A. SEM Surface and fracture cross-sectional studies

Diamond films grown at the lowest substrate temperature (740 °C) showed a typical cauliflower structure [Fig. 1(a)] as also conferred by other researchers, as, e.g., in Refs. Reference Zhang, Zimmer, Howe and Maboudian7 and Reference Haubner and Lux9. The reason of predominance of cauliflower diamond structure at lower substrate temperatures can be explained in terms of gas phase kinetic model suggested by Frenklach and Wang.Reference Frenklach and Wang¹⁹ The growth rate of this film was 0.25 μm/h as determined from its cross section [Fig. 2(a)]. The quality of both diamond films deposited with cauliflower and cubic structures were determined from the Raman spectra (Fig. 3). The diamond film with cauliflower structure has a high nondiamond content and intrinsic compressive stress indicated by the positive shift of Raman band of diamond.

FIG. 1.

SEM micrographs showing surface morphologies of diamond films deposited at substrate temperatures (a) 740 °C, (b) 780 °C, (c) 830 °C (x10 k), and (d) 830 °C (x5 k).

FIG. 2.

SEM interface micrographs showing (a) cauliflower (growth rate ∼0.25 μm/h), (b) formation of cubes on cauliflower (growth rate ∼0.27 μm/h), (c) well-faceted cubes (growth rate ∼0.4 μm/h), and (d) multilayer structures (1st layer: 4.5 h, ∼0.25 μm/h; 2nd layer: 2.5 h, ∼0.27 μm/h; 3rd layer: 2 h, ∼0.4 μm/h; total deposition time: 9 h, total growth rate ∼0.3 μm/h).

FIG. 3.

Raman spectra of (a) well-faceted cubic and (b) cauliflower diamond structures.

By bringing substrate closer to filament assembly from 8 to 7 mm, the substrate temperature increased from 740 to 780 °C. As a result of increased temperature, some of the grains of cauliflower structure partially transform into pseudocubic form [Fig. 1(b)]. Small crystalline clusters (cauliflowers) amalgamated into cubic form, as the substrate temperature increased, and enlarged them according to the size of the cauliflower, over which they reside and grow thereon [Fig. 2(b)].

Next, the substrate temperature was raised from 780 to 830 °C by reducing the substrate–filament distance from 7 to 6 mm. As a result, partially cubic morphology [Fig. 1(b)] merged into well-faceted cubic morphology [Figs. 1(c) and 1(d)] exhibiting crystalline aggregates with relatively small (100) facets (<1 μm) and uniform distribution. The growth rate in this case was higher (0.4 μm/h) compared with that at 780 °C, as determined from the fracture cross sections. The transformation of cauliflower structure to cubic can be explained by the enhanced surface diffusion with the increase of temperature and also higher dissociation of hydrogen atoms as proposed by Sokolov et al.Reference Skokov, Weiner and Frenklach²⁰ By changing the substrate–filament distance and keeping filament power constant, the chemistry of the gas phase may change particularly the concentration of atomic hydrogen.

Formation of the cubic morphology requires planar growth,Reference Liu and Dandy¹⁵ which becomes possible with the elimination of nondiamond content in the film and the presence of appropriate atomic hydrogen on the growing diamond surface.Reference Ali and Ürgen²¹

Various studiesReference Clausing, Heatherly, Specht, More and Begun^22–Reference Wild, Kohl, Herres, Müller-Sebert and Koidl³⁵ have been presented on (100) orientation diamond films grown by hot-filament CVD and enlighten different point of view for having cubic morphology. As an example, Chen et al.Reference Chen, Yang and Lin²³ observed that for (100)-oriented diamond films growth, additional process modifications, such as application of negative bias, are required. Zhang et al.Reference Zhang, Shi, Wang and Zhang²⁴ concluded that under optimum growth conditions, (100) orientation improved with increasing deposition time. According to Yu and Flodström,Reference Yu and Flodström²⁵ at higher pressure, diamond {100} is the favorable growth surface, whereas {111} is the favorable growth surface at lower pressure. Huang et al.Reference Huang, Yeh, Hwang and Chang²⁶ grew (100)-oriented diamond films by a four-step process in hot-filament CVD system. Taher et al.Reference Taher, Schmidt, Naseem, Brown, Malshe and Nasrazadani²⁷ reported that CH₄ percentage is responsible for the change in morphology: “the increase in CH₄ concentration resulted in two variations. The first was at 3 vol% CH₄ where the (100) orientation became dominant at the surface instead of the mixed (111) and (100) orientations exhibited at 1 vol% CH₄.” Zhang et al.Reference Shang, Fang, Liao and Cui²⁸ reported that (100)-textured diamond films are due to manual scratching of Si substrates because using the same conditions and the pretreatment of 1 μm diamond powder suspension in acetone in an ultrasonic bath lead to non-(100)-textured diamond films. But in the present case, 30–40 μm diamond powder suspension in acetone in an ultrasonic bath leads to (100)-textured diamond films. Ma et al.Reference Ma, Wang, Liu, Su, Xu, Peng, Shi and Xia³² observed that a positive dc bias can increase the nucleation density and (100)-oriented growth in hot-filament CVD. Again, in the present work, neither the negative nor the positive substrate biases were used for achieving (100) orientation of diamond film. According to Wild et al.,Reference Wild, Kohl, Herres, Müller-Sebert and Koidl³⁵ for {100}-oriented films, large values of alpha-parameter, α (e.g., α ≈ 2.5), result in a suppression of twin formation, whereas low values of α (e.g., α ≈ 1.5) lead to a deterioration in the film orientation and morphology due to twinning. Comparing the surface image shown in Fig. 1(d) with the published data, it appears that the best (100) orientation in hot-filament CVD has been achieved so far in this work.

