Plasma Cleaning of Metal Surfaces: From Contaminant Removal to Surface Functionalization

Abstract

The cleanliness and functionalization of metal surfaces are critical factors to determining their performance in high-performance microelectronic packaging, reliable biomedical implants, advanced composite bonding, and other fields. Compared to traditional wet cleaning methods, plasma cleaning technology has emerged as a research hotspot in surface engineering due to its unique advantages, such as high efficiency and environmental friendliness. It operates under versatile conditions (e.g., power: tens of watts to several kilowatts; pressure: atmospheric to low vacuum; treatment time: seconds to minutes), enabling not only efficient contaminant removal but also targeted surface functionalization, including dramatically enhanced hydrophilicity (e.g., contact angles from >80° to <10°), significantly improved adhesion (e.g., up to 40% increase in bond strength), and modifications in surface roughness, corrosion resistance, and biocompatibility. This review systematically elaborates on the physical, chemical, and synergistic mechanisms of plasma cleaning technology as it acts on metal surfaces. It focuses on plasma cleaning applied to copper, aluminum, titanium and their respective alloys, as well as alloy steels, providing a detailed analysis of contaminant types, plasma cleaning methodologies, common challenges, surface functionalization responses, and subsequent functional applications. Furthermore, this review discusses the current challenges faced by plasma cleaning technology and offers perspectives on its future development directions. It aims to systematize the research progress in plasma cleaning of metal surfaces, thereby facilitating the transition of this technology towards large-scale industrial applications for metal surface functionalization.

1. Introduction

Metal surface treatment is a critical component of modern manufacturing, as its quality directly influences the performance and reliability of materials in advanced sectors such as microelectronic packaging, biomedical implants, aerospace structures, and new energy equipment [1,2,3]. This relationship arises because surface quality is fundamentally connected to essential properties of metals, including mechanical strength, wear resistance, hardness, corrosion resistance, and biocompatibility [4]. Therefore, achieving high performance necessitates the thorough cleaning of metal surfaces to eliminate contaminants and ensure optimal surface quality. Traditional wet chemical cleaning methods, such as solvent cleaning, acid pickling, and alkaline cleaning, despite their widespread use, are inherently limited by several factors. These include risks of environmental pollution, issues related to chemical residue, challenges in treating complex geometries, and the potential for substrate damage [5,6,7]. Such drawbacks significantly restrict their applicability in the advancement of high-tech industries. Consequently, there is an urgent need for an efficient and environmentally friendly metal surface cleaning technology that can achieve effective surface functionalization.

Plasma cleaning technology, which utilizes energetic particles and reactive species within the plasma to remove surface contaminants through physical bombardment and chemical activation reactions, has emerged as an ideal choice for cleaning metals and their oxides. Compared to traditional wet cleaning methods, plasma cleaning, as a dry process, eliminates the generation of hazardous chemical waste at the source, offering significant environmental benefits. Furthermore, it eliminates the need for the consumption, storage, and disposal of chemicals and can be easily integrated into automated production lines, effectively reducing economic costs and enhancing process efficiency [8,9,10,11,12]. The combined physical and chemical mechanisms of plasma action on metal surfaces enable not only the highly efficient removal of various organic and inorganic contaminants, such as oils and greases [13], paints [14], flux residues [15], and inorganic oxides [16], but also allow the technology to transcend traditional cleaning boundaries. This technology facilitates the endowment of new functional characteristics to metal surfaces, including enhanced surface energy [17,18], the introduction of specific functional groups [19,20,21], and nanoscale cleaning without substrate damage [22]. These advantages render plasma cleaning an irreplaceable critical process in diverse fields such as microelectronics, biomedicine, and aerospace. For instance, compared to traditional wet cleaning, plasma cleaning of copper components achieves an approximately 10% improvement in cleaning efficacy and an approximately 25% enhancement in electrical conductivity [23].

This work provides a systematic review of plasma cleaning for key engineering metals, grouped as copper, aluminum, titanium and their respective alloys, as well as alloy steels. These four metal categories were selected for their broadly representative nature, as evidenced by shared commonalities in contaminant types, underlying cleaning mechanisms, and resultant functional outcomes. However, due to their distinct physicochemical properties, different metallic materials exhibit varying surface response behaviors during plasma cleaning. For instance, copper surface oxides are relatively easily reduced by hydrogen plasma but suffer from the unique issue of residual carbon films. The alumina layer on aluminum surfaces is exceptionally stable, making it difficult to remove effectively via reductive plasma. Titanium and its alloys are highly prone to surface re-oxidation post-cleaning, yet the resulting oxide layer can significantly influence biocompatibility. Steel commonly experiences a pronounced aging effect after treatment. Consequently, this review aims to provide a systematic presentation of the process of plasma cleaning metal surfaces, from contaminant removal to surface functionalization. It delves into the underlying physical, chemical, and synergistic mechanisms, offers a categorized evaluation of plasma cleaning methodologies applied to different metal surfaces, and discusses the associated challenges and future prospects, thereby seeking to propel innovation and application in plasma surface engineering technology.

2. Cleaning Mechanisms

Plasma cleaning of metal surfaces results from complex interactions between various particles such as electrons, ions, free radicals, and photons within the plasma and contaminants on the metal surface as well as the substrate. The cleaning mechanism can primarily be attributed to physical bombardment, chemical reactions, and synergistic effects arising from the coexistence of both mechanisms.

2.1. Physical Mechanism

The primary physical mechanism involves positively charged ions (e.g., Ar⁺) in the plasma accelerating under the influence of an electric field, thereby gaining kinetic energy (Figure 1a). This energy is subsequently imparted to the metal surface through impact. When high-energy ions collide with the surface, they transfer their kinetic energy to the atoms or molecules present on the surface of contaminants or the substrate. If the transferred energy exceeds the binding energy of these particles, they are ejected from the surface, resulting in effective removal [24].

Intense ion bombardment not only removes contaminants but also alters the nanoscale morphology of the metal surface, resulting in increased surface roughness. This microscopic ‘anchoring’ effect provides a larger contact area for subsequent bonding or coating processes, enhances mechanical interlocking, and significantly improves adhesion [25,26,27]. Inert gas plasmas, such as argon, serve as classic examples of purely physical cleaning methods. Due to their chemical inertness, they primarily eliminate contaminants through physical sputtering via ion bombardment, without introducing new chemical species. The underlying reaction mechanisms are detailed in Equations (1) and (2) [10,28]:

Ar + e⁻ → Ar⁺ + 2e⁻

(1)

Ar⁺ + contaminant → volatile contaminant

(2)

2.2. Chemical Mechanism

The chemical mechanism is primarily driven by chemical reactions between electrically neutral active free radicals (e.g., ·O, ·OH), excited molecules in the plasma, and the contaminants [24].

To remove organic contaminants, such as oils, greases, and flux residues, oxygen plasma is frequently utilized [23]. Within the plasma, oxygen-free radicals (·O) engage in oxidation-reduction reactions with organic compounds, which are typically composed of carbon and hydrogen. This interaction decomposes the organic contaminants into small volatile gaseous molecules, such as carbon dioxide and water (Figure 1b). These gaseous products are subsequently evacuated by the vacuum system, thereby achieving effective cleaning [25]. The representative reaction mechanism is illustrated in Equation (3) [29]:

C_mH_n + O^∗ → CO₂ + H₂O

(3)

To remove metal oxides, such as CuO and Fe₂O₃, hydrogen plasma or argon/hydrogen/nitrogen gas mixtures can be employed [8,30]. Hydrogen atoms and hydrogen-free radicals (·H) effectively reduce the oxides to elemental metal and water vapor, thereby cleaning the surface. For instance, the reaction mechanisms involving copper are detailed in Equations (4)–(6) [24,31,32]:

Cu₂O + 2H → Cu₂ + H₂O

(4)

CuO + 2H → Cu + H₂O

(5)

Cu(OH)₂ + 2H → Cu + 2H₂O

(6)

Another crucial function of the chemical mechanism is the alteration of the surface chemical composition. Reactive species introduce new polar functional groups onto the cleaned surface, such as hydroxyl (-OH), carboxyl (-COOH), and carbonyl (C=O). The introduction of these hydrophilic groups significantly enhances the surface free energy and hydrophilicity of the metal, thereby creating optimal conditions for subsequent biomolecule immobilization, coating, or bonding processes [33].

