CMOS Shortwave Infrared Sensors Set – Interpreting the Unseen

In a new study, researchers created a new high-performance SWIR image sensor based on non-toxic colloidal quantum dots. Shortwave infrared or SWIR is a wavelength band of light between 900 nm and 2500 nm far beyond the visible spectrum that ranges between 400nm to 700nm. SWIR imaging requires sensors that are capable of operating in this range. 

While conventional systems using charge-coupled devices (CCD) and complementary metal-oxide semiconductor (CMOS) image sensors can capture images within the visible spectrum of light pretty effectively, those leveraging SWIR technology can capture images in deeper wavelengths within the infrared spectrum. This allows the systems to see details even outside the visible spectrum.

Wavelengths that are longer than the visible ones can only be seen by dedicated sensors. And while light in the shortwave infrared (SWIR) region is invisible to the eye, this light interacts with objects in a similar manner as visible wavelengths. 

Much like visible light, SWIR light bounces off of objects and is reflective light, and as a result, it has shadows and contrast in its imagery. Because SWIR images are not in color but only in black and white, it makes objects easily recognizable and allows for individual identification. SWIR imaging also has the ability to image through glass.

So, SWIR has a lot of potential due to its small size, low power, high sensitivity and resolution, and low-cost visible spectrum lenses, along with the ability to see beacons and lasers that are covert and in the light of night sky radiance.

As such, softwave infrared imaging makes a lot of things possible in industrial and scientific applications, including silicon wafer inspection, laser beam profiling, medical imaging, machine vision imaging, chemical and plastics sensing, agricultural sensing, surveillance systems, and hyperspectral imaging in addition to being used for anti-counterfeiting, moisture detection, and sorting. Moreover, it also finds its use case in facial recognition sensors used by cell phones, for target identification & camouflage detection in defense, and by autonomous vehicles in shadowed environments.

Typically, InGaAs sensors are used in SWIR imaging. InGaAs, or indium gallium arsenide, is an alloy used in the manufacturing of certain semiconductors for photonics use. These sensors are used in applications that require wavelengths in the 900-1700 nm range. However, they are expensive and encounter challenges in regard to higher resolution arrays, sensitivity to temperature variations, and limited spectral range.

Click here to learn how lasers are set to play a pivotal role in the coming decades.

Non-toxic Quantum Dots-based SWIR Image Sensors

So, the new study developed a new non-toxic colloidal quantum dots-based high-performance SWIR image sensor. Image sensors with SWIR sensitivity can not only reliably operate under adverse conditions such as fog, bright sunlight, haze, and smoke, but their range also provides a source of illumination that is safe for the eyes. This way, SWIR opens up the possibility of determining the qualities of material through molecular imaging. 

Colloidal quantum dots (CQD) based image sensor technology here offers a promising tech platform to provide high-volume compatible image sensors in the near-infrared.

Quantum dots are human-made tiny particles or nanoscale crystals that were first discovered in 1980 and exhibit unique electronic and optical properties. This includes the ability to transport electrons and produce distinctive colors. These artificially synthesized semiconductor nanoparticles’ size is in the range of 2 to 10 nanometers, about 10-50 atoms in diameter, and have a wide range of applications, including solar cells, LEDs and solid-state lighting, displays, photovoltaics, transistors, ultrafast all-optical switches, logic gates, fluorescent biological labeling, quantum computing, medical imaging, biosensors, and more.

The latest study was published in Nature Photonics and involved researchers from ICFO, the Institute of Photonic Sciences, which conducts experiments in classical and quantum communication systems, and QURV, which is ICFO’s spin-off that develops wide-spectrum image sensor technologies to enable enhanced computer vision applications.

The study presented a new method for manufacturing non-toxic colloidal quantum dots that are functional, high-quality, and integrable with CMOS technology. CMOS is the technology most broadly used today in the chip or microchip industry to form integrated circuits (ICs). This semiconductor technology is used by the latest CPUs and smartphones.

By integrating CQDs with CMOS, the SWIR range can be accessed, though there’s a barrier to transforming these SWIR-sensitive quantum dots into a technology that can be used commercially. This is due to the presence of heavy metals like lead or mercury, which are subject to regulations by the Restriction of Hazardous Substances (RoHS), a European ruling that regulates the usage of these materials in commercial consumer electronic applications.

