The Microwave Plasma Enhanced Engineering Essay

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When diamond is mentioned, people will automatically think about the cost that valued by the society. Why diamond, a type of gemstones, will cost so much more than others? Diamond is not only a shinny stone. It has a lot of great and unique properties such as highest hardness and thermal conductivity of any bulk material. These properties determine the major industrial application of diamond in cutting and polishing tools. Also, the optical characteristic is something that must be discuss in diamond. With extremely rigid lattice, the optical characteristics become significant. However, diamond still can contaminate by few types of impurities, such as boron and nitrogen, which results in some color for diamond. In this paper, a review of diamond will be presented.

How diamond is made

Naturally, diamonds are formed at high pressure and high temperature conditions existing at depths of 140-190 km in the earth mantle. They are bought close to earth surface through volcanic eruptions by a magma, which cools into igneous rock known as kimberlites and lamproites. Figure 1shows a phase diagram of carbon. From Figure 1, diamond is stable at high pressures and temperatures. Graphite, however, is the stable form of carbon under ordinary temperature and pressure conditions. One method of synthesizing diamond is to subject graphite to conditions of approximately 55,000 atmospheres and temperatures of about 2000°-C. However, even though carbon is not at the minimum energy state, it does not spontaneously convert from diamond to graphite. Since we know that diamonds are form at high pressure and high temperature. Research originally synthesis diamonds under same conditions, high pressure high temperature (HPHT).

Figure 1 Phase diagram for carbon. At sufficiently high temperatures and pressures diamond is the stable. At lower temperatures and pressures, graphite is the stable form. Under ordinary conditions for temperature and pressure, near 1 atm and room temperature, diamond may be considered a metastable form of carbon.(Reinhard)

The unremarkable properties of diamond such as hardness and high thermal conductivity make it an important new material in a wide range of applications. However, the high cost of material production has limited the commercial used of diamond thin films to a few applications. Today the technology is able create artifactual diamond by chemical vapor deposition (CVD). CVD is a method of producing synthetic diamond by creating the circumstances necessary for carbon atoms in a gas to settle on a substrate in crystalline form. It is common to find well-formed crystals with almost equal development of (111) and (100) faces (Figure 2). (DeVries) Diamond grows by CVD often involves feeding varying amounts of gases into a chamber, energizing them and providing conditions for diamond growth on the substrate. The gases include a carbon source and typically include hydrogen as well. However the amounts used depends on the type of diamond being grown. In CVD of diamond, the factors driving cost include low reagent utilization, low deposition rates, high-energy consumption, large thermal management loads at the substrate, and capital equipment costs. For successful result, diamond deposition depends on different chemical and transport processes occurring in the gas phase and on the surface. (See Figure 4 for the observed shapes of as-grown natural diamonds, high temperature high pressure (HPHT) grown synthetics and chemical vapour deposition (CVD) grown diamonds, including the step patterns on the different faces)

Figure 2 Diamond crystals from CVD deposition. The largest dimension is about 20 jlm. (DeVries)

All diamond CVD processes under a highly energetic activation stage in the gas phase. It lead to two purposes which are to dissociate the hydrocarbon precursor molecule into fragments that react more readily at the deposition surface and to dissociated molecular hydrogen to create a superequilibrium concentration of gas-phase hydrogen atoms. (See Figure 3 for different technique of carbon dissolved in hydrogen vs. temperature) Hot-filament reactors, microwave plasma reactors, DC arcjet reactors, and combustion are most commonly energy used as diamond CVD reactors. These reactors have a few common features and that's why they are able to produce high quality diamond films. They all have a large amount of energy, in the form of electrical or chemical free energy, is input to achieve dissociation of molecular hydrogen and the hydrocarbon feedstock. Moderately low pressures are used to prevent three-body recombination of H to form molecular hydrogen. High gas-phase temperature is produced in the activation zone, and passive or active cooling is employed to maintain a substrate temperature in the neighborhood. However, they are different from the transport processes. Hot-filament and microwave plasma are dominated by diffusion which mean there is no thermal, velocity, or concentration boundary layer. Linear gradients in temperature, velocity, or species concentration between the excitation region (hot filament or plasma ball) and the deposition surface in both reactors are often found. However, the disadvantage is growth rate is slow. DC arcjet CVD and combustion is characterized by high velocities; thin boundary layers in temperature, velocity, and concentration are formed near the growth surface. In the following, the detail of each technique will be introduced.

