Imec, a belgian research and innovation hub in nanoelectronics and digital techno­logies, presents the successful integration of a pinned photodiode structure in thin-film image sensors. With the addition of a pinned-photogate and a transfer gate, the superior absorption qualities of thin-film imagers beyond one micron wavelength can finally be exploited, unlocking the potential of sensing light beyond the visible in a cost-efficient way. Detecting wavelengths beyond visible light, for instance infrared light, offers clear advantages. Appli­cations include cameras in autonomous vehicles to see through smoke or fog and cameras to unlock your smartphone via face recognition. Whilst visible light can be detected via silicon-based imagers, other semi­conductors are necessary for longer wavelengths, such as short-wave infrared (SWIR).

Use of 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 potential for integration with conventional (CMOS) readout circuits. None­theless, such infrared sensors have an inferior noise performance, which leads to poorer image quality. 

Already in the 1980’s, the pinned photodiode (PPD) structure was intro­duced for silicon-CMOS image sensors. This structure introduces an additional transistor gate and a special photo­detector structure, by which the charges can be completely drained before integration begins – allowing reset operation without kTC noise nor the effect of the previous frame. Conse­quently, because of lower noise and improved power performance, PPDs dominate the consumer market for silicon-based image sensors. Beyond silicon imaging, incor­porating this structure was not possible up until now because of the difficulty of hybridizing two different semiconductor systems.

Now, Imec demonstrates 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 a superior thin-film SWIR image sensor. “The prototype 4T image sensor showed a remarkable low read-out noise compared to the conventional 3T sensor, demons­trating its superior noise performance” stated Nikolas Papa­dopoulos, project leader at imec. As a result, infrared images can be captured with less noise, distortion or interference, and more accuracy and detail.

Pawel Malinowski, Imec program manager pixel innovations adds: “At Imec, we are at the forefront of bridging the worlds of infrared and imagers, thanks to our combined expertise in thin-film photo­diodes, IGZO, image sensors and thin-film transis­tors. 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.” (Source: Imec)

Reference: J. Lee et al.: Thin-film image sensors with a pinned photodiode structure, Nat. Elec., online 14 August 2023, DOI: 10.1038/s41928-023-01016-9

Link: Integrated Photonics, Imec, Leuven, Belgium