The Effect of Passivation SiON layer on the Data Retention Reliability of NAND Flash Memory
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1195-B07-03
The Effect of Passivation SiON layer on the Data Retention Reliability of NAND Flash Memory Younggeun Jang1, Kwangwook Lee1, Eunsoo Kim1, Jonghye Cho1, Jungmyoung Shim1, Sangdeok Kim1, Junggeun Kim1, Sangwook Park1, Byungseok Lee1, Jinwoong Kim1 1
Flash Process Development, R&D Division, Hynix Semiconductor Inc., Icheon-si, Kyoungkido, 467-701 Korea ABSTRACT Data retention is one of the major device reliabilities of NAND Flash memory. We found that the lower Refractive Index (RI) of the Passivation Silicon Oxynitride (SiON) layer deposited by PECVD, the better data retention behavior was achieved. The hydrogen concentration and the stress analysis of the films are analyzed to find out which is more important in this case. Generally, when the RI of SiON decreases, both parameters also decrease, so it is impossible to find out which parameter is major factor of data retention. To analyze the effects of two parameters separately, we applied two conditions which had the same hydrogen concentration but quite different stress values. The final data retention levels are same in both conditions. In addition, even if the layer has the same hydrogen concentration, the retention characteristic is changed by how hydrogen is bonded in the film. In conclusion, the data retention characteristic can be explained by mobile ions generated by the hydrogen weakly bonded in PECVD SiON films in our experiment. INTRODUCTION The data retention of NAND Flash is specified as the difference of the threshold voltage (Vt) caused by the decrease of the electron amount in floating gate over time without any external bias, or by the change of trapped charge amount at the interface of tunnel oxide or InterPoly Dielectric (IPD). Changes in Vt over a certain value induce the retention fails. Typically, the charge trap/detrap due to the mobile ions and the change in the trap activation energy by stress have been referred as the main factors to influence the data retention characteristic [1-5]. Silicon Oxynitride (SiON) or Silicon Nitride (SiN) is generally used as the passivation layer for the memory device. However, PECVD SiON or SiN is inherently a source of hydrogen since SiH4 and NH3 are used as the source gases for the reaction. It is reported this hydrogen, a source of the mobile ion, plays an important role on the data retention characteristic of the memory devices. It is found that the lower hydrogen concentration of the passivation SiON layer shows the better data retention behavior. Yet, the mechanical stress of the films moves down to less compressive level as the hydrogen concentration decreases, which makes it difficult for us to draw the final conclusion. Accordingly, we took a different approach to identify the exact mechanism. EXPERIMENT
Full processed 48nm node NAND flash wafers were prepared to split Refractive Index (RI) of passivation SiON. We could control the RI by N2O flow rate. The hydrogen concentration and stress changes were measured on blanket wafers. The hydrogen concentrations were measured in two ways; de
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