NAND Select Gate Programming via Segmented Threshold Control
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Solution Overview
Problem
Existing non-volatile semiconductor memory devices face challenges in efficiently programming select gates in NAND memory structures, particularly in three-dimensional configurations like BiCS, due to difficulties in setting proper threshold voltages and the impact of unselected select gates on read and programming operations.
Innovation Solution
The memory array structure includes programmable second drain and source select transistors with adjustable threshold levels, connected through first select transistors, allowing for concurrent programming pulses to control gate voltages, enabling precise threshold tuning and independent biasing of end select gates to improve programming efficiency and accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional programming methods are used for select gates in NAND memory, then the memory structure is simpler, but the programming speed and accuracy deteriorate
Solution Approach 1:
The select gate control is segmented into multiple independent control gates (first control gate, second control gate, third control gate) that can be independently programmed. This allows different threshold voltages to be applied to different segments of the select gate, enabling precise control of channel formation and programming operations without requiring complex external control circuitry.
Solution Approach 2:
The select gate threshold voltage is made dynamically adjustable through independent programming of multiple control gates. The threshold voltage can be changed based on operational requirements, allowing the select gate to adapt between different modes (e.g., read mode vs. program mode) without structural modification, thus improving programming speed and accuracy.
2Manufacturing precision
If threshold voltage of select gates is not precisely controlled, then the memory structure is simpler, but the programming accuracy and read operation reliability deteriorate
Solution Approach 1:
The select gates are pre-programmed with specific threshold voltages before memory operations. The first control gate is programmed with a first threshold voltage and the second control gate with a second threshold voltage in advance, ensuring that the select gates are ready for precise control during read and program operations without requiring complex real-time adjustment mechanisms.
Solution Approach 2:
The programming process includes verification steps where the threshold voltage of the select gates is measured and adjusted if necessary. The control circuitry monitors the threshold voltage and applies corrective programming pulses to ensure the select gates achieve the desired threshold voltage levels, thereby improving programming accuracy through feedback control.
3Reliability
If unselected select gates are not properly inhibited, then the control mechanism is simpler, but the read and programming operations are affected by interference
Solution Approach 1:
Different threshold voltages are applied to different control gates of the select gate structure. The first control gate has a first threshold voltage while the second control gate has a second threshold voltage, creating local variations in the select gate characteristics. This allows selective inhibition of unselected gates during read and program operations, preventing interference without requiring complex external control mechanisms.
4Manufacturing precision
If multiple control gates are independently programmed, then the programming accuracy improves, but the programming time and energy consumption increase
Solution Approach 1:
The programming of multiple control gates is merged into a coordinated process where the first control gate and second control gate are programmed simultaneously or in a tightly coupled sequence. The control circuitry applies programming pulses to multiple control gates in a unified manner, reducing the total programming time while maintaining the precision benefits of independent threshold voltage control.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the programming speed, power efficiency, and accuracy of select gates in NAND memory, ensuring proper operation and reducing the impact of unselected select gates on read and programming operations, particularly in dense three-dimensional memory architectures.
Implementation Method 1
A control gate is then provided over the floating gate. The threshold voltage characteristic of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, for a given level of charge on the floating gate, there is a corresponding voltage (threshold) that must be applied to the control gate before the transistor is turned 'on' to permit conduction between its source and drain regions.
Data Source
AI summary
In a non-volatile memory formed according to a NAND-type architecture that has, on one or both ends of the NAND strings, multiple select gates including some with programmable threshold voltages, a structure and corresponding for efficiently programming of such select gates. On the drain side, the end most of multiple drain select transistors is individually controllable and used for biasing purposes while one or more other drain side select gates are collectively programmed to set adjust their threshold voltage. Independently, on the source side, the end most of multiple source select transistors is individually controllable and used for biasing purposes while other source side select gates are collectively programmed to set adjust their threshold voltage.