Parasitic Light Sensitivity Limits in Stacked Backside Illuminated Global Shutter Image Sensors
Stacked BSI global shutter sensor parasitic light sensitivity depends on silicon depth, metal shield alignment, and active illumination wavelength.
Charge Transfer Drift
Degradation Dynamic
Image sensors experience progressive signal loss across pixel arrays when charges fail to move completely between potential wells during high-frame-rate operation. This progressive loss in charge transfer efficiency over operational time or temperature shifts is known as charge transfer drift. Solid-state image sensors rely on potential gradients created by gate electrodes to shift electron packets from photodiode nodes to readout nodes.
Physical Origin
Trapping sites within the silicon-silicon dioxide interface slow down charge movement. When charge transfer drift occurs, residual charge remains in storage nodes, causing image lag between successive exposures. High readout frequencies exacerbate this residual voltage build-up because the dwell time per gate transfer falls below the thermal release time of deep-level traps.
Interface state density varies with fabrication parameters, making initial screening essential for sensors designated for high-precision radiometric duty.
Metrological Evaluation
Testing protocols quantify signal retention degradation across operating temperature ranges from sub-zero space environments to elevated industrial conditions. A standard test pattern exposes the sensor to uniform illumination, measures signal deficit across successive shifts, and computes the fractional loss per gate transfer. Thermal cycling accelerates deep-level trap activation, producing a measurable shift in background noise floor.
Test bench sweeps verify whether charge transfer drift stays within manufacturer limits across all operational clock speeds and bias voltages.
Mitigation Boundary
System integration limits this instability through clock pulse shaping and adjusted substrate bias.