Influence of precursors gases on LPCVD TFT's characteristics

In this work, undoped amorphous silicon films were deposited on Coming glass substrates by low pressure chemical vapour deposition using pure disilane as source gas. The deposition temperature and pressure varied from 450 to 525 degreesC and 20 to 70 Pa, respectively. Silicon films were annealed by solid phase crystallisation at 600 degreesC and were compared with silicon films deposited from silane at 550 degreesC and 90 Pa (standard deposition conditions). Results show that layers obtained using disilane gas exhibit higher quality than those from silane. Especially, Hall effect mobility increased from 20 to 55 cm(2)/V s as a consequence of a higher nucleation time for optimised deposition temperature and pressure. So, two different types of unhydrogenated thin film transistors were made. A 150 nm thick active layer made from silane at 90 Pa and 550 degreesC permits obtaining an average mobility of 45 cm(2)/Vs and a threshold voltage of 6 V (with a gate oxide thickness of 40 nm) for N-type. The use of disilane as the precursor gas produces an increase in the mobility of 25% and reduces the threshold voltage to 4 V Identical CMOS-TFTs devices as inverters were made from silane and disilane for the N- and P-type, respectively. These structures present a good behaviour mainly due to the reduction of the threshold voltage difference. (C) 2002 Elsevier Science B.V. All rights reserved.