A low temperature amorphous silicon (a-Si) thin film transistor (TFT) and amorphous silicon PIN photodiode technology for flexible passive pixel detector arrays has been developed using active matrix display technology. The flexible detector arrays can be conformed to non-planar surfaces with the potential to detect x-rays or other radiation with an appropriate conversion layer. The thin, lightweight, and robust backplanes may enable the use of highly portable x-ray detectors for use in the battlefield or in remote locations. We have fabricated detector arrays up to 200 millimeters along the diagonal on a Gen II (370 mm x 470 mm rectangular substrate) using plasma enhanced chemical vapor deposition (PECVD) a-Si as the active layer and PECVD silicon nitride (SiN) as the gate dielectric and passivation. The a-Si based TFTs exhibited an effective saturation mobility of 0.7 cm2/V-s, which is adequate for most sensing applications. The PIN diode material was fabricated using a low stress amorphous silicon (a-Si) PECVD process. The PIN diode dark current was 1.7 pA/mm2, the diode ideality factor was 1.36, and the diode fill factor was 0.73. We report on the critical steps in the evolution of the backplane process from qualification of the low temperature (180°C) TFT and PIN diode process on the 150 mm pilot line, the transfer of the process to flexible plastic substrates, and finally a discussion and demonstration of the scale-up to the Gen II (370 x 470 mm) panel scale pilot line.
The U.S. Army, Arizona State University (ASU) and commercial industry have joined forces to create the Flexible Display Center (FDC) at Arizona State University, a large-scale collaborative venture designed to rapidly advance flexible display technology to the brink of commercialization. The Center has completed its startup phase and is now engaged in an intensive and aggressive applied research and development program that will produce high quality, high performance active matrix reflective and emissive flexible display technology demonstrators (TDs). Electrophoretic ink and cholesteric liquid crystals have been selected as Center reflective imaging layer technologies; these technologies are attractive because they are fully reflective and bistable (extremely low power) and because the materials are environmentally robust and intrinsically rugged. Organic light emitting devices (OLEDs) have been chosen as the emissive imaging layer technology. These three electro-optic subsystems will be integrated with a flexible a-Si thin film transistor active matrix backplane platform. We have created the integrated design, backplane fabrication, display assembly, test and evaluation capability to enable rapid cycles of learning and technology development. Backplane fabrication is currently accomplished on a 6” wafer scale pilot line linked to a Manufacturing Execution System and supported by a comprehensive suite of in-fab metrology tools. We are currently installing a GEN II pilot line, with qualified operation slated for 2006. This line will be used to demonstrate process and display form factor capability, while providing high yield low volume manufacturing of pilot-scale levels of technology demonstrators for the Army and our commercial partners.
The authors report on the angle resolved light scattering characteristics of individual polystyrene spheres on three silicon surfaces. A He-Ne laser (632.8 nm) focused to a 15 micrometers 1/e2 diameter was employed to illuminate 0.804 micrometers diameter spheres on optically smooth ((sigma) >= (lambda) ) silicon surfaces: monocrystalline silicon (bare silicon), polycrystalline silicon (polysilicon), and roughened silicon ('black silicon'). These surfaces provided a roughness spectrum ranging between the smooth, virtually featureless surface of the bare silicon to one of dense, very coarse needle- like features on the black silicon. Scattering was measured as a function of incident beam polarization for incident angles of 30, 45, and 75.3 degrees (Brewster's angle). Experimental measurements show that the beam incident angle and polarization are important factors controlling substrate background scatter. The substrate influences sphere detectability in two ways: first, by directly scattering incident radiation into the detector and second, by reflecting a portion of the forward scattered light originating from the sphere. The results obtained are qualitatively explained with reference to the silicon surface reflectance which varies significantly as beam incident angle and polarization are changed. As surface roughness approached a value comparable to the sphere size, detectability diminished; that is, the measured cross sections were lower under these conditions. Surface roughness not only added to the background signal, but also reduced the amount of energy scattered by the sphere reaching the detector. A modification to Lorenz-Mie theory is introduced to explain the experimental findings. The approach taken is to calculate the scattering components for a sphere in free space, then attenuate those components which are reflected from the substrate into the detector. The computation includes the variation in substrate reflectivity as the ray incident angle and polarization change. The assumptions and validity of this approach are discussed, as well as future possible improvements to the model.
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