Hot-wire Chemical Vapor Deposition Process Research Articles

Effects of the Size of Charged Nanoparticles on the Crystallinity of SiC Films Prepared by Hot Wire Chemical Vapor Deposition

Non-classical crystallization suggests that crystals can grow with nanoparticles as a building block. In this case, the crystallization behavior depends on the size and charge of the nanoparticles. If charged nanoparticles (CNPs) are small enough, they become liquid-like and tend to undergo epitaxial recrystallization. Here, the size effect of SiC CNPs on film crystallinity was studied in the hot-wire chemical vapor deposition process. To do this, SiC nanoparticles were captured under different processing conditions—in this case, wire temperature, precursor concentration and the filament bias. Increasing the temperature of tungsten wires and decreasing the ratio of (SiH4 + CH4)/H2 reduced the size of the SiC nanoparticles. When the nanoparticles were small enough, an epitaxial SiC film approximately 100-nm-thick was grown, whereas larger nanoparticles produced polycrystalline SiC films. These results suggest that the size of the CNPs is an important process variable when growing films by means of non-classical crystallization.

Generation of Charged SiC Nanoparticles During HWCVD Process

Charged nanoparticles have been shown to be spontaneously generated in the gas phase in various chemical vapor deposition (CVD). Furthermore, it has been shown that these charged nanoparticles can contribute to the growth of thin films, nanowires, nanotetrapods and so on. Here, the generation of charged silicon carbide (SiC) nanoparticles in the gas phase during a hot wire CVD process was studied by capturing nanoparticles with a different delay time on a silicon monoxide membrane of the copper mesh grid for transmission electron microscope. The average size of SiC nanoparticles captured for 30 s increased from 2.9 to 6.1 nm with increasing delay time from 0 to 60 min. The deposition behavior of SiC films was affected by the applied bias on a substrate holder. A homo-epitaxial SiC film as thick as ~ 200 nm was grown under the substrate bias of − 200 V, whereas polycrystalline SiC films were grown under 0 V and + 15 V. It indicates that nanoparticles generated in the gas phase should be charged.

DSMC – Simulation of the influence of hydrogen addition on the properties of silicon deposited by HWCVD

The gas phase chemistry in a hot-wire chemical vapor deposition (HWCVD) process, which utilizes an array of heated tungsten wires is analyzed via Direct Simulation Monte Carlo simulation. Process gases are silane (SiH4) and hydrogen (H2). SixHy radical partial pressures are simulated as a function of H2 gas flow for given gas inlet compositions in order to optimize radical partial pressures, leading to higher quality and more ordered Si:H films. The simulation results are compared with a series of deposition runs using an array of 10 heated tungsten wires (600 mm long, 0.53 mm diameter, held at 2100 °C) providing a deposition area of 500 x 600 mm2. H2 gas flows are varied to maximize the crystallinity of Si:H films as measured through Raman spectroscopy. The experimental sets are further compared to the simulation results by means of in-situ mass spectrometry, film thickness measurement, and process pressure measurement.

Crystalline tantalum carbide and ditungsten carbide formation via hot wire chemical vapor deposition using the precursor of 1-methylsilacyclobutane

An efficient method for the preparation of crystalline tantalum carbide (TaC) was reported using a new methyl-substituted precursor, 1-methylsilacyclobutane (MSCB), in the hot-wire chemical vapor deposition process with a tantalum filament. The ability of MSCB to produce methyl radicals on heated Ta filament allows for the formation of crystalline TaC (cubic Fm-3m) in a wide temperature range of 1600–2200°C. This corroborates the key role of the radical species formed via the precursor's decomposition on the hot metal wire in the type of metal alloys formed on the wire surface. The growth rate of TaC achieved at 0.12μm/min and 0.35μm/min, respectively, for 1600°C and 2200°C is much higher than the value obtained from using CH4/H2 mixtures as the C-containing precursor. Crystalline ditungsten carbide (W2C) was formed by treating the tungsten filament with MSCB at the relatively high temperatures of 2200–2400°C, with a deposition rate of 2.1μm/min at 2400°C. The method reported in this work is a useful method to produce crystalline TaC or W2C without the use of the corrosive metal halide precursors.

