Sony develops stacked CMOS image sensor technology with 2-layer transistor pixel

Article by: Sony Semiconductor Solutions Corp.

Sony Semiconductor has developed the world’s first stacked CMOS image sensor technology with 2-Layer Transistor Pixel.

Sony Semiconductor Solutions Corp. succeeded in developing the world’s first stacked CMOS image sensor technology with 2-Layer Transistor Pixel.

While the photodiodes and pixel transistors of conventional CMOS image sensors occupy the same substrate, Sony’s new technology separates the photodiodes and pixel transistors on different substrate layers. This new architecture approximately doubles the saturation signal level compared to conventional image sensors, widens the dynamic range and reduces noise, thereby significantly improving imaging properties. The pixel structure of the new technology will allow pixels to maintain or improve their existing properties not only at current but also smaller pixel sizes.

A stacked CMOS image sensor adopts a stacked structure consisting of a pixel chip composed of back-illuminated pixels stacked on a logic chip where signal processing circuits are formed. In the pixel chip, photodiodes for converting light into electrical signals and pixel transistors for controlling the signals are located next to each other on the same layer. Increasing the saturation signal level within the form factor constraints plays an important role in achieving high image quality with wide dynamic range.

Cutaway photo of the CMOS image sensor with 2-Layer Transistor Pixel technology.

Sony’s new architecture is a breakthrough in stacked CMOS image sensor technology. Using its proprietary stacking technology, Sony packaged the photodiodes and pixel transistors on separate substrates stacked on top of each other.

In conventional stacked CMOS image sensors, on the other hand, the photodiodes and pixel transistors are side by side on the same substrate. The new stacking technology enables the adoption of architectures that allow each of the photodiode and pixel transistor layers to be optimized, thus approximately doubling the saturation signal level compared to conventional image sensors and, in turn , expanding the dynamic range.

Additionally, since pixel transistors other than transfer gates (TRG), including reset transistors (RST), select transistors (SEL), and amplifier transistors (AMP), occupy one layer without a photodiode, the size of the amplifying transistors can be increased. By increasing the size of the amplifier transistors, Sony has managed to significantly reduce the noise that nighttime images and other dark places are prone to.

The expanded dynamic range and noise reduction available with this new technology will prevent underexposure and overexposure in settings with a combination of bright and low lighting (e.g. backlit settings) and enable images of high quality and low noise even in low light (e.g. indoor, nighttime).

Sony announced this breakthrough at the recent IEEE International Electron Devices Meeting.

Michael C. Garrison