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. 2025 Aug 28;10(35):40076–40088. doi: 10.1021/acsomega.5c04758

Plasma-Assisted Surface Functionalization for High-Strength, Thermally Stable Copper Plating on Cycloolefin Polymer Films

Jing Sang †,‡,*, Sumio Aisawa †,‡, Chika Chida †, Dexin Chen §, Di Wang ∥, Hidetoshi Hirahara †
PMCID: PMC12423972  PMID: 40949195

Abstract

With the advancement of beyond 5G technologies, miniaturization of electronic devices, and proliferation of Internet of Things applications, transparent and flexible materials such as cycloolefin polymers (COPs) have garnered increasing attention as candidate antenna substrates. However, the inherent lack of chemically reactive functional groups on COP surfaces poses challenges for their use in antenna and circuit board fabrication. In this study, we propose a surface modification strategy that combines mild low-pressure plasma treatment with the subsequent introduction of functional molecular layers to enhance interfacial adhesion between copper (Cu) and COP substrates. Importantly, this approach maintains the inherent low surface roughness of COP while increasing chemical reactivity. We systematically investigated the influence of plasma treatment time (30 s to 10 min) on the COP surface properties, molecular layer bonding, and the adhesion performance of Cu/COP. A peel strength exceeding 1.2 kN/m was achieved when plasma treatment time was kept below 5 min, and adhesion values above 1.0 kN/m were retained even after thermal aging at 120 °C for 7 days. To clarify the mechanisms underlying adhesion performance, we thoroughly characterized the chemical structure at the Cu/COP interface and evaluated the effects of plasma treatment on both interfacial strength and thermal durability. Notably, atomic force microscopy–nanoscale infrared spectroscopy (AFM-nanoIR) was employed to directly probe chemical changes at the Cu/COP cross-section, providing submicron spatial resolution. This analysis revealed that oxidation at the interfaceparticularly intensified after thermal exposurewas a primary factor contributing to interfacial degradation and reduced adhesion performance. These findings highlight the critical role of plasma-induced surface activation in enhancing metal–polymer adhesion and offer insights into the limitations of current modification strategies. The results further suggest potential strategies for improving interfacial reliability in chemically inert polymer systems, paving the way for their broader use in diverse metal adhesion applications.

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Introduction

Cycloolefin polymer (COP) resins have become increasingly popular in the design and manufacturing of high-performance electronic products, particularly in applications requiring low costs, low dielectric constants, high transparency, and enhanced performance. , Specifically, given their exceptional transparency, good chemical stability, low water absorption capacity, and excellent electrical insulating capability, COP resins have shown considerable potential for enhancing device performance. − Moreover, with the growing demand for high-performance materials in electronics applications, the use of COP resins in this sector is expected to expand further.

However, the low surface energy of COPs limits their adhesion to other materials, such as metals. This limitation is especially critical in electronic packaging and flexible circuit board applications, where COPs are used in combination with other materials. Generally, adhesion between resin substrates and metal circuitry is achieved through a combination of surface activation processes, chemical surface treatments, and adhesive application. Among these, during chemical surface treatment, the substrate is partially dissolved using solvents to create a roughened surface. The applied adhesive then relies on mechanical interlocking with this roughened surface to achieve strong adhesion. However, given the chemical stability of COPs, they are highly resistant to solvent-based surface treatments, which makes it difficult to induce surface roughness without compromising their inherent properties. Moreover, highly roughened COP surfaces pose challenges for their applications in high-frequency electronic devicessuch as beyond 5G systems and artificial intelligence hardwareowing to the skin effect, which limits transmission efficiency at high frequencies. Additionally, the presence of adhesives at the COP–metal interface can degrade the inherent properties of COPs, limiting their advantages in electronic devices. Consequently, solvent-free methods capable of bonding COPs to copper circuits while minimizing changes in their surface roughness have garnered considerable research interest. For instance, vacuum ultraviolet (VUV) irradiation or plasma treatment can be used to activate the COP surface, enhancing its reactivity with metal surfaces. −

Given that COPs comprise entirely of carbon–carbon (C–C) and carbon–hydrogen (C–H) bonds and lack reactive functional groups, activation methods such as UV and plasma treatment can be employed to introduce surface-active functional groups. Plasma treatment is particularly favored in this context owing to its broad applicability, operational simplicity, and environmental compatibility. − It is a widely used surface pretreatment method that improves adhesion by modifying the surface chemical composition. However, while plasma or UV treatment is commonly employed to improve adhesion by introducing functional groups, achieving strong interfacial bonding often requires high energy input. Such intensive treatment not only promotes functionalization but also increases surface brittleness, , which negatively affects both the bond strength between COPs and Cu and the thermal resistance at device interfaces. Therefore, there is a strong demand for surface modification strategies that enable high-strength Cu adhesion to COPs while preserving the polymer’s intrinsic surface roughness and mechanical properties. Moreover, the mechanisms underlying the degradation of interfacial strength during the plasma or UV treatmentand in particular, the decline in thermal resistance at the Cu/COP interfaceremain insufficiently understood and warrant further investigation.

