The nanoelectronics and digital technology research center imec in Belgium recently announced that they have successfully integrated a pinned photodiode structure into thin-film image sensors. By adding a pinned photogate and a transfer gate, the superior absorption qualities of thin-film imagers at wavelengths beyond one micrometer can finally be effectively utilized to unlock the potential of sensing light beyond the visible spectrum in a cost-efficient manner.
Detecting wavelengths beyond the visible range, such as infrared light, has distinct advantages. Applications include enabling cameras on autonomous vehicles to “see through” smoke or fog, and cameras that unlock smartphones via facial recognition. While visible light can be detected with silicon-based imagers, other semiconductor materials are needed to detect longer wavelength light, such as short-wave infrared (SWIR).
Using III-V materials can overcome this detection limitation. However, manufacturing these absorbers is expensive, limiting their use. In contrast, sensors using thin-film absorbers (such as quantum dots) have recently emerged as a promising alternative. They have superior absorption characteristics and the potential for integration with conventional CMOS readout circuits. Nevertheless, such infrared sensors have inferior noise performance, resulting in poorer image quality.
As early as the 1980s, the pinned photodiode (PPD) structure had already been introduced in silicon CMOS image sensors. This structure adds an extra transistor gate and a special photodetector structure that can completely drain charges before integration begins (allowing reset operation without kTC noise or effects from the previous frame). As a result, due to lower noise and improved power performance, PPDs dominate the consumer market for silicon-based image sensors. Outside of silicon imaging, integrating this structure has not been possible until now because of the difficulty of hybridizing two different semiconductor systems.
Now, imec demonstrates the successful integration of a PPD structure into the readout circuits of thin-film image sensors, a first. A SWIR quantum dot photodetector was monolithically hybridized with an indium-gallium-zinc oxide (IGZO) thin film transistor into a PPD pixel. This array was then processed on a CMOS readout circuit to form a superior thin-film SWIR image sensor. Nikolas Papadopoulos, project leader of “Thin-Film Pinned Photodiode” at imec, stated: “The prototype 4T pixel sensor showed remarkably low read noise of 6.1e-, compared to >100e- for conventional 3T sensors, demonstrating its superior noise performance.” As a result, infrared images can be captured with less noise, distortion or interference, and higher accuracy and details.
Imec Pixel Innovations program manager Pawel Malinowski added: “At imec, we stand at the forefront of the intersection between infrared and imagers, thanks to our combined expertise in thin-film photodiodes, IGZO, image sensors and thin-film transistors. By achieving this milestone, we have surpassed current pixel architecture limitations and demonstrated an approach to combine best-performing quantum dot SWIR pixels with affordable manufacturing methods. Next steps include optimizing this technology for various types of thin-film photodiodes, and expanding its applications in sensors beyond silicon imaging. We look forward to furthering these innovations in collaborations with industry partners.”
In summary, imec’s research demonstrates that thin-film pinned photodiode structures can achieve noise performance comparable to silicon photodiodes, enabling wider-band image sensing that opens up new opportunities in this field. As related technologies continue to mature, we can look forward to seeing their extensive application across areas like automotive cameras, security surveillance, biometric identification, and more.