Segmented Memory References for Column Loading and Read Margins
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Solution Overview
Problem
Memory devices such as MRAM, PCRAM, and RRAM face challenges with small read margins due to column loading effects like bit line and source line resistance variations, leading to yield loss and higher read error rates in global references, while local references incur significant area overhead.
Innovation Solution
A segmented reference structure is implemented, where one reference cell set is shared by a segment of word lines, typically 16 to 64 memory cell word lines, to closely track memory cells and reduce trim code, thereby improving read margins and reducing device area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If global reference structure is used, then device area is reduced, but read margin deteriorates due to column loading effects
Solution Approach 1:
The memory array is divided into multiple segments, each with its own reference cells. This segmentation allows reference cells to be placed closer to memory cells within each segment, reducing the impact of column loading effects while maintaining reasonable area overhead. The segmented approach tracks local variations in bit line and source line resistance better than a single global reference.
Solution Approach 2:
Different segments of the memory array are served by different reference cell sets, allowing each segment to have reference values optimized for its local characteristics. This local quality approach ensures that reference cells experience similar column loading conditions as the memory cells they reference, improving read margin without requiring every memory cell to have its own dedicated reference.
2Reliability
If local reference structure is used, then read margin is improved, but device area increases significantly
Solution Approach 1:
Instead of providing dedicated reference cells for every memory cell (full local reference), the memory array is segmented into groups where each segment shares a common reference cell set. This segmentation reduces the total number of reference cells needed while still maintaining local reference benefits within each segment, thereby reducing device area compared to full local reference structures.
Solution Approach 2:
Each reference cell set serves multiple memory cells within its segment, making the reference cells universal for that group rather than dedicated to individual cells. This multi-functionality allows the same reference cells to be used for reading multiple different memory cells, reducing the overall reference cell count and device area while maintaining adequate read margin.
3Reliability
If reference cell density is increased to track column loading effects, then read margin is improved, but device area increases
Solution Approach 1:
The memory array is divided into segments with a controlled number of memory cells per segment (e.g., 4 to 8 word lines). This segmentation achieves adequate reference cell density for tracking column loading effects within each segment while limiting the total number of reference cells needed, as each reference cell set serves only its local segment rather than the entire array.
Data Source
AI summary
A semiconductor device including at least one sense amplifier, a first memory array that includes first segments of first memory cells situated on a first side of the at least one sense amplifier, a second memory array that includes second segments of second memory cells situated on a second side of the at least one sense amplifier, first reference cells connected to first reference word lines in the first memory array for sensing data from the second memory cells and second reference cells connected to second reference word lines in the second memory array for sensing data from the first memory cells. The first reference cells connected to one of the first reference word lines for sensing data from one of the second segments and the first reference cells connected to another one of the first reference word lines for sensing data from another one of the second segments.