Old barriers to mass SWIR sensors production are tumbling fast, with multiple sectors set to benefit. And, says Quantum Solutions’ Dr. Sergio Lentijo Mozo, supplying top-quality quantum dots will be a key part of the process.

Short-wave infrared (SWIR) sensing in the range from 900 – 2,500 nm is attracting attention in multiple fields because the imaging solutions it offers promises a range of unique features.

Among the many sectors eager to capitalise on SWIR’s many benefits are the machine vision, automotive and consumer electronics industries. But complex production procedures mean that the creation of current SWIR sensors is not easily scaled.

This is because the majority are based on InGaAs technology – resulting in a complicated and non-scalable fabrication process. As a result, this leads to a high production cost, as well as a high price per sensor. Sensor prices can reach as much as tens of thousands of dollars.

Additionally, the current generation of sensors have a narrow sensitivity range up to 1,700 nm, as well as limited camera resolution (<1.36 MP). These two factors, plus the cost, hinder SWIR’s widespread adoption.

New technology heralds major changes

New sensor technology based on QDot™ quantum dots promises to democratise the SWIR image sensor camera market by offering a new benchmark in performance and cost. This is because QDot™ quantum dots can be monolithically integrated on silicon CMOS read-out integrated circuits (ROIC).

As a result, they are an ideal solution for businesses searching for economical, high-resolution image sensors with the broadband sensitivity they need.

Several companies have already commercialized Quantum Dot SWIR image sensors. These include Emberion and SWIR Vision Systems, whose cameras are sold for machine vision applications.

Compared to sensors created using the InGaAs production method, QDot-based SWIR image sensors offer a wider range sensitivity (300-2,500nm), smaller pixel size (<5μm), higher resolution, and a sensor fabrication process that is a 3-10x times more cost-efficient.

Taking SWIR production to the next level

Adoption of SWIR cameras in a mass market requires the price for these sensors to drop below 50-100 dollars. Such cost optimisation is only possible by making use of high throughput wafer-level processing. Despite the common belief that the QDot technology is at an early stage, it is, in fact, validated and mature enough for scaled-up SWIR image sensors fabrication.

Imec, a nanotechnology hub based in Belgium, is developing a high throughput technology for 200mm wafer-level processing and has already demonstrated SWIR image sensors with sub-2μm pixel pitch. The work is being done in partnership with stakeholders across the value chain.

For such a throughput, quantum dots production quality and reproducibility is one of the most crucial requirements. And in order to guarantee the replicability of the camera’s spectral range, EQE, dark current and processability, it is imperative that the quantum dots quality must be consistent. This is where Quantum Solutions is coming into play by providing a continuous supply of quantum dots qualified for SWIR sensors manufacturing.

As Dr. Sergio Lentijo Mozo, director of manufacturing at Quantum Solutions noted: “We are able to accommodate strict requirements of the optoelectronic industry because of unique customised approach, a proprietary manufacturing technology and high standards of quality control.”

Image sensor prototype:

To showcase the technology possibilities, Imec engineers fabricated the SWIR image sensor prototype using Quantum Solutions’ QDot™ PbS quantum dots with 5.5 nm particle size corresponding to 1,420 nm absorbance peak (QDot™ PbS-1420-abs). A quantum dot stack was deposited on CMOS ROIC chip to make the SWIR camera prototype having 640 x 480 px resolution with 5μm px pitch. The prototype demonstrated the solid performance metrics with the spectral sensitivity range of 300 – 1,500 nm with the EQE reaching over 45% at 1,450 nm that is getting closer to the industrial standards based on InGaAs technology. But the unique advantages of quantum dots technology is its spectral range tunability. In principle, this high throughput technology platform allows to utilize PbS quantum dots in a wide spectral ranges up to 2,500 nm.

QDot™ SWIR sensor specification (using QDot™ PbS-1420-abs):

Parameter	Performance
Range	400 – 1500 nm
Pixel pitch	5 μm
Array size	640×480 px
CG (HCG/LCG)	8.3/1.5 μV/e–
DR	84 dB
PD swing	0.7 V
FWC	470,000 e–
Dark current (at room temperature)	< 1 μA/cm2
PRNU	1.3 %
RN (dark random noise, min integration time in dark)	33 e–
QE @ 1450 nm	45 %

QDot™ SWIR sensor spectral sensitivity (using QDot™ PbS-1420-abs):

SWIR sensor prototype in use:

The SWIR sensor technology using QDot™ PbS quantum dots has unprecedented advantages over traditional InGaAs sensors. Quantum dot sensor technology is scalable, tunable and enables high resolution imaging, and can be used in many devices starting from machine vision cameras to consumer electronics. For showcasing purposes, Quantum Solutions and Imec demonstrated how SWIR sensors might be utilized.

