Panasonic Announces High - Dynamic - Range Cmos Image Sensor

Panasonic Announces High-dynamic-range CMOS Image SensorPanasonic Corp developed an image sensor with a dynamic range of 123dB, which is 100 times higher than that of the company's previous product.
Panasonic announced the sensor at ISSCC 2016, which runs from Jan 31 to Feb 4, 2016, in San Francisco, the US. When the sensor is applied to vehicles, it can display backlit people and vehicles without over-exposure.
It was realized by adding (1) the "two-cell-per-pixel composition technology" and (2) a circuit technology to improve S/N (signal versus noise) ratio at the time of taking images in the dark to an organic thin-film CMOS image sensor being developed by the company.

Each pixel of the organic thin-film CMOS image sensor uses an organic thin film capable of photoelectric conversion. Unlike backside-illuminated (BSI) sensors, which are the mainstream in the market for digital cameras and camcorders, the organic thin film is formed on the side of the chip's trace layer.

In the case of BSI sensors, photo diodes that convert light into electric signals are placed on the backside of the trace layer in the aim of prevening the layer from blocking incident light.

On the other hand, in the case of the new organic thin-film CMOS image sensor, the front side of the trace layer is covered with an organic thin film. Its light-receiving area is larger than that of the photo diodes of BSI sensors. Therefore, the sensitivity of the new sensor can be easily improved by taking in more light. Also, because it is thinner, even when light with an incident angle of ±60° is received, the light is not leaked to adjacent cells. In the case of BSI sensors, light with an incident angle of up to ±30-40° can be received without leakage, Panasonic said.

This time, to further improve the performance of the organic thin-film CMOS image sensor, Panasonic expanded the dynamic range by making each pixel with two types of cells (high- and low-sensitivity cells). The apertural area of the high-sensitivity cell was increased while that of the low-sensitivity cell was decreased. And the capacity of the low-sensitivity cell was increased so that the cell is not easily saturated with electric charge even when a strong light is applied to it.

As a result, there are a 10 times difference in cell area and a 10 times difference in capacity, increasing dynamic range by 100 times.

Characteristics of organic thin film

The expansion of the dynamic range was realized by utilizing the characteristics of the organic thin-film CMOS image sensor, which BSI sensors do not have. The organic thin film has a photoelectric conversion function, and electric charge is stored in circuits formed on a different layer. Therefore, it becomes possible to increase the area of the organic thin film and reduce the size of the corresponding area used to store electric charge.

On the other hand, the photo diodes of BSI sensors have both photoelectric conversion and electric charge storage functions. So, if the size of the storage area is increased to prevent it from being saturated with electric charge due to a strong light, the size of the apertural area also increases.

It is possible to take multiple images with different exposure times within a frame and synthesize them in the aim of expanding dynamic range. However, the new method does not cause blur even at the time of shooting fast-moving objects because the new CMOS sensor features a "global shutter function," which takes images at a time by turning on/off the organic film.

Panasonic considers that the new method is superior when pictures are taken from the inside of a moving vehicle. Also, it is possible to prevent "LED flicker," which is becoming a problem as more and more LEDs are being used for traffic signals and lighting apparatuses, by increasing exposure time within a frame.

Noise reduced to about 1.6 electrons

As a circuit technology to reduce noise at the time of shooting objects in the dark, Panasonic developed a capacitive-coupling noise-canceling technology. Image sensors reset and eliminate electric charge stored in the readout circuit before starting shooting each frame and measure the amount of electric charge equivalent to the amount of newly incoming light. The new circuit technology reduces the amount of electric charge remaining after the resetting to about 1.6 electrons at most.

Panasonic made improvements to the composition of the amplifier circuit used to measure the amount of electric charge. In the area that stores electric charge, there is a fluctuation caused by noise. Though the fluctuation is conventionally canceled by a negative feedback circuit, there remain up to about 25 electrons.

This time, the company adjusted the parameter that determines the gain of the negative feedback circuit, which causes the problem, by adding a capacitor. This measure also improves sensitivity and contributes to reducing blur at the time of high-speed shooting.

Panasonic plans to use the organic thin-film CMOS image sensor in the fields of automobiles and robot vision by utilizing its characteristics, which BIS sensors do not have, such as the capability of independently controlling photoelectric conversion and electric charge storage.