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.
Embedded Tungsten Light Shield
Shielding Structure
Solid-state image sensors incorporate metallic barriers directly within their backend metallization stacks to prevent unwanted photo-generation in optical black reference areas and storage nodes. An embedded tungsten light shield blocks incoming photons through high material density and broad spectral attenuation across visible and near-infrared wavelengths. Silicon substrates allow photon penetration that creates spurious charge in non-photosensitive zones unless shielded by dense refractory metals.
Integration Mechanism
Tungsten layers are deposited through chemical vapor deposition and patterned with sub-micron lithography during wafer fabrication. When integrated into global shutter CMOS image sensors, an embedded tungsten light shield isolates the memory node from parasitic light sensitivity during readout cycles. Insufficient thickness or pinhole defects in the tungsten layer lead to parasitic light sensitivity, corrupting stored analog voltages while the array reads out row by row.
Refractory tungsten provides high thermal stability, preventing metal migration or structural deformation during subsequent high-temperature annealing cycles.
Performance Verification
Testing procedures assess optical opacity by illuminating the sensor under high intensity while measuring parasitic response in shielded pixels. High-intensity pulse light sources evaluate light leakage ratios down to fractions of a percent of nominal full-well capacity. Test arrays compare unshielded photodiode signal against adjacent shielded storage nodes to quantify attenuation ratios.
Measurement protocols report parasitic light sensitivity as a ratio of storage node sensitivity to photodiode sensitivity.
Process Boundary
Layer thickness limits total attenuation efficiency.