Canon Paper on Global Shutter Sensor

Canon researchers have written a paper on a 3.4 μm pixel pitch global shutter CMOS image sensor with dual in-pixel charge domain memory.

While this technology is most suited for industrial, video camera, or automotive purposes; there are still real-world impacts upon the camera side of the business.  Global shutters would remove the last mechanical parts in a camera, removing the need for a mechanical shutter.

A normal shutter reads a line of pixels at a time sequentially from the top of the sensor down to the bottom.   When a sensor is read sequentially, then artifacts such as rolling shutter can occur with moving objects because each line of pixels is read at a slightly different time.  You simply don't get this artifact with a global shutter because all the pixels are read at the same time, and the value stored in memory.

As with all good things, there's a price to be paid, global shutter sensors are far more complex than a normal sensor because each pixel must have a corresponding memory cell.  This also reduces the dynamic range because each pixel has to be made smaller to have room for the memory cell.

Global shutter sensors and also organic sensors are most likely the "next big thing" to hit stills cameras.

Now about this paper.  This paper deals with a specific sensor that reads the sensor twice in an HDR mode.  This allows it to overcome some of the dynamic range inefficiencies with global shutter sensors.  By incorporating dual memory, Canon quickly reads the sensor twice and stores the values into the memory.  Those two values then are used to form the HDR image from the sensor.  They use special lightguides to take a 2.4um pixel pitch down to a 1.1um actual pixel size.  Using the dual memory, Canon eliminates ghosting from appearing on fast moving objects when taking the double images required for HDR.

Does this technology have any stills image practical purpose? Most likely not, because with stills you have a wide-ranging shutter speed, and even then taking double exposures with a stills sensor is hugely problematic with any motion in the scene.

Several other sensor notes were gleaned from this paper, confirming much in regards to current Canon technology;

1. While they are still working on back side illuminated sensors in patent literature, this sensor is still front side illuminated.
2. Canon used 130nm design rules with this sensor, most likely they are using 130nm with all their sensors now.  This is still a ways off from 95nm and 60nm that Sony is using, but certainly better than the last confirmed 500nm that they used in most of the pre-dual pixel AF sensors.
3. Canon also used copper wiring, this is particularly important since using copper wiring in the actual sensor itself allows the sensor to be more efficient, and also have a faster readout.  It's a relatively new process even with Sony sensors.

Canon states in the paper with respects to this global shutter sensor;

In this paper, we describe a newly developed 3.4  μ m pixel pitch global shutter CMOS image sensor (CIS) with dual in-pixel charge domain memories (CDMEMs) has about 5.3 M effective pixels and achieves 19 ke⁻ full well capacity, 30 ke⁻/lxs sensitivity, 2.8  temporal noise, and −83 dB parasitic light sensitivity. In particular, we describe the sensor structure for improving the sensitivity and detail of the readout procedure. Furthermore, this image sensor realizes various readout with dual CDMEMs. For example, an alternate multiple-accumulation high dynamic range readout procedure achieves 60 fps operation and over 110 dB dynamic range in one-frame operation and is suitable in particular for moving object capturing. This front-side-illuminated CIS is fabricated in a 130 nm 1P4M with light shield CMOS process.

via Image Sensor World