Superconformal silicon carbide coatings via precursor pulsed chemical vapor deposition

In this work, silicon carbide (SiC) coatings were successfully grown by pulsed chemical vapor deposition (CVD). The precursors silicon tetrachloride (SiCl 4 ) and ethylene (C 2 H 4 ) 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 SiCl 4 pulse — H 2 purge — C 2 H 4 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 SiCl 4 pulse, a layer of Si is deposited. In the following C 2 H 4 pulse, this Si layer is carburized


I. INTRODUCTION
Cubic silicon carbide, also named 3C-or β-SiC, is known for its high temperature stability, corrosion resistance, and superior mechanical and thermal properties.The state-of-the-art technique for fabricating 3C-SiC coatings is chemical vapor deposition (CVD).The precursors employed in the SiC CVD process can be classified into two categories.Multicomponent precursors, such as methyltrichlorosilane (CH 3 SiCl 3 ) [1][2][3] and hexamethyldisilane [HMDS, Si 2 (CH 3 ) 6 ], [4][5][6] contain both C-and Si-atoms.Single-component precursors, on the other hand, contain only C or Si, and thus, two single-component reactants are required for deposition of SiC.The commonly utilized Si precursors are silane (SiH 4 ) and silicon tetrachloride (SiCl 4 ), whereas short-chain hydrocarbons, e.g., methane (CH 4 ), [7][8][9] propane (C 3 H 8 ), [10][11][12] and ethylene (C 2 H 4 ), 13,14 are often used as C precursors.Both types of precursors have been used successfully in the preparation of 3C-SiC coatings and epitaxial films via CVD. 15he utilization of multiple single-component precursors allows for alternate precursor pulsing, creating an additional degree of freedom in the process design.If pulses from different precursors are fully separated, no gas phase interaction between them would occur, and the growth process would be only dictated by surface reactions, which can change the character of the deposition process.Such pulsed CVD processes have shown benefits for some deposition processes.One example is the deposition of GaN nanowires without metal catalyst on a SiN x -masked Si substrate using trimethyl gallium [Ga(CH 3 ) 3 ] and ammonia (NH 3 ) as precursors. 16n the conventional CVD mode, where both precursors are flowing into the chamber simultaneously, a continuous GaN film was obtained, while alternate pulses of precursors resulted in uniform arrays of GaN nanowires.The extreme end of the pulsed CVD spectrum enables conformal film growth when pulsing of gases renders a self-limiting surface chemistry, i.e., chemisorption stops when no more surface sites are available.This type of pulsed CVD is normally called atomic layer deposition (ALD). 179][20][21][22] However, they are not pure ALD processes because the surface chemistry is less likely to maintain self-limiting at such high temperatures and the resulting high growth rate also indicates that CVD growth is involved. 23The self-limiting surface chemistry of ALD also makes it impossible to achieve a superconformal growth 24 necessary for applications where recessed features should be filled.The conformality of a coating or film is assessed by step coverage (SC), which is defined as the ratio of the growth rate at the lower sidewall (close to the bottom) to that at the upper sidewall (close to the top) of a trench or a via.A SC < 1 indicates a subconformal growth, whereas SC = 1 and SC > 1 imply a conformal and a superconformal growth, respectively.
In this work, we explore the possibility of growing 3C-SiC coatings via a precursor-pulsed CVD process using SiCl 4 and C 2 H 4 as precursors.A two-step framework was proposed for the SiC growth via precursor pulsing.Furthermore, with precursor-pulsed CVD, it is possible to deposit superconformal SiC coatings with a SC between 3.5 and 9 at T = 1200 °C.

