A 2" matrix of 12288 light sensors was realized on a plastic substrate by low temperature thin film technology. In comparison to a glass realization the sensor matrix has a reduced weight, is unbreakable and flexible. Each sensor cell contains a photoconductor as a resistive sensor and a TFT for reading out the resistor values line by line. By using amorphous silicon for both the channel of the TFT and the photoconductor it is possible to fabricate the sensor matrix with a low number of masks. As gate insulator amorphous silicon nitride is used. The above mentioned films were deposited in a PECVD reactor in the same vacuum at 180°C which is suitable for PES-foils. The mismatch of the thermal expansion coefficients of the thin films and the plastic substrates causes peeling of the films and cracks. The remedy is the use of sputtered adhesion layers for the metallization and PECVD layers. To qualify the different adhesion layers a tape test method was used. Cracks were avoided by reducing the internal stress. For this reason a new layout was designed in which all PECVD layers are patterned in islands providing a minimum of internal stress. The TFTs show an ON/OFF ratio of 6 decades and a mobility of 0.3 cm2/Vs. The change of the a-Si:H conductivity under illumination is 6 nS/W/cm2. This value allows for a dynamic range of the sensor matrix of 60 dB. The new flexible sensor offers a wide variety of applications like an electronic eye with a round view for the observation of work pieces and traffic signs.
One of the major challenges during the processing of AMLCDs on plastic substrates is to guarantee the structure stability of the substrate. For the addressing of the active matrix, a-Si:H-TFTs are used. The a-Si:H layer is deposited by using PECVD at substrate temperatures between 150 and 250°C, whereas the photolithographic steps for structuring the layers are usually carried out at room temperature. Because the thermal expansion of the plastic substrate is roughly 10 times higher than the expansion of the PECVD layer, there is a high strain in the foil and in the layer after the substrate is cooled down to room temperature again. We have carried out an analytical and experimental investigation on the occurring film stress and the resulting structure distortion during the processing of AMLCDs on plastic substrates. We established design rules minimizing distortion and stress and built the first TN display with 12,288 pixels addressed by a-Si:H TFTs on a PES substrate.
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