3D NOR String Source Segmentation for Leakage-Limited Reads
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Solution Overview
Problem
In 3-dimensional semiconductor structures with NOR-type memory strings, the cumulative off-state source-drain leakage current from numerous thin-film transistors interferes with read current, necessitating shorter NOR strings, which increases the number of sense amplifiers and decoders, thereby raising chip costs.
Innovation Solution
The implementation of segmented shared source regions within NOR strings, where each thin-film storage transistor is grouped into segments with electrically isolated source line segments, allowing only the addressed segment to contribute to leakage current, while maintaining a continuous bit line for efficient data transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If NOR string length is increased to reduce the number of sense amplifiers and decoders, then chip cost decreases, but cumulative off-state source-drain leakage current increases and interferes with read current
Solution Approach 1:
The NOR string is divided into multiple segments by introducing isolation regions that electrically separate the source line into multiple isolated segments. Each segment contains a subset of the total TFTs, and only the segment containing the addressed TFT becomes conductive during read operations. This segmentation reduces the cumulative leakage current from thousands of TFTs to a manageable level from a smaller subset, while still allowing long NOR strings to maintain reduced decoder complexity.
2Object-generated harmful factors
If NOR string length is decreased to reduce leakage current, then read current interference decreases, but the number of sense amplifiers and decoders increases, raising chip cost
Solution Approach 1:
By segmenting the source line into isolated regions, the patent enables long NOR strings to be divided into manageable segments that can be independently controlled. This allows the system to maintain long string lengths (reducing decoder complexity) while keeping each segment's leakage current manageable through selective activation of only the relevant segment during read operations.
3Productivity
If segmented source line structure is implemented, then leakage current is reduced and pre-charge speed is improved, but manufacturing complexity increases
Solution Approach 1:
The segmentation is achieved by introducing isolation regions (such as oxide or nitride layers) between adjacent source line segments. These isolation regions are formed using standard semiconductor fabrication techniques including selective deposition and etching processes. The segmented structure allows independent pre-charge of each source line segment, significantly improving pre-charge speed by reducing the capacitive load, while the manufacturing complexity remains manageable through integration with existing fabrication processes.
Data Source
AI summary
A NOR string includes a number of individually addressable thin-film storage transistors sharing a bit line, with the individually addressable thin-film transistors further grouped into a predetermined number of segments. In each segment, the thin-film storage transistors of the segment share a source line segment, which is electrically isolated from other source line segments in the other segments within the NOR string. The NOR string may be formed along an active strip of semiconductor layers provided above and parallel a surface of a semiconductor substrate, with each active strip including first and second semiconductor sublayers of a first conductivity and a third semiconductor sublayer of a second conductivity, wherein the shared bit line and each source line segment are formed in the first and second semiconductor sublayers, respectively.