New-Tech Magazine - Europe | January Digital edition

detail. ● No image correction: All these demands have to be implemented without impacting the raw image quality so that no extra off-chip cor- rection is needed. Global Shutter for CMOS Sensors Combining a much smaller pixel layout with a global shutter is another major progress that imagers from CMOSIS have achieved in the last few years. A global shutter exposes all pixels of a sensor at the same time and over the same duration. It is a more complex concept - and it was ini-tially more costly to implement in CMOS sensors - because it requires some kind of a local storage element (usually a capacitor) inside each pixel, plus some control function to start and stop the exposure. All this enlarges pixel size. A rolling shutter, in a marked difference, will expose an image sequen-tially, row by row, top to bottom, at different moments in time, much like the mechanical curtain rolling down behind the lens in an old-fashioned analog photo camera. This rolling effect causes time-related artifacts, which can skew the image

as offered by CMOSIS has greatly improved. This primarily pertains to the capacitive storage nodes need- ed inside each pixel to hold the pixel values for reading them out se- quentially after the exposure stops. Smaller storage nodes are now fea- sible with reasonably small pixel sizes and at lower price points. Ad-vanced global-shutter CMOS sensor designs feature pixels down to 5.5 x 5.5 µm² as available in the CMOSIS CMV product family. The goal is having 3.6-µm pixels with a low noise global shutter in the near future. Of course, this scaled-down CMOS layout requires fabs or foundries with wafer-processing capabilities that can accommodate these small pixel dimensions. It also needs comprehensive design know-how to create the appropriate pixel architecture and technology.

of fast-moving objects as the expo- sure follows or deviates from their horizontal or vertical position at any given moment across the image plane (Figure 5). Another rolling shut-ter artifact occurs when illuminating the scene with a short-burst flash. The result is that only a few rows or parts of the image are exposed, whereas other areas remain dark. The rolling shutter concept has been the traditional method also in digital imaging since it is much easier to build a pixel architecture adapted to the row-by-row exposure scheme. Therefore CMOS sensors with four- transistor pixel architecture usually come with a rolling shutter. The lower number of pixel transistors and state- of-the art layout results in pixels with excellent specs in regard of dynamic range and dark current. CMOSIS has developed high-end sensors for the M camera model of Leica Camera based on these rolling shutter pixels. Providing a global shutter in a CMOS sensor is more complicated since it involves placing the storage capacity inside each pixel. This takes up space and leads to a larger pixel layout, which is more expensive. However, global-shutter technology

Eight-transistor Pixel Architecture

Fitting a low-noise global shutter to a CMOS image sensor requires a complex pixel architecture. But a specific new architecture is set to overcome this obstacle. The eight-transistor (8T) global-shutter

Figure 5: A rolling-shutter design (a) causes moving objects to be depicted with skewed lines, so fast-moving objects to appear skewed. Also, a flashlight exposes only a part of a frame. This is not the case with a global- shutter design (b)

New-Tech Magazine Europe l 41