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Growth of nanocrystalline diamond by dual radio frequency inductively coupled plasma jet CVD
Diamond & Related Materials 73 (2017) 67–71
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Growth of nanocrystalline diamond by dual radio frequency inductively coupled plasma jet CVD Yong-gang Zuo, Jia-jun Li, Yang Bai, Hao Liu, He-wei Yuan, Guang-chao Chen ⁎ College of Materials Sciences and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 28 June 2016 Received in revised form 28 November 2016 Accepted 7 December 2016 Available online 16 December 2016 Keywords: Dual RF inductively coupled plasma jet Diamond Nanocrystalline CVD
a b s t r a c t A dual RF inductively coupled plasma jet without the sheath gas was designed to grow diamond. The used radio frequencies were 13.56 MHz and 4 MHz, respectively. In this tandem RF system, the high electron temperature (~2.48 eV) and the high electron density (~6.65 × 1021 m−3) were found by analyzing the gas phase diagnosis results of optical emission spectra (OES). Nanocrystalline diamond films were deposited with the typical cauliflower-like morphology and the individual nodules morphology observed by the field emission electron microscopy (SEM). The surface roughness and crystalline phase of samples were measured by Atomic force microscopy (AFM) and X-ray diffraction (XRD). The Williamson-Hall (W-H) analysis was used to study the individual contribution of crystallite sizes and lattice strain on the peak broad of diamond, inferring that probably presence of stress and defects in nanodiamond. Raman spectra with an intense broad band near 1140 cm−1, confirmed the presence of the nanocrystalline diamond phase with few of sp2 carbon. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nanocrystalline diamond films (NCD), which retain to a large extent the extreme properties of polycrystalline diamond films but overcome their surface roughness disadvantages, have therefore been recognized as the promising candidates for those applications, such as filters, laser windows and field emission display [1,2]. Recently, single photon emission has been fabricated from color centers of nanodiamond [3]. So far, several deposition methods have succeeded in depositing NCD, such as the microwave plasma CVD (MW) [4], hot filament CVD (HF) [5] and d.c. arcjet [6]. Radio frequency inductively coupled plasma (RF-ICP), known as clean plasma free from contamination, has been widely applied into synthesis of ultra-fine powders and nanoparticles [7–9]. Actually, it is in 1987 that conventional one stage RF-ICP was first reported to be utilized to deposit micro- polycrystalline diamond [10]. Just 6 years later, it achieved 30 μm/h of the growth rate and 100 mm of the diameter in size [11]. In 2002, the growth rate increased up to 70 μm/h [12]. The RF-ICP also achieved the successful deposition of NCD [13,14]. In these researches, RF-ICP showed the feature of high plasma density about 1018 m−3 [15], and large volumetric growth rate rather than HF and MW [16].
⁎ Corresponding author. E-mail addresses: [email protected] (Y. Zuo), [email protected] (J. Li), [email protected] (Y. Bai), [email protected] (H. Liu), [email protected] (H. Yuan), [email protected] (G. Chen).
http://dx.doi.org/10.1016/j.diamond.2016.12.006 0925-9635/© 2016 Elsevier B.V. All rights reserved.
However, there still exist some drawbacks in this conventional one stage RF-ICP. Firstly, the “hollow plasma” in annular discharge zone caused by skin effect easily results in the non-diamond composition [17]. Secondly, “sheath gas” for heating-controlling in RF results in huge gas consumption and increase of costs [18]. Thirdly, “ion bombardment” caused by self-bias voltage of plasma damages the diamond growth [19]. Finally, the “recirculation eddy” in conventional RF seriously affects the stability of plasma [20]. Overcoming of these problems is doubtless to promote the application of RF-ICP in low-costed growth of high-quality diamond. RF-RF hybrid plasma jet, also called as dual RF or tandem RF, was proposed to synthesize powders in 1986 [21]. It offers a high velocity and enthalpy plasma as well as precise control of plasma [22,23]. However, this kind of plasma source has been not applied in diamond deposition yet. In this study, a dual RF-ICP plasma jet source consisting of a RF (13.56 MHz) assisted by RF (4 MHz) is proposed to try to overcome the problems of the conventional one stage RF-ICP mentioned above, and perform the expected advances of ICP in diamond CVD. To the best of our knowledge, this is the first report about the growth of NCD films by using this dual RF-ICP jet CVD. 2. Experiment A self-made dual RF-ICP jet CVD system is schematically illustrated in Fig. 1. It mainly consists of a plasma generator and a stainless-steel growth chamber. The plasma generator is composed of two watercooling quartz plasma confinement tubes with different internal diameters (6 mm and 35 mm, respectively). Outside of the quartz plasma
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Y. Zuo et al. / Diamond & Related Materials 73 (2017) 67–71
Fig. 1. Schematic of the experimental setup, showing the detect location of optical fiber of OES.
