Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Construction and characterization of an amorphous silicon flat-panel detector based on ion-shower doping process
Introduction
Since Wilhelm Roentgen's discovery of X-ray, analog films have been used to acquire medical especially chest X-ray images. In recent years, it has become technically and economically feasible to use solid-state detector technology to display, store and transfer X-ray images [1], [2]. Flat-panel X-ray detectors using an amorphous silicon (a-Si) thin-film-transistor (TFT) array are currently the most promising devices, and they have been under development by several groups for a number of years [3], [4], [5], [6], [7]. They may be divided into two categories depending on the method of generating the pixel signal. In direct detection devices, electron–hole pairs are generated by X-ray and are collected in the photoconductive layer (e.g., a-Se) deposited on the TFT array. In indirect detection devices as shown in Fig. 1, a photosensor is built into each pixel and the entire array is covered by a scintillating layer, where X-ray interact and produce visible light. Electron–hole pairs are generated by this light from the scintillating layer and are collected in the photosensor. In this paper, we introduce a new indirect-type a-Si flat-panel detector for medical imaging application. The important feature of this flat panel detector is that the p-layer of the p–i–n diode is formed using an ion shower doping method instead of the conventional Plasma-Enhanced Chemical Vapor Deposition (PECVD) method.
Section snippets
Characteristic of a p–i–n photodiode
In the conventional PECVD method, silane gas and other added gases for doping are decomposed by plasma and deposited on a glass substrate. p- and n-type a-Si:H layers can be obtained by adding dopant gases such as diborane (B2H6) or phosphine (PH3). In this study, we used an ion-shower doping method to form the p-layer of a-Si:H p–i–n diodes. For the source and drain doping of TFT in large-area processing, an ion shower doping technique has been used instead of conventional ion implantation
The array characteristics
The detector consists of a -thick CsI:Tl layer coupled to the a-Si diode array. The array consists of two half-panels which contain 2560×1536 pixels. The whole array as seen in Fig. 3 thus contains 2560 data lines and 3072 gate lines. In this array, the pixel pitch is and the sensing area occupies about 57% of the area of the pixel (fill factor). The diode capacitance is about , giving a maximum possible charge of . The length and width of TFT channel are 20
Dark current uniformity
The variation of dark current among the individual electronics channels is of interest as large channel-to-channel variation could compromise the useful operating range of the system. It has been shown that the variation in dark current for pixels on small arrays is within 10% [9]. Fig. 4 shows dark current variations among the 1536 pixels on the single gate line and the 2560 pixels on the single data line. The variations of readout ASIC chips make the dark current variation of pixels on the
X-ray image
Fig. 5 shows the X-ray image acquired using this flat-panel detector. No gain and offset correction is performed, so we can see the distinction between ASIC chips. The spatial resolution of an imaging system is usually characterized by the modulation transfer function (MTF), which is defined as the magnitude of the Fourier transform of the line spread function divided by the magnitude at zero frequency. In this paper, we measured MTF using an edge method [10]. Using an edge device, an edge
Conclusion
We made a prototype a-Si flat-panel detector using an ion-shower doped p–i–n diode array. The detector shows good uniformity with 2.5% dark current variation. The ion-shower doping process has an advantage over the conventional PECVD in large-area device manufacturing.
Acknowledgements
This work was supported by the Korea Ministry of Commerce, Industry and Energy.
References (10)
Phys. Med. Biol.
(1997)- J.A. Rowlands, J. Yorkston, Flat panel detectors for digital radiography, in: Handbook of Medical Imaging, Vol. 1, SPIE...
SPIE Proc.
(1998)SPIE Proc.
(1998)SPIE Proc.
(1998)
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