. The partial wafers can be sent to different die mounting machines such as Die Mounting machine  1  or Die Mounting machine n.       
   When the quarters Q 2 , Q 3 , Q 4  or H 2  are used on line no further teaching of locator die is required at the die mounting machine as the locator die coordinate will be down loaded to the bonder machine or die mounting machine. There no longer is the need for steps 6 and 7 in  FIG. 1  to teach the two limit dies in the X and Y directions. 
     FIG. 8  illustrates a block diagram of the system according to one embodiment of the present invention. The system includes a personal computer PC  11  where the image processing software is used to recognize the locator die and a scanner  13  for acquiring the partial wafer image. A mentioned in the steps above in Step 4 the first quarter Q 1  or first half H 1  is scanned and the locator die location as well as exact quantity of dies will be uploaded to a wafermap server (WISH)  15 . The software can be broken down into the seven major parts of
         1) User interface.   2) Image processing engine.   3) Telnet client   4) FTP client   5) Flatbed scanner interface   7) Barcode label printer.       
   The user interface is designed such that it will provide easy operation and require the least number of keystrokes. Key feature of the user interface includes a zoom window for magnified view of scanner wafer, square box cursor that is automatically resized based on die geometry and a cross hair. 
   A good set of image processing library is used to perform the computationally intensive task in a timely manner. Extensive image analysis method is utilized to automatically scan the wafer image, perform basic image processing, and perform higher level image processing to recognize the locator die coordinates with respect to reference die coordinates. The system at the PC  11  includes as input the partial wafer image from the scanner and the wafermap file based on wafer identification number (to identify the reference die, die size, wafer diameter, and bin quantity). 
   The image processing method used to recognize the locator die includes the steps method of recognizing the wafer diameter, method of recognizing quarter or half wafer from the scanned images, method of verifying the reference die validity and method of recognizing the locator die coordinates. 
   The method of recognizing the wafer diameter is achieved by converting the wafer image to a binary image and by counting the white pixels.  FIG. 9  illustrates the image of a quarter wafer after binary operation. The black area represents the area beyond the wafer and dies. A window is set after finding the tangent of the window. See FIG.  10 . The white pixel count is the pixels within the window. The number of white pixels determines the wafer diameter. The white pixel count determines the diameter of the wafer by means of a lookup table and image resolution (Dot per inch) method. 
   The method of recognizing quarter or half wafer from the scanned images is uses the image aspect ratio. Typical image aspect ratio for partial wafer is 1:1 (Horizontal: vertical), and typical image aspect ratio for half wafer is 1:2. Based on this concept, any aspect ration of less than 0.7 is used to decide that the particular wafer being scanned is half wafer. 
   The selection of reference die is based on an operator judgment. However, there is a software check made to verify whether the selection is valid or otherwise. A cross section is selected across the selected reference die, and the pixel profile is analyzed. There should be no valley of the left side of the selected die.  FIG. 11  is a graph to show the pixel profile of a cross section to determine the location of a reference die. It is a plot of pixel values vs. linear distance. 
   There are two methods of recognizing the locator die coordinates with one a fast way and the second a slower but more accurate way. 
   The fast method employs pixel profiling/histogram method. For the example of a quarter die with the corner of the partial wafer at the bottom right the histogram pixel profile should not have any valley on the right side of the locator die when the selected horizontal cross section for the histogram is taken.  FIG. 12  is a graph to show the pixel profile of a horizontal cross section to determine the location of a locator die. For the vertical cross section of the locator die the locator die coordinates are checked by the absence of a valley at the bottom of the locator die.  FIG. 13  is a graph to show the pixel profile of a vertical cross section to determine the location of a locator die. The lowest part of the valley is used to determine the edges of the die. The formula to calculate the grayscale value of each pixel is described as Gray=(222*IR+707*IG+71*IB)/1000. 
   The more accurate method in determining the coordinates of the locator die the following steps are taken:
         1) Threshold process where the image threshold is automatically adjusted using the common gray threshold function.   2) Binary gradient to enhance the edges,   3) Dilated gradient to enhance the edges,   4) Image fill to further outline the die edges, the noises were further eliminated using common image processing method,   5) Image erosion techniques to further outline the edges, and   6) Vertical/horizontal edge detection using gradient north and gradient west the edges vertical and horizontal is separated for the final measurement. From this step, the vertical and horizontal edges are clearly defined and measured using the edge profile method.       