II. EXPERIMENT
3C-SiC coatings were deposited at p total = 1 kPa, T = 1000-1200 °C via pulsed CVD using SiCl 4 , (99.998%, Sigma-Aldrich Chemie GmbH) and C 2 H 4 (99.95%,Linde Gas AB) as precursors.In this precursor-pulsed CVD process, the substrate was exposed to SiCl 4 (100 sccm) and C 2 H 4 (50 sccm) in alternating pulses with a H 2 -purge after each precursor exposure.Purified H 2 with a constant flow rate of 3200 sccm was used both as a carrier gas for precursors and as a purge gas.During the pulse, H 2 functioned as a carrier gas and entered the reactor together with the precursor.During the purge step, H 2 was responsible for flushing out residual precursors and reaction byproducts.Moreover, dihydrogen and atomic hydrogen, which have been reported to be produced in high amounts by gas-phase CVD chemistry, 25 also participate in the decomposition of SiCl 4 and C 2 H 4 in the gas phase.The atomic hydrogen has even been reported to take part in the SiC CVD process by creating surface dangling bonds and, consequently, promoting adsorption of active growth species. 26The liquid SiCl 4 was stored at 24 °C in a stainless-steel bubbler placed in a thermostat-controlled water bath, rendering a SiCl 4 vapor pressure of 30.7 kPa.The flow of SiCl 4 vapor from the bubbler was controlled by regulating the flow of H 2 through the bubbler and the headspace pressure in the bubbler.
A pulsed-CVD cycle consisted of x seconds (s) SiCl 4 , z s purge, y s C 2 H 4 , and another z s purge.These steps will be represented as Si-pulse-purge-C-pulse-purge in the rest of this work.Two continuous CVD reference depositions with a SiCl 4 flow of 100 sccm and C 2 H 4 flow of 50 sccm were also performed at p total = 1 kPa and T = 1000 and 1200 °C.During deposition, the graphite susceptor holding the substrate was heated inductively, and its temperature was monitored by an optical pyrometer.In this work, the growth rate is represented by growth per cycle (GPC, unit: nm/ cycle).Structured graphite substrates (25 × 50 × 3 mm) with trenches having a depth of 1000 μm and various widths from 250 to 1000 μm were used in the conformal studies, whereas flat graphite substrates (25 × 60 × 1.5 mm) were used in the GPC studies.Both types of graphite substrates have a surface roughness Ra of 2.5 μm.The error bars for SC in conformal studies were calculated by the propagation of uncertainty using the following equation: 27 where σ is the standard deviation and GPC lower and GPC upper denote the growth per cycle at lower and upper sidewalls of a trench, respectively.The chemical phase of deposited coatings was investigated by an x-ray diffractometer operating at a voltage of 45 kV and a current of 40 mA.The x ray with a characteristic wavelength of 1.54 Å generated from a Cu target was used to probe the samples at θ/2θ geometry with 2θ scanning from 20°to 140°.A nickel filter was placed in front of the detector during the measurement to remove undesired Cu-Kβ radiation.The resulting diffractograms were compared with the powder diffraction file (PDF) cards for identifying chemical phases in the coatings.The scanning electron microscope (SEM) was utilized to investigate the surface morphology and cross sections of the samples.

