Patent Document

BACKGROUND 
     1. Technical Field 
     This disclosure relates to semiconductor memories and more particularly, to an apparatus for testing memory devices using an on chip data pattern generator. 
     2. Description of the Related Art 
     The rapid growth in circuit complexity has increased the difficulty and cost of testing memories. Development of high density memories introduces a new dimension in testing complexity. For example, higher speed synchronous DRAMs need includes more complex and more time consuming pattern testing. Using test systems for memory testing may require additional equipment to maintain current levels of throughput. It is typically expensive to add additional testers at to maintain the throughput needed for more complex high-speed memory devices. 
     Another issue concerning the testing of both the current and future generations of high density memories involves chip frequencies relative to the speed and accuracy of the testers. It is becoming more difficult to find high-speed test systems that can keep up with the chips being tested. Typically, device frequency has been growing faster than the accuracy of testers. At the same time, the test equipment is getting more complex. The pin counts are getting higher and therefore the accuracy needs to be managed over more pins. Further, maintaining costs at a reasonable level and performing the tests in a reasonable time frame are also an issue for manufactures and testers. 
     In semiconductor memory testing, a chip is tested by writing a known data pattern to memory cells in the array by an external testing device. The data pattern is then read back to the device and compared to the known data pattern. Data patterns may include a physical pattern, a logical pattern and/or a checker pattern, for example. Referring to FIGS. 1A-1C, for semiconductor memory devices, such as dynamic random access memories (DRAMs), bitlines BL and complementary bitlines {overscore (BL)} (hereinafter BL bar) paired and coupled to a sense amplifier SA. To activate (read from or write to) a memory cell (denoted by circles), a sense amplifier SA and a wordline WL need to be selected. BL and BL bar each have memory cells associated therewith. For example, in FIG. 1A, a physical “1” data pattern is stored as a 1 on memory cells associated with BLs and as a zero on memory cells associated with BL bars. This means all memory cells have charged capacitors. For FIG. 1B, the data pattern is that for a logical “1”. In this case, all is are stored in the array which means that half of the memory cells have charged capacitors and half do not. In FIG. 1C, a checkerboard pattern is implemented having alternating is and Os and alternating charged and discharged memory cell capacitors. 
     As illustrated in FIGS. 1A-1C, physical data corresponds to the content or meaning or the storage capacitor. In the case of a physical 1, the capacitor is charged and for a physical 0, the capacitor is discharged. For logical data, only the value at an input/output pin (DQ) is important. The term logical 1 (0) means if the memory cell is connected to BL or BL bar, a 1 (0) is read/written from/to the I/O-pin. A checkerboard pattern is also a physical data pattern having alternating charged or discharged capacitors. Logical patterns are more easily implemented since the address of the memory cells is not as important as for physical data patterns. For physical data patterns, BL or BL bar connection information is needed to provide appropriate testing. Therefore, address information is needed to correlate BL/BL bar to each memory cell and the data pattern. Due to the address information and the density of memory cells difficulties arise with respect to testing. This is due in part to the number of memory cells and the need to keep track of not only the data pattern addressed to each memory cell but also the locations of failed memory cells. 
     Chip manufacturing processes are not error free. Therefore each memory chip has to be carefully tested, typically using the data patterns described above. Testing costs are presently a major contributor to overall manufacturing costs of memory chips. The test costs may be reduced either by reducing the time required to test a chip and/or to increase the number of chips tested in parallel. The number of chips tested in parallel is usually limited by the number of input/output (I/O) channels a memory tester can handle. One way to increase the number of chips tested in parallel is to reduce the number of connections between the external tester and the chip under test. Assuming a tester can handle 1024 I/O channels and 130 channels are needed to test one chip, then 7 chips can be tested in parallel. 
     Therefore, a need exists for an apparatus for testing memory cells to both reduce costs of testing and reduce test time. A further need exists for an apparatus which reduces the number of channels needed to test each chip. 
     SUMMARY OF THE INVENTION 
     A semiconductor memory chip in accordance with the present invention includes a first memory array to be tested including a plurality of memory cells arranged in rows and columns, the memory cells being accessed to read and write data thereto by employing bitlines and wordlines, the data provided on input/output pins, and a pattern generator formed on the memory chip. The pattern generator further includes a programmable memory array including a plurality of memory banks, the memory banks having memory cells arranged in rows and columns, each bank being capable of storing data for a pattern to be generated for each of the input/output pins of the first memory array. A means for addressing the data stored in the programmable memory array to address individual data to be transmitted to and from the first memory array is included. 
