Experimental Extraction of the Charge Centroid in SiCN-Based Charge Trapping Memories Using the Constant-Current Carrier Injection Method

Abstract

Nitride-based charge trapping memories, such as the metal-oxide-nitride-oxide-semiconductor (MONOS) devices, have received considerable attention for next generation nonvolatile memory (NVM) applications.1-3,7,8 The threshold voltage shifts of memory transistors are induced by electrons and holes trapped in the silicon nitride charge trapping layers. The memory cell size in the MONOS-type devices is rapidly becoming smaller. The number of electrons and holes trapped in the charge trapping layer is reduced in such small memory cells. It is essential to achieve sufficient memory effects even in the small size transistors of NVMs. There are still some challenging issues regarding the performance and reliability for the MONOS-type memories. We have previously proposed the application of silicon carbonitride (SiCN) films to the charge trapping layers.4-6 The relative dielectric constant of the SiCN films is 4.8-4.9, which is lower than that of silicon nitride films (~7). It has been suggested that larger threshold voltage shifts can be achieved by using the low-k SiCN films in the charge trapping memories. In order to improve the device performance in the SiCN based-NVM devices, it is important to understand the carrier trapping mechanism in the programming and erasing operations. In the present study, we investigated the hole trapping characteristics in the SiCN charge trapping layer using the constant-current carrier injection method and extracted the charge centroid in the SiCN layer. Two types of stacked film were formed on p-type (100) silicon substrates: (a) blocking oxide-silicon nitride-tunnel oxide (17.2/30.4/2.4 nm) stacked films and (b) blocking oxide-SiCN-tunnel oxide (17.3/31.6/2.4 nm) stacked films. The SiCN film was grown at 400 °C using Si(CH3)4 and NH3 gases by PECVD. The silicon nitride film was deposited at 600 °C by LPCVD. An aluminum film was deposited to form the gate electrode. After the sample fabrication, all the memory capacitors were baked at 235 °C for a long period of time to emit electrons and holes trapped in the stacked films. The flat-band voltage Vfb was determined by analyzing the CV curve of the baked capacitors. At the next step, the gate electrode was negatively biased to inject holes from the silicon substrate into the charge trapping film under a constant current density (Jg = -4.2 × 10-9 A/cm2). The number of injected holes per unit area Finj was derived by combining Jg and carrier injection time. Again the CV characteristics were measured to estimate the flat-band voltage shift ΔVfb,h which is induced by the trapped holes in the charge trapping layer. The charge centroid xtctl was calculated from the injected charges per unit area Qinj and ΔVfb.h as the injected charges can be assumed to be fully captured. Figs. 1(a) and 1(b) show xtctl as a function of Finj in the silicon nitride and SiCN layers during the constant current hole injection. As shown in Fig. 1(a), xtctl was initially located around the middle of the silicon nitride layer at smaller Finj and shifted towards the blocking oxide-silicon nitride interface at larger Finj. On the other hand, xtctl in the SiCN layer was located near the blocking oxide-SiCN interface as compared to that of the silicon nitride layer, as shown in Fig. 1(b). As mentioned above, the dielectric constant of the SiCN films is low. Therefore, in the two types of stacked film, the electric field in the SiCN layer is higher than that of the silicon nitride layer when a fixed voltage is applied to the gate electrodes. It is suggested that holes injected into the SiCN layer were easily transported towards the blocking oxide-SiCN interface by Poole–Frenkel conduction because of the higher electric field in the SiCN layer. This would result in the charge centroid near the blocking oxide-SiCN interface. The constant-current carrier injection method is useful to evaluate the hole trapping characteristics in the stacked films of charge trapping memories. Acknowledgement We would like to express gratitude to S. Tanaka, K. Hukuyama and K. Ozaki for their valuable discussions. This work was partly supported by JSPS KAKENHI Grant Number 26420280.