A single-beam plasma source was developed and used to deposit hydrogenated amorphous carbon (a-C:H) thin films at room temperature. The plasma source was excited by a combined radio frequency and direct current power, which resulted in tunable ion energy over a wide range. The plasma source could effectively dissociate the source hydrocarbon gas and simultaneously emit an ion beam to interact with the deposited film. Using this plasma source and a mixture of argon and C2H2 gas, a-C:H films were deposited at a rate of ∼26 nm/min. The resulting a-C:H film of 1.2 µm thick was still highly transparent with a transmittance of over 90% in the infrared range and an optical bandgap of 2.04 eV. Young’s modulus of the a-C:H film was ∼80 GPa. The combination of the low-temperature high-rate deposition of transparent a-C:H films with moderately high Young’s modulus makes the single-beam plasma source attractive for many coatings applications, especially in which heat-sensitive and soft materials are involved. The single-beam plasma source can be configured into a linear structure, which could be used for large-area coatings.

Amorphous carbon thin films have many superior properties, such as high mechanical hardness, chemical inertness, and optical transmittance.1 These properties strongly depend on the fraction of sp3 and sp2 carbon bonds as well as the hydrogen concentration. Films with high sp3 content are generally harder and have higher resistivity than those with high sp2 content. The sp2-rich carbon films are interesting for electrical applications (e.g., chemically stable electrodes), while the sp3-rich films are often used as hard coatings (e.g., tool coatings). By the way, they are prepared, hydrogenated amorphous carbon (a-C:H) films are softer but have higher transparency than hydrogen-free films. Hence, a-C:H films are particularly attractive for applications in which highly transparent coatings with sufficient wear-resistance are required. Examples include a-C:H coatings on plastic foils for packaging and perovskite solar cells as a moisture barrier. These applications require dense films prepared at low temperatures.

Amorphous carbon films have been deposited by plasma-enhanced chemical vapor deposition (PECVD),2,3 sputtering,3–5 arc discharge,6,7 and ion beam.8,9 PECVD is one of the most common technologies used for the fabrication of a-C:H because of its scalability and ability to modulate the sp3/sp2 ratio and H concentration. In PECVD, hydrocarbon source gas (e.g., CH4) is decomposed by the plasma, creating a variety of radicals that contribute to film growth. An example reaction is CH4 + e → H + CH3 + e.

A distinct characteristic of PECVD is that the chemical species are not under thermal equilibrium. This means the average electron energy (e.g., 3 eV) is much higher than the energies of the ions (e.g., ∼0.07 eV) and the neutral species (e.g., ∼0.03 eV). Therefore, the ions and neutral species have little kinetic energy as they reach the substrate. In a typical capacitively coupled PECVD, the bulk plasma potential is ∼10 V higher than the electrode and substrate. Ions would gain ∼10 eV kinetic energy assuming they are accelerated across the sheath region without collisions. Hence, conventional PECVD requires substrate heating to facilitate atom diffusion and produce dense films. Producing highly transparent a-C:H with high density and hardness is a fundamental challenge to conventional PECVD. Alternatively, a negative bias applied to the substrate can effectively densify the a-C:H by attracting the positively charged ions to bombard the growing film.10 However, applying a negative bias could be difficult in some applications, such as coatings on a plastic sheet in a continuous roll-to-roll process.

Ion beam enhanced chemical vapor deposition has the potential to produce highly transparent and dense a-C:H films at low temperatures. Ion sources are plasma generation devices that emit ions to interact with the film surface atoms as they are deposited. By controlling the ion energy transferred to the atoms, the film microstructures could be modulated.11–15 Since pioneering work of Aisenberg and Chabot,16 ion beam deposition of amorphous carbon thin films has continuously advanced with the development of ion sources. Two major types of ion sources have been widely used for thin-film processing—gridded and end-Hall ion sources.17,18 The gridded ion source has a relatively low current and high ion energy (e.g., >300 eV), which usually leads to large stress and graphitization of the carbon films. Gridless end-Hall ion sources can produce ions with a wide range of average energy (e.g., 40–200 eV) at a much higher current than gridded ion sources. These characteristics are beneficial for the high-rate deposition of dense a-C:H films. The requirement for a closed-loop drift of the electrons in end-Hall ion sources leads to circular or racetrack beam patterns, while some applications would demand the ions be focused on a specific area. Furthermore, end-Hall ion sources have a narrow ion-emitting slit, which could be coated quickly, resulting in unstable discharges.

