Synthesis of Polycrystalline Diamond Films in Microwave Plasma at Ultrahigh Concentrations of Methane

Abstract

Polycrystalline diamond (PCD) films are usually grown by chemical vapor deposition (CVD) in hydrogen–methane mixtures. The synthesis conditions determine the structure and quality of the grown material. Here, we report the complex effect of the microwave plasma CVD conditions on the morphology, growth rate and phase composition of the resulting PCD films. Specifically, we focus on the factors of (i) increased methane concentrations (${\nu }_c$) that are varied over a wide range of 4%–100% (i.e., pure methane gas) and (ii) substrate temperatures (${\mathrm{T}}_{\mathrm{s}}$) varied between 700–1050 °C. Using scanning electron microscopy, X-ray diffraction and Raman spectroscopy, we show that diamond growth is possible even at ultrahigh methane concentrations, including ${\nu }_c$ = 100%, which requires relatively low synthesis temperatures of ${\mathrm{T}}_{\mathrm{s}}$ < 800 °C. In general, lower substrate temperatures tend to facilitate the formation of higher-quality PCD films; however, this comes at the cost of lower growth rates. The growth rate of PCD coatings has a non-linear trend: for samples grown at ${\mathrm{T}}_{\mathrm{s}}$ = 800 °C, the growth rate increases from 0.6 µm/h at ${\nu }_c$ = 4% to 3.4 µm/h at ${\nu }_c$ = 20% and then falls to 0.6 µm/h at ${\nu }_c$ = 100%. This research is a step toward control over the nature of the CVD-grown PCD material, which is essential for the precise and flexible production of diamond for various applications.

1. Introduction

High hardness and thermal conductivity, wear resistance and chemical inertness are the key properties of diamond-based materials [1,2,3], so polycrystalline diamond (PCD) films and composites [4,5,6] can be used in heat sinks [7,8,9,10], hard coatings for cutting tools [11,12,13] and protective layer applications [14,15,16]. Based on the average size of diamond grains, polycrystalline diamond materials are classified as microcrystalline (MCD), nanocrystalline (NCD) and ultrananocrystalline (UNCD) [17,18,19,20]. MCD diamond consists of micrometer-sized grains with good crystalline perfection and a low content of the non-diamond phase in the intergranular space. Films and thick plates of high-quality microcrystalline diamond have a high thermal conductivity and low optical absorption. Therefore they are preferred for various heat sinks [21,22,23] and optical elements [24,25,26,27]. Nanocrystalline diamond consists of diamond grains 10–100 nm in size and is characterized by a reduced crystalline quality and a higher concentration of non-diamond phases compared to MCD diamond [28,29]. However, such films usually have low roughness and a low friction coefficient, combined with a relatively high hardness, and therefore are used in the formation of wear-resistant cutting tools; for example, based on tungsten carbide or ceramics [30,31]. UNCD coatings consist of diamond grains <10 nm and have the highest sp²/sp³ rate out of all diamond materials [32,33]. Such UNCD films are widely used as biocompatible protective coatings [34,35], while electrically conductive nitrogenated UNCD films are used as electrodes [36,37,38]. Specific design formation of PCD material structure well-adapted to one’s need requires using of precise and flexible methods of diamond synthesis.

In the most commonly used method, chemical vapor deposition (CVD), the structure type of polycrystalline diamond film, is determined by the intensity of the secondary nucleation process: the formation of new nanometer-sized diamond grains on top of already-formed ones [28,29,39,40]. It is well known that secondary nucleation processes can be stimulated by small additions of nitrogen [28,41]. Using cyclic small additions of nitrogen, it is possible to reduce the relative roughness of diamond coatings by more than three times and, at the same time, maintain the overall high quality and phase purity of the material [42]. Another main factor affecting this process is the content of methane in the reaction gas mixture: high concentrations stimulate secondary nucleation [43,44,45,46] and lead to the formation of NCD films. The majority of papers on high-quality MCD diamond use methane concentrations below 10% [9,47], whereas less numerous research studies on NCD diamond usually use the [CH₄] range of 10%–20% [17,20,48]. Thus, hereinafter, we will call the region of the methane concentration above 20% ultrahigh concentrations. In our recent work [49], we found that it is possible to grow decent-quality MCD films in microwave plasma, even at such ultrahigh methane concentrations of up to 40%. In this case, a more flexible variation in the synthesis conditions is possible for the formation of layered MCD–NCD films, as well as multilayer diamond coatings [42]. An interesting development of previous work is a more detailed study of synthesis regimes at ultrahigh methane concentrations. Thus, by varying the substrate temperature and methane concentration in microwave plasma, the activation or suppression of secondary diamond nucleation is possible. So far, this effect remains unexplored in the range of high methane concentrations. Experimental data on the conditions for switching between the modes of synthesis of micro- (MCD) and nanocrystalline (NCD) diamond will allow for the synthesis of PCD layers at high methane concentrations with improved growth rates but without the critical degradation of their microcrystalline structure due to the unintentional activation of secondary nucleation mechanisms.

