Hot-wire Chemical Vapor Deposition Process Research Articles

Opportunities for new materials synthesis by hot wire chemical vapor process

Over the last 20 years the hot wire chemical vapor deposition (HWCVD) technique is being explored as an effective alternative to the conventional plasma enhanced chemical vapor deposition (PECVD) for silicon based thin film devices and is claimed to have various advantages. An important point to be appreciated is the mixed nature of the HWCVD process. On one hand it offers all the benefits of being a chemical vapor process and on the other it has the flavor of physical vapor deposition due to the generation of precursors at a line/plane like source (wire array) far away from the substrate albeit with subsequent transport accompanied by secondary gas phase reactions. The possible control over the secondary gas phase reactions gives a unique feature to the HWCVD process. Apart from employing HWCVD for the preparation of a-Si:H and μc-Si:H with high deposition rates we have extended the applicability of the HWCVD to the synthesis of piezoresistive microcrystalline silicon, diffusion barriers of a-SiC:H, silicon nanowires, boron carbide for thermal neutron detectors, stress free a-SiN:H thin films for MEMS devices and metal nano-templates for semiconductor nanowire synthesis. We also established the applicability of HWCVD in surface nano-engineering to incorporate different functionalities (without actually depositing any film) i.e. nano-engineering or nano-modification of the surface to avoid electromigration on low-k dielectric layers and reduce surface defects in crystalline silicon and also bring about nano-crystallization of metallic thin films. Hence I would like to coin a more general nomenclature for this technique and refer to it as hot wire chemical vapor process (HWCVP). This paper discusses the results and outcomes of some of the case studies that we have carried out employing the HWCVP.

Gas-phase reaction chemistry of 1,1-dimethyl-1-silacyclobutane as a precursor gas in the hot-wire chemical vapor deposition process — Formation of tetramethylsilane and trimethylsilane

The secondary gas-phase reaction products of 1,1-dimethyl-1-silacyclobutane (DMSCB) and its isotopomer, 1,1-di(perdeuteratedmethyl)-1-silacyclobutane (DMSCB-d6), in a hot-wire chemical vapour deposition reactor were investigated using vacuum UV laser single photon ionization with time-of-flight mass spectrometry. Dimethylsilylene, one of the primary decomposition products, undergoes π-type addition across the double and triple C–C bond and an insertion reaction into the Si–H bond. A short-chain reaction mechanism, initiated by methyl radicals produced in the primary decomposition, is found to exist for both the source DMSCB molecule and its stable secondary products. The formation of tetramethylsilane and trimethylsilane via the reaction of 1,1-dimethylsilene with a methyl radical and an H2 molecule, respectively, has been demonstrated. These are two new reaction channels involving 1,1-dimethylsilene in secondary gas-phase reactions.

Rf excited optical emission spectrum of radicals generated during hot wire chemical vapour deposition for the preparation of microcrystalline silicon thin film

To study the radicals behavior in the hot wire chemical vapour deposition (HWCVD) process for the preparation of microcrystalline Si (μc-Si: H) thin film, a weak radio frequency (rf) power was introduced to excite the radicals generated in HWCVD chamber. The spectrum of rf-excited HWCVD (rf-HWCVD) was obtained by subtracting the emission of hot wires from the spectrum measured by OES. The influence of the rf power on the rf-HWCVD spectrum can be neglected as the rf power density was less than 0.1 W/cm2. Under the same deposition parameters,the emission spectra for rf-HWCVD and plasma enhanced CVD (PECVD) processes are different. Under the low deposition pressure (7.5 Pa), the intensities of SiH* and Hα vary with the hot wire temperature reversely, which is characteristic of HWCVD with high gas dissociation rate and high concentration of atomic H. The ratio of intensity of Hα to SiH* in the emission spectrum of rf-HWCVD varying with deposition pressure is consistent with the crystalline fraction of μc-Si: H film. The results indicate that the optical emission spectroscopy measurement is a suitable method for the investigation of the HWCVD process excited by a weak rf-power.