This work reports a single-beam plasma source developed recently to address the needs described above. This single-beam plasma source combines several desired features:

  • A single beam of ions;

  • Widely tunable discharge voltage from 0 to 250 V for optimal ion–surface interactions;

  • High ion current density (e.g., >30 mA/cm2) to achieve sufficient deposition rates;

  • Wide range of operation pressure (0.13–66.66 Pa) compatible with chemical vapor depositions in inert and reactive gases; and

  • Wide anode opening for long-term operation in a thin-film manufacturing environment and easy to maintain.

This initial study aims to validate the feasibility of the single-beam plasma source enhanced chemical vapor deposition. The basic characteristics of the single beam plasma source will be demonstrated by depositing a-C:H thin films at low temperatures.

A round single-beam plasma source (model SPR-10, Scion Plasma LLC) was used in this study. This plasma source is illustrated in Fig. 1. It consists of an anode with a center cavity and a closed bottom. The anode opening is 12 mm in diameter. A cathode cover is located above the anode. A magnet assembly generates a magnetic field that is substantially parallel to the side wall in the hollow anode region and can effectively confine the energetic electrons to sustain the discharge. To ignite plasma discharge, a positive voltage is applied to the anode relative to the cathode. As an electron is accelerated toward the anode, it experiences a Lorenz force and subsequently drifts around the anode side wall [Fig. 1(b)]. The axisymmetric electric field drives ions toward the center axis and subsequently out of the source as a single beam. The ion source discharge can be sustained at low voltages due to the strong confinement of the electrons by the magnetic field across almost the entire anode surface, creating a high-density plasma with tunable ion energies over a wide range (e.g., 0–200 eV). Details of this single beam plasma source can be found in Ref. 19.

FIG. 1.

(a) Profile side view and (b) top view of the single-beam plasma source. Reproduced with permission from Tran et al., J. Phys. D: Appl. Phys. 55, 395202 (2022). Copyright 2022 IOP Publishing.

FIG. 1.

(a) Profile side view and (b) top view of the single-beam plasma source. Reproduced with permission from Tran et al., J. Phys. D: Appl. Phys. 55, 395202 (2022). Copyright 2022 IOP Publishing.

Close modal

The plasma source was set at ∼38 mm away from the substrate. Before the film deposition, the system was pumped down to a base pressure below 6.60 × 10−4 Pa. The process gas was a mix of argon (Ar) and acetylene (C2H2, 99.6% minimum absorption grade, dissolved in acetone, Airgas) at a ratio of 2:1 (Ar:C2H2), and the deposition pressure was 1.60 Pa. Ar was introduced to stabilize the discharge. The process gas was input from the chamber rather than from the plasma source as in most of the previous ion beam depositions. All the carbon films were deposited at room temperature. The substrates were soda-lime glass (thickness: 1.0 mm), n-type Si(100) wafer (thickness: 0.5 mm), and polyethylene terephthalate (PET, thickness: 0.1 mm) of 25 × 25 mm2 area. During deposition, the substrates were placed on an insulative quartz plate so that they were at the same floating potential, if not otherwise stated.

The single-beam plasma source can be excited by 13.56 MHz radio frequency and direct current (RF+DC) or RF power. The RF+DC excitation allows the control of the ion energy over a wide range and was used in the deposition of the carbon films in this study. To simultaneously apply RF and DC to the plasma source, a DC-RF filter as shown in Fig. 2 was developed and used. The DC-RF filter includes (1) a capacitor C1 that is normally part of a matching network and allows the RF to pass through to the plasma source and blocks the DC from entering the RF source, and (2) two inductors (L1 and L2) and a shunt capacitor C2, which act together to block the RF and allow the DC passing through to the plasma source.

FIG. 2.

A DC-RF filter used for combining RF and DC power for the excitation of the single-beam plasma source.

FIG. 2.

A DC-RF filter used for combining RF and DC power for the excitation of the single-beam plasma source.

Close modal

The film thickness was measured using a Dektak 150 profilometer. The optical transmittance was characterized using a spectrophotometer (F20, Filmetrics). A laser acoustic surface wave spectroscopy instrument (LAWave, Fraunhofer) was used to measure the elastic properties and density of the coatings. Raman spectroscopy was measured using a Horiba LabRAM HR Evolution system with a 532 nm laser wavelength. The optical emission spectra of the plasma discharges were measured using an Ocean Optics 4000 spectrometer.

