04 Apr 2013

Sensors need better technology, not more resolution

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The Sensor of the Future The Sensor of the Future Alternative Vision/Lumisense Photonics

Index

Recently I've been expressing the view that all cameras are pretty much the same, because they all use the similar technology (give or take) for their sensors. I don't mean to be unnecessarily cruel about the work of camera manufacturers – taking an electronic component such as an imaging sensor and making it into a usable tool is far from trivial. Still, the absolute performance of cameras is determined by what you can get off the lump of silicon behind the lens

According to some people, though, creating sensors with ever larger numbers of pixels – is also becoming somewhat commoditised.

I first encountered this opinion in a discussion about Red's upcoming Dragon sensor, in which someone offered, for $2.5M, to produce a sensor of equivalent performance in a year and a half, even adding global shuttering. The brave soul making this claim was David Gilblom of Alternative Vision Corporation, based in Tucson, Arizona, a company specialising in imaging sensors. Among many other things, they're the reseller for the well known Foveon X3 sensor, which avoids the problems of bayer-filtered sensors by stacking the red, green and blue pixels one atop the other, but isn't seen much in the world of film and TV.

A bold claim

Supporting this bold claim is a new technique for manufacturing imaging sensors which allows the light sensitive and processing parts to be treated separately. Developer Lumiense Photonics calls it SiLM, for silicon film (without, I suspect, any intention to compare it to the photochemical imaging technology). The aim of SiLM is to “remove most of the need to make compromises among the various important imaging performance parameters, [allowing] them to be optimised nearly independently.”

In English, and not to get too deep into the details of manufacturing microchips, the crucial advancement is that this new approach allows the array of photodiodes which form the pixels to be stacked on top of a completely separate device – or perhaps more than one device. This is a technique distinct from the Foveon sensors, and would still require a Bayer filter, but there are significant advantages in separating the light-sensitive photodiodes from the controlling electronics,  providing, as Gilblom says, “greatly improved linear dynamic range and elimination of motion artefacts”.


Stacking

Current approaches to stacking up semiconductor devices such as memory lack the fineness of pitch necessary to connect such a large number of tiny pixels to the control electronics beneath, meaning that the electronics have to be on the same layer as the light sensitive devices. This means that the entire surface of a current CMOS sensor cannot be light sensitive; part of it must be used for control electronics, with the result that the pixels do not meet edge to edge – in the terminology of the field, the fill factor is less than 100%. This exacerbates aliasing and reduces the size of the pixel, thereby reducing dynamic range and sensitivity. It also suggests one reason why many modern cameras lack a true global shutter, because the single switching transistor that is required for each pixel consumes still more area on the silicon and reduces performance still further. Even then, the shutter control transistor is itself somewhat light sensitive, as are all silicon semiconductors, meaning that the ratio of shutter off to shutter on is possibly only about 50,000 to 1 – the shutter is, in effect, slightly transparent, and a bright sun might be able to punch through in a way that would be impossible with a mechanical shutter.

More crucially, the method by which a conventional CMOS sensor is built must be the same for all parts of the device. The familiar acronym refers to a semiconductor manufacturing process, a particular combination of materials and techniques. The materials and techniques used in the CMOS process are not ideally suited to producing high performance photodiodes.



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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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