Redundancy techniques for high-density DRAMs

This paper describes the redundancy techniques for high-density DRAMs to solve the following two problems which arise with the increase in memory capacity: (1) the increase in memory-array division reduces the replacement flexibility between defective lines and spare lines; (2) the defects causing DC-characteristics faults, especially excessive standby current faults cannot be repaired with the conventional redundancy techniques. First, two approaches to solve the first problem are discussed: enhancing the replacement flexibility within the limits of intra-subarray replacement, and the introduction of inter-subarray replacement. Next, the recent proposals to solve the second problem are reported. The DC-characteristics faults are repaired through the modification of bitline precharge circuit or the subarray-replacement redundancy.