One of the greatest temptations in reviewing cameras is to put too much emphasis on the pixel count. As we are learning (sometimes the hard way), it is the quality of the pixels that matters most. In the last ten years, the minimum pixel size has decreased by a factor of ten due to advances in sensor manufacturing methods. This allows dramatic increases in the number of pixels that can be arrayed onto a given sensor size. We are, however, beginning to reach fundamental limitations in the number of pixels we can cram onto a sensor. In this article, we’ll describe a technique to help improve the quality and quantity of the pixels on the sensor – backside illumination.
In order to understand backside illuminated sensors (BIS), we must first understand the normal method – frontside illumination. Camera sensor fabrication starts the same way most other microchips do – with a silicon wafer. Note there is no “e” on the end of silicon so unlike the squishy stuff that seals your bathroom tub, a silicon wafer is a stiff, brittle crystalline structure that breaks if you look at it. Therefore, although only the top few microns of material is actually used to do work, the silicon wafer starts out at thickness of around 750 microns for mechanical stability. We’ll skip the magic of how the pixels are constructed, but suffice to say that after many delicate operations, the light sensitive pixels are formed atop the silicon wafer.
As it turns out, the pixels are only the bottom layer in what turns out to be a fairly large layer cake. The rest of the layers are used to wire all the pixels together and connect them to the associated read-out circuitry. Because the wiring is so dense and there are so many connections to be made, it takes several layers of hairlike metal wires to connect everything together. Each layer is encased in a sheet of glass-like material to separate it from the layers above and below. As the wiring is routed, care must be taken not to block the light sensitive areas below. The result is a layout organized much like city blocks – with a grid of wire streets surrounding areas of light sensitive transistors. The diagram below shows an exploded view (left) and a finished cross section (right) of a normal frontside illuminated sensor. Only six of the many millions of pixels are shown on the left for simplicity. A single pixel is shown on the right.
Ironically, it is the density of metal wiring that ultimately begins to cause problems. As shown in the diagram above, the light path to a given pixel is limited by the wiring that surrounds it from above. This can be addressed to some extent by adding micro lenses atop the sensor which gather light and focus them onto the light sensitive area. Light guides can also be added to steer the light down to the area of interest. But ultimately, the diminished light path becomes a limiting factor for frontside illuminated sensors.
Enter backside illuminated sensors. As shown in the diagram below, backside illuminated sensors start out the same way that normal sensors do. The difference is that when all the metal wiring is complete, they are flipped upside down such that the light sensitive areas are on the top instead of the bottom. The entire bulk of the silicon wafer is then ground away so that the light sensitive pixels are only a few microns away from the freshly ground surface. Then the color filters and micro lenses are patterned atop the former backside (which due to the flipping and grinding exercise is now the front). All of this is a delicate operation given the aforementioned fragility of the silicon wafer and the immense challenge of grinding the wafer uniformly over its entire breadth. In the end though, the result is a much simpler light path to the active area due to the lack of overlying obstructions.
The advantages of backside illuminated sensors are clear. The lack of obstruction allows for more efficient light collection for each pixel. This means much better low light performance for the end user. It also allows for smaller pixel dimensions while maintaining a given performance level. Small camera phone sensors can now incorporate high pixel counts and render acceptable low light performance.
The disadvantages? First of all, cost. All the complex flipping, grinding, and mounting operations mean higher wafer costs. For this reason, backside illuminated sensors have long been limited to high end applications such as astrophotography and surveillance equipment. However, the high demand for consumer grade sensors is now beginning to force this price point downward. There are also performance issues to be overcome with backside illuminated sensors. A quick glance at the diagrams above shows that while the frontside sensor had a natural aperture formed by the overlying metal, the backside sensor has no equivalent. Therefore, there is the chance of an off angle light ray registering on the wrong pixel.
Manufacturers seem to be overcoming the cost and performance problems of backside sensors. Sony is incorporating the technology into its point-and-shoot sensors and OmniVision’s backside illuminated sensors are purported to be the magic behind the popular iPhone4 camera. Most other manufacturers are either implementing BIS or at least talking about it as part of their strategy. Backside illuminated sensors are particularly attractive for point-and-shoot and camera phone sensors since they allow for a much better low light performance on smaller sensors. But the technology is likely to drift towards SLR cameras eventually. Some analysts expect the backside illuminated sensors will take 70% of the sensor market share by 2015.
What does this mean for you? BIS is just one of many technology trends that will continue to enable better and better photography. As technology becomes more complex, it is becoming more important to “look beyond the megapixel” and examine the technologies behind the electronics that you buy. If you see the term backside illumination used on a spec sheet, hopefully we’ve taken some of the mystery out of it.
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