The recently published Omnivision patent application US20100140675 discloses local laser annealing idea on the sensor backside:
Here are the advantages of this proposal:
"Reducing the silicon thickness introduces process complexities in terms of laser annealing the backside dopant layer. ...the temperature rise at the front surface will be greater for thin silicon. Accordingly, in some embodiments, a laser anneal mask (not illustrated) is deposited on the die or wafer backside and patterned to only expose those areas immediately below PD regions 415. When the annealing laser is scanned across the die or wafer backside (see FIG. 6C) only the uncovered portion of dopant layer 405 is exposed to the laser, so only the boron in this exposed area is activated. The thickness and composition of the laser anneal mask are laser and process dependent; however, the composition and thickness should be chosen to have a high reflectively to the laser wavelength.
Masking the die backside prior to the laser anneal of dopant layer provides a number of advantages. The amount of energy absorbed by the silicon is reduced, thereby reducing the temperature rise of the die. There are typically no metal/silicide contacts above photodiode region 415 and therefore the front surface can tolerate a larger temperature rise than regions with metal contacts. Metal contacts will typically degrade above 400 C, but dopant profiles typically do not diffuse below 800 C. The periphery circuit around photodiode region 415 and pixel array is protected from the harmful effects of the laser anneal process. The P+ type ion implant under the masked back surface area is not exposed and therefore is not activated creating a high recombination region. Electrons that migrate to or are formed in this region will recombine easily reducing crosstalk."
There is also "a backside surface treatment layer is applied to further passivate the backside of epi layer and cure surface defects... Surface treatment may include exposing the backside silicon surface to a gas or liquid containing a reducing or oxidizing species to reduce backside surface states. These species may include H2, H2O, N2O, O2, O3, and H2O2, among others. Curing defects and reducing surface states advantageously can reduce dark current and white pixels."