Pixel Array Control Wire Fault Detection Circuit
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Solution Overview
Problem
Existing methods for testing CMOS image sensors fail to detect non-catastrophic faults in pixel arrays, which may evolve into significant image defects under varied field conditions, particularly in critical applications like automotive and medical fields.
Innovation Solution
A method and circuit that apply a signal transition to control wires of a pixel array, detecting the duration of the signal transition and comparing it to a reference duration to identify resistive opens or shorts, using a combination of low threshold inverters and half Schmitt triggers to assert signals at specific voltage thresholds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If optical testing is used to detect catastrophic defects in pixel arrays, then manufacturing yield is improved by identifying HFPN and VFPN defects, but non-catastrophic faults that may evolve into image defects during field operation remain undetected
Solution Approach 1:
The pixel array's control wires are used to serve dual purposes: both controlling pixel operations and carrying test signals for fault detection. The existing control wire infrastructure is leveraged to distribute test signals without requiring separate dedicated test wiring, enabling self-testing capability within the existing architecture.
Solution Approach 2:
The invention monitors temporal parameters (signal transition duration, rise time, fall time) and electrical parameters (voltage thresholds) of control wire signals to detect faults. By measuring these parameters and comparing them against reference values, the system can identify non-catastrophic faults that optical methods miss, transforming the testing approach from spatial/optical to temporal/electrical domain.
2Measurement precision
If traditional optical testing methods are employed, then catastrophic wire defects are detected, but the testing process lacks the capability to identify resistive opens or shorts that do not cause immediate failures
Solution Approach 1:
The test circuit applies signal transitions and measures their characteristics during the manufacturing process, performing fault detection before products are shipped. By conducting electrical parameter measurements preliminarily, the system identifies potential faults that would manifest as image defects later, preventing field failures without requiring extended field testing periods.
Solution Approach 2:
The invention replaces optical/mechanical testing methods with electrical measurement techniques. Instead of using optical systems to capture images and analyze pixel responses, the patent uses electrical circuits to measure signal transitions, durations, and voltage thresholds directly on control wires, providing more precise and faster fault detection.
3Reliability
If signal transition duration monitoring is implemented on control wires, then non-catastrophic electrical faults are detected, but additional circuit components are required for threshold detection and duration measurement
Solution Approach 1:
The test circuit is designed to be integrated with the existing pixel array control infrastructure, making the control wires serve multiple functions: pixel control during normal operation and fault detection during testing. The same control wire that drives pixel rows during imaging is also used to carry test signals and provide fault detection data, eliminating the need for completely separate dedicated test wiring.
Solution Approach 2:
The invention merges the fault detection functionality with the existing control wire infrastructure and pixel array architecture. By combining the control signal path with the test signal path and integrating threshold detection circuits within the control logic, the system achieves fault detection capability without adding completely separate independent test systems, thereby limiting the increase in overall device complexity.
Data Source
AI summary
Testing of control wires of a pixel array of an image sensor is performed by applying a signal transition to a control wire and detecting, based on a voltage signal detected on the control wire, the duration of at least part of the signal transition on the control wire. An electrical fault in the control wire is indicated based on a comparison of the detected duration to a threshold.