imec integrates thin-film pinned photodiode in SWIR sensors

20 Aug 2023

Results described in Nature Electronics unlock potential of thin-film sensors in IR imaging.

Imec, the Leuven, Belgium-based research and innovation hub in nano- and digital technologies, has announced the successful integration of a “pinned photodiode structure” in thin-film image sensors.

imec commented that “with the addition of a pinned-photogate and a transfer gate, the superior absorption qualities of thin-film imagers – beyond 1µm wavelengths – can now be exploited, unlocking the potential of sensing light beyond the visible in a cost-efficient way.”

The achievement is described in the August 2023 edition of Nature Electronics.

Detecting wavelengths beyond visible light, such as the infrared, offers clear advantages. Applications include cameras for autonomous vehicles that perceive their surroundings through smoke or fog, and sensors that could unlock a smart phone by face recognition.

The researchers note that “whilst visible light can be detected via silicon-based imagers, other semiconductors are necessary for longer wavelengths, such as short-wave infrared.”

The use of III-V materials can overcome this detection limitation. However, manufacturing such absorbers is usually expensive, limiting their use. In contrast, sensors using thin-film absorbers (such as quantum dots) have emerged as a promising alternative.

They have superior absorption characteristics and potential for integration with conventional (CMOS) readout circuits. Nonetheless, such infrared sensors typically have an inferior noise performance, which leads to poorer image quality.

In the 1980s, the pinned photodiode (PPD) structure was introduced for silicon-CMOS image sensors. This structure introduces an additional transistor gate and a special photodetector structure, so that charges can be completely drained before integration begins.

PPDs have dominated the consumer market for silicon-based image sensors. Beyond silicon imaging, incorporating this structure was not possible until now because of the difficulty of hybridizing two different semiconductor systems.

Successful PPD integration

Now, imec has demonstrated the successful incorporation of a PPD structure in the readout circuit of thin-film-based image sensors – the first of its kind. A SWIR quantum-dot photodetector was monolithically hybridized with an indium-gallium-zinc oxide (IGZO)-based thin-film transistor into a PPD pixel.

This array was subsequently processed on a CMOS readout circuit to form what imec calls “a superior thin-film SWIR image sensor.”

Nikolas Papadopoulos, project leader, Thin-Film Pinned Photodiode at imec, commented, “The prototype 4T image sensor shows a remarkable low read-out noise of 6.1e-, compared to more than 100e- for the conventional 3T sensor. As a result, infrared images can be captured with less noise, distortion or interference, and more accuracy and detail.”

Pawel Malinowski, imec’s Program Manager, Pixel Innovations, added, “By achieving this milestone, we surpassed current pixel architectural limitations and demonstrated a way to combine the best performing quantum-dot SWIR pixel with affordable manufacturing.

“Future steps include optimization of this technology in diverse types of thin-film photodiodes, as well as broadening its application in sensors beyond silicon imaging. We are looking forward to further these innovations in collaborations with industry partners.”