In this study, we developed a COP surface modification technique capable of achieving strong adhesion without substantially altering surface roughness or requiring the use of adhesives. The technique combines low-pressure, weak-energy plasma treatment with a triazine-based silane coupling agent that not only chemically bridges the plasma-activated COP surface and Cu, but also exhibits high thermal resistance. This dual-functional molecular modification approach was designed to preserve the mechanical integrity of COPavoiding surface brittlementwhile still achieving a practical bonding strength exceeding 1.0  kN/m. The synergy between plasma activation and targeted chemical modification enables a simple yet highly effective process for robust interfacial adhesion. We further investigated how plasma pretreatment conditions influence the bond strength and thermal resistance of the resulting COP/Cu composite, as well as the interfacial reactions and mechanisms occurring during thermal durability testing.

Beyond the technical implementation, this work provides fundamental insights into how plasma pretreatment modulates surface chemistry, interfacial strength, and thermal durability at polymer–metal interfaces. Although the study focuses on COPs, the proposed strategy may also be applicable to other chemically inert polymers such as polypropylene and polyethylene, offering a generalizable solution to their well-known adhesion limitations. From an engineering perspective, the method shows strong potential for enabling the reliable integration of such polymers in advanced applications requiring lightweight, flexible, and thermally resilient materials, including next-generation flexible electronics and high-frequency packaging systems.

Experimental Section

Materials and Reagents

COP films (ZF16, thickness = 100 μm; Zeon Co.) were cut into 3 cm × 6 cm specimens for Cu plating experiments in this study. Further, two triazine-based silane coupling agents, namely N-[3-(triethoxysilyl)­propyl]-1,3,5-triazine-2,4,6-triamine (aTES) and monosodium N-[3-(triethoxysilyl)­propyl]-1,3,5-triazine-2,4-dithiol-6-amine (nTES) (Sulfur Chemical Institute Co., Ltd.) were diluted using distilled water, were used as surface treatment agents to graft a molecular layer onto the COP surface.

A 0.28 wt % Predip solution was prepared by dissolving Predip 404 (DuPont Japan) in distilled water. Subsequently, a catalyst solution was prepared by combining 100 mL of the Predip solution with 1.5 mL of Cataposite 44 (DuPont Japan) and an Accelerator 19E solution (DuPont Japan) to obtain a final concentration of 5 vol %.

For electroless plating, an Adcopper solution (ATS Adcopper, Okuno Chemical Industries Co., Ltd.), prepared according to the manufacturer’s instructions, was used. Electroplating was performed using a copper sulfate electroplating solution.

TES-Based Surface Modification and Cu Plating on COP Substrates

Before depositing the TES functional layer, COP substrates were pretreated using plasma exposure. The substrates were first cleaned with ethanol in an ultrasonic bath for 10 min, followed by air drying. Surface functionalization of these substrates was carried out in a plasma reactor (Pico, Diener Electronic Co., Japan). They were then placed in the reaction chamber, which was evacuated and then filled with O2 (flow rate: 20 sccm) to create an gas atmosphere. Plasma was generated at 0.4 mbar (150 W), and the substrates were exposed to plasma treatment for 30 s, 1, 3, 5, or 7 min. Immediately after plasma treatment, the COP substrates were immersed in 2.4 mM solutions of aTES and nTES at 28 °C for 10 min each, as this concentration was found to be optimal for molecular adsorption. , They were then rinsed with distilled water. The treated samples were subsequently subjected to copper plating.

The TES-bonded COP substrates were immersed in a palladium-free Predip solution for 1 min, followed by immersion in the catalyst solution for 1 min at 50 °C. The samples were then rinsed under running water and subsequently in distilled water. After catalyst treatment, the substrates were washed with an accelerator to remove excess Sn from the catalyst colloid, thereby creating a catalyst-activated surface.

The activated substrates were immersed in a proprietary copper electroless plating solution for 15 min at 35 °C, in accordance with the manufacturer’s recommended procedure. To facilitate peel strength measurements, an electrolytic Cu layer was deposited to a thickness of 28 μm.

Characterization and Testing

Adhesion Strength

Mechanical adhesion between the metallic layer and the COP substrates was evaluated using a 90° peel test conducted on a tensile testing machine operating at 50 mm/min. The Cu-plated COP composites (COP/Cu) were subjected to thermal aging at 120 °C for 1, 3, 4, 7, 14, and 21 days. Peel strength was measured at each time point to assess the thermal durability of the composites. The reported adhesion strength values are based on 5 repeats.