confinement tubes are two independent induction coils of three-turn and nine-turn corresponding to high frequency (HF) 13.56 MHz and low frequency (LF) 4 MHz, respectively. The HF power is continuously supplied from a 6 kW generator through an impedance marching network. The LF power (~10 kW) is to maintain and heat upstream plasma ignited by HF discharge. This design of cascading of two induction coils aims at producing a high enthalpy plasma jet with a more uniform temperature and increasing energy efficiency of RF [24]. Meanwhile, the “recirculation eddy” found in conventional one stage RF plasma is improved due to HF discharge, which enhances the steady of plasma [25]. In the growth chamber, a quartz sample holder is placed 35 mm far away from plasma generator exit. A grounding conductor is set axially behind the sample holder. The background pressure of the growth chamber is usually pumped up to 2.8 × 10−2 Pa. The working feed gasses are the mixture of Ar\H2\CH4, and their flow rates are controlled by the mass-flow controllers. The sheath gas, usually utilized in conventional RF-ICP jet, is not used in this setup. The working feed gasses axially pass through the plasma generator along the centerline of the discharge, and form a plasma jet, which then expands into the growth chamber striking the grounded conductor. The mirror-polished p-type silicon (100) wafers of 8 mm diameter and 0.5 mm thickness are used as the substrate. Prior to the deposition, the Si substrates are ultrasonically cleaned in an acetone bath and subsequently scratched with diamond powder (5 μm) diluted in ethanol for 30 min, followed by a rinse in ethanol and distilled water to remove excess residues. The substrate is mounted on the quartz sample holder and immersed in the plasma, which effectively avoid the “ion bombardment” against substrate due to insulation of quartz holder. The substrates are plasma heating ones, of which the temperature is in the range of 1000-1100 K depending on the input power and working feed gasses flow rate as well as the chamber working pressure. Detailed deposition conditions of diamond films were listed in Table 1. As-grown deposits were characterized for structure, surface morphology and quality by X-ray diffraction (XRD, Rigaku Ultima IV), field emission scanning electron microscopy (SEM, KYKY-8000F), Atomic force microscopy (AFM, Micronano AFM-III) and Raman spectroscopy (inVia-Reflex).
Horizontal spatially resolved OES measurements were carried out positioning the optical quartz fiber on a movable support along the plasma generator's axis. As the settlement of grounding conductor blocking, the optical emission from different radial position was not done. Optical emission from the detected region (substrate center) was monitored through a quartz window using an adjustable lens/slit system with an optical fiber and detected by a 7ISU301 monochromator (1200 grooves/mm grating and focal length of 300 mm) with a R928 photomultiplier. The entrance slit width is 0.01 mm, which gave a wavelength resolution of 0.10 nm. 3. Results and discussion 3.1. Plasma diagnosis by OES The dependence of the activated species in the plasma and the composition of the working feed gasses are analyzed by the OES in the range of 400–850 nm. The typical OES result is shown in Fig. 2. It can be seen that the major optical emission lines are from atomic Ar (the range of 696.5 to 850 nm), H atom (Hα @656.3 nm, Hβ @486.1 nm) as well as hydrocarbon related radicals (CH @431.5 nm, CH+ @422.1 and 417.1 nm, C2 @Swan band 474.7, 516.1, 563.6 and 619.2 nm). Similar OES can be seen in other experimental cases [26]. The intensity ratio of Hβ, CH and C2 to Hα versus methane concentration is plotted in Fig. 3. It can be found that the intensity of optical emission from hydrocarbon radicals and C2 radicals become strong obviously with the increase of the concentration of methane. Among them, the intensity of C2 emission increases very faster than that of other radicals, which facilitates the formation of nano-diamond clusters in rich Ar
Table 1 Deposition parameters of nanocrystalline diamond films in this study. Parameter
Value
Argon flow rate Hydrogen flow rate The ratio of CH4 to H2 Temperature of the substrates Chamber pressure Deposition time Plasma power
4.5 slm 0.5 slm 2%–10% 1000–1100 K 6000 Pa 6–17 h 1500 W (HF) + 5000 W (LF)
Fig. 2. Typical optical emission spectrum with methane concentrations of 3%.
Y. Zuo et al. / Diamond & Related Materials 73 (2017) 67–71
Fig. 3. The optical emission intensity ratios of C2, CH, Hβ to Hα (left-hand axis) and CH to C2 (right-hand axis) as a function of the methane concentration.
