The deposition of high-quality silicon nitride (SiNx) layers is required in the manufacturing of state-of-the-art field-effect transistors (FETs), which are used everywhere throughout modern computing. An important application of SiNx in FETs is as a side-wall gate spacer, where it functions as a barrier layer to protect sensitive inner layers, against for example oxygen ingress and etch processing steps. From a literature study further opportunities and applications of SiNx films in future three-dimensional FET architectures will be discussed. It will be shown that a low thermal budget is essential for these applications of SiNx, in addition to obtaining a high quality and a high conformality. A novel precursor Di(Sec-Butyl)AminoSilane (DSBAS, SiH3N(C4H9)2) was employed to develop a plasma-assisted atomic layer deposition (ALD) process to grow high-quality SiNx at low substrate temperatures. Material properties have been characterised over a wide temperature range and have been compared with properties of a similar organosilane precursor Bis(Tert-Butyl-Amino)Silane (BTBAS, SiH2[NH(C4H9)]2). Compared to the BTBAS process and other plasma-assisted ALD processes reported in the literature, the obtained growth per cycle was low. However, typically high-quality SiNx films were obtained as was demonstrated by low wet etch-rates, low C, O, and H content, high mass densities and N/Si values close to stoichiometric Si3N4. The high-quality of the SiNx was also investigated in trenches with aspect ratio 1:4.5, relevant for FET applications. High quality was confirmed at horizontal surfaces in the trench, while at vertical surfaces the quality was slightly poorer. However, the conformality was proven to be not sufficient. The reduced thickness and quality at vertical side-walls also suggest that ions play a role in the quality and growth of SiNx films.The results in this work provide support for a growth mechanism, previously proposed for BTBAS. This mechanism explains how the split-off of the only amino-ligand in a DSBAS-molecule in the precursor half-cycle, reduces redeposition of ligand fragments in the subsequent plasma half-cycle leading to high film quality. The new insights obtained in this work have resulted in new proposed experiments, in order to further investigate the reaction mechanisms involved.