XPS

The chemical composition and structure of the COP surfaces, as well as the surfaces exposed after the peel test, were analyzed using X-ray photoelectron spectroscopy (XPS) with a Versa Probe ESCA system (ULVAC-PHI Inc.). For XPS analysis of the delaminated interface, samples were peeled prior to vacuum introduction to obtain both the COP side and the Cu side, ensuring the preservation of a fresh interfacial state. All specimens for XPS measurements were cut into dimensions of 4 mm × 5 mm and subjected to XPS measurements using a multifunctional spectrometer equipped with a focusing monochromator and an Al Kα X-ray source (spot size: 100 μm × 100 μm). For high-resolution scans, the analyzer pass energy was set to 93.9 eV at 24.3 W. Angle-resolved measurements were recorded at an electron takeoff angle of θ = 45°.

During the XPS measurements, the analysis chamber was maintained at a pressure of 3.0 × 10–6 Pa, and charge neutralization was applied. The XPS spectra were processed using Shirley background subtraction, and energy calibration was performed by referencing the C 1s peak at 284.8 eV. The full width at half-maximum (FWHM) of the C–C/C–H component was allowed to vary freely, while the FWHM of the other components was kept fixed. All spectral fittings were performed using variable-ratio Gaussian–Lorentzian line shapes with Multipak (ULVAC-PHI Inc.).

Contact Angle and Surface Free Energy

The wettability and surface free energy of COP surfaces subjected to various treatments were evaluated using an automated contact angle (Kyowa DMo-702). Wettability was assessed by measuring the contact angle with a 2 μL droplet of distilled water. Surface free energy was calculated using the measured contact angles with distilled water and diiodomethane, based on the Owens–Wendt method. The reported contact angle values are based on 5–8 repeats.

Surface Roughness and Morphology Analysis Using Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM)

AFM analysis was conducted using an SPM-9700HT system (Shimadzu) equipped with silicon probes (NCHR-20, Silicon SPM Sensors) with an Al coating. The probes had a resonant frequency of 320 kHz and a force constant of 42 N/m. Measurements were carried out in air using tapping mode. Surface roughness was evaluated from 1 μm × 1 μm height images acquired by AFM with the scan speed of 1 Hz. The reported Ra values are based on 5 repeats.

Interfacial morphologies of the COP/Cu samples were characterized using a TEM (JEM-2100, JEOL, Japan) system operated at an accelerating voltage of 200 kV. TEM specimens were prepared by Focused Ion Beam (FIB)­milling to obtain high-magnification cross-sectional images (90°) of the COP/Cu interfacial layer before and after the thermal resistance test.

Infrared (IR) and AFM-Nanoscale IR (nanoIR) Spectroscopy

The chemical structure of the surface exposed by peeling the COP/Cu sample, both before and after the thermal resistance test, was analyzed using Fourier transform infrared (FTIR) spectroscopy (JASCO FT/IR-4600AC) with an Attenuated Total Reflectance attachment (ATR PRO650G).

The interfacial chemical structure of COP/Cu was further examined using nanoIR spectroscopy. The corresponding measurements were performed using a Nano-IR platform (Nano-IR EasyScan 2 AFM, Nanosurf AG Co.) equipped with a quantum cascade laser and a high-resolution, low-noise AFM system integrated with a tunable infrared laser. Notably, this system enables nanoscale measurements of infrared absorption spectra as a function of wavenumber, providing spatial resolutions beyond the limits of conventional FTIR. The corresponding measurements were conducted in contact mode using a probe with a resonant frequency of 13 ± 4 kHz and a spring constant of 0.07–0.4 N/m.

Results and Discussion

Characteristics and Features of the Functionalized COP Surfaces

Effect of Plasma Exposure Duration and TES Treatment on COP Surface Roughness

The surface roughness of materials used in flexible printed circuit board substrates affects both high-frequency signal transmission speed , and adhesion strength with conductive lines. Surface treatments such as plasma and corona discharge introduce functional groups on polymer surfaces, thereby enhancing adhesion between the metal film and the polymer substrate. However, the surface roughness introduced by these treatments can reduce signal transmission speed in communication applications. Therefore, achieving strong adhesion while maintaining low surface roughness is critical. In this study, a low-energy, low-pressure plasma treatment was applied to investigate its effect on the surface roughness of COP films. Surface roughness was evaluated using AFM. As depicted in Figure a,c, the surface roughness (Ra) of the COP film after alcohol cleaning alone was 0.3 nm. After plasma treatment, a slight increase in roughness was observed: Ra increased to 0.4 nm after 30 s and remained below 0.5 nm even after 10 min of treatment.

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(a) Effect of plasma treatment duration and triazine ethoxysilane (TES) molecular layer modification on the surface roughness of COP samples. AFM height images of COP surfaces under different treatment conditions are shown as insets. (b) Effect of plasma treatment duration and subsequent TES modification on the surface free energy of COP substrates. (c) COP surfaces AFM images and Ra with different treatments.

