The new Omnivision patent application US20100159632 proposes to use a diffusion from heavy doped substrate to epi as an alternative to the backside implantation and laser annealing to passivate the backside surface:
"The typical thermal budget for the epi layer growth and front side CMOS processing causes diffusion tail to grow 1.5 to 2.0um into epi layer 410. Conventional image sensor fabrication techniques start with an initial epi layer thickness L3 of approximately 5um or greater to provide adequate sacrificial space within the bottom portion of epi layer 410 for diffusion tail to be completely removed. Embodiments of the technique disclosed herein may start with an initial epi layer thickness L3 of approximately 3-5um (in one embodiment L3 is selected to be 4um). In these embodiments, epi layer 410 is thinned to a target thickness L1 of approximately 2.0-2.3um for epi layer 410 and a target thickness L2 of approximately 0.5-0.8um for dopant gradient band 405. Accordingly, the initial thickness L3 of epi layer 410 is selected such that diffusion tail migrates upward to within 3.0um or less of the top surface of epi layer 410, and in the illustrated embodiment to within 2.0um of the top surface of epi layer 410."