What Is a Back-Illuminated CMOS Sensor?
A Back-Illuminated CMOS sensor is a type of CMOS sensor.
They are used in digital cameras and smartphones as image sensors that convert light information received through the lens into electrical signals.
There are two types of CMOS sensors: surface-illuminated and back-illuminated.
Conventional front-illuminated CMOS sensors have photodiodes on the opposite side of the surface from the incident side, whereas back-illuminated CMOS sensors have the structure of a photodiode on the incident side by irradiating light from the backside of the substrate.
Uses of Back-Illuminated CMOS Sensors
CMOS sensors are widely used as image sensors in digital cameras, smartphone cameras, and image inspection equipment.
Back-illuminated CMOS sensors are also used as distance image sensors in augmented reality (AR)/virtual reality (VR), robots and drones that require autonomous operation, and object recognition and obstacle detection devices, due to their high sensitivity and low noise compared to conventional CMOS sensors.
They are also used in automotive applications such as advanced driver assistance systems (ADAS) and camera monitoring systems (CMS) that replace rearview mirrors.
Principle of Back-Illuminated CMOS Sensors
A CMOS sensor consists of one photodiode, amplifier, and switch per pixel, and operates on the principle of converting and storing the received light into an electric charge with the photodiode, converting the charge into a voltage signal with the amplifier, and reading out the signal per pixel by switching one switch after another to select photodiodes arranged in a grid pattern.
The principle of the sensor works by switching a series of switches that select photodiodes arranged in a grid and reading out the signal from each pixel.
Conventional surface-illuminated CMOS sensors, like ordinary CMOS circuits, have a structure of photodiodes on the bottom layer of the silicon substrate, a wiring layer above the photodiodes, and a color filter and on-chip lens on the layer above that.
The problem with conventional CMOS sensors with this structure is that the light focused by the on-chip lens reaches the photodiode, but is blocked by the wiring and transistor circuitry, resulting in reduced sensitivity.
On the other hand, in Back-Illuminated CMOS sensors, the photodiode and wiring layers are installed on a silicon substrate, the substrate is inverted and shaved, and a color filter and on-chip lens are mounted on the backside.
In this structure of Back-Illuminated CMOS sensors, the light-illuminating surface is the back of the silicon substrate, and the amount of light received by a unit pixel is increased compared to the front-illuminated type because the focused light is not affected by the wiring and transistors. It also has a higher sensitivity to light incident from oblique angles.