Sony has developed a new backside-illuminated stacked sensor equipped with global shutter functionality with 1.46-million-pixel resolution.
While 1.46 megapixels doesn’t sound like much, it’s the global shutter functionality that is the real breakthrough here and the resolution will surely improve dramatically in a short amount of time.
‘Global shutter’ means that the sensor can capture an image by exposing from every pixel at the same time.
The sensor is able to read every pixel simultaneously thanks to its ‘stacked’ design, which comprises a top chip containing the active pixels, and below that a bottom chip which contains an analog-to-digital converter tied to each pixel in the photo-sensitive top chip.
Confused? It’s a little confusing. In short: most CMOS image sensors use rolling shutters, which means the sensor scans your scene from top to bottom. For videographers, this can produce artefacts in certain situations – typicall those where you are capturing motion.
A global shutter like Sony’s new sensor, and Panasonic’s new 8K global shutter sensor, scan the frame all at once, minimising these artefacts.
So why have a rolling shutter? Rolling shutters willy typically produce less noise and offer a wider dynamic range. A global shutter can do this too, but historically it’s been more expensive to produce. So a rolling shutter has been a nice compromise between image quality and overheads. Sony says:
The new Sony sensor comes with newly developed low-current, compact A/D converters positioned beneath each pixel. These A/D converters instantly convert the analog signal from all the simultaneously exposed pixels in parallel to a digital signal to temporarily store it in digital memory. This architecture eliminates focal plane distortion due to readout time shift, making it possible to provide a Global Shutter function*1, an industry-first for a high-sensitivity back-illuminated CMOS sensor with pixel-parallel A/D Converter with more than one megapixel*3.
The inclusion of nearly 1,000 times as many A/D converters compared to the traditional column A/D conversion method*2 means an increased demand for current. Sony addressed this issue by developing a compact 14-bit A/D converter which boasts the industry’s best performance*4 in low-current operation.
Both the A/D converter and digital memory spaces are secured in a stacked configuration with these elements integrated into the bottom chip. The connection between each pixel on the top chip uses Cu-Cu (copper-copper) connection*5, a technology that Sony put into mass production as a world-first in January 2016.
In addition, a newly developed data transfer mechanism is implemented into the sensor to enable the high-speed massively parallel readout data required for the A/D conversion process.