Kinetic study on heterogeneous nucleation and incubation period during chemical vapor deposition.

The kinetics of heterogeneous nucleation during chemical vapor deposition (CVD) is still unclear despite its importance. Nucleation delay is often observed in many CVD processes, which is known as the incubation period (τi). In this study, the effects of concentration (C) and sticking probability (η) of film-forming species on τi were formulated based on our kinetic model. To discuss the kinetics, τi -1 with the rate dimension was used and formulated using C and η. Because η onto heterogeneous surfaces (ηhetero) is difficult to evaluate, the study was initiated with η onto homogeneous surfaces (ηhomo), followed by a discussion on its reasonability. The formulation was validated using the experimental dataset for SiC-CVD from CH3SiCl3/H2 onto BN underlayers because CVD involves multiple film-forming species with different ηhomo ranging from 10-6 to 10-2 and thus is a suitable system for studying the effect of ηhomo. High-aspect-ratio (1000:1) parallel-plate microchannels consisting of τi-involving BN and a τi-free Si surface were utilized to separate these film-forming species along the microchannel depth. τi was exceptionally long, up to several hours, depending on the CVD conditions. τi -1 was found to be proportional to Cn, where n is the reaction order. n was quantified as ≈1.6, suggesting the initial nucleation was triggered by the impingement of two adspecies in the second order and lowered possibly by the discrepancy between C in the gas-phase and that actually producing adspecies on the surface. τi -1 was also found to be proportional to ηhomo. The exceptionally long τi was likely originated from the significantly lower ηhetero than ηhomo and the higher activation energy for ηhetero than that for ηhomo.

Conformal and Stoichiometric Chemical Vapor Deposition of Silicon Carbide onto Ultradeep Heterogeneous Micropores by Controlling the Initial Nucleation Stage.

Conformal chemical vapor deposition (CVD) of silicon carbide (SiC) from methyltrichlorosilane (MTS) and hydrogen (H2) onto high-aspect-ratio (HAR; typically >100:1) three-dimensional features has been a challenge in the fabrication of ceramic matrix composites. In this study, the impact of heterogeneous underlayers on the initial nucleation of SiC-CVD was studied using HAR (1000:1) microchannels with a tailored wetting underlayer of Si(100) and dewetting underlayers of thermally formed amorphous silicon dioxide (a-SiO2) and turbostratic boron nitride (t-BN). Incubation periods were distributed in the microchannels on a-SiO2 and t-BN underlayers, with the longest period of 70 min found at the feature-bottom due to a decreased concentration (C) of film-forming species. The longer incubation periods with more dewetting underlayers arose due to demoted initial nucleation. Prolonged incubation at the feature bottom led to poor conformality because thick films had already formed at the inlet when film formation began at the feature bottom. The incubation periods were eliminated by increasing the supply of MTS/H2, in accordance with classical heterogeneous nucleation theory. In the meantime, carbon-rich SiC films formed in the vicinity of dewetting a-SiO2 and t-BN underlayers at the feature bottoms, with greater carbon segregation on more dewetting underlayers. This was probably due to the deposition of pyrocarbons (CH4, C2H2, and/or C2H4) generated from MTS/H2 in the gas phase. Decreasing the temperature (T) from 1000 to 900 °C prevented carbon-rich film formation, and the expected deposition rate of pyrocarbon decreased to 0.6% for the case of CH4. A higher C of MTS/H2 combined with a lower T enabled conformal and stoichiometric film formation on the heterogeneous HAR features.

Porous Membranes as Sacrificial Layers Enabling Conformal Chemical Vapor Deposition Involving Multiple Film-Forming Species.

We propose a new, concise method for conformal chemical vapor deposition (CVD) using sacrificial layers (SLs) to fill three-dimensional features with microscopic pores. SLs are porous membranes (e.g., ceramic felts) that filter film-forming species having high sticking-probability (η). CVD processes with multiple film-forming species generally suffer from poor conformality due to preferential film deposition at the inlets of features by the high-η species, such as reactive intermediates. An SL traps such high-η species before they reach the target features and selectively supplies film-forming species with lower η (e.g., source precursors or stable intermediates) that enables conformal film deposition. Here the trapping efficiency of an SL was predicted and a procedure for designing an optimal SL was established. The procedure was demonstrated by CVD of silicon carbide (SiC) with multiple film-forming species of high-η species (η = 8.0 × 10-3) and lower-η species (η = 5.9 × 10-5 and 2.2 × 10-7). The trapping of 99.2% of incident high-η species was achieved with an optimized SL, wherein the deposition rate (m/s) contribution by high-η species declined from 0.546 at the SL inlet to 0.014 at its outlet. Finally, using these optimized SLs, SiC-CVD filling of micron-scale trenches was demonstrated with an aspect-ratio of 16:1.

Gas-phase reaction mechanism in chemical dry etching using NF3 and remotely discharged NH3/N2 mixture.

Modeling of dry etching processes requires a detailed understanding of the relevant reaction mechanisms. This study aims to elucidate the gas-phase mechanism of reactions in the chemical dry etching process of SiO2 layers which is initiated by mixing NF3 gas with the discharged flow of an NH3/N2 mixture in an etching chamber. A kinetic model describing the gas-phase reactions has been constructed based on the predictions of reaction channels and rate constants by quantum chemical and statistical reaction-rate calculations. The primary reaction pathway includes the reaction of NF3 with H atoms, NF3 + H → NF2 + HF, and subsequent reactions involving NF2 and other radicals. The reaction pathways were analyzed by kinetic simulation, and a simplified kinetic model composed of 12 reactions was developed. The surface process was also investigated based on preliminary quantum chemical calculations for ammonium fluoride clusters, which are considered to contribute to etching. The results indicate the presence of negatively charged fluorine atoms in the clusters, which are suggested to serve as etchants to remove SiO2 from the surface.