   Upon the initialization of the application, a telnet session is created to run in the background. In this method, all interaction with WISH is executed by entering a remote command from the application where the response will be captured and analyzed for further processing. The most important command used is a command to upload the locator die information as well as to update wafer status. The telnet session is created using net ActiveX control provided in Visual Basic 6.0. A background VT100 emulator performs data transaction WISH and application. Using this method, immediate response and status of every command sent to WISH can be received in real time. Other more common method of using RSH will not provide the same advantage if compared to this background telnet/VT100 approach. A typical command sent through the telnet session:
         “find/wish5.0/img/I*/* &amp; Wafer.Lot.No &amp; “*” which will list down   all available wafer in the die lot   “uws5.8-n” &amp;no_of_part &amp; “&amp;-w” &amp; wid &amp; &#39;−C”   the above command, a Unix script was written by IT support to list   wafer id and wafer number according to the wafer lot number.       

   FTP client is used to download wafermap file from WIH during the processing of the wafer image. Information such as reference die coordinate, die size, reference die type, die bin quantity and other information is extracted from the downloaded file. This session used FTP ActiveX control. 
   An embedded web browser is included in the application to simplify the access to WEB WAMP. 
   For each scanning job, a label containing quantity of each part as well as other information such as wafer number and lot number is automatically printed. A canned command is sent to a printer through a printer port. A method of direct access is employed to query the status of the printer. This method uses IO ActiveX control. 
   A custom made Off-line Partial Wafer Scanner system described below is used according to one embodiment of the present invention. The partial wafer (Q 1  or H 1 ) will be placed in a custom-made wafer scanner jig illustrated in FIG.  14 .  FIG. 14  is a side view of an apparatus  20  with the flat-bed scanner  21  according to one embodiment of the present invention. The apparatus comprises a generic flat bed scanner  21 , a scanner holder  23  that tilts the flat bed scanner  21  at an angle, and a custom designed flap  25  with a handle  27  and hinged with hinge  29  to a resting base  23   a  of the scanner holder  23 . The scanner holder  23  in the side elevation view is in the shape of a right angle triangle with a scanner resting base support  23   a  extending perpendicular to the slanted surface for receiving. The hinge  29  is connected to the extend end of the support  23   a . The scanner is positioned this way for accessibility, a small footprint, and to adapt to generic design of most flatbed scanners which use a guide rail on one side, but a roller on the other side for its scanning head. In this apparatus, the scanner is tilted at 50 to 60 degrees. Placing the scanner at a steeper degree can prevent from operating properly. Placing the scanner at an angle of 0 to 50 degrees increases the risk of the wafer falling onto the glass surface causing wafer breakage or damage. The custom designed flap  25  with a handle  27  and hinged at the base replaces the original flap of the scanner. A flap stopper  3  extends from the scanner to keep the flapper spaced from the scanner. A vacuum inlet  33  in the flap  25  is attached to a vacuum source to hold the wafer&#39;s flip side while the active surface if the wafer is facing toward the scanner glass surface  21   a . A sufficient amount of sealing about the flap  25  is provided by the flap  25  or by the flap stopper  31  or both to achieve a vacuum between the scanner and the flap  25 . The angle at which the flap is positioned against the scanner glass surface is critical as it can determine the image quality of the wafer being scanned due to reflectivity problem. This is achieved by using adjustable stopper at the top of the scanner holder. Due to the fact that the wafer is being held in place by the vacuum, the active surface of the wafer is prevented from touching the glass surface of the scanner. This prevents any circuit metalization damage on the wafer active surface. The wafer will be held back against a plate by vacuum that exists in the plate. 
   Other partial wafers from other types of cuts likewise may be used having the locator die position as discussed in the referenced patents. 
   The benefits of the new process are:
         1) Eliminate manual teaching of locator die, which is human dependent and prone to error.   2) Eliminate the need to issue H 1  or Q 1  part before any other part.   3) Eliminate Q 2 , Q 3 , Q 4  or H 2  hold lot due to missing locator die reference caused by missing H 1 , Q 1  or improper teaching or locator die.   4) Exact part quantity can be determined at lot start eliminating wrong lot quantity.       

   Other partial wafers from other types of cuts likewise may be used having the locator die position as discussed in the referenced patents. 
   Although the present invention and its advantages have been described, it should be understood that various changes, substitutions and alterations could be made without departing from the spirit and scope of the invention as defined by the appended claims.