This article appeared earlier in Laser Focus World.
Overcoming silicon’s limits
Silicon-based imagers excel at capturing visible light. However, for longer wavelengths such as (short-wave) infrared (SWIR or IR), offering applications ranging from a camera supporting the sorting of household waste to autonomous vehicles 'seeing' through smoke or fog, alternative semiconductors are necessary.
While III-V materials are addressing this need, their high manufacturing costs limit their widespread use, crucial for large-scale roll-out in domains like automotive. Enter the promising alternative of thin-film-based absorbers, including organic compounds or quantum dots. These materials feature superior absorption characteristics and can be more easily integrated with conventional CMOS readout circuits.
Breaking barriers with pinned photodiode structures
Nonetheless, the adoption of infrared sensors utilizing thin-film absorbers faces challenges, primarily related to low signal-to-noise ratios leading to poor image quality. Recent developments have overcome these issues for the first time. Not by changing the choice of materials, but by building on the rich history of pinned photodiode (PPD) structures in silicon-CMOS image sensors, dating back to the 1980s.
The PPD structure, with its additional transistor gate and specialized photodetector structure (termed photogate), significantly reduces noise, making it a dominant force in the consumer market for silicon-based image sensors. It allows reset operation of the transistor without kTC noise nor the effect of the previous frame, providing a kind of ‘super-switch’ by which charges can be completely drained. However, incorporation of a PPD in thin-film-based sensors has never been possible because of the complexity of integrating two different semiconductor systems.
Imec's pinned photodiode structure integrated in thin-film image sensors.
A leap in image sensor architecture
In a recent Nature Electronics paper, researchers from imec presented the successful incorporation of a PPD structure in the read-out circuitry of a thin-film pixel. The proof-of-concept leverages imec’s in-house expertise in both thin-film materials and photodiodes, surpassing current thin-film pixel architectural limitations. While the individual components of this pixel have been around for a long time and have proven their worth in other applications, this proof of concept demonstrates their collective value and versatility in sensing light beyond visible.
For the photogate transistor, indium-gallium-zinc-oxide (IGZO) was used, due to its compatibility with thin-film photodiodes and because it can be integrated into silicon read-out circuits. Such transistors have already demonstrated their effectiveness in various applications, including memory technologies.
Next, this IGZO transistor was monolithically hybridized with a thin-film organic photodiode directly on top of the complementary metal-oxide semiconductor (CMOS) readout circuitry. This wafer-level post-processing enables scalable and affordable manufacturing, paving the way for large-scale applications.
The resulting 4T SWIR imager showed a low read-out noise of 6.1e-, compared to >100e- for the conventional 3T reference sensor. With such a reduction in noise, higher signal-to-noise ratios (SNR) can be achieved. As a result, images can be captured with more accuracy and detail. Moreover, as fewer photons are necessary to induce signals, even weak signals can be converted into images.
Comparison of 3T and 4T pixel images.
Advancements in linearity: the key to precision
In follow-up research, the team showed that the introduction of this PPD pixel structure also resulted in a more linear response – a measure for the accuracy of image reproduction. Moreover, the photodiode bias was fixed, effectively reducing dark current (by 72%) and increasing the linearity (error reduction of 59%) for organic thin-film photodiode-based image sensors. Thus, adding a photogate ensured a more faithful representation of the captured scene, critical for applications where accurate image reproduction is paramount.
In addition to this, the initial pixel was designed to maximize charge to voltage conversion gain – with a resulting limited floating diffusion capacitance – thus not able to demonstrate high full-well capacity. A second-generation pixel structure did enable a high full-well capacity exceeding 1 Mega electrons of the proposed pixel (with a pitch of 5 µm). This ensures a dynamic range of > 100 dB, compared to the 82 dB of the 3T pixel, representing the ability of this high SNR pixel to capture a wide range of light intensities – from the darkest to the brightest.
Future horizons: accelerating thin-film-based image sensor deployment
This pixel architecture offers opportunities for other promising thin-film photodiodes beyond organic, including perovskite absorbers or quantum dots. Since the photodiode is separated from the readout circuit and integrated via post-processing, the used absorption materials do not need to undergo stringent optimization before integration with the readout circuitry. This novel 4T pixel architecture can accelerate the deployment of monolithic thin-film-based image sensors. The benefits of this new architecture could serve as a catalyst for novel applications that are either not covered or inadequately addressed by silicon imagers.
Imec's pinned photodiode structure integrated in thin-film image sensors.
Explore the peer-reviewed studies underpinning this article:
- J. Lee et al., Nat. Electron., 6, 590–598 (2023); https://doi.org/10.1038/s41928-023-01016-9.
- J. H. Kim et al., Sensors, 23, 8803 (2023); https://doi.org/10.3390/s23218803.
- M. Jin et al., IEEE Electr. Device L., 44, 12, 2007–2010 (Dec. 2023); https://doi.org/10.1109/led.2023.3325830.
