ST Leverages Back-side-illuminated Sensors for Driver Monitoring
Human error causes 95% of traffic accidents. ST hopes to reduce that figure with a new global-shutter image sensor for driver monitoring systems.
One important aspect of vehicle autonomy and safety is driver monitoring systems (DMS). Beyond designing a car that can drive itself, the automotive industry is looking for ways to ensure that the passengers inside them are still paying attention and behaving safely.
It’s for this reason that many companies are investing in hardware and software to support the rollout of new DMS. Last week, STMicroelectronics announced a new global-shutter image sensor for cheaper and more reliable DMS.
The new backside-illuminated global-shutter image sensor. Image from STMicroelectronics (PDF)
In this article, we’ll discuss different methods behind image sensors and how ST’s newest product is designed to improve the size, accuracy, and sensitivity of a new wave of DMS.
Front-side-illuminated Image Sensors
Historically, most conventional CMOS image sensors are employed using a “front-side-illuminated” (FSI) architecture.
From top to bottom, the pixel structure of an FSI consists of a sensor lens, a color filter, multiple metal gate layers, and a photodiode. In an FSI, light must pass through multiple layers of circuitry before reaching the light-receiving surface of a photodiode, where it can then be converted into useful electrical signals. This architecture has been the most widely used in CMOS imaging and hence has become inexpensive and abundant.
The structure of an FSI. Image from Stemmer Imaging
Despite their popularity, FSIs are far from an ideal solution.
One challenge with FSIs is that light must pass through many layers before reaching the photodiodes. Gate structures in the circuitry layers of an FSI are prone to absorb many of the incident photons, effectively reducing the number of photons that reach the photodiode. This creates a theoretical limit on the total quantum efficiency of the device, particularly in the 350–400nm region of the spectrum.
Further, the structure of FSI sensors makes them more susceptible to both optical and electrical crosstalk.
Back-side-illuminated Image Sensors
As engineers strive for more accurate and reliable image sensors, many have turned away from FSIs in favor of back-side-illuminated (BSI) image sensors.
FSI vs. BSI pixel structure. Image from Scientific Imaging
In a BSI image sensor, the pixel structure is reversed from an FSI sensor, where the photodiode comes directly after the color filter, with the metal layers and circuitry existing underneath it. In this structure, the light needs to travel through fewer layers to reach the photodiode, decreasing the overall attenuation and increasing the total light reaching the photodiode.
Because of this, BSIs are less susceptible to optical and electrical crosstalk, further enhancing their reliability compared to an FSI sensor. The aggregate result is that BSI sensors are more sensitive than FSI sensors. They have also been shown to achieve quantum efficiencies of over 90%.
ST’s Newest DMS Sensor
This week, STMicroelectronics announced the release of a new DMS imaging sensor built to provide higher reliability and performance compared to conventional solutions.
Functional block diagram of the VB56G4A. Image from STMicroelectronics
The new product, the VB56G4A, is a 1.5 megapixel automotive-grade imaging sensor built on ST’s 3D-stacked, back-side illuminated (BSI-3D) technology. According to ST, the device was designed from a global shutter architecture with a 2.61 µm x 2.61 µm pixel size, resulting in a small form factor and improved sensitivity compared to FSI solutions.
According to the datasheet, the device achieves 24% quantum efficiency in the 940nm (infrared) region and a linear dynamic range of up to 60 dB. It can also capture up to 88 frames per second at full resolution. Prioritizing power efficiency and size in the new device, ST says the VB56G4A records a power consumption of 145 mW at 60 fps and occupies a 6.2 mm x 6.9 mm OBGA package.