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.
Total Internal Reflection
Optical Phenomenon
Light striking a boundary between two transparent media returns completely into the denser medium when specific conditions are met. The condition for total internal reflection occurs when the angle of incidence exceeds the critical angle defined by the refractive indices of the materials. This effect is the fundamental principle behind the operation of optical fibers and certain types of rain sensors.
Critical Angle
Snell’s law determines the specific point where the refracted ray would theoretically travel parallel to the interface. Beyond this angle, the interface acts as a perfect mirror for the total internal reflection of the energy. Surface quality at the boundary must be high to prevent scattering.
Evanescent Field
Energy penetrates a fraction of a wavelength into the second medium beyond the interface. This evanescent wave allows total internal reflection to be used in sensing applications where changes in the external medium affect the reflected signal. Attenuated total reflection spectroscopy leverages this field to analyze thin films or liquid samples.
The sensitivity depends on the refractive index contrast and the wavelength of the light. Precise alignment of the light source is needed to maintain the necessary incident angle.
Fiber Transmission
Communication cables rely on a core with a higher refractive index than the surrounding cladding. This keeps the light trapped within the core over long distances with very little loss of signal.