Abstract:
A test method tests a memory device including a memory array having a plurality of symmetric memory cells, a plurality of word lines and a plurality of bit lines. In testing a first word line, a first bit line is charged to test a single bit of a first half of an adjacent first symmetric memory cell; and a second bit line is charged to test a single bit of a second half of an adjacent second symmetric memory cell. In testing a second word line, the first bit line is charged to test a single bit of the second half of an adjacent third symmetric memory cell; and the second bit line is charged to test a single bit of the first half of an adjacent fourth symmetric memory cell. In testing each of the word lines, each of the bit lines is charged once.

Description:
TECHNICAL FIELD 
     The disclosure relates in general to a test method for a memory, and more particularly to a method for testing a memory by half page read. 
     BACKGROUND 
     Flash memory plays an important role in an electronic device. For example, a memory card having the flash memory may be used to increase the storage capacity of a mobile device. After the memory chips are manufactured, the memory chips are tested. Therefore, how to fast test the memory is one of the targets. 
     SUMMARY 
     The disclosure is directed to a method for testing a memory which reduces test time by half page read. In half page read, a single half of each of the memory cells is read and tested. 
     According to one embodiment, a test method for testing a memory device including a memory array is provided. The memory array includes a plurality of symmetric memory cells, a plurality of word lines and a plurality of bit lines. In testing a first word line of the word lines, a first bit line of the bit lines is charged to test a single bit of a first half of a first symmetric memory cell adjacent to the first bit line; and a second bit line of the bit lines is charged to test a single bit of a second half of a second symmetric memory cell adjacent to the second bit line. In testing a second word line of the word lines, the first bit line of the bit lines is charged to test a single bit of the second half of a third symmetric memory cell adjacent to the first bit line; and the second bit line of the bit lines is charged to test a single bit of the first half of a fourth symmetric memory cell adjacent to the second bit line. In testing each of the word lines, each of the bit lines is charged once. 
     According to another embodiment, a test method for testing a memory device including a memory array is provided. The memory array includes a plurality of symmetric memory cells, a plurality of word lines and a plurality of bit lines. A half page read is performed on the memory array, wherein there are a first number of at least one defective lines of the memory array found during the half page read, and in the half page read, either one of a first half and a second half of each of the symmetric memory cells is read. The at least one defective line found during the half page read is repaired. A whole page read is performed on the repaired memory array and a defective status is recorded, wherein there are a second number of the at least one defective lines of the memory array found during the whole page read, and in the whole page read, both the first half and the second half of each of the symmetric memory cells are read. Whether the memory device passes test is determined based on the defective status and a relationship between the first and the second number. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a function block for a memory device. 
         FIG. 2  shows a memory array. 
         FIGS. 3A-3B  show test according to an embodiment of the application. 
         FIG. 4  shows a test flow according to another embodiment of the application, which is performed before mass product. 
         FIG. 5  shows a test flow according to still another embodiment of the application, which is performed after mass product. 
     
    
    
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     DETAILED DESCRIPTION 
     Technical terms of the disclosure are based on general definition in the technical field of the disclosure. If the disclosure describes or explains one or some terms, definition of the terms is based on the description or explanation of the disclosure. 
     Each of the disclosed embodiments has one or more technical features. In possible implementation, one skilled person in the art would selectively implement part or all technical features of any embodiment of the disclosure or selectively combine part or all technical features of the embodiments of the disclosure. 
       FIG. 1  shows a function block for a memory device. As shown in  FIG. 1 , the memory device  100  at least includes: a memory array  110 , a first redundancy circuit  120 , a second redundancy circuit  130  and an error correction circuit (ECC)  140 . 
     The memory array  110  includes a plurality of memory cells, a plurality of word lines and a plurality of bit lines. The memory cells are arranged in an array. Each memory cell is a symmetric memory cell. The symmetric memory cell includes a left half and a right half; and each of the left half and the right half stores at least one bit. The bit number of the left half is the same as that of the right half. 
     In the memory array  110 , the word lines are, for example, along x direction (i.e. horizontal direction) and the bit lines are, for example, along y direction (i.e. vertical direction). 
     The first redundancy circuit  120  is, for example, an x-direction redundancy circuit. After test, if the number of the defective memory cells on the word line reaches a first threshold, then the first redundancy circuit  120  may be used to replace (repair) the memory cells on the word line. 
