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Overview
A CMOS image sensor (CIS) converts the color and intensity of light captured by a lens into electronic signals and delivers them to a processor. It functions as the "eye" of devices such as smartphones and tablets. With the rise of virtual reality (VR), augmented reality (AR), and autonomous driving, CIS technology has become a key enabler for Industry 4.0 and is expected to evolve beyond basic imaging functions.
1. Frontside-Illuminated (FSI) Technology and Its Limitations
Early CIS products used a frontside-illuminated (FSI) pixel structure, in which the optical stack is placed above CMOS-based circuitry. This structure is suitable for most CIS solutions with pixel sizes of 1.12 μm and larger, and has been widely used in mobile devices, closed-circuit television (CCTV), dashcams, digital SLRs, and automotive sensors.
To produce bright, clear images under low-light conditions, a high-performance image sensor must maximize pixel quantum efficiency (QE). In FSI designs, metal interconnect layers and other circuitry beneath the pixel can obstruct or scatter incoming light, so wiring layout must minimize such interference.
Diffraction occurs when light passes through apertures or around objects; as aperture size decreases, diffraction increases and light spreads. When external light reaches an individual pixel, diffraction is unavoidable. In FSI structures, the presence of metal wiring layers beneath the pixel makes the design more susceptible to diffraction. Even as pixel pitch shrinks, the area covered by metal remains roughly constant, reducing the effective light-collecting region, increasing diffraction, and causing color mixing in the captured image.
2. Emergence of BSI-Based Pixel Technology
In 2011, one of the early mainstream applications of backside-illuminated (BSI) CIS appeared in a commercial smartphone. BSI places the photodiode closer to the incoming light by locating the pixel above the metal and circuitry, which increases the amount of light reaching the photodiode compared with typical FSI layouts.
BSI enabled practical implementation of pixel sizes at or below 1.12 μm and opened the path to high-resolution sensors of 16 megapixels and above. Unlike FSI, whose performance is limited by underlying wiring, BSI optical processing offers greater freedom for pixel architecture. As a result, various optical pixel structures have been developed to improve QE, including backside deep trench isolation (BDTI), W Grid, and Air Grid.
3. Summary
Moving from FSI to BSI is more than a simple reordering of internal layers; it represents a manufacturing and process innovation. Placing the photodiode above the metal layers requires a very thin supporting substrate, on the order of 1/100 the thickness used in typical FSI pixels. These process advances enabled BSI pixel production, though they also lead to higher manufacturing complexity and cost.