The plasma source could be set at any angle relative to the substrate, such as 0°–60°. At all these angles, the discharge characteristics were similar. Figure 3(a) shows a typical discharge image of the plasma source set at an angle perpendicular to the substrate. As indicated in the experimental section, this plasma source was excited by RF+DC. The DC I–V characteristics depended on the RF power. Figure 3(b) illustrates the DC I–V relationship under 10 W RF power. Therefore, the DC voltage could vary over a wide range, from 0 to 200 V. The plasma source had a round opening of ∼12 mm in diameter.

FIG. 3.

(a) Discharge image and (b) I–V characteristics of the single-beam plasma source excited by RF (10 W) and DC power.

FIG. 3.

(a) Discharge image and (b) I–V characteristics of the single-beam plasma source excited by RF (10 W) and DC power.

Close modal

Interestingly, the plasma source emitted a pronounced current even when the DC voltage was zero. This result implies that the plasma source excited by RF power also emitted ions, which was because the bulk plasma had much higher potential than the ground shield and substrate stage. Figure 4 illustrates the voltage waveform applied to the ion source collected with the oscilloscope in Fig. 2. The ground potential is at 0 V. It shows that the plasma source can be excited by a wide range of DC voltage superimposed on an RF voltage.

FIG. 4.

RF+DC voltage waveform applied to the beam plasma source. The RF voltage is scaled down by 10× for better illustration.

FIG. 4.

RF+DC voltage waveform applied to the beam plasma source. The RF voltage is scaled down by 10× for better illustration.

Close modal

Carbon films were deposited under 10 W RF power and 80 V DC voltage for 45 min on the glass, Si wafer, and PET substrates. The resulting film thickness was 1.2 µm, translating to a 26 nm/min deposition rate. The films deposited on the Si wafer were used for the LAWave measurement, which yielded Young’s modulus of ∼80 GPa and a density of 1.95 g/cm3 for the carbon film. These properties are better than the reported a-C:H films with similarly high transmittance as discussed in the next paragraph.1,20 In addition, the PET substrate did not exhibit any deformation after being coated, which implied that the heating to the substrate was negligible, and the substrate temperature was below 80 °C.

Raman spectrum excited at a wavelength of 532 nm indicated the nature of a-C:H film as evidenced by the characteristic G-peak and D-peak (Fig. 5). The G-peak and D-peak positions were found by Lorentzian fits with 1522 and 1308 cm−1, respectively. The I(D)/I(G) ratio results at 0.68. The position of the G-peak and the I(D)/I(G) ratio confirm that the deposited coating belongs to the group of hydrogenated (tetrahedrally) amorphous carbon coatings [(t)a-C:H] rather than polymeric a-C:H.21 

FIG. 5.

Raman spectrum of the carbon films deposited using the single-beam plasma source.

FIG. 5.

Raman spectrum of the carbon films deposited using the single-beam plasma source.

Close modal

The film appeared to be highly transparent. Figure 6 shows the optical transmittance and reflectance spectra of the 1.2 µm thick film deposited on glass. The transmittance T is close to 90% in the near-infrared range, where the reflectance R is about 10%. The optical absorption coefficient α was subsequently determined from the expression,22 

α=1tln1RT,

where t is the film thickness. The Tauc plot (Fig. 7) for the a-C:H film was derived from the absorption spectrum.23 The result indicates an optical bandgap of 2.04 eV.

FIG. 6.

Transmittance and reflectance of the carbon film deposited using the single beam plasma source.

FIG. 6.

Transmittance and reflectance of the carbon film deposited using the single beam plasma source.

Close modal
FIG. 7.

Tauc plot of the a-C:H film deposited using the single beam plasma source.

FIG. 7.

Tauc plot of the a-C:H film deposited using the single beam plasma source.

Close modal

It is worth noting that conventional ion beam deposition of highly transparent a-C:H films with similar mechanical properties (Young’s modulus ∼80 GPa) usually resulted in relatively low growth rates (e.g., 2.3 nm/min)10 than that achieved with this single beam plasma source. Of course, the process gas also has effects on the deposition rates. The high deposition rate obtained from the single beam plasma source indicates effective dissociation of the process gas, which was a mix of Ar:C2H2 of 2:1. Figure 8 illustrates the optical emission spectra of different process gases. The carbon precursor C2H2 was dissociated into various carbon, hydrogen, and hydrocarbon species that could contribute to the growth of a-C:H films. The significant peaks for these species could be identified and cross referenced by optical emission spectra of a pure Ar and C2H2 plasma, which are shown in Fig. 8.

FIG. 8.