plasma [27]. Although May [28] further believe that C1 species (including CH3) play a key role in renucleation rather than C2 species through model calculation, the amount of CH3 cannot be evaluated in our experiments due to no optical emission of CH3 being detected by OES. The ratio of Hβ/Hα often works as an indication of the electron mean temperature in plasma [29]. As the value of Hβ/Hα increases slightly with the increase of the concentration of methane, it indicates that electron temperature is influenced slightly by the flow of methane. This optical emission feature is almost as same as that in DC Arcjet plasma [30]. By assuming that the plasma is in a partial local thermal equilibrium (PLTE), the electron mean temperature Te is estimated using the Boltzmann plots [31], viz.: Iji λkl Aji g j Ek −E j ¼ exp Ikl λji Akl g k kB T e
ð1Þ
where Iji and λji are the intensity and wavelength of the spectral line corresponding to the transition between Ej and Ei energy level; kB is the Boltzmann constant; Aji and gj are the corresponding Einstein transition probability and statistical weight, which can be acquired from the literature [32]. The Stark broadening of the hydrogen Balmer Hβ line is applied to measure the electron density (ne). In our case for the cold plasma, the relevant sources of broadening necessary to consider include the Stark effect (ΔλStark) and Van der Waals broadening (ΔλWaals). The Doppler broadening is so weak that be neglected in cold plasma. Here, we assumed that the gas temperature (Tg) is room temperature, and then the ΔλWaals of Hβ is 0.0332 nm. Therefore, the FWHM of ΔλS was obtained by subtracting the ΔλWaals part from the total Lorentz broadening (ΔλL). Special equations [33] are following: ΔλStark ¼ 2 10−11 ðne Þ2=3
ð2Þ
ΔλL ¼ ΔλStark þ ΔλWaals
ð3Þ
ΔλWaals ¼
1:8 T1:8 g
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accords well to that of Hβ/Hα. The ne in our plasma is one order of magnitude higher than that of the conventional one stage RF-ICP (4 MHz) in atmospheric pressure [34]. Fig. 4 shows spatially resolved OES ratios as a function of the horizontal distance from the substrate surface center to the outlet of HF discharge. It can be seen that the emission line intensity of CH slightly increases with the distance increase and then decreases when reaches to 5 cm. However, the ratio of CH/C2 has opposite tendency and minimum value appears in 10 cm. The value of Hβ/Hα changes little and almost remains constant. It can be deduced that there is slightly change for the plasma composition, density and electron temperature in plasma channel with 15 cm length. According to the calculation Eqs. (1)–(4), the electron temperature is in the range of 2.32–2.48 eV while ne is in the range of 6.55– 5.30 × 1021 m−3 corresponding to distances of 0–150 mm. The gradient of electron density (ne) is approximately 8.2 × 1018 m− 3/mm. This means that this type of dual RF-ICP jet can generate rather long and uniform high temperature tail jet region. 3.2. Characterization of diamond film Fig. 5 exhibits surface and cross-sectional morphology of as-deposited films with the methane concentrations of 5% and 3%, respectively. It can be seen that a typical cauliflower-like morphology exists in Fig. 5(a), whilst the individual nodules morphology with an average diameter of 3 μm appears in Fig. 5(d). Both the cauliflower-like morphology and the individual nodules morphology consist of large quantities of tiny grains, proven by magnification pictures in Fig. 5(b) and (e). It is well known that the cauliflower-like morphology consists of nanocrystalline grains in the film [35]. Therefore, grains in the as-grown films are in the nanometer scale. Fig. 5(c) and (f) are the images of the cross sections of the films. Therefore, the growth rates are about 1 μm/h in both cases. The surface roughness of the films was measured using AFM in a 7 × 7 μm2 scanning area. The typical AFM images of the diamond films are presented in Fig. 6. The root mean square (Rm) and average (Ra) values of the surface roughness of the film in 5% are 57.1 and 45.2 nm, respectively. The Rm and Ra values for the film in 3% are observed to be 79.2 and 63.5 nm, respectively. The XRD patterns of as-deposited films in Fig. 5 are shown in Fig. 7. It is showed that three diamond diffraction peaks at (111), (220) and (311) could be identified, in addition to those of Si and SiC phases from substrate. There is no evidence for the presence of graphite related diffraction peaks in the XRD patterns of all samples. The intensity ratio of (311)/(111) is very weaker than that (16/100) of randomly oriented diamond powder, meaning that probable presence of highly damaged diamond crystallites. The increasing of methane concentration leads to
ð4Þ
where ΔλStark, ΔλWaals and ΔλL are the Stark broadening, Van der Waals broadening and Lorentz broadening of Hβ in terms of the full-width at half-maximum (FWHM), respectively. Tg is the gas temperature. From the results in Fig. 3, the ne and Te of the plasma can be calculated to be about 2.18 eV and 5.45 × 1021 m3 for 2%, 2.23 eV and 5.06 × 1021 m3 for 3%, 2.32 eV and 5.59 × 1021 m3 for 5%, 2.48 eV and 5.47 × 1021 m3 for 10%, respectively. So, the change tendency of Te
Fig. 4. Optical emission intensity of CH to Hα and C2 (left-hand axis) and Hβ to Hα (righthand axis) as a function of different distances.
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Fig. 7. XRD patterns of the NCD samples deposited with methane concentration of 5% and 3%, respectively. The diamond (111), (220), (311) peaks, the silicon and silicon carbide peaks are marked. The line broadening is analyzed using the Williamson-Hall plot (inset).
