Pre-program Threshold Voltage Detection for Select Gate Transistors
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Solution Overview
Problem
Semiconductor memory devices face challenges in maintaining optimal threshold voltages for select gate transistors, which can lead to operational failures due to factors like read disturb stress, data retention loss, and process variations, affecting the memory device's ability to correctly program and read data.
Innovation Solution
The evaluation of threshold voltage distributions for select gate transistors before executing commands, detecting deviations from acceptable ranges, and adjusting control gate voltages to optimize transistor operation, thereby preventing failures and ensuring reliable data storage and retrieval.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If select gate transistors are operated without pre-program threshold voltage detection, then the memory device can execute commands faster, but operational failures occur due to threshold voltage deviations caused by read disturb stress, data retention loss, and process variations
Solution Approach 1:
The patent applies preliminary action by detecting the threshold voltage distribution of select gate transistors before executing program or read commands. The controller evaluates whether the threshold voltages are within acceptable ranges and performs pre-program operations if deviations are detected, ensuring reliable operation before actual data operations begin.
2Reliability
If threshold voltage evaluation is performed before every command execution, then operational failures are prevented, but the memory device operation becomes slower
Solution Approach 1:
The patent implements feedback by continuously monitoring the threshold voltage distribution of select gate transistors and using this information to adjust control gate voltages dynamically. The controller receives feedback on threshold voltage deviations and modifies operating parameters to maintain optimal performance, creating a closed-loop control system.
Solution Approach 2:
The patent applies partial action by performing threshold voltage evaluation selectively rather than before every command. The controller determines whether evaluation is necessary based on detected deviations and performs pre-program operations only when threshold voltages fall outside acceptable ranges, avoiding unnecessary overhead for normal operations.
3Reliability
If control gate voltages are adjusted based on threshold voltage detection, then transistor operation is optimized, but the control circuit complexity increases
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting control gate voltages based on detected threshold voltage distributions. The controller modifies voltage parameters in response to measured threshold voltages, optimizing transistor operation by matching control signals to actual device characteristics rather than using fixed voltage levels.
Data Source
AI summary
A memory device includes memory cells arranged in NAND strings between select gate transistors. A threshold voltage (Vth) distribution of the select gate transistors is evaluated, such as in response to a program, erase or read command involving a block or sub-block of memory cells. For example, a lower tail and an upper tail of the Vth distribution can be evaluated using read voltages. If the Vth is out-of-range, such as due to read disturb, data retention loss or defects in the memory device, the block or sub-block is marked as being bad and previously-programmed data in the block or sub-block can be copied to another location. If the Vth is in range, the command can be executed. Also, a control gate voltage for the select gate transistors can be set based on a Vth metric which is obtained from the evaluation.