By Nicolas Vandencasteele, vice president, Research & Design, Plasmalex Group

Plasma technologies long have been used to modify materials and deposit coatings. Plasma enhanced chemical vapor deposition (PECVD) has been used routinely since the 1970s in various industries to deposit mostly inorganic coatings for passivation and protection. Since the 1980s, developments led to the use of PECVD methods to deposit organic coatings; in the 1990s, roll-to-roll (R2R) processes to deposit thin films on flexible materials gained traction. In the 2000s, atmospheric-pressure PECVD (AP-PECVD) processes began to appear. More recently, PECVD techniques also have been explored in emerging fields such as battery technology, where thin, uniform coatings on flexible substrates (e.g., battery separators or electrodes) can enhance performance significantly and provide protection. This paper will focus on the use of both vacuum-based and atmospheric-pressure PECVD technologies used in R2R processes, starting with a brief technology description and following with detailed application examples.

Plasma can be described as an ionized gas. It’s characterized by multiple parameters, the main ones being:

1. The electron density n_e, ion density n_i and neutra density n_n [m^-3]
2. The particles’ energies (or temperature): electrons (T_e), ions (T_i), neutrals (T_n) [eV] or [K]
3. The degree of ionization
4. The mean free path  [m], the average distance a particle travels before colliding with another particle.

In plasma, those variables extend over huge ranges of values covering multiple orders of magnitudes, going from weak energies (»10^-1 eV) and low densities (»10¹² m^-3) found in the ionosphere up to extremely high densities (»10²⁴ m^-3) and energies (»10⁴ eV) found in nuclear fusion reactions.

The plasma used in PEVCD usually is characterized by medium electron densities and high electron temperatures (T_e) but low ions and neutrals temperatures (T_i and T_n). The high T_e gives its chemical reactivity to the plasma while the low T_i and T_n ensure that the overall gas temperature remains low (i.e., 40-100° C), enabling the treatment of heat-sensitive materials, such as polymers. Plasma, where T_e >>> T_i and T_n, are referred to as non-local thermal equilibrium plasmas (Non-LTE) (see Figure 1).

The mean free path (λ) is related directly to the working pressure. For low-pressure plasma, typical values are in the μm range, while for atmospheric plasma, they are in the nm range.

For a more scientific description of the plasma and PEVCD processes, the reader is invited to consult the abundant scientific literature available [1,2].

Atmospheric vs. vacuum

The AP-PEVCD process has been developed to simplify the PECVD process by removing complex vacuum equipment and allowing continuous operation.

As there is no vacuum, continuous injection of gas in the reactor is required to control the gas composition and, therefore, the chemistry of the plasma. It is crucial to limit the contamination of the plasma gas by ambient oxygen and/or water vapor. Indeed, those gases are so reactive in plasma state that they would mask any other chemistry injected into the system and compromise the coating quality. The system consists of multiple gas diffusor/electrode assemblies, which each can be controlled independently, allowing for multi-zone plasma treatment with no interference between the different zones and no exposure to ambient atmosphere between each zone (see Figures 2 and 3).

The obvious difference between the two processes is the working pressure. The higher working pressure of the AP-PECVD has several impacts on the process:

1. The process is not suited to treat porous materials, such as paper, textile and non-wovens, as the mean-free path is reduced dramatically. This prevents coating inside the structure of textiles, etc.
2. The increased particle density increases the deposition rate, allowing for higher treatment speed. 
3. According to the Paschen law [3, 4], the breakdown voltage (the voltage that needs to be applied between the ground and powered electrode to ignite the plasma) increases according to the product of the pressure times the inter-electrode distance. Therefore, to keep this voltage in a reasonable range (< 50 kV), the interelectrode distance must be kept low, typically a few mm. Therefore, AP-PECVD mostly is used to treat polymeric film with thickness < 1,000 μm.

Applications

Atmospheric-Pressure PECVD

All the following application examples are carried by AP-PECVD. Treatment speeds range from approximately 10 m/min to more than 100 m/min. The typical roll width is around 1.5 m. Roll length is up to 9,000 m, and film thickness ranges from 12 µm to 100 µm.

Stable hydrophilic coating (atmospheric pressure)

The aim is to avoid droplet formation on the surface of low-surface-energy (SE) polymers and instead have the formation of a continuous water film. Applications are in evaporative-cooling towers and greenhouses.

