Hot-filament method

A large number of CVD diamond deposition technologies have emerged these can be broadly classified as thermal methods (e.g., hot filament methods) and plasma methods (direct current, radio frequency, and microwave) [79]. Film deposition rates range from less than 0.1 pm-h to 1 mm-h depending upon the method used. The following are essential features of all methods. 

Like synthetic diamond, C-BN is normally obtained by high-pressure processing. Efforts to synthesize it by CVD at low pressure are promising. It is deposited in an electron-cyclotron-resonance (ECR) plasma from a mixture of BF3 and either ammonia or nitrogen at 675°C on an experimental basis.F l Like CVD diamond, it is also deposited by the hot-filament method using diborane and ammonia diluted with hydrogen at 800°C.P 1... 

Diamond deposition by the hot filament method consists of a carbon containing gas and hydrogen, which undergo dissociation by passing through a hot filament usually made of tungsten wire. The dissociated molecules then deposit on a substrate (at approximately 900°C) where a carbon matrix grows in the form of diamond. Deposition dynamics is described by the CVD process. 

Hydrogen abstraction from single CHx species has been the preferred mechanism for diamond growth. In the hot-filament method that we employed, substrate temperature and processing pressure are similar to the MW-CVD method. It is conceivable that fullerene may form readily, but quickly fragment due to the diamond growth environment. If this is the case, then diamond film growth due to C2 addition may be more important in the CVD process than previously suspected. 

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

A number of other deposition methods have been used for growing diamond, with varying degrees of success. These include oxyacetylene welding torches, arc jets and plasma torches, laser ablation and hquid phase crystallisation, but none of these yet reahstically compete with the hot filament or microwave systems for reliability and reproducibility. 

Analysis of thermal decomposition of molecules on hot surfaces of solids is of considerable interest not only for investigation of mechanisms of heterogeneous decomposition of molecules into fragments which interact actively with solid surfaces. It is of importance also for clarifying the role of the chemical nature of a solid in this process. Furthermore, pyrolysis of molecules on hot filaments made of noble metals, tungsten, tantalum, etc., is a convenient experimental method for producing active particles. Note that it allows continuous adjustment of the intensity of the molecular flux by varying the temperature of the filament [8]. 

The surface ionization (SI) method refers to ions thermally evolving from the surface of a hot filament. The vapor of a volatile inorganic compound may be passed over a heated filament where it decomposes and ionizes to form bare metal... 

Fig. 2. A method of measuring the contractile force in a hot filament 1. This force is equilibrated by the weight 4 which can be rolled along the balance beam 3. The filament is attached also to base 2...

The method of choice for the preparation of Pa metal is a somewhat modified van Arkel-De Boer process, which uses protactinium carbide (Section II,C) as the starting material. The carbide and iodine are heated to form protactinium iodide, which is thermally dissociated on a hot filament 12-15). An elegant variation is to replace the filament with an inductively heated W or Pa sphere 109). A photograph of a 1.4-g sample of Pa metal deposited on a radiofrequency-heated W sphere is shown in Fig. 6. From the analytical data presented in Table V, the impurities present before and after application of this modified iodide transport process (Sections II,D and III,C) can be compared. 

When the gases are introduced into the chamber, they are heated. Heating may be accomplished by many methods. In a plasma-assisted process, the gases are heated by passing them through a plasma. Otherwise, the gases may be passed over a series of wires, such as those found in a hot filament reactor. 

The principal methods of gas activation are thermal and electrical much less common are chemical and photochemical activation. In the most commonly used thermal activation technique - the hot filament technique - a W or Ta wire is arranged in the immediate vicinity of the substrate to be coated by diamond (Fig. 1). The wire is heated until it reaches the temperature when H2 molecules dissociate readily. The gas phase is a mixture of a carbon-containing gas (e.g. methane, acetone or methanol vapor), at a concentration of a few per cent, and hydrogen. Upon the contact of the gas with the activator surface, excited carbon-containing molecules and radicals are produced, in addition to the hydrogen atoms. They are transferred to the substrate surface, where deposition occurs. Table 2 gives an indication of the hot-filament deposition process parameters. 

There are also numerous methods depending on the measurement of the potential differences which must be applied to various parts of a thermionic valve in order to prevent, or otherwise modify, the flow of electrons from the hot filament these have been briefly summarized by Oatley6 in a recent paper this author describes a new method baaed on the measurement of the magnetic field requisite to prevent the passage of electrons from a straight hot filament, placed axially inside a cylinder, to the cylinder. 

Guo, L. and Chen, G. (2007a), High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method. Diam. Relat. Mater., 16(8) 10. 

Physical methods include plasma treatments, UV irradiation, corona discharge, and flame treatment. Among these, plasma treatment is widely used for the surface modification of synthetic polymers. Plasma can be obtained by exciting gases into an energetic state by radio frequency, microwave, or electrons from a hot filament discharge. Generation of plasma requires a vacuum, which normally poses several... 

Stephan was the first to attempted direct synthesis of the B and N multi walled carbon nanotubes (BCN-MWNTs) in 1994 [15-17]. Since then, considerable progress has been made in the synthesis of BCN-MWNTs by different means of arc-discharge [16-18], laser ablation [18-20], piyolysis methods [18,21], and chemical vapor deposition [18,20-24]. Aligned BNC nanotubes have been sueeessfully fabricated by bias assisted hot filament chemieal vapor deposition [27,28]. Up to now, the only existing BCN-SWNTs synthesis was achieved via an... 

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