Sensor breakthrough: Sony has developed a backlit CMOS sensor with global shutter

Sony has made something of a breakthrough in sensor development, announcing a new backside-illuminated stacked sensor that can read out every pixel simultaneously to enable global shutter. While the company has only made it work with a 1.46-million-pixel sensor so far, the nascent technology has significant potential.

The sensor is able to read out from every pixel instantly because each pixel has its own analog-to-digital converter (ADC) buried in a 'bottom chip,' which is stacked beneath a 'top chip' containing the active, photosensitive pixels. This allows all exposed pixels to be read simultaneously, rather than sequentially row-by-row as is done with traditional CMOS sensors containing far fewer 'column parallel' ADCs.

This instant read-out avoids the rolling shutter distortion caused by the time delay as each row of pixels is recorded one after the other. In most existing chips, fast-moving objects become warped as they progress across the frame, because the pixels at the top of the sensor were read earlier than those at the bottom. This can also lead to banding under certain types of artificial lighting.

Global shutter—reading out all of the pixels at once—solves both these problems.

Shot with an exposure time of 0.56ms

Sony claims its sensor is the first back-illuminated high-sensitivity CMOS sensor with pixel-parallel ADCs and a pixel-count greater than 1 million.

While one million pixels may not be much good to photographers, this is a big step towards the production of a photographic quality sensor. Chips with 'global shutter' need only an electronic shutter to record undistorted action pictures, boast the ability to use short electronic shutter speeds with flash, and are able to work under fluorescent and solid state (LED) lighting without banding.

In the end, a global shutter sensor like this be useful for both still and movie photographers.

It's also a major improvement over current global shutter CMOS sensors, which have a photosensitive pixel, and then a 'storage' pixel that the charge is transferred to after the exposure is made. This storage pixel holds the charge until the column ADCs read out, row by row. The problem with this approach is that your active pixel area now has a bunch of dead space per pixel taken up by the 'storage pixel'.

By going BSI and stacked, we believe this technology eliminates the need for the storage pixel entirely, because you can read all the pixels at once at the end of your exposure.

The company says it has had to include 1000x more ADCs than it would normally in a 1MP sensor. The extra ADCs would require far more current, so Sony developed low current, compact ADCs for this chip. Additionally, new high speed data transfer construction allows for the fast read and write speeds required to operate all the ADCs in parallel and transfer the digital data.

While it might be some time before one is ready for use in a standard camera, this is a big step forward for global shutter sensor technology, which has traditionally been plagued by higher noise levels and lower dynamic range.

When will we see it scaled up to larger, smaller pixel pitch higher-resolution sensors? Hard to tell, but we're keeping our fingers and toes crossed.

Press Release

Sony Develops a Back-Illuminated CMOS Image Sensor with Pixel-Parallel A/D Converter That Enables Global Shutter Function

Sony CorporationSony Semiconductor Solutions Corporation Tokyo, Japan – Sony Corporation today announced that it has developed a 1.46 effective megapixel back-illuminated CMOS image sensor equipped with a Global Shutter function*1. The newly developed pixel-parallel analog-to-digital converters provide the function to instantly convert the analog signal from all pixels, simultaneously exposed, to a digital signal in parallel. This new technology was announced at the International Solid-State Circuits Conference (ISSCC) on February 11, 2018 in San Francisco in the United States.

CMOS image sensors using the conventional column A/D conversion method*2 read out the photoelectrically converted analog signals from pixels row by row, which results in image distortion (focal plane distortion) caused by the time shift due to the row-by-row readout.

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.

*1:A function that alleviates the image distortion (focal plane distortion) specific to CMOS image sensors that read pixel signals row by row.*2:Method where the A/D converter is provided for each vertical row of pixels in a parallel configuration.*3:As of announcement on February 13, 2018.*4:As of announcement on February 13, 2018. FoM (Figure of Merit): 0.24e-?nJ/step. (power consumption x noise) / {no. of pixels x frame speed x 2^(ADC resolution)}.*5:Technology that provides electrical continuity via connected Cu (copper) pads when stacking the back-illuminated CMOS image sensor section (top chip) and logic circuits (bottom chip). Compared with through-silicon via (TSV) wiring, where the connection is achieved by penetrating electrodes around the circumference of the pixel area, this method gives more freedom in design, improves productivity, allows for a more compact size, and increases performance. Sony announced this technology in December 2016 at the International Electron Devices Meeting (IEDM) in San Francisco.

Main FeaturesGlobal Shutter function*1 achieved in a high-sensitivity back-illuminated CMOS image sensor by using the following key technologies:

Low-current, compact pixel-parallel A/D converter In order to curtail power consumption, the new converter uses comparators that operate with subthreshold currents, resulting in the industry's best-performing*4, low current, compact 14-bit A/D converter. This overcomes the issue of the increased demand for current due to the inclusion of nearly 1,000 times as many A/D converters in comparison with the traditional column A/D conversion method*2.

Cu-Cu (copper-copper) connection*5 To achieve the parallel A/D conversion for all pixels, Sony has developed a technology which makes it possible to include approximately three million Cu-Cu (copper-copper) connections*5 in one sensor. The Cu-Cu connection provides electrical continuity between the pixel and logic substrate, while securing space for implementing as many as 1.46 million A/D converters, the same number as the effective megapixels, as well as the digital memory.

High-speed data transfer construction Sony has developed a new readout circuit to support the massively parallel digital signal transfer required in the A/D conversion process using 1.46 million A/D converters, making it possible to read and write all the pixel signals at high speed.

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