BSI

With a conventional front-illumination structure, the metal wiring above the sensor’s photo-diodes impede photon gathering. A back-illuminated structure (figure below) increases the amount of light that enters each pixel due to the lack of obstacles such as metal wiring and transistors that have been moved to the reverse of the silicon substrate.

Stacked

Newest sensors combe both BSI and stacking. Conventional CMOS image sensor technology creates the pixel function and analog logic circuitry on the same chip. The motivations for stacked chip CIS include: optimization of each function in the stack, adding functionality to the stack and decreasing form factor.

Now stacked sensors look:

Via http://electroiq.com/insights-from-leading-edge/2016/09/iftle-303-sony-introduces-ziptronix-dbi-technology-in-samsung-galaxy-s7/