Role of free-radical chain reactions and silylene chemistry in using methyl-substituted silane molecules in hot-wire chemical vapor deposition

The reaction chemistry of four methyl-substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS), and tetramethylsilane (TMS), has been studied in the hot-wire (catalytic) chemical vapor deposition (HWCVD) process, both on heated tungsten or tantalum filaments and in the gas phase. The four molecules dissociate catalytically on hot W or Ta surfaces to form ·CH3, ·H and H2. In a HWCVD reactor setup where the pressure is relatively high at 8–533Pa, two reaction mechanisms involving silylene and free radical intermediate, respectively, operate with the four precursor gases. For MMS, its reaction chemistry is characterized by the exclusive involvement of methylsilylene intermediate. Reaction products when using DMS as a precursor gas come from silylene chemistry and free-radical chain reactions. Filament temperature and pressure affect strongly the competition between the two mechanisms. Specifically, silylene chemistry dominates at low temperatures of 1200–1300°C and a low pressure of 16Pa, and free-radical chain reactions take over at high temperatures and pressures. With the increasing methyl substitutions in TriMS and TMS, free-radical chain reactions become predominant. A clear transition in the operating reaction mechanism from silylene chemistry to free-radical chain reactions has been shown when the number of SiCH3 bonds increases in the methyl-substituted silane molecules.

Non-classical crystallization of silicon thin films during hot wire chemical vapor deposition

The deposition behavior of silicon films by hot wire chemical vapor deposition (HWCVD) was approached by non-classical crystallization, where the building block of deposition is a nanoparticle generated in the gas phase of the reactor. The puzzling phenomenon of the formation of an amorphous incubation layer on glass could be explained by the liquid-like property of small charged nanoparticles (CNPs), which are generated in the initial stage of the HWCVD process. Using the liquid-like property of small CNPs, homo-epitaxial growth as thick as ~150nm could be successfully grown on a silicon wafer at 600°C under the processing condition where CNPs as small as possible could be supplied steadily by a cyclic process which periodically resets the process. The size of CNPs turned out to be an important parameter in the microstructure evolution of thin films.

Effect of argon ion energy on the performance of silicon nitride multilayer permeation barriers grown by hot-wire CVD on polymers

Permeation barriers for organic electronic devices on polymer flexible substrates were realized by combining stacked silicon nitride (SiNx) single layers (50nm thick) deposited by hot-wire chemical vapor deposition process at low-temperature (~100°°C) with a specific argon plasma treatment between two successive layers. Several plasma parameters (RF power density, pressure, treatment duration) as well as the number of single layers have been explored in order to improve the quality of permeation barriers deposited on polyethylene terephthalate. In this work, maximum ion energy was highlighted as the crucial parameter making it possible to minimize water vapor transmission rate (WVTR), as determined by the electrical calcium test method, all the other parameters being kept fixed. Thus fixing the plasma treatment duration at 8min for a stack of two SiNx single layers, a minimum WVTR of 5×10−4g/(m2day), measured at room temperature, was found for a maximum ion energy of ~30eV. This minimum WVTR value was reduced to 7×10−5g/(m2day) for a stack of five SiNx single layers. The reduction in the permeability is interpreted as due to the rearrangement of atoms at the interfaces when average transferred ion energy to target atoms exceeds threshold displacement energy.

Industrialization of Hot Wire Chemical Vapor Deposition for thin film applications

The consequences of implementing a Hot Wire Chemical Vapor Deposition (HWCVD) chamber into an existing in-line or roll-to-roll reactor are described. The hardware and operation of the HWCVD production reactor is compared to that of existing roll-to-roll reactors based on Plasma Enhanced Chemical Vapor Deposition. The most important consequences are the technical consequences and the economic consequences, which are both discussed. The technical consequences are adaptations needed to the hardware and to the processing sequences due to the different interaction of the HWCVD process with the substrate and already deposited layers. The economic consequences are the reduced investments in radio frequency (RF) supplies and RF components. This is partially offset by investments that have to be made in higher capacity pumping systems. The most mature applications of HWCVD are moisture barrier coatings for thin film flexible devices such as Organic Light Emitting Diodes and Organic Photovoltaics, and passivation layers for multicrystalline Si solar cells, high mobility field effect transistors, and silicon heterojunction cells (also known as heterojunction cells with intrinsic thin film layers). Another example is the use of Si in thin film photovoltaics. The cost perspective per unit of thin film photovoltaic product using HWCVD is estimated at 0.07€/Wp for the Si thin film component.