In machine vision, SWIR cameras can be used for various purposes:

• Silicon inspection: to identify cracks and voids in silicon wafers and to see through silicon wafers for packaging inspection.
• Plastics sorting: to sort out white or black plastics according to their types.
• Food quality inspection and control: to detect and sort contaminants, identify spoiled food, etc.

Watch the video to see how the sensor prototype works:

Another unique feature of SWIR vision is its transparency through various obstructions, such as mist, fog, haze, smoke or dust. We demonstrated how the SWIR camera can image the objects through smoke. This feature can be use for firefighting, security and even automotive applications:

In automotive, the SWIR technology can be used to improve tenfold the precision of 3D aerial and geographic mapping, as well as advanced driver-assistance systems at night and in adverse weather conditions (mist/fog/snow).

There are many other areas where this technology shows promise, for example defence, night vision and security. New SWIR cameras will certainly be a more affordable solution for night vision, for example. Also they can be used in airports or train stations for security purposes – face recognition through sun-glasses, identification of fake skin or wig. Ultimately, one of the most significant mass scale applications for new-generation SWIR cameras is in consumer smartphones. The image sensors of the phones’ cameras can be used for biometrics (face recognition and security) as well as 3D photography.

Stay tuned to see such applications in action…

• About Quantum Solutions

Quantum Solutions is a global leader in providing quantum dot components and technology for wide range image sensors: from infrared to X-ray ranges. Their SWIR QDot™ quantum dots have high optoelectronic grade quality and precise batch-to-batch production reproducibility, which are qualified to manufacture SWIR image sensors, from small to large industrial scales. Contact at [email protected] to discuss your needs in enabling the next generation image sensors with quantum dot technology

• About imec

Imec is a world-leading research and innovation hub in nanoelectronics and digital technologies. The combination of our widely acclaimed leadership in microchip technology and profound software and ICT expertise is what makes us unique. By leveraging our world-class infrastructure and local and global ecosystem of partners across a multitude of industries, we create ground breaking innovation in application domains such as healthcare, smart cities and mobility, logistics and manufacturing, energy and education. As a trusted partner for companies, start-ups and universities we bring together more than 5,000 brilliant minds from almost 100 nationalities. Imec is headquartered in Leuven, Belgium and has distributed R&D groups at a number of Flemish universities, in the Netherlands, Taiwan, USA, and offices in China, India and Japan. In 2019, imec’s revenue (P&L) totalled 640 million euro. Further information on imec can be found at www.imec-int.com.

How Quantum Dot Scintillators create a more efficient, cost-effective answer to X-ray inspection

X-ray imaging has contributed to major developments in medicine, but the benefits are not limited to one industry. As technology advances, the application possibilities for X-ray scanning continues to widen.

Growing potential

Since its discovery more than a century ago, X-ray scanning has predominantly been used as a diagnosis tool in the medical field. According to the World Health Organisation as many as 3.6 billion diagnostic medical examinations are performed across the globe each year. There have also been technological advances and more widespread use of X-ray scanners in areas such as security, to minimise the risk of threat and identify concealed items.

However, X-ray scanning is expanding into new industries, predominantly through its ability to perform non-destructive quality inspection and control. Here, the power of seeing inside an object, without damaging it, can be harnessed to ensure quality standards are met and safety is guaranteed, introducing a new, efficient, reactive assessment process. 

Examples of X-ray scanning in non-destructive testing and inspection include:

• Food manufacturing – Through the use of X-ray scanners, foods can be inspected, contaminants can be detected, and defects can be identified without any damage to the product or its packaging. 
• Automotive industry – X-ray scanners are able to check various parts of the vehicle including brakes, tyres, wheels, batteries, electromechanical components and so on. This can be undertaken throughout the manufacturing process. 
• Electronics – X-ray scanners are now being used to ensure electronic products meet standards of reliability and safety, (testing bonding areas, wires, solder joints and components).
• Welding – X-ray scanners can be used for quality assurance for welding inspection, to ensure weld connections are of a suitable standard.
• Aerospace industry – X-Ray scanners can also be used in the detection of cracks, corrosions, pores, or welding defects in aircraft manufacturing. The utilization of X-ray imaging for outer space exploration is another big thing. Today, tens of X-ray satellites and telescopes are observing the space, for example to probe matter as it falls into a black hole, as well as probe the nature of dark matter and dark energy by observing the formation of clusters of galaxies.

How X-Ray scanners work

Typically, an X-ray scanner comprises of two elements, an X-ray Source– which is the device that emits the X-ray beams, and the X-Ray Detector, which is the component that measures the properties of X-rays and converts them into a visual form.