Optical emission spectra of different gases during the discharges of the single beam plasma source.

FIG. 8.

Optical emission spectra of different gases during the discharges of the single beam plasma source.

Close modal

The energies of the electrons and ions created by the single beam plasma source depend on the excitation power source (RF or RF+DC) and voltage, as well as the substrate being a conductor or insulator. Figure 9 shows the simulated electron energy probability function (EEPF) and ion energy distribution function (IEDF). The simulation was performed using an established particle-in-cell Monte Carlo collision code ASTRA. The detail of this simulation scheme is described in previous work.24,25 The electron energy probability function includes two regions: inside and outside the anode cavity of the single-beam plasma source.26 The results indicate that higher RF peak voltage and DC bias lead to the increased energetic tail of the electrons. The ion energy distribution function shows that the ion energy is proportional to the RF and DC voltages on an electrically conductive substrate. On the other hand, the ion energy is much reduced if the substrate is an insulator. In this case, the RF excitation is more critical to modulating the ion energy.

FIG. 9.

(a) and (b): Electron energy probability function (EEPF) inside and outside of the anode cavity. (c) and (d): Ion energy distribution function (IEDF) on substrates. Reproduced with permission from Tran et al., J. Phys. D: Appl. Phys. 55, 395202 (2022). Copyright 2022 IOP Publishing.

FIG. 9.

(a) and (b): Electron energy probability function (EEPF) inside and outside of the anode cavity. (c) and (d): Ion energy distribution function (IEDF) on substrates. Reproduced with permission from Tran et al., J. Phys. D: Appl. Phys. 55, 395202 (2022). Copyright 2022 IOP Publishing.

Close modal

Comparison experiments were also conducted by setting the glass and n-type Si wafer substrates on a metal plate that was connected to ground potential. The a-C:H film on the glass substrate was similar to the previous one. However, nearly no film was deposited on the Si wafer. These results imply that the ion energy on the wafer (conductive material) could be much higher than the ion energy reaching the glass (insulative material). The high ion energy resulted in intensive etching of the film. These results also indicated that the film microstructures could be tuned by the beam plasma source discharge parameters.

This study used a round single-beam plasma source that could deposit a-C:H films with uniformity better than 95% over a substrate area of ϕ25 mm when the source was set at a distance of 38 mm away from the substrate. The single-beam plasma source can be designed into a linear structure of any custom length for coating larger areas. Figure 10 illustrates a linear source of 78 mm effective length together with a round beam plasma source.26 The linear source can deposit films on a rectangular area of about 25 mm wide and the effective length of the source. Hence, it could be scaled up for large-area inline coatings.

FIG. 10.

Round and linear (78 mm effective length) single beam plasma sources and discharges. Reproduced with permission from Tran et al., J. Phys. D: Appl. Phys. 55, 395202 (2022). Copyright 2022 IOP Publishing.

FIG. 10.

Round and linear (78 mm effective length) single beam plasma sources and discharges. Reproduced with permission from Tran et al., J. Phys. D: Appl. Phys. 55, 395202 (2022). Copyright 2022 IOP Publishing.

Close modal

This research demonstrates the use of a single-beam plasma source to deposit hydrogenated amorphous carbon films at low temperatures. The single-beam plasma source enables a high deposition rate of ∼26 nm/min due to the effective dissociation of the source C2H2 gas. The a-C:H films are highly transparent with an optical bandgap of ∼2.04 eV. The ion beam effectively densifies the a-C:H films as evidenced by the relatively high Young’s modulus of ∼80 GPa and high transmittance. The single-beam plasma source enhanced chemical vapor deposition is particularly attractive for low-temperature high-rate growth of transparent a-C:H films on heat-sensitive and soft materials, in which wear-resistance and barrier performance are desirable.

This work was partly supported by the National Science Foundation Award Nos. 1917577 and 1724941. In addition, acknowledged was the U.S. Department of Energy Award No. DE-EE0009018.

Thomas Schuelke and Qi Hua Fan are the inventors of the single-beam plasma source (U.S. Patent No. 11,049,697) used in this research.

Young Kim: Data curation (equal); Writing – original draft (equal). Nina Baule: Data curation (equal); Writing – review & editing (equal). Maheshwar Shrestha: Data curation (equal); Investigation (equal); Methodology (equal). Bocong Zheng: Data curation (equal); Investigation (equal). Thomas Schuelke: Conceptualization (equal); Project administration (equal); Resources (equal); Supervision (equal). Qi Hua Fan: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.

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