After plasma treatment, a surface chemical modification process known as TES treatment was applied. TES treatment increased the surface roughness from 0.3 to 0.6 nm, likely owing to the adsorption of TES molecules. For COP pretreated with 30 s of plasma, TES treatment increased the roughness to 1.2 nm. Surface roughness continued to increase with longer plasma durations, reaching 2.0 nm after 5 min. However, additional increases in plasma duration had minimal effect on surface roughness. The change in roughness due to TES treatment was approximately 1.0 nm for plasma durations under 5 min and approximately 1.5 nm for longer durations. Because the estimated molecular length of TES is approximately 1 nm. This increase is consistent with the formation of a monolayer coating. Further increasing the plasma treatment time beyond 5 min did not lead to additional changes in Ra, suggesting that the silane layer had saturated via adsorption or reaction on the COP surface.

These results suggest that, in addition to physical adsorption, chemical reactions or specific functional group interactions may occur between TES molecules and the COP surface following plasma treatment. After the plasma treatment, more functional groups (e.g., C–O, CO, COOH) is generated on the COP surface under the O2 atmosphere (confirmed by the rising O ratio in XPS, Table and Figure ). These groups provide additional adsorption sites for TES molecules and form covalent Si–O–C bonds, thereby enhancing chemical bonding at the interface. The formation of a TES molecular layer contributes to the observed increase in surface roughness. However, the roughness increase induced by plasma pretreatment and TES modification remains within a few nanometers. These minor changes in roughness are unlikely to affect the performance of modern high-frequency signal transmission substrates.

1. Chemical Element Ratio and Chemical Structure of Cop Surfaces Treated by Plasma and TES (atom %).
plasma-treated COP
 plasma- and TES-treated COP
plasma treatment time (min) C O Si 285.0 eV
 286.5 eV
 287.8 eV
 289.4 eV
 plasma treatment time (min) C N O Si S
–C*C/–C*H –C*O –C*O –C*OO
0 100 0 0 100       0 68 15.8 9.8 3.3 3.1
0.5 83.8 16.0 0.2 85.7 4.4 4.4 5.5 0.5 66.8 16.7 12.5 2.9 1.2
1 82.9 16.9 0.2 85.0 5.3 4.0 5.7 1 64.9 17.6 13.2 3.1 1.2
3 83.4 16.5 0.1 84.1 5.5 4.8 5.6 3 65.3 17.6 12.5 3.2 1.4
5 83.2 16.7 0.1 82.6 6.0 4.8 6.6 5 57.8 20.4 14.9 3.9 3.0
7 81.3 18.5 0.2 83.2 6.4 4.1 6.2 7 63.2 18.3 13.8 3.2 1.5
10 82.0 17.8 0.2 83.3 6.0 4.4 6.3 10 66.0 17.0 13.0 2.8 1.2
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Deconvolution of the C 1s spectra of COP surfaces after plasma treatments of varying durations.

Effect of Plasma Treatment Duration and TES Modification on the Contact Angle and Surface Free Energy of COPs

The surface roughness results suggest that plasma treatment of the COP surface promotes adsorption, chemical reactions, or other molecular interactions with TES molecules. To further evaluate surface properties under different treatments, the contact angle (Table S1) and surface free energy of the COP samples was measured, as depicted in Figure b. The distilled water contact angle relative to blank COP after plasma activation (higher surface polarity), and rises again after TES treatment, reflecting the replacement of polar oxygen groups by the less-polar TES layer. The surface free energy of untreated COP was 37.3 mJ/cm2. Plasma treatment led to an immediate increase in surface free energy, likely due to the conversion of CH groups on the COP surface into oxidized functional groups such as COH, CO, and COOH. However, surface free energy did not increase further with longer plasma durations, remaining stable between 30 s and 10 min.

When TES treatment was applied to untreated COP, the surface free energy increased to 47.2 mJ/cm2. TES treatment after plasma pretreatment resulted in a slight additional increase, with surface free energy stabilizing around 50 mJ/cm2. This increase can be explained by the fact that untreated COP contains only nonpolar C–C and C–H bonds, while TES introduces polar Si–OH groups, thereby increasing hydrophilicity. However, since TES is still an organic molecule, its overall surface polarity is lower than that of the plasma-treated surface, which contains a high density of oxygen-containing functional groups. Accordingly, the surface free energy of COP treated with TES alone lies between that of untreated and plasma-treated samples.

Chemical Analysis of COP Surfaces

The chemical properties of the COP surface under different treatment conditions were analyzed using XPS. The results are presented in Table and Figure . The untreated COP surface exhibited a C content of 100%, as expected for a material composed entirely of C–C/C–H bonds. After plasma treatment, the O content on the surface increased, rising from 16.0% after 30 s of treatment to 18.5% after 7 min. Si was also detected at approximately 0.1%, likely originating from minor additives used during the COP forming process.

The above increase in O content is attributed to the formation of surface oxides. To identify the specific functional groups responsible for the formation of these oxides, deconvolution of the C 1s peak was performed. This analysis revealed the presence of C–O, CO, and COOH groups on the COP surface − after 30 s of plasma treatment. With longer treatment durations, the CO content remained unchanged, while that of C–O and COOH groups increased slightly by 1–2% before stabilizing after 5 min.