Fig. 5. Surface and cross-sectional SEM morphology of as-grown films (a) (b) (c) methane concentration of 5% at 17 h; (d) (e) (f) methane concentration of 3% at 6 h. The (b) and (e) are the corresponding high magnification image, respectively.
broaden of diamond peaks of samples. The broadening effect can be caused by either grain size effect or substantial lattice strains, or both [36]. By measuring the broadening of the diffraction peak using the Williamson-Hall (W-H) analysis [37] (inset) one can estimate the crystal size (L) as well as potential lattice strains (ε). Both contribute to an increase in the linewidth β given by β = βL + βε, with βL = Kλ / Lcosθ and βε = 4εtanθ, where K is a constant reflecting the particle shape (K = 1 for spherical particles), θ is the scatting angle and λ is the wavelength of the X-ray radiation (λ = 0.154 nm). Combining the above equations we obtain: β cosθ 1 4ε sinθ ¼ þ λ L λ
shrinkage, indicating the presence of compressive stress in samples. The average crystal size (L) is estimated be 14.95 nm and 12.85 nm with 3% and 5% methane concentration, respectively. This calculated results are obviously smaller than one directly observed from SEM images, probably due to presence of defects (e.g. lattice dislocation) in the nanodiamond. For another, compressive stress in sample destroys the diamond crystal grains, and maybe result in tinier grains formed, which is not neglected. Raman analysis was performed to evaluate the quality of NCD. Fig. 8 shows the Raman spectra of these two samples. Both spectrum show an intense and broad peak at 1334 cm−1, which is typical Raman peak of diamond. In addition, there exists several extra broad bands near 1139 cm−1 and ~1440 cm−1 in the spectra related to trans-polyacetylene (TPA), which can evidence the presence of NCD [38]. The peaks corresponding to graphitic carbon, D-peak (at 1351 cm−1 and 1357 cm−1) and G-peak (1570 cm−1 and 1584 cm−1), are also found in both spectrum. As the cross-section of Raman scattering of graphitic carbon is usually 50–60 times higher than that of diamond, it can be estimated that small amount of graphitic carbon may exist between nano-diamond grains in the films. On the other hand, it can be found that the diamond peak (1334 cm−1) position from samples have same shifts relative to one (1332 cm−1) taken from type IIb natural diamond. This indicates that there is approximately same compressive stress in both films, which is consistent with above calculation of the slope from W-H plot. 4. Conclusions
Plotting βcosθ/λ vs. 2sinθ/λ gives a straight line with the slope 2ε and an intercept of 1/L, known as Williamson-Hall plot. Accordingly, lattice strains (ε) are negative values, which arise from the lattice
A dual RF inductively coupled plasma jet is designed to grow diamond. The electromagnetic energy was input to ignite and sustain the
Fig. 6. Three-dimensional AFM images scanned in an area 7 × 7 μm2 of NCD films with methane concentration of (a) 5% and (b) 3%.
Y. Zuo et al. / Diamond & Related Materials 73 (2017) 67–71
Fig. 8. Raman spectra of diamond films with methane concentration of 5% and 3%, respectively.
Ar/H2/CH4 plasma by two tandem coils with the radio frequencies of 13.56 MHz and 4 MHz, respectively. Water-cooling quartz tube was used as the plasma confinement so that the sheath gas was saved. The plasma compositions were diagnosed by OES. CH, C2, Hβ and Hα were the main radicals, of which the optical emission intensity was depending on the concentration of methane in the working feed gasses. Under the condition of high working pressure (~ 6000 Pa), the high electron temperature (~ 2.48 eV) and the high electron density (~6.65 × 1021 m−3) were found by calculation based on the gas phase diagnosis results. Using this plasma jet source, diamond films were deposited on single crystal silicon substrates with the cauliflower-like morphology and individual nodules morphology. Increasing methane is proved to helpful in decreasing surface roughness of the diamond film, but broadening diamond XRD peaks. The nanodiamond may possess compressive residual stress and defects by W-H analysis. Raman spectrum confirmed the presence of the nanocrystalline diamond phase again, but small quantity of amorphous graphite was scatter along the grain boundary.
