Abstract:
The present invention provides a built-in self-repairable memory. The invention repairs a faulty IC through hard fuses, as well as through available redundancy in memories on chip. As the faults are not present in all the memories, the invention uses a lesser number of fuses to actually make a repair and thus results in a yield enhancement. The fuse data is stored in a compressed form and then decompressed as a restore happens at the power on. The fuse data interface with the memory to be repaired is serial. The serial links decreases the routing congestion and hence gain in area as well as gain in yield (due to lesser defects and reduced area).

Description:
TECHNICAL FIELD OF THE INVENTION 
     The instant invention relates to a built-in self-repairable memory. 
     BACKGROUND OF THE INVENTION 
     The memories are becoming a major part of any Application Specific Integrated Chip (“ASIC”) today. In fact, it is predicted that in coming years the memories will occupy ninety percent (90%) of the total chip area. Therefore, the performance of any ASIC will mostly be dependent upon performance of memories. Since memories cannot test themselves, they are tested either using Built In Self Test (BIST) or any other external circuitry. Redundancy is provided in the memories in the form of extra rows or columns so that the memories can be repaired if they are faulty. 
     In the present scenario, fuses are implemented on a chip with small/medium size memories, and the same number of fuses is allocated as the number of redundant bits required for all memories. The BIST runs to first calculate the faulty locations and makes a decision whether the faults can be repaired depending upon the available redundancy in the memory or on the chip. In case the memory is not repairable, the BIST declares the memory as dead. After the first BIST run on memory the following configurations can arise. 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                 CONFIGURATIONS 
                 BBAD 
                 REPAIRABLE 
                 Repair Status 
               
               
                   
               
             
             
               
                 Memory is DEAD 
                 ‘1’ 
                 ‘0’ 
                 “0” 
               
               
                 Memory is GOOD 
                 ‘0’ 
                 ‘0’ 
                 “0” 
               
               
                 Memory is BAD but 
                 ‘1’ 
                 ‘1’ 
                 “1” 
               
               
                 repairable 
                   
                   
                   
               
               
                 Memory is 
                 ‘0’ 
                 ‘1’ 
                 “0” 
               
               
                 repaired 
               
               
                   
               
             
          
         
       
     
     Faulty Memory Repairable Bitmap (FMRB) consists of the faulty address (to be replaced by a available redundancy) and repair status. The repair status is ‘1’ when the memory is to be repaired and 0 if the memory is good. The calculated FMRB present in the BIST is shifted out to the fuse through a shift register. The FMRB shift register of all the BISTs are connected in big single chain, which is further connected to the fuse box as well as to the chip output port. The above shifted data is stored in the fuse and after the memory is repaired the BIST is run again to approve that. 
     But since only a very small number of the cuts are faulty on the same die, it is not an economical solution to have the fuse bits for all memories on chip, as it leads to an increase in area as a greater number of fuse macros are required on the chip. In order to overcome the above drawback, the concept of fuse sharing is used. In this methodology, the data is stored in the fuse with very few fuses actually present on the chip. The concept of fuse sharing has been shown in  FIG. 1 . The number of fuses used are “k” while “M” cuts with redundancy are present on the chip, which as total FMRB length of “N” bits. The “k” bit fuse data is converted into the data form the “M” cuts present on memory or N bits. In reality only “M” memories are failing with the total FMRB length less than “k” bits. The k bit fuse data can be divided into the following main parts: 
     1. Header: single bit data giving the repair status, the number “m”. 
     2. Address data (Kad): the address information relating to m faulty memories out of M memories. 
     3. Repair data (Krd): the repair data bit stream of “m” repairable and faulty memories. 
     But to store the data in k fuses the FMRB length of each memory should be known to the central controller, which will store the information in the “k” fuses. This information can be stored in the chip in a hard Read Only Memory (ROM), or a soft ROM. This information can also be sent along with the repair data, but then it will take more time to shift in the data. The die will be repairable if the value of k is greater than or equal to the summation of the value of header data, address data and repair data. 
     The advantages the above approach offers are reduced fuse area, reduced yield loss due to increased area, less time required for programming the fuse and reduced efforts at the user end. Yet it has a lot of drawbacks, as the problem of routing congestion is not addressed. Yield loss due to metal routing for M cuts is still present and the k to N converter requires a lot of area. An extra ROM is used which increases the overhead in terms of area and testing time. 
     To reduce routing congestion the fuse data can be serially shifted out as shown in the  FIG. 2 . The serial shifting improves the yield as a lot of parallel bus routing is reduced. Also yield loss due to parallel bus routing is reduced. But serial loading of fuse data for M cuts takes a lot of clock cycles at power on. The ROM utilized to store the repair data length increases overhead in terms of area and bits to be tested through dedicated BIST. Further extra area overhead in Test Manager Design (not yet included) is required with complex finite state machines (FSM) for encoding and decoding. 
     The present invention aims at solving problems created due to the above mentioned disadvantages. The ROM is completely eliminated in the present invention due to which extra overhead in terms of area is reduced as well as state machines for fuse encoding and decoding have become simpler. The repair data compression before storing it in the fuse had also been proposed so that more memories can be repaired with the same number of fuses in case die yield goes down. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, a primary object of the instant invention is to reduce overhead in terms of area and reduce routing congestion. 
     Another object of the invention is to use fewer fuses and maintain the repair coverage. 
     Yet another object of the invention is to shift the data for the faulty memories only after loading the same in the memory or the BIST via a serializer. 
     A further object of the invention is to save the power on time. 
     To achieve the aforesaid objects the instant invention provides a built-in self-repairable memory comprising:
         an array of fuses shared between repairable memory blocks;   a repairable memory block status logic for storing the status of each memory block;   mapping means connected to said array and said logic for providing the mapping data pattern; and   computation means for detecting the repairability of said memory;
 
