The High Pressure Collision Of Atoms

Although PVD coating of DLC, and CVD deposition of diamond films are preformed in partial vacuum, the action of diamond's sp3 bond formation is actually taking place at very high pressure and temperature. In the case of PVD process, carbon atoms are smashed together to form sp3 bonds. Locally, the pressure and temperature are higher than that for conventional high pressure synthesis of diamond (Fig. 4.12). Hence, the vacuum environment and low temperature condition refer to the carbon atoms that have already been quenched from the extreme conditions of deposition.

When a carbon containing ion smashes on a group of carbon atoms that already bonded to form diamond, the kinetic energy will be transmitted to many atoms. Hence, the smashing atoms must possess an excess amount of energy so they can overcome the activation energy for the formation of sp3 bonds even after losing the energy to other atoms as heat. However, if the kinetic energy is too high, the vibrations of atoms are excessive so even diamond's sp3 bonds are formed instantaneously, they will immediately be back converted to graphitic sp2 bonds at high temperature. Hence, the kinetic energy of the impinging ions must be optimized to maximize the diamond content of a DLC coating. In most cases, the optimized energy is around 100 eV. At such an energy, new sp3 bonds will be formed with the joining of the carbon cation to the existing DLC coating, at the same time the surrounding carbon

Figure 4.12. The extreme conditions of PVD deposition of DLC. The distributions are pressures (top) and temperatures (bottom) of carbon atoms near the vicinity of collision by a methane cation that traveled at a kinetic energy of 100 eV. Note that although the coating is performed in vacuum and low temperature, locally, sp3 bonds are actually formed at pressures and temperatures way higher than that for conventional diamond synthesis under ultra high pressure.

Figure 4.12. The extreme conditions of PVD deposition of DLC. The distributions are pressures (top) and temperatures (bottom) of carbon atoms near the vicinity of collision by a methane cation that traveled at a kinetic energy of 100 eV. Note that although the coating is performed in vacuum and low temperature, locally, sp3 bonds are actually formed at pressures and temperatures way higher than that for conventional diamond synthesis under ultra high pressure.

atoms will absorb the extra energy by vibrations (i.e. heated up), but the shaking is not strong enough as to break the bonds already formed.

Free carbon ions can be generated by several ways. For example, positively charged carbon ions may be derived from vaporized graphite cathode that is bombarded by an electric arc. If these carbon ions are accelerated toward a target in an electric field, they may acquire enough kinetic energy and attach to the target to form nano-crystalline (amorphous) diamond or DLC. This method of making diamond is known as cathodic arc.

Cathodic arc is a popular PVD method for depositing nano-crystalline diamond that does not contain hydrogen. During the ion bombardment, the ambient pressure is kept in a vacuum (e.g. 10-5 torr), and the substrate temperature is relatively low (e.g. 100°C). Hence, such a PVD process is viewed to be a metastable synthesis of imperfect diamond crystallites. However, at the site where carbon atoms are smashed to form diamond bond, the local pressure may exceed 15GPa, and temperature, more than 1000°C. Hence, such PVD processes are actually microscopically under high pressure, but macroscopically at low pressure.

In contrast, a CVD process starts with a carbon atom that is already compressed by four hydrogen atoms to yield sp3 configuration. The strategy is then to link this single atomic diamond to form a molecular diamond without being decompressed to form sp2 bonds. The C-H bond has an energy that is equivalent to a pressure of 200 GPa; and the C-C diamond bond, at a somewhat less pressure. Hence, during the CVD growth of diamond, carbon atoms are always under high pressure throughout the entire synthesis route. Thus, it is a misnomer to call a CVD process the metastable growth of diamond. Although gas molecules are under partial vacuum during the growth of CVD diamond, carbon atoms are always in the diamond stability field.

It would appear that all diamond synthesis routes are intrinsically high pressure methods. With this new insight in mind, the major commercial diamond synthesis routes can be classified according to the durations of pressure and temperature applied as depicted in Table 4.3.

Table 4.3. P/T duration's for diamond synthesis

Process

C-Atoms

Pressure

Temperature

High Pressure

All

Steady

Steady

Explosion

All

Instant

Instant

PVD

Few

Instant

Instant

CVD

Few

Steady

Steady

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