NAND Read Pass Voltage Control for Disturb Reduction
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Solution Overview
Problem
Existing non-volatile semiconductor memory devices, such as EEPROM and flash memory, face the issue of 'read disturb' where unselected memory cells can be inadvertently programmed during read operations due to the application of read pass voltages to unselected word lines, leading to unintended changes in threshold voltage and data loss.
Innovation Solution
The solution involves applying a first set of lower read pass voltages to unselected data memory cells at the ends of a NAND string and a second set of higher read pass voltages to unselected data memory cells between the ends and on both sides of the selected memory cell, with a read compare voltage applied to the selected memory cell, to minimize the risk of read disturb by controlling the voltages applied to unselected memory cells during read operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If read pass voltage is applied to unselected word lines to enable reading operations, then reading capability is achieved, but read disturb occurs causing unintended programming of unselected memory cells
Solution Approach 1:
The patent applies different read pass voltages to different groups of unselected word lines based on their position relative to the selected memory cell. Specifically, a first read pass voltage is applied to unselected word lines in a first group, and a second read pass voltage (different from the first) is applied to unselected word lines in a second group. This local differentiation of voltage levels reduces read disturb effects while maintaining reading capability.
2Measurement precision
If higher read pass voltage is applied to unselected word lines, then better signal strength for reading is achieved, but increased read disturb causes more unintended programming
Solution Approach 1:
The patent implements local quality by assigning different voltage levels to different spatial groups of unselected word lines. The first read pass voltage is applied to unselected word lines in the first group, while the second read pass voltage is applied to unselected word lines in the second group. This localized voltage differentiation optimizes signal strength for reading while minimizing read disturb in specific regions.
Solution Approach 2:
The patent segments the unselected word lines into at least two distinct groups (first group and second group) and applies different read pass voltages to each segment. This segmentation allows independent optimization of voltage levels for different regions, achieving better signal strength where needed while reducing read disturb in other regions.
Data Source
AI summary
A apparatus and process for reading data from non-volatile storage includes applying a read compare signal to a selected data memory cell of a NAND string, applying a first set of one or more read pass voltages to unselected data memory cells at both ends of the NAND string and applying a second set of one or more read pass voltages to unselected data memory cells between both ends of the NAND string and on both sides of the selected data memory cell. The second set of one or more read pass voltages are all higher than the first set of one or more read pass voltages.