Nonvolatile Memory Read Voltage Compensation
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Solution Overview
Problem
Nonvolatile memory devices face reliability issues due to electron leakage in floating gates, caused by external stimuli and wear, leading to decreased threshold voltage and read errors, especially in multi-level cells where small changes in threshold voltage can cause significant errors.
Innovation Solution
A nonvolatile memory device with a voltage generator that supplies specific read voltages to word lines based on the program/erase cycle or elapsed time of a memory block, adjusting the voltages to maintain accurate threshold voltages and prevent errors during read operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional read voltage is applied to all word lines during read operation, then the read operation is simple to perform, but threshold voltage changes due to electron leakage cause read errors and degrade reliability
Solution Approach 1:
The patent applies dynamic voltage adjustment by changing the read voltage level applied to adjacent word lines based on the program/erase cycle count or elapsed time. The voltage generator dynamically selects between a first read voltage (for memory blocks with fewer cycles/shorter time) and a second read voltage (for memory blocks with more cycles/longer time), making the system adaptive to degradation rather than static
Solution Approach 2:
The patent changes the voltage parameter of the read signal applied to adjacent word lines based on memory block usage history. By monitoring program/erase cycle count or elapsed time, the system adjusts the read voltage level to compensate for threshold voltage drift, directly addressing reliability issues through parameter modification
2Reliability
If the read voltage level is increased to compensate for threshold voltage decrease, then read accuracy improves, but power consumption increases
Solution Approach 1:
The patent applies different voltage levels to different groups of word lines based on their usage characteristics. Memory blocks with higher program/erase cycle counts or longer elapsed times receive a second read voltage level, while others receive a first read voltage level. This localized adjustment compensates for threshold voltage changes only where needed, avoiding unnecessary power consumption in unaffected areas
Solution Approach 2:
The system monitors program/erase cycle count or elapsed time and adjusts the read voltage parameter accordingly. By changing the voltage level based on actual degradation indicators rather than applying a fixed high voltage, the system achieves reliable reads while minimizing power consumption
3Reliability
If a single read voltage is used for all memory blocks, then the voltage generation is simple, but it cannot compensate for varying threshold voltage changes across different memory blocks
Solution Approach 1:
The voltage generator dynamically adjusts read voltages based on real-time or historical data about memory block usage (program/erase cycles or elapsed time). This dynamic adaptation allows the system to compensate for varying threshold voltage changes across different memory blocks without requiring complex per-cell calibration
Solution Approach 2:
The patent segments the word lines into different groups based on their program/erase cycle counts or elapsed times, applying different read voltage levels to each segment. This segmentation allows targeted compensation for threshold voltage changes in specific memory blocks while maintaining simpler voltage generation for other blocks
Data Source
AI summary
A nonvolatile memory device has improved reliability by compensating a threshold voltage of a flash memory cell. A nonvolatile memory device includes a memory cell array and a voltage generator for supplying a select read voltage to a select word line and an unselect read voltage to unselected word lines when a read operation is performed, and supplying a verify voltage to a select word line and the unselect read voltage to unselected word lines when a program operation is performed. The voltage generator supplies a first unselect read voltage to at least one between an upper word line and a lower word line adjacent to the select word line when the program operation is performed, and supplies a second unselected read voltage to at least one between the upper word line and the lower word line adjacent to the select word line when the read operation is performed.