In view of above, a question arises; then which parameter(s) is/are determining the diamond grain morphology. It is a well-established fact that changing hot-filament CVD process parameters allows modifying film morphology and texture of diamond film. The concentration of gaseous precursors at the growing diamond surface, and the temperature as well, can be tuned to grow {100} diamond films by hot-filament CVD and to affect the deposit’s phase purity and deposition rate. Such tailoring of process parameters depends on the specific design of the CVD reactor. Besides that, authors believe that presence of (100) orientation of diamond film may also be linked with the residence time,Reference Skokov, Weiner and Frenklach^20, Reference Heimann, Raiko and Buck³⁶ purity level of gases, rotation speed of substrate, and direction of gases flow. While varying the substrate/filament distance, affect of [H] at the surface of growing diamond may also account in the formation of pseudocubes and {100}-oriented cubes; Yarbrough et al.Reference Yarbrough, Tankala, Mecray and DebRoy³⁷ stated that when the filament/substrate distance is varied, the concentration of [H] at the surface is changing and this has important effects on the temperature (exothermic recombination of [H] at the surface).

B. Kinetics and thermodynamics of diamond growth

This section presents a study of kinetics and thermodynamics of diamond growth versus variation in substrate distance. Thermodynamically artificially grown diamond is in a metastable phase, so enough driving force is required to put into effect constructive transformation. On the other hand, kinetics of diamond growth determines the rate, i.e., transition from unstable to stable state or between two stable states. So it involves energy barrier to terminate the transition. As several parameters affect the growth of diamond in CVD, it is rather complex to determine kinetics and thermodynamics of diamond growth.

According to Cheesman et al.,Reference Cheesman, Harvey and Ashfold³⁸ the dominant process for carbon addition, in the form of methyl radicals, is predicted to be addition to a surface radical site, opening of the adjacent C–C dimer bond, insertion, and ultimate ring closure. Other steps such as insertion across the trough between rows of dimer bonds or addition to a neighboring dimer leading to formation of a reconstruction on the next layer may also contribute.

In early stages of the synthesis of diamond, clusters of carbon atoms are formed on the substrate surface. Due to thermal conditions, a change in the bonding structure occurs, which converts sp ¹ to sp ² and at a later stage into sp ³.Reference Singh³⁹

During transformation in various structures, atomic/ionic rearrangements take place. As a result, new bonds are formed and nearby atoms are displaced, which lead in the transition from their equilibrium positions. So there is temporary increase in local energy, forming an activated state. Now, interatomic interaction plays important role in determining the system energy. The 3rd layer in Fig. 2, i.e., (100) orientation, is at lower free energy compared to the 2nd layer. Similarly, 2nd layer is at a lower free energy compared to the 1st layer. Because of this, the 1st layer tends to be converted into the 2nd layer and 2nd layer into the 3rd.

Variation in free energy of these three layers is a thermodynamic behavior, while activation energy is the maximum energy that separates these layers/equilibrium states. Thus, activation energy is a rate determining step, which is normally given in transition state. Surface energy depends on substrate temperature. As indicated in Fig. 2, by bringing substrate closer to the filament assembly, the temperature increases resulting in greater mobility of atoms/molecules and thus more chance (probability) for them to overcome the energy barrier.

As the substrate is brought closer to the filament assembly, thermal disassociation of hydrogen becomes higher, which can now effectively etch graphite growing along with diamond (graphite has a lower free energy compared to diamond). However, it is indicated from Fig. 2 (3rd layer) that [H] also acts as a catalyst, as it speeds up the reaction by decreasing the activation energy. Again, by bringing substrate nearer to the filament assembly, carbon-containing species exhibit higher kinetic energy and hence larger surface mobility on the surface of growing diamond film.