2.3. Physicochemical Synergistic Mechanism

In practical applications, physical and chemical mechanisms often do not operate independently; rather, they interact synergistically to enhance the overall effect. For instance, when employing an argon/oxygen mixture, physical bombardment by Ar⁺ ions initially disrupts the dense surface layer of contaminants or weakens their chemical bonds, thereby creating pathways for subsequent oxidative reactions by oxygen radicals. Concurrently, the heat and defects generated by ion bombardment accelerate the diffusion and reaction rates of these radicals. Conversely, chemical reactions convert strongly bound contaminants into weakly bound substances, facilitating their removal through physical sputtering. Furthermore, based on the “Penning ionization” mechanism, argon significantly enhances the efficiency and yield of oxygen decomposition into active oxygen atoms (Figure 1c). The relevant reaction mechanisms are detailed in Equations (7) and (8) [29,34,35,36]:

Ar^∗ + O₂ → O + O^∗ + Ar

(7)

Ar^∗ + O → O^∗ + Ar

(8)

This synergistic effect typically results in more efficient and uniform cleaning compared to either mechanism alone, proving particularly advantageous for treating complex or stubborn contaminants.

3. Metal Categories and Functionalization Responses

Plasma cleaning technology encompasses a variety of excitation methods (e.g., DC, RF, microwave), each with its own characteristics in terms of energy and plasma density [10]. While the fundamental principles of plasma cleaning are universal, different metallic materials exhibit distinct surface response behaviors in plasma environments due to their unique physicochemical properties (e.g., electronegativity, oxide structure, chemical reactivity). This section focuses on four key categories of engineering metals—copper, aluminum, titanium and their respective alloys, as well as alloy steels—providing a detailed analysis of their characteristic surface contaminants, tailored plasma cleaning methodologies, and the resulting functionalization responses. To facilitate a direct comparison of these representative cases across different material categories, the key parameters and outcomes are summarized in

Table 1. Summary of Representative Plasma Cleaning Cases for Metal Surfaces.

No.	Metal Category	Contaminant Category	Plasma Reactor	Plasma Cleaning Parameters	Surface Functionalization Effect	Challenges and Limitations	Ref.
1	2A12 Al Alloy	Organic: Acrylic Polyurethane Paint	Atmospheric pressure plasma jet	Gas: Compressed Air Flow rate: 50 L/min Power: 800 W Inlet pressure: 0.5 MPa	The treatment significantly decreased surface carbon content, increased oxygen content, formed new C=O bonds, and increased surface roughness.	Combination with physical wiping was necessary for complete paint removal.	[14]
2	Cu (C19400)	Organic: Benzotriazole (BTA)	RF plasma system (Panasonic PSX303-S)	Gas: Ar Flow rate: 5 mL/min RF Power: 600 W Time: 30 s	BTA was effectively removed, resulting in a cleaner and slightly smoother surface with a significantly reduced water contact angle.	Cross-contamination occurred due to the mutual redeposition of Ag and Cu.	[37]
3	Cu	Organic Contaminants	Plasma asher	Gas: O2 Flow rate: 50 sccm Power: 50 W Time: 20 s	The treatment improved crystallinity, refined grain size, and enabled strong Cu diffusion bonding, which increased shear strength by approximately 40%.	The process intentionally formed a surface CuO layer, which was beneficial for bonding in this context.	[38]
4	Cu	Inorganic: Oxides	Not specified	Gas: NH3 Temperature: 350 °C RF Power: 550 W Pressure: 3.0 Torr Time: 5 s	Copper surface oxides were effectively removed, leading to a longer electromigration lifetime without significant degradation of RC performance.	The process requires precise parameter optimization to achieve the desired effect.	[39]
5	Al Alloy (AA7075)	Organic Contaminants	Dielectric barrier discharge (two parallel aluminum electrodes, 4 mm gap)/Atmospheric pressure plasma jet (horn-like nozzle, 1 mm gap)	(DBD) Gas: Air AC power: 60 Hz Voltage: 30 kVp-p Airflow: 8 L/min Time: ≥ 5 min /(APPJ) Gas: Ar Voltage: 12 kVp-p Time: ≥ 30 s	Hydrocarbon contaminants were effectively removed. The surface exhibited significantly improved wettability, increased surface free energy, enhanced corrosion resistance, and dramatically stronger coating adhesion.	No significant improvement in corrosion resistance was observed on uncoated samples after plasma treatment alone.	[40]
6	Cp-Titanium	Biofilm Contamination	PICO-UHP, Diener electronic, Germany	Gas: O2 Power: 50 W Pressure: 0.2–0.3 mbar Time: 15 min	The treatment reliably removed biofilm and hydrocarbon contamination without damaging the surface microstructures.	The plasma cleaning process requires a preceding solvent cleaning step and must be performed under controlled conditions (e.g., clean room).	[41]
7	Titanium	Organic Contaminants	Atmospheric pressure plasma jet (INP Greifswald, Greifswald, Germany)	Gas: 1.0%O2/Ar Time: 60 s	The surface became superhydrophilic. This dramatically enhanced the spreading of human osteoblastic cells, with cell area increasing by >57% on most surfaces.	The safety of direct plasma application in the oral cavity requires further validation in future studies.	[42]
8	Stainless Steel	Organic Contaminants	Atmospheric pressure plasma jet (Agrodyn Plasma Treat GmbH, Bielefeld, Germany)	Gas: N2/O2 (4:1) Nozzle-to-surface gap: 10 mm Nozzle moving velocity: 5 mm/s	The treatment significantly increased hydrophilicity and surface energy (up to 71.49 mN/m). Surface oxidation and the formation of new functional groups (C=O, C-O) were confirmed.	The induced high surface energy and hydrophilicity were not stable in ambient air, decaying within approximately 15 h.	[43]
9	Fe6.5Si Alloy (High silicon steel)	Inorganic: Oxide Scale	Electrolytic plasma processing (Fe6.5Si cathode, dual graphite anodes, and NaHCO3 electrolyte)	Voltage: 120 V Electrolyte: Saturated Sodium Bicarbonate at 75 °C Time: 40 s	The oxide scale and impurities were effectively removed, significantly reducing oxygen content and exposing the metallic Fe-Si matrix without damaging the bulk properties.	The high-power discharge altered the surface morphology, resulting in crater features.	[44]
10	Cold Rolled Steel (SPRC 440)	Organic Contaminants	Atmospheric pressure plasma	Gas: Ar/O2 Flow rate: O2 at 1 sccm, Ar at 500 sccm Power: 350 W Time: 75 s	Carbon contaminants were removed, and wettability was significantly improved, leading to a 23% increase in adhesive bonding strength.	Excessive power or treatment time led to the formation of non-active particles that hindered surface activation.	[45]

.

3.1. Copper and Its Alloys

Copper and its alloys exhibit exceptional advantages, including excellent electrical and thermal conductivity, a low coefficient of thermal expansion, a high melting point, and corrosion resistance. These properties lead to their widespread application in microelectronic packaging, electrical interconnects, and heat dissipation components [46]. However, their surfaces are prone to oxidation, resulting in the formation of inert copper oxide and cuprous oxide and they are also susceptible to contamination by organic contaminants. Such surface issues contribute to the degradation of key properties, including solderability, bondability, and coating adhesion.

