Redundancy

Abstract

For designing redundancy circuit, the estimation of the advantages and disadvantages is indispensable. The introduction of redundancy in a memory chip results in yield improvement and fabrication-cost reduction. However, it also causes the following penalties. First, spare memory cells to replace faulty cells, programmable devices to memorize faulty addresses, and control circuitry to increase chip size. Second, the time required for the judgment whether the input address is faulty or not is added to the access time. Third, special process steps to fabricate the programmable devices and test time to store faulty addresses into the devices are required. Therefore, the design of redundancy circuit requires a trade-off between yield improvement and these penalties. The estimation of yield improvement requires a fault-distribution model. There are two representative models, Poisson distribution model and negative-binomial model, which are often used for the yield analysis of memory LSIs. The “replacement” of normal memory elements by spare elements requires checking whether the accessed address includes faulty elements, and if yes, inhibiting the faulty element from being activated and activating a spare element instead. These procedures should be realized with as small penalty as possible. One of the major issues for the replacement is memory-array division. Memory arrays are often divided into subarrays for the sake of access-time reduction, power reduction, and signal/noise ratio enhancement. There are two choices for memories with array division: (1) a faulty element in a subarray is replaced only by a spare element in the same subarray (intrasubarray replacement) and (2) a faulty element in a subarray may be replaced by a spare element in another subarray (intersubarray replacement). The former has smaller access penalty, while the latter realizes higher replacement efficiency. It is also possible that a subarray is replaced by a spare subarray. The devices for memorizing faulty addresses and test for finding out an effective replacement are also important issues for redundancy.