Figure 3 Differential solubility of carbon in hydrogen for different CVD diamond methods.

Figure 4 Idealised morphologies of natural, HPHT-grown and CVD-grown diamonds. For the {111} and {100} faces characteristic orientations o f growth steps are also indicated. (Nazare and Neves)

High pressure and high temperature (HPHT)

Artificial Diamond is original made by high pressure and high temperature (HPHT) It is still widely used because of it's relatively low cost. It is typically processed under a pressure of 5GPa at 1500°C. There are two common systems; Belt system and Bars system. In belt system, a huge hydraulic press with anvils and a ring shaped structure are used. The upper and lower anvils supply the pressure load to a cylindrical inner cell and a belt of pre-stressed steel bands confines the internal. Anvils serve as electrodes and provide electrical current to the compressed cell. A variation of the belt press uses hydraulic pressure to confine the internal pressure. Figure 5 is a schematic example of a belt system where diamond seeds are placed at the bottom of the press. While the internal part of press is heated, the molten metal dissolves the high purity carbon source. The molten metal then transports to the diamond seeds and precipitates. Colorless diamond can be synthesized if the nitrogen is removed by mixing small amount of Ti with the metal. (International Diamond Lab)

Figure 5 
This is a schematic example of a Belt type HPHT press.  (International Diamond Lab)

BARS system is developed at the Russian Academy of Sciences in Novosibirsk. It is very similar to the belt type system. It is made up by eight outer anvils with a spherical outer shape to which pressure is applied and six inner anvils to multiply the pressure to the sample. BARS system is the most compact, efficient, and economical of all the diamond-producing presses. (International Diamond Lab)

Hot-filament CVD

Hot-filament CVD is also called thermally activated CVD. It is one of the earliest developed approaches to low pressure synthesis of diamond. A refractory metal, usually tungsten, is used as a filament, is heated to high temperature around 2300°C. The temperature can be reach by resistance heating and the high temperature help to activate the hydrocarbon-hydrogen gas mixture. The filament is located a few millimeters above the substrate also provides heating for the substrate. The hydrocarbon-hydrogen gas mixture is allowed to flow across the hot filament, where it is activated. Hot-filament CVD reactors are inexpensive and easy to construct. The filament temperature, the position of the substrate with respect to the filament, and the gas flow dynamics play important factors in the process. However, there are several disadvantages of this technique such as contamination of the diamond film by the filament, erosion and sagging of the filament, and a relatively slow growth rate. It is also necessary to supply constant power throughout a deposition using a proper power controller but the uniformity of the substrate temperature is difficult to maintain when using multiple filaments. (Reinhard)

Figure 6 Schematic diagram of the hot filament CVD process showing the basic elements.

Microwave plasma-enhanced CVD (MPECVD)

Microwave plasma enhanced CVD is widely used for diamond deposition. A magnetron is usually used to generate microwave energy at 2.45 GHz and a wave-guide assembly is used to couple the energy to a resonant cavity. MPECVD is an electrodless process, which is an advantage over other techniques, and there is no contamination from the electrode material. The microwave plasma excitation of hydrogen generates superequilibrium concentrations of atomic hydrogen. The collisions of electrons with gas atoms and molecules generate high ionization fractions. (Reinhard)

Figure 7 Schematic setup of the CVD synthesis of diamond. (Markus)

Direct current (DC) arcjet discharge technique

DC arcjet discharge is a very high growth rate process. Usually, this technique will be use to synthesis thick and freestanding diamond substrates. A DC arcjet discharge reactor for diamond deposition consists of a gas injection nozzle, composed of a rod cathode, which is usually made of tungsten, concentric with a tube anode. Gases are allowed to flow between the cathode and anode. Gases will be spray out from an orifice in the anode then a high temperature discharge jet is created and sustained by a DC voltage across the electrode. The substrate is located downstream from the jet stream on a water-cooled substrate stage. Carbon precursor and graphite etchant gases would be introduced at different locations depending on the desired activation temperature. Although this technique is often used because of the high growth rate, there is several disadvantages of it such as the film can undergo from high compressive stresses, microvoids, and high surface roughness. (Reinhard)

Combustion

Combustion is well know for its scalable nature, minimal utility requirements, and significantly reduce capital costs relative to plasma aided processes. The most important parameter in combustion synthesis is the oxygen-to-acetylene ratio, defined as

R = O2:C2H.

