04 Apr 2013

Sensors need better technology, not more resolution

  • Written by 
The Sensor of the Future The Sensor of the Future Alternative Vision/Lumisense Photonics


100% fill factor

The ability to separate the layers and treat the photodiodes independently is therefore valuable, as is the option to put the electronics on a layer behind the sensitive parts. Crucially, all of the electronics can be shielded from interfering light, producing a 107 shuttering efficiency, and a reflector can be placed behind the photodiodes such that any light which goes straight through can be reflected back out, for a second chance at detecting it. This is rather like the tapetum lucidum in the eyes of animals, which creates the characteristic rabbit-in-the-headlights shine, although whereas animals suffer a resolution falloff due to the varying refractive index of the retina and the material within the eye, the refractive index of silicon is so high that no such problem is anticipated in imaging sensors. Perhaps most enticingly of all for film and TV applications, the big pixels provide a near 100% fill factor and therefore greater dynamic range, lower noise, and “eliminates the need for microlenses thus always presenting a smooth surface with a high acceptance angle to the optics”. This means that the classic, film-oriented lenses cinematographers sometimes choose, which don't necessarily present light to the sensor at right angles (strictly are not image-space telecentric), should not vignette.

Lumiense have already made test devices using a three-wafer stack, with one for the photodiodes, one with control electronics, and a third backing wafer which is kept reasonably thick for mechanical handling purposes, but can still have devices on it. This represents an absolutely enormous increase in the amount of space that's available for electronics, so expect smart sensors to get even smarter and global shutter to become the norm. Sony are also talking about stacked sensors – it wouldn't be surprising to find that the sensors in the new F5 and F55 cameras use the technique – but not with the same level of sophistication.

Much as Kodak can tell you about nitrates, T-grains and colour couplers, Alternative Vision can talk the hind leg off a donkey about diodes, oxide layers, indium ball bonds (yeah, yeah), wafer sizes, and all the other minutiae of sensor manufacturing.

"Removes the need to make compromises"

Alternative Vision’s David Gilblom told RedShark:

 The SiLM sensor technology removes most of the need to make compromises among the various important imaging performance parameters since it allows each of these to be optimized nearly independently.  Removing the circuitry from around the photodiodes permits application of high-efficiency anti-reflectance coatings for maximum photon capture and eliminates the need for microlenses thus always presenting a smooth surface with a high acceptance angle to the optics.  Freeing the circuitry layer from area-hogging by the photodiodes opens up room for global shuttering (with sub-microsecond shuttering times), increased charge storage and in-pixel noise cancellation.  The result - increased natural ISO speed, greatly improved linear dynamic range and elimination of motion artefacts.

And when can we have cameras with this high performance new technology? Well, the prototypes were made in 2009, funding for productisation is apparently imminent, and after three to six months the first device will be a 4K by 4K sensor. Presumably an enthusiastic company could have a camera on set some time in 2015.

This is not about Pixel Count

The thing to realise about all this, though, is what we're talking about here is a way to offset some of the problems of existing CMOS sensors. This is not about pixel count, it's about global shuttering, sensitivity, dynamic range, and possibly frame rate – the important stuff that isn't really addressed by the race for resolution that's characterised sensor design to this point. One of the things about CMOS sensors is that more or less anything is available to someone with a shockingly small among of R&D money, especially in terms of scaling pixel count and frame rate (since frame rate is just a matter of how many parallel pipes of data are coming off the chip). As I think has been proven recently, ever increasing pixel count is less useful than it once was: that war is won, and it'll be interesting to see what the next few battles are.

Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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