Reduction of amorphous incubation layer by HCl addition during deposition of microcrystalline silicon by hot-wire chemical vapor deposition

The amorphous incubation layer, which is formed in the initial growth stage of hydrogenated microcrystalline silicon (μc-Si:H) thin film deposited at low temperature, is harmful to the electric properties of film. In this study, the effect of the addition of HCl gas on the reduction of such an amorphous incubation layer was investigated during the silicon deposition on a glass substrate at 220 °C by hot-wire chemical vapor deposition process using the Raman spectroscopy, the X-ray diffraction and the field-emission scanning electron microscopy. In the initial stage of deposition where the silicon film deposited without HCl addition consisted almost entirely of the amorphous incubation layer; highly crystalline silicon films could be deposited with HCl addition. As the flow rate of HCl increased, the crystallinity of silicon films increased but the film growth rate decreased. The surface morphology of films prepared with HCl addition became smoother with smaller grain size than that prepared without HCl.

Dual Catalytic Purpose of the Tungsten Filament During the Synthesis of Single-Helix Carbon Microcoils by Hot-Wire CVD

We report on the deposition of crystalline single-helix carbon microcoils, in the as-deposited state, by the hot-wire chemical vapor deposition process without any special preparation of nano-sized transition metal catalysts and subsequent post-deposition annealing. Tungsten, originating from the heated tungsten filament, is identified as the catalyst material responsible for the growth of the microcoils. High-resolution transmission spectroscopy, combined with Raman spectroscopy, confirm that the as-deposited microcoils are crystalline, which is induced by the high deposition temperature in the vicinity of the heated filament. These results suggest a simplified, less tedious deposition process for the growth of carbon microcoils, once the process has been optimized.

Microcrystalline silicon carbide alloys prepared with HWCVD as highly transparent and conductive window layers for thin film solar cells

Crystalline silicon carbide alloys have a very high potential as transparent conductive window layers in thin-film solar cells provided they can be prepared in thin-film form and at compatible deposition temperatures. The low-temperature deposition of such material in microcrystalline form (µc-Si:C:H) was realized by use of monomethylsilane precursor gas diluted in hydrogen with the Hot-Wire Chemical Vapor Deposition process. A wide range of deposition parameters has been investigated and the structural, electronic and optical properties of the µc-SiC:H thin films have been studied. The material, which is strongly n-type from unintentional doping, has been used as window layer in n-side illuminated microcrystalline silicon solar cells. High short-circuit current densities are obtained due to the high transparency of the material resulting in a maximum solar cell conversion efficiency of 9.2%.

Hot-wire chemical vapor deposition of silicon nanoparticles on fused silica

Silicon nanoparticles on fused silica have potential as recombination centers in infrared detectors due quantum confinement effects that result in a size dependent band gap. Growth on fused silica was realized by etching in HF, annealing under vacuum at 700–750 °C, and cooling to ambient temperature before ramping to the growth temperature of 600 °C. Silicon particles could not be grown in a thermal chemical vapor deposition (CVD) process with adequate size uniformity and density. Seeding fused silica with Si adatoms in a hot-wire chemical vapor deposition (HWCVD) process at a disilane pressure of 1.1 × 10 − 5 Pa followed by thermal CVD at a disilane pressure of 1.3 × 10 − 2 Pa, or direct HWCVD at a disilane pressure of 2.1 × 10 − 5 Pa led to acceptable size uniformity and density. Dangling bonds at the surface of the as-grown nanoparticle were passivated using atomic H formed by cracking H 2 over the HWCVD filament.