Depth-dependent crystallinity of nano-crystalline silicon induced by step-wise variation of hydrogen dilution during hot-wire CVD

To induce an amorphous surface in a nano-crystalline silicon (nc-Si:H) thin film, the hydrogen dilution was reduced step-wise at fixed time intervals from 90 – 50% during the hotwire chemical vapour deposition process. This contribution reports on the structural properties of the resultant nc-Si:H thin film as a function of the deposition time. Raman spectroscopy, confirmed by high resolution transmission spectroscopy, indicates crystalline uniformity in the growth direction, accompanied by the progression of an amorphous surface layer as the deposition time is increased. The silicon- and oxygen bonding configurations were probed using infrared spectroscopy and electron energy loss spectroscopy. The growth mechanism is ascribed to the improved etching rate by atomic hydrogen in nano-crystalline silicon towards the film/substrate interface region. The optical properties were calculated by applying the effective medium approximation theory, where the existence of bulk and interfacial layers, as inferred from cross-sectional microscopy, were taken into account.

Effect of pressure on the gas-phase chemistry when using monomethylsilane and dimethylsilane in hot-wire chemical vapor deposition

The effect of source gas pressure on the gas-phase reaction chemistry of dimethylsilane (DMS) and monomethylsilane (MMS) in the hot-wire chemical vapor deposition process has been studied by examining the secondary gas-phase reaction products in a reactor using a soft laser ionization source coupled with mass spectrometry. For DMS, the increase in sample pressure has resulted in the formation of small hydrocarbons, including ethene, acetylene, propene, and propyne. This leads to a switch from silylene dominant chemistry to a free radical dominant one with the pressure increase at low filament temperatures of 1200 and 1300 °C. At the lower pressure of 0.12 Torr, the formation of 1,1,2,2-tetramethyldisilane by dimethylsilylene insertion reaction into the Si–H bond in DMS is favored over trimethylsilane produced from a free radical recombination reaction for a short reaction time. However, when the pressure is increased by 10 times, the gas-phase chemistry becomes dominated by the formation of trimethylsilane. We have demonstrated that trapping of the corresponding active intermediates by the small hydrocarbons produced in situ is responsible for the observed switch. In the study with MMS, the gas-phase chemistry is dominated by the formation of 1,2-dimethyldisilane and 1,3-disilacyclobutane at both pressures of 0.48 and 1.2 Torr. Unlike DMS, the gas-phase reaction chemistry with MMS does not involve free radicals, which are the precursors to produce small hydrocarbons. The absence of small hydrocarbons formed in situ with MMS explains the preservation in chemistry upon the increase in pressure when MMS is used as a source gas.

A Novel SU8 Polymer Anchored Low Temperature HWCVD Nitride Polysilicon Piezoresitive Cantilever

In this paper, we present a novel approach for fabricating piezoresistive silicon nitride cantilevers using polymer as an anchor. In our approach, the silicon nitride structural layers, as well as the polycrystalline silicon piezoresistive layer, are deposited by a low temperature hot-wire chemical vapor deposition process. The novelty of this process is that the silicon wafer is not consumed and is reusable, and the process also allows use of alternate materials for cantilever fabrication in place of silicon substrate. The fabricated silicon nitride cantilevers are characterized for their mechanical and electromechanical behavior.

Moisture barrier enhancement by spontaneous formation of silicon oxide interlayers in hot wire chemical vapor deposition of silicon nitride on poly(glycidyl methacrylate)

We deposited a silicon nitride (SiNx)–polymer hybrid multilayer moisture barrier in a hot wire chemical vapor deposition (HWCVD) process, entirely below 100 °C. The polymer, poly(glycidyl methacrylate) (PGMA), was deposited by initiated chemical vapour deposition and the SiNx in a dedicated HWCVD reactor. Line profile investigation of our barrier structures by cross-sectional scanning transmission electron microscopy and energy dispersive X-ray spectrometry reveals that, upon deposition of SiNx on top of our polymer layer, an intermediate layer of silicon oxide (SiOx) like material is formed. X-ray photoelectron spectroscopy measurements confirm the presence of this material and indicate the epoxy rings in the PGMA material open upon heating (to 100 °C) and exposure to atomic hydrogen and amine species in the HWCVD process. The oxygen atoms subsequently react with silicon and nitrogen containing radicals to form SiOxNy. The interlayer turns out to be highly beneficial for interlayer adhesion and this is considered to be one of the reasons for the excellent barrier properties of our multilayer.

Dominance of silylene chemistry in the decomposition of monomethylsilane in the presence of a heated metal filament.