At values of R near 1.0, a neutral flame is achieved, which is defined as the condition where the feather region just disappears because all the acetylene is consumed in the primary flame. The diamond growth regimes as a function of composition are showed in Figure 9. The highest quality diamond is obtained in slightly rich flames, when oxygen-to-acetylene ratio is about 0.85-1.0. The value of R at which a neutral flame occurs is dependent on both burner design and total flow rate. Substrate temperature is control in a range from 950-1650K during combustion CVD. With high temperatures, substrates has been limited to materials such as silicon, alumina, and diamond. However, it is not easy to measure the substrate temperature in combustion CVD due to the extreme heat fluxes present. Substrate temperature controls growth rate and morphology. As the substrate temperature increases, the growth rate is proportional. (See Figure 9) However, after the growth rate reaches its maximum, an expression of a rapid decline in both the quality and the growth rate is observed. (Reinhard)

Figure 8 Two designs of atmospheric flat flame burners: (a) a coflow design and (b) a trumpet bell design (Reinhard)

Figure 9 The effect of substrate temperature on growth rate observed in combustion CVD of diamond. In atmospheric torches the maximum growth rate occurs at substrate temperatures between 1450 and 1650 K (Reinhard)

Metastable

Diamond is uncommon because of two reasons. First, the kinetics of graphite formation are much faster than the kinetics of diamond formation in normal condition. Second, a large activation energy barrier between graphite and diamond prevents thermal activation of diamond into graphite. (See Figure 10) When diamond is synthesized under conditions where graphite is the stable phase of carbon, the result of synthesizing diamond is usually failed. It is because the density of diamond is greater than the density of graphite. (Anthony)

At ordinary temperatures and pressures, although diamond is not the minimum energy state of carbon, it is also not an unstable stage of carbon. (see Figure 1) Therefore, if carbon atoms are in the diamond lattice spatial arrangement, the solid does not spontaneously convert to graphite under low temperature, low pressure conditions. Formation of diamond from nascent carbon containing species under metastable conditions is both thermodynamically allowed and readily achieved under proper deposition conditions. It is the lower temperatures and pressures associated with this method of diamond synthesis that have offered the capability of direct deposition of diamond on a variety of substrates and have opened the possibility of new applications of diamond. For many such applications, the diamond thickness need be only on the order of micrometers; hence the structures are referred to as diamond films. (Reinhard)

Figure 10 Energy diagram of carbon (Anthony)

Structure

Pure diamond is composed only by carbon and arranged in the diamond lattice.(See Figure 11 In theory, pure diamonds are transparent and colorless.) In diamond lattice, each carbon atom has four nearest neighbors in the tetrahedral arrangement associated with sp3 chemical bonds. The nearest neighbor distance is 1.54 Å and the unit cell dimension is 3.567 Å. The density of diamond is 3.515g/cm3. The quantity of diamond is usually referred to carats, where one carat is equal to 200mg. (Reinhard) However, quality of diamond is considerable because both natural and synthetic diamond may contain impurities and defects. Diamonds occur in various color and these are caused by defects, including replaced impurities and structural defect. These defects will affect the light absorption. Therefore, diamonds are characterized into type I, type II and some subtypes, with the former containing nitrogen as an impurity and the latter being essentially nitrogen free. (John, Polwart and Troupe)

(a) (b)

Figure 11 a) Schematic diagram of carbon-carbon bonding in diamond and graphite (Anthony) b)3D diamond lattice

