Surface modification of polyimide by combining swelling and TiO2 photocatalytic treatments for adhesion improvement of electroless Cu

In order to enhance the adhesion strength between the PI film and the electroless copper film, a combination of swelling and TiO₂ photocatalytic treatments was used to modify polyimide (PI) film. The effects of the swelling solution composition and TiO₂ photocatalytic condition on the surface performance were investigated. After the optimal swelling and photocatalytic treatment, the surface contact angle of the PI film decreased from 85 to 28.7°, and the surface average roughness of the PI film only increased from 1.3 to 13.6?nm, indicating no obvious change for the surface topography of PI film after the photocatalytic treatment. However, the adhesion strength between electroless copper film and the PI film reached to 0.6?KN·m^?1. The FT-IR spectra and XPS analyses indicated that –COOH group was formed on the PI surface after the treatment, and the surface hydrophilicity was improved, which improved the adhesion strength between the PI film and the electroless copper film.     

Role of polymer chain end groups in plasma modification for surface metallization of polymeric materials

How to improve adhesion between poly(oxybenzoate‐co‐oxynaphthoate) (Vecstar OC and FA films) and copper metal by Ar, O₂, N₂ and NH₃ plasma modification was investigated. The mechanism of adhesion improvement is discussed from the viewpoint of chemical and physical interactions at the interface between the Vecstar film and copper metal layer. The adhesion between Vecstar OC film and copper metal was improved by chemical rather than physical interactions. Polymer chain end groups that occur at Vecstar OC film surfaces contribute effectively to adhesion. This improvement in adhesion is due to interactions between copper metal and O?C groups formed by plasma modification. Aggregation of the O?C groups to the copper metal/Vecstar OC film interface is a key factor for good adhesion. From this aspect, heat treatment of plasma‐modified Vecstar OC films on glass plates is effective in the aggregation, and the peel strength for the copper metal/Vecstar OC film system reached 1.21 N (5 mm)^?1. Copyright © 2009 Society of Chemical Industry     

Surface modification of tetrafluoroethylene–hexafluoropropylene (FEP) copolymer by remote H2, N2, O2, and Ar plasmas

Tetrafluoroethylene–hexafluoropropylene (FEP) copolymer sheets were modified by remote H₂, N₂, O₂, and Ar plasmas, and the effects of the modification on adhesion between FEP sheets and copper metal were investigated. The four plasmas were able to modify the FEP surfaces' hydrophilicity. Defluorination and oxidation reactions on the FEP surfaces occurred with exposure to the plasma. The hydrophilic modification by H₂ plasma was best, followed by modification by O₂, Ar, and N₂ plasmas. The surface modification of FEP by all four remote plasmas was effective in improving adhesion with copper metal. The peel strength order of the FEP/Cu adhesive joints was H₂ plasma > Ar plasma > N₂ plasma > O₂ plasma. Mild surface modification is important for the adhesion improvement of FEP with Cu metal. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1258–1267, 2002     

Plasma surface modification of polyimide for improving adhesion to electroless copper coatings

The influences of oxygen plasma treatment of polyimide (PI) films on the adhesion of electroless copper coatings as well as on the chemical composition of the film surface and the PI surface morphology were investigated. The plasma operating parameters were 1800 W forward power with O₂ flowing at a rate of 300 cm³/min at a pressure of 200 mTorr. The peel strength increased with decreasing plasma treatment temperature. However, extension of the treatment time at higher temperatures had a positive effect on adhesion. A correlation between the enhancement in peel strength and the content of oxygen-containing groups at the PI surface (investigated using XPS) was observed. A change in the morphology as a result of plasma etching was also observed, in the formation of pits in the film surface. The pits ranged from 3 to 6 μm in depth and the diameter varied from 10 to 200 μm. Comparison of the data obtained after plasma treatment with the results of chemical etching in alkaline solutions of permanganate showed approximately the same adhesion increase (to 0.6 kN/m) in both cases. However, chemical etching did not affect the surface morphology and increased the oxygen content at the PI surface less than the plasma treatment.     

