Cold wall CVD

Figure 4 (a) Cold-wall CVD reactor with parallel vapor flow (b) hot-wall CVD reactor with perpendicular... 

N3)2Ga (CH2)3NMe2 ] liquid precursor, nonpyrophoric, nonexplosive Horizontal cold-wall CVD Transparent 1 1 stoichiometric films. Growth temperature 773-1323 K, no additional N source 286... 

Cl2Ga(N3)(NMe3) Cold-wall CVD 1 1 GaN grown at 700 °C on Si and sapphire substrates, chlorine and carbon contamination 293... 

In compounds 7 and 8, we have studied the influence, on the thermal behavior, of the presence of Me3Si substituents on the Cp ring of Ti(lV) trichloro derivatives. Such groups should help in the elimination of the Cl atoms through formation of volatile Me3SiCl in the gas phase. Thermal analyses showed that both compounds are volatile and decompose below 623 K. Cold wall CVD experiments run at 973 K and normal pressure yielded in both cases thick films (up to 15 pm) that were identified by XRD as TiC. The deposits were free of chlorine but slightly contaminated with... 

All our preliminary CVD experiments were carried out under an atmosphere of N2 gas in order to determine the intrinsic ability of the studied molecules to serve as precursors to ceramic materials. Further studies could be run under a reactive medium such as H2 in order to reduce the C and O content of the films. This was done in the case of compound 19. Cold-wall CVD experiments on this molecule were performed at 973 K and normal pressure under H2 carrier gas. These experiments resulted in the formation of highly pure VC films.38 XPS and EPMA-WDS analyses of these films showed both the free carbon and oxygen contents to be lower than the limits of detection of the techniques. Various factors can account for these results diminution of the C content induced by H2, stabilization of the Cp ligand in the gas phase due to the presence of f-butyl groups, and decomposition mechanism involving a methyl activation leading to the formation of V = CH2 species.38... 

When we speak of a cold wall CVD reactor, we refer to a continuous flow system where the wafer is kept at the required high temperature, but all other surfaces bounding on the reacting gases are cold. The objective here is to cause the desired reaction only on the hot wafer and keep all other surfaces free of deposits. In practice this is a goal that can only be partially attained. Although reactions will proceed more slowly on colder surfaces, they will proceed-and films will build up. At the same time the films that form on the colder surfaces may be more porous than the normal film and may spall off more easily. All of which says that in spite of our best efforts, cold walled reactors may have their cold walls an undesirable source of particulates which may end up on the hot substrate. The occurence of such particulates can be minimized by frequent cleaning of the chamber walls to remove deposits. 

The problem of assuring uniform depositions on many wafers closely spaced in a long uniform tube was solved when operation of the reactor at low pressure was considered.22 Normally, in an atmospheric pressure cold wall CVD system, the reactant gas is heavily diluted in N2 in order to reduce gas phase nucleation. At the pressures used for low pressure CVD (0.5-1.0 Torr), this is less of a problem so less diluent is needed. The net effect then is that deposition rates only fall by a factor of five. However, as many as 100 wafers can be processed in such a reactor at one time (see Figure 26), and this more than compensates for the lower deposition rate. In addition, due to the low pressure, diffusion occurs at high rates and the deposition tends to be controlled by the surface temperature. Given the very uniform temperatures available in a diffusion furnace, the deposition uniformity tends to be excellent in such a system. 

Using a cold-wall CVD reactor similar to the internally-heated barrel described in Figure 22 of Chapter 1, tungsten silicide was deposited from WF6 and SiH4,5 which is often described by the overall reaction... 

In a pyrolytic or thermal LCVD experiment, the gas is transparent and the substrate absorbs the laser energy. This creates a so - called hot - spot on which a normal thermal CVD process occurs. Pyrolytic LCVD allows a very precise localization of the coating. In a sense, this technique may be compared to the cold - wall CVD technique in which the substrate may be heated by passing an electric current through it (resistance heating), or by induction, where the substrate itself acts as a susceptor. In these cases, the gas volume is not heated significantly (hence the name cold - wall CVD). The main difference between the cold-wall CVD and the pyrolytic laser CVD is that in the latter, the heated area can be localized and scanned very precisely. 

For example, a hotter and less dense gas above a hot substrate will rise, whereas a cooler and denser gas will sink. Such a situation is often encountered in cold-wall CVD reactors. 

Typically, this reaction is carried out at atmospheric pressure in a cold-wall CVD system. The growth rate by the silane process is rather high, usually 8-17 A/s. 

In a cold-wall CVD only the substrates are heated either inductively or resistively and the wall of the reactor is colder than that of the substrate. Therefore, the deposition mainly occurs on the heated substrate, and negligible deposition on the walls of the reactor. Cold-wall reactors are mainly used for continuous deposition of fibres and depositions where a thermal gradient is required to facilitate CVI. Hot-wall CVD reactors represent one of the major categories of CVD reactors. In such systems, the chamber containing the parts is heated by a furnace from outside. In general, hot-wall reactors have the advantages of being... 

Nanopowders have been of particular interest in recent years as they can be used to make high-performance products due to their superior properties. Manufacturing of these powders is therefore important. Such a system using a laser-induced cold-wall CVD reactor is shown in Figure 3.28. It mainly consists of five parts a reactor with two reaction zones, an oven for vaporising hexamethyl disilylamine (HMDS-... 

Figure 3.28. Schematic of laser-induced cold-wall CVD reactor [57] (1 C02 laser, 2 reflector, 3 laser beam, 4 GaAs lens, 5 cooling water, 6 reactor, 7 nozzle, 8 reaction flame, 9 particle plume, 10 board, 11 window, 12 throttling valve, 13 powder collector, 14 pump, 15 pressure gauge, 16 water-cooled Cu block, 17 temperature controller, 18 oven, 19 heater, 20 precursor vessel, 21 liquid HMDS, 22 needle valve, 23 flow meter, 24 preheating tube, 25 co-axial protection gas, 26 lens protective gas)...

Figure 3.30. Schematic diagram of a process for SCS fibre by cold-wall CVD [58]...

The hot-wall CVD process has a number of advantages over the cold-wall CVD process. These advantages are as follows (1) the thickness uniformity of the coating is better since the deposition temperature and temperature uniformity can be easily controlled in a relatively small chemical reaction chamber and (2) there is no requirement for the fibre and coating to be electrically conductive, hence this technique can be used to coat a large number of materials. 

A vertical cold wall CVD furnace was employed to prepare the B-C coatings. Boron trichloride (BCl3>99.99 vol.% and iron<10 ppm) was used as the boron source. The carbon source was provided by the methane (CH4>99.95 vol.%) and propylene (CjH6>99.95 vol.%) gas. Hydrogen (H2>99.999 vol.%) was used as a dilution gas of BCb.The deposition parameters are listed in Table 1. T-300 carbon fiber from Toray, Japan was employed as substrate. 

During this period, a redesigned cold-wall CVD system was constructed and tested. The initial results indicate that the expected results of faster deposition and reduced powder formation have been achieved however, only amorphous material has been produced at temperatures up to 775°C. 

A previous study by us [14] showed the feasibility of depositing silicon—based ceramic films from methylsilazane. In our current work, clean silicon nitride and silicon carbonitiide thin ms were formed by reacting gaseous methylsilazane vdth ammonia or hydrogen in a cold—wall CVD reactor at moderate temperatures. 

Reactor, cold wall (CVD) A reactor furnace where the CVD gases are heated by the hot substrate and the walls of the containing structure are cold. 

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