Although the effect of the methane concentration on the process of CVD synthesis was approached on multiple occasions (e.g., [45,46,48,50,51,52]), the CVD diamond growth region of ultrahigh methane concentration (up to pure methane gas mixtures) was not investigated in detail for such a wide temperature range. Thus, in this study, we demonstrate the effect of the methane-to-total-flow ratio ${\nu }_c$ (equivalent volume methane concentration) in the range of 4%–100% and substrate temperature ${\mathrm{T}}_{\mathrm{s}}$ (700–1050 °C) on the morphology and phase composition of PCD films.

2. Materials and Methods

Polished single-crystal (100) silicon wafers with dimensions of 10 × 10 × 0.35 mm³ were used as the initial substrates. Substrates were seeded using a suspension of nanodiamond particles (particle sizes 3–7 nm, Zeta potential >+50mV, Cardiff University) [53,54] by spin coating the wafers at 3000 rpm. Synthesis of diamond films was carried out on a microwave plasma CVD reactor ARDIS 100 (2.45 GHz, 5 kW, Optosystems Ltd., Moscow, Russia) [9,49]. All samples were grown using methane and hydrogen gas mixture with a fixed total gas flow at 500 sccm. In this work, the concentration of methane was calculated as the ratio between methane gas flow and the total gas flow, or ${\nu }_c$ = [CH₄]/([H₂] + [CH₄]). Thus, ${\nu }_c$ = 100% means pure methane and ${\nu }_c$ = 50% means gas mixture with 250 sccm CH₄ and 250 sccm H₂. The film thickness was controlled directly during synthesis by a laser interferometer [9] and the final thickness of all PCD films was 2 μm for all samples. The growth rates are given only for diamond films. The total deposition time varied between 23 min and 3 h 33 min for different samples. Substrate temperatures ${\mathrm{T}}_{\mathrm{s}}$ were measured with the two-color pyrometer METIS M322 (SensorTherm GmbH, Steinbach, Germany) with accuracy ±25 °C.

This paper investigates three series of experimental samples:

1.

In the first one, polycrystalline diamond films were grown at a fixed ${\nu }_c$ = 40% (an order of magnitude higher than the standard one) and various substrate temperatures ${\mathrm{T}}_{\mathrm{s}}$ (700–1050 °C). ${\mathrm{T}}_{\mathrm{s}}$ was adjusted by adjusting the pressure (54–86 Torr) and microwave power (3.5–5 kW). Higher power and pressure correspond to higher substrate temperature.

2.

In the second one, growths were carried out at a fixed ${\mathrm{T}}_{\mathrm{s}}$ = 800 °C and various ${\nu }_c$. Constant temperatures were maintained by pressure and microwave power (with increase in ${\nu }_c$ from 4% to 100%, the pressure and microwave power decreases 73 → 45 Torr and 4.5 → 2.2 kW, respectively, because higher ${\nu }_c$ value leads to a higher substrate temperature.

3.

In the third one, substrate temperature and microwave power were fixed at 900 °C and 4 kW while methane concentrations varied ${\nu }_c$ = 3 → 100% only by pressure (80 → 45 Torr).

