Image Sensor Redundant Bitline Segmentation for Defect Tolerance
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Solution Overview
Problem
Image sensors with increasing pixel density face challenges in defectivity due to manufacturing defects, particularly in column bitlines, which can lead to open-circuit errors affecting multiple pixels and are difficult to correct without degrading pixel performance or increasing sensor size.
Innovation Solution
The implementation of at least two column bitlines per pixel column, with each bitline connected to the readout amplifier and optionally positioned on different planes, provides redundancy to maintain electrical connectivity and correct for defects, thereby mitigating the impact of open-circuit defects without compromising sensor area or sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single column bitline is used for each pixel column, then the sensor area is minimized and manufacturing is simplified, but open-circuit defects in the bitline will cause column-wide failures affecting multiple pixels
Solution Approach 1:
The single column bitline is segmented into multiple parallel bitlines (first column bitline and second column bitline) that are connected at multiple points along their lengths. This segmentation allows the system to tolerate open-circuit defects in individual bitline segments while maintaining connectivity through alternative paths, thereby improving reliability without requiring complete redundancy of the entire bitline structure.
Solution Approach 2:
The patent introduces a spatial dimension by positioning bitlines on different planes (e.g., different metal layers in CMOS architecture). The first and second column bitlines are disposed on different planes and connected through vertical interconnects, creating a three-dimensional routing structure that provides defect tolerance while maintaining planar integration benefits.
2Reliability
If wider metal traces are used for column bitlines to avoid open circuit defects, then defect immunity is improved, but light reaching the sensor is blocked and pixel performance degrades
Solution Approach 1:
Instead of using a single wide trace, the bitline connection is segmented into multiple narrower parallel traces (first and second column bitlines) that are connected at multiple points. This segmentation allows each individual trace to be narrow enough to allow light transmission while the collective structure provides defect immunity through redundant connection paths.
Solution Approach 2:
The patent implements partial redundancy by providing multiple bitlines that are connected at multiple points along their lengths, rather than providing complete redundancy throughout. This partial action approach achieves sufficient defect immunity for practical purposes while minimizing the additional area and complexity overhead compared to full redundancy.
3Difficulty of detecting and measuring
If additional receivers are placed at the top of the device to detect bitline faults, then defect detection capability is improved, but sensor area increases significantly
Solution Approach 1:
The redundant bitline structure provides self-diagnostic capability through its inherent architecture. When an open-circuit defect occurs in one bitline, the connected-at-multiple-points structure automatically maintains signal integrity through alternative paths, and the defect effectively identifies itself by the failure of one redundant path while others remain functional, eliminating the need for separate detection circuits.
Data Source
AI summary
The image sensor includes a plurality of pixels, arranged in rows and columns to form a pixel array, each pixel having a pixel output. The image sensor includes at least two column bitlines for each pixel column, each column bitline connected to each pixel output in the pixel column and to a common readout amplifier. In one embodiment, the bitlines are not in the same plane and multiple bitlines are positioned one above the other, maximizing pixel efficiency.