INFLUENCE OF THE RELAXATION CURRENT IN BAXSR(1-X)TIO3 THIN-FILM CAPACITORS ON DRAM OPERATION

This paper describes influence of the relaxation current in BaxSr(1-x) TiO3 (BST) thin films on dynamic random access memory (DRAM) operation . The relaxation current is a transient content of dielectric leakage currents. In BST thin films (expected to be a cell capacitor dielectri c in 256 Mb DRAM and beyond), the relaxation current often displays th e power law behavior I(t)similar to t(-1). This leads to the singulari ty near the time zero. When one attempts to evaluate precisely the inf luence of this leakage on DRAM operation, the behavior should be estim ated on a time-dependent bias. However, such a singular behavior makes analysis based on a linear response difficult. In this analysis, we s tart by assuming that the behavior of the relaxation current can be mo deled as a linear equivalent circuit. We also assume that the relaxati on current follows the power law, I(t)similar to t(-1) for 1 ns<t<1 s. Then, the voltage drop across the capacitor due to the relaxation cur rent is expressed by a integral equation. The singularity is isolated after the integral equation is transformed. All the parameters in the transformed integral equation are related to finite measured quantitie s. The transformed equation is solved by iteration, and the problem of the logarithmic divergence is consistently solved. The BST films fabr icated in our experiment show an average voltage drop of 7 to 10% duri ng the refresh period of 1 s. This will be a concern for 256 Mb DRAMs and beyond.