Plasma modification of poly(oxybenzoate-co-oxynaphthoate) film surfaces for copper metallization

Poly(oxybenzoate-co-oxynaphthoate) (POCO) film surfaces were modified by four plasma gases, Ar, O₂, N₂ and NH₃, and the effects of the plasma modification were investigated in order to understand the adhesion with copper metal. The Ar, O₂, N₂ and NH₃ plasmas converted the POCO surfaces from hydrophobic to hydrophilic. The effect of the plasma on the hydrophilic modification was in the order: Ar plasma > O₂ plasma > N₂ plasma > NH₃ plasma. The plasma modification contributed to the adhesion between the deposited copper metal and the POCO film. The NH₃ plasma was most effective in improving the adhesion, and the Ar plasma was ineffective. The plasma-modified POCO film surfaces showed quite different C_ls spectra from that of the original POCO film. There were large differences in the C_ls and N_ls spectra between the NH₃ and Ar plasma modifications. The NH₃ plasma modification did not show C_ls component #5 due to π–π^* shake-up satellite, but the Ar plasma modification did show this component. Furthermore, NH₃ plasma modification led to a new N_ls spectrum. The plasmas caused etching of the POCO film surfaces, and the etch rate depended on what plasma was used and how much RF power was used. The NH₃ plasma-modified POCO film surface showed a larger R _a (25.5 nm) than the other plasma-modified surfaces (R _a = 16.4–19.0 nm), which were comparable to that of the original surface (R _a = 14.8 nm). The NH₃ plasma led to a highly-undulated surface, and the other plasmas did not alter the surface roughness. The roughened surfaces showed contribution to enhancement of the adhesion to the deposited copper metal.     

Plasma surface modification of poly(phenylene sulfide) films for copper metallization

Poly(phenylene sulfide) (PPS) films were modified by Ar, O₂, N₂ and NH₃ plasmas in order to improve their adhesion to copper metal. All four plasmas modified the PPS film surfaces, but the NH₃ plasma modification was the most effective in improving adhesion. The NH₃ plasma modification brought about large changes in the surface topography and chemical composition of the PPS film surfaces. The peel strength for the Cu/plasma-modified PPS film systems increased linearly with increasing surface roughness, R _a or R _rms, of the PPS film. The plasma modification also led to considerable changes in the chemical composition of the PPS film surfaces. A large fraction of phenylene units and a small fraction of sulfide groups in the PPS film surfaces were oxidized during the plasma modification process. Nitrogen functional groups also were formed on the PPS film surfaces. The NH₃ plasma modification formed S—H groups on the PPS film surfaces by reduction of S—C groups in the PPS film. Not only the mechanical interlocking effect but also the interaction of the S—H groups with the copper metal may contribute to the adhesion of the Cu/PPS film systems.     

Adhesion of SiO2 thin films to plasma-treated SUS304 stainless steel substrates

The adhesion strength of the coated SiO₂ thin film to SUS304 stainless steel substrates with various surface treatment conditions is studied in this research. The surface of the SUS304 stainless steel substrate is first treated with 1000-W plasma and then a SiO₂ thin film is deposited onto the surface via radio-frequency magnetron sputtering. Scanning electron microscopy is employed to observe the surface and cross section of the coating and X-ray diffraction is used to analyze the crystallographic structure. Moreover, a nanoscratch test instrument was employed to examine the indentation, scratches, coating hardness, modulus of elasticity, coefficient of friction, and critical adhesion of the SiO₂ film and to obtain surface profiles. A comparison of the coating adhesion of the substrate surfaces with and without plasma treatment indicates that critical adhesion increases significantly after Ar/N₂/O₂ plasma treatment.     

Surface modification of ABS by photocatalytic treatment for electroless copper plating