     The second redundancy circuit  130  is, for example, a y-direction redundancy circuit. After test, if the number of the defective memory cells on the bit line reaches a second threshold, then the second redundancy circuit  130  may be used to replace (repair) the memory cells on the bit line. 
     After test, if the number of the defective memory cells on the word line or the bit line is smaller than the first/second threshold, then the ECC  140  may be used to replace (repair) the defective memory cells. 
       FIG. 2  shows the memory array  110 . As shown in  FIG. 2 , in the memory array  110 , the word line WL 0  is coupled to the memory cells  210 _ 0 _ 0 ,  210 _ 0 _ 1 ,  210 _ 0 _ 2 , . . . ,  210 _ 0 _N,  210 _ 0 _(N+1),  210 _ 0 _(N+2), . . . . N is a positive integer. Similarly, the word line WL 1  is coupled to the memory cells  210 _ 1 _ 0 ,  210 _ 1 _ 1 ,  210 _ 1 _ 2  . . .  210 _ 1 _N,  210 _ 1 _(N+1),  210 _ 1 _(N+2) and so on. For simplicity,  FIG. 2  shows two word lines WL 0  and WL 1  but the application is not limited by this. 
     The memory cell  210 _ 0 _ 0  includes a gate coupled to the word line WL 0 , a source and a drain. One of the source and the drain of the memory cell  210 _ 0 _ 0  is coupled to the bit line BL 0 , and the other of the source and the drain of the memory cell  210 _ 0 _ 0  is coupled to the bit line BL 1 . The coupling of other memory cells is similar. 
     The memory cells coupled to the same word line may be grouped in a plurality of pages. Furthermore, the left half bits and the right half bits of the same memory cell may be of different pages. As shown in  FIG. 2 , the left half bit of the memory cell  210 _ 0 _ 0  is of page  0  but the right half bit of the memory cell  210 _ 0 _ 0  is of page  32 . In  FIG. 2 , the number shown in the half bit of the memory cell refers to the page number of the half bit of the memory cell. 
     How to perform memory test according to the embodiment of the application is described as follows. For simplicity, the word lines WL 0 , WL 1  . . . are sequentially tested. The application is not limited thereby. 
     In the embodiment, in half page read, a single half (either one of the left half and the right half) of each memory cell is read. That is to say, not both the left half and the right half of the memory cell are read and tested. Further, in testing the same word line, each bit line is charged/sensed once. Further, in testing, the left half bits of about 50% of the memory cells on the same bit line are read and tested, and the right half bits of about the other 50% of the memory cells on the same bit line are read. Similarly, in testing, the left half bits of about 50% of the memory cells on the same word line are read, and the right half bits of about the other 50% of the memory cells on the same word line are read. 
       FIGS. 3A-3B  show test according to an embodiment of the application. For simplicity, in testing, the word line which is applied by a test voltage is marked with “+V”; and on the contrary, the word line which is applied by a ground voltage is marked with “GND”. Further, in testing the word line, the memory cells of the same page are concurrently read and tested. 
     As shown in  FIG. 3A , in testing page  0  of the word line WL 0 , the bit lines BL 1  and BLN are concurrently charged to test the left half bit of the memory cell  210 _ 0 _ 0  and the right half bit of the memory cell  210 _ 0 _N, respectively. In  FIGS. 3A and 3B , the dotted arrow refers to that, the left/right half bit of the memory cell is tested by the charged bit line. 
     Similarly, in testing page  48  of the word line WL 0 , the bit lines BL 2  and BL(N+1) are concurrently charged to test the left half bit of the memory cell  210 _ 0 _ 1  and the right half bit of the memory cell  210 _ 0 _(N+1), respectively. In testing page  8  of the word line WL 0 , the bit lines BL 3  and BL(N+2) are concurrently charged to test the left half bit of the memory cell  210 _ 0 _ 2  and the right half bit of the memory cell  210 _ 0 _(N+2), respectively. 
     Further, in testing the word line, the bits of the same page on the same word line are concurrently test. After the whole page on the same word line is tested, the next page on the same word line is tested. For example, as shown in  FIG. 3A , in testing the word line WL 0 , the test sequence may be page  0 , page  2  (not shown) . . . and so on. 
