Adaptive Sense Time for Memory Cell Threshold Voltage Stability
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Solution Overview
Problem
As semiconductor memory cells undergo program-erase cycles, the threshold voltage distribution widens due to increased subthreshold swing, leading to issues like over-programming, and existing methods struggle to maintain an optimal Quick Pass Write window across varying cycling times and die-to-die variations.
Innovation Solution
An adaptive sense time method is implemented, where sense times are dynamically adjusted based on cycling counts to maintain a desired verify voltage window by using an adaptive sense time table that increments or decrements sense times, and die-level dynamic sense tables are generated to account for fabrication variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fixed sense time is used for verify operations, then the initial QPW window works efficiently for fresh cells, but the QPW window shifts after many program cycles causing over-programming
Solution Approach 1:
The patent applies dynamics by making the sense time variable rather than fixed. The sense time is dynamically adjusted based on the cycling count of the memory block, allowing the verify operation to adapt to the changing characteristics of the memory cells as they undergo program-erase cycles. This resolves the contradiction by enabling the system to maintain programming accuracy across different cycling states.
Solution Approach 2:
The patent changes the sense time parameter based on the cycling count. By modifying the sense time parameter according to the number of program-erase cycles a block has undergone, the system compensates for the shifting QPW window and maintains accurate verify operations throughout the memory device's life.
2Reliability
If the sense time is increased to account for cycling variations, then the QPW window stability improves, but the programming time increases
Solution Approach 1:
The sense time is dynamically adjusted based on cycling count rather than being uniformly increased. This allows the system to optimize the balance between QPW window stability and programming speed by applying longer sense times only when necessary (at higher cycling counts) and maintaining shorter sense times for fresh blocks.
Solution Approach 2:
The sense time parameter is changed selectively based on the cycling count of each block. This parameter adaptation allows the system to maintain QPW window stability when needed while minimizing the impact on programming time for blocks that don't require extended sense times.
3Measurement precision
If different sense times are used for different verify voltages, then programming precision is maintained, but the system complexity increases
Solution Approach 1:
The patent segments the memory device into multiple blocks, each with its own cycling count and corresponding sense time. This segmentation allows precise control of verify operations at the block level while managing complexity through localized adjustments rather than system-wide changes.
Solution Approach 2:
The system uses feedback from the cycling count to automatically adjust the sense time for each block. This feedback mechanism maintains verify voltage precision by continuously adapting to the state of each block while simplifying management through automated adjustment based on stored cycling information.
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 stabilizes the threshold voltage distribution throughout the memory device's life, preventing over-programming and ensuring optimal performance by maintaining the Quick Pass Write window, even as cycling numbers increase and die variations occur.
Implementation Method 1
the threshold voltage ("Vt") of a memory cell can be determined by controlling the time it takes for the sense node to discharge through a bit line coupled to the memory cell
Data Source
AI summary
A memory device with adaptive sense time tables is disclosed. In order to maintain a desired (initial or preset) threshold voltage distribution, the sense time is adjusted as the program-erase cycle count increases. The program-erase cycle process tends to wear down memory cells, causing the QPW window to expand and the threshold voltage to widen. However, by adjusting (i.e., reducing) the sense time for increased program-erase cycles, the QPW window and the threshold voltage can be at least substantially maintained. Additionally, systems and methods for adjusting sense time based on die-to-die variations are also disclosed.