Flash Memory Program Operation Narrowing Threshold Voltage Distribution
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Solution Overview
Problem
NAND flash memory devices face errors due to wide threshold voltage distributions when storing 2-bit data, leading to overlapping voltage levels and potential data separation issues, which limits the ability to narrow the distribution width during program operations.
Innovation Solution
A method is introduced to adjust the program operation conditions based on the threshold voltage levels of memory cells, involving a first program operation followed by a verifying operation to separate cells into different categories, and a second program operation that increases threshold voltages inversely proportional to their levels, using specific bit line voltages and program voltages to minimize distribution width.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If a memory cell stores 2 bit data with four threshold voltage levels, then data storage capacity is improved, but threshold voltage distribution width increases causing overlapping levels and data separation errors
Solution Approach 1:
The patent segments the program operation into multiple stages: a first program operation to initially program memory cells, a verifying operation to classify cells by threshold voltage levels, and a second program operation to further program only those cells needing adjustment. This segmentation allows differential treatment of cells based on their individual threshold voltage levels, narrowing the overall distribution width while maintaining 2-bit storage capacity.
Solution Approach 2:
The patent applies local quality by making the program operation conditions dependent on the specific threshold voltage level of each memory cell. Cells are divided into different groups (first, second, third, fourth memory cells) with different threshold voltage ranges, and different program voltages are applied to different groups. This localized approach ensures each cell receives the appropriate programming strength to achieve uniform threshold voltage distribution.
2Ease of operation
If uniform program operation conditions are applied to all memory cells, then operation simplicity is maintained, but threshold voltage distribution width cannot be sufficiently narrowed
Solution Approach 1:
The patent introduces dynamics into the program operation by making program conditions variable rather than static. The verifying operation dynamically determines which memory cells require further programming based on their threshold voltage levels. The second program operation is selectively applied only to specific cell groups, and program voltages are dynamically adjusted based on cell classification. This dynamic approach narrows threshold voltage distribution while adding manageable complexity.
3Manufacturing precision
If multiple verifying operations are performed to classify memory cells by threshold voltage levels, then threshold voltage distribution precision is improved, but operation time increases
Solution Approach 1:
The patent applies preliminary action by performing a first program operation that programs all memory cells to a preliminary threshold voltage level before the verifying operation. This preliminary programming ensures all cells have a baseline threshold voltage, reducing the range of variation that needs to be corrected in subsequent operations. The verifying operation then only needs to identify cells that need adjustment, rather than programming all cells from scratch, thereby reducing total operation time.
Data Source
AI summary
A method of operating a flash memory device wherein the width of threshold voltage distribution of memory cells is adjusted by setting different conditions of a program operation in accordance with levels of threshold voltages of the memory cells. As a result, width of the threshold voltage distribution of memory cells may be narrowed.