Patent Publication Number: US-6667915-B2

Title: Semiconductor memory device having redundancy structure with defect relieving function

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a semiconductor memory device, and more particularly, to a semiconductor memory device having a redundancy circuit for relieving a defective memory cell. 
     DESCRIPTION OF THE BACKGROUND ART 
     In recent years, it has been essential for a semiconductor memory device with increasing capacity to have a redundancy circuit mounted therein to improve yields of products. The redundancy circuit is provided for replacing and relieving a defective memory cell, which is detected by an operation test, with a spare memory cell provided as an extra. Information related to the process of relieving the defective memory cell is programmed into the redundancy circuit and stored inside in a non-volatile manner. 
     The information programmed in the redundancy circuit includes a defective address indicating an address of a defective memory cell. At the time of using, when an access to the defective memory cell is required, the redundancy circuit is used in place of the defective memory cell to execute data reading and data writing. 
     Such a configuration allows the entire semiconductor memory device to normally operate using the redundancy circuit constituted by a spare memory cell, even if a defective memory cell occurs due to a defect at manufacturing. This allows secured yields of products. 
     However, the operation test for the semiconductor memory device is executed at different stages, such as a wafer test executed in a wafer state after fabrication of the wafer, and a product test executed in a product state after the subsequent step of assembling. In particular, the product test is performed after execution of a screening test (accelerated test) for revealing a potential failure. 
     Thus, if a defective memory cell is detected at the product test performed subsequent to the stage of the wafer test where a defective memory cell was detected and once relieved by a redundancy circuit, the process of replacing and relieving by the redundancy circuit must be programmed in consideration of the results of the both operation tests. This will require a more complicated analysis for relieving of the defective memory cell. 
     In particular, the analysis for relieving becomes complicated when a failure is detected again in a memory cell that was once relieved at the product test, after relieving of a defective memory cell detected at the wafer test had been programmed into the redundancy circuit. Thus, if the defective memory cell detected at the product test was once relieved at a row-related redundancy circuit after the wafer test, the defective memory cell must be relieved by a column-related redundancy circuit. On the contrary, if the defective memory cell detected at the product test was once relieved by a column-related redundancy circuit after the wafer test, the defective memory cell must be relieved by a row-related redundancy circuit. In such a case, the analysis for relieving performed after the product test becomes particularly complicated. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a semiconductor memory device which is configured that a defective memory cell detected by an operation test can be relieved without the need for a complicated analyzing process. 
     According to an aspect of the present invention, a semiconductor memory device includes a plurality of memory cells; a plurality of redundancy circuits, each of which is used to relieve a defective memory cell that occurred in the plurality of memory cells; and a redundancy control circuit to selectively activate one of the plurality of redundancy circuits when the defective memory cell is designated as an accessing object. The redundancy control circuit includes a first redundancy decoder arranged corresponding to one of the plurality of redundancy circuits and activating the corresponding one redundancy circuit when a defective address stored inside is designated as the accessing object; and a second redundancy decoder arranged corresponding to another one of the plurality of redundancy circuits and activating the corresponding another one redundancy circuit when a defective address stored inside is designated as the accessing object, except for the case where the defective address stored in the first redundancy decoder is designated as the accessing object. 
     Preferably, when the defective address stored in the first redundancy decoder is designated as the accessing object, the second redundancy decoder inactivates the corresponding another one redundancy circuit, irrespective of whether or not the defective address stored the inside is designated as the accessing object. 
     Such a semiconductor memory device can execute a replacement and relieving process adapted to the defective address stored in the first redundancy decoder used with a higher priority, without consideration given to the defective address stored in the second redundancy decoder used with a lower priority. Therefore, a final defective memory cell can be replaced and relieved by a redundancy circuit corresponding to the first redundancy decoder (a high-priority decoder) without the need for any consideration given to the defective memory cell detected in the past or the replacement and relieving process corresponding thereto. Thus, a problem can be avoided in that a plurality of memory cell rows or memory cell columns are selected within the same memory array, which may be caused by the problem in the analysis for relieving, without a complicated analyzing process. 
