AbstractPolysilicon Thin Film Transistors (TFT's) have a vast potential in a wide range of applications including, flat panel displays, large area electronics and 3D integrated circuits. This thesis describes advances made in the technology for the fabrication of TFT's. A Multi-Process reactor has been designed to produce device quality layers on Coming 1733 glass substrates, which are incompatible with standard high temperature processing techniques. A variety of processes can be performed sequentially in the system without removal of the substrates from vacuum. This integrated processing ensures good quality interface and film properties.
High quality silicon films have been deposited and characterised. The grain size and the impurity content of the layers have been determined from Atomic Force Microscopy (AFM) imaging techniques and Secondary-Ion Mass Spectroscopy (SIMS). Controlled in-situ doping of poly silicon by phosphorous to produce a conductive layer has also been demonstrated.
By careful control of the process conditions PECVD silicon nitride can be utilised for a dual purpose in TFT applications. A silicon rich film can be used as a passivation layer and prevent the diffusion of impurities from a glass substrate into active device layers. A high quality nitrogen rich film is suitable for use as a gate dielectric in a TFT.
TFT's have been fabricated on oxidised silicon substrates. Devices with a silicon nitride dielectric had comparable characteristics to those with a thermal Si02 dielectric. A number of analytical techniques were investigated for the calculation of trap state density in the active silicon layer. These techniques are useful for evaluating the effect of passivation methods and novel processing steps. A suitable glass substrate for TFT fabrication has been selected and a full low temperature process has been developed. TFT's have successfully been fabricated on glass substrates using this process.
|Date of Award||Dec 1995|