Type I

Type I diamonds in which impurity-related optical and paramagnetic absorption are dominated by nitrogen defects. Normally, type I diamonds are transparent to 300 nm. (Robertson R.) In general, the impurity content of natural type I diamonds is more varied compared to that of type II diamonds. The most evident difference between type I and II diamonds comes from IR absorption spectra, which are considered to be the main criterion for this differentiation. (See Figure 12 for Refraction index of type I and type II) About 98% of natural diamonds contain nitrogen with concentrations detectable in optical absorption. 74% of them have a nitrogen content high enough to be definitely classified as type I. Nitrogen is regularly present in natural diamonds at levels as high as 200 to 4000 ppm. (Zaitsev) In type I, there are three subtypes, type Ia, type Ib, and type Ic. Type Ia contains nitrogen in farily substantial amounts of the order of o.1% which most natural diamonds belong to this type. Type Ib also contains nitrogen but in dispersed substitutional form which most of synthetic diamonds are this type. (Markus) Type Ic include diamonds that contain high concentration of dislocations. Even type Ic doesn't really related to contaimination of nitrogen but the characteristic of type Ic is categorized in type I. Type Ic has the absoption continuum at wavelength below 900nm and a peak at 560nm. (Zaitsev)

Figure 12 Refraction index of type IIa and type I natural diamonds in the UV spectral region(Zaitsev)

Type II

Type II includes diamonds showing no optical and paramagnetic absorption due to nitrogen-related defects. The quantity of nitrogen found in type II is very little. (Below 1017cm-3) Type II diamonds are exhibited optical transparency up to 230 nm (Robertson R.). However, it is rare to find natural diamonds without nitrogen-related defects in absorption. Only 1 to 2% of type II diamonds are found in nature. (Zaitsev) There are two subtypes are in type II, type IIa and type IIb. Type IIa is not effective by nitrogen and this type of diamonds has enhanced optical and thermal properties. However, they are rare to find in natural. Type IIb is a very pure type which has semiconducting properties and this type of diamond is usually find in blue and extremely rare in nature. (Markus)

Influence of defects and impurity

Nitrogen does not strongly influence the refractive index of diamond in the visible spectral region. Therefore the refractive index for types I and II natural diamonds may not differ by more than 1%. (Robertson R.)

Since there is no definite tendency for preferential birefringence of diamonds of any type, it indicated that nitrogen impurity does not directly influence the birefringence of diamond. However there is a trend such that natural diamonds of average size, with an enhanced birefringence, are ultraviolet transmitting. Diamonds with a low birefringence are usually ultraviolet-opaque and nitrogen is the caused for this effect. Diamond with low nitrogen, type II, have a very deformed stressed crystal lattice. (Zaitsev)

The birefringence of diamond is caused by plastic deformation, elastic deformation near inclusion, growth striations, growth sector boundaries, dislocations, grain boundaries, and diamond-substrate boundaries. The phenomena occur in both types of diamonds. The highest birefringence is found in fragments of natural diamonds where dodecahedral diamonds shows the least birefringence. Defects arises from sheets of stacking faults are expected to resulting the birefringence contrast weaker than from partial dislocations. However, partial dislocations or stacking fault sheets will be seen only the background birefringence is very low. (Zaitsev)

Properties

Diamonds have some great properties that other material still cannot be compared and that is the reason why people would like to understand how diamond is formed and synthesis diamond to reduce the cost of the material. Diamond is well known for high thermal conductivity, high electrical resistivity, low coefficient of friction, high degree of chemical inertness, high optical dispersion, large energy gap, low infrared absorption, and high breakdown voltage. See Table 1 for outstanding properties of diamond.

Table 1 Some properties of diamond (Markus)

Thermal Properties

As mentioned, diamond has high thermal conductivity. For high quality single crystals, usually type IIa, the thermal conductivity, κ, is about 22W/cm°C at room temperature. This property is due to the stiffness of diamond bond and the diamond structure, which rise to a high acoustic velocity and a very high characteristic temperature. Recently, researcher has reported the best thermal conductivity of the film is about 11W/cm°C. For thick film, the conductivity is about17W/cm°C at room temperature. (J.E. Graebner) Figure 13 indicates the relation between thermal conductivity and film thickness, where thermal conductivity increases with film thickness. Thermal conducitivy also depends on grain boundary. Diamond's thermal conductivity increases with decreasing temperature, reaching a maximum of 42 W/cm-K near 80 K, after reaching the maximum the thermal conductivity decreases. Impurities, such as nitrogen, reduce the thermal conductivity. Type I diamonds with 0.1% nitrogen only have 50% thermal conductivity values of type II diamonds in room temperature. Isotropic purity increases the thermal conductivity. Synthetic diamond crystals grows with pure carbon-12 have thermal conductivities 50% higher than those of natural diamond for which the atomic weight is 12.01 because the material contains 1.1% carbon-13. (Zaitsev)

