Active-matrix OLED using 150°C a-Si TFT backplane built on flexible plastic substrate

Sarma, Kalluri R.; Chanley, Charles; Dodd, Sonia R.; Roush, Jared F.; Schmidt, John A.; Srdanov, G.; Stevenson, Matthew; Wessel, Richard; Innocenzo, J.; Yu, Gang; O’Regan, Marie B.; MacDonald, William; Eveson, R.; Long, Ke; Glesková, Helena; Wagner, S.; Sturm, James C.

doi:10.1117/12.497638

Cited by 38 publications

(23 citation statements)

References 4 publications

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“…Over projection angles typical of DBT, the angular dependence of the NNPS is minimal. For example, comparing 30 incidence to normal incidence at 5.0 lp=mm in a front-screen configuration, NNPS decreases by 8% in a phosphor with no optical absorption and by 4% in a phosphor with high optical absorption. In a back-screen configuration, NNPS increases slightly by 5% and 2%, respectively.…”

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confidence: 99%

“…Figure 2 (a) indicates that increasing the angle of incidence decreases the MTF, giving rise to poorer spatial resolution in the front-screen configuration. For example, comparing 30 incidence to normal incidence at 5.0 lp=mm (a ¼ 0 ), the MTF decreases by 17% in a phosphor with no optical absorption and by 15% in a phosphor with high optical absorption. As expected, the MTF has minimal angular dependence orthogonal to the ray of incidence [ Fig.…”

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confidence: 99%

“…19,20 More significantly, Figures 2(b) and 2(d) further show that the angular dependence of the MTF is much less pronounced in the back-screen configuration than in the front-screen configuration. For example, comparing 30 incidence to normal incidence at 5.0 lp=mm along a 0 polar angle, the back-screen MTF decreases by a mere 3%. Unlike the front-screen, the backscreen MTF increases slightly with projection angle for measurements orthogonal to the incident ray.…”

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confidence: 99%

“…The high frequency DQE for the front-screen configuration is lower than that of the back-screen configuration, and its degradation with projection angle is much more pronounced. For example, at 5.0 lp=mm in a front-screen irradiated at a 30 angle, the DQE decreases by 20% relative to normal incidence. In the back-screen configuration, the relative decrease in DQE is less than 5%.…”

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confidence: 99%

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Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Acciavatti

Maidment

2011

Medical Physics

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Purpose: In digital breast tomosynthesis (DBT), a volumetric reconstruction of the breast is generated from a limited range of x-ray projections. One trade-off of DBT is resolution loss in the projections due to non-normal (i.e., oblique) x-ray incidence. Although degradation in image quality due to oblique incidence has been studied using empirical data and Monte Carlo simulations, a theoretical treatment has been lacking. The purpose of this work is to extend Swank's calculations of the transfer functions of turbid granular phosphors to oblique incidence. The model is ultimately used as a tool for optimizing the design of DBT detectors. Methods: A quantum-limited system and 20 keV x-rays are considered. Under these assumptions, the modulation transfer function (MTF) and noise power spectra (NPS) are derived using the diffusion approximation to the Boltzmann equation to model optical scatter within the phosphor. This approach is applicable to a nonstructured scintillator such as gadolinium oxysulfide doped with terbium (Gd 2 O 2 S:Tb), which is commonly used in breast imaging and which can reasonably approximate other detector materials. The detective quantum efficiency (DQE) is then determined from the Nishikawa formulation, where it is written as the product of the x-ray quantum detection efficiency, the Swank factor, and the Lubberts fraction. Transfer functions are calculated for both front-and back-screen configurations, which differ by positioning the photocathode at the exit or entrance point of the x-ray beam, respectively. Results: In the front-screen configuration, MTF and DQE are found to have considerable angular dependence, while NPS is shown to vary minimally with projection angle. As expected, the high frequency MTF and DQE are degraded substantially at large angles. By contrast, all transfer functions for the back-screen configuration have the advantage of significantly less angular dependence. Using these models, we investigated the possibility for optimizing the design of DBT detectors. As an example optimization strategy, the phosphor thickness which maximizes the DQE at a fixed frequency is analyzed. This work demonstrates that the optimal phosphor thickness for the frontscreen is angularly dependent, shifting to lower thickness at higher angles. Conversely, the backscreen is not optimized by a single thickness but instead attains reasonably high DQE values over a large range of thicknesses. Although the back-screen configuration is not suited for current detectors using a glass substrate, it may prove to be preferred in future detectors using newly proposed plastic thin-film transistor (TFT) substrates. Conclusions: Using the diffusion approximation to the Boltzmann equation to model the spread of light in a scintillator, this paper develops an analytical model of MTF, NPS, and DQE for a phosphor irradiated obliquely. The model is set apart from other studies on oblique incidence in being derived from first principles. This work has applications in the optimization of DBT detector design.