    
    
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram showing the configuration of a semiconductor memory device according to an embodiment of the present invention; 
     FIG. 2 is a circuit diagram showing the configuration of a redundancy control circuit shown in FIG. 1; 
     FIG. 3 is a block diagram showing the configuration of low-priority redundancy decoders  60 - 1  to  60 -n; 
     FIG. 4 is a flow chart illustrating the relation between a manufacturing process of the semiconductor memory device according to the first embodiment and a program of a defective address for a redundancy decoder; and 
     FIG. 5 is a block diagram showing the configuration of a redundancy control circuit according to the second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below in detail with reference to the drawings. It is noted that the same reference characters in the drawings denote the same or corresponding portions. 
     First Embodiment 
     Referring to FIG. 1, a semiconductor memory device  1  according to the present embodiment includes a memory array  10 , an address terminal  20 , a control signal terminal  25 , a data terminal  27 , a control circuit  30 , a decode portion  40 , a data input/output circuit (data I/O circuit)  45 , and a redundancy control circuit  50 . 
     Memory array  10  includes a plurality of memory cells. Memory array  10  is divided into a normal memory cell region  11  in which normal memory cells MC are arranged in a matrix of rows and columns and a spare memory cell region  12  in which spare memory cells that are provided as backup are arranged for reliving a defective memory cell occurred in the normal memory cells. A plurality of spare memory cells provided in spare memory cell region  12  are used to constitute a plurality of redundancy circuits (not shown). Each of the redundancy circuits corresponds to, for example, a spare row or a spare column, which is activated when a defective memory cell is to be accessed, to replace and relieve a normal memory cell row or column including the defective memory cell. 
     Address terminal  20  receives an input of an address signal ADD for designating an object to be accessed (hereinafter referred to as accessing object). Address signal ADD designates a row address and a column address to designate a selected memory cell in normal memory cell region  11 . Control signal terminal  25  receives an input of a command control signal CMD for giving an operation instruction to semiconductor memory device  1 . Data terminal  27  exchanges read/write data DQ that are input/output to/from memory array  10  with the outside. Control circuit  30  controls the entire operation of semiconductor memory device  1  for executing the designated operation in response to command control signal CMD input into control signal terminal  25 . 
     Decode portion  40  executes row selection and column selection in normal memory cell region  11  in response to address signal ADD input into address terminal  20 . Data I/O circuit  45  outputs the data that is read from memory array  10  at data reading to data terminal  27 , and also writes the data that is input into data terminal  27  at data writing to memory array  10 . 
     Redundancy control circuit  50  stores a defective address indicating a defective memory cell by an external program input. For example, these defective addresses are stored in a non-volatile manner into redundancy control circuit  50  by a fuse element that can be trimmed by laser. In such a case, a laser input is used as a program input. 
     Redundancy control circuit  50  compares address signal ADD input into address terminal  20  with the defective address information stored inside, and when a defective memory cell is designated as an accessing object, redundancy control circuit  50  instructs decode portion  40  to stop accessing to normal memory cell region  11  by a normal access stop control signal NSTP. Moreover, redundancy control circuit  50  selectively activates a redundancy circuit corresponding to the defective address that is designated as an accessing object. For example, when (n+1) redundancy circuits (n is a natural number) are provided, spare selection signals /ES 1  to /ESn+1 corresponding to the respective (n+1) redundancy circuits are selectively activated, to allow activation of a redundancy circuit corresponding to the defective address designated as an accessing object. 
     Thus, when a defective memory cell in normal memory cell region  11  is designated as an accessing object, the defective memory cell can be replaced and relieved by accessing to the activated redundancy circuit in spare memory cell region  12 . 
     Referring to FIG. 2, redundancy control circuit  50  includes redundancy decoders  60 - 1  to  60 -n provided corresponding to redundancy circuits RC( 1 ) to RC(n), respectively, of the (n+1) redundancy circuits; and a redundancy decoder  70  provided corresponding to a redundancy circuit RC (n+1). 
