NAND Flash Read Level Control via Distribution Filtering
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Solution Overview
Problem
Semiconductor memory devices, particularly NAND-type flash memory, face challenges in reducing data reading errors due to noise sources like thermal and Random Telegraph Signal (RTS) noises, and difficulties in setting an optimal read level due to multiple local minima in threshold voltage distributions, leading to reduced data reliability and durability.
Innovation Solution
A storage device and method that includes a read level controlling unit with a filter to measure and filter the distribution of memory cells, removing local minima and setting a new read level based on the filtered distribution, using a Finite Impulse Response (FIR) filter and modifying the distribution with saturation, interpolation, or mean values to improve data reading accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple read levels are used to read data from multi-bit memory cells, then data storage capacity is improved, but data reading errors increase due to noise and difficulty in setting optimal read levels
Solution Approach 1:
The patent applies preliminary action by measuring the threshold voltage distribution of memory cells before setting read levels. The controller performs an initial measurement of the voltage distribution across all memory cells, then uses this measured distribution to determine optimal read levels that account for actual device characteristics and noise conditions, rather than using fixed predetermined levels
Solution Approach 2:
The patent implements feedback by using the measured threshold voltage distribution to adjust and optimize read levels. The controller continuously monitors the distribution characteristics and modifies read level settings based on this feedback, creating a closed-loop system that adapts to actual memory cell behavior and minimizes reading errors
2Measurement precision
If read levels are set based on measured distribution of memory cells, then data reading accuracy is improved, but device complexity increases due to additional measurement and control operations
Solution Approach 1:
The patent applies universality by designing the controller to perform multiple functions: it既能 measure threshold voltage distributions,既能 analyze distribution characteristics,既能 determine optimal read levels,又能 execute read operations with adjusted levels. This multi-functional approach consolidates complexity into a single controller unit rather than requiring separate dedicated circuits for each function
Solution Approach 2:
The patent changes parameters by adjusting read level voltages based on measured distribution characteristics. Instead of fixing read levels at predetermined values, the system dynamically modifies voltage parameters according to actual memory cell behavior, transforming static operation into adaptive parameter-based control
3Ease of operation
If filtering is applied to measured distribution to remove local minima, then read level setting becomes easier, but processing time increases
Solution Approach 1:
The patent extracts and removes problematic local minima from the measured threshold voltage distribution through filtering operations. By eliminating these spurious local minima that complicate read level determination, the system simplifies the distribution into a form where optimal read levels can be more easily identified, typically leaving only the significant minima that correspond to actual data states
Data Source
AI summary
The storage device includes a storage unit configured to store data, an error controlling unit configured to correct an error of the data read out from the storage unit according to at least one read level, and a read level controlling unit configured to control the at least one read level when the error is uncorrectable. The read level controlling unit is configured to measure a distribution of memory cells of the storage unit, configured to filter the measured distribution, and configured to reset the at least one read level based on the filtered distribution.