Surface roughness of acrylonitrile–butadiene–styrene (ABS) resin prior to metallization is treated generally with sulphuric/chromic acid system. However, the presence of chrominum (VI) ion imposes serious environmental problems. In this work, TiO₂ photocatalytic treatment was used to enhance the adhesion strength between the ABS surface and the electroless copper film. Effects of the TiO₂ content, irradiation time and UV power upon the surface topography, surface characterization and the adhesion strength were investigated. The results indicated that the surface hydrophilicity of ABS resin and the adhesion strength between the electroless copper film and ABS surface increased with an increase in the UV power and a prolongation in irradiation time, and did not increase linearly with an increase of TiO₂ content. Though the surface topography of ABS changed little, the adhesion strength reached 1.25?kN/m, which was higher than that in the optimal H₂SO₄–MnO₂ colloid. The surface chemistry results indicated that –COOH and –OH groups formed with the photocatalytic treatment and the absorption strengths increased with the UV power. XPS analysis results further demonstrated that the contents of C=O and –COOH reached 6.4 and 4.9% with the photocatalytic treatment, which was much higher than that of the H₂SO₄–MnO₂ colloid (3.9 and 3.1%). The high contents of C=O and –COOH groups enhanced the surface hydrophilicity of the ABS resin and improved the adhesion strength between the electroless copper film and ABS resin. The results indicated that the photocatalytic treatment was an environment-friendly and effective method to replace the commercial wet chemical process for ABS surface modification.     

Improved adhesion property and electromagnetic interference shielding effectiveness of electroless Cu‐plated layer on poly(ethylene terephthalate) by plasma treatment

To develop high‐quality electromagnetic interference (EMI) shielding materials, the effect of plasma pretreatment with various gases prior to Cu plating was investigated. Plasma treatment increased the surface roughness in the decreasing order of Ar > O₂ > NH₃, but adhesion of the Cu layer on poly(ethylene terephthalate) (PET) film increased in the following order of O₂ < Ar < NH₃, indicating that the appropriate surface roughness and introduction of an affinitive functional group to Pd on the surface of the PET film were key factors for improving adhesion of the Cu layer. As investigated by XPS analysis, plasma treatment with NH₃ produced N atoms on the PET film, which enhances the chemisorption of Pd²⁺ on PET film, resulting in improved adhesion and shielding effectiveness of the Cu layer deposited on the Pd‐catalyzed surface, because of the high affinity of Pd²⁺ for nitrogen. Comparatively, O₂ plasma treatment allowed the chemisorption of more Sn²⁺ than of Pd²⁺ due to a lack in the affinity of Pd²⁺ for oxygen, resulting in the lowest Pd_3d/Sn_3d ratio; thereby, the lowest EMI–shielding effectiveness (SE) value was obtained. In addition, fairly low adhesion was obtained with Ar plasma‐treated PET, even though the PET surface was significantly etched with Ar plasma, due to introduced oxygen groups on the PET surface. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1369–1379, 2002; DOI 10.1002/app.10272     

Plasma etching and plasma polymerization coating of carbon fibers. Part 2. Characterization of plasma polymer coated carbon fibers

Unsized AS-4 carbon fibers were subjected to RF plasma etching and/or plasma polymerization coating in order to enhance their adhesion to vinyl ester resin. Ar, N₂ and O₂ were utilized for plasma etching, and acetylene, butadiene and acrylonitrile were used for plasma polymerization coating. Etching and coating conditions were optimized in terms of plasma power, treatment time, and gas (or monomer) pressure by measuring the interfacial adhesion strength. Interfacial adhesion was evaluated using micro-droplet specimens prepared with vinyl ester resin and plasma etched and/or plasma polymer coated carbon fibers. Surface modified fibers were characterized by SEM, XPS, FT-IR, α-Step, dynamic contact angle analyzer (DCA) and tensile strength measurements. Interfacial adhesion between plasma etched and/or plasma polymer coated carbon fibers and vinyl ester resin was reported previously (Part 1), and characterization results are discussed is this paper (Part 2). Gas plasma etching resulted in preferential etching of the fiber surface along the draw direction and decreased the tensile strength, while plasma polymer coatings altered neither the surface topography of fibers nor the tensile strength. Water contact angle decreased with plasma etching, as well as with acrylonitrile and acetylene plasma polymer coatings, but did not change with butadiene plasma polymer coating. FT-IR and XPS analyses revealed the presence of functional groups in plasma polymer coatings.     