The gas-phase reaction chemistry of the decomposition of monomethylsilane (MMS) has been studied in the presence of a heated metal filament in a hot-wire chemical vapor deposition (HWCVD) reactor. A 10.5 eV vacuum ultraviolet laser single-photon ionization time-of-flight mass spectrometry was employed in combination with isotope labeling and chemical trapping to examine the mechanistic details in the reaction chemistry. We have demonstrated the dominant involvement of the methylsilylene (HSiCH3) intermediate in the gas-phase reaction chemistry. Free radical and silene intermediates do not play a role. Major products are found to be H2, 1,2-dimethyldisilane (DMDS), and 1,3-disilacyclobutane (DSCB). The formation of DMDS proceeds by the insertion reaction of methylsilylene, whereas DSCB originates from the dimerization reaction of methylsilylene. Similar reaction chemistry has been observed when using the different filament materials of tungsten and tantalum in the HWCVD reactor. This indicates that changing the filament material from Ta to W does not affect the gas-phase reaction chemistry when using MMS in the HWCVD process. Finally, comparison of the reaction chemistry of MMS with those of dimethylsilane, trimethylsilane, and tetramethylsilane sheds light on the influence of increasing Si-H bonds. A switch in the dominated chemistry from free-radical short-chain reactions to silylene insertion/dimerization reactions occurs as the number of Si-H bonds increases in the four methyl-substituted silane molecules.

Effect of trimethylsilane pressure on hot-wire chemical vapor deposition chemistry using vacuum ultraviolet laser ionization mass spectrometry

In this study, the authors investigated the effect of sample pressure on the reaction chemistry of trimethylsilane (TriMS) in the hot-wire chemical vapor deposition (CVD) process. The secondary gas-phase reaction products were examined in a reactor with varying TriMS pressures. The reaction products were analyzed using a laser ionization source with a vacuum ultraviolet wavelength of 118 nm, coupled with mass spectrometry. By increasing TriMS pressure, methane formation was observed. To our knowledge, this is the first successful use of either open-chain alkylsilanes or four-membered-ring (di)silacyclobutane molecules as an independent precursor gas in the hot-wire CVD reactor to achieve methane formation. Our results showed that methane was formed mainly from the radical chain reactions with minor contributions from molecular elimination. The increase in the sample pressure also led to the formation of other small hydrocarbon molecules including acetylene, ethene, propyne, and propene. The formation of hydrogen molecules was enhanced when the sample pressure was increased. In addition, the change in the sample pressure had a direct effect on the radical recombination and disproportionation reactions. This is reflected in the different behavior assumed by the main products from these two types of reactions, i.e., tetramethylsilane, hexamethyldisilane from the former, and three methyl-substituted disilacyclobutanes from the latter. The trapping of free radicals resulting from the in-situ produced ethene and propene molecules is responsible for the observed difference.

Hot-wire chemical vapor deposition of WO3−x thin films of various oxygen contents

We present the synthesis of tungsten oxide (WO3−x) thin films consisting of layers of varying oxygen content. Configurations of layered thin films comprised of W, W/WO3−x, WO3/W and WO3/W/WO3−x are obtained in a single continuous hot-wire chemical vapor deposition process using only ambient air and hydrogen. The air oxidizes resistively heated tungsten filaments and produces the tungsten oxide species, which deposit on a substrate and are subsequently reduced by the hydrogen. The reduction of tungsten oxides to oxides of lower oxygen content (suboxides) depends on the local water vapor pressure and temperature. In this work, the substrate temperature is either below 250 °C or is kept at 750 °C. A number of films are synthesized using a combined air/hydrogen flow at various total process pressures. Rutherford backscattering spectrometry is employed to measure the number of tungsten and oxygen atoms deposited, revealing the average atomic compositions and the oxygen profiles of the films. High-resolution scanning electron microscopy is performed to measure the physical thicknesses and display the internal morphologies of the films. The chemical structure and crystallinity are investigated with Raman spectroscopy and X-ray diffraction, respectively.

Growth of Crystalline Tungsten Carbides Using 1,1,3,3-Tetramethyl-1,3-disilacyclobutane on a Heated Tungsten Filament

A method of forming crystalline tungsten carbides was reported by exposing the heated tungsten filament to 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB) in a hot-wire chemical vapor deposition process. Methyl radicals produced from the decomposition of TMDSCB on the filament serve as the carbon source. The formation of tungsten carbides was investigated by X-ray diffraction, cross-sectional scanning electron microscopy, and in-situ filament resistance measurements. A pure W2C phase was formed at a high temperature of 2400 °C after 1–2 h exposure time with a growth rate of 4.4 μm min–1. The growth of the W2C layer is found to be a diffusion-controlled process. Our study at longer deposition time of 3–4 h shows that once the metal filament is fully carburized to form W2C, the carbon-rich WC phase starts to form on the outside layer upon further exposure to TMDSCB. A WC layer with no contamination from the W2C phase was found to be formed at 2400 °C and 4 h deposition time.

