Experimental Extraction of the Charge Centroid of Holes Trapped in Metal–Oxide–Nitride–Oxide–Semiconductor Memories

Abstract

In metal-oxide-nitride-oxide-silicon (MONOS)-type memories with an ultrathin tunnel oxide film, electrons and holes are injected from the silicon substrate to the silicon nitride charge trapping film owing to the quantum mechanical tunneling and are trapped in the film. In the previous studies, the constant-voltage carrier injection method has been used to determine the charge centroid of trapped carriers [1-4]. This method requires the application of an auxiliary pulse before injecting carriers. To determine the auxiliary pulse level, another set of measurements of the programming and erasing characteristics is needed in preliminary step. Such a sequence would generate an error in the charge centroid estimation due to the variation of the programming and erasing characteristics among individual memory elements. Therefore, we have developed the constant-current carrier injection (CCCI) method which does not require the application of the auxiliary pulse and any additional measurements [5]. The CCCI method is effective to obtain the accurate charge centroid of trapped carriers. On the other hand, these two methods can be used only in the case that the number of carriers passing through the silicon nitride film during the carrier injection is negligible as compared to that of carriers trapped in the nitride film. Therefore, neither of the two methods is usable with high gate voltages which induce a large number of carriers passing through the nitride film. In the present work, in order to obtain the charge centroid of holes trapped in the silicon nitride film at high gate voltages, we proposed a method based on the analysis of Fowler-Nordheim (F-N) tunneling current through the blocking oxide film. We determined the charge centroid and the density of holes trapped at high gate voltages by means of the proposed method and the CCCI method. Memory capacitors with the blocking oxide (SiOx)/silicon nitride/tunnel oxide stacked films were used in this work. A tunnel oxide film of 2.4 nm in thickness was grown by rapid thermal oxidation of silicon. A 30.4-nm-thick silicon nitride film was deposited at 600 °C in an LPCVD reactor. A blocking oxide film of 17.2 nm in thickness was grown at 400 °C using a PECVD method. The thickness of the blocking oxide film was designed to be sufficiently thick for electrons to obey the F-N tunneling mechanism and to gain a high kinetic energy in the oxide film. High-energy electrons entering the nitride film from the blocking oxide film would pass through the nitride film without interaction with nitride trap centers. Figure 1 shows the charge centroid of holes trapped in the nitride film, which was obtained by means of the analysis of F-N current under negative gate bias and the CCCI method. The charge centroid of trapped holes was initially located near the middle of the nitride film, and moved toward the blocking oxide-nitride interface as the flat-band voltage shift ΔVfb,h increased. Finally, the charge centroid reached 2.3 nm from the blocking oxide-nitride interface after the ΔVfb,h value was saturated. We also have estimated the density of trapped holes. A maximum of the density of trapped holes was determined to be 1.0×1013 holes/cm2 by means of the analysis of F-N tunneling current. The proposed method is useful to determine the maximum density of trapped holes which the charge trapping film in MONOS-type memories can achieve. Acknowledgment- We would like to express gratitude to S. R. A. Ahmed, K. Kato and H. Koizumi for their valuable discussions. This work was partly supported by JSPS KAKENHI Grant Number 26420280.