Figure 13Measured thermal conductivity at 25°C for five samples (squares). Solid circles show the derived conductivity. The horizontal dashed line indicates the typical conductivity reported for single crystal high quality (Type IIa) diamond. (J.E. Graebner)

Optical Properties

Diamond films are usually transparent in the infrared, with the exception of the carbon-hydrogen absorbing bands centered at about 2900cm-1, weak absorbing in the visible spectrum, and increasing absorbing with decreasing wavelength in the UV light. The optical gap value is range from 0.38 to 2.72 for diamond films. (A.)

The index of refraction, both the real part n and imaginary part k, and its spectroscopic variation has been found to be dependent on the preparation conditions and hydrogen content of the films. Its value at 632.8 nm can be adjusted from 1.7 to 2.4 by adjusting the deposition conditions. (A.) This refractive index is large compare to other transparent material. With large refractive index, it is also found large reflection coefficient and a small angle for total internal reflection. (Zaitsev)The index of refraction is also affected by the hydrogen content in the diamond films and generally increases with decreasing concentration of bound hydrogen. It is, however, dependent on the concentration of bound hydrogen and not total hydrogen content in the film. A higher index of refraction usually indicates diamond with stronger crosslinking, higher hardness, and better wear resistance. (A.)

Diamond is also photoconductive. There is a strong photoconductive peak at 225 nm due to excitation of electrons across the band gap in pure diamond, and in boron doped diamond there are also peaks from 1.4 to 3.5 μm due to excitation of the deep-lying acceptor levels. (Reinhard)

Electrical properties

The electrical property of diamond film is well known for large band gap. Diamond have a modest bandgap. The energy band structure of diamond exhibits an indirect energy gap with a value of 5.47 eV at 300 K. This is sufficiently large that at near room temperature the intrinsic carrier concentration is negligible and the material is an insulator with a dielectric constant of approximately 5.7. (Zaitsev) (See Figure 14 for band structure) In an insulator the valence electrons form strong bonds between neighboring atoms and consequently these bonds are difficult to break. Thus, the bandgap is large and there are no free electrons to participate in current conduction at or near room temperature.(Markus)

Figure 14 Activation energies for some impurities in diamond. B is boron for p type, P is phosphorus for n-type, and N is nitrogen. (Markus)

The band structure of diamond film is assumed to consist only a mobility gap, where carriers residing in gap states are localized. The mobility gap produces semiconductor behavior, however, the high density of localized gap states leads to low apparent carrier mobilities and significantly degrades the semiconducting properties of material. Diamond films usually have high electrical resistivities from 102-1016Ω, depending on the deposition condition(A.) The electrical conductivity of diamond is more sensitive to impurities than the thermal conductivity. The electrical resistivity can be reduced by several orders of magnitude through incorporation of metals or nitrogen in the films. The decrease of resistivity by incorporation of dopants maybe related to a dopant induced graphitization. However, more evidences are needed to prove.

Boron doped p-type diamond exists in nature. The growth of boron doped diamond films by CVD techniques has been achieved by adding B containing molecules to the gas mixture in either a microwave or in a hot filament reactor resulting in the growth of B containing p-type diamond films. (A.)(R.)

N-type doping is much more complicated. It is still questionable about the potential donor atom that will yield a shallow enough energy level in the gap to be sufficiently ionized at room temperature. Most recently clear donor activity is phosphorus doped for n-type diamond. In Figure 15, the dependence of the resistivity on measurement temperature. Similar slopes are obtained for all samples proves that in this temperature range the conduction mechanism is thermally activated, with an activation energy of 0.46 eV, rather independent of growth conditions. (R.)