To remove the sp² phase of carbon, the samples were annealed in a laboratory-made electric heater at atmospheric pressure and a temperature of 590 °C for 10 h. The surface morphology of the synthesized films were examined with the scanning electron microscope (SEM, Tescan, Brno, Czech Republic) Tescan MIRA3. The phase composition of the films was analyzed at room temperature with micro-Raman spectroscopy using the LABRAM HR-800 spectrometer equipped with a diode-pumped solid-state laser (λ_exc = 473 nm). The spectrometer operated in a confocal mode, while the laser beam was focused in a spot of ≤1 μm in diameter on the sample surface. The Raman spectra were recorded on each sample from three random points. The variance of values between different points did not exceed 5%; therefore, below, the average values of the intensities are shown. X-ray diffraction (XRD) measurements were performed with the Bruker D8 Discover A25 DaVinsi Design instrument (Bruker, Karlsruhe, Germany, CuKα radiation at 40 kV voltage, focus spot size of 0.4 × 12 mm²) in Bragge–Brentano geometry in the 2θ angle range of 20°–80° with a step size of 0.02° and exposition time of 1.2 s for a single step. The total exposure time for one sample was increased to 90 min to enhance the signal-to-noise ratio: thin carbon films weakly scatter x-rays. The XRD spectra were processed with the EVA v.2.1 software and analyzed using the PDF-2 database. The XRD results were quantified by defining the texture coefficient $T_{c\left(hkl\right)}$. This factor can be calculated for each orientation using the following equation [55,56]:

$$T_{c\left(hkl\right)}=\frac{I_{\left(hkl\right)}∕I_{0\left(hkl\right)}}{\left(1∕N\right)·{{\displaystyle \sum }}_N{I}_{\left(hkl\right)}∕I_{0\left(hkl\right)}},$$

where $T_{c\left(hkl\right)}$ is the texture coefficient of the hkl plane, $I_{\left(hkl\right)}$ is the measured intensity, $I_{0\left(hkl\right)}$ is the relative intensity of the corresponding plane given in PDF-2 data (for randomly oriented crystallites) and N is the number of reflections.

3. Results and Discussion

3.1. SEM Characterization

Figure 1 shows the surface morphologies of the diamond films deposited at fixed ${\nu }_c$ = 40% and different ${\mathrm{T}}_{\mathrm{s}}$ (700–1050 °C) before and after air annealing.

At the fixed (40%) methane concentration, an increase in temperature from 700 to 750 °C firstly leads to a change in the faceting of diamond crystallites, which is evidence of the effect of CVD growth conditions on the texturing of PCD film. With a further increase in ${\mathrm{T}}_{\mathrm{s}}$ up to 850 °C, the film morphology transforms to nanocrystalline (NCD) diamond. In addition, at higher temperatures, only the deposition of graphite sheets occurs. Evidence of the more defective structure of PCD films grown at ${\nu }_c$ = 40% is their weakness to the annealing in air at a reasonable 590 °C. While the high-quality PCD films are usually stable in such conditions, for the PCD films grown at 40%, even the most well-faceted grains tend to be covered in etch pits—note in Figure 1 the degradation of diamond grains after annealing.

Figure 2 shows the surface morphologies of the diamond films deposited at two fixed substrate temperatures—${\mathrm{T}}_{\mathrm{s}}$ = 800 ± 25 °C and ${\mathrm{T}}_{\mathrm{s}}$ = 900 ± 25 °C—and different ${\nu }_c$.

Increasing the methane concentration leads to the MCD-to-NCD transition. However, the critical methane-concentration-to-NCD transition depends on the substrate temperature. The MCD-to-NCD transition occurs earlier at higher substrate temperatures. Thus, at ${\mathrm{T}}_{\mathrm{s}}$ = 900 °C, the critical point is in the region of ${\nu }_c$ = 20%, and, at ${\mathrm{T}}_{\mathrm{s}}$ = 800 °C, it shifts to the region of ${\nu }_c$ = 70%. Note that the synthesis regime at 900 °C and ${\nu }_c$ = 43% (Figure 2n) corresponds to the formation of an NCD film, while, in the first experimental series at the same temperature (Figure 1e), the NCD-to-graphite transition has already occurred. We explain this by the fact that, in the first series, the NCD-to-MCD transition occurred just between samples 850 and 900 °C (Figure 1d,e), i.e., 850–900 °C is the threshold temperature at ${\nu }_c$ = 40%. Thus, we consider the conditions of ${\mathrm{T}}_{\mathrm{s}}$ = 900 °C and ${\nu }_c$ = 40% to be a borderline between NCD and MCD regimes, so even slight deviations may lead to a change in the type of grown material. This means that there is no disagreement, taking into account the error in temperature control. In addition, note that, at 800 °C, even in pure methane gas (methane concentration of 100%), the deposition of NCD diamond films is possible.

