| Panasonic claims that the reduced dispersion of light and charge between pixels allows for better differentiation between colors. |
Panasonic has published a blog post promoting the claimed benefits of the organic film CMOS sensor it has been developing since 2013. While the company has previously talked about the organic film sensor’s ability to deliver a global shutter, wide dynamic range, and a variable ND effect, the new blog post discusses low color crosstalk, something Panasonic says the sensor can demonstrate. Could.
Low color crosstalk means that the red, green, and blue pixels of the sensor only collect their intended color and that light or charge does not spill over from different colored pixels. This promises greater color accuracy, especially under contrast-colored light sources that fall between these two primary colors (especially very yellow, cyan or magenta light).
Panasonic highlights three technologies that enable this low cross-talk:
Part of the benefit comes from the fact that the photoconductive film layer is more effective at absorbing electrons than silicon (up to 10x for green light, the company says), allowing the film layer to be much thinner. This thinness means that there is a narrow range of angles at which light striking an adjacent pixel can travel through the photosensitive section and into a neighboring pixel. This makes the physical separation of different colored pixels more effective.
Another aspect of cross-talk reduction is a series of discharge electrodes at the edges of the pixels: these carry away the charge generated at the boundaries between pixels, so that charge from adjacent pixels does not accumulate. These discharge electrodes were not shown on Panasonic’s previous diagrams, and may result in a reduction in sensor efficiency, as some of the charge generated by the organic layer moves away rather than toward image formation.
Panasonic claims that the final benefit is that the electrode behind the organic layer prevents long wavelength light (especially red light) from entering the sensor’s circuitry, beyond the photosensitive area. In Panasonic’s design, the electrode reflects unabsorbed light back into the organic layer, allowing its absorption; The company claims that only 1% of the red light (measured at 600nm wavelength) enters the sensor circuitry, compared to 20% in some CMOS designs.
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| Minimizing pixels collecting light from their neighbors should be especially valuable under challenging lighting conditions that fall between the sensor’s two priorities. |
The organic photoconductive film used in the sensor was originally patented By Fujifilm in 2011Who Then cooperated with Panasonic To develop a sensor in 2013. After this Panasonic 8K-capable sensor built on technology And released a camera that uses,
However, this latest blog post proposes commercial broadcasting, industrial machine vision, medical, automotive, and healthcare as applications in which the sensor could provide benefits: the company does not include photography in the list.
blog post:
Panasonic has developed Organic Photoconductive Film (OPF) CMOS image sensor technology that achieves excellent color reproduction capability under any light source irradiation.
Osaka, Japan – Panasonic Holdings Corporation announced that it has developed excellent color reproduction technology that suppresses color crosstalk by using the high light absorption rate of organic photoconductive file (OPF) and thinning the photoelectric conversion layer using electrical pixel separation technology. Is. In this technology, the OPF part that performs the photoelectric conversion and the circuit part that stores and readouts the electrical charge are completely independent. This unique layered structure dramatically reduces the sensitivity of each pixel to green, red and blue in wavelength regions outside the target range. As a result, color crosstalk is reduced, excellent spectral characteristics are achieved, and accurate color reproduction becomes possible regardless of the type of light source.
abstract
Traditional Bayer array-type silicon image sensors do not have sufficient color separation performance for green, red and blue. So, for example, under light sources that have peaks at specific wavelengths, such as cyan light and magenta light, it has been difficult to reproduce, identify, and judge colors accurately.
Our OPF CMOS image sensor has a unique structure in which the photoelectric conversion part that converts light into an electrical signal is an organic thin film, and the function of storing and reading the signal charge is carried out in the circuit part, which is completely are independent of each other (Figure 1). As a result, unlike conventional silicon image sensors, it is possible to provide photoelectric conversion characteristics that do not depend on the physical properties of the silicon. OPF with its high light absorption rate enables thinning of the photoelectric conversion part ((1) Photoelectric conversion film thinning technology). By providing a discharge electrode at the pixel boundaries, the signal charge caused by incident light at the pixel boundaries is discharged, and the signal charge from adjacent pixels is suppressed ((2) Electrical pixel isolation technique). Furthermore, since the bottom of the OPF is covered with pixel electrodes to collect the signal charge generated in the OPF and electrodes to discharge the charge, incident light that cannot be absorbed by the OPF does not reach the circuit side. It suppresses transmission ((3) Light transmission suppression structure). With the above three technologies, it is possible to suppress light and signal charge entering from adjacent pixels. As a result, color crosstalk can be reduced to nearly ideal size, as shown in the spectral characteristics shown in Figure 2, and accurate color reproduction is achieved regardless of the color of the light source (Figure 3).
This technology enables accurate color reproduction and inspection even in environments where it is difficult for conventional image sensors to reproduce the original colors, such as plant factories that use magenta light. It is also possible to accurately reproduce the colors of substances with subtle color changes, such as living organisms. It can also be applied to the management of skin conditions, monitoring of health conditions and inspection of fruits and vegetables. Furthermore, in combination with the high saturation characteristics of our OPF CMOS image sensor and the global shutter function,This can contribute to extremely robust imaging systems that are highly tolerant to changes in light source type, illumination, and motion.
