Flexible X-Ray Detector Array Fabricated With Oxide ... - IEEE Xplore

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IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 5, MAY 2012

Flexible X-Ray Detector Array Fabricated With Oxide Thin-Film Transistors R. A. Lujan and R. A. Street

Abstract—A flat-panel flexible X-ray image sensor fabricated with oxide thin-film transistors (TFTs) is described. Bottom-gate GaInZnO TFTs are fabricated on a plastic substrate and integrated with amorphous silicon p-i-n photodiodes, with a maximum process temperature of 170 ◦ C. X-ray images obtained with a 160 × 180 pixel array in the indirect detection mode are reported. Index Terms—Oxide semiconductor, thin-film transistor (TFT), X-ray detector.

I. I NTRODUCTION

T

HE RECENT development of metal–oxide thin-film transistors (TFTs) provides a new technology choice for flatpanel displays [1], and the high mobility that can be achieved is of particular interest for organic light-emitting diode displays [2]. A part of the attraction of the oxides is that the TFT fabrication process is simpler and therefore less expensive than lowtemperature polysilicon, which is the current technology for high-mobility TFT backplanes [3]. Digital X-ray detectors also rely on TFT backplane technology and currently use amorphous silicon (a-Si) for the pixel TFT and the photodiode [4]. Highmobility TFTs enable pixel amplifiers to reduce electronic noise [5] or allow high-frame-rate imaging for specialized medical diagnostic procedures. This letter describes a prototype digital X-ray detector based on GaInZnO (GIZO) TFTs. The detector is fabricated on a flexible plastic substrate with an a-Si continuous photodiode [4]. Operating in the indirect detection mode, a phosphor is placed in contact with the array; the X-rays excite fluorescence in the phosphor which is detected and imaged by the photodiode and TFT backplane [4]. Backplanes made on plastic flexible substrates are of interest for both displays and X-ray detectors. In the case of the detector, the plastic substrate renders it more robust than with a glass substrate and allows for curved detectors for applications such as computed tomography, and a sufficiently thin substrate may enable higher sensitivity detectors combining both front and back phosphors. A wide range of oxide compounds has been shown to give high-mobility TFTs [6]–[9], but GIZO is one of the most widely studied [10]–[14] in part because it is amorphous and is therefore thought to provide a higher device uniformity than the polycrystalline materials. TFTs can be fabricated over a wide temperature range from room temperature upward, and in general, the TFT properties improve with either deposition or

Manuscript received January 17, 2012; revised February 16, 2012; accepted February 17, 2012. Date of publication April 3, 2012; date of current version April 20, 2012. The review of this letter was arranged by Editor Z. Chen. The authors are with the Palo Alto Research Center, Palo Alto, CA 94304 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/LED.2012.2188825

annealing temperature. Temperatures below 200 ◦ C are needed for devices on plastic. Oxide TFTs are sensitive to exposure to the ambient and to a plasma and/or UV light [15]. Furthermore, the TFTs exhibit the bias-stress effect with a magnitude that is sensitive to the detailed processing of the oxide surface and its exposure to the ambient and/or illumination [16], [17]. The main challenges for a flexible oxide TFT image sensor are therefore, first, to maintain high TFT performance after the integration of the photodiode with the TFT backplane and, second, to fabricate both device elements using the low process temperature that is required for the plastic substrate. However, a successful imager fabrication process results in a backplane in which the oxide TFT channel material is isolated from the ambient by a combination of a barrier layer and a gate dielectric underneath the TFT and the photodiode on top, which is expected to help make an electrically stable device with long lifetime. II. A RRAY FABRICATION The X-ray detector array was fabricated on a polyethylene naphthalate (PEN) substrate supported on a holder. Conventional photolithography was used to pattern the layers. A barrier layer is deposited first by plasma-enhanced chemical vapor deposition (PECVD) followed by the MoCr metal gate for the bottom-gate staggered TFT structure. The 160-nm SiO-based gate dielectric is also deposited by PECVD in part to make the fabrication process as similar as possible to the conventional aSi backplane process. Both the barrier and the gate dielectric are deposited at 160 ◦ C–170 ◦ C consistent with the requirements of the PEN substrate. The 15-nm GIZO channel material is sputtered at room temperature using a target with 1 : 1 : 1 composition of Ga:In:Zn and is followed by deposition of the MoCr source and drain contacts. The array was completed with a 1-μm parylene interlayer dielectric, an Al/MoCr contact metal, a conventional a-Si p-i-n photodiode deposited by PECVD at 170 ◦ C and with a thickness of about 1.2 μm, and an ITO top contact. The detector array has a 160 × 180 pixel format with a 200-μm pixel size for an overall dimension of 3.2 × 3.6 cm. The GIZO TFT array was annealed in clean dry air at 170 ◦ C for 10 h. The pixel circuit is shown in Fig. 1. III. R ESULTS AND D ISCUSSION Test TFT devices with channel length from 3 to 15 μm were fabricated on the array substrate using the complete process including the photodiode layer. Fig. 1 shows a typical example of the oxide TFT transfer characteristics with a width-to-length ratio of 100/10 μm. The mobility is 10–15 cm2 /V · s, with the saturation mobility slightly higher than the linear mobility,

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LUJAN AND STREET: FLEXIBLE X-RAY DETECTOR ARRAY FABRICATED WITH OXIDE TFTs

Fig. 1. Transfer characteristics for a GIZO TFT fabricated with the complete image sensor process. The inset shows the same data on a linear plot. The image sensor pixel circuit comprising the TFT and photodiode is shown.