3.2. X-ray Diffraction

The third series of experimental samples (grown at a fixed ${\mathrm{T}}_{\mathrm{s}}$ = 900 °C) was chosen for XRD analysis because it consists of texture PCD films and shows a transition from MCD to NCD. The X-ray diffraction patterns recorded in the standard θ/2θ scanning mode had a wide range of peaks caused by the decomposition of the X-ray tube spectrum on a single-crystal Si (100) substrate. A scanning mode was chosen with a deviation in the substrate plane from the position 2θ/2 by 3°, which makes it possible to eliminate the reflection from the substrate. The XRD pattern for the PCD films synthesized at substrate temperatures ${\mathrm{T}}_{\mathrm{s}}$ = 900 ± 25 °C and different CH₄ concentration ${\nu }_c$ (Figure 3a) demonstrates four reflexes for diamond from crystallographic planes (111) at 2θ = 43.93°, (220) at 75.30°, (311) at 91.49°, (400) at 119.50° and (331) at 140.56°, and a wide set of peaks caused by the decomposition of the X-ray tube spectrum on a single-crystal Si (100) substrate in the region 2θ~69°.

The values of the texture coefficient $T_{c\left(hkl\right)}$ were calculated. The effect of methane concentration on the texture coefficients $T_{c\left(hkl\right)}$ along the (111), (220), (311), (400) and (331) planes is shown in Figure 3b. At standard methane concentration ${\nu }_c$ = 3% (111), (220) and (311) planes have the same $T_{c\left(hkl\right)}$ value, which is higher than (400) and (331). With an increase in ${\nu }_c$ up to 22%, a sharp decrease in $T_{c\left(111\right)}$ is observed. Note the simultaneous change in the film structure at the same concentration (Figure 3). With a further increase in ${\nu }_c$, an increase in the reflection (220) is observed due to a further decrease in the reflection (111). Thus, films obtained at an ultrahigh methane concentration have a 220 texture.

3.3. Growth Rates

It is well known that the growth rate of diamond increases with the substrate temperature. We confirm an almost linear increase in the growth rate from 0.8 to 4.6 µm/h with ${\mathrm{T}}_{\mathrm{s}}$ elevation for the films deposited at ${\nu }_c$ = 40% (Figure 4a).

Despite the transition from MCD to NCD for the samples grown at an ultrahigh methane concentration, a decrease in the growth rate is observed (Figure 4b). For samples grown at ${\mathrm{T}}_{\mathrm{s}}$ = 800 °C, the growth rate increases from 0.6 µm/h at ${\nu }_c$ = 4% to 3.41 µm/h at ${\nu }_c$ = 20% and then decreases again to 0.6 µm/h at ${\nu }_c$ = 100%. For samples grown at ${\mathrm{T}}_{\mathrm{s}}$ = 900 °C, the same trend is observed, but with much higher growth rates and a shift in the critical point towards higher methane concentrations. The growth rate increases from 2.8 µm/h at ${\nu }_c$ = 3% to 4.1 µm/h at ${\nu }_c$ = 40% and then decreases to 2.1 µm/h at ${\nu }_c$ = 100%.

3.4. Raman Spectroscopy

The Raman spectra for the films deposited at fixed ${\nu }_c$ = 40% and different ${\mathrm{T}}_{\mathrm{s}}$ (700–1050 °C) before and after air anneal are shown in Figure 5.

The spectra reveal the following features common for polycrystalline diamond films: the arrow diamond peak at 1333 cm⁻¹, two wide bands from trans-polyacetylene (t-PA) at 1140 and 1480 cm⁻¹ and D- and G-peaks at 1350 and 1580 cm⁻¹, respectively, from graphitic carbon. At fixed ${\nu }_c$ = 40% and substrate temperatures above ${\mathrm{T}}_{\mathrm{s}}$ = 900 °C, graphite wins in the competitive coprecipitation of diamond and graphite. Annealing in the air for 24 h at 590 °C leads to the complete removal of the sp² phase of carbon, as confirmed by empty Raman spectra for these samples. Thus, at high methane concentrations, low synthesis temperatures promote diamond growth. At a temperature of 800 °C, the growth of the PCD diamond is possible at any concentration of methane from 4 to 100%. At a temperature of 900 °C, with an increase in the concentration of methane, the portion of the sp³ carbon phase to the sp² phase decreases. Samples grown at 900 °C and ${\nu }_c$ = 29% and higher could be referred to as NCD diamond films.

