NAND Flash Defect Screening for Program Reliability
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Solution Overview
Problem
Non-volatile semiconductor memory devices, such as NAND flash memory, often contain defective storage elements due to manufacturing variations, leading to reliability issues and increased program disturb, which cannot be corrected by error correction codes, causing errors during programming and erasing processes.
Innovation Solution
A method to detect defective NAND strings and modify programming or erasing processes by identifying and storing status bits to prevent programming of defective elements, adjusting verify test conditions based on defective string counts, and using modified write data to avoid errors, thereby improving reliability and yield.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional programming methods are used without defect detection, then the programming process is simple and fast, but defective storage elements cause reliability issues and cannot be corrected by error correction codes
Solution Approach 1:
The patent applies preliminary action by performing defect detection before the actual programming operation. The system first identifies defective NAND strings through read operations and stores their locations in status bits, then uses this information to modify the programming process by skipping defective locations. This prevents defective elements from causing reliability issues while maintaining programming efficiency.
Solution Approach 2:
The patent implements feedback by using the defect detection results to modify the programming operation. The status bits containing defect information are fed back to the control logic, which then adjusts the programming process accordingly - specifically, the control logic uses the status bits to determine which NAND strings should be skipped during programming, creating a closed-loop system that adapts to the actual device state.
2Productivity
If defective storage elements are attempted to be programmed, then complete programming coverage is achieved, but program disturb increases and additional errors occur
Solution Approach 1:
The patent applies the extraction principle by identifying and separating defective NAND strings from the programming operation. Through defect detection, the system extracts information about which strings are defective and stores this in status bits. During programming, the control logic then excludes these defective strings from the programming process, effectively removing the harmful elements that would cause program disturb and additional errors.
3Reliability
If error correction codes are used, then some errors can be corrected, but defects in NAND strings cannot be corrected and cause errors during programming and erasing
Solution Approach 1:
The patent extends the preliminary action concept by performing defect detection before programming or erasing operations. The system identifies defective NAND strings and stores their locations in status bits, then uses this information to modify subsequent programming and erasing operations. This allows the system to adapt to the actual device state and handle defects that ECC cannot correct.
Solution Approach 2:
The patent applies dynamics by making the programming and erasing process adaptive based on defect detection results. The control logic dynamically adjusts the programming/erasing operation based on the status bits containing defect information. This dynamic adaptation allows the system to handle different defect configurations and provides versatility in dealing with various failure modes that static ECC cannot address.
Data Source
AI summary
A programming process evaluates NAND strings of a block to detect a defective NAND string, e.g., a NAND string with a defective storage element. Status bits can be stored which identify the defective NAND string. Original data which is to be written in the NAND string is modified so that programming of the defective NAND string does not occur. For example, a bit of write data which requires a storage element in the defective NAND string to be programmed to a higher data state is modified (e.g., flipped) so that no programming of the storage element is required. Subsequently, when a read operation is performed, the flipped bits are flipped back to their original value, such as by using error correction code decoding. In an erase process, a count of defective NAND strings is made and used to adjust a pass condition of a verify test.