ALD/ALE 2025

The Mechanism of Thermal ALD of Silicon Carbonitride from Chloroalkylsilanes and Ammonia – Theory Meets Experiment

Speaker:
Simon Elliott, Director of Atomic Level Process Simulation, Schrödinger

Abstract:
Carbon-doping of silicon nitride or silicon oxide results in a lowering of the dielectric constant and etch rate. The C-doped derivatives are therefore being used to replace the parent films in many applications in FEOL semiconductor manufacturing, such as for spacers and gap fill. Traditionally these C-doped films are deposited by CVD methods, but recently ALD methods have been formulated using silicon precursors with Si-CHx bonds. Maintaining the Si-CHx bond has been challenging due to its propensity to cleave under plasma conditions or at high temperatures. For these reasons, precursors that can be used in a low temperature thermal ALD process are highly desired.In this work we investigate whether ammonia is viable as a thermal ALD co-reagent for this application and what mixture of chloro and alkyl ligands are most favorable in the silicon precursor.

We use density functional theory (DFT) to compute the atomic-scale structure of surface intermediates and the mechanism of potential ALD reactions and complement this with detailed characterization during substrate exposure experiments.

Looking first at the co-reagent, we investigate the kinetics of proton transfer from ammonia to a model silyl fragment on an aminated surface. Four proton transfer pathways are obtained for HCl elimination, all with high barriers (>1.2 eV) that are indicative of low Bronsted acidity and suggest that side-reactions will compete with the ALD process. Three pathways are obtained for CH4 elimination with even higher barriers (>2.5 eV). Therefore, up to moderate temperatures, terminal-CH3 is likely to survive ammonia treatment and be incorporated into the film, and this could be a route to C-doping. Residual gas analysis (RGA) and quartz crystal microbalance is used to monitor whether HCl or CH4 is in fact evolved during each half reaction.

The second part of the study looks at the effect of various combinations of chloro and methyl ligands in candidate Si precursors, both monomers (SiCl4, SiHCl3, SiCl3Me, SiH2Cl2, SiHCl2Me, SiCl2Me2) and dimers (Si2Cl6, Si2Cl4Me2). Structural models are generated efficiently of the >100 surface intermediates that can potentially occur through physisorption, chemisorption via ligand elimination and etching by exchange with surface amines. For each precursor, DFT-based thermodynamics reveal that the most favorable intermediate is surf-SiCl3 up to about 250C, surf-Cl2 up to 450C and surf-Cl at higher temperatures, with associated predictions of ALD growth rate. Film composition and growth rate is then validated experimentally by ellipsometry (SE) and infrared red spectroscopy (ATR-FTIR), ex-situ but with an inert atmosphere glove box.