To achieve this, an inorganic coating with a thickness < 10 nm has been deposited by AP-PECVD on the polymeric film. For this process, the multi-zone treatment showed a clear advantage compared to classical systems. For the greenhouse application, the coating had to be applied on ETFE, a fluoropolymer with poor adhesion properties; the surface needed to be plasma-activated before the coating was deposited. Figure 4 compares the results of two processes: In the first one, the film is first plasma activated and, in a second step, plasma coated. The coating shows poor performance as evidenced by the numerous condensation droplets visible on the film. In the second process, the activation and coating were performed in one step with three plasma zones, the first zone being used for activation, the second for the deposition of an anchorage layer and the third one for coating. In this case, minimal droplet formation is visible on the film. Instead, the water condenses as a continuous transparent water film on the surface of the ETFE film.

Release coating (XIMO ®): The aim is to avoid adhesion of reactive components or glues on the film. Applications are PU film casting and demolding films for composite manufacturing.

This is achieved by depositing an ultra-thin (1-5 nm), silicone-based coating with low surface energy and low adhesion properties. The main advantages of the coating is the controllable surface energy, allowing for easy application of the resin to be casted (no dewetting); excellent thermal stability of the coating, allowing curing temperature above 200° C; and ultra-low extractables (ng/cm²), preventing the contamination of the components by release agents (see Figure 5). 

The release force, as well as the surface energy of the coating, can be modified by controlling the coating thickness, as shown in Figure 6 and Figure 7.

Adhesion of rubber

Aim: Direct adhesion of the rubber compound to a polymeric film during molding of the rubber components. Markets: Medical and solar.

An ultra-thin coating (1-2 nm) is deposited on the polymeric film. This coating is covalently bound to the polymeric film and its functional groups allowed to create covalent bonding with the rubber during the vulcanization process, giving an excellent adhesion between the rubber and the polymeric film.

Contrary to common assumption, in this case, a higher surface energy does not mean the adhesion will be better. In fact, the coating decreases the surface energy of most polymers but still dramatically improves the adhesion between the polymer and rubber (see Figure 8). This is, of course, because the nature of the grafted functional groups is the relevant parameter allowing the treated polymer film to become part of the vulcanization reaction.

Low-pressure PECVD

All processes were carried out at working pressure ranging from 30 to 200 mTorr. Speed ranged from 1 to 10 m/min, maximum web width was 1,600 mm.

Improvement of filtration media

Aim: Improving filtration performance and shelf life of electrets materials without impacting the pressure drop. Markets: Air and gas filtration

A thin conformal coating is applied to the fibers of the material. Thanks to the high mean-free path, the bulk of the material is treated as well. On top of imparting hydrophobic and oleophobic properties, this coating increases the lifetime of the static charge incorporated into the material using technology based on the (expired) 3M patent US5496507A [5] (see Table 1). 

Alternative to PFAS membrane

Aim: Prevent liquid penetration through microporous membranes.

Similarly to the previous application, a thin (»20 nm) conformal coating is applied on/in the membrane. This time the coating is PFAS free. The overall performances of the coated membranes are linked strongly to the characteristics of the membrane (materials used and pore size). but generally, the coating improves the performance of the membrane (see Table 2). The high-water contact angle (WCA) measured on the membrane is due to the roughness of the samples (as described by the Wenzel [6] or Cassie Baxter [7] models). The WCA of the coating on a flat substrate (glass) is 105°.

Battery-specific applications

Aim: Enhance the performance, safety and lifespan of battery components (separators, electrodes, etc.) through ultra-thin, uniform PECVD coatings.

Both vacuum and atmospheric-pressure PECVD techniques are being applied in battery manufacturing. For instance, AP-PECVD can deposit nanoscale organosilicon coatings onto polymeric battery separators to improve their thermal stability and chemical resistance. Such plasma-deposited layers have been shown to increase separator heat resistance and electrolyte wettability, which boosts the battery’s safety and electrochemical performance [8].

Vacuum PECVD, on the other hand, is used to coat electrode surfaces. A notable example is the deposition of thin amorphous silicon films onto graphite anodes, which significantly increases the anode’s capacity and maintains high coulombic efficiency without requiring high processing temperatures [9]. Even at hundreds of nanometers thick, these PECVD layers are uniform and conformal. In general, such coatings add minimal weight or volume while providing improved mechanical robustness to the electrodes and enhanced chemical stability in the battery’s harsh reactive environment.

