Edge Wordline Read Disturb Reduction in Flash Memory
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Non-volatile semiconductor memory devices, such as EEPROM and flash memory, face the issue of 'read disturb' where unselected word lines during read operations can inadvertently program adjacent memory cells due to over-voltage, leading to unintended changes in threshold voltage and data loss.
Innovation Solution
The system reduces read disturb by using lower initial program pulses and step sizes for edge word lines and applying a higher bias on neighboring word lines during read operations, while maintaining default voltages for other word lines, thereby minimizing voltage-induced programming of unselected memory cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If unselected word lines are raised to read pass voltage during read operations, then memory cells on unselected word lines operate as pass gates enabling read functionality, but voltage-induced electron injection into floating gates causes read disturb and unintended threshold voltage changes
Solution Approach 1:
The patent applies different pass voltages to different groups of unselected word lines based on their position relative to the selected word line. Neighbor word lines receive a first pass voltage (e.g., 6V), while other unselected word lines receive a second, lower pass voltage (e.g., 4V). This local differentiation reduces read disturb on edge word lines that are most susceptible to voltage-induced programming, while maintaining sufficient conduction for read operations on interior word lines.
2Reliability
If higher pass voltage is applied to reduce read disturb on edge word lines, then threshold voltage stability improves, but programming precision deteriorates due to wider threshold voltage distributions
Solution Approach 1:
The patent modifies programming parameters specifically for edge word lines to compensate for the reduced pass voltage. This includes applying higher program voltages and adjusting pulse widths for edge word lines during programming operations. These parameter changes ensure that edge word lines achieve the same programming precision as interior word lines, despite receiving lower pass voltage during read operations to reduce read disturb.
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 results in tighter threshold voltage distributions and reduced read disturb, ensuring data integrity by preventing unintended programming during read operations.
Implementation Method 1
When programming an EEPROM or flash memory device, typically a program voltage is applied to the control gate and the bit line is grounded. Electrons from the channel are injected into the floating gate.
Implementation Method 2
Because the unselected word lines receive the pass voltage, memory cells along unselected word lines during a read operation will receive a voltage on their control gate which over many read operations may cause electrons to be injected into their floating gate, thereby, raising the threshold voltage of those memory cells. This effect is called Read Disturb.
Data Source
AI summary
A system for reducing read disturb on edge word lines in non-volatile storage is disclosed. In one embodiment, the memory cells on edge word lines are programmed using a series of pulses that have an initial magnitude and step size between pulses that are lower than for memory cells on word lines that are not edge word lines. Additionally, when reading memory cells on word lines that are not edge word lines, the edge word lines receive a lower pass voltage than the default pass voltage applied to other unselected word lines. In another embodiment. the system applies a higher than normal bias on a neighboring word lines when reading memory cells on an edge word line.