Non-equal Threshold Voltage Ranges in MLC NAND Flash Memory
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Solution Overview
Problem
Traditional solid-state memory devices face inefficiencies in storing and retrieving multiple bits of information due to the need for multiple program and read operations, which become increasingly cumbersome as more bits are stored, as they operate on binary signals rather than utilizing the full range of threshold voltage levels for data representation.
Innovation Solution
The memory device employs non-equal threshold voltage ranges, allowing for the storage and retrieval of data as analog signals representing complete bit patterns, rather than discrete bits, enabling a single operation to return multiple bits of information, such as eight bits with a single read operation, by using sample and hold circuitry to manage and process these signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple program and read operations are used to store and retrieve multiple bits of information, then data storage capacity increases, but operation time and complexity increase significantly
Solution Approach 1:
The patent changes the parameter of threshold voltage range sizing from equal to non-equal, where lower threshold voltage ranges are made larger and higher ranges are made smaller. This parameter change allows the system to store multiple bits per cell while using fewer read operations by optimizing the distribution of voltage states, thereby reducing operation time while maintaining storage capacity.
Solution Approach 2:
The patent implements dynamic read operations where the number and positioning of read voltage levels are adjusted based on the specific data retrieval needs. Instead of always performing multiple fixed read operations, the system dynamically determines the optimal number of reads required, reducing unnecessary operations and improving overall access speed for multi-bit storage.
2Ease of manufacture
If equal threshold voltage ranges are used for all data values, then simplicity of implementation is maintained, but program disturb and data retention issues increase
Solution Approach 1:
The patent applies local quality by making each threshold voltage range have different sizes tailored to its specific requirements. Lower threshold voltage ranges are made larger to provide better noise margins and stability, while higher ranges are made smaller. This localized optimization of range sizes improves data retention and reduces program disturb without requiring complete redesign of the entire system.
3Quantity of substance
If more bits are stored per cell, then storage density increases, but the number of required read operations increases making the process cumbersome
Solution Approach 1:
The patent applies partial action by performing only the necessary number of read operations to retrieve multi-bit data, rather than always performing a fixed maximum number of reads. The system determines the optimal number of reads based on the stored data pattern, performing fewer reads when possible, which simplifies the operation process while maintaining high storage density.
Data Source
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AI summary
Memory devices adapted to process and generate analog data signals representative of data values of two or more bits of information facilitate increases in data transfer rates relative to devices processing and generating only binary data signals indicative of individual bits. Programming of such memory devices includes programming to a target threshold voltage range representative of the desired bit pattern. Reading such memory devices includes generating an analog data signal indicative of a threshold voltage of a target memory cell. Threshold voltage ranges of the memory cells have a larger range size for ranges that include lower threshold voltages and a smaller range size for ranges that include higher threshold voltages since program disturb is lower at higher threshold voltages.