Log H

Figure 11.17. The relative deposition rate of diamond and graphite is controlled by the concentration of hydrogen atoms in the gas mixture.

atoms are fooled to maintain its pseudo sp3 configuration with nearby hydrogen atoms until they join together to form diamond. In order to maintain this dynamic balance, it is critical that the carbon atoms must hit hydrogen atoms all the time and in no cases that carbon atoms should hit themselves, in particular, they must not run into one another consecutively. Hence, the mean free path of hydrogen atoms must be much shorter than the collision distance of carbon-carbon atoms. Consequently, for growing diamond films, there is a maximum carbon concentration that can be supported by the presence of hydrogen atoms; in other words, there is an optimal ratio of CH4/H2 in the original gas mixture (Fig. 11.18).

Thus, the higher the concentration of hydrogen atoms, the faster the diamond film can grow. Alternatively, if the deposition rate is maintained at optimal, then the higher the concentration of hydrogen atoms, the better the quality of the deposited film. The dependence of diamond's growth rate (G) and the film's defect

Figure 11.18. Diamond formation rate and its conversion rate as a function of CH4/H2 ratio in the original gas mixture.

concentration (D) on the concentration [H] of hydrogen atoms may be expressed by:

where K is dependent on the deposition conditions other than the concentration of hydrogen atoms. Thus, for a given concentration of hydrogen atoms in the reaction chamber, an optical grade diamond film must be grown very slowly, whereas a tool grade diamond film can be deposited at a higher rate.

As both the growth rate and the quality of diamond film are dependent on the concentration of hydrogen atoms, hence, all CVD processes for growing diamond are designed to enhance the dissociation of hydrogen molecules in the gas mixture to form hydrogen atoms. The dissociation of hydrogen molecules are highly dependent on temperature and pressure (Table 11.1), hence, most CVD

Table 11.1. The concentration of hydrogen atoms as a function of temperature.

Temperature (°C)

[H] (kg mol/m3)

[H] (kg mol/m3)

P = 20 torr

P = 200 torr

1600

7.62 x 10-8

2.41 x 10-7

1800

4.43 x 10-7

1.40 x 10-6

2000

1.78 x 10-6

5.66 x 10-6

2200

5.47 x 10-6

1.75 x 10-5

2400

1.35 x 10-5

4.43 x 10-5

2600

2.75 x 10-5

9.50 x 10-5

2800

4.69 x 10-5

1.77 x 10-4

3000

6.65 x 10-5

2.92 x 10-4

processes are aimed to heat the gas mixture more effectively at a higher pressure.

A common gas mixture for CVD growth of diamond film contains CH4 that is fully diluted in H2. The dissociation of this gas mixture will produce various species of C and H radicals; all of them are temperature dependent (Fig. 11.19).

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