Pixel is the smallest element on the sensor that converts the energy from light (photons) to electrons which are then digitized on the sensor itself to provide us with an image. The size of each pixel indicates how many photons it can accept during the time the sensor is exposed to the light. The more the pixel size, the more light it accepts, thereby, providing us with better image.
However, we are seeing technology improving leaps and bounds to enable us to get higher quality images with lower pixel sizes. This improvements helps us to reduce the sensor size. At this point, we would like to introduce you to sensor architectures such as FSI, BSI, BSI-II.
They stand for:
1. FSI – Front Side Illumination
2. BSI – Back Side Illumination
3. BSI II – Back Side Illumination II
Before we get into the details, we can quickly have a glance at a term called well capacity.
As we know, each pixel accepts the light energy falling on them in the form of photons and generates electrical charge in terms of electrons. The maximum electrical charge that each pixel can hold is termed as “full well capacity”. The full well capacity determines the dynamic range. The fill factor is determined by the area of the pixel that can gather light. The sensor architecture determines how much area of the pixel substrate is active area and how much area reduces the fill factor as it is not receptive to photons or it deflects photons.
FSI – Front Side Illumination
In this case, the sensor architecture is such that the light is falling on the front side of the substrate. The substrate, apart from the active pixel area also comprises of other interconnects and circuitry that are essential for charge transfer and sensor functioning. The photon shall have to fall on the active area that lies in-between the above mentioned inactive regions as shown in the below image.
This results in loss of higher percentage of photons. Thus, the architecture demanded higher pixel size to enable more light to fall on the active region.
BSI – Back Side Illumination
In this case, the sensor architecture is such that the light is falling on the back side of the substrate. In this scenario, the photons are directly falling on the active area and we do not lose photons due to other inactive substrate region as they are on the front side. Please refer the image above for better understanding.
This enables more percentage of photons to fall on the active area even with a small pixel size. With BSI architecture, the pixel size were brought down to 1.1um. To add to that, the current BSI II technology enables more well depth at the same 1.1um, thereby, enabling more responsivity and more dynamic range.
Examples
FSI Sensors - OV5640 from Omnivision
BSI sensors - AR0330, AR0230 from On Semiconductor
BSI II Sensors - AR1335, AR1337 from On Semiconductor
When we talk about pixel architecture, we have to look at microlenses. What is a microlens? Microlens are designed in the form of arrays that that are designed on top of the sensor area. They are designed to focus the light onto the active pixel area.
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Sarvesh Rajagopal
