Satoshi Tokuda

Preparation and characterization of polycrystalline CdZnTe films for large-area, high-sensitivity X-ray detectors

This article reviews the preparation and characterization of polycrystalline CdTe and CdZnTe films to be used in large-area, high-sensitivity X-ray panel detectors intended for medical diagnostics. The films, deposited by closed-spaced sublimation, are expected to exhibit excellent efficiency at low X-ray doses because of their high sensitivity. The detectors constructed using these films incorporated a novel hybrid technique, in which zinc-doped CdTe was pre-deposited onto a ceramic substrate and then connected to a TFT circuit substrate. The sensor substrate material was specially selected to avoid both incident X-ray attenuation in the substrate and micro-cracks in the film. Zinc doping, which was used to grow the CdTe film, also served to form the heterojunction diode structure that suppressed leakage current. Moreover, the quality of a polycrystalline CdZnTe film deposited on a 9″×9″ substrate was characterized, revealing its applicability to large-area X-ray detectors. Further investigation and improvements are in progress.

Experimental evaluation of a-Se and CdTe flat-panel x-ray detectors for digital radiography and fluoroscopy

Described are two types of direct-detection flat-panel X-ray detectors utilizing amorphous selenium (a-Se) and cadmium telluride (CdTe). The a-Se detector is fabricated using direct deposition onto a thin film transistor (TFT) substrate, whereas the CdTe detector is fabricated using a novel hybrid method, in which CdTe is pre-deposited onto a glass substrate and then connected to a TFT substrate. The detector array format is 512 X 512 with a pixel pitch of 150 micrometer. The imaging properties of both detectors have been evaluated with respect to X-ray sensitivity, lag, spatial resolution, and detective quantum efficiency (DQE). The modulation transfer functions (MTFs) measured at 1 lp/mm were 0.96 for a- Se and 0.65 for CdTe. The imaging lags after 33 ms were about 4% for a-Se and 22% for CdTe. The DQE values measured at zero spatial frequency were 0.75 for a-Se and 0.22 for CdTe. The results indicate that the a-Se and CdTe detectors have high potential as new digital X-ray imaging devices for both radiography and fluoroscopy.

Improvement of temporal response and output uniformity of polycrystalline CdZnTe films for high-sensitivity x-ray imaging

X-ray detectors that we developed utilizing polycrystalline CdZnTe films exhibited superior sensitivity, but inadequate temporal response and output uniformity for medical imaging purposes. In order to improve those deficiencies, we tested new procedures for deposition and post-deposition chemical-heat treatment of polycrystalline CdZnTe films, in addition to investigating new device structures. We doped the polycrystalline CdZnTe films with Cl in a new manner so as to achieve effective grain boundary passivation. Polycrystalline CdZnTe films that were Cl-doped by our new procedure were found to have a finer and more uniform grain structure. We fabricated and evaluated devices with a replaced barrier layer against charge injection under negative bias. All these measures helped reduce the temporal lag of a 300 μm thick polycrystalline CdZnTe film exposed to X-ray irradiation. Moreover, utilizing this film in a detector reduced the detectors output uniformity. We have succeeded in improving the X-ray temporal response and output uniformity of a 300 μm thick CdZnTe film. This study will also discuss countermeasures against a number of problems that were encountered, including MTF degradation and short range image lag.

A CMOS detector readout front-end for X-ray digital radiography systems

Described is a 32-channel CMOS readout front-end for X-ray pixel detectors. This front-end is intended for use in digital radiography systems that employ photon counting methods. In many photon counting systems, counting losses become problematic at higher counting rates. To minimize counting losses and ensure true counts at high rates, a synchronous charge sampling method with analog pipelining has been developed and integrated in standard 3-/spl mu/m CMOS technology. The device operates with a single 5-V power supply and dissipates 4 mW/channel. The equivalent noise charge measured was less than 400 electrons at a counting rate of 8.3 Mcps (with no detector capacitance).

Large-area deposition of a polycrystalline CdZnTe film and its applicability to x-ray panel detectors with superior sensitivity

This paper describes our investigation of the X-ray detective characteristics of a thick polycrystalline CdZnTe film deposited on a large-area substrate. We deposited a polycrystalline CdZnTe film on a 9 inch by 9 inch substrate, and investigated its quality. It was verified to be quite uniform within the substrate. We also cut the film and connected it to a 3 inch by 3 inch TFT panel for evaluating the X-ray imaging performance. The TFT array format was 512 by 512 pixels with a pixel pitch of 150 micrometers . The thickness of the CdZnTe film was about 300 micrometers after lapping and polishing, and the film density per unit area was higher than 170 mg/cm2. The average sensitivity was 1.5E9 e-/mR/mm2; the beam condition was 80 kV with 26-mm Al filtration. The MTF measured at 1 lp/mm was 0.82. The time response and uniformity of X-ray sensitivity were not still adequate, and further improvements are in progress. In conclusion, we have demonstrated the applicability of the polycrystalline CdZnTe film to a large-area detector, although further investigations and improvements are needed.

Close-Spaced Sublimation Growth and Characterization of Polycrystalline Cd1-xZnxTe Thick Films for Flat-Panel X-ray Detectors

Polycrystalline Cd1-xZnxTe thick films (x ~0.05) with thicknesses above 400 µm were prepared by the close-spaced sublimation (CSS) as a conversion layer for next-generation highly efficient flat-panel X-ray detectors. The effects of the substrate temperature on the properties of the Cd1-xZnxTe layer were investigated, and it was found that the surface morphology and preferred crystallographic orientation depended on the substrate temperature. Furthermore, the control of the initial stage of deposition of Cd1-xZnxTe films was attempted, and high-quality Cd1-xZnxTe thick films were obtained by deposition after removing the surface layer of the sintered CdTe and ZnTe powder source.

Grain growth and characteristics of chlorine doped thick polycrystalline CdZnTe films

Chlorine (CI)-doped polycrystalline CdZnTe is expected to be used as photoconductor material for Flat Panel Xray detectors (FPDs) due to its high detection efficiency and environmental resistance. However, polycrystalline film causes great variability among pixels in the X-ray image of FPDs. To reduce the variability among pixels, chlorine doping methods and their effects were investigated. By combining the two methods, the addition of CdCl2 to the source and the introduction of vapor chloroform during deposition, we were able to reduce the variability among pixels.