Influence of the Relaxation Current in BaxSr(1-x) TiO3 Thin Film Capacitors on DRAM Operation

Ken NUMATA  Yukio FUKUDA  Katsuhiro AOKI  Yasutoshi OKUNO  Akitoshi NISHIMURA  

Publication
IEICE TRANSACTIONS on Electronics   Vol.E80-C   No.7   pp.1043-1055
Publication Date: 1997/07/25
Online ISSN: 
DOI: 
Print ISSN: 0916-8516
Type of Manuscript: PAPER
Category: Recording and Memory Technologies
Keyword: 
cell capacitor dielectric,  relaxation current,  BST,  power law,  divergence,  

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Summary: 
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 dielectric in 256 Mb DRAM and beyond), the relaxation current often displays the power law behavior I(t)t-1. This leads to the singularity near the time zero. When one attempts to evaluate precisely the influence of this leakage on DRAM operation, the behavior should be estimated on a time-dependent bias. However, such a singular behavior makes analysis based on a linear response difficult. In this analysis, we start by assuming that the behavior of the relaxation current can be modeled as a linear equivalent circuit. We also assume that the relaxation current follows the power law, I(t)t-1 for 1 ns<t<1 s. Then, the voltage drop across the capacitor due to the relaxation current 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 quantities. The transformed equation is solved by iteration, and the problem of the logarithmic divergence is consistently solved. The BST films fabricated in our experiment show an average voltage drop of 7 to 10% during the refresh period of 1 s. This will be a concern for 256 Mb DRAMs and beyond.