3.1.1. Plasma Cleaning Methodologies

The surface contaminants on copper and its alloys primarily include organic contaminants (e.g., anti-tarnish layers, lubricating oils) and inorganic contaminants (e.g., oxides). For organic contaminants, Li, W. [37] demonstrated that argon plasma cleaning at an RF power of 600 W for 30 s successfully removed anti-tarnish chemicals like benzotriazole (BTA) from copper surfaces. Nakamura, T. et al. [47] achieved surface cleaning and lubricant removal from fine copper wires (0.2 mm diameter) using atmospheric pressure plasma, highlighting its capability for treating intricate geometries, with the cleaning efficacy shown in Figure 2a. Park, S. et al. [38] reported that oxygen plasma can also effectively remove organic contaminants, ensuring a clean bonding interface. Cvelbar, U. et al. [23] employed oxygen plasma to eliminate a thick carbon film (likely residues from oils, lubricants, or coolants after mechanical processing), confirming the effectiveness of plasma in completely reacting with and removing the organic impurity layer. Regarding inorganic oxides, the efficacy of plasma cleaning is equally notable. Yi, C.X. et al. [48] observed significant morphological changes on copper surfaces and cross-sections before and after argon plasma cleaning via SEM. After 30 min of treatment, the macroscopic surface morphology (Figure 2(b2)) showed that plasma bombardment dispersed surface oxides into finer particles. Cross-sectional analysis (Figure 2(b4)) indicated that the oxide layer thickness was substantially reduced from approximately 20 μm to 6 μm, providing clear evidence of successful oxide removal. Baklanov, M.R. et al. [31] employed vacuum thermal treatment and hydrogen plasma for the reductive cleaning of copper surfaces. Initially, Cu(OH)₂ and CuCO₃ underwent thermal decomposition, releasing water and carbon dioxide, which resulted in the copper surface being primarily coated with CuO. Subsequently, atomic hydrogen reduced the oxides present on the copper surface, such as CuO and Cu₂O, thereby achieving effective cleaning. Ito, F. et al. [39] found that optimized NH₃ plasma conditions (e.g., high RF power and pressure) could efficiently remove copper surface oxides without increasing the dielectric constant of adjacent low-k films, thereby enhancing interconnect reliability metrics.

Plasma cleaning methodologies primarily employ single gases—such as argon, nitrogen, oxygen, and ammonia—or gas mixtures like N₂/H₂, Ar/H₂/N₂, and Ar/H₂/O₂. Yi, C.X. et al. [48] compared Ar and N₂ plasmas for removing surface oxides and adsorbed hydrocarbons, finding Ar more effective. After argon plasma cleaning, X-ray photoelectron spectroscopy (XPS) analysis revealed elements like Na, N, and Cl were undetectable, the carbon concentration dropped from 71.88% to 13%, and copper concentration increased markedly from 2.11% to 43.9% (Figure 2c). This result aligns with Park, M. et al. [49], who observed a surface composed primarily of metallic Cu with minimal Cu₂O after Ar plasma treatment. Park, S. et al. [38] optimized O₂ plasma conditions (50 sccm, 50 W, 20 s) for Cu-Cu bonding. This treatment formed a dense nanocrystalline CuO layer, enhancing crystallinity and refining grain size with minimal impact on sheet resistance. Furthermore, it enabled effective diffusion bonding, increasing bond strength by approximately 40% due to interfacial oxide removal. Similarly, Ito, F. et al. [39] demonstrated that NH₃ plasma efficiently removes copper oxides while minimizing surface damage, beneficial for structures with sensitive low-k dielectrics. Mixed-gas plasmas often leverage synergistic effects. Cvelbar, U. et al. [23] achieved optimal cleaning using a sequential O₂/H₂ plasma approach: O₂ plasma effectively removed organic layers, while subsequent H₂ plasma reduced the remaining oxide, yielding an almost impurity-free surface. Baklanov, M.R. et al. [31] used N₂/H₂ plasma, finding that within a substrate temperature range of 150–350 °C, the copper oxide reduction rate surpassed the hydrogen diffusion rate, enabling effective surface cleaning without significant hydrogen embrittlement. Wang, J. et al. [50] showed that H₂/Ar plasma pretreatment facilitated the adsorption of self-assembled molecules (1-hexanethiol, C6-SAM) due to the resulting high surface activity and cleanliness. Jeong, B.H. et al. [30] reported that both Ar/H₂N₂ and Ar/H₂/O₂ plasmas effectively reduced CuO_X on Cu-Mn surfaces. The Ar/H₂/N₂ mixture was particularly advantageous, causing less damage and effectively suppressing oxygen diffusion into the underlying Cu–Mn/TaN barrier, thereby mitigating oxidation-related reliability issues in back-end-of-line (BEOL) processing.

However, plasma cleaning of copper and its alloys faces several common challenges: (1) Formation and Persistence of Residual Carbonaceous Films. Dong, B. et al. [51], using in situ XPS, demonstrated that various plasma sources and gases leave behind 1–3 monolayers of graphitic or carbide-like carbon on copper surfaces. This residual film cannot be eliminated by further plasma treatment. Notably, this phenomenon appears intrinsic to the metal substrate and is independent of plasma type (ICP or CCP), as it was not observed on silicon oxynitride surfaces. (2) Surface Re-oxidation and Contamination. For instance, XPS analysis revealed that the surface oxygen concentration increased from 26% to 43.1% after argon plasma cleaning [48]. This oxygen uptake is primarily attributed to exposure to ambient air, as the activated, plasma-treated surface readily oxidizes at room temperature [47,52]. It is important to note that surface oxidation is not necessarily detrimental. Park, S. et al. [38] reported that oxygen plasma treatment intentionally forms a several-nanometer-thick nanocrystalline CuO layer. This oxide layer possesses a high surface energy, which can be advantageous for promoting strong initial bonding. (3) Re-deposition and Surface Damage from Prolonged Treatment. Yi, C.X. et al. [48] observed that material from the Ta cathode used to generate the plasma was inevitably deposited on the copper surface during prolonged cleaning. Furthermore, etched copper atoms or clusters can be reflected back to the sample surface through collisions with other species in the vacuum, leading to their re-deposition and the subsequent growth of three-dimensional islands. Li, W. [37] detected the mutual re-deposition of silver and copper onto each other’s surfaces during plasma processing. Generally, optimizing process parameters, such as reducing treatment time and increasing bias voltage, can help mitigate the occurrence of re-deposition [48].

3.1.2. Surface Functionalization Response

Plasma cleaning of copper and its alloys not only removes organic and inorganic contaminants but also achieves surface functionalization through the reconstruction of surface states, morphology modulation, and energy modification. This functionalization manifests as enhanced hydrophilicity, increased surface roughness, reduced grain size, and improvements in electrical, mechanical, and electrical insulation properties. (1) Enhanced Hydrophilicity: For instance, Li, W. et al. [37] reported that the water contact angle on a copper surface decreased sharply from 93.9° to 9.4° after argon plasma treatment (100 W RF power, 10 s), transforming it into a highly hydrophilic surface. The contact angle was observed to decrease further and eventually stabilize with increasing plasma power and process time. Park, S. et al. [38] attributed this effect to the highly activated state of the plasma-cleaned copper surface, which promotes hydrophilicity. (2) Increased Surface Roughness: Kim, S. et al. [53] used atomic force microscopy (AFM) to show that the root-mean-square roughness (Rrms) of a copper oxide surface increased from 6.8 nm to 9.4 nm after 20 treatment cycles at 250 W plasma power. This surface roughening improves adhesive strength by inducing mechanical interlocking and increasing the area for chemical interaction between surfaces. (3) Reduced Grain Size: Park, M. et al. [49] observed a broadening and decrease in intensity of the Cu(111) peak in X-ray diffraction (XRD) patterns with increasing RF power and pressure, indicating a reduction in grain size at the surface. (4) Enhanced Electrical Performance: Cvelbar, U. et al. [23] demonstrated that the resistance of copper components cleaned via a combined oxygen-hydrogen plasma sequence was only 52.7 mΩ. This represents about 10% improvement over traditional wet chemical cleaning (which reduced resistance from 70.1 mΩ to 58.4 mΩ) and about 25% enhancement in overall conductivity, offering a significant advantage for electronic applications. (5) Enhanced Mechanical Properties: Frayssines, P.-E. et al. [54] showed that CuCrZr/CuCrZr joints fabricated with a plasma cleaning process could achieve an impact energy of 100 J/cm², compared to only 80 J/cm² for joints prepared using standard laboratory cleaning methods. (6) Enhanced Electrical Insulation Properties: Saressalo, A. et al. [55] found that cleaning copper electrodes with O₂ and Ar plasma effectively reduced surface impurities and significantly improved dielectric withstand performance. The electric field strength achievable before the first breakdown event increased by over 90% for both hard and soft copper electrodes.