A mechanistic study of gas-phase reactions with 1,1,3,3-tetramethyl-1,3-disilacyclobutane in the hot-wire chemical vapor deposition process

The gas-phase chemical species produced from both the direct decomposition of 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB) on a tungsten filament and the secondary reactions in the HWCVD reactor were identified by vacuum ultraviolet laser single-photon ionization coupled with time-of-flight mass spectrometry. TMDSCB decomposes on the filament to methyl and 1,1,3-trimethyl-1,3-disilacyclobutane-1-yl radicals. Subsequent hydrogen abstraction reactions from the parent molecule by methyl radicals and biradical combination reactions are dominant in the reactor. The formation of dimethylsilene ( m/ z = 72) through the ring Si–C bond cleavage is indirectly confirmed by the observation of the signal from 1,1,3,3,5,5-hexamethyl-1,3,5-trisilacyclohexane at m/ z = 216. Tetra- and tri-methylsilane are also found to be formed in the HWCVD reactor.

Flexible electrochromic devices based on crystalline WO 3 nanostructures produced with hot-wire chemical vapor deposition

Crystalline WO 3 nanoparticles are employed in the development of flexible electrochromic (EC) devices. The nanoparticles are synthesized at high-density with a hot-wire chemical vapor deposition process where the hot filament provides the source of the tungsten metal. Polyethylene terephthalate coated with indium tin oxide is employed as a transparent flexible substrate. A simple electrophoresis technique is employed to deposit the WO 3 nanoparticles on the polymer, resulting in a uniform thin film. The EC performance is optimized for WO 3 particles that were baked at ~ 300 °C for 2 h prior to electrode fabrication. The transmittance is modulated between ~ 94% and ~ 28% without degradation for 100 cycles.

Effect of wire temperature on the microstructural evolution of Si films in hot-wire chemical vapor deposition for digital display and photovoltaic devices

The formation of crystalline phase in Si by hot-wire chemical vapor deposition (HWCVD) was investigated, focusing on the microstructural evolution as a function of hot-wire temperature. The microstructure of films deposited on a Si wafer was compared between hot-wire temperatures of 1590, 1670, and 1800 °C. A heavily twinned structure was observed at the wire temperature of 1670 °C, which resulted in the apparent intensity peak of (1 1 1) hexagonal-closed packed (HCP) crystalline Si from a typical face-centered cubic (FCC) crystalline Si structure in the X-ray diffraction analysis. The twin-related HCP crystalline phase was markedly diminished at 1800 °C and hardly observed at 1590 °C. The observed deposition behavior was approached by the effect of the wire temperature on the size of charged nanoparticles formed in the gas phase in the HWCVD process.

Problem of catalyst ageing during the hot-wire chemical vapour deposition of thin silicon films

The filament in a hot-wire chemical vapour deposition (HWCVD) reactor is an important component. When tantalum (Ta) filaments are used for the deposition of thin silicon films, strong degradation takes place: there is a large amount of silicon not only at the surface but also in the bulk of the tantalum catalyst. Ta–Si phases form on the filament surface and in the bulk, which can lead to a porous structure of the catalyst filament. Filament contamination (silicide formation and thick silicon deposits (TSDs)) is the reason for the changes in filament resistance. It also reduces filament lifetime, which is a serious concern for HWCVD deposition technology. A cleaning procedure for the filament at high-temperatures in a vacuum (about 2000 °C) can neither remove the thick silicon deposits nor fully restore the filament surface properties. In order to decrease the silicon content in the tantalum catalyst and suppress TSD formations on the filament surface, we use radio-frequency alternating current (RF, 13.5 MHz) instead of direct current (DC) to heat the filament. The skin effect of the RF current reduces the formation of TSDs on the surface and silicon diffusion into the filament. We show that it is possible to clean the filament surface of TSDs by means of a high-frequency current. Combined RF + DC filament heating allows us to increase the lifetime of the catalyst (almost twofold) and to improve HWCVD process reproducibility without any deterioration in the quality of the deposited film.