Following TES treatment, elements such as N, Si, O, and Soriginating from TESwere detected on the COP surfaces. After TES treatment, the O and N contents were higher on plasma-treated COP surfaces compared to untreated COP, indicating that TES molecules more effectively reacted with the functional groups introduced by plasma activation. This suggests that chemical bonding between TES and the surface occurs predominantly on plasma-functionalized substrates. Furthermore, among the plasma-treated samples, the surface elemental composition after TES modification remained relatively constant across treatment durations ranging from 30 s to 10 min. This implies that the density of reactive surface groups reaches a plateau at short plasma exposure times, which is consistent with the surface free energy and contact angle results.

Effect of Plasma Treatment Duration on the Adhesion Strength of COP/Cu Composites

Adhesion Strength of COP/Cu Composites

Cu was plated onto COP surfaces that had undergone plasma treatment followed by TES treatment, and the adhesion strength of the resulting COP/Cu composites was evaluated. The effects of plasma treatment duration and the presence or absence of TES treatment on adhesion strength are illustrated in Figure a-1.

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(a-1) Effect of plasma treatment time on adhesion strength of COP/Cu composite; (a-2) FTIR spectra of peeled COP side and (a-3) Cu side from COP/Cu composites prepared with different plasma treatment durations; (b-1) effect of thermal resistance test duration on adhesion strength of COP/Cu composites prepared with plasma treatment times of 30 s and 5 min; FTIR spectra of peeled surfaces of COP/Cu composites prepared with plasma treatment durations of (b-2) 30 s and (b-3) 5 min, after thermal resistance testing.

As shown in the inset image, copper plating could not be successfully achieved on the blank COP surface. The black curve represents the adhesion strength of samples treated with plasma only. Although plasma activation enabled copper deposition on the COP surface, the resulting adhesion strength remained below 0.2 kN/m, with no clear dependence on plasma treatment duration. In contrast, plasma treatment followed by TES treatment led to a substantial increase in adhesion strength, reaching 1.2 kN/m after 30 s and remaining above 1.0 kN/m even after 5 min of plasma treatment. However, with extended plasma treatment, adhesion strength declined, dropping to 0.69 kN/m after 10 min.

XPS results revealed that the amount of oxidized functional groups on the COP surface remained nearly constant during plasma treatment from 30 s to 5 min, with only a slight increase observed beyond 5 min. This suggests that the decrease in adhesion strength with extended plasma treatment may be attributed not only to functional group formation but also to the breaking of C–C or C–H bonds within COP molecules, resulting in oxide formation. Additionally, polymer chain scission may occur, making the surface layer more fragile than the bulk material, , which may influence the thermal resistance, as discussed in the following section.

Adhesion Strength of COP/Cu Composites after Thermal Resistance Testing

In practical systems such as electronic devices and flexible printed circuits, thermal resistance is a critical requirement. We examined the effect of plasma pretreatment conditions on thermal resistance, as depicted in Figure b-1. Specifically, the conditions at plasma treatment durations of 30 s and 5 min were compared. For the 30 s treatment, adhesion strength decreased slightly after 1 day but remained above 0.8 kN/m even after 21 days of thermal testing. In contrast, the adhesion strength of copper-plated samples treated for 5 min continuously declined during thermal testing, dropping below 0.4 kN/m after 21 days. These results reveal a substantial difference in thermal resistance between the 30 s and 5 min plasma treatments. The initial adhesion strength of both samples was above 1.0 kN/m. However, the long-term thermal durability differs significantly depending on the plasma treatment duration. This finding highlights the importance of optimizing plasma exposure time to avoid excessive oxidation and degradation at the interface. While short plasma treatment is sufficient to introduce reactive sites for TES bonding, prolonged treatment may weaken the interface, reducing thermal reliability.

To explore the underlying causes, the interfaces of copper-plated COP samples subjected to different plasma durations were analyzed after peel testing, as discussed in the following section.

Characteristics and Features of COP/Cu Interfaces

Characteristics of COP and Cu Side Surfaces after Peel Testing

FTIR Results before and after Thermal Resistance Testing

Figure a-2, a-3 presents the FTIR spectra of the peeled surfaces of COP/Cu composites prepared with different plasma treatment durations. The spectra of the COP sides treated under various conditions closely resemble that of the untreated COP surface (blank). The absorption bands at 2853 and 2914 cm–1 are attributed to the symmetric and asymmetric stretching vibrations of sp3-hybridized C–H bonds, respectively. The IR peak at 1452 cm–1 corresponds to the bending vibration of −CH2– groups. Absorption peaks in the 1327–1347 cm–1 region are associated with the stretching vibrations of the C–C backbone. An extremely broad band between 3750 and 3050 cm–1, centered around 3450 cm–1, is attributed to O–H stretching. Additionally, the absorption peak of 1017 cm–1 corresponds to the ring deformation vibrations of norbornene units in the COP structure.

