Authored By:
Ji-Seok Lee and Ilgu Yun
Yonsei University
Seoul, Korea
Summary
Recently, as the data usage increases, the memory usage for storing the data also increases. A lot of researches have been
done to reduce the production cost of the solid state drive (SSD) using NAND structure, which is widely used as a storage device along with the hard disk drive (HDD). One of them is to change the NAND structure from 2-D to 3-D by using the charge trap flash (CTF) technology, which is using nitride material (i.e., TANOS) in the floating gate instead of the conventional poly-silicon (i.e., SONOS).
As the structure and material are changed, the characteristics of the device are also changed. One of the important functions of memory is the ability to preserve the data. Thus, in this paper the long-term evaluation of TANOS structure is investigated and the prediction of retention characteristic can be evaluated through the accelerated tests. We analyzed the behavior characteristics through experiments and Technology Computer Aided Design (TCAD) simulation to improve the accuracy of long-term data retention in TANOS (Tantalum-Alumina-Nitride-Oxide-Silicon) which is one of 3-D NAND. We also examined the effects of time and temperature about data retention by dividing them into four mechanisms: Schottky emission, Fowler-Nordheim (FN)
tunneling, Poole-Frenkel (PF) emission, and trap-assisted tunneling.
Conclusions
In this paper, as NAND Flash memory is changed from 2-D structure to 3-D structure to increase storage capacity, data retention characteristics change due to material change of the floating gate. We analyzed changes in retention characteristics and tried to accurately and quickly predict changes in long-term device reliability. For this purpose, the experiments for data retention were performed at three temperatures, and the results confirmed that there was an acceleration factor according to the temperature. When using the Arrhenius equation, which is used for the acceleration evaluation due to the temperature factor, it was confirmed that the activation energy is ranged from 0.53 to 0.65 eV.
In order to classify them by mechanism, we simulated them using TCAD and analyzed their behaviors by comparing them with actual experiments. First of all, the simulation confirmed that the results could represent the experimental results. Based on this, the compact modeling was applied to make it easier to apply the equations. In addition, this study classified the behavioral characteristics by mechanism. In the early stage after the program, the effect of Schottky emission was big, but it was confirmed that detrapping of electrons due to TAT had the biggest effect over time.
By analyzing precisely the mechanism influence on the data retention behavior with time and temperature, the accurate initial behavior of data retention was analyzed by increasing the accuracy of compact modeling. Through compact modeling, data was measured for the actual behavior of the device with only performing the relatively short initial test, and the result of the long-term reliability estimation can be calculated for a long time of more than 3 years, which is the actual life of the current electronic products.
Initially Published in the SMTA Proceedings
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