Effects of plasma gas composition on bond strength of hydroxyapatite/titanium composite coatings prepared by rf-plasma spraying

The effects of plasma gas composition on the bond-strength of HA/Ti composite coatings were investigated. HA/Ti composite coatings were deposited on titanium substrates by a radio-frequency (rf) thermal plasma spraying method with input powers of 10–30 kW. The ratio of the HA and Ti powders supplied into the plasma was precisely controlled by two microfeeders so as to change the coating's composition from Ti-rich at the bottom to HA-rich at its upper layer. The bond (tensile) strength of the obtained HA/Ti composite coatings was 40–65 MPa when sprayed with plasma gas containing N₂ (i.e., Ar–N₂). On the other hand, HA/Ti composite coatings prepared with plasma gas containing O₂ (i.e., Ar–O₂) had significantly lower bond strength (under 30 MPa). XRD patterns of Ti coatings without HA showed that titanium nitride and titanium dioxide formed, respectively, on titanium deposits sprayed with Ar–N₂ and Ar–O₂ plasma. Scanning electron microscopic (SEM) observation showed an acicular texture on the Ti deposits prepared with Ar–N₂ plasma. SEM observations implied that, when sprayed with Ar–O₂ plasma, a thin TiO₂ layer formed at the interfaces between the Ti splats in the deposits.     

Effect of TiO2 on thermal and adhesive characteristics of poly(imide siloxane)/TiO2 hybrid films

A novel poly(imide siloxane)/titania (PIS/TiO₂) hybrid film was fabricated by sol‐gel process via in situ formation of TiO₂ within PIS matrix. Poly(amic acid siloxane) (PAAS) was prepared from 4,4′‐oxydiphthalic anhydride, 2,2‐bis [4‐(4‐aminophenoxy) phenyl] propane, and α,ω‐bis(3‐aminopropyl)polydimethylsiloxane (APPS). Chelating agent, acetylacetone, and catalyst‐free polymerization were used to reduce the rate of hydrolysis of titanium alkoxide in the PAAS. X‐ray photoelectron spectroscopy data showed that the presence of APPS promotes the Ti surface composition of PIS/TiO₂ hybrid film. The effects of TiO₂ and APPS contents on the characteristics of surface, thermostability, coefficient of thermal expansion (CTE), and the strength of adhesion were investigated. The presence of TiO₂ on the surface of the hybrid films enhanced the adhesive strength at the interface of PIS/TiO₂ hybrid film and copper foil. When more TiO₂ was incorporated into the PIS matrix, the PIS/TiO₂ hybrid film exhibited lower CTE while retaining favorable mechanical and thermal properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Adhesion Properties and Structure of Ion-Beam Modified Polyimide Films

Polyimide (PI) surface modification was carried out by an ion beam treatment to improve the adhesion between the polyimide film and copper. The PI film surface was treated with an ion-beam source at ion doses ranging from 1.96 × 10¹³ to 2.38 × 10¹³ ions/cm² using a mixture of nitrogen (N₂) and hydrogen (H₂). Contact angle measurement, atomic force microscopy and X-ray photoelectron spectroscopy, respectively revealed an increase in the surface roughness, a decrease in contact angle, and the formation of oxygen complexes and functional groups on the treated PI surfaces. Adhesion between the copper and PI film treated with the beam was superior to that of the untreated PI film. The 90° peel test revealed the highest peel strength of 7.8 N/cm.     

The effect of TiO2 morphology on the surface modification of poly (ethylene terephthalate) for electroless plating

In this study, a surface modification of the poly (ethylene terephthalate) (PET) film using TiO₂ photocatalytic treatment was investigated. In order to enhance the adhesion strength between the PET film and the electroless copper film, the effects of TiO₂ crystal forms, TiO₂ particle sizes, and TiO₂ content, as well as treatment condition, upon the surface contact angle, surface characterization, and adhesion strength were investigated. Anatase TiO₂ with a particle size of 5 nm had a high catalytic activity and dispersibility in aqueous solution. After the optimal photocatalytic treatment, the surface contact angle of the PET film decreased from 84.4° to 19.8°, and the surface roughness of the PET film increased from 36 to 117 nm. The adhesion strength between the PET film and the electroless copper film reached 0.89?KN?m^?1. X-ray photoelectron spectroscopy analyses indicated the carbonyl group was formed on the PET surface after photocatalytic treatment, and the surface hydrophilicity was improved. Consequently, TiO₂ photocatalytic treatment is an environmentally friendly and effective method for the surface modification of the PET film.     