Acknowledgments This work is supported by NSFC (51272281), Hundred Talents Program of the Chinese Academy of Sciences, Scientific Research Equipment Project of the Chinese Academy of Sciences (yz201356), Hebei Science and Technology Plan (No. 14291110D), Beijing Science and Technology Project (No. Z151100003315024). References [1] T. Sharda, M.M. Rahaman, Y. Nukaya, T. Soga, T. Jimbo, M. Umeno, Structural and optical properties of diamond and nano-diamond films grown by microwave plasma chemical vapor deposition, Diam. Relat. Mater. 10 (2001) 561–567. [2] N. Jiang, K. Eguchi, S. Noguchi, T. Inaoka, Y. Shintani, Structural characteristics and field electron emission properties of nano-diamond/carbon films, J. Cryst. Growth 236 (2002) 577–582. [3] A. Khalid, K. Chung, R. Rajasekharan, D. Lau, T. Karle, B. Gibson, S. Hanic, Lifetime reduction and enhanced emission of single photon color centers in nanodiamond via surrounding refractive index modification, Sci. Rep. 5 (2015) 1–12. [4] C.J. Tang, A.J. Neves, S. Pereira, A.J.S. Fernandes, J. Grácio, M.C. Carmo, Effect of nitrogen and oxygen addition on morphology and texture of diamond films (from polycrystalline to nanocrystalline), Diam. Relat. Mater. 17 (2008) 72–78. [5] E. Hamzah, T.M. Yong, M.A. Mat Yajid, Surface morphology and bond characterization of nanocrystalline diamonds grown on tungsten carbide via hot filament chemical vapor deposition, J. Cryst. Growth 372 (2013) 109–115. [6] G.C. Chen, B. Li, Z.Q. Yan, J. Liu, F.X. Lu, H. Ye, Growth of ultrananocrystalline diamond film by DC Arcjet plasma enhanced chemical vapor deposition, J. Cryst. Growth 349 (2012) 1–5. [7] M.I. Boulos, The Inductively Coupled Radio Frequency Plasma. 1, 1997 17–39.
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Growth of nanocrystalline diamond by dual radio frequency inductively coupled plasma jet CVD Yong-gang Zuo, Jia-jun Li, Yang Bai, Hao Liu, He-wei Yuan, Guang-chao Chen ⁎ College of Materials Sciences and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 28 June 2016 Received in revised form 28 November 2016 Accepted 7 December 2016 Available online 16 December 2016 Keywords: Dual RF inductively coupled plasma jet Diamond Nanocrystalline CVD
a b s t r a c t A dual RF inductively coupled plasma jet without the sheath gas was designed to grow diamond. The used radio frequencies were 13.56 MHz and 4 MHz, respectively. In this tandem RF system, the high electron temperature (~2.48 eV) and the high electron density (~6.65 × 1021 m−3) were found by analyzing the gas phase diagnosis results of optical emission spectra (OES). Nanocrystalline diamond films were deposited with the typical cauliflower-like morphology and the individual nodules morphology observed by the field emission electron microscopy (SEM). The surface roughness and crystalline phase of samples were measured by Atomic force microscopy (AFM) and X-ray diffraction (XRD). The Williamson-Hall (W-H) analysis was used to study the individual contribution of crystallite sizes and lattice strain on the peak broad of diamond, inferring that probably presence of stress and defects in nanodiamond. Raman spectra with an intense broad band near 1140 cm−1, confirmed the presence of the nanocrystalline diamond phase with few of sp2 carbon. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nanocrystalline diamond films (NCD), which retain to a large extent the extreme properties of polycrystalline diamond films but overcome their surface roughness disadvantages, have therefore been recognized as the promising candidates for those applications, such as filters, laser windows and field emission display [1,2]. Recently, single photon emission has been fabricated from color centers of nanodiamond [3]. So far, several deposition methods have succeeded in depositing NCD, such as the microwave plasma CVD (MW) [4], hot filament CVD (HF) [5] and d.c. arcjet [6]. Radio frequency inductively coupled plasma (RF-ICP), known as clean plasma free from contamination, has been widely applied into synthesis of ultra-fine powders and nanoparticles [7–9]. Actually, it is in 1987 that conventional one stage RF-ICP was first reported to be utilized to deposit micro- polycrystalline diamond [10]. Just 6 years later, it achieved 30 μm/h of the growth rate and 100 mm of the diameter in size [11]. In 2002, the growth rate increased up to 70 μm/h [12]. The RF-ICP also achieved the successful deposition of NCD [13,14]. In these researches, RF-ICP showed the feature of high plasma density about 1018 m−3 [15], and large volumetric growth rate rather than HF and MW [16].
⁎ Corresponding author. E-mail addresses: [email protected] (Y. Zuo), [email protected] (J. Li), [email protected] (Y. Bai), [email protected] (H. Liu), [email protected] (H. Yuan), [email protected] (G. Chen).
http://dx.doi.org/10.1016/j.diamond.2016.12.006 0925-9635/© 2016 Elsevier B.V. All rights reserved.