     Redundancy decoders  60 - 1  to  60 -n store therein different defective addresses FAD 1  to FADn respectively in a non-volatile manner. Redundancy decoder  70  has a defective address FAP input therein. It is noted that defective address FAP stored in redundancy decoder  70  may be the same as any one of defective addresses FAD 1  to FADn stored in redundancy decoders  60 - 1  to  60 -n respectively. 
     Redundancy decoders  60 - 1  to  60 -n output, respectively, agreement determination signals HT 1  to HTn for indicating the result of agreement comparison between defective addresses FAD  1  to FADn stored therein and address signal ADD input into address terminal  20 . Redundancy decoder  70  outputs an agreement determination signal PHT indicating the result of agreement comparison between defective address FAP stored therein and address signal ADD. In the present embodiment, for each of redundancy decoders  60 - 1  to  60 -n and  70 , a corresponding agreement determination signal is set to be at a logic high or “H” level when the defective address stored inside agrees with address signal ADD. 
     Agreement determination signals HT 1  to HTn and PHT output by redundancy decoders  60 - 1  to  60 -n and  70 , respectively, are inverted by inverters and thereafter are transmitted to redundancy circuits RC( 1 ) to RC(n) and to RC(n+1), respectively, as spare selection signals /ES 1  to /ESn and /ESn+1. Each of redundancy circuits RC( 1 ) to RC (n+1) is activated when a corresponding spare selection signal is activated to be at a logic row or “L” level, and is to be accessed in place of a memory cell row or memory cell column including the defective memory cell. 
     Redundancy control circuit  50  further includes a normal access stop control portion  80  for stopping the operation of decode portion  40  when address signal ADD agrees with any one of the defective addresses. Normal access stop control portion  80  is constituted by an NOR logic gate producing an output of the result of an NOR operation for agreement determination signals HT 1  to HTn and PHT output from redundancy decoders  60 - 1  to  60 -n, respectively, as normal access stop control signal NSTP. Thus, when address signal ADD agrees with a defective address in any one of redundancy decoders  60 - 1  to  60 -n and  70 , normal access stop control signal NSTP is set to be at the “L” level to stop the accessing operation by decode portion  40  to normal memory cell region  11 . 
     Redundancy decoders  60 - 1  to  60 -n and  70  are classified into redundancy decoder  70  of which the agreement determination result is used with a higher priority, and other redundancy decoders  60 - 1  to  60 -n with lower priorities. Hereinafter, redundancy decoder  70  of which the agreement determination result is used with a higher priority is also referred to as a “high-priority redundancy decoder.” Moreover, the other redundancy decoders  60 - 1  to  60 -n are also referred to as “low-priority redundancy decoders.” 
     Agreement determination signal PHT from high-priority redundancy decoder  70  is input into each of low-priority redundancy decoders  60 - 1  to  60 -n. Agreement determination signals HT 1  to HTn output by low-priority redundancy decoders  60 - 1  to  60 -n are forcibly set to the “L” level indicating disagreement, when agreement determination signal PHT is set to the “H” level, i.e., when defective address FAP stored in high-priority redundancy decoder  70  agrees with address signal ADD. 
     Thus, when defective address FAP held in high-priority redundancy decoder  70  is designated as an accessing object, activation of a redundancy circuit by low-priority redundancy decoders  60 - 1  to  60 -n are forcibly stopped even when the same defective address is stored in any one of low-priority redundancy decoders  60 - 1  to  60 -n. Moreover, the operation test for obtaining defective address FAP stored in high-priority redundancy decoder  70  is executed later than the operation test for obtaining defective addresses FAD 1  to FADn stored in low-priority redundancy decoders  60 - 1  to  60 -n. As a result, a new replacement and relieving process can be programmed by high-priority redundancy decoder  70 , irrespective of what kind of replacement and relieving process has been programmed into low-priority redundancy decoders  60 - 1  to  60 -n in the past, i.e., what defective address has been stored therein. 