     That is, as shown in  FIG. 3A , the bit lines BL 1 , BLN, . . . and so on are concurrently charged to test bits of the page  0  of the word line WL 0 . In testing page  8  of the word line WL 0 , the bit lines BL 3 , BL(N+2), . . . and so on are concurrently charged to test bits of the page  8  of the word line WL 0 . In testing page  48  of the word line WL 0 , the bit lines BL 2 , BL(N+1), . . . and so on are concurrently charged to test bits of the page  48  of the word line WL 0 . 
     Similarly, in  FIG. 3B , for testing the page  32  of the word line WL 1 , the bit lines BL 0  and BL(N+1) are concurrently charged to test the right half bit of the memory cell  210 _ 1 _ 0  and the left half bit of the memory cell  210 _ 1 _N, respectively. For testing the page  16  of the word line WL 1 , the bit lines BL 1  and BL(N+2) are concurrently charged to test the right half bit of the memory cell  210 _ 1 _ 1  and the left half bit of the memory cell  210 _ 1 _(N+1), respectively. For testing the page  40  of the word line WL 1 , the bit lines BL 2  and BL(N+3) are concurrently charged to test the right half bit of the memory cell  210 _ 1 _ 2  and the left half bit of the memory cell  210 _ 1 _(N+2), respectively. 
     Further, in testing the same page on the same word line, the left half bits of 50% of the memory cells of the same page on the same word line are concurrently read and tested, and the right half bits of the other 50% of the memory cells of the same page on the same word line are concurrently read and tested. 
     In the embodiment of the application, in order to reduce the test time, in testing the same word line, each of the bit lines is charged/sensed once. Thus, in testing the same word line, not every page is read and tested. Of course, during the test of the whole memory array  110 , all pages are read and tested. For example, in testing the word line WL 0 , the page  0  is read and tested, but the page  32  is neither read nor tested. Similarly, in testing the word line WL 1 , the page  32  is read and tested, but the page  0  is neither read nor tested. 
     Besides, in the embodiment, in testing the same word line, about 50% of the bit lines (or said, the first bit line group) are concurrently charged to read and test the left half bits of the memory cells on the left side of the first bit line group; and about the other 50% of the bit lines (or said, the second bit line group) are concurrently charged to read and test the right half bits of the memory cells on the right side of the second bit line group. In testing the next word line, the bit lines of the first bit line group are concurrently charged to read and test the right half bits of the memory cells on the right side of the first bit line group; and the bit lines of the second bit line group are concurrently charged to read and test the left half bits of the memory cells on the left side of the second bit line group. This is referred as “reverse read”. 
     In the embodiment of the application, “half page read” is defined as that, if the bit on a single half of each memory cell is read and tested, then the bit on the other half of each memory cell is neither read nor tested. 
     In the application, “whole page read” is defined as that, the bits on both the left and the right halves of each memory cell are read and tested. 
     In prior test, the bits on both the left half and the right half of each memory cell are read and tested, and thus, in testing the same word line, each of the bit lines are charged twice. This results in a long test time in prior test. In the embodiment of the application, the bit on a single half of each memory cell is read and tested; and in testing the same word line, each bit line is charged once. Thus, the test time in the embodiment of the application may be reduced to 50%, compared with the prior test time. 
     In order to have an uniform test result, in the embodiment of the application, as for the same bit line, in testing the word line, the bit line may test bit of the left half of the memory cell on the left side of the bit line; but in testing the next word line, the bit line may test bit of the right half of the memory cell on the right side of the bit line. This test may have uniform test on a plurality of memory cells for assuring test quality and reliability. 
       FIG. 4  shows a test flow according to another embodiment of the application, which is performed before mass product. In step  410 , the half page read is performed on the memory array  110  to find all defective lines. For example, if the number of the defective memory cells on the word line WL 0  reaches the first threshold, then the word line WL 0  is checked as a defective line. Step  410  is for finding all defective lines on the word lines and on the bit lines. After the defective lines are found, the defective lines are repaired. For example, the first redundancy circuit  120  is used to repair/replace the defective word line (i.e. the whole defective word line is replaced by the redundancy word line of the first redundancy circuit  120 ); and the second redundancy circuit  130  is used to repair/replace the defective bit line (i.e. the whole defective bit line is replaced by the redundancy bit line of the second redundancy circuit  130 ). The number of the defective lines found in the step  410  is recorded as R1. 
