Hot filament CVD reactor

This reactor was also invented at NIRIM (NIMS). The structure is very simple. It has a hot filament of W or Ta that is heated to 1900-2200 °C, and placed 5-10 mm above the substrate. The gas pressure P is usually 20 30 Torr. In the electron-assisted hot filament CVD (EACVD) reactor, a negative bias is applied to the filament [219, 448]. 

Diamond films were synthesized by DC plasma for the first time by Sawabe et al. [449]. The negative electrode with about 1-kV voltage is positioned at 20-25 mm above the grounded substrate. The gas pressure is about 200Torr, so that the growth rate of diamond film is very high, typically 20 pm/h. 

It is known that there are three modes of film growth  

1) Frank van der Merwe mode The atoms are deposited on the substrate layer by layer. 

2) Volmer-Weber mode Islands of atoms are discretely nucleated on the substrate. 

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A.7.1. Microwave plasma CVD reactors A.7.2. Hot filament CVD reactor A.7.3. DC plasma CVD reactor A.8. Crystal growth modes A.9. Carbon materials A. 10. Miscellaneous notations... 

Menningen et al. (1995b) have studied the effects of oxygen on the CH density by adding a small amount of O2 to a feed gas composed of a hydrocarbon diluted in H2 in a hot-filament CVD reactor. They found that at higher filament temperatures, the CH3 and CH densities are lower when O2 is added to the feed gas than when there is no O2 present. They proposed that this is due to rapid reactions that form CO and thereby tie up some of the carbon. 

If significant temperature gradients exist, thermal diffusion can cause the gas mixture to separate, so that it has a different composition at different locations. Toyoda et al. (1994) analyzed the possible separation of the gas in their hot-filament CVD reactor. They found that if their system were static, then significant thermal separation would occur. This was not the case for their reactor, however, and the flow of fresh feed gas was sufficient so that the C2H2 concentration differed by no more than 10% throughout their CVD reactor despite the presence of large thermal gradients in the CVD reactor. 

Figure 6 shows a hot-filament CVD reactor together with a sketch of a stagnation-point flow the simulation is based on. A mixture of H2 and CH4 passes a hot fllament placed at a distance L = 1 cm away from the substrate. The pressure is 33.3 mbar, the temperature of the filament 2430 K, the temperature of the substrate is varied between 800 and 1240 K, and the gas composition is 0.4 mole% CH4 in H2. The corresponding Navier-Stokes equations [10,16] of... 

Hiqh and low pressure LCVD fibers (a) Reactor pressure vs. growth rate (t>) Tip temperature vs. properties (c) Side growth versus rip growth (a) Versatility versus whisker processes Commercial hot filament CVD fibers... 

Figure 5.2. Two of the more common types of low pressure CVD reactor, (a) Hot Filament Reactor - these utilise a continually pumped vacuum chamber, while process gases are metered in at carefully controlled rates (typically a total flow rate of a few hundred cubic centimetres per minute). Throttle valves maintain the pressure in the chamber at typically 20-30 torr, while a heater is used to bring the substrate up to a temperature of 700-900°C. The substrate to be coated - e.g. a piece of silicon or molybdenum - sits on the heater, a few millimetres beneath a tungsten filament, which is electrically heated to temperatures in excess of 2200 °C. (b) Microwave Plasma Reactor - in these systems, microwave power is coupled into the process gases via an antenna pointing into the chamber. The size of the chamber is altered by a sliding barrier to achieve maximum microwave power transfer, which results in a ball of hot, ionised gas (a plasma ball) sitting on top of the heated substrate, onto which the diamond film is deposited.

Microwave Plasma CVD reactors use very similar conditions to hot filament reactors, and despite being significantly more expensive, are now among the most widely used techniques for diamond growth. In these... 

Besides the MPCVD reactors, other CVD reactors are also used for diamond deposition. They are hot filament, DC plasma, radio-frequency (rf) plasma, thermal rf plasma, plasma jet, and combustion CVD reactors. In the following, hot filament and DC plasma CVD reactors will be described, because they have been used for oriented growth of diamond. 

Childs et al. (1994b) have measured both the CH3 and CH column density in a hot-filament diamond film CVD reactor. They measured the CH column density using the CH absorption spectra due to the C L (0,0) band near... 

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