There are two types of X-ray detector, called direct and indirect. The first has a higher level of sensitivity, and is the type often used in medical applications. These detectors apply a direct conversion of X-ray photons into an electrical signal and are the more expensive of the two.

The second utilises an indirect conversion of photons, achieved by using scintillators, (materials which convert X-rays into visible light). These scintillators convert ionising radiation into visible photons that later can be detected by a silicon photodiode. Whilst these detectors are less sensitive, they are also a lot less expensive.

There are certain advantages and disadvantages to using both detectors, depending on the application required. For the purposes of quality control and inspection, indirect conversion using scintillators, is most suitable because it is cost effective and can be easily scaled-up for large area scanning.

X-ray Scintillators

Scintillator materials are very useful for non-destructive quality inspection and detection of product failures and inconsistencies, due to their high light output and impressive resolution.

There are many scintillator materials available, but the most commonly used are thallium-doped Cesium Iodide – CsI(Tl) scintillators, and terbium-doped gadolinium oxysulfide (GADOX(Tb)) scintillators.

The commonly used CsI(Tl) scintillators are usually the columnar type, grown in small columns (10 μm in thickness between two substrates). This kind of scintillator offers very high resolution with up to 10 – 20 lp/mm and excellent brightness (of around 10 ph/keV).

However, one big disadvantage of these CsI(Tl) scintillators is that they are extremely expensive, costing between $30,000-$40,000 per m2. Another disadvantage is their low radiation hardness, meaning they degrade very fast at high X-ray doses and have high afterglow (the scintillation light that exists once the X-ray has ceased). A more affordable, and significantly brighter scintillator is GADOX, which costs around $1,000 -$4,000 per m2. This scintillator is most frequently used in non-destructive testing. Whilst this scintillator has high brightness (20 ph/keV), its disadvantage is that it provides a lower resolution of image (reaching up 10 lp/mm max).  

The benefits of Quantum Dot Scintillators

Scintillators using ‘Perovskite’ Quantum Dots (QDs) combine the advantages of both CsI (Tl) scintillators, and GADOX scintillators. They offer an extremely high resolution and brightness of image, (equivalent to those that the CsI(Tl) scintillators), but they are as cost-effective as GADOX scintillators). Their ability to provide higher resolution images allows them to show the smaller structures in objects, making them ideal for machine vision.

An additional performance advantage is that perovskite scintillators are more robust, durable and work more efficiently when exposed to hard X-rays and gamma rays.

However, possibly the most important advantage of Perovskite QD scintillators is their super-fast response time, allowing them to cease scintillation when the X-ray is switched off. The quantum confinement of Perovskite QDs means it has a response time measured in ‘ns’ as opposed to the standard response time for conventional scintillators, which is in within µs and ms ranges. This unique property allows for ultrafast X-ray scanning that is perfect when examining objects that are moving (again, ideal for non-destructive testing and inspection). 

Lutfan Sinatra, the VP of Product Development of Quantum Solutions noted: “QDot™ Perovskite Scintillators demonstrate the unprecedented advantages over CsI(Tl) and GADOX, offering a new type of highly performing scintillators”.

As a result of these impressive features, perovskite materials are quickly gaining a lot of attention. Marat Lutfullin, the CEO of Quantum Solutions confirmed this stating: “In the last year, we have seen an enormous response from the X-ray industry as the qualities and advantages of QDot™ scintillators have become more widely recognised. Industries are getting convinced about the benefits in terms of overall efficiency and cost. Quantum Solutions has focused on meeting the needs of our clients working in this particular field.”

The Future…

As X-ray technology continues to advance, it is predicted that evaluation cameras with novel perovskite X-ray scintillators will be available from as early as 2023.

Furthermore, there are many other exciting explorations and developments around perovskite materials that are taking place in the field of medical X-ray.

Since the invention of X-rays, the biggest challenge has always been developing technology that reduces exposure to radiation, without the X-ray image being compromised. Achieving an X-ray with sufficient resolution and depth has naturally led to administering a higher radiation dose, which, as we know, is detrimental to a patient’s health, (and the reason why only a limited number of X-rays per year are recommended).  Reducing the dose without compromising resolution relies on the development of a higher level of sensitivity in the sensor.

Perovskite semiconductors have already demonstrated great potential in direct X-ray detection due to their exceptional properties (large X-ray attenuation, ultrafast, extremely high sensitivity and cost-efficiency).

Quantum Solutions is currently exploring and investigating the possibilities in terms of ways in which Perovskite and Quantum Dot technology can play an important role in this process, and we have already seen some very positive results.