A. Framework of SiC growth by precursor pulsed CVD
A series of SiC coatings were deposited on flat graphite substrates at 1000 °C via pulsed CVD using different pulsing schemes to study the pulsed process.A first group of samples was prepared with a fixed Si-pulse of 3 s and a varying C-pulse of 0-5 s.A second group of samples was grown with a fixed C-pulse of 3 s and a varying Si-pulse of 0-5 s.The H 2 purge was 1 s for all samples.The total deposition time for all samples was 30 min, and their x-ray diffractograms are displayed in Fig. 1.Peaks from graphite substrates and 3C-SiC are present in both groups of samples, indicating that it is possible to grow 3C-SiC without having the two precursors in the chamber at the same time.The sample prepared using only a C-pulse of 3 s (and 0 s Si) does not show any sign of 3C-SiC phase, whereas on the sample deposited with a Si-pulse of 3 s (and 0 s C), XRD shows a coating consisting mostly of Si with a slight incorporation of 3C-SiC, which may result from the reaction of Si species with the surface of graphite substrates in the beginning of deposition.
From these observations, we propose a two-step framework for the growth of 3C-SiC deposited via pulsed CVD.In the first step, i.e., during the Si-pulse, a layer of Si is deposited on the graphite surface.In the second step, i.e., during the C-pulse, the Si layer is carburized.Here, the carbon species would react with Si to form 3C-SiC. From Fig. 1(a), the strong Si (111) peak at 2θ = 28.4°,seen in the sample deposited with 3 s Si + 0 s C, disappears when a C-pulse is added, supporting the assumption that the C-pulse may act as a carburization step.In Fig. 1(b), a weak Si (111) peak appears in the sample prepared with 5 s Si + 3 s C, suggesting that with such a pulsed-CVD cycle there may not be enough carbon species to carburize the previously grown Si layers or that carburation could only proceed to certain distance in the Si layer.
In the proposed framework, we assume that the Si-pulse would be the primary step that contributes to the overall growth per cycle.This assumption is supported by the significant increase in GPC with elongated Si-pulse at a fixed C-pulse as shown in Fig. 2, where the GPC is plotted against durations of both C-pulse (with a fixed Si-pulse of 3 s) and Si-pulse (with a fixed C-pulse of 3 s).One can notice that with a fixed C-pulse of 3 s, the GPC increases with increasing duration of the Si-pulse.In contrast, a fixed Si-pulse of 3 s results in a Si layer with a GPC of 7 nm/cycle without the C-pulse (C-pulse of 0 s).A longer C-pulse only results in a slight increase in GPC.

B. Surface chemistry
The surface chemistry of the pulsed process was probed by using graphite substrates with trenches to study the influence of precursor pulsing on the coating step coverage (SC).The initial pulsed-CVD cycle was chosen as 3 s Si-pulse-1 s purge-3 s C-pulse-1 s purge, which was one of the combinations that did not result in co-deposition of elemental Si (Fig. 1).In contrast to a subconformal SiC coating deposited at 1200 °C by continuous CVD reported in our previous studies, 28 a superconformal SiC growth with a SC of nearly 9 was achieved at 1200 °C in a trench with an aspect ratio of 4 via pulsed CVD.The thickness difference between the top and the bottom corner of a trench in 3C-SiC coatings deposited by pulsed and continuous CVD can be seen in Fig. 3.The coating prepared by pulsed CVD is thicker at the bottom corner than at the top, the one prepared by continuous CVD exhibits the opposite result.To understand surface chemistry leading to such a drastic improvement in coating conformality by pulsed CVD, the GPC and the SC in 1-mm deep trenches with widths ranging from 250 to 1000 μm were investigated for H 2 purge durations of 1 and 5 s.The total time during which the substrate surface was exposed to respective Si-and C-precursors was maintained at 675 s.Figures 4(a  GPC and SCs, respectively, for increasing trench width.It can be observed that a longer purge generally results in higher GPCs of both upper and lower sidewalls of the trench, whereas a shorter purge results in higher SCs. A possible explanation for these results is a gas trapping effect, where the precursors are trapped in the lower part of trenches, rendering a local continuous CVD leading to a higher SC.The GPC with 1 s purge in the lower part of trenches [black solid line in Fig. 4(a)] seems to be independent of the trench width, while the GPC with 5 s purge in the lower part of the trench decreases with decreasing trench width [red solid line in Fig. 4(a)].Since a gas trapping effect should be more pronounced in a narrower trench, resulting in a higher GPC in the lower part of trenches with smaller widths, these results speak against a gas trapping effect.Furthermore, the modelling results from computational fluid dynamics also implied the absence of a gas trapping effect because the gas in the trench was shown to be replaced within milliseconds during the H 2 -purge.
Instead, we consider a surface chemical inhibition effect to explain the improved conformality.In a seminal work, Kumar et al. reported an enhanced conformality of TiB 2 thin films deposited by continuous CVD using Ti(BH 4 ) 3 (dme) as the precursor and dme as the inhibitor, where dme = 1,2-dimethoxyethane. 29Later, Yanguas-Gil et al. have proposed several possible mechanisms explaining how the addition of an inhibitor would improve the film conformality. 30In our SiCl 4 + C 2 H 4 process, Cl atoms on the surface are regarded as growth inhibitors.It has been shown by Schulberg et al. that HCl molecules would inhibit the growth of 3C-SiC by the site-blocking mechanism, 31 and HCl is also one of the products from hydrogenation of SiCl 4 . 32Formation of HCl is

