Omnivision's patent application US20130200396 "Prevention of light leakage in backside illuminated imaging sensors" by Wei Zheng, Vincent Venezia, Hsin-Chih Tai discusses the ways to protect the pixel array from the IR light generated in periphery circuits:
"Certain elements, such as light emitting element 123 within periphery circuit region 122 may emit light. Light emitting element 123 may emit light by various mechanisms, for example, through electroluminescence of biased p-n junctions, and produce light having wavelength approximately in the infrared (“IR”) or near-IR (“NIR”) spectrum. For example, light emitting element 123 may be a MOS tunnel diode emitting light that includes a wavelength near 1.1 μm. In one embodiment, light emitting element 123 includes a forward biased diode with ion implant induced dislocations, emitting light that includes a wavelength near 1.5 μm."
So, Omnivision proposes a backside trench to stop the light: