In this work, silicon carbide (SiC) coatings were successfully grown by pulsed chemical vapor deposition (CVD). The precursors silicon tetrachloride (SiCl4) and ethylene (C2H4) were not supplied in a continuous flow but were pulsed alternately into the growth chamber with H-2 as a carrier and a purge gas. A typical pulsed CVD cycle was SiCl4 pulse-H-2 purge-C2H4 pulse-H-2 purge. This led to growth of superconformal SiC coatings, which could not be obtained under similar process conditions using a constant flow CVD process. We propose a two-step framework for SiC growth via pulsed CVD. During the SiCl4 pulse, a layer of Si is deposited. In the following C2H4 pulse, this Si layer is carburized, and SiC is formed. The high chlorine surface coverage after the SiCl4 pulse is believed to enable superconformal growth via a growth inhibition effect.

The approaches to conformal and superconformal deposition developed by Abelson and Girolami for a low-temperature, low-pressure chemical vapor deposition (CVD) setting relevant for electronic materials in micrometer or submicrometer scale vias and trenches, are tested here in a high-temperature, moderate pressure CVD setting relevant for hard coatings in millimeter-scale trenches. Conformal and superconformal deposition of polycrystalline silicon carbide (SiC) can be accomplished at deposition temperatures between 950 and 1000 degrees C with precursor partial pressure higher than 20 Pa and an optional minor addition of HCl as a growth inhibitor. The conformal deposition at low temperatures is ascribed to slower kinetics of the precursor consumption along the trench depth, whereas the impact of high precursor partial pressure and addition of inhibitor is attributable to surface site blocking. With the slower kinetics and the site blocking from precursor saturation leading the growth to nearly conformal and the possibly preferential inhibition effect near the opening than at the depth, a superconformal SiC coating with 2.6 times higher thickness at the bottom compared to the top of a 1 mm trench was achieved. Published under an exclusive license by the AVS.

Highly ⟨111⟩-oriented 3C-SiC coatings with a distinct surface morphology consisting of hexagonally shaped pyramidal crystals were prepared by chemical vapor deposition (CVD) using silicon tetrachloride (SiCl4) and toluene (C7H8) at T ≤ 1250 °C and ptot = 10 kPa. In contrast, similar deposition conditions, with methane (CH4) as the carbon precursor, resulted in randomly oriented 3C-SiC coatings with a cauliflower-like surface of SiC crystallites. No excess carbon was detected in the highly ⟨111⟩-oriented 3C-SiC samples despite the use of aromatic hydrocarbons. The difference in the preferred growth orientation of the 3C-SiC coatings deposited by using C7H8 and CH4 as the carbon precursors was explained via quantum chemical calculations of binding energies on various crystal planes. The adsorption energy of C6H6 on the SiC (111) plane was 6 times higher than that on the (110) plane. On the other hand, CH3 exhibited equally strong adsorption on both planes. This suggested that the highly ⟨111⟩-oriented 3C-SiC growth with C7H8 as the carbon precursor, where both C6H6 and CH3 were considered the main active carbon-containing film forming species, was due to the highly preferred adsorption on the (111) plane, while the lower surface energy of the (110) plane controlled the growth orientation in the CH4 process, in which only CH3 contributed to the film deposition. 

SiC multilayer coatings were deposited via thermal chemical vapor deposition (CVD) using silicon tetrachloride (SiCl₄) and various hydrocarbons under identical growth conditions, i.e. at 1100 °C and 10 kPa. The coatings consisted of layers whose preferred growth orientation alternated between random and highly 〈111〉-oriented. The randomly oriented layers were prepared with either methane (CH₄) or ethylene (C₂H₄) as carbon precursor, whereas the highly 〈111〉-oriented layers were grown utilizing toluene (C₇H₈) as carbon precursor. In this work, we demonstrated how to fabricate multilayer coatings with different growth orientations by merely switching between hydrocarbons. Moreover, the success in depositing multilayer coatings on both flat and structured graphite substrates has strengthened the assumption proposed in our previous study that the growth of highly 〈111〉-oriented SiC coatings using C₇H₈ was primarily driven by chemical surface reactions.