Atomic oxygen erosion resistance of titania–polyimide hybrid films derived from titanium tetrabutoxide and polyamic acid

A series of polyimide/titania (PI/TiO₂) hybrid films have been successfully synthesized based on titanium tetrabutoxide (Ti(OEt)₄), 3,3′,4,4′‐bezonphenone tetracarboxylic dianhydride (BTDA), 4,4′‐oxydianiline (ODA), and 1,3‐bis(aminopropyl) tetramethyldisiloxane (APrTMOS) by a sol–gel process. The atomic oxygen (AO) exposure tests were carried out using a ground‐based AO effects simulation facility. The chemical structure of PI/TiO₂ films was characterized by Fourier transform‐infrared (FT‐IR) spectroscope before and after AO exposure. The glass transition temperature (T_g) and mechanical properties were examined by dynamic mechanical analysis (DMA) and universal mechanical testing machine, respectively. The tensile strength and elongation of the hybrid film decreased with the increase of TiO₂ content, whereas the T_g increased with the increase of TiO₂ content. The effects of TiO₂ content on the morphology and structure evolvement of PI/TiO₂ hybrid films were also investigated using field emission scanning electron microscopy (FE‐SEM) and X‐ray photoelectron spectroscope (XPS), respectively. The results indicated that a TiO₂‐rich layer resulting from the Ti(OEt)₄ formed on the PI film after AO exposure, which decreased the mass loss rate and obviously improved the AO resistance of PI films. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Surface modification of low‐density polyethylene (LDPE) film and improvement of adhesion between evaporated copper metal film and LDPE

To improve the interfacial adhesion between evaporated copper film and low‐density polyethylene (LDPE) film, the surface of LDPE films was modified by treating with chromic acid [K₂Cr₂O₇/H₂O/H₂SO₄ (4.4/7.1/88.5)]/oxygen plasma. Chromic‐acid‐etched LDPE was exposed to oxygen plasma to achieve a higher content of polar groups on the LDPE surface. We investigated the effect of the treatment time of chromic acid in the range of 1–60 min at 70°C and oxygen plasma in the range of 30–90 sec on the extent of polar groups created on the LDPE. We also investigated the surface topography of and water contact angle on the LDPE film surface, mechanical properties of the LDPE film, and adhesion strength of the evaporated copper metal film to the LDPE film surface. IR and electron spectroscopy for chemical analysis revealed the introduction of polar groups on the modified LDPE film surface, which exhibited an improved contact angle and copper/LDPE adhesion. The number of polar groups and the surface roughness increased with increasing treatment time of chromic acid/plasma. Water contact angle significantly decreased with increasing treatment time of chromic acid/plasma. Combination treatment of oxygen plasma with chromic acid drastically decreased the contact angle. When the treatment times of chromic acid and oxygen plasma were greater than 10 min and 30 sec, respectively, the contact angle was below 20°. With an increasing treatment time of chromic acid, the tensile strength of the LDPE film decreased, and the film color changed after about 10 min and then became blackened after 30 min. With the scratch test, the adhesion between copper and LDPE was found to increase with an increasing treatment time of chromic acid/oxygen plasma. From these results, we found that the optimum treatment times with chromic acid and oxygen plasma were near 30 min and 30 sec, respectively. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1677–1690, 2001     

Plasma etching and plasma polymerization coating of carbon fibers. Part 1. Interfacial adhesion study

Unsized AS-4 carbon fibers were etched by RF plasma and then coated via plasma polymerization in order to enhance their adhesion to vinyl ester resin. Gases utilized for plasma etching were Ar, N₂ and O₂, while monomers used in plasma polymerization coating were acetylene, butadiene and acrylonitrile. Plasma etchings were carried out as a function of plasma power (30–70 W), treatment time (1–10 min) and gas pressure (20–40 mtorr). Plasma polymerizations were performed by varying the treatment time (15–60 s), plasma power (10–30 W) and gas pressure (20-40 mtorr). The conditions for plasma etching and plasma polymerization were optimized by measuring interfacial adhesion with vinyl ester resin via micro-droplet tests. Plasma etched and plasma polymer coated carbon fibers were characterized by SEM, XPS, FT-IR and α-Step, dynamic contact angle analyzer (DCA) and tensile strength measurements. In Part 1, interfacial adhesion of plasma etched and plasma polymer coated carbon fibers to vinyl ester resin is reported, while characterization results including tensile strength of carbon fibers are reported in Part 2. Among the treatment conditions, a combination of Ar plasma etching and acetylene plasma polymer coating provided greatly improved interfacial shear strength (IFSS) of 69 MPa, compared to 43 MPa obtained from as-received carbon fiber. Based on the SEM analysis of failure surfaces and load-displacement curves, the failure was found to occur at the interface between plasma polymer coating and vinyl ester resin.     