The functional applications of this surface functionalization primarily focus on low-temperature bonding enhancement and high-reliability interconnects. (1) Low-Temperature Bonding Enhancement: Plasma cleaning enables robust bonding at significantly lower temperatures. Park, S. et al. [38] demonstrated that oxygen plasma-treated samples achieved a shear strength of 27.86 MPa, representing an approximately 40% improvement over untreated samples, indicating a substantial increase in the mechanical strength of Cu bonds. Wang, J. et al. [50] successfully achieved defect-free Cu–Sn bonding at a low temperature of 200 °C by employing plasma cleaning combined with a self-assembled monolayer pretreatment, attaining a high bond strength of 68.7 MPa. Furthermore, Kim, S. et al. [53] used Ar/H₂ plasma to clean copper oxide layers, obtaining surfaces suitable for Cu-Cu bonding. Notably, even as the bonding temperature was reduced from 415 °C to 300 °C, the interfacial adhesion energy increased from 1.8 J/m² to 5.55 J/m² after plasma treatment. In contrast, wet-chemical treatment resulted in a decrease in adhesion energy from 2.71 J/m² to 0.74 J/m². (2) High-Reliability Interconnects: Plasma processing is critical for enhancing the longevity and stability of advanced interconnects. Ito, F. et al. [39] developed a process using NH₃ plasma under high-RF power and -pressure conditions, which produced 22 nm-node low-k/Cu interconnect structures (L/S = 40/40 nm) with a longer electromigration lifetime and no significant degradation in resistance–capacitance performance. Similarly, Wang, J.-P. et al. [56] reported that optimizing argon plasma cleaning parameters—specifically using a higher pre-clean bias power and a shorter pre-clean time—effectively cleaned the via bottom. This optimization minimized the negative impact on via resistance while simultaneously significantly improving the electromigration and stress migration reliability of the Cu interconnects.

Plasma cleaning provides an efficient and versatile surface treatment strategy for copper and its alloys. By utilizing inert gases (e.g., Ar) for physical sputtering or reactive gases (e.g., O₂, H₂, NH₃) and their mixtures (e.g., N₂/H₂) for chemical reduction and oxidation, the cleaning process significantly enhances key surface properties, including hydrophilicity, roughness, active site density, and electrical performance. Such enhanced surfaces are ideal for low-temperature, high-strength Cu–Cu bonding and highly reliable interconnect technologies, thereby establishing plasma cleaning as a key surface engineering solution for microelectronic packaging and integration.

3.2. Aluminum and Its Alloys

Aluminum and its alloys are characterized by good ductility, high specific strength, ease of formation, and adequate thermal and electrical conductivity, making them suitable for a wide range of applications in the aerospace, automotive, architectural, and high-speed rail industries [57,58]. However, their surfaces are naturally covered with a loose native alumina (Al₂O₃) layer and are often contaminated with processing oils and greases. These surface conditions severely hinder subsequent processes such as bonding, painting, and welding [59].

3.2.1. Plasma Cleaning Methodologies

Surface contaminants on aluminum and its alloys primarily consist of organic pollutants (e.g., hydrocarbons) and inorganic pollutants (e.g., oxides). For organic contaminants, Wang, Z. et al. [14] utilized an atmospheric pressure plasma jet to clean acrylic polyurethane paint from an aluminum alloy surface, finding that organic substances like resins were effectively removed, primarily through reactions with reactive plasma species. Similarly, Prysiazhnyi, V. et al. [60] employed atmospheric pressure diffuse coplanar surface barrier discharge (DCSBD), confirming its efficacy in reducing hydrocarbon contamination on aluminum. Azioune, A. et al. [61] reported that after plasma cleaning, XPS results showed increased aluminum and oxygen content, decreased carbon content, and a water contact angle of less than 10°, indicating a surface clean enough for most industrial processes. Mui, T. et al. [40] corroborated the removal of hydrocarbon contaminants via Fourier Transform Infrared (FTIR) spectroscopy. Ikeda, A. et al. [62] demonstrated that wet cleaning was ineffective against carbide residues for small 5 × 5 μm² aluminum pads, whereas plasma cleaning successfully removed carbon contamination, enabling the subsequent successful growth of electroless nickel plating. Furthermore, Bónová, L. et al. [63] showed that even a thick layer of machine oil could be successfully cleaned from an aluminum surface using air-based microwave plasma discharge at 600 W power and a substrate speed of 2.5 mm/s. For inorganic oxides, Kim, D. et al. [64] used a low-oxygen thermal plasma (LO-ITP) to clean aluminum micro-powder, reducing the surface oxide layer thickness to approximately 1.9 nm and achieving a high electrical conductivity of 2.9 × 10⁷ S/m, comparable to cast aluminum bulk, confirming successful surface cleaning. Chen, G. et al. [65] leveraged argon plasma cleaning to remove the oxide layer from an aluminum foil surface, thereby activating its surface chemical state. This activation successfully facilitated aluminum doping, improved the quality of the subsequent Cu₂ZnSnS₄ (CZTS) film, and reduced the resistance at the Al/CZTS interface.

Plasma cleaning methodologies for aluminum and its alloys primarily employ single gases or gas mixtures, including air, argon, oxygen, Ar/O₂, Ar/H₂, and N₂/O₂. Prysiazhnyi, V. et al. [60] demonstrated that air plasma not only effectively reduced surface contaminants on aluminum but also significantly increased the surface hydroxyl (-OH) group content. Similarly, Wang, Z. et al. [14] used compressed air plasma to treat an aluminum alloy surface, reporting that a 360 s treatment reduced 35 μm paint layer to approximately 5 μm (Figure 3). Bónová, L. et al. [63] attributed the efficacy of compressed air plasma to its abundance of oxygen radicals and hydroxyl radicals, which play a key role in the chemical removal of surface contaminants. Research on argon and oxygen-based plasmas reveals distinct advantages. Azioune, A. et al. [61] found that pure argon plasma was the most effective method for cleaning aluminum surfaces, leaving almost no oxygen or carbon contamination. However, they noted that oxygen or Ar/O₂ mixtures were more useful for generating aluminum hydroxide compounds. Wang, F. et al. [4] confirmed the role of oxygen, stating that its addition to argon plasma generates reactive oxygen species (ROS) such as H₂O₂, O₂⁻, ·OH, HO₂⁻, and O₃, which react with and rapidly decompose contaminants. For specialized applications, other gas mixtures prove beneficial. Ikeda, A. et al. [62] reported that Ar/H₂ plasma effectively removed contaminants from small aluminum pads, making the technique suitable prior to zinc displacement plating and electroless nickel deposition processes. Kim, M. et al. [66] achieved optimal adhesion for aluminum using an atmospheric pressure plasma jet with a N₂/O₂ mixture under specific conditions: a nozzle-to-surface distance of 10–20 mm and a nozzle travel speed of 5 mm/s.

However, plasma cleaning of aluminum and its alloys faces several common challenges: (1) High Stability of Aluminum Oxides and Difficulty in Effective Removal by Reductive Plasmas: Prysiazhnyi, V. et al. [60] observed, based on spectral changes, that while DCSBD plasma treatment modifies the aluminum surface by increasing the number of chemically active hydroxyl groups, this may subsequently lead to re-oxidation upon air exposure. Kim, D. et al. [64] explicitly stated that alumina (Al₂O₃) cannot be completely removed via hydrogen reduction. Corroborating this, Yasuda, H.K. et al. [67] found that while oxygen plasma effectively removes organic contaminants, plasma treatments using mixtures of argon and hydrogen were difficulty to alter the state of the oxide layer on these alloy surfaces. This resistance is attributed to the exceptional stability of aluminum oxides, which can withstand plasma cleaning and modification efforts. (2) Poor Universality of the Cleaning Process Across Different Alloys: Yasuda, H.K. et al. [67] highlighted that the adhesion of polymers and their effectiveness in protecting aluminum alloy substrates against corrosion depend on both the initial substrate cleanliness and the state of the surface oxide layer. Both of these factors are influenced by the specific type of aluminum alloy used. Consequently, the optimal plasma surface treatment method varies with the alloy type. Furthermore, prolonged plasma treatment can induce thermal effects on the aluminum alloy, potentially leading to changes in the concentration of alloying elements at the surface.

