So What is a Backside Illuminated Sensor?
In last week's discussion of camera sensors I mentioned something called a Backside Illuminated sensor. Based on some questions since then, I should have explained better what this means. Here's the details I left out last week.
As a reminder of where we left things last week, one problem of traditional digital camera sensor design is that it takes a lot of circuitry and wires to make a sensor with millions of pixels operate, and all that circuitry and all those wires have to go somewhere. Everything works fairly well so long as the sensor doesn't have that many photosites, or if it's big enough to accommodate everything without crowding. But as sensor resolution goes up or sensor size decreases, the density of photosites reaches a point where all those wires start to block some of the light from reaching its intended photosite targets. And that's a bad thing.
A Backside Illuminated (or BSI) sensor represents a novel reimagining of the digital sensor to solve that problem.
At first consideration, one might assume that somehow the placement of tiny light bulbs or LEDs placed behind the camera sensor somehow enabled the gnomes living inside each photosite to better see things and thus create a better image. Obviously, this isn't so.
Instead, the illumination referred to in the name of the BSI sensor is the light travelling through the lens on its way to record the image. And no, there isn't a mirror behind the sensor reflecting it back onto the rear of the sensor. Instead, the camera sensor is pretty much turned backwards so that now the light hits the backside of the sensor without mirrors. The traditional sensor design places the wiring on top of the photodiodes. A BSI sensor flips everything upside down so that the light sensitive side is now on top rather than on the bottom.
You might ask why they ever put all that wiring on top if it were possible to reverse things and put it on the bottom. The short answer is "because." The longer answer isn't much better. That's just the way they've always been fabricated. It all comes down to the common factor of silicon. Digital integrated circuits have used silicon as the substrate for chip design for years. But silicon is also part of what makes photosites able to record an image. So the easiest design for early camera sensors was to mount the circuitry components on top of the silicon substrate. If this design still sounds downright stupid to you, there is a precedent — that's the way our own eyes are built. The light sensitive pigments in the eyes of human beings and those of most other animals are on the side opposite the lens rather than right behind it. I doubt early camera sensor designers pattered their creations after the human eye, but it is a curious coincidence. Rather, it was just that imaging sensors were traditionally built the same way most other modern digital components are: start with a silicon chip and pile everything else on top.
A BSI sensor (also called a "Back Illuminated" or BI sensor) starts out life much the same as a traditional sensor, but once all the photodiodes have been embedded and the circuitry are mounted, the whole thing is flipped over and ground down until nothing much remains on top of (what used to be underneath) the photodiodes. The thinned silicon wafer is now just a few microns thicker than the the photodiodes themselves. The colored (Bayer mosaic) filters are then affixed on top of the light sensitive layer just as in a traditional sensor, with the array of micro-lenses on top of that to focus the light. But these are fastened to what was originally the bottom of the silicon layer rather than the top.
As you might expect, this level of manufacturing complexity does add to the cost of the resulting sensor, but the advantages are worth it if you want the best image possible with small, crowded sensors. So far BSI sensors have been used mainly in cell phone cameras and other places where high resolution but small sized sensors are truly needed. It seems likely that this approach will see its way into an increasing number of cameras as resolution expectations continue to go up.