Formation of aminosilanes in the hot-wire chemical vapor deposition process using SiH4-NH3 gas mixtures

Vacuum ultraviolet laser single photon-ionization coupled with time-of-flight mass spectrometry has been used to detect the gas-phase reaction products from the HWCVD reactor with SiH4 NH3 mixtures with various partial pressure ratios ranging from 1: 1 to 200 : 1 for NH3 : SiH4. The identity of the products depends strongly on the relative amounts of SiH4 and NH3 in the mixture. Low NH3 content favors the production of disilane and trisilane. When the NH3 content is high, the formation of aminosilanes becomes the primary pathway. The production of sufficient amount of NH2 (or ND2) radicals is found to play a key role in the competition between the two pathways. The formation of aminosilanes when using mixtures with an ammonia to silane pressure ratio greater than 49 : 1 is confirmed through the use of the isotopomer ND3 in place of NH3 in the gas mixtures. A stepwise amination reaction scheme, initiated by the reaction between the SiH3 and NH2 radicals, is responsible for the formation of aminosilanes. The effect of filament temperature on the formation of aminosilanes and di-/trisilane has also been investigated.

Real-time study ofa−Si:H∕c−Siheterointerface formation and epitaxial Si growth by spectroscopic ellipsometry, infrared spectroscopy, and second-harmonic generation

The performance of many devices based on Si thin films deposited on crystalline Si $(c\text{\ensuremath{-}}\mathrm{Si})$ is highly governed by interface quality. For many of these applications, only fully epitaxial films or fully amorphous films having an abrupt interface with the substrate are desired. However, the realization of these perfectly sharp interfaces and the mechanisms governing their formation are not fully understood yet. In this study, the interface formation between Si thin films and $c\text{\ensuremath{-}}\mathrm{Si}$ has been investigated by simultaneously applying three complementary optical techniques in real time during low temperature Si film growth. The films were deposited in a hot-wire chemical vapor deposition process by using both native oxide covered and H terminated Si(100) substrates. The formation of hydrogenated amorphous Si $(a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$, epitaxial Si, and mixed phase Si has been detected with spectroscopic ellipsometry by measuring the optical properties of the growing films. The evolution of the hydrogen content and hydrogen bonding configurations in the films has been monitored by attenuated total reflection infrared spectroscopy. A clear dependence of the hydrogen content on film morphology is observed with the amorphous films containing significantly more hydrogen. The surface and interface sensitive technique of second-harmonic generation (SHG) has been applied both spectroscopically and in real time. The SHG spectra of $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ films on Si(100) obtained in the SHG photon energy range of $2.7--3.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ revealed a dominant contribution originating from the film/substrate interface related to ${E}_{0}^{\ensuremath{'}}∕{E}_{1}$ critical point (CP) transitions of $c\text{\ensuremath{-}}\mathrm{Si}$. The real-time behavior of the SHG response is shown to strongly depend on differences in initial film morphology, which allows for identification of direct $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}∕c\text{\ensuremath{-}}\mathrm{Si}$ heterointerface formation, nanometer-level epitaxial growth, and fully epitaxial growth at a very early stage of film growth. On the basis of the results obtained by the three optical techniques, the $c\text{\ensuremath{-}}\mathrm{Si}$ surface passivation mechanism by $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ thin films is addressed and it is demonstrated that the combination of the techniques provides a profound method to control processes occurring during Si thin film growth.

Structural order of thin film silicon made at 100 °C

AbstractSpectroscopic ellipsometric measurements showed that amorphous silicon thin films made by very high frequency plasma enhanced chemical vapour deposition at 100 °C using an optimum H2 dilution has denseness (A parameter) and structural order (C parameter) that are comparable to the sample made at 200 °C. The materials made by hot wire chemical vapour deposition (HWCVD) at low hydrogen dilution conditions have a less dense structure and higher roughness compared to the plasma deposited samples. The C parameter reaches the lowest value (C=1.67) for the HWCVD sample made at an optimum H2/SiH4 ratio of 20, even though it was made only at 100 °C. The importance of filament to substrate distance in HWCVD process in suppressing the undesirable species reaching at the growing surface has been put forward to achieve materials with very small structural disorder and fabrication of thin film silicon solar cells with high stability. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Decomposition of hexamethyldisilane on a hot tungsten filament and gas-phase reactions in a hot-wire chemical vapor deposition reactor

