NAND Flash Memory Programming With Inhibit Pulse Fine-Tuning
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Solution Overview
Problem
Modern NAND flash memory devices face challenges in achieving precise programming of cell threshold voltages as they transition to multi-level cell storage, requiring higher precision due to limited threshold voltage availability and increased array density.
Innovation Solution
The use of inhibit voltage pulses to fine-tune the programming of selected cells in a memory array by applying incremental inhibit pulses to adjacent unselected word lines, allowing for precise control of threshold voltages, which is achieved by using a combination of programming and inhibit pulses in a predictable ratio, and employing two eight-bit digital-to-analog converters (DACs) to generate these pulses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple levels are programmed into each memory cell to increase storage capacity, then the storage density is improved, but the precision required for threshold voltage control increases
Solution Approach 1:
The programming process is segmented into multiple stages: initial programming to reach a target threshold voltage, followed by fine-tuning stages using incremental inhibit pulses to achieve precise voltage levels. This segmentation allows the system to first achieve coarse programming and then refine the threshold voltage with high precision, resolving the contradiction between storage capacity and precision requirements.
Solution Approach 2:
The patent employs dynamic adjustment of pulse voltages and timings during the programming process. The inhibit pulse voltage is incrementally adjusted based on verification results, allowing the system to adaptively achieve the desired threshold voltage precision. This dynamic approach enables precise control of multiple threshold voltage levels required for multi-level cell storage.
2Manufacturing precision
If a high precision DAC is used to generate programming pulses, then the threshold voltage control precision is improved, but the device complexity and size increase
Solution Approach 1:
The voltage control function is segmented between a lower-resolution DAC and incremental inhibit pulses. The DAC generates base programming pulses with moderate precision, while the fine-tuning is achieved through controlled inhibit pulses. This segmentation allows use of a simpler, smaller DAC while still achieving the required overall precision through the combined approach.
Solution Approach 2:
The inhibit pulse mechanism acts as an intermediary that bridges the gap between coarse DAC-generated pulses and the required fine precision. By using the inhibit pulse as a mediator for fine-tuning, the system achieves high precision without requiring the DAC itself to have extremely high resolution, thereby reducing DAC complexity and size.
3Quantity of substance
If the threshold voltage range is divided into more levels for multi-bit storage, then the bits per cell is improved, but the precision requirement for each level increases
Solution Approach 1:
The system dynamically adjusts inhibit pulse voltages in incremental steps during fine-tuning, allowing precise navigation between multiple threshold voltage levels. This dynamic adjustment enables accurate establishment of multiple distinct voltage levels required for multi-bit storage, with each level achieving the necessary precision through iterative verification and adjustment.
Solution Approach 2:
The patent employs verification processes that provide feedback on the actual threshold voltage achieved after programming pulses. This feedback is used to determine subsequent programming actions and inhibit pulse adjustments, ensuring that each of the multiple threshold voltage levels is precisely established. The feedback mechanism enables precise control of multiple levels by continuously monitoring and adjusting the threshold voltage.
Data Source
AI summary
Methods of programming memory cells, and memories incorporating such methods, are disclosed. In at least one embodiment, programming is accomplished by applying a set of incrementing program pulses to program a selected cell to a first target threshold voltage, and applying a set of incrementing inhibit pulses to an unselected cell to fine-tune program the selected cell to a second threshold voltage.