C. Raman spectra

From the Raman spectra, the band shifting to wave number lower or higher than natural diamond (1332.1 cm⁻¹) indicates that the film is under stress (tensile or compressive).Reference LeGrice, Nemanich, Glass, Lee, Rudder, Markunas, Glass, Messier and Fujimori⁴⁰ The residual stress (σ) value is calculated by σ (GPa) = −0.567 (v − v ₀), where v ₀ = 1332.1 cm⁻¹ and v is the wave number of the diamond peak.Reference Kuo, Lin and Lien⁴¹ The “σ” calculated by above formula in diamond film having cauliflower structure is −3.23 GPa and for cubic morphology is −0.17 GPa. Raman spectra of the coating, occupied with well-faceted cubic crystals, consisted of a strong band at 1332.4 cm⁻¹, close to natural diamond (1332.1 cm⁻¹), indicating low intrinsic stress, and also almost free of nondiamond species demonstrating the good purity of the film [Fig. 3(a)]. This clearly indicates that film deposited at substrate temperature of 740 °C has higher level of compressive stress compared to film grown at substrate temperature of 830 °C but both films exhibit compressive stress as σ > 0. The higher value of compressive stress actually indicates poor quality of diamond. According to Prawer et al.,Reference Prawer, Nugent, Jamieson, Orwa, Bursill and Peng⁴²sp ²-related grain boundary contributions arise from either disordered amorphous carbon (around 1380–1520 cm⁻¹) or from nanosized grains-/tetra-polyacetylene-like fragments (around 1150–1250 cm⁻¹), which are also visible in cauliflower structure spectrum of Fig. 3. Therefore, the Raman spectra of diamond film with cubic morphology reveal better purity compared to the diamond film deposited with cauliflower structure, which is also in-line with SEM investigations. On one side, cauliflower structure (of multilayer) is helpful in preventing crack propagation and annihilation of film. On the other hand, cubic morphology of diamond film with lower degree of stress is helpful in improving surface roughness and quality of the film.

D. Proposed application of multilayer diamond films

After determining temperature-dependent growth morphology, multilayer films were grown, simply, by changing the substrate distance from filament assembly. Growing of films was started with parameters for cauliflower growth, followed by partially (pseudo) cubic and then cubic growth for a total deposition period of 9 h [Fig. 2(d)]. The thickness of each layer showed correlation with the coating rates determined from the experiments with single layer coatings. Starting the coating process with suitable parameters for achieving structures possessing compressive stress [Fig. 3(b), cauliflower] and then growing cubic films on the transition layer (pseudocubic) are expected to increase the adhesion of the films to the substrate.

Base on the published work,Reference Schäfer, Höfer and Kröger^16, Reference Takeuchi, Oda and Murakawa¹⁷ the present approach can be used for solving the adhesion problems of diamond coatings deposited not only on Si substrate but more likely on hard tool materials/substratesReference Sein, Ahmed, Jackson, Ali and Gracio^5, Reference Salgueiredo, Almeida, Amaral, Fernandes, Costa, Silva and Oliveira^6, Reference Porro, Temmerman, Lisgo, John, Villalpando, Zimmer, Johnson and Wilson^8, Reference Köpf, Haubner and Lux^10, Reference Haubner and Kalss^11, Reference Huang, Yeh, Hwang and Chang^26, Reference Taher, Schmidt, Naseem, Brown, Malshe and Nasrazadani^27, Reference Chattopadhyay, Sarangi and Chattopadhyay^43, Reference Amaral, Almeida, Fernandes, Costa, Oliveira and Silva⁴⁴ along with extra advantage to have lower surface roughness of the top layer. The increase of performance of these diamond multilayers is largely due to reduced crack propagation. Multilayers of fine-grained and coarse-grained diamond have been developed,Reference Schäfer, Höfer and Kröger¹⁶ showing increased shock resistance due to reduced crack propagation. In another study, diamond secondary nuclei are grown on a substrate by applying bias current to the substrate repeatedly and intermittently; multilayer diamond films, in which the continuity of grain growth is suppressed, are synthesized,Reference Takeuchi, Oda and Murakawa¹⁷ but the procedure was complex and expensive.

Due to certain limitations, this study does not provide any demonstration that such procedure allows attaining better adhesion and/or resistance to crack propagation. But on the basis of Raman spectra and morphology results along with published literature, authors strongly believe that multilayer diamond growth, which is presented here, is useful, where smooth diamond surface, annihilating crack propagation, and increasing the adhesion are major challenges.