Adhesion and interface studies between copper and polyimide

The adhesion and interface structure between copper and polyimide have been studied. Polyimide films were prepared by spinning a polyamic acid solution (Du Pont PMDA-ODA) in an NMP solvent onto a Cu foil, followed by thermal curing up to 400°C. The adhesion strength was measured by a 90° peel test. The peel strength of 25 μm thick Cu foil to 25 μm thick polyimide substrate was about 73 g/mm with the peel strength decreasing with increasing polyimide thickness. Cross-sectional TEM observation revealed very fine Cu-rich particles distributed in the polyimide. Particles were not found closer than 80-200 nm from Cu boundary. These Cu-rich particles were formed as a result of reaction of polyamic acid with Cu during thermal curing. We attribute the high peel strength to interfacial chemical bonding between Cu and polyimide. This behavior is in contrast to vacuum-deposited Cu onto fully cured polyimide.     

Adhesion of PMDA-ODA polyimide to silicon oxide surface: sensitivity to fluorine contamination

The adhesion of pyromellitic dianhydride-oxydianiline polyimide (PMDA-ODA) to clean and fluorine-contaminated silicon oxide (SiO₂ and F-SiO₂, respectively) surfaces was studied by the peel test. A 50% drop in the peel force was observed for PMDA-ODA on F-SiO₂ as compared to that on SiO₂ measured at low (11-22%) ambient humidity. It was also found that the PMDA-ODA adhesion strength to F-SiO₂ was a strong function of the humidity. A 21/2-fold increase in the humidity caused over an order of magnitude decrease in the peel force of PMDA-ODA on F-SiO₂, which was only about 5% of the peel force measured under the same ambient conditions for PMDA-ODA on the SiO₂ surface. A careful analysis further demonstrated that the basic mechanical properties of the polyimide film were unaffected to within experimental error. The most probable cause of the adhesion loss is thus attributed to moisture attack in the polyimide-substrate interface region. The locus of failure in the case of PMDA-ODA on SiO₂ is in an apparent weak boundary layer of the polyimide, while for polyimide on F-SiO₂ the failure locus is mixed within the polyimide and the F-SiO₂. An estimate of the work of adhesion (interface fracture energy) was made using the high (40-55%) humidity peel data for PMDA-ODA on the F-SiO₂ surface, giving a value of 6 J/m².     

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.     

Changes in the Peeled Surfaces of Copper Thin Films Deposited on Polyimide Substrates After Heat Treatment

Peel strength between a copper (Cu) thin film and a polyimide (pyromellitic dianhydride-oxydianiline, or PMDA-ODA) substrate is reduced by heat treatment at 150°C in air. In this work, we investigated the peel strength, the morphology of the interface between Cu films and polyimide substrates using optical microscopy and electron microscopy, and chemical change of the interface using Auger electron spectroscopy (AES) and micro X-ray photoelectron spectroscopy (XPS). The analysis showed that CuO “lumps” were present on the peeled surface of PMDA-ODA after heat treatment at 150°C in air. The peeled surfaces of other polyimide substrates were also analyzed: biphenyl dianhydride-para phenylene diamine (BPDA-PDA) and biphenyl dianhydride-oxydianiline (BPDA-ODA). CuO lumps were present on the peeled surface of BPDA-ODA after the heat treatment, but not that of BPDA-PDA. Compared with the adhesion strength for the Cu thin film, the adhesion strength was high for the Cu/PMDA-ODA and Cu/BPDA-ODA laminates, but the adhesion strength was very low for the Cu/BPDA-PDA laminate. This low strength is the reason that CuO lumps were not detected on the peeled surface of the BPDA-PDA substrate. These CuO lumps were related to the adhesion degradation of the Cu/polyimide laminates after the heat treatment.     