PECVD also enables the creation of effective moisture- and chemical-barrier layers on flexible battery-related substrates. Thin inorganic films (e.g., SiNx or SiO₂) deposited by PECVD serve as barrier coatings on polymeric battery packaging and other components, protecting them from humidity ingress and chemical degradation. This added protection helps prolong the lifespan of battery cells and prevents performance loss due to environmental factors. Furthermore, there is a growing interest in using PECVD for next-generation energy storage – for example, depositing interfacial layers in solid-state batteries to improve the electrode–electrolyte contact, or applying protective films on advanced lightweight current collectors. These emerging applications highlight the expanding role of PECVD in the battery field.

Conclusions

The use of PECVD technologies, both vacuum-based and atmospheric pressure (AP-PECVD), has expanded the capabilities of thin-film deposition, particularly in roll-to-roll (R2R) processing. The flexibility of AP-PECVD in enabling continuous, high-speed deposition without the need for vacuum systems makes it particularly advantageous for large-scale industrial applications. However, vacuum-PECVD remains indispensable for treating porous materials and achieving deeper penetration of coatings.

By carefully selecting process conditions and leveraging multi-zone plasma treatments, it is possible to fine-tune coatings to meet specific application requirements. The continued advancement of PECVD technologies will drive further innovation in functional coatings, particularly as sustainability and environmental regulations push for alternatives to traditional chemical processes. Additionally, battery technology has emerged as a significant and expanding field for PECVD applications; plasma-deposited coatings on battery components increasingly are used to enhance safety, durability and performance in energy storage devices.

Future work will focus on optimizing process efficiency and coating performance to expand PECVD applications into new industries, including biomedical, battery technology and next-generation electronic materials. 

References

1. Snyders, R.; Hegemann, D.; Thiry, D.; Zabeida, O.; Klemberg-Sapieha, J.; Martinu, L. Foundations of Plasma Enhanced Chemical Vapor Deposition of Functional Coatings. Plasma Sources Sci. Technol. 2023, 32 (7), 074001. https://doi.org/10.1088/1361-6595/acdabc.
2. Merche, D.; Vandencasteele, N.; Reniers, F. Atmospheric Plasmas for Thin Film Deposition: A Critical Review. Thin Solid Films 2012, 520 (13), 4219–4236. https://doi.org/10.1016/j.tsf.2012.01.026.
3. Direct Current (DC) Discharges. In Principles of Plasma Discharges and Materials Processing; John Wiley & Sons, Ltd, 2005; pp 535–569. https://doi.org/10.1002/0471724254.ch14.
4. Paschen, F. Ueber Die Zum Funkenübergang in Luft, Wasserstoff Und Kohlensäure Bei Verschiedenen Drucken Erforderliche Potentialdifferenz. Ann. Phys. 1889, 273 (5), 69–96. https://doi.org/10.1002/andp.18892730505.
5. Angadjivand, S. A.; Jones, M. E.; Meyer, D. E. Method of Charging Electret Filter Media. US5496507A, March 5, 1996. https://patents.google.com/patent/US5496507A/en (accessed 2025-01-30).
6. Wenzel, R. N. RESISTANCE OF SOLID SURFACES TO WETTING BY WATER. Ind. Eng. Chem. 1936, 28 (8), 988–994. https://doi.org/10.1021/ie50320a024.
7. Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40 (0), 546–551. https://doi.org/10.1039/TF9444000546.
8. Wu, L.-Y.; Chung, F.-Y.; Huang, C. Synthesis and Application of Nano-Organosilicon Coating through Cyclonic Plasma Deposition on a Polymeric Separator for Lithium-Ion Batteries. J. Coat. Technol. Res. 2022, 19 (4), 1159–1170. https://doi.org/10.1007/s11998-021-00595-6.
9. Lee, C.-Y.; Yeh, F.-H.; Yu, I.-S. A Commercial Carbonaceous Anode with A-Si Layers by Plasma Enhanced Chemical Vapor Deposition for Lithium Ion Batteries. J. Compos. Sci. 2020, 4 (2), 72. https://doi.org/10.3390/jcs4020072.

Dr. Nicolas Vandencasteele is a distinguished scientist specializing in plasma surface treatments and thin film deposition. He earned his Ph.D. from the Université Libre de Bruxelles (ULB), where he conducted extensive research on plasma-modified polymer surfaces, contributing significantly to the field. Following his doctoral studies, Dr. Vandencasteele was awarded fellowships from the Belgium American Education Foundation and the Fulbright Commission, enabling him to join the NESAC/BIO group at the University of Washington in 2009. There, he focused on characterizing DNA surfaces for microarray applications. Currently, he serves as vice president of R&D for the Plasmalex Group after joining the group in 2016. Dr. Vandencasteele can be reached at email: Nicolas.vandencasteele@plasmalex.com, www.plasmalex.com.