     FIG. 3 shows a block diagram showing the configuration of low-priority redundancy decoders  60 - 1  to  60 -n. Each of low-priority redundancy decoders  60 - 1  to  60 -n has a similar configuration, and thus the configuration of low-priority redundancy decoder  60 - 1  is representatively shown in FIG.  3 . 
     Referring to FIG. 3, low-priority redundancy decoder  60 - 1  includes an address comparison circuit  61 , an inverter  62 , and an agreement comparison invalidiation circuit  65 . 
     Address comparison circuit  61  stores defective address FAD 1  in a non-volatile manner by an external program input, while generating an address agreement signal HIT 1  indicating the result of an agreement comparison between address signal ADD input into address terminal  20  and defective address FAD 1 . Address agreement signal HIT 1  is set to the “H” level when address signal ADD agrees with defective address FAD 1 , whereas it is set to the “L” level when they disagree with each other. 
     High-priority redundancy decoder  70  only has a portion corresponding to address comparison circuit  61 , which stores defective address FAP in a non-volatile manner while generating agreement determination signal PHT indicating the result of agreement comparison between address signal ADD input into address terminal  20  and defective address FAP. Agreement determination signal PHT is set to the “H” level when address signal ADD agrees with defective address FAP, whereas is set to the “L” level when they disagree with each other. 
     Inverter  62  inverts and outputs address agreement signal HIT 1 . As already described, the operation of each of low-priority redundancy decoders  60 - 1  to  60 -n is different depending on the agreement determination result (agreement determination signal PHT) at high-priority redundancy decoder  70 , so that the operation timing of each redundancy decoder is adjusted by appropriately designing signal propagation time of inverter  62 . Agreement comparison invalidation circuit  65  is constituted by a logic gate that receives agreement determination signal PHT from high-priority redundancy decoder  70  and an output of inverter  62 , and outputs the result of an NOR operation thereof as agreement determination signal HT 1 . 
     Therefore, when agreement determination signal PHT from high-priority redundancy decoder  70  is set to the “H” level, i.e., when address signal ADD agrees with defective address FAP stored in high-priority redundancy decoder  70 , agreement determination signal HT 1  from redundancy decoder  60 - 1  is forcibly fixed at the “L” level by agreement comparison invalidation circuit  65 . Accordingly, spare selection signal /ES 1  of redundancy circuit RC( 1 ) corresponding to redundancy decoder  60 - 1  is forcibly inactivated to the “H” level. 
     On the other hand, when agreement determination signal PHT from priority redundancy decoder  70  is at the “L” level, i.e., when address signal ADD disagrees with defective address FAP of high-priority redundancy decoder  70 , agreement determination signal HT 1  is set in accordance with address agreement signal HIT 1  output from address comparison circuit  61 . Thus, when address agreement signal HIT is at the “H” level, i.e., when address signal ADD agrees with defective address FAD  1 , agreement determination signal HT 1  is set to the “H” level by inverter  62  and agreement comparison invalidation circuit  65 . In response thereto, corresponding spare selection signal /ES 1  is activated to be at the “L” level. This activates redundancy circuit RC( 1 ) corresponding to redundancy decoder  60 - 1  to be an accessing object. 
     Referring to FIG. 4, circuits constituting semiconductor memory device  1  according to the first embodiment are fabricated on a wafer (process P 100 ), and thereafter an operation test in a wafer state, i.e., a wafer test, is executed (process P 110 ). A defective memory cell detected at the stage of the wafer test is replaced and relieved by storing, i.e., programming, a corresponding defective address into low-priority redundancy decoders  60 - 1  to  60 -n (process P 120 ). Low-priority redundancy decoders  60 - 1  to  60 -n each stores a defective address by, for example, a fuse element or an anti-fuse element that is blown by a laser input. In such a case, the process P 120  corresponds to a so-called laser blowing step. 
     After the defective memory cell detected by the wafer test is replaced and relieved, an assembly step is executed (process P 130 ), and semiconductor memory device  1  is enclosed in a package to be in a product state. For semiconductor memory device  1  in the product state, a defect accelerated test (burn-in test) is executed for revealing a potential defect by applying a stress such as a high voltage or high temperature (process P 140 ). 
