Matrix of light sensors addressed by a-Si:H TFTs on a flexible plastic substrate

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Abstract
A 2 inch 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 degree 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 around view for the observation of wok pieces and traffic signs.

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