What's the difference? In construction, not much. But in use, they're like night and day — almost literally. The always fascinating Preston Scott over at Camera Technica has the in-depth explanation, but the quick version is surprisingly interesting.
Frontside and backside refer to where the circuitry sits on the chip. Delicate silicon is made thick to stop it from breaking, and then the pixels are added. On top of this goes the circuitry. To stop the wires which connect each pixel from getting in the way, they are routed around the edges, “like city blocks,” in Preston's words. As these stacks get higher with ever bigger pixel-counts, they shade the pixels in the same way Manhattan's tall buildings shade the streets.
The solution? Flip it over and grind away the silicon layer to reveal the pixels. It's like putting all those skyscrapers underground and then enjoying the uninterrupted sunshine in a beautiful flat field. More light gets to the sensor, making it much more sensitive.
There's more too it than that, of course, and you should read Preston's great article. In fact, photo nerds should just subscribe to the Camera Technica RSS feed.
Technology Demystified: Backside Illuminated Sensors [Camera Technica]
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