However, there still exist some drawbacks in this conventional one stage RF-ICP. Firstly, the “hollow plasma” in annular discharge zone caused by skin effect easily results in the non-diamond composition [17]. Secondly, “sheath gas” for heating-controlling in RF results in huge gas consumption and increase of costs [18]. Thirdly, “ion bombardment” caused by self-bias voltage of plasma damages the diamond growth [19]. Finally, the “recirculation eddy” in conventional RF seriously affects the stability of plasma [20]. Overcoming of these problems is doubtless to promote the application of RF-ICP in low-costed growth of high-quality diamond. RF-RF hybrid plasma jet, also called as dual RF or tandem RF, was proposed to synthesize powders in 1986 [21]. It offers a high velocity and enthalpy plasma as well as precise control of plasma [22,23]. However, this kind of plasma source has been not applied in diamond deposition yet. In this study, a dual RF-ICP plasma jet source consisting of a RF (13.56 MHz) assisted by RF (4 MHz) is proposed to try to overcome the problems of the conventional one stage RF-ICP mentioned above, and perform the expected advances of ICP in diamond CVD. To the best of our knowledge, this is the first report about the growth of NCD films by using this dual RF-ICP jet CVD. 2. Experiment A self-made dual RF-ICP jet CVD system is schematically illustrated in Fig. 1. It mainly consists of a plasma generator and a stainless-steel growth chamber. The plasma generator is composed of two watercooling quartz plasma confinement tubes with different internal diameters (6 mm and 35 mm, respectively). Outside of the quartz plasma
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Fig. 1. Schematic of the experimental setup, showing the detect location of optical fiber of OES.
confinement tubes are two independent induction coils of three-turn and nine-turn corresponding to high frequency (HF) 13.56 MHz and low frequency (LF) 4 MHz, respectively. The HF power is continuously supplied from a 6 kW generator through an impedance marching network. The LF power (~10 kW) is to maintain and heat upstream plasma ignited by HF discharge. This design of cascading of two induction coils aims at producing a high enthalpy plasma jet with a more uniform temperature and increasing energy efficiency of RF [24]. Meanwhile, the “recirculation eddy” found in conventional one stage RF plasma is improved due to HF discharge, which enhances the steady of plasma [25]. In the growth chamber, a quartz sample holder is placed 35 mm far away from plasma generator exit. A grounding conductor is set axially behind the sample holder. The background pressure of the growth chamber is usually pumped up to 2.8 × 10−2 Pa. The working feed gasses are the mixture of Ar\H2\CH4, and their flow rates are controlled by the mass-flow controllers. The sheath gas, usually utilized in conventional RF-ICP jet, is not used in this setup. The working feed gasses axially pass through the plasma generator along the centerline of the discharge, and form a plasma jet, which then expands into the growth chamber striking the grounded conductor. The mirror-polished p-type silicon (100) wafers of 8 mm diameter and 0.5 mm thickness are used as the substrate. Prior to the deposition, the Si substrates are ultrasonically cleaned in an acetone bath and subsequently scratched with diamond powder (5 μm) diluted in ethanol for 30 min, followed by a rinse in ethanol and distilled water to remove excess residues. The substrate is mounted on the quartz sample holder and immersed in the plasma, which effectively avoid the “ion bombardment” against substrate due to insulation of quartz holder. The substrates are plasma heating ones, of which the temperature is in the range of 1000-1100 K depending on the input power and working feed gasses flow rate as well as the chamber working pressure. Detailed deposition conditions of diamond films were listed in Table 1. As-grown deposits were characterized for structure, surface morphology and quality by X-ray diffraction (XRD, Rigaku Ultima IV), field emission scanning electron microscopy (SEM, KYKY-8000F), Atomic force microscopy (AFM, Micronano AFM-III) and Raman spectroscopy (inVia-Reflex).
Horizontal spatially resolved OES measurements were carried out positioning the optical quartz fiber on a movable support along the plasma generator's axis. As the settlement of grounding conductor blocking, the optical emission from different radial position was not done. Optical emission from the detected region (substrate center) was monitored through a quartz window using an adjustable lens/slit system with an optical fiber and detected by a 7ISU301 monochromator (1200 grooves/mm grating and focal length of 300 mm) with a R928 photomultiplier. The entrance slit width is 0.01 mm, which gave a wavelength resolution of 0.10 nm. 3. Results and discussion 3.1. Plasma diagnosis by OES The dependence of the activated species in the plasma and the composition of the working feed gasses are analyzed by the OES in the range of 400–850 nm. The typical OES result is shown in Fig. 2. It can be seen that the major optical emission lines are from atomic Ar (the range of 696.5 to 850 nm), H atom (Hα @656.3 nm, Hβ @486.1 nm) as well as hydrocarbon related radicals (CH @431.5 nm, CH+ @422.1 and 417.1 nm, C2 @Swan band 474.7, 516.1, 563.6 and 619.2 nm). Similar OES can be seen in other experimental cases [26]. The intensity ratio of Hβ, CH and C2 to Hα versus methane concentration is plotted in Fig. 3. It can be found that the intensity of optical emission from hydrocarbon radicals and C2 radicals become strong obviously with the increase of the concentration of methane. Among them, the intensity of C2 emission increases very faster than that of other radicals, which facilitates the formation of nano-diamond clusters in rich Ar
Table 1 Deposition parameters of nanocrystalline diamond films in this study. Parameter
Value
Argon flow rate Hydrogen flow rate The ratio of CH4 to H2 Temperature of the substrates Chamber pressure Deposition time Plasma power
4.5 slm 0.5 slm 2%–10% 1000–1100 K 6000 Pa 6–17 h 1500 W (HF) + 5000 W (LF)
Fig. 2. Typical optical emission spectrum with methane concentrations of 3%.
