Two-dimensional numerical model of memory devices with a corrugated capacitor cell structure

A mathematical model of a corrugated capacitor cell (CCC) structure for megabit-class dRAM's is proposed. This model is constructed using a numerical analysis of time-dependent current continuity equations and Poisson's equation. An electrically floating electrode and nonplanar geometries are modeled by zero-current boundary conditions and by Gauss's theorem for Poisson's equation. Transient analyses by a full two-dimensional simulation reveal a complicated device operation mechanism. The storage charge in a floating capacitor is strongly affected by the nearest neighboring capacitor potential, if two storage capacitors are laid out with small pitches. The potential in the floating capacitor significantly shifts when the drive capacitor is charged-up, compared with the reverse case, i.e., a discharge operation for the drive capacitor. A leakage current between two cells was generated due to the electric coupling between them. The current decreases exponentially with respect to the spacing between cells. That is, the electrical coupling strength rapidly decreases with an increase in the spacing. The present model can be highly applicable for designing dRAM devices with complicated and highly integrated structures. Design criteria are clarified.