Effect of electric field polarity on inter-poly dielectric during cell operation for the retention characteristics

Keywords: Inter-poly dielectric (IPD) ONO Leakage current Bias polarity a b s t r a c t Retention characteristic represents a capability to maintain the storage data and it is related with the reliability of memory device. The retention characteristic is degraded by the leakage of charges from a floating gate to a control gate, and thus the leakage current at low and moderate electric field of inter-poly dielectric (IPD) is one of the important characteristic for floating gate type flash memories. In addition, it is necessary to investigate the effects of the electric field polarity on the electric characteristics of IPD because the electric field polarity is changed as the cell operations such as the programming and erasing. Therefore, in this paper, the variation of the leakage current of IPD at moderate electric field region is measured with varying the previously applied electric field polarity. Based on the result, the effect of sequential change of an applied electric field polarity on the electrical characteristics is analyzed. A non-volatile NAND flash memory has been scaled down to improve its performance in terms of the speed, density, and power consumption. The multi-level cell (MLC), which has several program states such as double and tri-program levels, has been also used for high programmable bits [1]. The threshold voltages (V th) of various program levels are distributed sequentially in the MLC and the margin of the V th is reduced compared to that of single-level cell (SLC). In MLC, even a small distortion of the V th distribution can induce a data error due to an incorrect decision of the cell bits [1,2]. Especially, the tail bits phenomena of the V th distribution can cause the serious problem. The distortion of the V th distribution can be induced by the leak of charges in the floating gate (FG) which is related to the leakage currents of the inter-poly dielectric (IPD) and tunneling oxide (T ox). To improve the data retention, the relationships between the data retention and electrical characteristics of IPD and T ox with various stress conditions have been previously investigated [3–7]. In addition, the degradation of IPD with various oxide thicknesses was investigated using the injected charge to breakdown and time to breakdown [3–5]. The V th distribution shift was also demonstrated with respect to the bake time [6]. Moreover, retention mod-eling with silicon oxide–silicon nitride–silicon oxide (ONO) scaling was investigated …