3.2.2. Surface Functionalization Response

Plasma cleaning of aluminum and its alloys effectively removes surface organic contaminants and induces various surface functionalization responses, including changes in surface morphology, grain size, surface activation phenomena, as well as improvements in corrosion resistance and electrical conductivity. (1) Changes in Surface Morphology: Plasma treatment can lead to either a decrease or an increase in surface roughness, depending on the cleaning conditions and the initial state of the surface. Prysiazhnyi, V. et al. [60] observed that an untreated aluminum surface featured numerous, randomly distributed “islands” over 100 nm in height. After 3 s of plasma cleaning, the island count dropped to 60% of the initial value; it decreased to 50% after 10 s, 10% after 40 s, and after 100 s, only a few islands remained, resulting in a final roughness below that of the untreated surface. Similarly, Wang, F. et al. [4] reported that the surface roughness of an aluminum plate contaminated with lycopene dropped significantly from 564.8 nm to 111.7 nm after plasma cleaning (Figure 4). Conversely, Wang, Z et al. [14] found that the arithmetical mean height (S_a) and the root mean square height (S_q) of an untreated sample were 481 nm and 616 nm, respectively. Within the first 10 s of plasma cleaning, both S_a and S_q increased rapidly to 822 nm and 1058 nm. The originally flat and dense surface became covered with non-removable, inactive particles of spherical, rod-like, and layered shapes, ranging from nano- to micro-scale, leading to the observed roughening. Terpilowski, K. et al. [68] also reported that argon plasma treatment increased the surface roughness (R_a) of an aluminum alloy (AMS 4049) from 107.0 ± 4.5 nm to 115.1 ± 5.2 nm. Furthermore, Chen, G. et al. [65] noted that prolonged plasma treatment caused over-etching of a CZTS film deposited on aluminum, creating defects and increasing surface roughness. Therefore, identifying optimal plasma cleaning conditions is crucial to strike a balance between achieving sufficient cleaning and avoiding surface damage. (2) Changes in Grain Size: The impact of plasma cleaning on grain size also exhibits variation, influenced by the material and processing parameters. Prysiazhnyi, V. et al. [60] reported grain refinement on aluminum surfaces, where the average grain size decreased from 700 nm (untreated) to about 160 nm after 3 s of plasma treatment, further reducing to approximately 130 nm after 40 s, and reaching 100 nm after 100 s. In contrast, Chen, G. et al. [65] observed the opposite effect in their system, where plasma-cleaned samples exhibited significantly larger grain sizes compared to untreated ones. Furthermore, the average grain size of the CZTS film increased with rising cleaning voltage. This phenomenon was attributed to plasma-enhanced diffusion doping of aluminum, which promoted grain growth in the CZTS layer. (3) Surface Activation Phenomena: Mui, T. et al. [40] reported that the water contact angle on untreated AA7075 aluminum alloy was 89° ± 2°, which dropped sharply to 5° ± 1° after DBD plasma treatment (Figure 5a). Concurrently, the surface free energy (SFE) increased markedly to approximately 70 mJ/m², nearly 2.5 times that of the untreated sample (Figure 5b). This improvement in wettability and SFE following plasma cleaning was also confirmed by Terpilowski, K. et al. [68]. Furthermore, Bónová, L. et al. [63] demonstrated that the hydrophilic character of the aluminum metal was retained for at least 24 h after plasma treatment (Figure 5c). (4) Enhanced Corrosion Resistance: Mui, T. et al. [40] found that plasma treatment prior to coating provided superior corrosion protection for AA7075 substrates. It significantly reduced the electrochemical corrosion rate (by approximately threefold) and shifted the corrosion potential from −0.67 V to −0.35 V, indicating superior corrosion resistance. (5) Improved Electrical Conductivity: Chen, G. et al. [65] stated that plasma cleaning produces a cleaner and more active surface, which improves electrical contact and reduces interfacial resistance. The clean surface minimizes charge carrier scattering, thereby enhancing carrier mobility and overall conductivity.

The functional applications of this surface functionalization primarily focus on coating adhesion enhancement and microelectronic packaging. (1) Coating Adhesion Enhancement: Mui, T. et al. [40] evaluated the adhesion according to the ASTM D3359 tape test, finding that the rating for plasma-treated samples improved from 0B (complete peeling) to 5B (no paint removal). This indicates a drastic enhancement in the bonding strength between the polyurethane coating and the AA7075 aluminum alloy substrate. The study attributed this significant improvement primarily to the surface activation and cleaning effects of the plasma, rather than to an increase in surface roughness. Bónová, L. et al. [63] highlighted an additional advantage, noting that the plasma cleaning method reduces chemical waste generation compared to traditional chemical treatments while simultaneously improving the adhesion of subsequent protective coatings. Furthermore, the work of Carrino, L. et al. [69,70,71] supports that the combined mechanical bombardment and chemical interactions from the plasma significantly improve the wettability and cleanliness of aluminum alloy surfaces. This activation leads to a slower decay of wettability over time, thereby promoting superior paint film adhesion. (2) Microelectronic Packaging: Ikeda, A. et al. [62] demonstrated its effectiveness for small-scale features, showing that plasma cleaning successfully removed carbon contamination from 5 × 5 μm² aluminum bond pads. Crucially, electroless nickel (Ni) successfully grew on these plasma-cleaned pads, whereas no Ni deposition occurred on pads cleaned with conventional wet chemicals. This makes plasma cleaning a critical pretreatment step for electroless nickel plating on small aluminum pads used in advanced packaging technologies like flip-chip and under-bump metallurgy (UBM).

For aluminum and its alloys, plasma cleaning methodologies utilizing air, argon (Ar), oxygen (O₂), and their mixtures (e.g., Ar/O₂, N₂/O₂) effectively remove surface organic contaminants and partial oxides through physical sputtering and chemical oxidation by reactive radicals. This process achieves surface purification while simultaneously inducing significant functionalization responses: a dramatic increase in surface hydrophilicity and free energy, alongside improved corrosion resistance and electrical conductivity. These optimized surface properties offer distinct advantages in enhancing coating adhesion and enabling selective metallization (e.g., electroless nickel plating) in microelectronic packaging, establishing plasma cleaning as a key surface engineering solution for industrial applications in aerospace, automotive, and related fields.

3.3. Titanium and Its Alloys

Titanium and its alloys are widely used as biomedical implant materials due to their excellent biocompatibility and corrosion resistance, which are attributed to a thin, stable surface titania (TiO₂) layer. Furthermore, an increase in the thickness of this oxide layer has been shown to significantly improve hemocompatibility (blood compatibility) [72,73]. However, the native oxide layer on untreated titanium alloys is often too thin to provide high bioactivity. Therefore, surface treatment, such as plasma cleaning, is necessary to enhance surface wettability and promote subsequent cell adhesion and proliferation [74].

3.3.1. Plasma Cleaning Methodologies

Plasma cleaning effectively removes surface contaminants from titanium and its alloys. Pászti, Z. et al. [75] noted that untreated titanium surfaces are almost invariably covered by a several-nanometer-thick hydrocarbon contamination layer, primarily consisting of CH₂ or CH₃ groups along with some carbon–oxygen bonds. Jun, J. et al. [76] demonstrated that over 90% of carbon atoms were removed from the TiO₂ surface following argon plasma treatment. The efficacy of this cleaning directly enhances bioactivity, as shown by Ou, K.-L. et al. [73], who reported that argon plasma successfully removed adsorbed contaminants and impurities, resulting in a titanium surface with superior bioactivity compared to the untreated state. Cools, P. et al. [77] highlighted that plasma cleaning achieves results comparable or superior to those of thermal and chemical cleaning within minutes. Lin, C.C. et al. [78] observed that argon plasma treatment effectively eliminates surface contaminants (e.g., particles), while also inducing phenomena such as atomic mixing, ion bombardment, and plasma-induced surface reactions. Furthermore, plasma is highly effective against biological contaminants. Fingerle, M. et al. [41] established that oxygen plasma is a necessary and effective method for eradicating biological contamination from titanium surfaces, achieving complete recovery. This capability extends to both plain and micro-structured surfaces, enabling the reliable removal of even thick biofilms. Moreover, the application of plasma cleaning technology can be extended to metallic substrates with functional coatings. Schwartz, A. et al. [79] systematically evaluated the efficacy of various methods for cleaning Plasma Electrolytic Oxidation (PEO) bioactive coatings on Ti-6Al-4V alloys. Their research demonstrated that plasma cleaning significantly removes surface organic contaminants (e.g., reducing the carbon content from 5.21 at.% to 1.01 at.%) and effectively enhances surface hydrophilicity, demonstrating its potential for treating such complex surfaces.