thereby avoiding the need to store the repair data length of each memory to be repaired.
       

     The data pattern is loaded in the memory using a serial interface thereby reducing routing congestion and defect probability in the repair circuitry. 
     The built-in self-repairable memory further comprises an encoding and decoding FSM for computing said repair data length. 
     The data pattern is compressed before being stored in fuses facilitating repair of a larger number of memory blocks. 
     The repairable memory blocks are repaired at power on by decompressing means that decompresses the compressed data pattern stored in fuses. 
     The data pattern is a faulty memory repairable bitmap. 
     A method for repairing memory comprising the steps of: 
     sharing an array of fuses between repairable memory; 
     storing the status of each memory block; 
     providing the mapping data pattern; and 
     detecting the repairability of the memory; 
     thereby avoiding the need to store the repair data length of each memory to be repaired. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; and the term “controller” may be used interchangeably and mean any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described with reference to and as illustrated by the accompanying drawings, in which like reference numerals represent like parts, and in which: 
         FIG. 1  shows fuse macro block with fuse sharing; 
         FIG. 2  shows serial loading of fuse data for reducing routing congestion; 
         FIG. 3  shows an exemplary block diagram in accordance with the invention; 
         FIG. 4  shows compression and decompression block; 
         FIG. 5  shows an exemplary block diagram of the invention; 
         FIG. 6  shows ten memories on a die as an example; and 
         FIG. 7  shows the serially shifted out repair status data. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 3 through 7 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged built in self-repairable memory. 
       FIG. 3  shows the architecture of the instant invention consisting of a fuse encoding and diagnostic FSM  30 , fuse decoding FSM  31 , Kad computation block  32 , Log 2 k bit counter  33 , Krd counter  34 , Computation block  35  and comparator  36  along with the input and output signals. 
     Fuse encoding and diagnostic FSM  30  is the control unit for the following functions: 
     Shift out the repairable status of each memory on a die and store Kad in the fuse. 
     Shift out the repair data of m repairable memories to the remaining bits of the fuse. 
     Computation of Krd. 
     Computation of the repair status of the die depending upon the condition of whether k&gt;Kd+Krd+Header, for repairability. 
     Fuse decoding FSM  31  is triggered depending upon the repair status of the die and performs the following functions: 
     Shifts out the repair status of all memories on the die from k bit fuse. 
     Then shifts out the Krd data stored in the k bit fuse for the m repairable memories on the die using Krd counter. 
     Kad computation block  32  computes the value of Kad depending upon whether the address of m repairable memories is stored or whether the sequence of 1 and 0 where position of 1 specifies a repairable memory is stored. 
     Log 2 k bit counter  33  is connected to Kad computation block  32  that counts till CountMax (k) until the fuse is completely filled with Kad and Krd. A Krd counter  34  is connected to Log 2 k bit counter  33  that determines Krd length. The computation block  35  in turn computes k−Kad−1. 
     The Krd counter  34  and the computation block  35  are connected to a comparator  36  which is used to determine the die repair status: 
       FIG. 4  shows a compression/decompression block  40 . The repair data coming from the repair data chain of m repairable memories may be long enough and occupy a large space in the k bit fuse. Therefore this repair data is subjected to compression using compression/decompression block  40  before storing it in the k bit fuse. This increases the probability of more memories to be repaired on die data which otherwise could not have been repaired if Krd was stored without compression. 
     The input pins are
         1. clk_m: is the system clock   2. rst_n: reset   3. shscout 1 : The chain  1  output to the above block conveying repair status   4. shscout 2 : The REPAIR DATA chain output to the above block conveying repair data bit stream.   5. Fusedata [k−1:0]: The data from fuse at power on.   6. Scan inputs: scan inputs like test_si, test_se, test_so.   7. Nomemdead: The test manager flag to convey that no memory is dead.       