While the team was exploring ways to synthesize AgBiTe2 or silver bismuth telluride nanocrystals in order to extend the AsBiS2 tech’s coverage for the purpose of increasing the performance of photovoltaic devices, they acquired a by-product during the process, which was silver telluride (Ag2Te).

Ag2Te demonstrated a strong quantum-confined absorption, much like quantum dots, which is adjustable. As the team realized silver telluride’s potential for SWIR photodetectors and image sensors, they shifted their focus and efforts to devise a new process to synthesize phosphine-free versions of these Ag2Te quantum dots. Phosphine was actually found to have a damaging effect on quantum dots’s optoelectronic properties relevant to photodetection.

To synthesize quantum dots of Ag2Te or silver telluride that doesn’t have any phosphine and is size adjustable, the study used a new method that also kept the advantageous properties of traditional heavy-metal counterparts, which laid the foundation for the SWIR colloidal quantum dot tech for large scale production.

In the new synthetic method, researchers used phosphine-free complexes such as silver precursors and tellurium. This enabled them to have quantum dots with well-controlled size distribution and excitonic peaks over a very broad range of the spectrum. These quantum dots showed remarkable performances. Compared to the previous quantum dot fabrication using phosphine-based techniques, the team had an unprecedented achievement in the form of distinct excitonic peaks over 1500 nm.

Using these phosphine-free quantum dots, the team then created a simple laboratory-scale photodetector on a glass substrate covered with Indium Tin Oxide (ITO), which is a common standard, in order to assess their properties and performance. 

The team then had a “challenging task” of reverting the device set up as the lab-scale devices operate by shining light from their bottom. The study’s co-author Yongjie Wang, a postdoc researcher at ICFO, noted that:

“For CMOS integrated CQD stacks, light comes from the top, whereas the bottom part of the device is taken by the CMOS electronics.” 

The photodiode initially demonstrated a low performance in sensing SWIR light, so the team made adjustments to include a buffer layer, which improved its performance significantly. The redesign resulted in a SWIR photodiode demonstrating a spectral range from 350 nm to 1600 nm. Meanwhile, the linear dynamic range surpassed 118 dB, and a -3dB bandwidth exceeded 110 kHz. The SWIR photodiode also had a room temperature detectivity of the order 1012 Jones.

“To the best of our knowledge, the photodiodes reported here have for the first time realized solution-processed, non-toxic shortwave infrared photodiodes with figures of merit on par with other heavy-metal containing counterparts.”

– said ICREA Prof. at ICFO, Gerasimos Konstantatos. 

Adding further, he said:

“These results further support the fact that Ag2Te quantum dots emerge as a promising RoHS-compliant material for low-cost, high-performance SWIR photodetectors applications.”

SWIR Image Sensors’ Potential Real-World Use-cases

After ICFO researchers Lucheng Peng, Aditya Malla, and Wang, led by Konstantatos, successfully developed a heavy-metal-free quantum dot-based photodetector, they collaborated with Qurv researchers Stijn Goossens, Yu Bi, Andres Black, and Julien Schreier to create a SWIR image sensor.

Speaking on the development of high-performance infrared photodetectors and a non-toxic colloidal quantum dots-based SWIR image sensor operating at room temperature, the researchers noted that they then integrated the redesigned photodetector with a CMOS-based ROIC (read-out integrated circuit is the main component of cameras and positioned behind the photodetectors) FPA.

To prove the sensor’s operation in the SWIR, it was tested by taking several pictures of silicon wafers’ transmission under the light. The team was also able to see the content of plastic bottles that were not observable in the visible light range.

This is not all, though. In the next step, the team is interested in enhancing photodiode performance by engineering the device’s different layers. Moreover, the researchers intend to look into new surface chemistries for silver telluride quantum dots to not only amplify their performance but also improve their environmental stability.

For now, though, the ability of the sensor to access the SWIR with a low-cost tech for consumer electronics, according to Konstantatos:

“Will unleash the potential of this spectral range with a huge range of applications including improved vision systems for the automotive industry (cars) enabling vision and driving under adverse weather conditions.”