To study the effect of an Si-Si bond on gas-phase reaction chemistry in the hot-wire chemical vapor deposition (HWCVD) process with a single source alkylsilane molecule, soft ionization with a vacuum ultraviolet wavelength of 118 nm was used with time-of-flight mass spectrometry to examine the products from the primary decomposition of hexamethyldisilane (HMDS) on a heated tungsten (W) filament and from secondary gas-phase reactions in a HWCVD reactor. It is found that both Si-Si and Si-C bonds break when HMDS decomposes on the W filament. The dominance of the breakage of Si-Si over Si-C bond has been demonstrated. In the reactor, the abstraction of methyl and H atom, respectively, from the abundant HMDS molecules by the dominant primary trimethylsilyl radicals produces tetramethylsilane (TMS) and trimethylsilane (TriMS). Along with TMS and TriMS, various other alkyl-substituted silanes (m/z = 160, 204, 262) and silyl-substituted alkanes (m/z = 218, 276, 290) are also formed from radical combination reactions. With HMDS, an increasing number of Si-Si bonds are found in the gas-phase reaction products aside from the Si-C bond which has been shown to be the major bond connection in the products when TMS is used in the same reactor. Three methyl-substituted 1,3-disilacyclobutane species (m/z = 116, 130, 144) are present in the reactor with HMDS, suggesting a more active involvement from the reactive silene intermediates.

Development of a new laser induced electron impact ionization source for studying the hot-wire chemical vapor deposition chemistry of silane–ammonia mixtures

A newly-developed laser induced electron impact ionization (LIEI) method is presented. In this method, electrons are generated by thermionic emission when directing a low-power, pulsed UV laser beam to a stainless steel rod exterior to the ion optics of a time-of-flight mass spectrometer. The electrons are collimated into a beam almost parallel to the ion optics by the electric field between the rod and a copper mesh placed in the vicinity and the utilization of a delayed ion extraction pulse. It has been demonstrated that EI-type mass spectra can be obtained with a mass resolution that is comparable to that using laser ionization. The detection sensitivity is very high, with a detection limit of < 32 attomole for NH3. The gas-phase chemistry of SiH4/NH3 mixtures in the hot-wire chemical vapor deposition (HWCVD) process was examined using the new LIEI source. It is found that this method is well suited for detecting stable species with high ionization potentials, such as H2 and N2, which characterizes the NH3decomposition chemistry in a HWCVD reactor. The suppression of NH3 chemistry by the presence of SiH4 in the mixture is demonstrated, as is the temperature dependence of the suppression extent.

Application of Laser Induced Electron Impact Ionization to the Deposition Chemistry in the Hot-Wire Chemical Vapor Deposition Process with SiH 4-NH 3 Gas Mixtures

The application of a laser-induced electron impact (LIEI) ionization source in studying the gas-phase chemistry of the SiH(4)/NH(3) hot-wire chemical vapor deposition (HWCVD) system has been investigated. The LIEI source is achieved by directing an unfocused laser beam containing both 118 nm (10.5 eV) vacuum ultraviolet (VUV) and 355 nm UV radiations to the repeller plate in a time-of-flight mass spectrometer. Comparison of the LIEI source with the conventional 118 nm VUV single-photon ionization (SPI) method has demonstrated that the intensities of the chemical species with ionization potentials (IP) above 10.5 eV, e.g., H(2), N(2) and He, have been significantly enhanced with the incorporation of the LIEI source. It is found that the SPI source due to the 118 nm VUV light coexists in the LIEI source. This allows simultaneous observations of parent ions with enhanced intensity from VUV SPI and their "fingerprint" fragmentation ions from LIEI. It is, therefore, an effective tool to diagnose the gas-phase chemical species involved with both NH(3) and SiH(4) in the HWCVD reactor. In using the LIEI source to SiH(4), NH(3) and their mixtures, it has been shown that the NH(3) decomposition is suppressed with the addition of SiH(4) molecules. Examination of the NH(3) decomposition percentage and the time to reach the N(2) and H(2) steady-state intensities for various NH(3)/SiH(4) mixtures suggests that the extent of the suppression is enhanced with more SiH(4) content in the mixture. With increasing filament temperatures, the negative effect of SiH(4) becomes less important.