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| BSI C CMOS Image Sensor | OPF CMOS Image Sensor |
| Figure 1. Comparison of pixel structure (cross-sectional image) | |
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| BSI C CMOS Image Sensor | OPF CMOS Image Sensor |
| Figure 2. Comparison of spectral characteristics | |
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| Figure 3. Comparison of color chart imaging under different light sources |
Main characteristics
This development is based on the following technologies.
- Photoelectric conversion film thinning technology with 10 times higher light absorption
- Electrical pixel isolation technology that discharges unnecessary charge at pixel boundaries
- Light transmission suppression structure that suppresses the transmission of light through the photoelectric conversion part
Description of Technologies
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Photoelectric conversion film thinning technology with 10 times higher light absorption
The light absorption coefficient of the OPF developed this time is approximately 10 times higher than that of silicon (Figure 4). The distance required for light absorption is reduced, allowing OPFs to be designed thinner than silicon photodiodes, and in principle, it is possible to reduce oblique incident light from adjacent pixels, which is a factor in color crosstalk. (Figure 5).
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| Figure 4. Optical absorption coefficient of OPF and C |
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| BSI C CMOS Image Sensor | OPF CMOS Image Sensor |
| Figure 5. Comparison of the effects of obliquely incident light. | |
- Electrical pixel isolation technology that discharges unnecessary charge at pixel boundaries
Charge generated at pixel boundaries includes signal charge originating from adjacent pixels due to oblique incident light, which contributes to color crosstalk and resolution degradation. In conventional silicon image sensors, a light-shielding layer is provided at the boundary between pixels to prevent oblique incident light. However, the light reflected by the light-shielding layer becomes stray light and enters adjacent pixels, and is diffracted to wrap around, resulting in insufficient light-shielding. Therefore, Panasonic has developed a structure that discharges the signal charge caused by incident light at pixel boundaries and suppresses the intrusion of signal charge from adjacent pixels by placing a new discharge electrode at the pixel boundaries. As shown in Figure 6, by providing a discharge electrode, the charge generated at the pixel boundaries is discharged, thereby preventing degradation of image quality.
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| Figure 6. Signal charge in OPF |
- Light transmission suppression structure that suppresses the transmission of light through the photoelectric conversion part
Light incident on a photoelectric converter (photodiode in silicon image sensors, OPF in CMOS image sensors) is photoelectrically converted into signal charge. However, some of the light is not photoelectrically converted and passes through, which contributes to chromatic crosstalk. Red light, which has a longer wavelength and lower energy than other light, is easier to distinguish and has more crosstalk. As shown in Figure 7, a silicon image sensor transmits about 20% of the light with a wavelength of 600 nm, while an OPF CMOS image sensor transmits only 1% of the light with the same wavelength. The bottom of the OPF is covered with a pixel electrode to collect signal charge and an electrode to discharge charge. Therefore, the incident light that cannot be completely absorbed by the OPF gets absorbed or reflected by the electrode, and the reflected light is again absorbed by the OPF. Furthermore, since the space between the pixel electrode and the discharge electrode is very small, it is difficult for light to pass through the bottom part of the OPF. As a result, OPF CMOS image sensors are structurally very tolerant of color crosstalk.
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| BSI C CMOS Image Sensor | OPF CMOS Image Sensor |
| 7. Simulating light intensity at pixel cross section | |
In the future, we will propose these OPF CMOS image sensor technologies for various applications such as commercial broadcast cameras, surveillance cameras, industrial inspection cameras and automotive cameras. We will also contribute to highly robust imaging systems that are highly tolerant to changes in light source type, illumination, and motion (Figure 8).
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| Figure 8. Application example of OPF CMOS image sensor |
Panasonic will present some of these technologies at the international academic conference Image Sensors Europe 2023, to be held in London, UK from March 15 to 16, 2023.
*(Press Release) Panasonic has developed the industry’s first 8K high-resolution, high-performance global shutter technology using an organic-photoconductive-film CMOS image sensor.https://news.panasonic.com/global/press/en180214-2
technical terms
(1) Bayer array
A series of color filters are installed in each pixel to obtain color information. It is arranged repeatedly in units of 4 pixels of RGGB. Since each pixel contains only R, G, or B color information, other color information is interpolated from surrounding pixels.
(2) color crosstalk
Mixing signals from one pixel to adjacent pixels. In Bayer array type image sensors, since adjacent pixels have different colors, the color signals are mixed, resulting in a situation in which accurate colors cannot be reproduced.
(3) Color reproduction
How accurately a captured image can reproduce the colors of the subject. It is affected by the spectrum of the image sensor, the spectrum of the light source, and the reflection spectrum of the target object.