LETTER
avs.scitation.org/journal/jva also expected from surface chemical reactions as the main Si-containing growth species, i.e., SiCl, 33 brings Cl to the film surface.This means that at the upper part of the trench, there is an inflow of HCl from gas phase reactions between SiCl 4 and H 2 during the Si-pulse as well as the HCl produced from surface chemical reactions between the SiCl species and the surface moieties terminated by H, e.g., CH x , at the lower part of trench, which diffuses upward.These effects may increase the partial pressure of HCl closer to the trench opening, resulting in a higher inhibition effect and a lower GPC in the upper part of the trenches, as seen in Fig. 4. On the other hand, the higher GPC observed in the lower part of trenches could be resulted from depletion of inhibitor species along the trench depth, leading to a less inhibited growth.
In a continuous CVD process, both Si-and C-precursor molecules arrive at the surface at a similar rate, leading to a mix of surface species.In our pulsed CVD process, only Si-or C-precursors arrive at the surface during a precursor pulse.During the Si-pulse, it is expected that the whole surface is covered with SiCl x species, rendering a stronger inhibition effect compared to a continuous CVD process, where there is a more even mixture of SiCl x and CH x surface species.This would explain the extremely high SC values of up to 9.
Comparing the GPC between trenches with different purge durations, it can be noticed that the GPC of trenches deposited with 5 s purge is higher than that with 1 s purge.We speculate that more Cl-termination on the surface would be removed and replaced with H if the H 2 purge after Si-pulse is longer.Although it has been reported that at 1200 °C the replacement of Cl-termination with H is thermodynamically unfavorable (with a free energy change of +62 kJ/mol), 34 an elongated purge time could be expected to promote the replacement.In the sample deposited with 5 s purge, the GPC at the lower part of trenches increases noticeably with increasing trench width, which could be explained by a larger gas volume being replaced in a wider trench during the purge, leading to a higher degree of Cl-termination replacement in a wider trench.This effect is less pronounced in the sample deposited with 1 s purge, presumably because this purge may not be long enough to effectively remove growth inhibitors.The growth framework proposed for 3C-SiC coatings deposited by pulsed CVD is summarized in Fig. 5.

IV. CONCLUSIONS
In this work, we have demonstrated the capability to grow 3C-SiC coatings on graphite via pulsed CVD.SiCl 4 and C 2 H 4 were pulsed alternately into the growth chamber with each precursor pulse being separated by a H 2 -purge.We suggest a framework based on deposition of Si from the SiCl 4 pulse followed by carburization to form SiC during the C 2 H 4 pulse.Superconformal SiC growth was achieved by the pulsed CVD process in trenches with ARs ranging from 1 to 4 at 1200 °C.Continuous CVD at the same temperature rendered subconformal SiC.We ascribed this superconformal growth behavior to enhanced Cl termination in the opening of the trench as a result of the pulsed precursor supply.

FIG. 3 .
FIG. 3. Cross-sectional SEM micrographs of 3C-SiC coatings deposited by pulsed CVD (a)-(c) and by continuous CVD (d)-(f ) in a 1 mm deep and 250 μm wide trench.(a) and (e) are upper sidewall/top corners, while (b) and (f ) are lower sidewall/bottom corners of the trench.(c) and (d) are the overview of trenches and share the same scale bar.

FIG. 4 .
FIG. 4. GPCs at 1200 °C of upper (approximately 75 μm from the opening) and lower (about 5 μm from the bottom) sidewalls of 1 mm deep trenches with different widths and with a purge duration of either 1 or 5 s (a).Step coverage of SiC coatings at 1200 °C in trenches with different widths.The circles and triangles denote the coating deposited by pulsed CVD with a purge length of either 1 or 5 s, while the rhombuses denote the coating prepared by continuous CVD (b).