We present a novel approach to the modeling of carrier energy relaxation during high-field phases in semiconductor-oxide-nitride-oxide-semiconductor (SONOS) flash memory gate stacks. We show that ...this method integrates well with TCAD simulators and that taking the energy relaxation of carriers into consideration solves two of the most prominent problems of trapping layer dynamics modeling: The missing slope degradation in incremental step-pulse programming (ISPP) simulations and the incompatibility of the resulting charge distributions with long-term room temperature charge retention measurements. This article consists of two parts where this part discusses the physical/TCAD level. The second part derives a semianalytical model specifically for programming that reduces the numerical complexity while still retaining the main physical assumptions and the applicability to experimental data.
In this letter, a Ni-Al alloy based metal induced lateral crystallization (MILC) process in a vertical Si channel, from 3-D NAND flash cell, is reported. Alloying Ni with Al improves NiSi2 ...nucleation, resulting in a favorable channel morphology after the silicide transport. We show that it is possible to achieve better ON-current and gate control over the existing Ni-only MILC process with no observable impact on the memory performance.
The introduction of the 3D NAND architecture brought new integration challenges, including the impact of fabrication-induced mechanical stress. If not controlled, the mechanical stress can result in ...high wafer warpage, incompatible with wafer handling and lithography steps. This work presents a study of the impact of annealing on the warpage and residual stress of blanket SiO2/Si3N4 stacks relevant to 3D NAND fabrication. It is shown that annealing promotes H outgassing from the Si3N4 layers and minimizes their residual stress. The optimal temperature is calculated by combining the warpage measurements with finite element modelling. That allows to calibrate the model on simplified samples and then expand it for stacks with a higher number of layers, which can be beneficial for the future 3D NAND generations.
3D NAND, fabrication, mechanical stress.
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•The impact of annealing on the warpage of 3D NAND-related SiO2/Si3N4 stacks is shown.•The study is performed by combining experiments with Finite Element Modelling.•Annealing promotes H outgassing from Si3N4, which minimizes wafer warpage.•The optimal annealing temperature was calculated for 8–128 bilayers stacks.
Vertical pitch scaling and channel splitting are under active research to increase bit density in 3D NAND flash memories. Here, we use 3D TCAD simulations to investigate the associated program charge ...interference from neighboring cells, both in single and multiple channel configurations. We find that interference-induced threshold voltage shifts increase significantly at scaled gate lengths and intergate spacings. In multiple channel configurations, additional sources of interference are present. We find the introduction of airgaps to be an essential mitigation strategy in these scaled devices and compare several possible configurations.
To present the most frequent occult pathologies unexpectedly encountered via cone-beam computed tomography (CBCT), with particular reference to the diagnostic role of the dentist and that of the ...radiographer, with a view to clarifying where the diagnostic responsibility lies.
A narrative literature review on the most diffused occult pathologies under CBCT was conducted, with iconographical guide as an example for each category.
The most frequent forms of unexpected pathologies encountered are: the presence of foreign bodies, airway anomaly, and the presence of radio-opacity or -transparency in the maxillofacial district.
The orthodontists must know that they are responsible to recognize these frequent, and potentially serious, pathologies of the head and neck. If the dentist feels unable to take on this responsibility, he or she should, however, be sure to have the scans read by a specialist radiologist.
► Experimental evidence of lateral charge migration in planar device is shown. ► A 2D model able to reproduce the retention transients is developed. ► The model is applied to vertical cylindrical ...device to simulate retention. ► Disturbs to neighboring cells due to lateral charge migration is evaluated. ► Impact of the lateral migrated charge on the cell’s string resistance is evaluated.
This paper investigates the impact of lateral charge migration on the retention performance of charge-trap memories whose storage layer is not patterned self-aligned with the channel area of each cell. Experimental results on planar SONOS devices reveal an important contribution of lateral charge migration at 150°C and are used to calibrate a new numerical model accounting for both the vertical and the lateral charge loss from the silicon nitride. This new model was used to simulate 3D vertical SONOS devices, where multiple cells are stacked in a cylindrical structure and process constrains the adoption of a self-aligned charge storage layer. A string of three cells and two select transistors has been simulated, evaluating, for different channel length and temperatures, the impact on string operations of the charge migrating in the nitride located between different gates.
A new characterization technique and an improved model for charge injection and transport through ONO gate stacks are used to investigate the program/retention sequence of silicon nitride-based ...(SONOS/TANOS) nonvolatile memories. The model accounts for drift-diffusion transport in the conduction band of silicon nitride (SiN). A priori assumptions on the spatial distribution of the charge at the beginning of the program/retention operations are not needed. We show that the carrier transport in the SiN layer impacts the spatial distribution of the trapped charge and, consequently, several aspects of program and retention transients. A few model improvements allow us to reconcile the apparent discrepancy between the values of silicon nitride trap energies extracted from program and retention experiments, thus reducing the number of model parameters.
We applied the developed trap spectroscopy by charge injection and sensing to validate the extraction of the silicon nitride trap distribution (both in space and energy) from the modeling of ...retention transients of charge-trapping memories. We compared three different types of silicon nitrides using these two techniques, and similar distributions were extracted, thus confirming the validity of the charge profiles resulting from the modeling of retention transients and the physics of the proposed model, based on two main mechanisms of charge loss: Poole-Frenkel emission (dominating at high temperature) and direct tunneling (dominating at room temperature).
A vertical cylindrical SONOS cell with a novel bilayer polysilicon channel down to 22-nm diameter for 3-D NAND Flash memory is successfully developed. We introduce a thin amorphous silicon layer ...along with the oxide-nitride-oxide (ONO) gate stack inside the memory hole. This silicon layer protects the tunnel oxide during opening of the gate stack at the bottom of the memory hole, after which it serves as the first layer of the bilayer polysilicon channel. This approach enables the 3-D architecture to achieve minimum cell area (4 F 2 , with F being the feature size) without the need for the so-called pipeline connections. The smallest functional cells have the memory hole diameter F = 45 nm, resulting in 22-nm channel diameter. In case 16 cells are stacked, F = 45 nm would correspond to an equivalent 11-nm planar cell technology node. Excellent program/erase and retention obtained with the all-deposited ONO stack are demonstrated.
We comprehensively investigate defects in 3-D SONOS devices (macaroni vs. full channel) in fresh state and after program/erase cycling endurance stress with three trap characterization techniques: ...charge pumping for SiO sub(2)/poly-Si interface traps assessment, fast random telegraph noise measurement for traps in the poly-Si channel, and trap spectroscopy by charge injection and sensing for assessment of traps in the tunnel oxide. The number of interface traps after endurance stress increases in the same way for both full and macaroni channels. For all devices and channel processes, RTN is dominated by traps near the Fermi level. A small improvement of near-interface poly-Si traps is detected for macaroni. No change in RTN trap properties is observed after endurance stress. Trap generation in the tunnel oxide after endurance stress follows similar trend in both channel type.
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