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Fig. 3. The optical emission intensity ratios of C2, CH, Hβ to Hα (left-hand axis) and CH to C2 (right-hand axis) as a function of the methane concentration.
plasma [27]. Although May [28] further believe that C1 species (including CH3) play a key role in renucleation rather than C2 species through model calculation, the amount of CH3 cannot be evaluated in our experiments due to no optical emission of CH3 being detected by OES. The ratio of Hβ/Hα often works as an indication of the electron mean temperature in plasma [29]. As the value of Hβ/Hα increases slightly with the increase of the concentration of methane, it indicates that electron temperature is influenced slightly by the flow of methane. This optical emission feature is almost as same as that in DC Arcjet plasma [30]. By assuming that the plasma is in a partial local thermal equilibrium (PLTE), the electron mean temperature Te is estimated using the Boltzmann plots [31], viz.: Iji λkl Aji g j Ek −E j ¼ exp Ikl λji Akl g k kB T e
ð1Þ
where Iji and λji are the intensity and wavelength of the spectral line corresponding to the transition between Ej and Ei energy level; kB is the Boltzmann constant; Aji and gj are the corresponding Einstein transition probability and statistical weight, which can be acquired from the literature [32]. The Stark broadening of the hydrogen Balmer Hβ line is applied to measure the electron density (ne). In our case for the cold plasma, the relevant sources of broadening necessary to consider include the Stark effect (ΔλStark) and Van der Waals broadening (ΔλWaals). The Doppler broadening is so weak that be neglected in cold plasma. Here, we assumed that the gas temperature (Tg) is room temperature, and then the ΔλWaals of Hβ is 0.0332 nm. Therefore, the FWHM of ΔλS was obtained by subtracting the ΔλWaals part from the total Lorentz broadening (ΔλL). Special equations [33] are following: ΔλStark ¼ 2 10−11 ðne Þ2=3
ð2Þ
ΔλL ¼ ΔλStark þ ΔλWaals
ð3Þ
ΔλWaals ¼
1:8 T1:8 g
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accords well to that of Hβ/Hα. The ne in our plasma is one order of magnitude higher than that of the conventional one stage RF-ICP (4 MHz) in atmospheric pressure [34]. Fig. 4 shows spatially resolved OES ratios as a function of the horizontal distance from the substrate surface center to the outlet of HF discharge. It can be seen that the emission line intensity of CH slightly increases with the distance increase and then decreases when reaches to 5 cm. However, the ratio of CH/C2 has opposite tendency and minimum value appears in 10 cm. The value of Hβ/Hα changes little and almost remains constant. It can be deduced that there is slightly change for the plasma composition, density and electron temperature in plasma channel with 15 cm length. According to the calculation Eqs. (1)–(4), the electron temperature is in the range of 2.32–2.48 eV while ne is in the range of 6.55– 5.30 × 1021 m−3 corresponding to distances of 0–150 mm. The gradient of electron density (ne) is approximately 8.2 × 1018 m− 3/mm. This means that this type of dual RF-ICP jet can generate rather long and uniform high temperature tail jet region. 3.2. Characterization of diamond film Fig. 5 exhibits surface and cross-sectional morphology of as-deposited films with the methane concentrations of 5% and 3%, respectively. It can be seen that a typical cauliflower-like morphology exists in Fig. 5(a), whilst the individual nodules morphology with an average diameter of 3 μm appears in Fig. 5(d). Both the cauliflower-like morphology and the individual nodules morphology consist of large quantities of tiny grains, proven by magnification pictures in Fig. 5(b) and (e). It is well known that the cauliflower-like morphology consists of nanocrystalline grains in the film [35]. Therefore, grains in the as-grown films are in the nanometer scale. Fig. 5(c) and (f) are the images of the cross sections of the films. Therefore, the growth rates are about 1 μm/h in both cases. The surface roughness of the films was measured using AFM in a 7 × 7 μm2 scanning area. The typical AFM images of the diamond films are presented in Fig. 6. The root mean square (Rm) and average (Ra) values of the surface roughness of the film in 5% are 57.1 and 45.2 nm, respectively. The Rm and Ra values for the film in 3% are observed to be 79.2 and 63.5 nm, respectively. The XRD patterns of as-deposited films in Fig. 5 are shown in Fig. 7. It is showed that three diamond diffraction peaks at (111), (220) and (311) could be identified, in addition to those of Si and SiC phases from substrate. There is no evidence for the presence of graphite related diffraction peaks in the XRD patterns of all samples. The intensity ratio of (311)/(111) is very weaker than that (16/100) of randomly oriented diamond powder, meaning that probable presence of highly damaged diamond crystallites. The increasing of methane concentration leads to
ð4Þ
where ΔλStark, ΔλWaals and ΔλL are the Stark broadening, Van der Waals broadening and Lorentz broadening of Hβ in terms of the full-width at half-maximum (FWHM), respectively. Tg is the gas temperature. From the results in Fig. 3, the ne and Te of the plasma can be calculated to be about 2.18 eV and 5.45 × 1021 m3 for 2%, 2.23 eV and 5.06 × 1021 m3 for 3%, 2.32 eV and 5.59 × 1021 m3 for 5%, 2.48 eV and 5.47 × 1021 m3 for 10%, respectively. So, the change tendency of Te
Fig. 4. Optical emission intensity of CH to Hα and C2 (left-hand axis) and Hβ to Hα (righthand axis) as a function of different distances.