Hot-Wire Chemical Vapor Deposition of Few-Layer Graphene on Copper Substrates

Chemical vapor deposition (CVD) of graphene on copper is an efficient technology for producing high-quality graphene for large areas. The objective of this work is to deposit graphene/few-layer graphene (FLG) using different types of copper substrate by a new hot-wire CVD process. We carried out the processes at temperatures below 1000 °C with acetylene (C2H2) as a precursor gas. After a general characterization of the samples, the results mostly indicate the formation of FLG on copper samples by this method. Nevertheless, the presence of pure, crystalline, and sufficiently flat surfaces is needed for depositing high-quality graphene layers.

Competition of Silene/Silylene Chemistry with Free Radical Chain Reactions Using 1-Methylsilacyclobutane in the Hot-Wire Chemical Vapor Deposition Process

The gas-phase reaction chemistry of using 1-methylsilacyclobutane (MSCB) in the hot-wire chemical vapor deposition (CVD) process has been investigated by studying the decomposition of MSCB on a heated tungsten filament and subsequent gas-phase reactions in a reactor. Three pathways exist to decompose MSCB on the filament to form ethene/methylsilene, propene/methylsilylene, and methyl radicals. The activation energies for forming propene and methyl radical, respectively, are determined to be 68.7 ± 1.3 and 46.7 ± 2.5 kJ·mol(-1), which demonstrates the catalytic nature of the decomposition. The secondary gas-phase reactions in the hot-wire CVD reactor are characterized by the competition between a free radical chain reaction and the cycloaddition of silene reactive species produced either from the primary decomposition of MSCB on the filament or the isomerization of silylene species. At lower filament temperatures of 1000-1100 °C and short reaction time (t ≤ 15 min), the free radical chain reaction is equally important as the silene chemistry. With increasing filament temperature and reaction time, silene chemistry predominates.

Effect of the Initial Structure on the Electrical Property of Crystalline Silicon Films Deposited on Glass by Hot-Wire Chemical Vapor Deposition

Crystalline silicon films on an inexpensive glass substrate are currently prepared by depositing an amorphous silicon film and then crystallizing it by excimer laser annealing, rapid thermal annealing, or metal-induced crystallization because crystalline silicon films cannot be directly deposited on glass at a low temperature. It was recently shown that by adding HCI gas in the hot-wire chemical vapor deposition (HWCVD) process, the crystalline silicon film can be directly deposited on a glass substrate without additional annealing. The electrical properties of silicon films prepared using a gas mixture of SiH4 and HCl in the HWCVD process could be further improved by controlling the initial structure, which was achieved by adjusting the delay time in deposition. The size of the silicon particles in the initial structure increased with increasing delay time, which increased the mobility and decreased the resistivity of the deposited films. The 0 and 5 min delay times produced the silicon particle sizes of approximately 10 and approximately 28 nm, respectively, in the initial microstructure, which produced the final films, after deposition for 300 sec, of resistivities of 0.32 and 0.13 Omega-cm, mobilities of 1.06 and 1.48 cm2 V(-1) S(-1), and relative densities of 0.87 and 0.92, respectively.

Effect of Si–H Bond on the Gas-Phase Chemistry of Trimethylsilane in the Hot Wire Chemical Vapor Deposition Process

The effect of the Si-H bond on the gas-phase reaction chemistry of trimethylsilane in the hot-wire chemical vapor deposition (HWCVD) process has been studied by examining its decomposition on a hot tungsten filament and the secondary gas-phase reactions in a reactor using a soft laser ionization source coupled with mass spectrometry. Trimethylsilane decomposes on the hot filament via Si-H and Si-CH(3) bond cleavages. A short-chain mechanism is found to dominate in the secondary reactions in the reactor. It has been shown that the hydrogen abstractions of both Si-H and C-H occur simultaneously, with the abstraction of Si-H being favored. Tetramethylsilane and hexamethyldisilane are the two major products formed from the radical recombination reactions in the termination steps. Three methyl-substituted disilacyclobutane molecules, i.e., 1,3-dimethyl-1,3-disilacyclobutane, 1,1,3-trimethyl-1,3-disilacyclobutane, and 1,1,3,3-tetramethyl-1,3-disilacyclobutane are also produced in reactor from the cycloaddition reactions of methyl-substituted silene species. Compared to tetramethylsilane and hexamethyldisilane, a common feature with trimethylsilane is that the short-chain mechanism still dominates. However, a more active involvement of the reactive silene intermediates has been found with trimethylsilane.