Adhesion of polyimide to fluorine-contaminated SiO2 surface. Effect of aminopropyltriethoxysilane on the adhesion

The adhesion of pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide to fluorine-contaminated silicon dioxide (F-SiO₂) with y-aminopropyltriethoxysilane (APS) adhesion promoter has been studied as a function of the peel ambient humidity. The peel strength was not affected by the change in peel ambient relative humidity (RH) from 11-17% to 35-60% when APS was used at the interface. Without APS, the adhesion degraded significantly with this change in RH. It was found that although the dip application of APS caused the removal of about 80% of the initial atomic percentage of fluorine on the surface, it could not be totally removed even after several days in water at elevated temperature.     

Mechanical effects in peel adhesion test

—Mechanical effects in the peel strength of a thin film have been studied both experimentally and theoretically. It has been found that the adhesion strength measured by the peel test is a practical adhesion (an engineering strength per unit width) and does not represent the true interface adhesion strength. The measured value may represent a multiplication of the true interface adhesion and other work expended in the plastic deformation of the thin film. The contribution of the latter to the peel strength is found to be, sometimes, of the order of 100 times higher than the former. It is found that the major controlling factors in the peel strength are the thickness, Young's modulus, the yield strength, the strain hardening coefficient of the film, and the compliance of the substrate as well as the interface adhesion strength. Even though the true interface adhesion strength is the same, a higher peel strength is obtained if the film is thinner or more ductile under the test conditions reported in this paper. The same effect can be obtained if the substrate is thinner in the case where the substrate is a soft elastic material, or if the substrate is thicker in the case where the substrate is a rigid material.     

Application of the Island Blister Test for Thin Film Adhesion Measurement

The island blister test has recently been proposed as an adhesion test which allows the peel of thin, well-adhered films without exceeding the tensile strength of the film. The island blister test site is a modification of the standard blister test site, consisting of a suspended membrane of film with an “island” of substrate at the film center. The membrane support and island are secured to a rigid plate and the film is pressurized, peeling the film inward off the island. A model for this inward or “annular” peel indicates that even for systems of good adhesion, peel can be initiated at low enough pressures to prevent film failure by making the center island sufficiently small relative to the size of the film.

We have fabricated island blister test sites using micromachining techniques and have used them to measure the debond energy of polymer films on various substrates. The peel data obtained from these island sites match well to the behavior predicted by a simple fracture mechanics analysis. This paper reports the fabrication of the island test sites, the experimental verification of the test, and the results of application of the test to polyimide films on metallic and polymeric substrates.     

Application of the Island Blister Test for Thin Film Adhesion Measurement

The island blister test has recently been proposed as an adhesion test which allows the peel of thin, well-adhered films without exceeding the tensile strength of the film. The island blister test site is a modification of the standard blister test site, consisting of a suspended membrane of film with an “island” of substrate at the film center. The membrane support and island are secured to a rigid plate and the film is pressurized, peeling the film inward off the island. A model for this inward or “annular” peel indicates that even for systems of good adhesion, peel can be initiated at low enough pressures to prevent film failure by making the center island sufficiently small relative to the size of the film.

We have fabricated island blister test sites using micromachining techniques and have used them to measure the debond energy of polymer films on various substrates. The peel data obtained from these island sites match well to the behavior predicted by a simple fracture mechanics analysis. This paper reports the fabrication of the island test sites, the experimental verification of the test, and the results of application of the test to polyimide films on metallic and polymeric substrates.     

Peeling of heterogeneous thin films: Effect of bending stiffness,adhesion energy,and level of heterogeneity

The peeling behaviour of a heterogeneous thin film bonded to a rigid substrate was investigated by using both experiments and finite element modeling. The enhancement in peel force was studied specifically for heterogeneous thin films with periodic stiff and compliant portions along the length. Peel tests with homogeneous thin films (uniform film thickness) showed that the maximum peel force can be observed before the onset of steady state peeling process. Moreover, this maximum peel force was observed to be a function of the bending stiffness of the film and adhesion energy at the film-substrate interface. For the heterogeneous thin films, maximum peel force can be observed either before the onset of steady state or when the peel front traverses from compliant to the stiff portion of the film. The three-dimensional finite element model, based on cohesive zone technique was developed, which provided further insight into the enhancement in peel force. The maximum force was shown to be dependent on the level of heterogeneity in addition to adhesion energy and bending stiffness as was observed with homogeneous films. The improvement in peel force was found to be prevalent at relatively low adhesion energy. This study may be helpful for the better design of homogeneous and heterogeneous thin film-substrate systems having improved bonding strength.     

