Pre-read Voltage Pulse for NAND Read Error Handling
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Solution Overview
Problem
Memory cells in NAND strings experience variability in threshold voltage (Vth) levels due to coupling up states of word lines during read operations, leading to errors as it is unclear if cells are in the first or second read condition, causing downshifts or upshifts in Vth levels over time.
Innovation Solution
A pre-read operation is performed to transition memory cells from the first read condition to the second read condition by maintaining specified voltages on selected and unselected word lines, ensuring Vth levels are in a predictable range during read operations, with adjacent unselected word lines receiving increased voltage to compensate for coupling reductions and minimize read disturb.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If read operations are performed without pre-read operation, then read operation speed is maintained, but Vth level variability causes read errors
Solution Approach 1:
A pre-read operation is performed before the actual read operation to transition memory cells from a first read condition to a second read condition. This preliminary action stabilizes Vth levels by maintaining specified voltages on selected and unselected word lines, ensuring cells are in a predictable state for accurate reading.
2Reliability
If word line voltages are not maintained during pre-read, then power consumption is reduced, but Vth downshifts cause read errors
Solution Approach 1:
Different voltage levels are applied to different word lines during the pre-read operation. Selected word lines maintain a first voltage level while unselected word lines maintain a second voltage level. This localized voltage application stabilizes Vth levels in the selected memory cells without unnecessarily consuming power in unselected cells.
3Reliability
If adjacent unselected word lines receive standard voltage, then device complexity is minimized, but coupling reductions cause Vth upshifts
Solution Approach 1:
The voltage parameter for adjacent unselected word lines is changed to a higher level during the pre-read operation to compensate for coupling reductions. This parameter change counteracts the Vth upshift effect caused by reduced coupling, stabilizing the threshold voltage levels without requiring complex control mechanisms.
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
The pre-read operation stabilizes Vth levels, reducing read errors by ensuring memory cells are in a consistent conductive or non-conductive state, thereby improving the accuracy of data retrieval and reducing the likelihood of read disturb.
Implementation Method 1
Memory cells in NAND strings experience variability in threshold voltage (Vth) levels due to coupling up states of word lines during read operations
Data Source
AI summary
Apparatuses and techniques are described for performing a pre-read operation in preparation for a read operation in a memory device. The pre-read operation transitions the memory cells from a first read condition to a second read condition so that their threshold voltages will be in a desired, predictable range when the read occurs. The pre-read operation can involve maintaining voltages on a selected word line and unselected word lines at specified levels and for a specified duration which is relatively long compared to a duration of the read operation. The word line voltages, in combination with bit line and source line voltages, provide the channels of a NAND string in a conductive state and gradually transitions the memory cells to the second read condition.