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.
Optical Absorption Depth
Spectral Attenuation
Electromagnetic radiation penetrating a semiconductor medium experiences exponential intensity decay as photons interact with electron structures inside the crystal lattice. Material scientists define optical absorption depth as the distance into a material at which incident light intensity drops to one over e of its surface value. Semiconductor image sensors exhibit strong wavelength dependency in light absorption, where short blue wavelengths absorb near the illuminated surface while long red and near-infrared wavelengths penetrate deep into the substrate bulk.
Sensor Implication
Physical silicon thickness determines quantum efficiency across different spectral bands. When optical absorption depth exceeds the depth of the pixel depletion region, photo-generated carriers form in neutral substrate zones where collection efficiency drops. Infrared imaging requires thicker active silicon regions or backside illumination structures to capture deeply penetrating photons before they diffuse away.
Conversely, ultra-violet applications require shallow junction designs because blue light absorbs within a few nanometers of the front surface.
Qualification Sweep
Optical bench testing measures spectral response curves using calibrated monochromators across visible and infrared spectrums. Instruments map quantum efficiency against wavelength to verify that active depletion depths match target optical absorption profiles. Reflection and transmission measurements confirm light attenuation characteristics of customized silicon formulations.
Laboratory measurements validate spectral sensitivity bounds across specified sensor operating temperatures.
Wavelength Boundary
Substrate thickness limits infrared photon collection capability.