Hydrothermal stability of Cr/polyimide interfaces

The hydrothermal stability of both Cr/polyimide and C₇₅Cr₂/polyimide interfaces has been studied using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and peel testing. It was found that RF sputter treatment of the polyimide surface prior to metal deposition leads to an enhancement of adhesion through chemical bonding of the metal with polyimide. Owing to the enhanced chemical bonding at the interface, failure always occurs cohesively in the polyimide. The RF sputter treatment of the polyimide surface also simultaneously modifies the polyimide underneath the surface. The adhesion strength of the Cr/polyimide interface is degraded significantly upon exposure to high temperature and high humidity (85°C/81% relative humidity, T/H) environment. It is suggested that this degradation results from the hydrolysis of polyimide. The hydrolysis is facilitated by the presence of unstable modified polyimide near the interface. This degradation of adhesion strength can be minimized by converting the unstable modified polyimide to a stable state by reheating the Cr/polyimide interface at 400°C for 40 min before exposure to the T/H environment.     

Low‐temperature thermal graft copolymerization of 1‐vinyl imidazole on fluorinated polyimide films with simultaneous lamination of copper foils

A simple technique of thermal graft copolymerization of 1‐vinyl imidazole (VIDZ) on pristine and argon plasma pretreated fluorinated polyimide (FPI) films with simultaneous lamination of copper foils was demonstrated. The simultaneous thermal grafting and lamination process was carried out in the temperature range of 80–140°C under atmospheric conditions and in the complete absence of a polymerization initiator. Three different FPI samples of different chemical structures were employed in the present study. An optimum T‐peel strength about 15 N/cm was achieved for the copper/FPI laminate. The adhesion strength, however, decreased with increasing fluorine content in the FPI film. The onset of cohesive failure occurred in the FPI film for assemblies with T‐peel strength greater than 6 N/cm. The T‐peel strengths are reported as a function of the argon plasma pretreatment time of the FPI films and thermal lamination temperature. The adhesion strengths were compared to that of the similarly prepared copper/polyimide (Kapton HN) laminate. Time‐dependent water contact angle (Θ) measurements indicated that the surfaces of FPI films are significantly more hydrophobic and more resistant to water diffusion or hydration than the Kapton HN films. The surface compositions of the pristine FPI films, as well as the delaminated FPI films and copper foils were studied by X‐ray photoelectron spectroscopy. The thickness of the graft VIDZ polymer layer was in the order of 200 nm, as derived from the cross‐sectional view of the scanning electron micrograph. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1478–1489, 1999     

Correlation of residual stress and adhesion on copper by the effect of chemical structure of polyimides for copper‐clad laminates

