Sony has announced what it claims is the world’s first vertically stacked CMOS image sensor design, which could potentially be a breakthrough in terms of low light imaging. According to Sony, the breakthrough was achieved by vertically stacking a layer of pixel transistors below a layer of photodiodes, which has benefits in the shape of stronger saturation signals of pixels, wider dynamic range and improved low light imaging performance. The hardware-level change is, according to the company, an entirely new design for CMOS (complimentary metal oxide semiconductor) image sensor design.
In a typical stacked CMOS image sensor, the photodiodes and pixel transistors are stacked next to each other — parallely and in one layer itself. A photodiode is the outermost layer of an image sensor, which captures the light data coming in through the lens of a camera, and converts it into electrical signals. The pixel transistors then control the signals and decode them into colour data on the sensor itself, before relaying this signal on to the logic chip of the image sensor. The logic chip then renders the data received from the pixel transistors, running it through an image processor to finally produce the end result — the photograph.
By stacking the photodiodes and pixel transistors in two layers, Sony increased the area and volume of each layer, while also optimising each layer individually. This, Sony says, has helped in doubling the typical saturation signal level — or the volume of colour and luminance data that each pixel can process. In turn, this higher saturation signal from each pixel accounts for far wider dynamic range than what a sensor of a certain size can achieve today.
The separate transistor layer also means that amp transistors, which are responsible for handling image signal noise, can also be increased in size as the stacked sensor offers a full layer for the pixel transistors. This, in turn, means that image noise can be significantly reduced as the image signal is amplified more than before — according to Sony.
“The widened dynamic range and noise reduction available from this new technology will prevent underexposure and overexposure in settings with a combination of bright and dim illumination (e.g., backlit settings) and enable high-quality, low-noise images even in low-light (e.g., indoor, nighttime) settings,” Sony said in a statement.
Sony stated that its two-layer stacked CMOS sensor design will come to smartphone image sensors in future, where it may play a massive role. Mobile image sensors are confined by how large they can be made, where vertical stacking may prove to be key.
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