Numerical and Compact Modeling of Embedded Flash Memory Devices Targeted for IC Design

In a semiconductor market dominated by portable consumer applications, embedded flash memory technology has experienced a rapid diffusion. It is now considered the preferred solid-state memory solution for its non-volatile characteristics, high read and write speeds and scalability properties. As technology scales down in the nanometer range, new accurate physical tools should be made available to circuit designers, to support the development and optimization of high-voltage circuit blocks. A surface potential-based model for the flash memory cell has been developed with the purpose of providing a comprehensive physical understanding of the device operation, suitable for performance optimization in memory circuit design. An accurate validation methodology also takes into account charge balance effects on the isolated floating gate node and parasitic couplings inside and between the memory cells. The compact model supports DC, AC and transient analyses, including program/erase bias scalability, temperature effects, process corners and statistical variations. The results have been compared to Technology Computer-Aided Design (TCAD) simulations demonstrating that short channel effects, overlap capacitances and velocity saturation dominate over the intrinsic behaviour of the cell in ultrascaled devices. The approach includes drain disturb and memory endurance degradation models due to oxide aging. These effects are becoming dominant in ultrascaled devices. The model has been implemented using the Verilog-A language for portability into common circuit simulators. Validation has been performed on measurement results of test structures integrated in a 65nm derivative NOR CMOS technology. The compact model development has been based on a rigorous modeling approach combining conventional TCAD simulation tools with physically-based analyses. A new TCAD tool has been proposed for the investigation of advanced quantum effects, band structure models, quantum tunneling and multiphonon-assisted charge trapping effects in dielectrics. The effects of charge trapping in oxide layers and Si/SiO2 interfaces have been studied, specifically focusing on flash technology, where high voltage biases represent a major issue for dielectric degradation. A multiphonon-assisted model has been coupled with a Poisson-Schrödinger quantum solver. A novel impedance calculation method has been applied to the analysis of DC and AC MOS characteristics. This approach permits the physical modeling of trap filling, frequency response and device electrostatics. Transient effects of trap filling and trap-assisted tunneling through the gate have also been investigated. The adoption of such a multilevel approach permits to apply the methodology to flash memory cells. This enabled the investigation of the role of defects on electrostatics and program/erase efficiency reduction. The flash compact model has been applied in a process development and IC memory design perspective. Technology development requires a profound understanding of trade-offs in flash devices, which affect DC, transient and long-term performances. The design, integration and characterization of a 40 KB memory sector for smart card applications has been performed to demonstrate the capabilities of the compact model.

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