Innovation and expertise often combine to produce groundbreaking discoveries. At Imec, a leading research and innovation hub focused on nanoelectronics and digital technologies, they recently made history by successfully incorporating PPD structures into thin-film image sensors – an incredible accomplishment which holds immense promise for infrared imaging applications.
Imec’s groundbreaking achievement lies in its potential to expand our understanding of light sensing beyond visible wavelengths. By integrating photogates and transfer gates into thin-film imagers, Imec has exploited their exceptional absorption characteristics extending their capabilities far beyond visible light wavelengths.
Practical applications of this achievement are immense: imagine autonomous vehicles equipped with cameras capable of seeing through smoke or fog, or smartphones equipped with cameras using infrared light for facial recognition. Traditional silicon-based imagers excel at detecting visible light but fall short when it comes to capturing longer wavelengths such as those present in shortwave infrared (SWIR) range.
Longer wavelengths have forced researchers to seek alternative solutions, and III-V materials could prove revolutionary in breaking through detection limitations associated with longer wavelengths; however, their expensive manufacturing processes prevent widespread adoption. Thin-film absorbers like quantum dots are emerging as promising alternatives that offer superior absorption characteristics as well as seamless integration into conventional readout circuits known as complementary metal-oxide-semiconductor (CMOS) circuits.
Thin-film absorbers demonstrate great promise; however, sensors based on them have historically suffered from subpar noise performance and image quality issues. This is where pinned photodiode (PPD) structures come into play: first introduced in the 1980s for silicon-CMOS image sensors, these structures add an additional transistor gate and special photodetector structure designed to drain all charges fully before integration begins, eliminating noise sources as well as sources of distortion.
PPDs have long dominated the consumer market for silicon-based image sensors due to their ability to mitigate noise and improve power performance, yet incorporating PPDs into thin-film-based sensors has proven challenging due to hybridizing multiple semiconductor systems.
Imec’s achievement transforms the landscape. They successfully integrated PPD structures into thin-film-based image sensor readout circuits – an unprecedented feat – making history. This integration involved pairing an SWIR quantum dot photodetector with an indium-gallium-zinc oxide (IGZO) thin film transistor to form PPD pixels; these were then processed onto a CMOS readout circuit for processing into an impressive thin-film SWIR sensor array.
Impact was dramatic. The prototype 4T image sensor demonstrated an amazing low read-out noise of only 6.1e-, in comparison to over 100e- for traditional 3T sensors. This remarkable noise performance enabled accurate, detailed images with reduced interference being captured for infrared imaging purposes.
Imec’s innovation extends far beyond technological progress; they specialize in thin-film photodiodes, IGZO image sensors and thin-film transistors – at the cutting-edge of infrared/imaging technologies that bridge infrared/imaging divisions. By breaking past current limitations they have created a path towards producing top performing quantum dot SWIR pixels at cost-effective manufacturing levels – this achievement marks an historic step that should continue its advancement across multiple thin-film photodiodes as well as expanding applications beyond silicon imaging applications.
Imec is proud of our partnership with industry partners and remains eagerly optimistic of further advancements. Together they bring expertise, innovation, and dedication – key ingredients in driving the realm of technology forward step-by-step.
What is a Thin-film Image Sensor (TFIS)
Thin-film technology employs image sensors that capture light and convert it to electronic signals for digital imaging devices like cameras, smartphones and industrial equipment. Thin-film technology involves depositing or growing thin layers of semiconductor materials onto substrates such as glass or silicon wafers in thin-film manufacturing technology.
Thin-film image sensors employ thin-film deposition techniques to integrate light-sensitive elements and readout circuits onto a thin substrate, offering several advantages:
Thin Film Image Sensors Are More Compact: Thin-film image sensors are smaller and thinner compared to their more bulky predecessors, making them suitable for applications where space constraints are a primary consideration, such as smartphones, wearable devices, and industrial equipment.
Flexible Design: Thin-film fabrication technology permits flexible designs and customizations that allow for customizations, such as the integration of different sensing elements, circuitry features, and features onto one chip.
Cost-Efficiency: Thin-film manufacturing processes may be more cost-efficient compared to traditional approaches, making them suitable for large-scale production.
Lightweight: Thanks to its reduced thickness, sensor devices become lighter devices – an advantage in portable and wearable applications.
Compatibility: Thin-film image sensors can be combined with other electronic components on a substrate for maximum compatibility and simplified assembly processes.
Thin-film image sensors utilize various light-sensitive materials, including organic semiconductors, amorphous silicon, or quantum dots. Quantum dots have become widely recognized due to their customizable optical properties and potential for high-efficiency light absorption.
Thin-film image sensors present certain challenges, including increased noise levels and potentially lower performance compared to traditional sensors. Recent advances in technology, as demonstrated by imec’s integration of pinned photodiode structures into thin-film sensors, aim to address these difficulties and unlock their full potential in various applications outside the visible spectrum, including those that involve light detection beyond this visible spectrum.
What is Shortwave Infrared Radiation (SWIR)
Shortwave infrared (SWIR) refers to a portion of the electromagnetic spectrum located just beyond visible light and into infrared ranges. It includes all forms of electromagnetic radiation such as radio waves, microwaves, visible light, ultraviolet light, X-rays and gamma rays; within this spectrum lie SWIR wavelengths which correspond approximately to frequencies from 400 THz up to 214 THz.
SWIR radiation may not be visible to human eyes, but it can still be detected and measured with sensors designed for operating within this wavelength range. SWIR imaging offers numerous unique properties and applications:
Penetration of Certain Materials: SWIR radiation has the capability of penetrating materials that are opaque to visible light, such as certain plastics and fabrics. This makes SWIR imaging particularly valuable in applications with limited visibility such as industrial inspection or security screening.
SWIR imaging provides images in low light environments, including nighttime or indoor settings where visible light might not provide sufficient illumination.
Temperature Difference Detection: SWIR radiation can be used to detect variations in surface temperatures, making it ideal for thermal imaging of industrial processes, building diagnostics and monitoring equipment performance.
Material Identification: Certain materials exhibit characteristic spectral signatures within the SWIR range that allow SWIR imaging to identify and differentiate them based on reflectance or absorption properties, making this technology useful in applications like remote sensing, agriculture and geology.
Medical Imaging: SWIR imaging has many potential uses in medicine, from imaging blood flow to identifying conditions based on differences in tissue properties.
Remote Sensing: SWIR imaging is used in satellite-based remote sensing to gather information about Earth’s surface, vegetation health and water quality.
SWIR imaging technology typically relies on sensors designed to detect and measure SWIR radiation emitted or reflected from objects, integrated into cameras or imaging systems for various uses. With its unique properties of SWIR radiation making it invaluable tools in fields like industrial inspection, agriculture, defense, astronomy and scientific research.