     After the burn-in test is terminated, a product test which is an operation test in the product state is executed to confirm presence/absence of a final failure (process P 150 ). The product test is also referred to as a final test, since it includes a final check before shipping. A defective memory cell detected at the product test is replaced and relieved by programming a corresponding defective address into high-priority redundancy decoder  70  (process P 160 ). High-priority redundancy decoder  70  stores a defective address using, for example, an electric fuse element or anti-fuse element that can be blown by a high voltage input from the outside the package. It is noted that the step of programming the defective addresses into low-priority redundancy decoders  60 - 1  to  60 -n (process P 120 ) and the step of programming the defective address into high-priority redundancy decoder  70  can be collectively executed as one step. 
     Such a configuration can allow replacement and relieving of the defective memory cell finally detected at the product test, without the need for any consideration given to the replacement and relieving process based on passed operation tests such as the wafer test. As a result, the final defective memory cell can simply be relieved to be used without consideration of consistency with the replacement and relieving process executed before the product test, i.e., without a complicated analyzing process (process P 170 ). This eliminates a problem in that a plurality of memory cells or memory columns are selected within the same memory array, which may be caused by the problem of analysis for relieving. 
     Second Embodiment 
     In the second embodiment, the configuration of a redundancy control circuit will be described that can accommodate to the case where a plurality of high-priority redundancy decoders are arranged. 
     Referring to FIG. 5, a redundancy control circuit  51  according to the second embodiment is different from redundancy control circuit  50  according to the first embodiment shown in FIG. 2, in that it further includes a plurality of high-priority redundancy decoders and a selection stop circuit  90 . Redundancy control circuit  51  includes high-priority redundancy decoders  70 - 1  to  70 -m (m is a natural number). 
     High-priority redundancy decoders  70 - 1  to  70 -m are provided corresponding to redundancy circuits RC(n+1) to RC(n+m) respectively. The configuration of each of high-priority redundancy decoders  70 - 1  to  70 -n is similar to that of the high-priority redundancy decoder according to the first embodiment. High-priority redundancy decoders  70 - 1  to  70 -m store therein different defective addresses FAP 1  to FAPm respectively. High-priority redundancy decoders  70 - 1  to  70 -m activate corresponding agreement determination signals PHT 1  to PHTm to the “H” level when address signal ADD input into address terminal  20  agrees with defective addresses stored respectively therein. This activates spare selection signals /ESn+1 to ESn+m for activating redundancy circuits RC(n+1) to RC(n+m) respectively, based on the result of agreement determination at high-priority redundancy decoders  70 - 1  to  70 -m. 
     In the configuration according to the second embodiment, agreement determination signal PHT is used as a signal indicating agreement between address signal ADD and a defective address in any one of high-priority redundancy decoders  70 - 1  to  70 -m. Thus, agreement determination signal PHT is generated by selection stop circuit  90  constituted by a logic gate producing an output of the result of an OR operation for agreement determination signals PHT 1  to PHTm corresponding to high-priority redundancy decoders  70 - 1  to  70 -m respectively. Low-priority redundancy decoders  60 - 1  to  60 -n and normal access stop control portion  80  operate in response to address signal ADD and agreement determination signal PHT in a manner similar to that in the first embodiment, so that the detailed description thereof will not be repeated. 
     Such a configuration allows execution of replacement and relieving operation similar to that in the first embodiment, even if a plurality of high-priority redundancy decoders, of which agreement determination results are used with higher priority, are arranged. This enables simple replacement and relieving of the defective memory cell detected finally, without any complicated analysis for relieving. 
     Moreover, the degree of priorities for the redundancy decoders can be made more hierarchical. That is, a redundancy decoder having a priority higher than that of the high-priority redundancy decoder shown in each of the first and second embodiments can further be arranged using a similar configuration. However, as also shown in the first and second embodiments, consideration must be given so as not to store the same defective address into a plurality of redundancy decoders having the same degree of priority. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.