Figure 15 Temperature dependence of the resistivity of n-type diamond, doped with different amounts (ppm) of phosphorus (300,800 K).(R.)

B. Mechanical Properties

Diamond is the hardest known substance. Diamond also has the lowest compressibility, the highest elastic modulus, and the highest isotropic speed of sound (18,000 m/sec) of any known materia (Nazare and Neves). The degree of hardness is quantified in terms of both resistance to indentation and abrasion (or scratch) resistance.

In terms of compressibility, the ratio of tensile stress to linear strain, or Young's modulus, is 1050 GPa, a value approximately five times higher than that of steel. However under different methods of testing, the Young's modulus is different and C11, C12, C44. Table 2 provides the Young's moduli of diamond with different test methods. Because of its brittle nature, diamond is not particularly strong. (Markus)

Table 2 Elastic moduli of diamond(GPa) (Nazare and Neves)

Method

C11

C12

C44

Ultrasonics

950

390

430

X-rays

1100

330

440

Brillouin

949

151

521

The mechanical strength of diamond is influenced by a number of significant factors, including the applied stress system, the ambient temperature and the degree of both internal (impurities) and external (surface finish) perfection. Fracture occurs when a certain level of stress is applied and the mode of failure will be that which requires the smallest stress. Materials, where the bonding is predominantly covalent or where there is a substantial degree of covalent bonding, have a large inherent lattice resistance to dislocation motion and failure occurs at low stresses, below the theoretical fracture stress. Diamond, as with any other crystalline material, can fail by either brittle fracture, cleavage, or in a ductile mode, flow by a shear process. Although thermal properties and electrical conductivity are both highly affected by nitrogen, there is no clear evidence found that mechanical properties are clear related to nitrogen. (Nazare and Neves)

Highly inert chemically

Diamond is highly inert chemically, except for two situations. It is susceptible to oxidizing agents at high temperatures. For example, if diamond is heated in the presence of oxygen, oxidation begins at around 900 K. Also, diamond is subject to chemical attack by certain metals at high temperatures. These include carbide formers such as tungsten, tantalum, titanium, and zirconium as well as solvents for carbon such as iron, cobalt, manganese, nickel, chromium, and platinum. (Zaitsev)

Applications

Diamond is a very useful material because of the outstanding properties including high thermal conductivity, high electrical resistivity, low coefficient of friction, high degree of chemical inertness, high optical dispersion, large energy gap, low infrared absorption, and high breakdown voltage. With these properties, diamond is used for diverse application besides jewelry. They are commonly used in mechanical application, optical applications, thermal applications, and detector applications. Diamond can be used for abrasive and wear resistance coating for cutting tools, lenses, windows for power lasers, diffractive optical elements, heat sinks for power transistors, semiconductor laser arrays, solar blind photodetector, and radiation hard and chemically inert detectors.

Table 3 Future application areas for diamond electronics.(Markus)

Cutting Tools

Single crystal diamond is used for coating of modulated or layered composition of two or more transition metal compounds. It is common to use diamond coating for certain types of grinding wheels or cutting of highly abrasive alloys and metals. There are two ways to apply diamond on to the cutting tools. First, growing relatively thick layers of CVD diamond from which separate freestanding pieces are obtained. These pieces are then brazed onto a cutting tool. Second, directly deposited diamond onto the cutting tools.(Markus) Often, high-quality diamonds are selected for use in dressing tools for non-ferrous alloys, aluminum, brass bronze, ceramics, graphite, and glass fiber-reinforced structure. (Markus) (Hammond and Evans)Single-point diamond is mounted in a metal matrix. They are usually used to dress and impart or restore the required geometric shape to certain abrasive wheels. Two typically forms of such cutting tools are single-point and multi-point. Today, single or multi-point cutters include milling, turning, boring, cutting-off and slitting. (Hammond and Evans)

Demonstrated coating diamond onto hardmetals

Hardmetals are the most valuable and important substrates for coated tools, due to their intrinsic properties and their wide range of mechanical properties. They consist of WC and Co with additions of TiC, (Ta5Nb)C, and VC, which mainly change their hardness and wear resistance. The amount of Co binder is largely responsible for ductility or brittleness. Hardmetals have been used as wear parts and cutting tools for decades, with and without coating applications.

