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Direct conversion hybrid-organic X-ray detectors on metal oxide backplane

Periodic Reporting for period 2 - DiCoMo (Direct conversion hybrid-organic X-ray detectors on metal oxide backplane)

Reporting period: 2016-07-01 to 2017-12-31

X-rays are the oldest and most frequently used form of medical imaging which helps doctors in the identification, diagnosis, and treatment of many types of medical conditions. Today, despite the undiscussed validity of X-Ray imaging, still too many medications errors due to incorrect diagnosis occur. DiCoMo aims to improve the specificity (thanks to a higher resolution) and sensitivity of digital flat panel X-ray detectors.
This is achieved by combining radical innovations from leading European players from various disciplines. On the backplane level IGZO thin film transistors enable the realization of an active pixel readout scheme. In combination with a custom made read-out IC this promises a significant enhancement in sensitivity. On the frontplane level a novel solvent-free processing technology for hybrid-organic semiconductors is used in conjunction with hybrid-organic core-shell structures to realize thick hybrid-organic photodetectors with integrated scintillator particles for minimal optical cross-talk and enhanced resolution.
The consortium brings together commercial and research partners from across the Organic and Large Area Electronics and medical imaging value chain. These partners are system provider Siemens Healthineers, materials supplier BASF, innovation centers TNO and imec, plus SMEs ICsense and Morphwize focusing on customized read-out ICs and modelling and simulation respectively.

DiCoMo tested new innovative design solutions to improve the dynamic range and sensitivity of the X-Ray imager. We switched from the commonly used passive pixel to an in-pixel scheme. Also, we introduced novel materials and a low-temperature process for low-cost direct-conversion layer fabrication. DiCoMo showcased a remarkable case of synergy between European high tech micro-SMEs, large enterprises and R&D excellence institutions, in the quest to develop solutions close to the market.
As a result, the DiCoMo consortium did successfully implement functional VGA-demonstrators. To the best of our knowledge it is the first time that an active pixel backplane with low-cost IGZO technology is used in a flat-panel X-ray detector. The combination with a low-cost directly converting X-ray absorber has the potential to increase the resolution and sensitivity of such detectors. While further improvement of several aspects (collection efficiency of front plane, noise performance of backplane) is required, DiCoMo has significantly enhanced the competence and scientific know-how in this domain and we expect to see a plurality of follow-up initiatives towards innovative products.
Active pixel backplane
It has successfully developed a new type of active pixel TFT backplane technology tailored to drive a high voltage X-ray detector frontplane. With its VGA size (640x480 pixels) demonstrator backplane, DiCoMo has delivered an essential building block in order to demonstrate industrially relevant integration toward better and more affordable healthcare.
A dedicated ROIC is designed to convert electrical signals from an Active Pixel Sensor into a digital image. The ROIC can be fully programmed to be able to convert different sensitivity levels at different speeds. This allows to trade-off resolution with accuracy. One ROIC can acquire 128 pixels simultaneously with 14bit resolution and a very low noise level of 1.2LSBrms.
The timing control is flexible and programmable to allow operation with any commercially available pixel driver ASICs. The highest achievable conversion speed for a 4K panel is up to 130FPS.
Fig. 2
Core shell particles
The new “direct conversion” mode developed within the DiCoMo project is based on innovative materials and processes that allow an efficient preparation of X-Ray conversion layers made of blends of X-ray scintillator particles and organic semiconductors within a single layer. The hybrid monolithic conversion layer thus formed can then be directly integrated on the backplane of the X-Ray imager. Careful and creative selection of materials, preparation methods and scale-up led to highly homogeneous converting layers.
Fig. 3
Frontplane processing and characterization
Fig. 4
Siemens has developed a process to “soft-sinter” semiconductor powders onto the VGA backplane. During the pressing process the powder is compacted and a high-quality homogeneous layer brought onto the backplane substrate (Fig. 3).
Layers with a thickness of 500µm have been achieved. During the project we reduced the leakage current below 1E-6 mA/cm² @ 150 V. Unfortunately, the conversion efficiency was reduced at the same time because the charge carrier transport was deteriorated.
As a mitigation action we tested the system with thin-film OPDs and perovskites which provide superior charge transport properties (Fig. 4). Fig. 5 displays a finished VGA-sized Perovskite detector and the corresponding MTF. The corresponding MTF is rather good with 0.4@2lp/mm which is comparable or even slightly better than CsI scintillator based detector at comparable X-ray converter thickness.
Fig. 5
System integration
The reading and driving electronics of the system has been designed by imec and consists of hardware, firmware and software developed to address and read out the X-ray imager. The final demonstrator electronics is able to drive and readout a VGA imager. The entire system is shown on the right side of Fig 6.
Fig. 6.
A first image based on OPDs and an active pixel IGZO TFT backplane is shown in Fig. 7. On the right side a linearity plot of the system is shown. The noise floor with ~120LSB is dominated by an RF noise which degrades the noise performance of the imager and which origin is still under evaluation. To the best of our knowledge this is the first time that an active pixel backplane with low-cost IGZO technology in a flat-panel X-ray detector setting has been presented.
Fig. 7.
Morphwize has set up computational tools to model the system and dominate the significant complexity of multiscale hybrid photodetectors. Advanced techniques such as kinetic Monte Carlo were coupled with Finite Element Modeling (FEM), to describe indirect photon to electron conversion and charge carrier collection. The software allowed gaining insight into intrinsic system limitations and strategies to improve conversion efficiency. The full development cycle allowed integrating the modules into the powerful in-memory mPER-INM platform. The result is an extended version of our computational platform, with unprecedented capabilities, suitable for describing a whole spectrum of photodetector types and effects.
Fig. 8.
Through-out the project we shared our technical results with the scientific community by contributing several talks and posters to conferences. We received many invitations to give key-note presentations at various conferences. We further shared our insights with the European expert community in the field of medical X-ray imaging buy publishing press releases and organizing networking events. A particular highlight was the workshop on organic materials for radiation detection at the IEEE NSS-MIC 2017 in Atlanta which was jointly organized by the LORIX, iFLEXIS and DiCoMo projects.
We are confident that many of the project results will significantly influence the roadmaps and future activities of the partners, not only in the direct use-case of the X-ray detector, but also in other domains such as display, sensor and energy applications.
SEM images of cross-section of two hybrid conversion layers made out of sintered hybrid core-shell p
a)VGA-COF active pixel backplane with MAPI perovskites OPDs, b) Modulation transfer function under X
(left) Image taken with thin film OPDs and an active pixel backpalne in visible light; (right) Linea
(left) ROIC with Chip-On-Flex (COF); (right) Imager with FPGA based readout hardware
a) 8" Wafer with 400 ROIC after bumping, b) Black image showing very low noise level
Overview of X-ray Photodetector software suite, based on Morphwize’s mPER-INM simulator
Pictures of high voltage VGA size final demonstrator backplanes. (Left) Chip on Foil (COF); (right)
Lifetime mobility plot for different semiconductor technologies relevant for X-Ray detection