Samsung patent application US20110193147 proposes BSI pixel with convex etched shape of photodiodes on the back side:
The planarisation layers 150 and 170 help to keep color filters 160 and microlens 180 flat in this embodiment. Other versions omit layer 150, so that color filter follows the PD shape.
The manufacturing process includes etching out the islands like this:
and then melting them into a spherical shape using "a thermal process":