Pre-Boost Inhibit Scheme for NAND Flash Memory Disturbance
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Solution Overview
Problem
Existing flash memory programming methods fail to effectively prevent unintended changes to the logic state of non-programmed memory cells on the same wordline during the programming process, leading to disturbances in stored data.
Innovation Solution
A pre-boost stage is introduced to elevate the channel voltage of a selected cell by biasing the inhibit bit line to a first level and the string select transistor gate to a higher second level, reducing the potential difference between the gate and channel, and then ramping down the string select transistor before applying a programming voltage on the selected wordline.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a program inhibit scheme is used to reduce disturbance to non-programmed memory cells, then programming reliability is improved, but the inhibition effectiveness is insufficient and data accuracy deteriorates
Solution Approach 1:
The patent applies preliminary action by raising the channel voltage of the selected cell to a high level before applying the programming voltage to the wordline. This pre-boosting action ensures that the selected cell is fully inhibited before the programming operation begins, preventing any unintended programming while maintaining data accuracy.
Solution Approach 2:
The patent changes the voltage parameter of the channel by raising it to a high level (first voltage level) before programming. This parameter change creates a sufficient potential difference between the gate and channel of the selected cell, ensuring effective inhibition and preventing disturbance to non-programmed cells.
2Ease of manufacture
If the channel voltage of the selected cell is not elevated before programming, then the programming process is simple, but the selected cell may be unintentionally programmed and data accuracy deteriorates
Solution Approach 1:
The patent introduces a preliminary action step that raises the channel voltage before the main programming operation. This adds a preparatory phase to the programming process, ensuring the selected cell is properly inhibited before programming begins, thus maintaining data accuracy without significantly complicating the overall process.
3Ease of operation
If the string select transistor is turned on during programming to enable cell selection, then programming control is improved, but the selected cell may be unintentionally programmed due to insufficient channel voltage
Solution Approach 1:
The patent applies preliminary action by raising the channel voltage to a high level before turning on the string select transistor and applying programming voltage. This ensures that even when the transistor is turned on during programming, the selected cell remains properly inhibited due to the pre-established high channel voltage, maintaining inhibition effectiveness.
Solution Approach 2:
The patent applies beforehand cushioning by pre-raising the channel voltage to create a protective voltage margin before the programming operation. This voltage cushion ensures that when the string select transistor is activated during programming, the selected cell is already protected against unintended programming, maintaining reliability.
Data Source
AI summary
A method of programming a NAND flash memory cell string. The method includes a pre-boost stage configured to elevate channel voltage of a selected memory cell, and a boost stage is introduced after the pre-boost stage. The pre-boost stage has at least the following steps of biasing a bit line to a first voltage, biasing a string select transistor to a second voltage; and ramping down the string select transistor to the first voltage. In particular, the second voltage is higher than the first voltage.