In addition to automotive applications, he noted that it could also provide an eye-safe window, free of background light under both day and night conditions, hence enabling LiDAR and 3D imaging for augmented reality (AR) and virtual reality (VR) applications as well.

In the field of robotics, these sensors can be particularly helpful for object detection and navigation due to their ability to capture images through certain materials and low-light conditions. This way, they can enhance the perception of the environment for robots, allowing them to navigate it more effectively. Moreover, these sensors can also be used for obstacle avoidance even in complete darkness, further helping ensure safe navigation.

Development and Integration of Infrared Technologies

FLIR Systems, which is part of Teledyne Technologies, is one company that has been working on the development and integration of infrared technologies like SWIR sensors in robotics. Late in 2023, Teledyne unveiled the ‘Black Recon’ Vehicle Reconnaissance System (VRS) at the Defence Security and Equipment International expo in London. 

In development for nearly five years, often in cooperation with the Norwegian Ministry of Defense, the device is designed to launch autonomously. It sends live imagery and targeting information back to vehicle crews and can be used for sweeping for mines, making inspections under bridges, and improvised explosive devices. It also boasts of high precision recon, surveillance, and target acquisition and can also perform GPS-denied operations. To ensure “very high” precision, the device uses a proprietary tracking system that utilizes LED technology with a SWIR camera.

Other companies leading the space include Rockwell Automation, iRobot, ABB, Yaskawa, Fanuc, Yamaha Motor, Midea Group, and Sony.

Now, in Electric Vehicles (EVs), the SWIR image sensor can improve their vision systems by providing reliable imaging to enable more efficient and safer driving under adverse conditions. These sensors can also contribute to the development of night vision systems and help recognize and distinguish objects in different lighting scenarios, making it beneficial for advanced driver assistance systems (ADAS), which are being increasingly used in autonomous vehicles (AVs) to enhance road safety.

Sensing Technologies for the Automotive Industry

Luminar Technologies is a company that works on sensing technologies for the automotive industry, especially LiDAR technology. The US lidar maker plans to launch 1 million Luminar-equipped cars in China in the next five years.

The public company has a market cap of 1.657 bln with its shares trading at $3.23, down about 3% year-to-date. The company has a revenue (TTM) of $58.791 mln, EPS (TTM) of -1.50, and P/E (TTM) of -2.17.

Meanwhile, the likes of Tesla, VW Group, GM, BMW Group, and Hyundai Motor are leading the EV industry.

Augmented Reality and Virtual Reality

Now, in the world of AR and VR, where accurate depth perception and object recognition are essential, these SWIR sensors can be very beneficial for providing more immersive AR and VR experiences. They can further be utilized to maintain image quality in challenging environments and allow AR applications to overlay digital information onto the user’s view even when objects are not directly visible in the visible light spectrum. Being a low-cost tech means by integrating SWIR image sensors into consumer devices, they can be brought to a wide audience, leading to new applications and experiences.

Sony is a prominent name in the AR world that makes use of imaging sensors, including CMOS technology. A few months ago, Sony announced the IMX992 SWIR image sensor with 5.32 effective megapixels and 3.45 μm pixel size for industrial equipment. To capture light efficiently, the sensor comes with an optimized pixel structure that allows for high-definition imaging across a broad spectrum. 

With a market cap of $115 bln, SONY’s shares are trading at $91.22. The company posted revenue (TTM) of $84.834 bln and has EPS (TTM) of 4.72, P/E (TTM) of 19.33, and ROE (TTM) of 12.45%. Sony also pays a dividend yield of 0.61%.

Other prominent names in this field include Microsoft, Google, AMD, NVIDIA, Samsung, AMD, Magic Leap, Meta Platforms, and Unity.

Click here for the list of the ten best augmented reality & virtual reality stocks.

Conclusion

As we saw, CMOS SWIR sensors’ sensitivity to light in the shortwave infrared spectrum allows them to capture images beyond the range of traditional visible light sensors, which makes them valuable in applications where visibility through certain materials or in low-light conditions is critical. 

This way, it can make unprecedented performance and reliability in high-volume, computer vision-first applications in different industries such as automotive, service robotics, AR, VR, and consumer electronics markets. In the future, as technology continues to advance, the integration of these sensors can expand in other areas, like space exploration, as well.