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Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Acciavatti

Maidment

2011

Medical Physics

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“…They suffer from a significant threshold voltage shift as a function of time with a gate bias applied, and deposition by standard rf plasma enhanced chemical vapor deposition (PECVD) typically requires the substrate to be heated to temperatures in excess of 200 8C, which is incompatible with most plastic substrates [21]. Although other techniques are being developed to allow deposition at lower temperatures [22][23][24][25][26] and compensation circuits are being devised to counteract the threshold voltage shift [27,28] to allow a-Si:H TFTs to be used in AMOLED displays, both of these approaches add complexity to the device fabrication process. Compared to the incumbent a-Si:H TFT technology, metal oxide TFTs have demonstrated a much higher field effect mobility (typically 10-50 cm 2 V À1 s À1 ) and enhanced stability, making them an attractive alternative for AMOLED display backplane applications.…”

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High‐density remote plasma sputtering of high‐dielectric‐constant amorphous hafnium oxide films

Bayer

Hofmann

et al. 2013

Physica Status Solidi (b)

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Hafnium oxide (HfO x ) is a high dielectric constant (k) oxide which has been identified as being suitable for use as the gate dielectric in thin film transistors (TFTs). Amorphous materials are preferred for a gate dielectric, but it has been an ongoing challenge to produce amorphous HfO x while maintaining a high dielectric constant. A technique called high target utilization sputtering (HiTUS) is demonstrated to be capable of depositing high-k amorphous HfO x thin films at room temperature. The plasma is generated in a remote chamber, allowing higher rate deposition of films with minimal ion damage. Compared to a conventional sputtering system, the HiTUS technique allows finer control of the thin film microstructure. Using a conventional reactive rf magnetron sputtering technique, monoclinic nanocrystalline HfO x thin films have been deposited at a rate of $1.6 nm min À1 at room temperature, with a resistivity of 10 13 V cm, a breakdown strength of 3.5 MV cm À1 and a dielectric constant of $18.2. By comparison, using the HiTUS process, amorphous HfO x (x ¼ 2.1) thin films which appear to have a cubic-like short-range order have been deposited at a high deposition rate of $25 nm min À1 with a high resistivity of 10 14 V cm, a breakdown strength of 3 MV cm À1 and a high dielectric constant of $30. Two key conditions must be satisfied in the HiTUS system for high-k HfO x to be produced. Firstly, the correct oxygen flow rate is required for a given sputtering rate from the metallic target. Secondly, there must be an absence of energetic oxygen ion bombardment to maintain an amorphous microstructure and a high flux of medium energy species emitted from the metallic sputtering target to induce a cubiclike short range order. This HfO x is very attractive as a dielectric material for large-area electronic applications on flexible substrates.

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OLED Displays on Plastic

Hildner

2005

Flexible Flat Panel Displays

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Active-matrix OLED using 150°C a-Si TFT backplane built on flexible plastic substrate

Cited by 38 publications

References 4 publications

Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

High‐density remote plasma sputtering of high‐dielectric‐constant amorphous hafnium oxide films

OLED Displays on Plastic

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