The CVD conditions with ${\nu }_c$ = 40% and ${\mathrm{T}}_{\mathrm{s}}$ = 850 °C (sample in Figure 5a) are near the threshold at which diamond growth is no longer possible. At the same time, the sample obtained at 43% and 900 °C (Figure 3d) is NCD diamond. Given the accuracy of the temperature measurements, this is a normal observation for a threshold value. At a threshold temperature of 900 °C, an increase in the methane concentration above 43% did not lead to the formation of graphite. Thus, in this case, lower temperatures facilitated preserving the growth of the diamond phase instead of the formation of the continuous graphitic film.

3.5. sp²/sp³ Ratio

The sp²/sp³ ratio, expressed as the ratio of the sum of D- and G-peaks’ integrated intensities (areas under the bands) to that for the diamond Raman peak, is usually used to determine the quality of PCD films [57,58] and is shown in Figure 6. For this analysis, the diamond line at 1333 cm⁻¹ was approximated by a Lorentzian profile, and the G- and D-peaks by two Gaussians.

An increase in the substrate temperature or methane concentration leads to a decrease in the quality of the diamond. However, while, for 900 °C, this is an exponential trend, for 800 °C, it is almost linear. In addition, note an order of magnitude difference in the sp²/sp³ ratio between Figure 6a,b. Thus, at ultrahigh methane concentrations, temperature has more of a significance than methane concentration.

3.6. Diamond Raman Peak width

The dependence of the diamond Raman peak width on substrate temperatures and methane concentrations is shown in Figure 7. For this analysis, Raman spectra were recorded in a narrow frequency range and the diamond peak at 1333 cm⁻¹ was approximated by a Lorentzian profile.

An increase in the substrate temperature or methane concentration leads to a broadening of the diamond Raman peak. The full width on half magnitude (FWHM) increases from 5.9 cm⁻¹ (${\mathrm{T}}_{\mathrm{s}}$ = 700 °C) to 8.7 cm⁻¹ $({\mathrm{T}}_{\mathrm{s}}$ = 850 °C) for films grown at fixed ${\nu }_c$ = 40%. The FWHM increases from 4.7 cm⁻¹ (${\nu }_c$ = 4%) to 12.2 cm⁻¹ (${\nu }_c$ = 100%) for films grown at fixed ${\mathrm{T}}_{\mathrm{s}}$ = 800 °C. The FWHM increases from 5.4 cm⁻¹ (${\nu }_c$ = 3%) to 11.9 cm⁻¹ (${\nu }_c$ = 58%) for films grown at fixed ${\mathrm{T}}_{\mathrm{s}}$ = 900 °C. The broadening of the diamond peak may be associated with an increase in the number of microdeformations and defects in films grown at high methane concentrations and temperatures. The more defective structure of films grown at high temperatures is also confirmed by the reduced annealing resistance (see Figure 1i–l).

4. Conclusions

In our work, we demonstrated that diamond growth is possible even at ultrahigh (>20%) methane concentrations, including pure methane gas mixtures, which requires relatively low synthesis temperatures of ${\mathrm{T}}_{\mathrm{s}}$ < 800 °C to obtain MCD coatings. In general, low substrate temperatures facilitate the formation of higher-quality PCD films; however, this comes at the cost of lower growth rates. The PCD films grown at ultrahigh methane concentrations have a lower resistance to annealing in the air in comparison to higher-quality PCD films grown at low ${\nu }_c$, which is evidence of a difference in the concentration of internal structural defects. Despite the possibility of growing diamond films at methane concentrations up to 100%, the growth rate of PCD coatings has a non-linear trend: for samples grown at ${\mathrm{T}}_{\mathrm{s}}$ = 800 °C, the growth rate increases from 0.6 µm/h at ${\nu }_c$ = 4% to 3.4 µm/h at ${\nu }_c$ = 20% and then decreases again to 0.6 µm/h at ${\nu }_c$ = 100%. With an increase in ${\nu }_c$, an increase in the XRD reflection (220) is observed due to a decrease in the reflection (111). An increase in the substrate temperature or methane concentration leads to a broadening of the diamond Raman peak and an increase in the sp²/sp³ ratio of diamond films. The manipulation of the methane concentration and temperature has a complex and combined effect on the morphology, growth rate and phase composition of polycrystalline diamond films. The obtained results play an important role in the task of the control over the nature of the CVD-grown PCD material, which is essential for the precise and flexible production of diamond for various applications.