On the Cu side, although the characteristic FTIR peaks of COP after 30 s of plasma treatment are relatively weak, COP-related signals are still detectable. As the plasma treatment duration increases, the intensity of these peaks becomes more pronounced. These results indicate that, after 30 s–10 min of plasma treatment, oxidized functional groups on the COP surface react with TES, which subsequently forms bonds with the copper, creating a strong interface. This strong interfacial bonding leads to cohesive failure within the COP, causing COP material to remain on the Cu side after the peel test. With increasing plasma treatment duration, the amount of COP transferred to the Cu side increases, resulting in stronger FTIR signals corresponding to COP.

Figure presents the FTIR spectra of peeled surfaces from COP/Cu composites prepared with plasma treatment durations of (b-2) 30 s and (b-3) 5 min, after thermal resistance testing. The results indicate that, regardless of plasma treatment duration, C–H peaks characteristic of COP are observed on both the COP and Cu sides after peeling. This suggests that even with a short plasma treatment of 30 s, interactions occur between copper and the oxidized functional groups introduced by plasma treatment during thermal exposure, leading to the detectable presence of COP-derived C–H peaks on the Cu surface.

XPS Findings before and after Thermal Resistance Testing

To investigate the interfacial structure of COP/Cu before and after thermal resistance testing, XPS analysis was performed on the peeled surfaces of COP/Cu composites prepared with 30 s and 5 min of plasma treatment. The results are shown in Figure and Table . Compared to the condition before thermal testing, the oxygen content on the peeled Cu side increased after testing, indicating that oxidation occurred at the composite interface. This suggests that the decrease in adhesion strength following thermal resistance testing is due to interfacial oxidation, which weakens the bond between COP and Cu, making the materials easier to separate. When comparing the effects of different plasma treatment durations, the O content on the COP side was observed to be similar before thermal resistance testing. However, the O content on the Cu side was higher after 30 s of treatment compared to that at 5 min, increasing from 3.9 to 11.6%. After thermal resistance testing, the O content on the COP side increased from 0% (after 30 s of treatment) to 3.6%, while the O content on the Cu side rose from 11.9 to 29%. As is well-known, XPS probes only the top ∼10 nm of the surface, whereas ATR-FTIR penetrates several micrometers, depending on the material. In our ATR-FTIR results, regardless of plasma treatment duration (30 s or 5 min) and whether thermal resistance test was performed, only characteristic peaks of COP were observed on both the Cu and COP sides of the peeled interface. This suggests that cohesive failure occurred within the polymer layer, rather than failure at the interface.

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C 1s and Cu 2p spectra of the COP side (upper) and Cu side (lower) of the peeled surfaces of COP/Cu composites before (solid line) and after thermal resistance testing (dotted line), obtained by XPS. Samples were prepared using plasma treatment durations of 30 s (red) and 5 min (green).

2. Element Concentration Ratio of the Peeled Surface before and after Thermal Resistance of COP/Cu Composites Prepared by Plasma Treatment for 30 s and 5 min.

    element concentration ratio (atom %)
plasma treated time   peeling side C N O Si S Cu Pd Sn
30 s before thermal resistance test COP 99.2 0 0.8 0 0 0 0 0
Cu 91.8 2.6 3.9 1.1 0 0.4 0.3 0
after thermal resistance test COP 100 0 0 0 0 0 0 0
Cu 73.0 1.2 11.9 0 0 13.9 0 0
5 min before thermal resistance test COP 99.5 0 0.5 0 0 0 0 0
Cu 75.9 0 11.6 0 0 12.5 0 0
after thermal resistance test COP 92.5 0.9 3.6 0 0 3.1 0 0
Cu 34.0 1.6 29 0.7 0 34.7 0 0
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In contrast, the XPS analysis revealed more pronounced differences depending on plasma treatment time and thermal exposure. Specifically, after the thermal resistance test, the oxygen content on the Cu side increased compared to the COP/Cu interface without thermal resistance test. For the 30 s plasma treatment, no detectable oxygen was found on the COP side, while for the 5 min treatment, higher oxygen content was observed on both the Cu and COP sides.

Although the initial XPS oxygen content was similar for both plasma durationsdue to comparable surface oxidation within the shallow XPS detection depthdifferences after thermal aging suggest that longer plasma treatment introduces deeper oxidation beyond the ∼10 nm XPS detection range. This subsurface oxidation likely contributes to interfacial reactions with Cu during thermal exposure, as reflected in the XPS data after the thermal resistance test. Additionally, Cu was detected on the COP side after thermal testing in samples treated for 5 min, indicating that under longer plasma treatment conditions, the interfacial oxidation or diffusion of Cu is more pronounced, resulting in detectable Cu on the COP side. As shown in Figure , the Cu 2p spectra obtained from both the COP and Cu sides exhibit a main Cu peak at around 932.5 eV, with no visible satellite peaks. This indicates the formation of Cu2O rather than CuO, as the absence of satellite features suggests that copper at the interface did not undergo significant CuO, due to the lack of air exposure at the COP/Cu interface.

