Numerical approach for retention characteristics of double floating-gate memories

Abstract

We report on a numerical investigation in which memory characteristics of double floating-gate (DFG) structure were compared to those of the conventional single floating-gate structure, including an interference effect between two cells. We found that the advantage of the DFG structure is its longer retention time and the disadvantage is its smaller threshold voltage shift. We also provide an analytical form of charging energy including the interference effect. PACS numbers:Floating-gate (FG) memories are widely used in computers because of their low cost and high density [1,2]. FG memories have progressed rapidly through downscaling using state of the art technology. However, several problems have been arising as a result of the progress of down-scaling of the FG structure to the nanoscale region. In particular, the interference between FGs due to Coulomb interaction is emerging as one of the largest obstacles for FG memories [3][4][5]. Stored charges in neighboring FGs interfere with one another, resulting in undesirable threshold voltage shifts in memory operations. In order to reduce this interference, complicated programming sequences are carried out in the current commercial FG arrays.In the case of locally charged materials such as dielectric materials, it is evident that the electric dipoles exist stably within mutual strong Coulomb interaction. This leads us to consider whether we can construct an "artificial dipole" using a FG system. One of the candidates might be a stacked double floating-gate (DFG) structure [6], in which an additional FG exists between the FG and the control gate in the conventional FG array as shown in Fig. 1(a). Moreover, if the FG becomes as small as a quantum dot [7,8], DFG can be used as a qubit [9], which is a basic element of a quantum computer [10]. Therefore, it is important to clarify the fundamental properties of the DFG structure. The purpose of this paper is to numerically compare the retention time of the DFG structure with that of the conventional single FG (SFG) structure in Si/SiO 2 system, including an interference effect between two cells. We clarify the unique transient behavior of DFG owing to the existence of the additional FG. In order to compare DFG with SFG impartially, we adopt the same equivalent oxide thickness (EOT) for both structures. We also compare read disturbs of both FG structures. Finally, we derive an analytical form of charging energy of DFG and SFG as a function of gate voltages and electron charges.Formulation.-We calculate transient behaviors of a cell where length L and width W of each FG and a distance between neighboring cells X D are set equal as L = W = X D = 23 nm and the height of all FGs is Z = 50 nm for two cases of oxide thickness ( Fig. 1(b)).(We obtain similar results for the L = W =11nm case.) We take dielectric constants and effective mass of Si and oxide SiO 2 as ǫ Si = 11.7, ǫ ox = 3.9, and m Si = 0.19, m ox = 0.5, respectively. The barrier height of SiO 2 is Φ b = 2.9eV. The capacitances are defined bywhere T ox1 , T ox2 a...

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