seems pretty clear to me - from the article originally linked to:
While the
silicon substrate itself is about 600 to 800 μm
thick, in the Sony “Exmor R” CMOS image
sensors, it was necessary to make of the silicon
substrate, including the metal wiring layer,
have a thickness of about 8 μm to allow light
to be received through the back of the substrate.
....
This meant that it would
not be possible to form the on-chip lenses and
color filters over 1 μm pixels. To resolve this
issue, Sony developed a new wafer thinning
technology to assure that such distortion and
warping does not occur.
In any case, the devices are back illuminated, which is what really matters - that's where the gain in performance comes from. If Sony managed to do this without thinning, then good luck to them, but they certainly claim that they do thin the chips to a very fine structure and they claim to have developed a superior thinning technology to boot.
We live in interesting times