Usually, successful diamond coatings on WC-Co substrates have no or a very low amount of cubic carbides (TiC) and also a relatively low Co content. Both Co and TiC additions increase the thermal expansion coefficient of the hardmetal and reduce the adhesion of the diamond coating. A high Co vapor pressure and its high mobility on the substrate surface influence diamond deposition. In the gas phase surrounding the substrate surface, Co catalyses the formation of nondiamond carbon phases, which can be deposited at the interface prior to the diamond formation. How and why the Co drops reach the diamond coating surface is not yet fully understood. Surface forces might play an important role(See Figure 16).

Figure 16 Co as part of the hardmetal binder phase and its influences on the diamond deposition

Electrochemical Applications

Electrochemical behavior of boron-doped CVD diamond is one of the most promising applications of conductive diamond. Boron doped diamond fits the need for an electrode operates inertly and without deterioration in harsh chemical environments. Compare to platinum electrodes, diamond electrodes provide a much wider potential range over which no significant water decomposition occurs. (Reinhard) Diamond electrodes are suitable substrates for reactions spanning a wide potential range in aqueous solutions. They also have the advantage of chemical stability, even in highly aggressive environments. In Figure 17 the I-V curves obtained with a B doped CVD diamond electrodes in various (KI, KBr, and HCl) solutions are shown. The behavior of the doped diamond electrode is much superior to that of the commonly used noble metal electrodes. Diamond bears as a material for the fabrication of cold cathode or other electron emitting devices requires the diamond to be electrically conductive, with no need for an accurately known doping level. (R.)

Figure 17 Current vs. Potential of a highly B doped CVD diamond electrode in (a) 1 M KI; (b) 1 M KBr and (c) 1 M HCl. Scan rate 150 mV/s.

The unique electron emission properties of diamond are the most promising applications of semiconducting diamond. Although, no clear understanding of the physics that determined the electron emission from diamond emerges. There are still many applications such as field emission from diamond surfaces using diamond to conductive.

Thermal Management

The high thermal conductivity of diamond, combined in some cases with its chemical inertness and high electrical resistivity, makes it of interest for a variety of thermal management applications. Laser diamond heat sinks and other thermal management substrates formed from CVD polycrystalline diamond are examples of available products. Because diamond combines exceptionally high thermal conductivity with exceptionally low electrical conductivity, it is of considerable interest in electrical packaging applications. It provides efficient paths for heat flow without compromising the electrical isolation of individual components. (Reinhard)

Transmission Applications

Diamond provides a window with high transmittance for various portions of the electromagnetic spectrum. It is an ideal radiation window material in particular for applications involving high power levels and mechanical, thermal or chemical load. Due to its large bandgap (5.5 eV) and the lack of infrared active fundamental vibrational modes, diamond is optically transparent over a large wavelength range. Even at elevated temperatures, diamond remains transparent, since the large bandgap does not allow the formation of free carriers. In the x-ray portion of the spectrum, diamond is of interest for x-ray lithography masks. The low atomic number of diamond results in low x-ray absorption. Another example is in high-power gyrotrons such as are used in fusion research. This application requires the transmission of very large powers (megawatts) at microwave frequencies (170 GHz) as well as the ability to dissipate heat rapidly. The ability to transmit high powers in the optical portion of the spectrum is of interest to laser designers because the design of high-power lasers is power limited by damage limits to laser optics rather than limitations of the laser medium or pumping mechanisms. The scratch resistance and chemical inertness make diamond of interest as an optical coating material as well. (Reinhard)

Diamond is known for its broadband optical transparency covering the UV, visible, near and far IR. In this range the optical transmission exhibits only minor intrinsic absorption bands arising from two- phonon (1332-2664 cm-1) and three-phonon (2665-3994 cm-1) transitions. The maximum absorption coefficient amounts to 14 cm-1 at 2158 cm-1. This holds true for optical grade CVD-diamond as shown in Figure 18. The absorption around 10 ^m is of particular interest for CO2-laser components and because many IR sensors operate within the 8-12 μm atmospheric window. (Nazare and Neves)

