Aptina's patent application US20120009723 titled "Range modulated implants for image sensors" talks about deep photodiode process and shows quite impressive PD isolation shape of a 1.1um-pitched PD extending all the way through the 4um-thick epi 66 to a heavy doped substrate 64:
The company describes the challenges in deep photodiode manufacturing: "When conventional methods are used for implanting isolation regions, it can be challenging to form very narrow and deep isolation regions. Isolation regions are typically formed by implanting ions through the openings of patterned photoresist. If very narrow openings are formed in photoresist, the walls of the openings may be unstable. Narrower openings could be formed in thinner photoresist, but thinner photoresist would limit the permissible energies used during ion implantation and resulting implants would be too shallow. Shallow isolation regions are undesirable because they would limit photodiode depth, reducing the quantum efficiency and sensitivity of the pixels."
So, the photodiode isolation implants are implemented by a set of repeating steps with complementary photoresist patterns where the boron implant penetrates through the photoresist:
The resulting deep PD isolation is composed of multiple regions 92 and 94: