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.     

Maintaining Cu metal integrity on low-k IMDs with a nanometer thick a-SiC:H film obtained by HWCVD

Hydrogenated amorphous silicon carbon (a-SiC:H) ultra thin films obtained by Hot wire chemical vapor deposition (HWCVD) have been shown to act as efficient diffusion barriers for copper on inter metal dielectric (IMD) layers which are of great significance for ultra-large scale integration (ULSI) circuits. In this work, we have studied the influence of the a-SiC:H barrier layer obtained by HWCVD which has implications towards issues related to the resistance to electromigration of Cu in the low dielectric (low-k) hydrogen silsesquioxane (HSQ) film. The presence of the ultra thin a-SiC:H film maintains the integrity of the Cu metal not only by suppressing Cu diffusion but also by increasing its crystallinity, which would have implications with respect to the mean time to failure (MTF) arising from metal electromigration. Though, we demonstrate this aspect on the low-k (HSQ)/Cu system, this should yield similar benefits for other low-k dielectric materials too.     

In–vitro calcium phosphate growth over functionalized cotton fibers

Biomimetic growth of calcium phosphate compound on cotton sheets treated with tetraethoxy silane and soaked in simulated body fluid solution was studied using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), micro-Fourier transform infrared spectroscopy (FTIR) and X-ray diffractometry (XRD). Micro-FTIR and EDAX results show that silicon was coupled to the cotton fiber when cotton was treated with tetra-ethoxy silane (TEOS) at 125°C for 1 h. Calcium phosphate nucleation started to occur on the surface of TEOS-treated cotton fibers upon immersion in 1.5×SBF (simulated body fluid solution) within 3 days and after 20 days, all the fiber surfaces were found covered with a thick and porous coating of calcium phosphate. The Ca and P determined by inductively coupled plasma spectroscopy (ICP) analysis revealed that the Ca/P ratio as well as the amount of calcium phosphate coating depends on the soaking time in SBF solution. © 1999 Kluwer Academic Publishers     

Low temperature crystallization of amorphous silicon carbide thin films for p–n junction devices fabrication

Metal induced crystallization technique was used to crystallize hydrogenated amorphous silicon carbide (a-SiC:H) thin films at low temperatures. Two types of substrates, silicon and silicon carbide were considered and the substrate effects on the final crystallized film were studied. About 200 nm a-SiC:H films were deposited and crystallized successfully on n-type Si and n-type 6H SiC substrates at a temperature of 600 °C. Fourier Transform Infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) analysis confirmed the crystallization of a-SiC:H film. Current–voltage (I–V) and capacitance–voltage (C–V) measurement confirms the formation of p–n junction with rectification over five orders of magnitude from ?2 V to 2 V.     

Aluminum-induced in situ crystallization of HWCVD a-Si:H films

Aluminum-induced in situ crystallization (AIC) of amorphous silicon films deposited by hot wire chemical vapor deposition (HWCVD) on glass is demonstrated. Aluminum was deposited at temperatures varying from room temperature to 300 °C on HWCVD a-Si:H films. The AIC was observed to take place in situ during the deposition of Al films, when the glass/a-Si:H temperature is kept 300 °C. A 20-nm Al film was effective in inducing crystallization of about 63% in the a-Si:H film. Thus, separate post-deposition annealing step can be avoided. For an Al film thickness comparable to the amorphous silicon film deposited at an optimum deposition rate, crystallization at temperature as low as 200 °C is observed. It was also observed that the growth pattern of c-Si in case of AIC without post-deposition annealing was identical to AIC with annealing step.     

Multi-phase structured silicon carbon nitride thin films prepared by hot-wire chemical vapour deposition

In this work, Silicon Carbon Nitride (Si-C-N) thin films were deposited by Hot Wire Chemical Vapour Deposition (HWCVD) technique from a gas mixture of silane (SiH₄), methane (CH₄) and nitrogen (N₂). Six sets of Si-C-N thin films were produced and studied. The component gas flow rate ratio (SiH₄:CH₄:N₂) was kept constant for all film samples. The total gas flow-rate (SiH₄ + CH₄ + N₂) was changed for each set of films resulting in different total gas pressure which represented the deposition pressure for each of these films ranging from 40 to 100 Pa. The effects of deposition pressure on the chemical bonding, elemental composition and optical properties of the Si-C-N were studied using Fourier transform infrared (FTIR) spectroscopy, Auger Electron Spectroscopy (AES) and optical transmission spectroscopy respectively. This work shows that the films are silicon rich and multi-phase in structure showing significant presence of hydrogenated amorphous silicon (a-Si:H) phase, amorphous silicon carbide (a-SiC), and amorphous silicon nitride (a-SiN) phases with Si-C being the most dominant. Below 85 Pa, carbon content is low, and the films are more a-Si:H like. At 85 Pa and above, the films become more Si-C like as carbon content is much higher and carbon incorporation influences the optical properties of the films. The properties clearly indicated that the films underwent a transition between two dominant phases and were dependent on pressure.     

