MPCVD

In this section, basics of diamond growth process will be briefly described for the readers who are not very familiar with diamond CVD. One will find more detailed descriptions for each process factor later in this monograph. Note that the descriptions below are only for typical cases using MPCVD, and do not always apply to all cases. 

AIXTRON AG, Germany, produces 6-kW (2.45 GHz) and 60-kW (915 MHz) MPCVD systems that have been developed in Fraunhofer Institute. It has a very unique oval shape microwave cavity, as shown in Figure 3.6. They can accommodate 3- to 6-inch substrates, respectively. 

SAIREM, France, produces a 1.2-kW, 2.45-GHz MPCVD reactor with a quartz bell jar, as shown in Figure 3.7 [71]. 

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. 

An uniaxially (lOO)-oriented growth was first found at NIRIM in 1985 (see Ref. [23]). It was intensively studied by Wild et al. [81-83] using a NIRIM-type MPCVD reactor. A SEM image of a (lOO)-oriented film surface is shown in Figure 5.6. It is seen that the (100) faces are co-planar (the angular spread was only 1°), and the... 

Figure 5.13. The approximate regions of uniaxial (100) and (111) growth obtained using an ASTeX 5-kW MPCVD reactor. The symbols express the data points of and c for given P and [67].

Figure 5.16 shows top views of cubo-octahedral diamond particles. The growth rates are calculated separately for a > 3/2 and a < 3/2. The reason for starting with a diamond particle of a = 3/2 is that such particles can be synthesized under the standard conditions of the NIRIM-type MPCVD reactor, i.e. c = 1-2%CH4/H2, P = 30-40 Torr, Ts-SOO C, and = 300 00 W. 

Figure 5.15. Diagram of a-parameter curves and film morphology. The SEM images (a)-(d) are the film surfaces grown under the CVD conditions indicated by a-d in the diagram. The numbers in the diagram are the a-parameter values associated with the regions encircled by the dashed curves. The horizontal axis is an effective carbon concentration, C, defined by the concentrations of CH4, H2, and CO2 in the source gas. For this experiments, the large MPCVD reactor shown in Figure 3.4 was used [68, 70].

Recently, a mosaic technology has been further developed in Refs, [135, 136]. In this case, 16 pieces of diamond Ib (100) plates of 4mm x 4mm in size were used, and a diamond layer of 1-mm thickness was deposited on them. Since a custom-made MPCVD reactor was used to achieve a fairly uniform... 

A homoepitaxial diamond layer deposited by MPCVD on a diamond Ib (111) surface was studied by STM [147]. See also Ref. [142]. The H-terminated (111) surface had a 1 x 1 structure with C-H bonds vertically sticking out of the surface. 

In Refs. [188, 189], diamond growth was done by MPCVD on polycrystalline Cu using the BEN technique, but the nucleation density was only 10 /cm, significantly smaller than that on Si, 10 /cm. The nucleation density was lO /cm on Cu that had been scratched with a 0.25-pm diamond paste. A surface analysis indicated that there existed a thin graphitic layer of 5-10 A thickness on Cu. The work of Ref. [183] presents presumably the best-optimized diamond... 

Later, diamond growth on graphite flakes was done by MPCVD using 50%CO/Fl2 [198]. The results indicated the presence of coalesced diamond crystals at the edge of the graphite flakes, consistent with the theory [197]. Also found were some mutually oriented diamond crystals with their (111) faces parallel to the G(OOOl) surface. So far, however, no epitaxial diamond films have been made on graphite. 

The nucleation density of diamond on as received Si wafer is only in the order of 10 cm by a NIRIM-type MPCVD reactor under standard CVD conditions, e.g. c = 0.5%CH4/H2, P = 30 Torr, and = 800 °C. To make a continuous diamond film within one hour or so under these CVD conditions, a nucleation density of >10 cm is necessary. To achieve this, the Si surface is scratched with diamond powder or paste. The powder size is usually 0.1-30 pm. The nucleation density is increased to approximately 10 cm by this treatment. Alternatively, the Si wafer is ultrasonically treated in alcohol with diamond powder suspension for several minutes. The nucleation density can be increased to 10 -10 cm by this treatment. In both cases, the Si surface is roughened. 

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