Stay tuned for further updates …

LCDs might be one of the oldest display technologies around, but going quantum has helped them paint a whole new picture.

Like most decades-old technologies, liquid-crystal displays (LCDs) had a fairly unglamorous start. The first LCDs emerged around the late 1960s, and were used mainly to represent the numbers on digital watches and pocket calculators. It would take almost 20 years for engineers to transform LCDs from one-dimensional arrays into two-dimensional, active-matrix displays. And it would take another dozen years to make the first LCD manufacturers’ dreams a reality; to create a wall-mountable display that could rival the cathode-ray tube television.

Since then, the LCD has been steadily refined to generate increasingly crisper, brighter, and overall more pleasing images. While different companies have put their own twist on how LCDs look and function, the principles are largely the same across the board.

A panel of white LEDs is used as the light source for image generation. This light contains a broad spectrum of wavelengths, including all colours in the visible and near-infrared ranges (Figure 1(a)). Each pixel within an LCD contains three colour filters, which allow red, blue, or green light to pass through to reach the viewer. When these subpixels allow all of the LED backlight through, the full pixel appears white. When the subpixels block all of the backlight, the pixel appears black. The trick to generating different colours is modulating the intensity of light that each subpixel allows to pass through.

That is where liquid crystals come in.

Liquid crystals are, quite literally, substances made up of many crystals that collectively behave as a liquid. Imagine each crystal as a tiny grain of rice. Spreading these grains into a thin layer will mean that some grains are oriented vertically, some are oriented horizontally, and all others oriented along some angle in between. This, it turns out, makes for a great light filter.

Sandwiched between two polarisers, a liquid crystal film can allow light with intensity values ranging from 0 to 255 to pass through the individual subpixels in an LCD. This is made possible by an electric field, which alters the orientation of the crystals in the film. Because the light intensity penetrating each subpixel can be altered individually (and each distinct combination of light intensity represents a unique colour), a standard LCD can produce around 16 million (256 x 256 x 256) different colours.

That means that LCDs cast a wide net across the full range of colours visible to the human eye, also known as the colour gamut.

But there’s a limit to how far traditional LCDs can stretch that colour net. And that’s due to the LED backlight. Because ultimately, a display’s output is only as vivid as its light source, and despite being broad-spectrum, the white LEDs in an LCD produce rather diffuse light that must pass through several filters. Overall, that makes for sub-optimal picture quality.

So how are modern LCD manufacturers dealing with this problem? By simply adding a quantum-sized tweak to the traditional backlighting system.

Instead of a single layer of white LEDs, the backlighting of next-generation LCDs is deconstructed into two layers: a panel of LEDs that shine blue light and a polymer film of quantum dots (QDs) that convert that light into red and green light (Figure 1(b)). Unlike the amalgam of diffuse blue, red, and green light emitted by an all-white LED, this deconstructed system produces high-intensity light of each wavelength. Due to the highly pure light generated by the QDs, these LCDs can produce around 25 million colours- about 50% more than a conventional LCD (Figure 2). This is a result of higher colour gamut coverage of QD LCD than standard LCD in CIE chromaticity diagram space (Figure 1(c)). While standard LCD systems can cover up to 70-75% of Rec2020 colour standard, QD LCD systems can reach up to 85 % of the colour standard. That translates to wider coverage of the colour gamut, higher contrast, and a brighter image—all while using less energy.

The quantum dots commonly used in today’s QD LCDs are made of cadmium selenide (CdSe) and indium phosphide (InP). Quantum Solutions is currently developing a new class of QDs for display applications made from perovskite quantum dots.

“Perovskite” refers to a crystal structure with the formula ABX3. The structure is composed of four octahedra containing a halogen (e.g., chlorine, bromine, or iodine) at their vertices and a transition metal (e.g., lead or tin) at their centre. At the center of the four octahedra is a cesium ion or an organic formamidinium [CH(NH₂)₂⁺] or methylammonium [CH₃NH₃⁺] ion [4].

Perovskites are garnering tremendous interest in the world of photovoltaics, due to their combination of broad absorption, a strong ability to shuttle charge carriers, high defect tolerability, and ease of manufacturing.

Perovskite QDs offer higher efficiency and colour purity than their CdSe and InP counterparts, and could represent the future of LCD technology. The technology is promised to deliver 80 – 95 % of Rec2020 standard colour gamut coverage, more than any other quantum dot technology.

Useful links

https://www.displaydaily.com/article/display-daily/it-s-2020-so-where-is-rec2020

https://www.wired.com/2015/01/primer-quantum-dot/

https://news.samsung.com/global/why-qled-matters-for-4k-hdr-content

https://quantum-solutions.com/blog/what-are-perovskite-quantum-dots/