Flexible X-ray Imager
July 11, 2012, FlexTech Alliance, San Francisco—Robert Street from PARC described a flexible X-ray image sensor using organic TFTs. This is yet another new technology from the research center that invented Ethernet, GUIs, and the mouse.
Digital X-ray imaging uses indirect detection to capture scintillation from phosphors in a photodiode. In radiometry, the imaging process takes from 0.1 to 1 second, and in fluoroscopy the radiologist is viewing the images at a video rate. The imagers are between 10×10 cm and 40×40 cm. They have pixel sizes from 75 to 250 microns and provide between 14 and 16 bit resolution.
Requirements for the active devices include low off current, low trapped charge—to minimize image lag and ghosting—sharp turn-on for linear response and low parasitic charge. The hard part is that the transistors have to be radiation immune. Comparing material characteristics of MOTFT, amorphous silicon, and polysilicon shows that the oxide materials are higher velocity and able to integrate more of the circuitry such as the read-out amplifiers.
The concerns for the oxide materials are bias stress, radiation hardness, and noise. The noise contributions include the transistor thermal noise, capacitance, and data line resistance. A high signal to noise ratio is needed to get the maximum photodiode signals, but the transistor noise can be compensated for by higher complexity circuit designs.
To test out the concept, they fabricated an OTFT on PEN using GaInZnO with component ratios of 1:1:1 for the bottom gate transistor. They etched the back channel and applied a 160nm of PECVD gate dielectric material. Then they added the 15nm GIZO channel and metalized the source and drain contacts. Next was a 1 micron layer of parylene as an interlayer dielectric. The whole structure is then annealed at 170 °C.
The resulting devices had gate lengths between 3 and 15 microns, mobility between 10 and 15 cm/V/S, on-off ratios of 109, and a threshold voltage of 2 +/- 1 V. Bias stress effects change the Vt 1.5 V after 5,000 seconds, but the effect is reversible after a few hours. They determined that the transistors are acceptable for pixel detectors, but not good enough for the amplifiers. The photodetectors are made with amorphous silicon in a P-I-N structure and produce 0.1 microamp on current and 0.1 femtoamps off current.
The final prototype had an array of 160 x 180 pixels. The pixels were 200 microns square and provided linear and low lag performance. This demonstration shows that there is potential for low noise fluoroscopy using oxide TFTs as the detectors. It is possible to integrate a good portion of the electronics including the drivers and amplifiers. All of these capabilities are possible on a low-cost, flexible substrate with better performance than low temperature polysilicon.