Characteristics of the COP/Cu Adhesion Interface

TEM Results of the COP/Cu Cross-Sectional Interface

TEM micrographs of the COP/Cu adhesion interface are shown in Figure . For the sample treated with plasma for 30 s, prior to thermal resistance testing, the interface between COP and Cu appears clean and well-defined, with a very thin, faint gray layerjust a few nanometers thickvisible between them, presumed to be the TES layer. After thermal resistance testing, this gray interfacial region containing TES-derived components becomes broader and extends into the COP side. Figure c,d shows the interface after 5 min of plasma treatment. In this case, a distinct interfacial layer is visible even before thermal resistance testing, differing from the thinner layer observed after 30 s of treatment. Based on the XPS results presented in the previous section, this interfacial layer is likely composed of copper oxide. After thermal resistance testing, the interfacial layer widens, extending into the COP with a thickness of approximately 10 nm, as observed in the TEM images. This thickness is greater than that of the interfacial layer on the Cu side after 30 s of treatment. In peel tests, no notable difference in adhesion strength is observed between samples treated for 30 s and 5 min prior to thermal exposure. However, during thermal resistance testing, Cu at the interface oxidizes, forming an oxide layer. With longer plasma treatment durations, this oxide layer becomes thicker, leading to fracture within the oxidized region of the COP, as observed as a gray band in the TEM images. This fracture contributes to the reduction in adhesion strength.

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TEM images of the cross-sectional interface of COP/Cu composites treated with plasma for 30 s (a) before and (b) after thermal resistance testing, and for 5 min (c) before and (d) after thermal resistance testing.

AFM-nanoIR Analysis of the COP/Cu Cross-Section

To further investigate interfacial chemical reactions between COP and Cu, the effects of plasma treatment, and changes induced by thermal endurance testing, an AFM-nanoIR analysis was performed on cross sections of COP/Cu samples treated with plasma for 30 s and 5 min. The corresponding results are presented in Figure . AFM imaging was conducted on the cross sections, and IR spectra were first collected at the interface (green points), followed by measurements on the COP side (red, orange, and yellow points) and the Cu side (light blue, blue, and purple points).

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AFM images and nano-IR spectra of the cross sections the COP/Cu sample treated with plasma for 30 s before (a-1, a-2, a-3) and after the thermal resistance test (b-1, b-2, b-3), and treated with plasma for 5 min before (c-1, c-2, c-3) and after the thermal resistance test (d-1, d-2, d-3).

From Figure , it can be observed that after 30 s of plasma treatment, an absorption peak at 1452 cm–1, characteristic of COP, was detected on the COP side of the cross-section. Conversely, little to no IR absorption was observed at the interface or on the Cu side (Figure a-3). However, after thermal endurance testing (Figure b-3), a broad absorption band appeared in the 1000–1260 cm–1 range near the interface and on the Cu side. This band is attributed to polar functional groupssuch as C–O, CO, and COOHintroduced onto the COP surface by plasma treatment, as well as possible NH groups from TES. These groups likely reacted with Cu ions during prolonged thermal exposure, forming coordination compounds such as Cu–O, Cu–CO, Cu–COOH, or Cu–N. − The thickness of these coordination layers was estimated to be on the micrometer scale.

The initial cross-section of the sample subjected to prolonged plasma treatment (5 min, Figure c-3) displayed a pronounced broad absorption band between 1000 and 1260 cm–1 near the interface and on the Cu side, in contrast to the 30 s treatment. This result suggests that the increased density and thickness of surface oxides promoted the formation of coordination compounds such as Cu–O, Cu–CO, Cu–COOH (reacted with plasma treated COP surfaces), Cu–N (from reaction to TES) and Si–O–Si (reaction between TES) even at the initial bonding stage. A blue shift of this broad band was observed on the Cu side farther from the interface, indicating that polar functional groups interacting with Cu were primarily concentrated at the interface. The reduced density of interactions farther from the interface likely contributed to this spectral shift.

After thermal endurance testing, the broad absorption band observed at the interface and on the Cu side exhibited a further blue shift (Figure d-3). This shift is presumed to result from changes in the electronic environment of copper. Near the interface, the chemical surroundings of Cu become more complex due to potential coordination with various oxidation products and TES molecules, leading to more localized and intricate vibrational modes. In contrast, Cu atoms farther from the interfaceparticularly after thermal exposureare likely subject to weaker interactions and exist in a chemically simpler environment, resulting in distinct vibrational characteristics.

