Referring to backlighting CMOS, there are many cameras or cameras have adopted this technology, SONY is no exception, there are more SONY autofocusing cameras are used this sensor, such as FCB-ER8530, FCB-EV7520,FCB-ER7100 and so on. Since it is so widely used, today I will tell you about backlit CMOS.
1. Development of CMOS
We are familiar with two kinds of sensors, one is CCD, the other is CMOS. Early CMOS is much worse than CCD, but with the development of technology, now the quality of CMOS has taken a qualitative leap, although not equal to CCD, but the gap has shrunk very small. In addition, CMOS is cheap, which is the most suitable for film and television shooting in terms of cost and power consumption.
2. The photosensitive principle of front-lit CMOS
Up to now, CMOS has evolved into backlit stack CMOS. However, in the early days of CMOS, such complex technology was not possible. At that time, the relatively mature CMOS was the structure of Front Side Illumination (abbreviated FSI).
The following is a classic, traditional front-shot CMOS structure diagram.
In the traditional front-facing CMOS structure, there are Micro lens, Color Filter, Wiring Layers and photodiode from top to bottom.
Microlenses are tiny convex lenses on each CMOS physical pixel that are used to focus light.
The color filters further down break down the incident light color in RGB mode, and the Bayer arrangement we sometimes hear is how these filters are arranged. Such as the most classic RGB permutations.
Further down are the circuitry layers, usually several, which are mainly used to transmit signals.
Further down, there’s the photodiode, which, in the case of CMOS, is really the photosensitive part where the photoelectric effect occurs.
This means that light needs to pass through a microlens, through a color filter, and through multiple layers of circuits before it reaches the photodiode. Among them, the circuit layer is the most interference.
Where does the interference come from? This is mainly because the transmittance of metal is limited, and it will reflect light. Even if the design is good, due to technological limitations, the light after passing through the metal circuit layer reaches the photodiode only about 70%. Now, low – and medium-grade CMOS circuit layer uses relatively cheap aluminum, which has about 90% reflectivity for visible band. Sometimes the reflected light also crosstalk adjacent pixels, causing color distortion.
3. Advantages of backlit CMOS
Due to these disadvantages of illuminated Illuminated CMOS, the design of Back-Illuminated CMOS was created. It places the circuit layer behind the photodiode, so that the light hits the photodiode directly, without being lost by blocking and reflecting the circuit layer.
As a result, backlit CMOS is capable of higher light utilization efficiency and thus, higher sensitivity in low-light environments. At the same time, since the circuit does not interfere with the photodiode’s ability to receive light, the circuit layer can be made thicker so that more processing circuits can be placed, which helps to speed up signal processing.
Devices equipped with back-lit sensors are capable of achieving approximately 30 to 50 percent more sensitivity in low-light environments than conventional forward-lit sensors, allowing them to take higher-quality photos or videos in low-light environments with less noise. And more abundant processing circuit, can do a larger amount of data of the original image signal processing.
Taken together, these features are the bottom reason why the camera equipment has better elevation, larger resolution and other parameters.
4. SONY Exmor R CMOS
SONY’s Exmor R refers to backlit CMOS. With the development of technology, SONY’s Exmor R has made a big leap forward.
Note, however, that Exmor is different from Exmor R. Exmor is still A front-shot CMOS, which incorporates an analog-to-digital A/D converter in each column to improve the image signal-to-noise ratio.
Exmor and Exmor R
SONY also introduced the Exmor RS trademark, which is a combination of backlit and stacked design. So what is a stack? What kind of sensor is used in SONY’s upcoming autofocus camera FCB-EV9520L? We’ll talk about it later.