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.
Minority Carrier Diffusion Length
Carrier Transport
Semiconductor materials transmit non-equilibrium charge carriers through crystal lattices via random thermal motion before recombination events neutralize them. Physicists define minority carrier diffusion length as the average distance an excess minority carrier travels through a semiconductor bulk before recombining with a majority carrier. Silicon substrates used in image sensors depend on this transport parameter to determine how far photo-generated electrons migrate prior to collection by a potential well.
Metrological Impact
Doping concentration and crystal defect density dictate carrier lifetime and diffusion characteristics. When minority carrier diffusion length exceeds the depth of the active depletion region, carriers generated deep within the bulk diffuse laterally into neighboring pixels, causing electrical crosstalk. Short diffusion lengths reduce quantum efficiency in the near-infrared spectrum because long-wavelength photons penetrate deeply into the silicon substrate before absorption.
High-purity silicon wafers require rigorous qualification to maintain uniform carrier transport parameters across entire fabrication lots.
Measurement Technique
Characterization relies on surface photovoltage and electron beam induced current measurement techniques to quantify transport distances. Test systems illuminate silicon samples with varying wavelengths while tracking open-circuit voltage response to extract diffusion distances. Laboratory instruments map spatial variations in carrier transport across the wafer area to identify structural defects or metallic contamination.
Results establish acceptable threshold boundaries for raw wafer procurement prior to sensor fabrication.
Thermal Boundary
Temperature increases accelerate recombination rates, reducing overall transport distance.