Improved adhesion between Kapton film and copper metal by silane-coupling reactions

Kapton film, poly[(N,N′-oxydiphenylene)pyromellitimide], was modified by silane-coupling reactions using 3′(trimethoxysilyl)propoxy-2-hydroxypro-pyl-1,3-diazole (Si–imidazole) to improve the adhesion with copper metal. The Kapton film surface was first treated with argon plasma for 30 s, then dipped into a methanol solution of Si–imidazole (0.01 wt %), followed by heating at 110°C for 90 min. The Kapton surfaces, modified by the argon plasma and Si–imizadole coupling reactions, were analyzed by water contact-angle measurement, atomic force microscopy, and XPS. The Si–imidazole modification showed a large increase in adhesion between the copper metal and the Kapton film. The peel strength of the copper metal/Kapton film joint increased from 0.94 to 2.4 N/5 mm. The failure occurred at the interface between the Si–imidazole and the Kapton film layer. We conclude that the Si–imidazole modification is an effective treatment for improvement of the adhesion between copper metal and Kapton film. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1645–1654, 1999     

Effect of the Plasma Gas Type on the Surface Characteristics of 3Y-TZP Ceramic

Plasma surface treatment can be an attractive strategy for modifying the chemically inert nature of zirconia to improve its clinical performance. This study aimed to clarify the effect of plasma gas compositions on the physicochemical surface modifications of 3 mol% yttria-stabilized zirconia (3Y-TZP). The cold, atmospheric plasma discharges were carried out by using four different plasma gases, which are He/O₂, N₂/Ar, N₂, and Ar from an application distance of 10 mm for 60 s. Static contact angles were measured to define the surface free energy. Changes in elemental composition, surface crystallinity, and surface topography were assessed with X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), confocal laser scanning microscopy (CLSM), and scanning electron microscopy (SEM), respectively. A significant decrease in water contact angle was observed in all plasma groups with the lowest value of 69° in the N₂/Ar group. CLSM and SEM investigations exhibited no morphological changes in all plasma groups. XPS revealed that a reduction in the surface C content along with an increase in O content was pronounced in the case of N₂/Ar compared to others, which was responsible for high hydrophilicity of the surface. XRD showed that the changes in crystallite size and microstrain due to oxygen atom displacements were observed in the N₂/Ar group. The N₂/Ar plasma treatment may contribute to enhancing the bioactivity as well as the bonding performance of 3Y-TZP by controlling the plasma-generated nitrogen functionalities.     

Changes in the adhesion strength between copper thin films and polyimide substrates after heat treatment

The adhesion strength between a copper (Cu) thin film and a polyimide [pyromellitic dianhydride-oxydianiline (PMDA-ODA)] substrate is reduced by heat treatment at 150°C in air. In this work, we determined the changes in adhesion strength between Cu films and polyimide substrates using Auger electron spectroscopy (AES), attenuated total reflection Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The analysis showed that the weak boundary layer (WBL) shifted towards the Cu interface as the heat treatment time was increased. To confirm this shift, we looked at two other polyimide substrates: biphenyl dianhydride-p-phenylene diamine (BPDA-PDA) and biphenyl dianhydride-oxydianiline (BPDA-ODA). Comparing the adhesion strength for the Cu thin film, the adhesion strength was high for the Cu/PMDA-ODA and Cu/BPDA-ODA laminates, but very low for the Cu/BPDA-PDA laminate. One of the possible reasons for this behavior could be that the ether moiety between the two benzene rings in ODA is related to the adhesion between a Cu film and an 0₂-plasma-treated polyimide (PI) substrate. The relationship between the adhesion strength and chemical bonding states is also discussed. We conclude that a Cu thin film sputtered onto a PI substrate is apt to peel at the oxidized interface, due to the heat treatment.