     Another semiconductor memory chip includes a first memory array to be tested including a plurality of memory cells arranged in rows and columns, the memory cells being accessed to read and write data thereto by employing bitlines and wordlines, the data provided on input/output pins. A pattern generator is formed on the memory chip. The pattern generator further includes a programmable memory array including a plurality of memory banks, the memory banks having memory cells arranged in rows and columns, each bank being capable of storing data for a pattern to be generated for each of the input/output pins of the first memory array. A means for addressing the data stored in the programmable memory array to address individual data to be transmitted to and from the first memory array is also included. A pattern decoder for selecting a pattern from a plurality of patterns, stored in the memory banks, in accordance with an input signal. Outputs are coupled to the input/output pins of the first memory array to provide the individual data to be transmitted to and from the first memory array. 
     A DRAM memory chip in accordance with the invention includes a first memory array to be tested including a plurality of memory cells arranged in rows and columns, the memory cells being accessed to read and write data thereto by employing bitlines and wordlines. The data is provided on input/output pins. A pattern generator is formed on the memory chip. The pattern generator further includes a programmable memory array including a plurality of memory banks, the memory banks having memory cells arranged in rows and columns, each bank being capable of storing data for a pattern to be generated for each of the input/output pins of the first memory array. An input means for inputting the pattern data from a source external to the memory chip to the memory banks of the programmable memory is included. The pattern data is provided to the programmable memory prior to testing the memory chip. A means for addressing the data stored in the programmable memory array is included to address individual data to be transmitted to and from the first memory array. A pattern decoder selects a pattern from a plurality of patterns, stored in the memory banks, in accordance with an input signal. Outputs of the pattern generator are coupled to the input/output.pins of the first memory array to provide the pattern data to be transmitted to and from the first memory array. 
     In alternate embodiments, the means for addressing may be included on or off the semiconductor memory chip. The means for addressing may be provided by an external testing device. The pattern generator may include a pattern address input to select a pattern stored in the programmable memory array. The programmable memory array may include read only memory having pattern data stored therein. The memory chip is preferably a dynamic random access memory chip. The programmable memory array preferably stores a plurality of data patterns, each data pattern of the plurality of data patterns may be stored on a number of memory banks. The addressing means preferably includes wordlines and sense amplifiers to activate the memory cells of the programmable memory array. The pattern to be generated may include a physical pattern, a logical pattern and/or a checker pattern. 
    
    
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     This disclosure will present in detail the following description of preferred embodiments with reference to the following figures wherein: 
     FIGS. 1A-1C are a top views of memory arrays showing typical data patterns stored in accordance with the prior art; 
     FIG. 2 is a block diagram of a memory device having a pattern generator formed thereon having programmable memory in accordance with the present invention; 
     FIG. 3 is a schematic diagram of the pattern generator of FIG. 2 showing memory banks and a pattern decoder in accordance with the present invention; 
     FIG. 4 is a schematic diagram of a memory bank of the pattern generator of FIG. 3 in accordance with the present invention; and 
     FIG. 5 is a schematic diagram of a set of memory banks for storing a complete pattern for the pattern generator of FIG. 3 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention relates to semiconductor memories and more particularly, to an apparatus for testing memory devices using a programmable on chip data pattern generator. The data pattern generator is preferably designed and built as part of the memory chip. The data pattern generator stores an arbitrary data pattern either supplied by a external tester or hard coded directly into the pattern generator. The on chip data pattern generator in accordance with the present invention provides a faster and more efficient method of testing semiconductor memory chips/devices since the data pattern is stored close to the memory cells prior to testing. 
     Referring now in specific detail to the drawings in which like reference numerals identify similar or identical elements throughout the several views, and initially to FIG. 2, a semiconductor memory device/chip  100  is shown. Semiconductor memory device  100  includes a memory array  102  including a plurality of memory banks  104 . Memory banks  104  include memory cells  106  which are accessed using wordlines WL and bitlines BL and BL bar. A pattern generator  108  is included on chip to provide a testing pattern for testing memory cells  106 . 