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Fig. 7. XRD patterns of the NCD samples deposited with methane concentration of 5% and 3%, respectively. The diamond (111), (220), (311) peaks, the silicon and silicon carbide peaks are marked. The line broadening is analyzed using the Williamson-Hall plot (inset).
Fig. 5. Surface and cross-sectional SEM morphology of as-grown films (a) (b) (c) methane concentration of 5% at 17 h; (d) (e) (f) methane concentration of 3% at 6 h. The (b) and (e) are the corresponding high magnification image, respectively.
broaden of diamond peaks of samples. The broadening effect can be caused by either grain size effect or substantial lattice strains, or both [36]. By measuring the broadening of the diffraction peak using the Williamson-Hall (W-H) analysis [37] (inset) one can estimate the crystal size (L) as well as potential lattice strains (ε). Both contribute to an increase in the linewidth β given by β = βL + βε, with βL = Kλ / Lcosθ and βε = 4εtanθ, where K is a constant reflecting the particle shape (K = 1 for spherical particles), θ is the scatting angle and λ is the wavelength of the X-ray radiation (λ = 0.154 nm). Combining the above equations we obtain: β cosθ 1 4ε sinθ ¼ þ λ L λ
shrinkage, indicating the presence of compressive stress in samples. The average crystal size (L) is estimated be 14.95 nm and 12.85 nm with 3% and 5% methane concentration, respectively. This calculated results are obviously smaller than one directly observed from SEM images, probably due to presence of defects (e.g. lattice dislocation) in the nanodiamond. For another, compressive stress in sample destroys the diamond crystal grains, and maybe result in tinier grains formed, which is not neglected. Raman analysis was performed to evaluate the quality of NCD. Fig. 8 shows the Raman spectra of these two samples. Both spectrum show an intense and broad peak at 1334 cm−1, which is typical Raman peak of diamond. In addition, there exists several extra broad bands near 1139 cm−1 and ~1440 cm−1 in the spectra related to trans-polyacetylene (TPA), which can evidence the presence of NCD [38]. The peaks corresponding to graphitic carbon, D-peak (at 1351 cm−1 and 1357 cm−1) and G-peak (1570 cm−1 and 1584 cm−1), are also found in both spectrum. As the cross-section of Raman scattering of graphitic carbon is usually 50–60 times higher than that of diamond, it can be estimated that small amount of graphitic carbon may exist between nano-diamond grains in the films. On the other hand, it can be found that the diamond peak (1334 cm−1) position from samples have same shifts relative to one (1332 cm−1) taken from type IIb natural diamond. This indicates that there is approximately same compressive stress in both films, which is consistent with above calculation of the slope from W-H plot. 4. Conclusions
Plotting βcosθ/λ vs. 2sinθ/λ gives a straight line with the slope 2ε and an intercept of 1/L, known as Williamson-Hall plot. Accordingly, lattice strains (ε) are negative values, which arise from the lattice
A dual RF inductively coupled plasma jet is designed to grow diamond. The electromagnetic energy was input to ignite and sustain the
Fig. 6. Three-dimensional AFM images scanned in an area 7 × 7 μm2 of NCD films with methane concentration of (a) 5% and (b) 3%.
Y. Zuo et al. / Diamond & Related Materials 73 (2017) 67–71
Fig. 8. Raman spectra of diamond films with methane concentration of 5% and 3%, respectively.
Ar/H2/CH4 plasma by two tandem coils with the radio frequencies of 13.56 MHz and 4 MHz, respectively. Water-cooling quartz tube was used as the plasma confinement so that the sheath gas was saved. The plasma compositions were diagnosed by OES. CH, C2, Hβ and Hα were the main radicals, of which the optical emission intensity was depending on the concentration of methane in the working feed gasses. Under the condition of high working pressure (~ 6000 Pa), the high electron temperature (~ 2.48 eV) and the high electron density (~6.65 × 1021 m−3) were found by calculation based on the gas phase diagnosis results. Using this plasma jet source, diamond films were deposited on single crystal silicon substrates with the cauliflower-like morphology and individual nodules morphology. Increasing methane is proved to helpful in decreasing surface roughness of the diamond film, but broadening diamond XRD peaks. The nanodiamond may possess compressive residual stress and defects by W-H analysis. Raman spectrum confirmed the presence of the nanocrystalline diamond phase again, but small quantity of amorphous graphite was scatter along the grain boundary.
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