Diamond deposition

Plasma-jet diamond techniques yield growth rates of about 980 p.m/h (163,164). However, the rate of diamond deposition is still one to two orders of magnitude lower than the HP—HT technology, which is about 10,000 p.m/h (165). Diamond deposition rates of ca 1 p.m/s have been reported usiag laser-assisted techniques (166). This rate is comparable to the HP—HT synthesis. 

Figure 15 shows the variation of diamond deposition rates by various activated CVD techniques as well as the HP—HT technique (165). It can be seen that the highest growth rate of activated CVD diamond synthesis is stiU an order of magnitude lower than the HP—HT technique. However, CVD has the potential to become an alternative for diamond growth ia view of the significantly lower cost of activated CVD equipmeat and lower miming and maintenance costs. 

Fig. 15. Variation of diamond deposition rates by various activated CVD techniques as well as the HP—HT technique (165).

Other topics recently studied by XPS include the effects of thermal treatment on the morphology and adhesion of the interface between Au and the polymer trimethylcy-clohexane-polycarbonate [2.72] the composition of the surfaces and interfaces of plasma-modified Cu-PTFE and Au-PTFE, and the surface structure and the improvement of adhesion [2.73] the influence of excimer laser irradiation of the polymer on the adhesion of metallic overlayers [2.74] and the behavior of the Co-rich binder phase of WC-Co hard metal and diamond deposition on it [2.75]. 

A large number of CVD diamond deposition technologies have emerged these can be broadly classified as thermal methods (e.g., hot filament methods) and plasma methods (direct current, radio frequency, and microwave) [79]. Film deposition rates range from less than 0.1 pm-h to 1 mm-h depending upon the method used. The following are essential features of all methods. 

CVD deposition except for the special and important case of diamond deposition, a topic which is reviewed in Ch. 7.P H34]... 

These compounds generally decompose into two stable primary species the methyl radical (CH3) and acetylene (C2H2).P 1 The methyl radical is considered the dominant compound in generating the growth of CVD diamond.P2][23] Direct deposition from acetylene, although difficult experimentally, has been accomplished, with a marked increase in the crystallinity of the diamond deposit.P" ... 

Atomic hydrogen plays an essential role in the surface and plasma chemistry of diamond deposition as it contributes to the stabilization of the sp dangling bonds found on the diamond surface plane. Without this stabilizing effect, these bonds would not be maintained and the diamond 111 plane would collapse (flatten out) to the graphite structure. 

Microwave-Plasma Deposition. The operating microwave frequency is 2.45 GHz. A typical microwave plasma for diamond deposition has an electron density of approximately 10 electrons/m, and sufficient energy to dissociate hydrogen. A microwave-deposition reactor is shown schematically in Fig. 5.18 of Ch. 5.P ]P°]... 

Plasma-arc diamond deposition is produced at a higher pressure than in a microwave plasma (0.15 to 1 atm). At such pressure, the average distance traveled by the species between collisions (mean free path) is reduced and, as a result, molecules and ions collide more frequently and heat more readily. 

In addition to microwave plasma, direct current (dc) plasma [19], hot-filament [20], magnetron sputtering [21], and radiofrequency (rf) [22-24] plasmas were utilized for nanocrystalline diamond deposition. Amaratunga et al. [23, 24], using CH4/Ar rf plasma, reported that single-crystal diffraction patterns obtained from nanocrystalline diamond grains all show 111 twinning. 

Whereas a microwave plasma is most commonly used for the PE-CVD of diamond films, an ECR is the only plasma that is used for diamond deposition below 1 Torr [27-29]. Although Bozeman et al. [30] reported diamond deposition at 4 Torr with the use of a planar ICP, there have been a few reports that describe the synthesis of diamond by low-pressure ICP. Okada et al. [31-33] first reported the synthesis of nanocrystalline diamond particles in a low-pressure CH4/CO/H2 ICP, followed by Teii and Yoshida [34], with the same gas-phase chemistry. 

In the conversion process, graphite dissolves on one side of the nickel film and diamond deposits on the other. The shaded area in Figure 15.6 gives the (p, T) conditions where the conversion can be made at a reasonable rate (usually on the order of minutes) while still keeping the pressure at a value that is obtainable without too much difficulty. Quenching the mixture leaves... 

They are charcoal, diamond, deposits on spark plugs, coke, graphite and soot. 

The non-diamond carbon phase in polycrystalline diamond films (often referred to as graphite, although this conclusion is far from accurate [23]) is first and foremost the disordered carbon in the intercrystallite boundaries. Their exposure to the film surface can be visualized by using a high-resolution SEM techniques [24] the intercrystallite boundaries thickness comes to a few nanometers. In addition to the intercrystallite boundaries, various defects in the diamond crystal lattice contribute to the non-diamond carbon phase, not to mention a thin (a few nanometers in thickness) amorphous carbon layer on top of diamond. This layer would form during the latest, poorly controlled stage of the diamond deposition process, when the gas phase activation has ceased. The non-diamond layer affects the diamond surface conduc-... 

Experimental process for diamond deposition is shown in Fig. 3.5. Before the CVD process, seeding pretreatment on the substrates, especially nondiamond... 

Okoli, S., Haubner, R. and Lux, B. (1991), Carburization of tungsten and tantalum filaments during low-pressure diamond deposition. Surf. Coat. Technol., 47(1-3) 585-599. 

Ramesham, R. and Rose, M. F. (1997), Electrochemical characterization of doped and undoped CVD diamond deposited by microwave plasma. Diam. Relat. Mater., 6(1) 17-27. 

Specific conductive silicon substrates have to be carefully prepared before use. For the diamond-deposition process, substrates have to be cleaned, seeded with diamond nanocrystalline seeds at high surface density, and then coated with a grown thick diamond film (from less than 1 pm up to several p,m) by hot filament chemical vapor deposition (HF-CVD). At Adamant, deposition processes are performed automatically in programmable controlled process units, which allow growing diamond on scale up to 0.5 m2. The process is performed under low pressure (1 < 0.1 bar) and high temperature (filament temperature 2,500°C and substrate temperature 800-1,000°C) with a gas mixture composed of CH4, H2 (CH4/H2 ratio <1%), and a boron source (typically trimethyl boron). 

N.G. Glumac and D.G. Goodwin, Diagnostics and Modeling of Strained Fuel-Rich Acetylene/Oxygen Flames Used for Diamond Deposition, Combustion... 

Most of the synthesis techniques used for diamond deposition fall in the category of... 

The processes involved in the low pressure synthesis of diamond are not yet clearly understood, but various models have been proposed on the basis of thermodynamic and / or chemical kinetic considerations and a number of in situ as well as ex situ diagnostic studies. Figure 3 is a schematic representation of the diamond deposition process as it is understood today. 

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