Apatite formation from simulated body fluid on various phases of TiO2 thin films prepared by filtered cathodic vacuum arc deposition

Hydroxyl (OH⁻)-free TiO₂ thin films with amorphous and crystalline phases were deposited onto (100) silicon substrates using filtered cathodic vacuum arc deposition in order to investigate the in vitro apatite formation in simulated body fluid (SBF). The surface morphology, composition and structure of the TiO₂ thin films were characterized. The X-ray photoelectron spectroscopy results confirmed the presence of calcium and phosphorus on all TiO₂ thin film surfaces after immersion in SBF at 37 °C. Fourier transform infra red results showed the presence of carbonated apatite on the surface of these films. Amorphous structured TiO₂ thin film showed poor ability to form apatite on its surface in SBF. Apatite formation was more pronounced on the surfaces of the anatase films in comparison to those of rutile. The carbonated apatite deposition rate increased significantly when the TiO₂ film was illuminated with UV light prior to immersing in the SBF. In particular, the UV-treated anatase and rutile films showed increased rates of carbonated apatite formation on their surfaces in comparison to samples not treated with radiation. The increase in hydrophilicity due to UV treatment appears beneficial for the apatite growth on these surfaces.     

Comparative study of annealing and oxidation effects in a-SiC:H and a-SiC thin films deposited by radio-frequency magnetron sputtering

Thermal annealing and oxidation effects in hydrogenated (a-SiC:H) and nonhydrogenated (a-SiC) amorphous silicon-carbon alloy films deposited by radio-frequency magnetron sputtering have been studied. The a-SiC:H and a-SiC films were thermally treated in dry Ar, wet Ar, and dry O₂ atmospheres at temperatures up to 1150 °C. The principal effects of thermal annealing in an inert atmosphere on a-SiC:H films were found to be redistribution of hydrogen bonds and formation of amorphous graphitic carbon clusters. Strong oxidation of a-SiC:H was observed after thermal treatment in oxygen at 700 °C while annealing in wet argon caused partial oxidation. Oxidation of the carbon clusters in porous a-SiC:H structures is suggested to be responsible for the higher oxidation efficiency of a-SiC:H in oxygen. In contrast, the structure of a-SiC films remained almost unchanged after annealing in dry argon up to 1000 °C. No oxidation of a-SiC was detected until 1000 °C. Water vapor was found to be more effective at oxidizing a-SiC at 1000 °C than dry oxygen, which is similar to the oxidation behavior of crystalline SiC. The high thermal and oxidation stabilities of a-SiC layers were attributed to the dense and nanovoid-free amorphous SiC network.     

Hot-wire chemical vapor deposition of epitaxial film crystal silicon for photovoltaics

We have demonstrated that hot-wire chemical vapor deposition (HWCVD) is an excellent technique to produce high-quality epitaxial silicon at high rates, at substrate temperatures from 620 to 800 °C. Fast, scalable, inexpensive epitaxy of high-quality crystalline Si (c-Si) in this temperature range is a key element in creating cost-competitive film Si PV devices on crystalline seed layers on inexpensive substrates such as display glass and metal foil. We have improved both the quality and rate of our HWCVD Si epitaxy in this display-glass-compatible T range. We understand factors critical to high-quality epitaxial growth and obtain dislocation densities down to 6 × 10⁴ cm⁻² by techniques that reduce the surface oxygen contamination at the moment growth is initiated. We have also developed and validated a model of the HWCVD silicon growth rate, based on fundamentals of reaction chemistry and ideal gas physics. This model enables us to predict growth rates and calculate the sticking coefficient of the Si radicals contributing to film formation between 300 and 800 °C. We obtain efficiencies up to 6.7% with a 2.5-micron absorber layer grown on heavily-doped ‘dead’ Si wafers although these cells still lack hydrogenation and light trapping. Open-circuit voltages up to 0.57 V are obtained on 2-μm cells. Efficient film crystal silicon photovoltaics will require dislocation spacing more than 6 times the cell thickness, or else effective H passivation of the dislocations.     

Formation of high bioactive nanoporous titania film by hybrid surface mechanical attrition treatment

Surface mechanical attrition treatment (SMAT) was used to induce grain refinement on the surface of the commercially pure Titanium (Ti). Pure nanocrystalline anatase thin film with nano pores was deposited on pre-SMATed Ti substrate via chemical oxidation by simply soaking in a mix solution of 8.8 mol/l H₂O₂ and 0.1 mol/l HCl at 80 °C for 30 min followed by heat treatment at 400 °C for 1 h in air. The nanoporous anatase showed excellent bioactivity while being soaked in simulated body fluid (SBF), which could be mainly attributed to the unique nanostructure on the SMATed Ti surface. Mechanism of the formation of nanoporous titania was also discussed.     

