Charge Loss Induced by Defects of Transition Layer in Charge-Trap 3D NAND Flash Memory
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-01-01 |
| Journal | IEEE Access |
| Authors | Fei Wang, Yuan Li, Xiaolei Ma, Jiezhi Chen |
| Institutions | Shandong University |
| Citations | 13 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study utilizes first-principles calculations (DFT) to analyze the impact of defects within the inevitable Silicon Oxynitride (Si2N2O) transition layer on charge loss in Charge-Trap (CT) 3D NAND flash memory.
- Lateral Charge Loss Mechanism: Shallow-trap centers, specifically intrinsic defects like Nitrogen Vacancy (VN, Et ~ 0.37 eV) and Interstitial Silicon (Sii, Et ~ 0.63 eV), and Titanium-related defects (Interstitial Ti, Tii), couple with the Si2N2O conduction band, leading to rapid lateral charge migration and fast data loss.
- Vertical Charge Loss Mechanism: Substitution defects (N substituting Si, NSi; Ti substituting Si, TiSi) in Si2N2O create resonant energy pathways, coupling with electron traps in the Si3N4 CT layer and significant defects (VO-H) in the SiO2 tunneling layer, thereby promoting vertical charge loss.
- Ti Doping Risk: While Ti doping is used in Si3N4, the resulting Ti-related defects in Si2N2O (e.g., ultra-shallow Tii, Et ~ 0.14 eV) introduce additional shallow traps, potentially worsening lateral charge loss if the transition layer is not optimized.
- H-Passivation Solution: Hydrogen (H) passivation is confirmed as an effective strategy to deepen shallow trap levels significantly (e.g., VN Et deepened from 0.37 eV to 1.33 eV), suppressing fast detrapping and improving device reliability.
- Engineering Implication: Appropriate treatment and H-passivation of the Si2N2O transition layer are critical for avoiding both lateral and vertical charge loss, especially when metallic doping strategies (like Ti) are employed in the CT layer.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Si2N2O Band Gap (Eg) | 5.97 | eV | Calculated value (consistent with 5.2-5.97 eV reported range). |
| Critical Shallow Trap Threshold (Et) | < 1.4 | eV | Energy level leading to fast charge loss (reference value from Si3N4 CT layer). |
| Intrinsic VN Trap Energy (Et) | 0.37 | eV | Shallowest intrinsic trap causing lateral loss. |
| H-Passivated VN Trap Energy (Et) | 1.33 | eV | Deepened trap level after H-passivation (VN-H). |
| Interstitial Si (Sii) Trap Energy (Et) | 0.63 | eV | Shallow trap in intrinsic Si2N2O. |
| Interstitial Ti (Tii) Trap Energy (Et) | 0.14 | eV | Ultra-shallow trap in Ti-doped Si2N2O. |
| Operating Temperature (T) | 85 | °C | Used for Poole-Frenkel (PF) emission rate modeling. |
| Attempt-to-Escape Frequency (ÎœPF) | 5 x 107 | s-1 | Used for PF emission rate modeling. |
| Relative Dielectric Constant (Δr) | 6.2 | N/A | Used for PF emission rate modeling (Silicon Oxynitride). |
| Si2N2O Supercell Size | 360 | atoms | Used for accurate trap energy level calculations. |
Key Methodologies
Section titled âKey Methodologiesâ- First-Principles Calculations (DFT): Electronic properties and defect formation energies were calculated using Density Functional Theory (DFT) via the GPU-accelerated PWmat package, based on plane wave and pseudopotential methods.
- Supercell Modeling: A 360-atom crystalline Si2N2O supercell (Base-Centered Orthorhombic) was used to model intrinsic and doped systems (e.g., Ti-doped).
- Geometric Optimization: Structures were optimized using the Generalized Gradient Approximation (GGA) Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.
- Band Gap Correction: The screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) was applied to achieve the experimentally consistent Si2N2O band gap of 5.97 eV (Hartree-Fock mixing constant α = 0.14).
- Defect Formation Energy Calculation: Formation energy (ÎH) was calculated for various defects (vacancy, interstice, substitution) in charged states (q), incorporating chemical potentials (ÎŒi) and electrostatic potential corrections (AV).
- Charge Loss Rate Modeling: The electron emission rate (RE) from traps to the conduction band was determined using the Poole-Frenkel (PF) emission model, simulating conditions relevant to data retention (T = 85 °C).
- Interface Band Alignment: Band alignment between Si3N4, Si2N2O, and SiO2 was obtained by calculating the Partial Density of States (PDOS) of constructed interface layers, with dangling bonds saturated by hydrogen.
Commercial Applications
Section titled âCommercial Applicationsâ- High-Density Non-Volatile Memory (NVM): Directly applicable to improving the reliability and data retention of commercial 3D NAND flash memory devices, particularly those utilizing the Charge-Trap (CT) ONO (SiO2/Si3N4/SiO2) stack structure.
- Semiconductor Manufacturing Process Control: Provides guidelines for optimizing N2-annealing and deposition processes to minimize the formation of shallow-trap defects (VN, Sii) in the Si2N2O transition layer, which is critical for yield and performance.
- Reliability Engineering and Testing: The calculated Poole-Frenkel emission rates and trap energy levels offer crucial input for device simulation models used to predict Read Disturb (RD) and Data Retention (DR) lifetimes during accelerated testing.
- Dielectric Material Design: Informing the design and selection of advanced silicon oxynitride (SixOyNz) materials for use as transition layers or gate dielectrics where charge trapping and leakage must be precisely controlled.
- Hydrogen Passivation Technology: Validating the use of hydrogen annealing or plasma treatments as a necessary post-fabrication step to passivate defects and stabilize charge storage in CT memory stacks, especially in Ti-doped systems.
View Original Abstract
In charge-trap (CT) three-dimensional (3D) NAND flash memory, the transition layer between Si<sub>3</sub>N<sub>4</sub> CT layer and SiO<sub>2</sub> tunneling layer is inevitable, and the defects in the transition layer are expected to cause both lateral and vertical charge loss. Here, by first-principles calculations, we present a detailed study on the defects in the transition layer Si<sub>2</sub>N<sub>2</sub>O to comprehend their impacts on charge loss in CT 3D NAND flash memory. It is shown that shallow-trap centers, such as intrinsic nitrogen vacancy (<inline-formula> <tex-math notation=âLaTeXâ>$\text{V}{\mathrm {N}}$ </tex-math></inline-formula>) and interstitial Ti (Ti<inline-formula> <tex-math notation=âLaTeXâ>${\mathrm {i}}$ </tex-math></inline-formula>), can couple with the conduction band of Si<sub>2</sub>N<sub>2</sub>O to lead to lateral charge loss. On the other hand, the N substituting Si atom (<inline-formula> <tex-math notation=âLaTeXâ>$\text{N}{\mathrm {Si}}$ </tex-math></inline-formula>) and Ti substituting Si atom (Ti<inline-formula> <tex-math notation=âLaTeXâ>${\mathrm {Si}}$ </tex-math></inline-formula>) defects in the transition layer can couple through resonance with the trap centers in Si<sub>3</sub>N<sub>4</sub>, leading to vertical charge loss from the CT layer to the transition layer. Our results strongly suggest that appropriate treatment of the transition layer and hydrogen passivation are both important for avoiding charge loss in CT 3D NAND flash memory.