Additionally, a broad absorption band in the 1000–1260 cm–1 region was detected on the COP side of the COP/Cu interface, with the affected zone extending up to 5 μm into the COP. These results suggest that prolonged plasma treatment not only modifies the surface functional groups of COP but also alters the chemical structure of both COP and Cu to a micrometer-scale depth. Notably, oxidation at the interface was more pronounced after the thermal endurance test than at the initial bonding stage.

Adhesion Mechanism Between COP and Cu and Its Relationship with Heating Trends and Plasma Treatment

Figure schematically illustrates the COP surface treatment process, which includes plasma activation, modification with an amino-functionalized silane coupling agent, coordination between amino groups (−NH2) and Cu ions, reduction of Cu2+, and deposition of metallic Cu, along with the corresponding interfacial reactions.

7.

7

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Schematic of the adhesion mechanism between COP and Cu, and the relationship between thermal behavior and plasma treatment conditions.

Following the thermal durability test, oxides formed during plasma treatment enhanced interfacial interactions between COP and copper, leading to an expanded oxidation zone from the interface. This interfacial oxidation reduced the adhesion strength. Although the initial adhesion strength exceeded 1.0 kN/m after 5 min of plasma treatment, prolonged thermal exposure caused further oxidation at the COP/Cu interface, which substantially reduced the interfacial strength. Before thermal resistance test, the relatively strong adhesion at the Cu/COP interface is primarily attributed to the interaction or coordination between Cu and the NH2 groups of the triazine moieties. However, after thermal resistance test, the formation and expansion of the oxidized layer on COP leads to a significant reduction in interfacial adhesion strength. Several factors may contribute to this deterioration, including the lower binding energy of COO– with Cu2+ compared to Cu–NH2 interactions, partial degradation of the COP surface due to prolonged plasma exposure, and further oxidation or diffusion processes occurring at the interface under thermal stress. Importantly, these effects can be traced back to the initial surface condition generated by plasma treatment, which introduces oxygen-containing functional groups. While these groups are essential for the subsequent reaction with TES molecules, excess unreacted oxidized species can remain at the interface and promote undesirable oxidation of both COP and Cu during thermal aging. As shown in our results, the interface formed with 30 s plasma treatment retained a higher postannealing adhesion strength compared to that treated for 5 min, despite both being subjected to TES modification. This highlights that in the surface chemical modification of composite materials, the use of minimal yet sufficient plasma pretreatment to introduce reactive siteswithout excessive oxidationis critical for achieving long-term thermal stability and robust interfacial bonding.

These findings suggest that interfacial oxidation and the resulting loss of adhesion may pose a general challenge for materials with inherently low surface polarity, particularly when they undergo surface activation followed by metallization. Therefore, optimizing plasma treatment conditions for such materials should not rely solely on achieving high initial adhesion strength. Instead, a comprehensive evaluation of long-term interfacial durability, particularly under thermal aging conditions, is essential to ensure reliability in practical applications.

Conclusions

In summary, we demonstrated a surface modification method for COP films that enables robust bonding at the COP/Cu interface, achieving an initial adhesion strength exceeding 1.0 kN/mmeeting the practical benchmark for circuit board applications in communication devices. Plasma treatment followed by functionalization with triazine-based silane coupling agents yielded an initial adhesion strength of 1.2 kN/m. Even after thermal aging at 120 °C for 3 weeks, the bonding strength remained above 1.0 kN/m, indicating excellent thermal durability.

The influence of plasma treatment duration on surface chemical functionalization, interfacial bonding strength, and thermal resistance of the COP surface was systematically investigated. The interfacial chemical structure was directly analyzed using AFM-nanoIR. Plasma treatment effectively activated the otherwise inert COP surface by introducing polar functional groups. However, prolonged treatment causes polymer degradation, weakens the mechanical integrity of COP, and reduces interfacial strength, even though bonding to the Cu surface is retained.

After thermal exposure, a broad absorption band in the 1000–1250 cm–1 range appeared at the interface, attributed to the formation of coordination complexes between copper and oxidized species or TES molecules on the COP surface. This band exhibited a blue shift with increasing distance from the Cu interface, suggesting a gradual weakening of Cu–oxide and Cu–TES interactions. Mechanistically, while plasma treatment improves the initial adhesion, prolonged exposure combined with thermal aging leads to further oxidation of both Cu and COP at the interface, which results in a decline in interfacial strength.

These findings offer valuable insights into the surface treatment and metallization of COP films and provide practical guidance for optimizing plasma activation processes in other organic materials lacking functional groups, such as polypropylene (PP) and polyethylene (PE). The demonstrated approach to dry-surface activation can play a critical role in promoting reliable adhesion with various materials, particularly metals. Consequently, the implications of this work extend beyond electronics and communication devices, offering potential utility in broader fields such as infrastructure materials, where long-term interfacial stability and material integration are essential.

Supplementary Material

ao5c04758_si_001.pdf (79.3KB, pdf)

Acknowledgments

This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) (24K08051).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c04758.

  • Contact angle and surface energy of COP surfaces after plasma treatment and TES treatment (Table S1) (PDF)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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