     In step  415 , the whole page read is performed on the repaired memory array to obtain ECC status. The ECC status refers to the ECC bit number which is used in repairing the memory array and the ECC array. The ECC status is output from the ECC  140 . The ECC array is in the ECC  140  and the memory cells in the ECC array may be defective. That is, the ECC status may refer as the defective status of the memory array and the ECC array of the ECC  140 . 
     As described above, if the number of the defective memory cells on the word line/bit line reaches the first/second threshold, then the word line/bit line is replaced by the first/second redundancy circuit  120 / 130 . Alternatively, if the number of the defective memory cells on the word line/bit line is under the first/second threshold, then defective memory cells on the word line/bit line are repaired by the ECC  140 . 
     In step  420 , the whole page read is performed on the memory array  110  to find the number (R2) of the defective lines. 
     In step  425 , whether the ECC status is smaller than or equal to 1 bit is determined. If the ECC status is smaller than or equal to 1 bit, then the defective memory cells in the memory array is few. Thus, the memory device may pass the test. Besides, in the embodiment, the memory device which passes the test is further analyzed. 
     In step  430 , whether R2=R1 is determined. If R2=R1, then the number of the defective lines found by the half page read is equal to the number of the defective lines found by the whole page read. That is, the defective status of the memory cells of the memory array  110  is not serious and thus in the whole page read, no new defective line is found. Thus, the memory device is determined as “test pass” (step  435 ). 
     On the contrary, if R2 is not equal to R1 in the step  430 , then it means that new defective line(s) is/are found in the whole page read. However, the memory device may be repaired by the ECC because there are few defective memory cells in the memory array. Thus, the memory device is determined as “test pass” (step  440 ). 
     If no in step  425 , then it means that the defective status of the memory device is more serious (because the ECC status is higher than 2 bits). The memory device is determined as “test failure” in the embodiment of the application. Besides, in the embodiment, the memory device which is failed in the test is further analyzed. 
     In step  445 , whether R2=R1 is determined. If R2=R1, it means that no new defective line is found in the whole page read. However, the embodiment determines that the ECC of the memory device has serious defects which result the ECC status higher than 2 bits. Thus, the memory device is determined as “test failure” (step  450 ). 
     If R2 is not equal to R1 in step  445 , then it means the memory array of the memory device has serious defects which result finding of new line(s) in the whole page read. Thus, the memory device is determined as “test failure” (step  455 ). 
       FIG. 5  shows a test flow according to still another embodiment of the application, which is performed after mass product. In step  510 , all bits of each of the memory cells are set as bit “1”. The half page read is performed on the memory array to find and repair all defective lines of the memory array. 
     In step  520 , the repaired defective line(s) is/are read again to determine whether the repair is successful. 
     In step  530 , all word lines of the memory array are grouped and tested, and the ECC status of each word line group is checked. For example, 32 word lines are grouped as a word line group, and the whole page read is performed on each of the word line groups to check the ECC status of each word line group. If the whole page read of the current word line group indicates that the ECC status is smaller than or equal to 3 bits, then the whole page read is performed on the next word line group. On the contrary, if the whole page read of the current word line group indicates that the ECC status is higher than 3 bits, then the memory device is determined as “test failure”. If any word line group is failed in the test, the memory device is determined as “test failure”. The step  530  is repeated until all word line groups of the memory array pass the test, and thus the memory device is determined as “test pass”. 
     In step  540 , the test result is output. 
     Besides, in test, the above embodiments of the application may be combined. For example, in performing the test flow of  FIG. 4  or  FIG. 5 , the half page read of  FIG. 4  or  FIG. 5  may be implemented by the half page read of  FIG. 2 . 
     Further, in testing, the test flow of  FIG. 4  may be performed first, and then the test flow of  FIG. 5  is performed on the memory device which is passed the test flow of  FIG. 4 . 
     As described above, the test flow of  FIG. 4  or  FIG. 5  applies the test method in  FIGS. 3A and 3B , and thus the test time is shortened. Besides, the embodiment uses ECC to assure correctness of the test. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.