CMOS Image Sensor Row Readout Delay Mechanics and Measurement
CMOS image sensor row readout delay dictates rolling shutter temporal skew, determined by physical word line RC delays and column ADC settling times.
Image Lag
Performance Artifact
A residual signal remains in a sensor pixel after the exposure period has completed and the readout sequence has finished. This persistent charge is defined as image lag, and it appears as a ghost image or trail behind moving objects in successive frames. This phenomenon degrades the dynamic resolution of cameras used in automated inspection and clinical diagnostics.
Chemical Root
The presence of potential barriers or traps at the transfer gate interface of a pinned photodiode prevents the complete transfer of photoelectrons to the sense node. When these electrons are held back, they gradually leak out during subsequent frames, creating the unwanted secondary signal. This delayed emission is highly dependent on temperature, which can accelerate the release of trapped charges.
Test Methodology
Quantitative evaluation of this sensor property utilizes a timed light source that can be switched off within a fraction of a microsecond. Technicians measure the signal output in the frames immediately following the termination of the light pulse to calculate the percentage of residual signal. A high performance industrial sensor must exhibit a value close to zero percent to pass qualification for high speed sorting lines, where any trailing artifact would otherwise lead to false rejects or blurred defect detection on the conveyor system, so a multi frame analysis is performed during factory testing to ensure the decay is complete.
Design Evolution
Sensor designers minimize image lag by optimizing the implantation profile of the transfer gate and adjusting the operating voltages. By creating a continuous potential gradient from the photodiode to the floating diffusion, electrons are driven out of the photodiode more efficiently. This approach is verified during wafer level probing to ensure consistent performance across production lots.