     Pattern generator  108  may be controlled off chip by an.external tester  110  which may be coupled to pattern generator  108  by, for example a serial interface  112 . The pattern generator  108  may be activated/deactivated by setting or resetting a test mode of operation. This may be performed using an enable switch or enable signal supplied on Enable line. Enable permits pattern generator  108  to output data patterns, such as those shown in FIG. 1, to be transmitted to memory array  102  through Data Out lines. Data Out lines are coupled to input/output (I/O) pins or DQs of memory chip  100 . 
     Data-In and Program lines permit direct access to memory  114  of pattern generator  108 . Pattern generator  108  may include read only memory or erasable memory or both. Data-In permits pattern data to be input and stored in memory  114  until it is transmitted into memory array  102  for testing memory cells  106 . Program permits programming operations to write or rewrite to memory  114 . Pattern Address lines permit specific patterns to be entered and programed into pattern generator  108 . Pattern Address is employed to select a pattern in which to write data to memory cells  106 . 
     Memory address lines  120  include row address lines and column address lines. Memory address lines  120  supply locations within memory  114  of memory cells to be written from by pattern generator  108  to memory cells  106  of array  102 . Pattern generator  108  manages the address information to write pattern data to memory cells  106  in accordance with the pattern specified, for example, a physical pattern (see FIGS. 1A and 1C) or a logical pattern (see FIG.  1 B), and the pattern data. The pattern topology (physical data scrambling or arrangement of data within memory array  102 ) is controlled by a subset of row and column addresses supplied to pattern generator  108  through row lines and column lines of memory address lines  120 . In one embodiment, only a single bit (1 or 0) is needed on row lines of memory address lines  120  to provide row address data scrambling as illustrated in FIG.  1 . Two or three bits (1s and/or 0s) may be needed on column lines of memory address lines  120  to provide column address data scrambling as illustrated in FIG.  1 . The actual number of bits for row/column data scrambling may be varied according to the chip architecture. 
     Referring to FIG. 3, on chip pattern generator  108  is shown schematically in greater detail. Memory  114  of pattern generator  108  includes a plurality of memory banks  115 , each of which includes information on a specific pattern, i.e. pattern &lt;0&gt;, pattern &lt;1&gt;, . . . or pattern &lt;k&gt;, an x address &lt;x&gt; for row information, all y addresses y &lt;0:n−1&gt;for column information and all data to be input/output to memory array  102  by DQ &lt;0:j&gt;. A pattern address is input to a pattern decoder  122  to select a specific pattern i.e. pattern &lt;0&gt;, pattern &lt;1&gt;, . . . or pattern &lt;k&gt;. Banks  115  are labeled in FIG. 3 according to the following convention: a bank is identified by a pattern (0−k) and a y address (0−n). For example, bank &lt;k&gt; &lt;n&gt; designates a pattern k, which may include a physical pattern or any other desired pattern, and a column n. 
     Referring to FIG. 4, a single bank &lt;k&gt; &lt;0&gt; is shown to illustrate banks  115  in greater detail. Bank &lt;k&gt; &lt;0&gt; receives an input signal, data pattern &lt;k&gt;, from pattern decoder  122  (FIG.  3 ), to enable bank &lt;k&gt; &lt;0&gt;. Bank &lt;k&gt;&lt;0&gt; stores information for data pattern &lt;k&gt; to be transmitted to memory array  102  (FIG.  2 ). Bank &lt;k&gt; &lt;0&gt; includes data to be output from bank &lt;k&gt; &lt;0&gt; through data-out for all data lines DQ &lt;0:j&gt; for a single x address (row address) and the entire y address (column address) space. Other storage arrangements are contemplated, for example, each bank  115  may include information for all data lines DQ &lt;0:j&gt; for a single y address (column address) and the entire x address (row address) space. For example, j may be equal to 4, 8, 16, 32, 64 or multiples thereof. 
     Referring to FIG. 5, banks &lt;k&gt; &lt;0:n&gt; are shown to further illustrate the arrangement of on chip pattern generator  108  in accordance with the present invention. A set of banks &lt;k&gt; &lt;0:n&gt; includes information for a complete data pattern for the entire address space i.e., x-address &lt;0:m−1&gt; and y-address &lt;0:n−1&gt; where m and n are the number of bits needed for the pattern in the x and y direction, respectively. 
