Unlike conventional CMOS image sensors in which photodiodes and pixel transistors occupy the same substrate, Sony’s new technology separates photodiodes and pixel transistors on different substrate layers. This new architecture, says the company, approximately doubles saturation signal level relative to conventional image sensors, widens dynamic range, and reduces noise, thereby substantially improving imaging properties, and the new technology’s pixel structure will enable pixels to maintain or improve their existing properties at not only current but also smaller pixel sizes.
A stacked CMOS image sensor adopts a stacked structure consisting of a pixel chip made up of back-illuminated pixels stacked atop a logic chip where signal processing circuits are formed. Within the pixel chip, photodiodes for converting light to electrical signals, and pixel transistors for controlling the signals are situated alongside each other on the same layer. Increasing saturation signal level within form-factor constraints plays an important role in realizing high image quality with wide dynamic range.
The company’s new architecture is offered as an advancement in stacked CMOS image sensor technology. The new stacking technology enables adoption of architectures that allow the photodiode and pixel transistor layers to each be optimized, thereby approximately doubling saturation signal level relative to conventional image sensors and, in turn, widening dynamic range.
In addition, because pixel transistors other than transfer gates (TRG), including reset transistors (RST), select transistors (SEL) and amp transistors (AMP), occupy a photodiode-free layer, the amp transistors can be increased in size. By increasing amp transistor size, the company says that it succeeded in substantially reducing the noise to which nighttime and other dark-location images are prone.
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. The company says that its two-layer transistor pixel technology will contribute to the realization of increasingly high-quality imaging such as smartphone photographs.
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