Polycrystalline cubic silicon carbide, 3C-SiC, has long been investigated in the field of hard coating materials. The typical synthesis method for 3C-SiC coatings is thermal chemical vapor deposition (CVD) using either multicomponent precursors, e.g. methyltrichlorosilane, or a combination of single component precursors, e.g. silane and propane. In this thesis, the fabrication of polycrystalline SiC coatings has been explored from the new aspects on the basis of thermal CVD utilizing silicon tetrachloride (SiCl₄) and various hydrocarbons, i.e. toluene (C₇H₈), methane (CH₄) and ethylene (C₂H₄) as the precursors. The goal of this thesis is to control the surface chemistry in the SiCl₄-based SiC CVD and has been accomplished by the following three different approaches: 

In the first approach to control the surface chemistry of SiC CVD, the difference in the adsorption energy of aromatic and aliphatic hydrocarbons on different SiC crystal planes was utilized. Under identical deposition conditions, a highly <111>-oriented 3C-SiC coating was deposited using C₇H₈ as the carbon precursor, whereas using CH₄ resulted in a randomly oriented 3C-SiC. The results from quantum chemical calculation showed that the active film forming carbon species, i.e. C₆H₆ in the C₇H₈ process and CH₃ in both C₇H₈ and CH₄ processes, behaved differently when they adsorbed on the 3C-SiC (111) and (110) planes. CH₃ is strongly chemisorbed on both planes, while C₆H₆ is chemisorbed on the (111) plane, but only physiosorbed on the other. The significant difference in the adsorption energy of CH₃ and C₆H₆ on the (111) and (110) planes therefore explains the resulting highly <111>-oriented 3C-SiC from the C₇H₈ process. Furthermore, the ability to deposit 3C-SiC coatings with alternating highly <111>- and randomly oriented layers by merely switching the carbon precursor between C₇H₈ and CH₄ or C₂H₄ in a single CVD deposition has further proven that the effect of aromatic hydrocarbons on the preferred growth orientation of 3C-SiC was controlled primarily by the surface chemistry.  

The second approach to the surface-controlled SiC CVD was based on the reduction of surface reaction probability (β) for conformal film growth via low-temperature, low-pressure CVD, which was originally proposed by Abelson and Girolami. Their strategies in reducing β, including lowering the temperature and increasing the precursor partial pressure, were successfully adapted to the SiC CVD growth using SiCl₄ and C2H₄ as the precursors in this thesis, where an elevated temperature and a moderate pressure were used. Moreover, the addition of Cl species as a growth inhibitor to the process further reduced the β, leading to a superconformal SiC growth.  

The third approach employed in this thesis for the SiC growth was pulsed CVD. Instead of a continuous and simultaneous SiCl₄ and C2H₄ flow, the precursors were pulsed alternately into the chamber with each precursor pulse being separated by a H₂ purge. In this precursor delivery mode, the gas phase reactions between SiCl₄ and C2H₄ were avoided and hence the SiC growth was mostly controlled by the surface chemistry. Altering the pulse durations of the precursors led to a variation of growth per cycle (GPC), which was explained by a two-step mechanism. During the SiCl₄ pulse, a thin layer of Si is deposited, which is carburized by carbon species produced during the C2H₄ pulse. Additionally, the separation of precursor pulses should lead to a large increase in the surface coverage of Cl species, further enhancing the inhibition effect and resulting in a superconformal SiC growth. By using this approach, superconformal SiC coatings were achieved at temperatures where conventional CVD only yielded nonconformal SiC coatings. The observed decline in coating conformality with an elongated purge implied that more surface Cl species were replaced by H during the H₂ purge and consequently the inhibition effect was diminished.