BACKROUND: Polyimide films coated on copper are a potential new substrate for fabricating printed circuit boards; however, adhesion between the copper and polyimide films is often poor. The relations between residual stress and adhesion strength according to the development of molecular orientation of polyimide films with different chemical backbone structure coated on copper were studied. RESULTS: The effect of chemical structures on properties including the residual stress and the adhesion strength were widely investigated for four different polyimides. Diamine 4,4′‐oxydianiline (ODA) and dianhydrides 1,2,4,5‐benzenetetracarboxylic dianhydride (PMDA), 4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,4′‐oxydiphthalic anhydride (ODPA) and 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride (BTDA) were used to synthesize polyimide. In an attempt to quantify the interaction of thermal mismatch with the polyimide films depending on various structures, residual stress experiments between polyimide film and Cu? Si wafer were carried out over a range of 25–400 °C using in situ thin film stress analysis. A universal test machine was used to conduct 180° peel test (ASTM D903‐98) of polyimide film from cooper foil. The residual stress on Cu? Si (100) wafer decreased in the order 6FDA‐ODA > BTDA‐ODA > ODPA‐ODA > PMDA‐ODA, and the interfacial adhesion strength decreased in the order BTDA‐ODA (5 N mm^?2) > ODPA‐ODA > PMDA‐ODA > 6FDA‐ODA. The results may suggest that the morphological structure, degree of crystallinity of chain orientation and packing significantly relate to the residual stress and adhesion strength in polyimide films. Wide‐angle X‐ray diffraction was used for characterizing the molecular order and orientation and X‐ray photoelectron spectroscopy was used for the analysis of components on copper after polyimide films were detached to confirm the existence of copper oxide chemical bonding and to measure the binding energy of elements on the copper surface. CONCLUSION: In this research, it is demonstrated that BTDA‐ODA polyimide has a low residual stress to copper, good adhesion property, good thermal property and low dielectric constant. Therefore, BTDA‐ODA would be expected to be a promising candidate for a two‐layer copper‐clad laminate. Copyright © 2007 Society of Chemical Industry     

Studies on PI/PI adhesion strength

The adhesion of polyimide to polyimide was studied by measuring the peel strength of various polyimide–polyimide composites. Different factors such as diffusion of polyamic acid to polyimide substrate, contact angle, wettability, and the thermal expansion coefficient of polyimide films and the presence of siloxane can affect this adhesion and are discussed in this article. © 1994 John Wiley & Sons, Inc.     

Surface modification of polyimide film by coupling reaction for copper metallization

In this study, Upilex-S [poly(biphenyl dianhydride-p-phenylene diamine)], one of polyimide films, was modified by coupling reactions with N,N-carbonyldiimidazole (CDI) to increase adhesion to copper for flexible copper clad laminate (FCCL). Imidazole groups show strong interaction with copper metal to make charge transfer complexes. Because polyimide film did not have active site with coupling agent, the film surfaces were modified by aqueous KOH solutions and reacted with dilute HCl solutions.Surface modified Upilex-S was analyzed by X-ray photoelectron spectroscopy (XPS) to examine the surface chemical composition and film morphology and investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Changes in the wettability were evaluated by measuring contact angle with the sessile drop method. After deposition of copper on surface modified Upilx-S, the adhesion strength of the copper/polyimide system was measured by a 90° peel test using the Instron tensile strength tester. The peel strength of the copper/polyimide system increased from 0.25 to 0.86 kg_f/cm by surface modification. This result confirmed that the CDI coupling reaction is an effective treatment method for the improvement of the adhesion property between copper metal and polyimide film.     

Peel strength and peel angle measured by the T-peel test on Cr/BPDA-PDA interfaces

The peel strength and peel angle of Cr/BPDA-PDA interfaces were measured using the T-peel test for Cu/Cr/BPDA-PDA structures. When the Cu/Cr metal film was thin, plastic bending of the metal film occurred during the T-peel test. With a thicker metal film, however, plastic bending of BPDA-PDA polyimide was observed. The critical thickness of the metal film for the transition from metal film bending to polyimide bending became thinner with decreasing yield strength of the metal film. Regardless of the metal film bending or polyimide bending during the T-peel test, the peel strength increased with higher peel angle and the failure mode of the Cr/BPDA-PDA interfaces was cohesive failure within BPDA-PDA.     

Indentation test for adhesion measurement of polyimide films

An indentation technique was used to determine the adhesion of polyimide films on a ceramic substrate. Experimental results were obtained by indenting 13 μm thick polyimide films with a conical indenter at different indentation loads. Among the process variables investigated were the amount of adhesion promoter added to the polyimide and the exposure to temperature and humidity. The technique provides a measure of the bond strength, based on the analysis of indentation debonding of thin films. For well adhered films, no debonding could be induced, indicating the usefulness of the test only for the poorly bonded films.     