     Referring again to FIG. 2, banks  115  may include storage space sufficient to store enough data for a complete pattern for a smallest repeatable pattern to be transmitted to memory array  102 . Likewise, banks  115  may include storage space sufficient to store enough data for a complete pattern for an arbitrarily sized pattern to be transmitted to memory array  102 . It is possible to fabricate pattern generator memory  114  in a similar way as memory array  102 . For example, memory  114  includes sense amplifiers SA, bitlines BL (BL bar) and wordlines WL as shown in FIG.  1 . In this way, y-addresses are employed to activate memory cells in memory  114  corresponding to wordlines while x-addresses are employed to activate sense amplifiers SA. The pattern generator  108  has structures, such as, sense amplifiers SA, bitlines BL (BL bar) and wordlines WL, formed simultaneously with the corresponding structures of memory array  102 . 
     Pattern Address signals may be generated on the chip or by external tester  110 . The more patterns stored in memory  114  the more Pattern Address lines are needed. For example, if 8 different data patterns are to be stored then 3 different address bits are needed. Further, address signals on memory address lines  120  may be generated on or off chip. Memory  114  may include read only memory having preprogrammed patterns stored therein for use. 
     To implement a test employing pattern generator  108  in accordance with the present invention, a pattern is selected to by choosing a test mode to be used for the test. This is input as a pattern address to pattern decoder  122  which selects memory banks  115  having the data corresponding to the selected pattern therein. The x-address which may be generated on or off chip is used to select a single bank and the y-address determined the data set to provide to the pattern generator output. To conserve chip area, circuitry for pattern generator may be reduced to a single, programmable data pattern. Each time a new pattern is needed it is downloaded into memory  114  of pattern generator  108 . In other embodiments, pattern data may be mixed, i.e., several patterns used for a single test, for example a checkerboard pattern and a ripple pattern may be used at arbitrarily selected locations to provide the test pattern for the memory array  102 . 
     EXAMPLE 
     The following Example illustratively describes a pattern generator in accordance with the present invention for a 16 bit DRAM chip. For a 16 bit chip DQs 0-15 are included. For this example, memory architecture permits a pattern with 2 column bits, i.e., n=2, and 4 bits in the row direction, i.e., m=4. Also, 8 patterns are desired to be stored in the patten generator memory, then k=7 (0-7 is 8 patterns). (WLs are decoded from the row address, m) m and n are related to the smallest repeatable structure in the array in terms of topology. It is desired to write a checkerboard pattern (See FIG.  1 C). The pattern generator has to provide the 0&#39;s and 1&#39;s of the pattern. Referring to FIG. 1C, in the case of activating sense amp SA &lt;0&gt;, the y-address would be 0, and x-address (WL) is also 0, and a 1 is applied to the bitline BL. Now, if the x-address is changed from 0 to 1 (to WL &lt;1&gt;), a 0 is needed at the output of the pattern generator. From WL &lt;0&gt; to WL &lt;1&gt; 1, the same y-address is used. 
     For the pattern of FIG. 1C, the information that the pattern generator has to write, includes a 1,0,0,1 pattern. In this case, for a fixed y-address 4 bits in the x-direction are needed. Then, the pattern repeats itself. These four 4 bits are stored in the memory of pattern generator already. 2 bits are needed for the y-direction because the pattern for SA &lt;0&gt; is different from the pattern of SA &lt;1&gt;. m and n (4×2) is the smallest unique pattern in this example. Advantageously, pattern generator stores this smallest repeatable pattern which is repeated by simply changing the address to read/write the pattern in memory cells of the DRAM chip. 
     In the example described above, an external tester can handle 1024 I/O channels and 130 channels are needed to test one chip, therefore 7 chips can be tested in parallel in the prior art. By incorporating a pattern generator in accordance with the present invention, channels normally used for pattern generation become available. For example, about 31 channels are available per chip. This means the tester may now test 10 chips in parallel thereby increasing throughput for acceptance testing of memory chips. 
     Having described preferred embodiments for on chip programmable data pattern generator for semiconductor memories (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.

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