Imec has successfully integrated a pinned photodiode structure in thin-film image sensors with comparable noise performance to a standard silicon pinned photodiode pixel. The device enables the superior absorption qualities of thin-film imagers – beyond one micrometer wavelength – to be exploited.
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 1980s, the pinned photodiode (PPD) was introduced for silicon-CMOS image sensors. 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.
Imec has now demonstrated successful incorporation of a PPD structure in the readout circuit of thin-film-based image sensors. 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. As a result, infrared images can be captured with less noise, distortion or interference, and more accuracy and detail.
“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,” commented Pawel Malinowski, Imec program manager ‘Pixel Innovations’.
The results have been published in Nature Electronics.