Electrostatic Effect of Localized Charge in Dual Bit Memory Cells with DiscreteTraps

L. Perniola(1, 2,*), S. Bernardini(3), G. Iannaccone(1,6), B. De Salvo (4), G. Ghibaudo (2), P. Masson(3), C. Gerardi(5)

(1)Dipartimento di Ingegneria dell’Informazione, Università degli Studi di Pisa, Via Caruso, 56122 Pisa, Italy, *perniola@enserg.fr
(2)IMEP-CNRS/INPG, Avenue de Martyrs 32, 38016 Grenoble, France
(3) L2MP-Polytech – IMT Technopôle de Château Gombert, 13451 Marseille Cedex 20 France
(4)CEA-LETI, Avenue de Martyrs 16, 38054 Grenoble, France
(5)STMicroelectronics, Catania, Italy
(6)IEIIT-CNR, Via Caruso, 56122 Pisa, Italy.

The following article, presented at ESSDERC 2004 conference, illustrates the capability of ATLAS in the validation and the comprehension of complex effects of new and promising devices like non-volatile discrete trap memory devices.

Abstract

In this paper the electrostatic impact of Channel Hot Electron (CHE) injection in discrete-trap memories is quantitatively addressed. The dual bit behavior of the transfer characteristic during forward and reverse read of a written cell is thoroughly analysed with the help of an analytical model. Such model allows, for the first time, to estimate the effective charged portion of the discrete storage layer, L₂, and the quantity of electrons, Q, injected in the trapping sites from the experimental parameters of the I_d-V_g characteristics, the reverse-forward threshold voltage shift V_RF, and the total threshold voltage shift Vtot. The viability of this model is confirmed with tests performed on nanocrystal memories, under different bias conditions. These results are confirmed with the help of a 2D drift-diffusion commercial code (ATLAS-SILVACO).

1. Introduction

Channel Hot Electron injection is widely used as a standard writing method for non-volatile discrete-trap memory products [1]-[2]. It provides the opportunity to localise the charges injected in a small region of the trapping medium, and two-bit operation is achieved through multilevel storage [3]. The basic principle on which two-bit operation resides, is common to NROM memories and nanocrystal memories [4]-[5]. It is possible to trap charges near one junction (drain or source) with a programming stress, and read them in the reverse mode, compared to the programming, enhancing the electrostatic effect of these charges on the conductivity of the active channel (see Figure 1).

It has been shown in the literature [1]-[3]-[6] that the threshold voltage during the forward read, V_th-F, is lower than the threshold voltage during the reverse read, V_th-R, when the cell is polarised in the saturation regime.

This is due to the strong two dimensional effects near the charged junction. If the injected charge, near the drain, is completely screened by the high V_ds applied in forward read (which induces a long pinchoff region), the I_d-V_g characteristic results very close to the characteristic of the fresh cell. In this case we have a low V_th-F. On the other hand, during the reverse read the high V_ds applied is not able to screen the effect of electrons and the conductivity of the active channel is lowered by the “bottleneck” near the low-voltage contact. In such a case we have a high V_th-R [2]-[6].