Non-Volatile Memory Pass Voltage Compensation for Read Accuracy
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Solution Overview
Problem
Non-volatile memory devices, particularly NAND flash memory, face challenges in read accuracy due to program disturb effects, where the threshold voltage of unselected storage elements is shifted during programming, affecting the accuracy of reading selected storage elements, especially in multi-level devices with closely spaced threshold voltage ranges.
Innovation Solution
The method involves applying different pass voltages during program-verify and read operations by using a higher verify pass voltage and a lower read pass voltage to reduce program disturb and improve read accuracy, optimizing the voltage difference based on specific memory device metrics to maintain accurate data states across storage elements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a common pass voltage is used during both program-verify and read operations, then the device operation is simplified, but read accuracy deteriorates due to program disturb effects shifting threshold voltage
Solution Approach 1:
The patent applies different pass voltage levels for program-verify and read operations. Specifically, a first pass voltage level is used during program-verify operations, and a second, lower pass voltage level is used during read operations. This parameter change compensates for front-pattern-effect and program disturb, thereby improving read accuracy without significantly complicating device operation.
2Quantity of substance
If threshold voltage ranges are placed close together in multi-level devices to increase storage density, then storage capacity is improved, but read accuracy deteriorates due to reduced margin for error
Solution Approach 1:
The patent uses different pass voltage levels for program-verify and read operations to compensate for threshold voltage shifts. By applying a lower pass voltage during read operations, the patent restores the original threshold voltage distribution, effectively increasing the margin between closely-spaced threshold voltage ranges and improving read accuracy while maintaining high storage density.
3Reliability
If program voltage is applied to program storage elements, then data storage is achieved, but program disturb effects occur that shift threshold voltage of unselected elements
Solution Approach 1:
The patent applies different pass voltage levels depending on the operation type. During program-verify operations, a first pass voltage level is used that is appropriate for programming. During subsequent read operations, a second, lower pass voltage level is used to compensate for and restore threshold voltage shifts caused by program disturb, thereby maintaining data storage reliability while reducing harmful effects.
Data Source
AI summary
Optimized verify and read pass voltages are obtained to improve read accuracy in a non-volatile storage device. The optimized voltages account for changes in unselected storage element resistance when the storage elements become programmed. This change in resistance is referred to as a front pattern effect. In one approach, the verify pass voltage is higher than the read pass voltage, and a common verify voltage is applied on the source and drain sides of a selected word line. In other approaches, different verify pass voltages are applied on the source and drain sides of the selected word line. An optimization process can include determining a metric for different sets of verify and read pass voltages. The metric can indicate threshold voltage width, read errors or a decoding time or number of iterations of an ECC decoding engine.