Plasma cleaning protocols for titanium and its alloys primarily utilize single gases or gas mixtures, including argon, oxygen, air, and Ar/O₂. Pászti, Z. et al. [75] demonstrated that hydrocarbon contaminants could be readily eliminated after a few minutes of plasma treatment using argon, oxygen, or air at 10–20 Pa with an AC voltage of 700–1500 V. Lin, C.C. et al. [78] reported argon plasma cleaning reduced surface contamination, with the enhanced cleanliness providing a larger effective surface area for cell attachment, thereby improving bioactivity. A particularly effective strategy involves sequential processing, as shown by Ou, K.-L. et al. [73], who first used argon plasma to remove contaminants, followed by oxygen plasma to oxidize the surface. This two-step process not only created a nanostructured titania layer but also transformed the near-surface microstructure, significantly enhancing biocompatibility. Cools, P. et al. [77] confirmed the high decontamination efficiency of non-thermal plasma in both air and argon environments, emphasizing that it modifies only the surface nanostructure without affecting the bulk material properties. Gas mixtures can leverage synergistic effects. Duske, K. et al. [42] demonstrated that an atmospheric-pressure Ar/O₂ plasma jet was markedly more effective than pure argon plasma in rendering titanium surfaces superhydrophilic and enhancing the spreading of osteoblastic cells, with its performance being unaffected by the surface’s original topography. The chemical efficacy of oxygen-based plasmas, as noted by Paredes, V. et al. [80], stems from reactions with highly reactive oxygen radicals that remove organic contaminants while simultaneously promoting surface oxidation and the formation of hydroxyl (-OH) groups, which are crucial for bioactivity.

However, plasma cleaning of titanium and its alloys faces a common challenge: the surface is prone to oxidation and contamination. Pászti, Z. et al. [75], using XPS and AES analysis, demonstrated that a plasma-cleaned surface immediately forms a several-nanometer-thick, stoichiometric TiO₂ film upon exposure to air. Oxygen plasma treatment naturally leads to the formation of an even thicker oxide film. These freshly oxidized surfaces are highly susceptible to rapid recontamination by atmospheric hydrocarbon layers. Furthermore, Cools, P. et al. [77] demonstrated that plasma cleaning/activation is more effective than the chemical cleaning methods and more mild than the thermal cleaning procedure. However, the highly active surface, when exposed to air, is prone to adsorb carbon-containing contaminants from the atmosphere, leading to surface recontamination. This inherent tendency for carbon uptake from the atmosphere presents a challenge for maintaining the highly active, clean state desired for applications like biomedical implantation.

3.3.2. Surface Functionalization Response

Plasma cleaning of titanium and its alloys not only removes surface contaminants but also induces significant functionalization responses, including dramatically enhanced hydrophilicity, surface smoothing, and the emergence of nanocrystallization. (1) Significantly Enhanced Hydrophilicity: Plasma treatment can induce a superhydrophilic state on titanium surfaces. Duske, K. et al. [42] reported that the water contact angle on untreated titanium disks was 68–117°, which dropped to nearly 0°—achieving superhydrophilicity—after treatment with a 1.0% O₂/Ar plasma for 60 or 120 s. Paredes, V. et al. [80] also observed superhydrophilicity on oxygen plasma-treated titanium alloys, with the surface energy increasing from 51.9 mN/m to 74.9 mN/m. This indicates enhanced surface polarity, attributed to the introduction of radicals or hydroxyl groups (OH⁻, O₂⁺, O₂⁻, O⁺, O⁻) and the removal of contaminants, a view supported by Terpilowski, K. et al. [68] and Winiecki, M. et al. [74]. Furthermore, Huang, M.-S. et al. [81] found that argon plasma effectively removed contaminants and that the optimal hydrophilicity was achieved under specific conditions: 100 W power, 25.33 Pa (190 mTorr) pressure, and a 12-min treatment time. (2) Surface Smoothing Effect: Lin, C.C. et al. [78] demonstrated that after 10 min of plasma treatment, the surface roughness of a titanium plate decreased from 17.20 nm to 2.02 nm. This smoothing effect was attributed to the bombardment by energetic free radicals and ions during the plasma process. In a related observation, Paredes, V. et al. [80] noted that oxygen plasma cleaning did not alter the surface roughness or bulk properties of their titanium alloy samples, which could be considered smooth as their average roughness (R_a) was less than 27 nm. (3) Nanocrystallization Phenomenon: Lin, C.C. et al. [78] reported that untreated titanium plate possessed a crystalline structure with relatively large grains. As the plasma treatment duration increased, the grain size gradually decreased, indicating that the reaction and bombardment by high-energy radicals and ions during plasma processing induced nanocrystallization. After 10 min of Ar plasma treatment, the surface grain size of titanium was reduced to approximately 20 nm. This nanostructured, short-range ordered crystalline phase is reportedly highly attractive due to its relatively high biocompatibility.

The functional applications of this surface functionalization primarily focus on enhancing biocompatibility for biomedical implants. Pászti, Z. et al. [75] used SFG spectroscopy to analyze bovine serum albumin (BSA) adsorption, finding that methyl groups of BSA on plasma-treated titanium exhibited greater disorder. This suggests the highly reactive surface formed stronger bonds with the protein, potentially stabilizing a more open structure that may improve subsequent cell adhesion. Lin, C.C. et al. [78] reported that plasma treatment significantly improved the biocompatibility of titanium plates. The osteoblast-like MC3T3-E1 cell/albumin/allylamine/TiO₂/Ti contact system maintained high activity after 48 h of culture, with cell growth rates on plasma-treated plates being markedly higher than on untreated plates. Duske, K. et al. [42] demonstrated that plasma treatment promoted cell growth regardless of surface topography, resulting in a significant about 57% increase in cell area (p < 0.001) without inhibiting metabolic activity, thereby eliciting a favorable cellular response. Fingerle, M. et al. [41] established oxygen plasma as a necessary and reliable tool for the complete cleaning and recovery of biofilm-contaminated titanium surfaces. They noted that while microstructured surfaces favored biofilm growth, they could be fully restored post-contamination without the plasma process affecting the microstructure itself. Paredes, V. et al. [80] observed that oxygen plasma cleaning increased the relative content of TiO₂ compared to other metal oxide species on the surface, which contributes to improved biocompatibility. Furthermore, Winiecki, M. et al. [74] showed that oxygen plasma treatment of a titanium alloy surface significantly enhanced the quality of subsequently deposited tannic acid coatings. These coatings were more uniform, exhibited fewer and finer cracks, and were thicker than those on untreated substrates, thereby improving the functional characteristics of biomedical implants.

For titanium and its alloys, plasma cleaning demonstrates highly effective surface purification and functionalization capabilities. Utilizing plasma generated from argon (Ar), oxygen (O₂), air, or their mixtures (e.g., Ar/O₂), this process effectively removes surface organic contaminants and biofilms while significantly modifying surface properties, including the induction of superhydrophilicity, reduction in roughness, and nanocrystallization. These surface modifications consequently enhance the material’s biocompatibility, promoting cell adhesion and proliferation, and facilitating the uniform deposition of functional coatings. Despite challenges such as susceptibility to recontamination, plasma cleaning remains a promising technique for the surface functionalization of titanium-based biomedical implants, owing to its high efficiency, mild processing conditions, and non-damaging nature to the substrate.

3.4. Alloy Steels

Steel encompasses a wide variety of types and finds extremely broad applications. For instance, stainless steel is utilized as the main frame for microchannel reactors due to its excellent corrosion resistance and thermal stability [82]. However, steel surfaces are invariably covered with native oxide layers and contaminants, and they are frequently contaminated with processing oils. These surface impurities often result in compromised adhesion of deposited hard coatings to the substrate [83,84].