Low Dielectric Polyimide/Fluorinated Ethylene Propylene (PI/FEP) Nanocomposite Film for High-Frequency Flexible Circuit Board Application

The low dielectric polymer films have drawn great attention to the application as the dielectric insulating materials in high-frequency circuit boards, while the weak adhesion to the copper foils and the poor processability resulted from the fluorinated or rigid structures limited their high-frequency application. In this work, the low dielectric and high adhesive polyimide/fluorinated ethylene propylene (PI/FEP) nanocomposite film for high-frequency flexible circuit board application is developed. It is indicated that the fluorocarbon surfactants can significantly improve the dispersion of FEP in PI substrate, and thus, the PI/FEP nanocomposite film exhibits excellent mechanical properties, including the tensile strength increases to 46.6 MPa and the elongation at the break increases to 13.7%. Importantly, at the high-frequency of 10 GHz, the 60 wt% FEP filled PI nanocomposite film displays an ultralow dielectric loss (0.006) and a reduced dielectric constant (2.69). In addition, the high-frequency flexible circuit board with the PI/FEP film as the dielectric insulating layer has a high peel strength of 0.75 N mm⁻¹, indicating this PI/FEP nanocomposite film can meet the requirements of the high-frequency flexible circuit board application.     

Adhesion of polyimide to silicon and polyimide

The characteristics of the adhesions of polyimide to silicon and to polyimide and the autohesion of a polyimide blend have been investigated. As found, the peel strength of pyromellitic dianhydride–4,4′-oxydianiline (PMDA–ODA) on silicon can be greatly improved by blending with 20 or 40% benzophenone tetracarboxylic dianhydride–p-phenylene diamine (BPDA–PDA). Exposing in air for a 2 day period resulted in a serious deterioration in adhesion for the pure PMDA–ODA system, while in no deterioration for the blend systems. Regardless of adhesion or autohesion, the resulting peel strength decreased markedly with the increase of the curing temperature. It was also found that based on the same curing temperature the diffusion of NMP is much faster in the film of PMDA–ODA than in the blend containing 20% BPDA–PDA. Beside curing temperature, imide-to-imide compatibility seems to play an important role in affecting the adhesion characteristics. © 1993 John Wiley & Sons, Inc.     

The Mechanical Behaviour of Polyimide/Copper Laminates Part 2: Peel Energy Measurements

The mechanical peel behaviour of laminates consisting of polyimide films adhered to copper foil using a modified acrylic adhesive has been studied over a wide range of test rates and temperatures. The laminates were prepared from polyimide films which had been subjected to either a “high-thermal history” or a “low-thermal history” treatment during the production of the film. The measured peel energies of the laminates could be superimposed to give a master curve of peel energy versus the reduced rate of peel test, Ra_T , where R is the rate of peel test and a_T is the time-temperature shift factor. The appropriate shift factors were a function of the test temperature and were mainly deduced from tensile tests conducted on the bulk adhesive. The “high-thermal history” laminates gave higher peel energies and the locus of failure of the laminates was mainly by cohesive fracture through the adhesive layer. At low values of log₁₀ Ra_T , i.e. Low rates of peel and high test temperatures, the “low-thermal history” laminates also failed in the adhesive layer and possessed similar peel energies to those measured for the “high-thermal history” laminates. However, at high log₁₀ Ra_T values, the peel energies measured for the “low-thermal history” laminates were lower and showed a wider scatter. These arose from a different locus of failure occurring in these “low-thermal history” laminates when tested under these conditions. Namely, it was found that most of these laminates failed in a weak boundary layer in the outer regions of the “low-thermal history” polyimide film.     

Metallization of polyimide film by wet process

The interfacial adhesion strength of metallized polyimide (BPDA/ODA/PDA) has been studied with respect to polyimide surface molecular structure, reactions during electroless nickel deposition, baking, copper electroplating, and thickness of polyimide film. Each factor is discussed in terms of its influence on the peel strength. For practical application, operation at optimized conditions for each step of the metallization process is essential for sustaining the mechanical integrity of the copper/polyimide laminate. © 1994 John Wiley & Sons, Inc.