Amorphous silicon carbide thin films (a-SiC:H) deposited by plasma-enhanced chemical vapor deposition as protective coatings for harsh environment applications

We investigated amorphous silicon carbide (a-SiC:H) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) as protective coatings for harsh environment applications. The influence of the deposition parameters on the film properties was studied. Stoichiometric films with a low tensile stress after annealing (< 50 MPa) were obtained with optimized parameters. The stability of a protective coating consisting of a PECVD amorphous silicon oxide layer (a-SiO_x) and of an a-SiC:H layer was investigated through various aging experiments including annealing at high temperatures, autoclave testing and temperature cycling in air/water vapor environment. A platinum-based high-temperature metallization scheme deposited on oxidized Si substrates was used as a test vehicle. The a-SiO_x/a-SiC:H stack showed the best performance when compared to standard passivation materials as amorphous silicon oxide or silicon nitride coatings.     

Revisiting the B-factor variation in a-SiC:H deposited by HWCVD

In order to understand material properties in a better way, it is always desirable to come up with new variables that might be related to the film properties. The B-parameter is such a variable, which relates to the quality of a-SiC:H films both in terms of electronic and optical properties. B (scaling factor) is essentially the slope of the straight-line part of the (E)^1/2–E (Tauc plot). Due to dependence on a large number of parameters and no detailed research, many previous authors have surmised that B has an ambiguous correlation with carbon content. We have made an attempt to establish the relation between the B-parameter as a quality-indicating factor of a-SiC:H films in both carbon- and silicon-rich material. For this we studied a-SiC:H films deposited by the HWCVD method with broad deposition parameters of substrate temperature (T_s), filament temperature (T_F) and C₂H₂ fraction. Our results indicate that the B-parameter varies considerably with process conditions such as T_F, total gas pressure and carbon content. An attempt is made to correlate the B-parameter with an opto-electronic parameter, such as the mobility edge, which has relevance to the device-quality aspects of a-SiC:H films prepared by HWCVD.     

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.     

Low temperature (< 100 °C) fabrication of thin film silicon solar cells by HWCVD

Amorphous silicon films have been made by HWCVD at a very low substrate temperature of ≤ 100 °C (in a dynamic substrate heating mode) without artificial substrate cooling, through a substantial increase of the filament-substrate distance (∼ 80 mm) and using one straight tantalum filament. The material is made at a reasonable deposition rate of 0.11 nm/s. Optimized films made this way have device quality, as confirmed by the photosensitivity of > 10⁵. Furthermore, they possess a low structural disorder, manifested by the small Γ/2 value (half width at half maximum) of the transverse optic (TO) Si-Si vibration peak (at 480 cm^− 1) in the Raman spectrum of ∼ 30.4 cm^− 1, which translates into a bond angle variation of only ∼ 6.4°. The evidence gathered from the studies on the structure of the HWCVD grown film by three different techniques, Raman spectroscopy, spectroscopic ellipsometry and transmission electron microscopy, indicate that we have been able to make a photosensitive material with a structural disorder that is smaller than that expected at such a low deposition temperature.Tested in a p-i-n solar cell on Asahi SnO₂:F coated glass (without ZnO at the back reflector), this i-layer gave an efficiency of 3.4%. To our knowledge, this is the first report of a HWCVD thin film silicon solar cell made at such a low temperature.     

Fabrication and characterization of silk fibroin/bioactive glass composite films

Composite films of silk fibroin (SF) with nano bioactive glass (NBG) were prepared by the solvent casting method, and the structures and properties of the composite films were characterized. Fourier transform infrared (FT-IR) spectroscopy analysis shows that the random coil and β-sheet structure co-exist in the SF films. Results of field emission scanning electron microscope (FESEM) indicate that the NBG particles are uniformly dispersed in the SF films. The measurements of the water contact angles suggest that the incorporation of NBG into SF can improve the hydrophilicity of the composites. The bioactivity of the composite films was evaluated by soaking in 1.5 times simulated body fluid (1.5 × SBF), and formation of a hydroxycarbonate apatite (HCA) layer was determined by XRD and FESEM. The results show that the SF/NBG composite film is bioactive as it induces the formation of HCA on the surface of the composite film after soaking in 1.5 × SBF for 7 days. In vitro osteoblasts attachment and proliferation tests show that the composite film is a good matrix for the growth of osteoblasts. Consequently, the incorporation of NBG into the SF film can enhance both the bioactivity and biocompatibility of the film, which suggests that the SF/NBG composite film may be a potential biomaterial for bone tissue engineering.     