Figure 18Transmission spectrum of a high-quality CVD-diamond window (thickness: 150 μm) (Nazare and Neves)

CVD-diamond is also used as vacuum windows for high-power microwave (Gyrotron) tubes. These Gyrotron tubes are used for the electron cyclotron heating of fusion plasmas. Power levels exceeding 1 MW at frequencies of around 100 GHz have been demonstrated. Until recently the output window of these devices has been the most critical component limiting the maximum output power or the pulse duration. In this context CVD-diamond window with water edgecooling is found to be very promising. The extremely high power level requires very low dielectric losses. CVD-diamond exhibits loss tangent values as low as 10-5 (at 140 GHz). Below 350-400°C there is practically no temperature dependence. In the 10- 145 GHz range the loss tangent decreases with frequency as 1/f [27] or as 1/f05. (Nazare and Neves)

X-ray lithography masks

The resolution limit of optical lithography is defined by diffraction and scattering as the feature size approaches the exposing wavelength. X-ray lithography, which uses significantly shorter radiation (0.8-1.5 nm versus 300-400 nm), offers a technical path to achieving the higher resolution. However, several factors have delayed the implementation of X-ray lithography on the production line for IC fabrication.

The major non-technical factor is related to the huge built-in optical technological infrastructure which has continued to make significant improvements by using step and repeat exposure tools, incorporating multilevel resist, employing contrast enhancers, using shorter wavelength radiation, designing higher numerical aperture optics, which has effectively delayed the implementation of X-ray lithography. The technical barriers to X-ray lithography implementation include the absence of a reliable, high volume, low defect density X-ray mask technology, a high speed X-ray resist, a high speed, low cost exposure/alignment tool.

The best mask material has low atomic number since the X-ray transparency improves with decreasing atomic number. TABLE 1 reveals the weakness of polymers as membrane material candidates. They are not only hygroscopic but are mechanically soft and therefore easily distorted. The metals Ti and Be are moderately stiff; however, their opacity is troublesome, but not pathological, since alignment windows can be etched in the membrane after overcoating with polyimide to support the alignment pattern. Beryllium would be excellent were it transparent, discounting its toxicity. Si and its nitride and oxide are good from an X-ray and optical transparency viewpoint but lack the mechanical stiffness of the refractories like SiC, BN and diamond. Si has the significant advantage of a large installed technology base and capital equipment availability. As can be seen, diamond has the highest stiffness factor S of any material. There are other factors to consider in selecting a material such as: scale-up of fabrication process, X-ray-induced degradation, surface smoothness, flatness, secondary electron emission induced by the X-rays, adhesion of metallization. Diamond's low mass absorption coefficient and low density make it compatible with a variety of X-ray sources. (Nazare and Neves)

Table 4 Comparison of materials based on X-ray transparency tx, optical transparency to, and mechanical stiffness S. (Nazare and Neves)

Material

tX(μm)

t0

S = Et0Z(I-V) (GPa

Polyimide

8.7

>5

43

Mylar

8.5

>5

42

Ti

1.09

Opaque

-

Be

42.0

Opaque

-

Si

9.6

2

362

Al2O3

0.8

1

448

SiO2

4.99

5

444

Si3N4

7.71

6

728

BN

7.94

4

420

SiC

6.81

3

1590

Diamond

5.9

2

2336

Detectors and Sensors

Diamond-based devices are also of interest for detecting a variety of radiation types as well as sensing various physical parameters such as temperature and pressure. For example, diamond thermistors have been proposed for temperature measurement in hostile environments such as chemical processing, gearbox oil, and cryogenics. The piezoresistive effect of diamond has been used to sense pressure, and p-type CVD polycrystalline diamond is reported to have a large piezoresistive gauge factor [12]. Diamond is extremely radiation hard, with a 55-eV displacement energy for a carbon atom in the diamond lattice. It also acts as an ionizing radiation detector and is therefore of interest for radiation measurements where exposure to large doses is required. The large band gap of diamond make it of interest as a UV detector, based on photoconductivity, which is blind to visible light.(Reinhard)

Conclusion

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