Extension of the lifetime of tantalum filaments in the hot-wire (Cat) Chemical Vapor Deposition process

One of the prime components of a hot-wire (Cat) Chemical Vapor Deposition system is the filament used to pyro-catalytically crack the gases like silane. Burnt out tantalum filaments were studied to determine the possible improvement of lifetime for these filaments. The structure and chemical distribution in the filament were investigated, using electron microscopy and elemental analysis. It was found that at the high temperature centre section of a filament a thin surface silicide layer develops around a central metal core. At the filament ends a surface layer of metal silicide develops, that eventually consume the metal completely, causing breakages. Changes in the pre- and post-treatments of gas type and pressure were tested to improve the filament lifetime, arriving at a very simple procedure that increases the lifetime of the filament to several months. By annealing the hot filament both before and after a deposition run in a low pressure hydrogen ambient, a surface layer of pure silicon developed at the lower temperature ends, essentially protecting the metal from further silicidation.

Surface evolution during crystalline silicon film growth by low-temperature hot-wire chemical vapor deposition on silicon substrates

We investigate the low-temperature growth of crystalline thin silicon films: epitaxial, twinned, and polycrystalline, by hot-wire chemical vapor deposition (HWCVD). Using Raman spectroscopy, spectroscopic ellipsometry, and atomic force microscopy, we find the relationship between surface roughness evolution and (i) the substrate temperature $(230--350\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and (ii) the hydrogen dilution ratio $({\mathrm{H}}_{2}∕\mathrm{Si}{\mathrm{H}}_{4}=0--480)$. The absolute silicon film thickness for fully crystalline films is found to be the most important parameter in determining surface roughness, hydrogen being the second most important. Higher hydrogen dilution increases the surface roughness as expected. However, surface roughness increases with increasing substrate-temperature, in contrast to previous studies of crystalline Si growth. We suggest that the temperature-dependent roughness evolution is due to the role of hydrogen during the HWCVD process, which in this high hydrogen dilution regime allows for epitaxial growth on the rms roughest films through a kinetic growth regime of shadow-dominated etch and desorption and redeposition of growth species.

Synthesis of multi-walled carbon nanotubes by combining hot-wire and dc plasma-enhanced chemical vapor deposition

Multi-walled carbon nanotubes (MWCNTs) have been grown on 7 nm Ni-coated substrates consisting of crystalline silicon covered with a thin layer (10 nm) of TiN, by combining hot-wire chemical vapor deposition (HWCVD) and direct current plasma-enhanced chemical vapor deposition (dc PECVD), at 620 °C. Acetylene (C 2H 2) gas is used as the carbon source and ammonia (NH 3) and hydrogen (H 2) are used either for dilution or etching. The carbon nanotubes range from 20 to 100 nm in diameter and 0.3 to 5 μm in length, depending on growth conditions: plasma intensity, filament current, pressure, C 2H 2, NH 3, H 2 flow rates, C 2H 2/NH 3 and C 2H 2/H 2 mass flow ratios. By combining the HWCVD and the dc PECVD processes, uniform growth of oriented MWCNTs was obtained, whereas by using only the HWCVD process, tangled MWCNTs were obtained. By patterning the nickel catalyst, with the use of the HW dc PECVD process, uniform arrays of nanotubes have been grown as well as single free-standing aligned nanotubes, depending on the catalyst patterning (optical lithography or electron-beam lithography). In the latter case, electron field emission from the MWCNTs was obtained with a maximum emission current density of 0.6 A/cm 2 for a field of 16 V/μm.