Low temperature growth of silicon nitride by electron cyclotron resonance plasma enhanced chemical vapour deposition

Abstract

Silicon nitride (a-SiN) is extensively used as the gate insulator layer in thin films transistors (TFTs) for active matrix liquid crystal displays. Such displays are currently produced on thin, glass substrates; plastic substrates offer a cheaper and more robust alternative, but have maximum process temperatures below 150°C. The deposition of a-SiN by rf plasma enhanced chemical vapour deposition (PECVD) requires the production of SiNxHy radicals in the plasma. However, as N2 is very stable, a gas mixture of SiH4 with NH3 is required for this to take place, which yields a hydrogen-rich material. High substrate temperatures (∼300°C) are therefore required to drive out the excess hydrogen. In electron cyclotron resonance (ECR)-PECVD, a highly ionised plasma (∼1016 m−3) with very low ion energies (∼10 eV) can be produced. This allows the dissociation of N2 into atomic N, and hence a gas phase reaction can take place directly with SiH4, which results in a material with a lower hydrogen content than is obtained with NH3 gas. Energy for further H removal is provided by the bombardment of He ions, allowing a reduction in the deposition temperature to 80°C. In this way a-SiN has been produced with a resistivity of 3×1014 Ω cm and an average breakdown strength of 4.5 MV cm−1.