Sony moved sensor wiring behind the pixel so cameras could see more light

Small cameras wanted more megapixels without bigger sensors, but shrinking pixels made each one collect less light. The obvious answer was better signal processing; the harder answer was changing the physical path of photons.

Sony's June 2008 announcement said its prototype back-illuminated CMOS sensor used 1.75 micrometer pixels, five effective megapixels and 60 fps operation. Compared with a same-pixel-size front-illuminated Sony sensor, it reported nearly twofold sensitivity and a +8 dB signal-to-noise improvement.

The clever move was structural. By illuminating the backside of the silicon substrate, Sony reduced the shading caused by front-side metal wiring and transistors. It then handled the usual backside penalties with a new photodiode, optimized on-chip lens and high-precision alignment.

In 2010 Sony said Exmor R had been in Sony digital imaging products since 2009 and announced mobile-phone sensors using the same back-illuminated idea.

Let light enter from the back

It attacked the optical bottleneck before the signal became noisy data.

The design preserved CMOS advantages such as low power and high-speed readout.

Backside entry made small pixels more useful instead of merely smaller.

Solving dark current, defects and color mixing turned a lab structure into a manufacturable camera part.

When miniaturization makes every component fight for the same surface, turn the stack into a three-dimensional design problem.

Back-illuminated CMOS became standard in premium phone and camera sensors. Sony's Exmor R branding made a buried semiconductor stack visible to consumers through better low-light photos.