3.4.1. Plasma Cleaning Methodologies

Surface contaminants on alloy steels primarily include organic pollutants (e.g., carbonaceous contaminants, anti-corrosion lubricants) and inorganic pollutants (e.g., oxides, mill scale). For organic contaminants, plasma cleaning demonstrates high efficacy. Kim, M. et al. [43] showed that an atmospheric pressure plasma jet (APPJ) could remove carbon contaminants from stainless steel, with XPS analysis revealing a decrease in surface carbon atomic concentration from 41.6 at.% to 24.7 at.%. Lin, J.-W. et al. [85] also reported reduced and more uniformly distributed carbon content on plasma-treated stainless steel surfaces. Ono, S. et al. [86] confirmed the effective removal of organic oil contaminants from stainless steel (SUS304) by plasma cleaning, evidenced by reduced carbon signals and enhanced substrate iron signals in XPS analysis. A quantitative study by Janík, R. et al. [87] using fluorescence detection found that DCSBD plasma cleaning (8 s) resulted in the lowest residual fluorescence intensity (3934 F.U.) on steel surfaces, significantly outperforming ultrasonic cleaning (9007 F.U.) and ethanol wiping (16576 F.U.), proving its superior performance in removing anti-corrosion lubricants. For inorganic contaminants, Meletis, E.I. et al. [84] demonstrated that electrolytic plasma processing (EPP) successfully removes oxides and mill scale from steel surfaces, with profilometry and EDAX confirming significantly reduced oxygen content on all EPP-treated samples—a finding corroborated by Schilling, P.J. et al. [88] for A-36 mild steel. Barshilia, H.C. et al. [83] reported that Ar + H₂ plasma pretreatment effectively removes the oxide and contaminant layer from stainless or mild steel (MS) substrates, significantly eliminating Fe₃O₄ and resulting in a surface composed primarily of metallic iron with low FeO concentration. Kumruoglu, L et al. [16] showed that electrolytic plasma cleaning removes and reduces iron oxides, rust, and other contaminants, applicable for general metal cleaning and coating removal, with results (Figure 6) visually confirming the elimination of scale, hydroxides, and various iron oxides. Cherenda, N. et al. [89] utilized a compressed plasma flow to remove an oxide layer containing F₂O₃/Fe₃O₄/FeO formed on steel by air annealing. Similarly, Gui, W. et al. [44] found that EPP effectively removed the mill scale and contaminants from rolled Fe6.5Si alloy surfaces, with nearly complete elimination achieved after 40 s of treatment.

Plasma cleaning methodologies for steel can be primarily categorized into gas-phase plasma and liquid-phase (or electrolytic) plasma cleaning. For gas-phase plasma cleaning, Yasuda, H.K. et al. [67] demonstrated that an Ar + H₂ plasma effectively removed oxides from a steel surface, providing a foundation for the subsequent plasma polymerization of trimethylsilane (TMS), ultimately achieving excellent adhesion and corrosion protection. Ono, S. et al. [86] utilized argon, Ar-O₂ and Ar-H₂O gas mixtures with microwave plasma technology at atmospheric pressure to clean stainless steel plates. Their results indicated that plasmas containing oxygen and water vapor molecules yielded superior surface cleaning. Specifically, the Ar-H₂O mixture exhibited stronger oxidative power, attributed to the presence of hydroxyl radicals (OH·). Furthermore, Tang, Z. et al. [90] reported that the oxide layer on low-carbon steel could be removed using low-pressure arc plasma technology. They also noted that during cleaning with nitrogen (N₂) plasma, a high-pressure environment favored the nitridation of iron. For liquid-phase plasma cleaning, the process involves plasma generation within an electrolyte. Meletis, E.I. et al. [84] described a method where plasma is continuously generated at a high rate through an electrolytic process in a solution, with the steel acting as the cathode immersed in the electrolyte, enabling surface cleaning. Schilling, P.J. et al. [88] employed an Electrophysical Cleaning and Deposition (EPCAD) process using a 14% sodium bicarbonate electrolyte at 220 V. In this setup, the plasma is generated continuously at high speed, producing fine sparks that cause local surface melting. This is accompanied by intense pressure fluctuations from bubble collapse and shock waves, effectively removing mill scale from the steel surface. Niemi, R. et al. [91] applied Plasma Electrolytic Tilting (PET) technology to treat stainless steel surfaces. Here, the stainless steel served as the anode and the electrolyte bath as the cathode, using a non-toxic aqueous solution of 1–5 wt.% ammonium sulfate ((NH₄)₂SO₄) as the electrolyte. Their findings showed that the stainless steel surface became smooth after just 1 min of Plasma Electrolytic Treatment.

However, plasma cleaning of alloy steels faces common challenges: (1) Aging Effect: The highly activated state achieved by plasma treatment is transient, leading to a significant aging effect where surface properties degrade over time upon air exposure. Tang, S. et al. [82] observed that the surface free energy of plasma-treated stainless steel began to decrease after just 10 min, a process predominantly driven by the increase in the non-polar component. Lin, J.-W. et al. [85] quantitatively tracked this phenomenon through water contact angle measurements: the contact angle increased approximately threefold from 7.1° to 20.63° after 10 min of aging, further rising to 61.86° after 30 min, 78.88° after 60 min, and reaching 81.75° after 1440 min (1 day). The limited stability of this activated state is a practical concern, as Kim, M. et al. [43] reported that the highly active state of the plasma-treated stainless steel surface remained stable in air for only about 15 h, a critical factor to consider for industrial applications involving long-term storage or processing. (2) Narrow Processing Window: Achieving optimal results requires precise control of both the plasma treatment duration and the timely utilization of the activated surface. Lin, J.-W. et al. [85] observed a non-monotonic trend in surface wettability, where the water contact angle on stainless steel decreased to 9.75° after 120 s of treatment but increased to 19.3° after 180 s. They attributed this degradation at longer times to reactions between active plasma species and surface radicals, which generate an unfavorable surface state. Mechanistically, Lee, C. et al. [45] proposed that prolonged plasma exposure covers the surface with a layer of inactive particles. This layer hinders adhesion by preventing direct contact and essential chemical bonding between the adhesive and the substrate. These findings collectively underscore that exceeding the optimal treatment timeframe can be counterproductive, necessitating careful process optimization.

3.4.2. Surface Functionalization Response

Plasma cleaning of alloy steels induces several key surface functionalization responses, including enhanced hydrophilicity, reduced grain size, changes in surface morphology, enhanced corrosion resistance, and improved adhesive bonding performance. (1) Significantly Enhanced Hydrophilicity: Lin, J.-W.et al. [85] reported that the water contact angle on an untreated stainless steel plate was 82.95°, which dropped to 1.60° under optimal plasma parameters (1000 W power, 80 mm torch-to-sample distance, 300 s treatment time, 1.5 wt% oxygen content), indicating a super-hydrophilic surface. This result is consistent with Ono, S. et al. [86], who also observed a significant reduction in the contact angle to nearly 0° after optimal cleaning, rendering the surface highly hydrophilic. Kim, M. et al. [43] showed that after plasma cleaning, the surface oxygen content on steel increased from 47.8 at.% to 59.3 at.%, accompanied by the generation of numerous oxygen-containing polar functional groups, which are directly responsible for the enhanced hydrophilicity. (2) Reduced Grain Size: Meletis, E.I. et al. [84] attributed this to local melting of the surface layer caused by plasma discharges within hydrogen bubbles generated during electrolytic plasma processing. The rapid cooling of this molten layer resulted in the formation of a fine-grained structure with a thickness varying between 150 and 250 nm and a grain size of approximately 10 to 20 nm, yielding a clean, nanostructured steel surface. Similarly, Lin, J.-W. et al. [85] reported that during treatment with an Ar/N₂/O₂ plasma, reactions between oxygen molecules/oxygen-containing species and the stainless steel surface generated oxides along with smaller and more uniformly distributed grains, contributing to a smoother surface topography. (3) Surface Roughness Modification: The effect of plasma on surface morphology and roughness is complex and process-dependent. Lee, C. et al. [45] observed a moderate increase in surface roughness after plasma treatment, with R_rms and R_a changing in the range of 3–7 nm and R_z changing by about 92 nm. They suggested that this increased roughness might expand the contact area, thereby enhancing mechanical adhesion. Barshilia, H.C. et al. [83] also reported that Ar + H₂ plasma treatment increased the surface roughness of stainless or mild steel substrates from 1.7 nm (untreated) to 6.5 nm, which favored mechanical interlocking for PVD coatings and enhanced coating adhesion. In contrast, Lin, J.-W. et al. [85] found that the average surface roughness of stainless steel decreased from about 174.62 nm (untreated) to approximately 128.49 nm after Ar/N₂ plasma treatment, with a more uniform distribution of surface peaks, indicating a smoothing effect. (4) Enhanced Corrosion Resistance: Meletis, E.I. et al. [84] demonstrated that the plasma-treated steel surface, characterized by its nanocrystalline structure, exhibited passive corrosion behavior. This passivation was a direct result of the physical processes during cleaning, rendering the steel significantly more corrosion-resistant than the base material. This characteristic of enhanced corrosion resistance prior to coating is crucial for the quality of subsequent coating processes and the allowable waiting time before coating application. (5) Improved Adhesive Bonding Performance: Lee, C. et al. [45] used an Ar/O₂ atmospheric pressure plasma for room-temperature pretreatment of cold-rolled steel sheets for automotive applications. Their results showed that under optimal conditions (350 W power, 75 s treatment time), the adhesive shear strength increased by approximately 23%. Furthermore, Barshilia, H.C. et al. [83] reported that the adhesion strength of TiAlN coatings increased from 191 mN on an untreated substrate to 298 mN on a substrate pretreated with Ar + H₂ plasma and an interlayer of Ti, indicating substantially improved coating adhesion.

