Lateral Distribution of Electrons Trapped in Nitride Layers
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Lateral Distribution of Electrons Trapped in Nitride Layers M. Lorenzini¹, M. Rosmeulen¹,², L. Breuil¹, L. Haspeslagh¹, J. Van Houdt¹, K. De Meyer¹,² ¹IMEC⎯Interuniversity Microelectronics Center, Kapeldreef 75, 3001 Leuven, Belgium ²Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, 3001 Leuven, Belgium ABSTRACT The recent advent of nitride-based localized charge-trapping storage has raised much interest in the lateral characterization of the trapped charge distribution, which is fundamental in defining scalability and retention properties. A direct characterization technique based on amplitude-sweep charge-pumping measurements is exploited here to investigate the lateral distribution of trapped electrons along the channel length. Further, the technique is applied to monitor the charge redistribution in time at high temperature.
INTRODUCTION Recently, nitride-based trapping storage non-volatile memory concepts, such as NROM [1] or MXVAND [2], have received an increasing interest due to their smaller cell size, simpler fabrication process and double density. In these flash cells, the two-bit storage capability is achieved by storing the charge locally in the nitride layer in the vicinity of the source/drain junctions. A reverse read scheme allows for the detection of each single bit [1], provided the charge distributions are sufficiently spaced apart. In this respect, the characterization of the lateral distribution of the trapped charge is fundamental in understanding the scalability and retention properties of these concepts. Previously reported characterization studies are based on the comparison between selected electrical monitors, i.e., subthreshold currents and gate-induced drain leakage characteristics, and carefully calibrated device simulations [3,4]. The resulting charge profile is selected at the end of an iterative, trial-and-error procedure as the one that, once incorporated into the device simulator, matches best the measured electrical monitors. In this work, the lateral charge profile in the channel region is calculated directly from amplitude-sweep charge-pumping measurements using a deconvolution-based procedure [5]. The charge-pumping method has been widely used for lateral profiling of oxide-trapped charge and interface states in hot-carrier degradation studies [6]. In stressed devices, the spatial distribution of interface states is non-uniform, thus complicating the interpretation of the measurements. However, in fresh samples of moderate channel length, the interface states can be assumed uniformly distributed across the device area. The latter assumption remains valid also for the injecting conditions considered in this study, as suggested by the generally negligible increase of the maximum value of the charge-pumping current in programmed samples. Remarkably, the number of interface states per unit area N it has a typical value of 4 to 10 1011 cm-2, yielding at least 4.000 interface states per micrometer device length, which guarantees sufficient lateral resolution. Here, the
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