Flash Memory Programming Start Bias Adjustment for Over-Programming
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Solution Overview
Problem
The existing Incremental Step Pulse Programming (ISPP) scheme for NAND flash memory devices fails to prevent widening or rightward movement of cell threshold voltage distribution, leading to over-programming and device failure due to parasitic effects and cycling.
Innovation Solution
A method is introduced to set a programming start bias by calculating the difference between the detected maximum threshold voltage level and a target maximum threshold voltage level, and adding this difference to a first programming voltage, allowing for incremental bias adjustments during programming to maintain a desired distribution and prevent over-programming.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the ISPP scheme is used for programming, then over-programming is suppressed, but the cell threshold voltage distribution widens or moves to the right due to parasitic effects and cycling
Solution Approach 1:
The patent performs pre-programming before the main ISPP programming process to intentionally shift the threshold voltage distribution to the left. This preliminary action compensates for the expected widening and rightward movement that will occur during subsequent programming cycles, ensuring the final distribution remains within acceptable bounds and prevents over-programming.
Solution Approach 2:
The patent dynamically adjusts the programming start bias voltage based on the detected maximum threshold voltage level from pre-programming. By calculating the difference between the detected maximum level and the target maximum level, the system modifies the start bias parameter to compensate for distribution widening, thereby maintaining precise control over the threshold voltage distribution throughout the programming process.
2Reliability
If the ISPP scheme is used for programming, then programming verification is performed between periods, but programming time increases due to multiple bias application cycles
Solution Approach 1:
The patent performs pre-programming with a first bias voltage before the main ISPP programming sequence. This preliminary action prepares the memory cells in advance, establishing an initial threshold voltage distribution that reduces the number of incremental bias steps needed during the main programming phase, thereby shortening the overall programming time while maintaining verification reliability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively reduces the occurrence of over-programming and shortens programming time by ensuring a sufficient margin in the cell threshold voltage distribution, even after cycling, thereby enhancing the reliability of the flash memory device.
Implementation Method 1
An operation for programming and/or erasing a NAND flash memory device is performed through tunneling such as Flower-Nordheim (F-N) tunneling.
Data Source
AI summary
A method for setting a programming start bias for a flash memory device to perform a programming operation is provided. First, the method performs pre-programming to change a threshold voltage distribution of a selected transistor using a first programming voltage and detects the maximum threshold voltage level of the changed threshold voltage distribution. The method then calculates the difference between the detected maximum threshold voltage level and a target maximum threshold voltage level and sets a start bias to a voltage obtained by adding the calculated difference to the first programming voltage.