The quest for superior digital images in evershrinking camera designs never ends. Now, CMOS-sensor specialist OmniVision Technologies has partnered with Taiwan Semiconductor Manufacturing Corp. (TSMC) to take a completely different approach to traditional CMOS image sensor technology.
OmniVision’s OmniBSI architecture consists of a novel sensor design that uses backside illumination (BSI) to improve image quality while shrinking pixel size down to 0.9 µm. This accomplishment is critical for the further miniaturization of digital imaging products.
The OmniBSI architecture turns the camera sensor chip upside down, allowing the device to receive light from what was previously the backside of the silicon substrate. This approach is a departure from traditional front-side illumination (FSI) image sensors where light reaching the photosensitive area is partially limited by the metal and dielectric layers necessary for the sensor to convert photons into electrons.
FSI VERSUS BSI
Conventional FSI CMOS sensors can block or deflect light from reaching the pixel, reducing the fill factor and causing more problems such as cross talk between pixels. Depending on the level, when cross talk occurs between pixels of two different colors, the colors blend. This decreases image sharpness and creates an unnatural color landscape.
Measuring 1.75 µm, FSI pixels are larger than BSI pixels. Consequently, they require certain camera components, particularly the length of the lens, to be larger. In the race for less space, every millimeter and micron is precious.
The OmniBSI architecture takes the FSI topology and reverses the arrangement of layers, situating the metal and dielectric layers under the sensor array (Fig. 1). Instead of passing through the metal layers, light hits the silicon layer directly without interference.
The first advantage of this approach is that light entering the sensor takes the shortest path to the detector, through the color filter only. There are no metal layers or transistors to block or reflect light.
Since light strikes the silicon directly, the sensor’s fill factor significantly improves, which in turn boosts low-light sensitivity dramatically. As this arrangement optimizes light absorption, it most notably creates a 1.4-µm BSI pixel, which OmniVision claims surpasses all the performance metrics of 1.4-µm and most 1.75-µm FSI pixels.
“Under current design rules, moving FSI pixels down to 1.4 µm and below poses real challenges because metal lines and transistors drive the pixel aperture close to the wavelength of light, its physical limit,” says Howard Rhodes, VP of process engineering at OmniVision.
“Overcoming this with traditional FSI pixel technology would require a migration to 65-nm copper process technologies, which would significantly increase the complexity and cost of manufacturing,” Rhodes continues. “As BSI allows for more than three metal layers, it achieves significant manufacturing benefits without moving to smaller process nodes that add complexities and costs. Routing is simpler and die sizes can be smaller than in FSI sensors.”
Other advantages of the OmniBSI architecture include increased sensitivity per unit area and improved quantum efficiency. It also reduces crosstalk and photo response non-uniformity, which helps to produce sharp images and better colors. Lastly, BSI accommodates a larger aperture size, which allows for lower camera-lens f-stops. The pixel then can collect more photons, translating into better pictures, especially in low-light situations.
The BSI approach has been around for a while. But due to cost issues, its application has been predominantly in the military and avionics fields.
“Although backside illumination concepts have been studied for over 20 years, up until now nobody has been able to successfully develop the process for commercial, high-volume CMOS sensor manufacturing,” says Ken Chen, senior director of mainstream technology marketing at TSMC. “Combining OmniVision’s imaging expertise with TSMC’s experience in process development, we have delivered a truly advanced technology that defines the future of digital imaging.”
OmniVision has demonstrated an 8-Mpixel Omni BSI sensor, built on its 0.11-µm process technology (Fig. 2). In addition to thinner camera modules, the device forecasts lower stack heights enabling a higher chief ray angle (CRA) for shorter lenses, lower z-height modules, easier zoom capabilities due to tolerances in the CRA, best in class aperture size, and best performance for price (Fig. 3). The Omni BSI sensor falls under the company’s line of CameraChip sensors and should be sampling now (Fig. 4).