Apatite wollastonite–poly methyl methacrylate bio-composites

Bio-composites consisting of sol–gel processed apatite wollastonite (AW) glass ceramics and poly methyl methacrylate (PMMA) were prepared by hot compaction method. Density of the composites decreased with increase in PMMA content, while, biaxial flexural strength (BFS) was 21 MPa for 20 wt.% PMMA and beyond which it decreased. A correlation between phase compositions of AW glass ceramics with BFS was attempted from the XRD results. In vitro bioactivity of the composites in a simulated body fluid (SBF) showed the formation of spherical globules on the surface within 7 days of soaking as observed by environmental SEM. Thin film XRD and EDX measurement confirmed these globules to be bone like apatite with Ca/P ratio 1.53 and FTIR measurement showed the corresponding peaks for phosphates. Results indicated the bone bonding ability of the composites by forming a surface apatite (calcium phosphate) layer in SBF and the growth increased with increase in soaking durations. ICP measurement of the remaining SBF after 7, 14 and 21 days soaking of samples was found to be in good agreement with the EDX analysis results.     

Preparation of Nanocrystalline ZrO2 Film Using a Zirconium Naphthenate and In Vitro Formation of Calcium Phosphate

Bioactive nanocrystalline ZrO₂ coatings were prepared onto (100) Si substrates by using a chemical solution deposition with a zirconium naphthenate as a starting material. Precursor sol was spin-coated onto the substrates and prefired at 500°C for 10 min in air. Formation of crystalline ZrO₂ film was observed at 800°C by X-ray diffraction. Surface morphology and surface roughness of the film were characterized by field emission-scanning electron microscope and atomic force microscope. After soaking for 5 days in a simulated body fluid, formation of the calcium phosphate was observed on ZrO₂ film annealed at 800°C by energy dispersive X-ray spectrometer. Fourier transform infrared spectroscopy revealed that carbonate was substituted into the calcium phosphate.     

NaOH treatment of vacuum-plasma-sprayed titanium on carbon fibre-reinforced poly(etheretherketone)

Carbon fibre-reinforced polyetheretherketone (CF-PEEK) substrates were coated with titanium by vacuum-plasma-spraying and chemically treated in 10 M sodium hydroxide (NaOH) solution. After NaOH treatment, the specimens were immersed in simulated body fluid (SBF) containing ions in concentrations similar to those of human blood plasma. Scanning electron microscopy, energy-dispersive X-ray analysis and diffuse reflectance Fourier transformed–infrared spectroscopy were used to analyse the NaOH-treated VPS-Ti surface and the calcium phosphate layer formed during immersion in SBF. It was observed that a carbonate-containing calcium phosphate layer was formed on the NaOH-treated VPS-Ti surface during immersion in SBF, whereas no calcium phosphate precipitation occurred on the untreated surfaces. It is therefore concluded that vacuum-plasma-spraying with titanium and subsequent chemical modification in 10 M NaOH solution at 60°C for 2 h is a suitable method for the preparation of bioactive coatings for bone ongrowth on CF-PEEK.     

通过仿生法在硅橡胶表面制备磷灰石薄膜的研究

用CaCl2的乙醇溶液和K2HPO4溶液对硅橡胶进行预处理,将处理过的硅橡胶分别浸渍于模拟体液和钙磷饱和溶液中来制备磷灰石薄膜.利用薄膜X射线衍射、红外吸收光谱和扫描电子显微镜对形成的薄膜进行了表征.结果表明,分别在模拟体液中7d和在钙磷饱和溶液中5d后,硅橡胶表面形成了一层磷灰石薄膜;在模拟体液中的薄膜表面呈网状并分布有许多球状晶粒,在钙磷饱和溶液中的薄膜为结晶良好的片状晶体.     

Microstructure of ceramic coating on titanium surface as a result of hydrothermal treatment

Hydroxyapatite coating on commercially pure titanium has been produced by a biomimetic method in order to improve osteointegration for medical implant purposes. A specific chemical treatment by etching titanium substrate with different concentrations of NaOH aqueous solution at 130 °C in an autoclave, followed by heat treatment at 600 °C was selected to obtain an activated titanium substrate. The microporous surface obtained has allowed the nucleation and growth of a calcium phosphate layer by soaking the substrate in a simulated body fluid (SBF). Scanning electron microscopy (SEM) together with energy dispersive analyzer for X-ray (EDS), X-ray diffraction (XRD) as well as Fourier transform infrared spectroscopy (FT-IR) were employed to evaluate the hydroxyapatite coating. A homogeneous structure coating without cracks defined the chemical treatment condition of the substrate. © 2000 Kluwer Academic Publishers