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Fig. 3. Dark and white-light-illuminated current–voltage characteristics of the low-temperature a-Si photodiode on a PEN plastic substrate.

Fig. 4. X-ray image of a resolution target obtained with an 80-kVp exposure. The only image correction is a background subtraction.

Fig. 2. Gate bias-stress and recovery data for a GIZO TFT fabricated with the complete detector process. Dashed line with crosses is the initial data. The gate stress voltage is +20 V, and the drain voltage is 5 V. (Squares) Threshold voltage shift after 5000 s is 1.5 V. Recovery data after (triangles) 10 min, (circles) 20 min, and (diamonds) 16 h are shown. The 16-h data are almost coincident with the initial data. The inset shows the decrease of drain current during the stress.

using parameter extraction methods as described in [3]. The threshold voltage VT is about 2 V, and the TFT turns on at a small negative voltage. The leakage current is 30 fA at a drain voltage of 1 V, increasing to about 300 fA at 20 V, and is independent of negative gate voltage. The subthreshold slope is ∼0.3 V/decade. The effective mobility increases with the channel length, indicating some contact resistance. Fig. 2 shows the bias-stress data with +20-V gate voltage applied in the dark ambient. The current drops to 80% of the initial value after about 5000 s which corresponds to a threshold voltage shift of about 1.5 V. The VT shift recovers in about 1 h at room temperature. These TFT characteristics are comparable to other good-quality GIZO TFTs and are well suited to an X-ray detector backplane which requires a high on/off ratio, low leakage, and a reasonably sharp turn-on region. The results show that the fabrication process is successful and, in particular,

that the sensor layer process does not degrade the GIZO TFT performance. Fig. 3 shows the forward and reverse currents for a typical 170 ◦ C test photodiode on PEN fabricated by the process used for the array. Previous results found that the low-temperature (170 ◦ C) a-Si p-i-n photodiode has comparable properties to the conventional high-temperature (∼250 ◦ C) devices but with a leakage current increased by about an order of magnitude and with a reduced mobility-lifetime product [18]. The process has been further improved for the present device so that the leakage current is now below 3 pA/mm2 at 5-V bias, which is within a factor 3 of the typical high-temperature conventional devices on glass. Some of the p-i-n diodes have a leakage current below 1 pA/mm2 at 5 V. Low leakage current is important to allow the long integration times used in radiographic medical imaging, without degrading the image quality. Fig. 3 also shows that the response to white-light illumination is almost independent of voltage in reverse bias, giving an excellent linear response. X-ray imaging was performed with a Gd2 O2 S:Tb (Lanex regular) phosphor placed in direct contact with the detector array and using an X-ray generator operating in the 50–100-kVp range. The phosphor emits at about 550 nm which is an excellent match to the a-Si photodiode. Fig. 4 shows the image of a standard X-ray resolution target obtained at 80 kVp and with a 0.2-s integration time. A sensor bias voltage of −5 V is applied directly to the top of the array, and the electrode

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IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 5, MAY 2012

contact is the origin of the black rectangle at the top right of the image. The image is corrected for the dark background but is not normalized to the flood exposure field, since the response uniformity was found to be sufficiently good. The image is also shown without any spatial filter correction for line and pixel defects. Aside from demonstrating good imaging properties, the image shows that the fabrication process is reasonably robust since there are relatively few defects. The detector line-spread function (LSF) was obtained by the angled slit measurement technique [19]. The LSF is well described by a Gaussian with the full-width at half-maximum of 1.8 pixels which is consistent with the Lanex screen resolution. The image lag was also investigated and gave a 10% first frame lag, decreasing to about 1% at the third lag frame. Both of these results are consistent with the typical properties of the a-Si photodiode and the Lanex screen. The data therefore show that the oxide TFT does not introduce any significant degradation of the imaging properties. The image sensor operated well only for the integration times of < 0.5 s due to significant TFT leakage. However, the pixel TFT has an excessively large W/L ratio of about 20 because the backplane was originally designed for a low-mobility TFT channel material. An optimized pixel design will greatly improve the range of frame times. The detector results demonstrate the potential of oxide TFT backplanes for X-ray imaging applications either on glass or on plastic substrates. Previous work has shown that a pixel amplifier made from high-mobility polysilicon TFTs can reduce the electronic noise [5] which is important for low X-ray dose medical imaging. Oxide TFTs offer the same opportunity but with a technology that is expected to have lower fabrication cost than polysilicon. However, at this point, we have no information about the radiation hardness of the oxide TFTs, which awaits further study. ACKNOWLEDGMENT The authors would like to thank S. Kor, B. Russo, and Q. Wang for helpful assistance and to the Palo Alto Research Center for internal funding. R EFERENCES [1] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, and H. Hosono, “Thin-film transistor fabricated in single crystalline transparent oxide semiconductor,” Science, vol. 300, no. 5623, pp. 1269–1272, May 2003. [2] J.-S. Park, T.-W. Kim, D. Stryakhilev, J.-S. Lee, S.-G. An, Y.-S. Pyo, D.-B. Lee, Y. G. Mo, D.-U. Jin, and H. K. Chung, “Flexible full color organic light-emitting diode display on polyimide plastic substrate driven by amorphous indium gallium zinc oxide thin-film transistors,” Appl. Phys. Lett., vol. 95, no. 1, pp. 013503-1–013503-3, Jul. 2009.

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