     The Output Pins are 
     1. repairable: Bit indicating the repair status of the die. 
     2. rstRepair_chain: to reset the repair data in BIST chain. To calculate the data in the Krd chain, the chain is first reset. After that ‘1’ is shifted in the chain to calculate after how many cycles it reaches back to initial data. This clock count is the number of repair data in the chain. 
     3. bend: The pin goes high when all the operations are completed. 
     4. shscin 1 : serial input to BIST to give repair status. 
     5. shscin 2 : serial input to BIST to give the repair data bit stream. 
     The FMRB from the m repairable memories is drawn out in serial fashion and stored in the k bit fuse. There are two separate chains for repair status and repair data. The complete configuration has been shown in  FIG. 5 . 
     The shift register  50  receives all the repair status bits and outputs CHAIN 1 . The shift register  51  receives the repair data of the entire BIST and outputs the second chain CHAIN 2 . The calculated FMRB present in the BIST is shifted out to the fuse in the following way:
         Repair status is serially shifted out through SHSCOUT 1  pin.   Repair data is serially shifted out through SHSCOUT 2  pin.       

       FIG. 6  shows ten memories on a die as an example. The repair status of each memory is drawn out first. If the memory is good, the repair status is set to ‘0’. If the repair status bit is ‘1’, the memory is repairable. The repairable status is shifted out serially by running counter 10(M) times. In this case the serially shifted out repair status data is “0010010001” as shown in  FIG. 7 . 
     The repair data chain is shortened by bypassing the repair data chain of good memories. Thus the repair data of only m repairable faulty memories out of total M memories is shifted out, hence requiring lesser number of clock cycles. 
     The instant invention uses a “k” bit register, which exactly represents the “k” bit fuse. Any data present in these registers is programmed in the fuses. These “k” registers are divided into three parts: 
     1. Header: single bit data giving the die repair status, the number “m”. 
     2. Address data (Kad): the address information relating to m faulty members out of M memories. 
     3. Repair data (Krd): the repair data bit stream of “m” faulty memories. 
     All the three values mentioned above are calculated as well as the information whether the die is still repairable with the “k” fuses. To determine this first the value of Kad is calculated. Then Krd is counted. Based upon the values of Kad as well as Krd, it is checked whether the die is repairable or not. 
     Determination of address data (Kad) length: 
     Repair status of each memory is shifted out first and there is “1” detector in the controller. There are two ways to store Kad:
         1. The clock count on which the output SHSCOUT 1  is “1” is stored as the address of the faulty memory.   2. The repair status sequence itself can be stored.
 
Depending upon which method is chosen, Kad length can be determined accordingly.
       

     Determination of repair data (Krd) length: 
     The Kad is shifted back to the corresponding memories. The repair status bit of each memory bypasses the good memory repair data from the CHAIN 2 .
         1. The repair data present in the serially connected REPAIR DATA chain of BIST of m repairable memories is shifted out and stored in the remaining bits of k bit fuse.   2. After the fuse is filled the serially connected REPAIR DATA chain is resetted.   3. Scan the bit ‘1’ through the REPAIR DATA chain to determine the length of Krd.       

     Determination of Repair Status of a Die: 
     Upon the determination of length of Kad and Krd, the repair status of a die can be known, since for repairability of the die k&gt;=kad+krd+Header. 
     To conclude, the solution that is provided by the invention is worth using on a die with a large number of memories with redundancy. Since all memories on the die will not be faulty, therefore providing fuse for all of them will not be an efficient technique. So fuse can be shared. Also the serial shifting of repair status and repair data to and from BIST will reduce the routing congestion. Compression technique used on the repair data before storing it in fuse will enable the repair of more memories if yield goes down. There is also reduction in area and power on settling time by using this concept. Efforts at the user end are also drastically reduced. 
     LEGENDS 
     
         
         
           
             k Number of fuses present on the dice 
             M Number of redundant memories on chip 
             m Number of bad but repairable memories on chip 
             N Sum of all redundancy bits per memory 
             Kad Address data for the repairable memories (m) on chip 
             Krd Repair status data of all the memories (M) on chip 
             Rdw Repair data length in worst case 
             Rdb Repair data length in the best case 
           
         
       
    
     It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.