The functional applications primarily focus on coating adhesion enhancement. Meletis, E.I. et al. [84] demonstrated that Electrolytic Plasma Processing (EPP) effectively cleans steel surfaces, creating an ideal anchor profile. This process enables the formation of strongly adherent, dense, and uniform zinc (Zn) and zinc–aluminum (Zn-Al) alloy coatings, which act as sacrificial anodes to protect the base steel. Simultaneously, the passive corrosion-resistant layer formed on the steel surface provides an additional protective mechanism. Schilling, P.J. et al. [88] showed via tensile adhesion tests that coatings applied to electrophysical cleaned surfaces exhibited adhesion performance comparable to traditionally grit-blasted samples. This improvement was attributed to the micro-roughness and unique surface morphology generated by plasma treatment. The enhancement is also rooted in improved chemical bonding. Tang, S. et al. [82] reported that after atmospheric plasma cleaning followed by polyacrylic acid (PAA) coating on stainless steel, the oxygen-containing groups generated by the plasma significantly promoted the formation of both hydrogen bonds and covalent bonds at the PAA–stainless steel interface. This led to a substantial increase in the bonding strength between the two plasma-treated stainless steel plates. Lee, C. et al. [45] found that plasma treatment of cold-rolled steel sheets significantly improved the bond strength of epoxy structural adhesive, achieving an approximately 23% increase over the untreated strength of 24.6 MPa under optimal parameters, with the adhesion state shown in Figure 7.

As an advanced surface engineering technique, plasma cleaning effectively removes various contaminants from alloy steels, including organic oils, oxides, and mill scale. By employing gas-phase (e.g., Ar, O₂, H₂) or liquid-phase (e.g., electrolytic plasma) technologies, this cleaning achieves efficient purification while significantly modifying the surface properties: inducing superhydrophilicity, promoting surface nanocrystallization, and modulating surface roughness. These functional changes effectively enhance coating adhesion and improve the corrosion resistance of the material, thereby boosting key performance metrics in applications such as microreactors, automotive bonding, and protective coatings. Despite challenges such as the aging effect and the narrow processing window, plasma cleaning remains a pivotal technology for achieving high-performance surface modification of steel materials.

4. Challenges and Future Perspectives

Plasma cleaning technology has demonstrated significant application potential and unique advantages in the field of metal surface treatment. However, its transition from the laboratory to large-scale industrial application still faces a series of scientific and technological challenges that require urgent solutions. Concurrently, the cross-integration of emerging technologies also points towards promising future directions for the development of this field.

4.1. Current Major Challenges

4.1.1. The Persistent Challenge of Surface Stability

Plasma-treated activated surfaces suffer from an aging effect. These surfaces, particularly the introduced polar functional groups, exist in a high-energy state. Upon storage in air, they undergo molecular rearrangement or adsorb hydrocarbons, leading to a decrease in surface energy and an increase in contact angle. Alternatively, exposure to air can cause surface re-oxidation and recontamination. For instance, the hydrophilicity of steel surfaces can significantly decay within hours to days, and titanium surfaces readily re-form contamination layers in air. This limits their application in industrial processes requiring long-term storage or specific handling sequences.

4.1.2. The Challenge Between Process Universality and Specificity

Although the principles of plasma cleaning are universal, the optimal treatment methods vary significantly for different metal surfaces, and a universal process standard is lacking. For example, removing the stable oxide layer from aluminum requires intensive physical bombardment or specific chemical reactions, whereas treating delicate copper circuits necessitates minimizing ion-induced damage. Current processes rely on empirical tuning, showing poor transferability between different alloys or even different material batches (e.g., aluminum alloys). A standardized, intelligent parameter library has not yet been established, posing a challenge for their scaled industrial application.

4.1.3. The Challenge of Treating Complex Contaminants and Structures

Plasma cleaning technology performs excellently in removing contaminants from single-component, flat surfaces. However, its efficacy is still inadequate for treating mixed contaminants of complex composition or workpieces with intricate 3D structures such as deep holes or micro/nano-features. For example, steel surfaces after processing may host a combination of mill scale, anti-rust oils, and coolant residues. These contaminants possess distinct physicochemical properties, demanding different reactive species and reaction pathways from the plasma. Furthermore, for complex aluminum alloy castings or porous titanium alloy implant structures, plasma-active species struggle to reach and act uniformly on all internal surfaces. This can lead to over-etching at protrusions and edges, while recessed areas remain insufficiently cleaned, ultimately compromising the quality of subsequent functionalization processes.

4.2. Future Development Directions and Perspectives

To address the aforementioned challenges, future research must seek breakthroughs across multiple dimensions, including fundamental mechanism studies, intelligent innovation, and technological synergy.

4.2.1. Long-Term Stabilization of Surface Activity

Future research will shift focus from merely creating highly active surfaces to locking in and maintaining this activated state. A core direction will be the development of integrated, continuous “cleaning–functionalization–passivation” processes. For instance, immediately following plasma cleaning, specific gaseous precursors could be introduced to fabricate an ultra-thin (nanoscale), dense, protective interfacial layer in situ on the activated clean surface via techniques like plasma-assisted chemical vapor deposition. This approach aims to achieve long-term stability of surface properties, thereby expanding the application prospects of plasma-treated components in fields demanding high long-term reliability.

4.2.2. Intelligent and Precision Process Control

The future lies in the integration of artificial intelligence and online diagnostics to develop intelligent, digital, and self-adaptive plasma systems. Such systems would leverage real-time feedback on metal type, contaminant composition, and surface morphology to dynamically adjust parameters like power, gas mixture ratio, and treatment time. This represents a crucial paradigm shift from experience-driven to data-driven operation, ultimately facilitating the standardization and precision of plasma cleaning processes for diverse applications.

4.2.3. Synergistic Innovation via Integrated Technologies

Acknowledging the inherent limitations of standalone plasma technology in dealing with complex contaminants and geometries, future advancements will emphasize a synergistic “plasma + X” approach. This includes hybrid strategies such as “plasma-supercritical fluid cleaning”, where supercritical fluids first remove bulk organic layers, followed by plasma for precision cleaning of molecular-level residues and oxides. Technologies like “immersed liquid plasma” can overcome the “line-of-sight” limitation, enabling uniform, omnidirectional treatment of complex 3D components. Furthermore, integration with other techniques (e.g., ultrasound, laser, micro-bubbles) can create powerful synergistic effects, unlocking new capabilities for challenging surface engineering tasks.

5. Summary

Plasma cleaning technology has emerged as an efficient, environmentally friendly, and versatile surface treatment solution, establishing itself as a paradigm in metal surface engineering. This review systematically elaborates on the complete process facilitated by this technology, ranging from fundamental contaminant removal to advanced surface functionalization, and reveals the complex mechanisms—through physical sputtering, chemical reactions, and their synergies—by which it influences metal surfaces.

By summarizing its applications across different metal systems—copper, aluminum, titanium, and steel—the review highlights the technology’s remarkable adaptability and superior treatment efficacy. Whether removing oxides from copper alloy surfaces to enhance the reliability of microelectronic packaging, or generating nanostructured titania layers on titanium alloys to improve their biocompatibility, plasma technology demonstrates advantages unparalleled by traditional wet chemical processes.

Nevertheless, the broader industrial adoption of this technology still faces challenges, which are anticipated to be overcome through the deep integration of novel processes and intelligent systems. With ongoing advancements in fundamental research and continuous breakthroughs in engineering technology, plasma cleaning is poised to provide indispensable technical support for the innovation and development of high-end manufacturing sectors such as microelectronics, aerospace, biomedicine, and new energy. Ultimately, it is destined to become a key engine for realizing green and intelligent manufacturing.