New Experimental Approaches for the Study of Polymer/Metal Interphases

In this short review we present recent experiments which can be used to infer the structural parameters of ultrathin polyimide and polyamide acid films as a function of distance from the substrate surface. The polyimide films are prepared by the Langmuir-Blodgett technique in a layer-by-layer fashion, and the orientation of the pyromellitic imide unit in the polyimide macromolecules is determined as a function of film thickness by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. Subsequent delamination experiments on these Langmuir-Blodgett (LG) deposited polyimide films reveal that the locus of failure does not occur in a “weak boundary-layer” adjacent to the silicon substrate as expected from delamination experiments with macroscopically thick films.

As a non-destructive method to study the orientation of polymer molecules during film growth, second harmonic generation (SHG) experiments on the deposition of polyamide acid (PAA) on gold and silver surfaces will be briefly described. In this particular case, the experiments reveal an influence on the interfacial chemical bond on the film structure up to a total thickness of 60-100 mm.     

Growth of ODPA-APB polyimide films using molecular beam deposition, and their characterization

We report the first growth of polyimide ODPA—APB films by the molecular beam deposition (MBD) technique and characterize these films using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. MBD is a dry, ultra-high vacuum technique based on the co-deposition of monomers from two different sources, followed by a thermally induced polycondensation reaction to form polyimide. This technique is an alternative to standard wet processing techniques involving the use of solvents. Asdeposited films consist largely of low-molecular-weight species. Heat-treated films are chemically indistinguishable from thermally cured, spin-coated films. Different heat treatment conditions are compared.     

Comparison of finite element stress analysis results with peel strength at the copper-polyimide interface

An exploratory stress analysis has been performed on dense wiring structures, such as those used for multichip modules, with the objective of anticipating possible delamination failure mechanisms. The structure consists of an array of parallel copper lines imbedded in a polyimide insulator, all of which is supported by an underlying silicon wafer. Different versions include thin layers of silicon nitrides on top of and at the base of the copper lines. Using the finite element technique, several models were constructed to explore where delamination could occur in this structure by calculating the driving force (strain energy release rate) which could act on any pre-existing microcrack or delamination. It was found that delaminations propagating from the base of the copper line up the sidewall were a strong possibility. The predictions of the stress analysis were substantiated by transmission electron microscopy photographs of real structures which showed the same type of delamination as that anticipated by stress analysis. For these parts, the adhesion between the copper line and the polyimide insulator was mediated by a liner of tantalum, which gives weak adhesion to copper under certain circumstances. X-ray photoelectron spectroscopy and peel test analyses were used to verify the suspected failure modes.     

Monolithic PDMS Laminates for Dielectric Elastomer Transducers through Open-Air Plasma Treatment

This study investigates the use of an open-air plasma treatment system for the surface modification of polydimethylsiloxane, to improve layer-to-layer adhesion. The procedure presented herein is more cost-efficient compared to conventional vacuum-based plasma treatment, and it is performed at different speeds and distances away from the nozzle, to investigate how these two parameters influence the resulting interfacial layer of two fully cured polydimethylsiloxane films. The plasma treatment is determined to not alter mechanical properties as compared to the single film, while peel forces are sufficient to avoid delamination during operation.     

Effect of organosilica isomers on the interfacial interaction in polyimide/aromatic organosilica hybrids

We report the effect of organosilica precursor isomers on the interfacial interaction between polyimide and organosilica in polyimide/organosilica hybrid composite films. Poly(4,4′‐oxydianiline biphenyltetracarboxamic acid) (BPDA‐ODA PAA) was used as the polyimide precursor, while the organosilica was made using o‐substituted, m‐substituted, and p‐substituted phenyl organosilica precursor isomers. For the preparation of precursor hybrid films, BPDA‐ODA PAA and organosilica precursors were mixed and then the organosilica precursors were converted to corresponding organosilica via sol–gel process. Finally, these precursor films were converted to corresponding polyimide/organosilica hybrid films by the thermal imidization of BPDA‐ODA PAA, which results in poly(4,4′‐oxydianiline biphenyltetracarboximide) (BPDA‐ODA PI). The polyimide/organosilica hybrid films were characterized using three distinctive nuclear magnetic resonance spectroscopies (¹H NMR, ¹³C‐CPMAS‐NMR, and ²⁹Si‐MAS‐NMR), wide‐angle X‐ray diffraction (WAXD), small‐angle X‐ray scattering (SAXS), and peel strength measurement. We found that the m‐substituted phenyl organosilica shows poorer interfacial interaction with BPDA‐ODA PI than do the o‐ or p‐substituted phenyl organosilicas. It was observed, however, that the peel strength of the hybrid films against an aluminum substrate increased with increasing contents of organosilicas, regardless of the nature of the organosilica isomers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2507–2513, 2007     

Interfacial delamination of thin-film PTFE (polytetrafluoroethylene) coatings

Interfacial adhesion is a major concern with respect to successful performance of thin polymer films in developing new thin-film processes. Micro-indentation was used to induce interfacial delamination of polytetrafluoroethylene (PTFE) films deposited on glass substrates using hot filament chemical vapour deposition (HFCVD). Film thickness (1, 2, 3, 5, 10 µm) and indentation load (0.5, 0.75, 1, 2, 3 N) effects on the delamination diameter were investigated. A three-dimensional finite element model using shear material failure criterion and cohesive zone model (CZM) was developed to simulate the delamination. A normalized load–delamination radius relationship was obtained to evaluate the interfacial fracture toughness. The experimental observations showed that the delamination diameter depends on film thickness and indentation load. The numerical simulation indicates the delamination diameter depends on film thickness, material properties, and indentation force. The predictions of interfacial fracture toughness for 5- and 10-µm PTFE films are much smaller than those values using Rosenfeld et al.’s equation, which excludes the energy spent during the penetration.     

Low-dielectric, nanoporous polyimide films prepared from PEO-POSS nanoparticles

Dielectric insulator materials that have low dielectric constants (k<2.5) are required as inter-level dielectrics to replace silicon dioxide (SiO₂) in future semiconductor devices. In this paper, we describe a novel method for preparing nanoporous polyimide films through the use of a hybrid PEO-POSS template. We generated these nanoporous foams are generated by blending polyimide as the major phase with a minor phase consisting of the thermally labile PEO-POSS nanoparticles. The labile PEO-POSS nanoparticles would undergoes oxidative thermolysis to releases small molecules as byproducts that diffuse out of the matrix to leave voids into the polymer matrix. We achieved significant reductions in dielectric constant (from k=3.25 to 2.25) for the porous PI hybrid films, which had pore sizes in the range of 10-40 nm.     

Adhesion at diamond-metal interfaces: a chemical composition perspective

Diamond films were chemically vapor deposited (CVD) on titanium, tungsten, molybdenum, copper and aluminum oxide substrates. In these studies, the interface formed between diamond and the substrate was exposed by mechanically deforming the metal substrate or diamond film to cause film delamination. The observed degree of adhesion for these interfaces can be ranked in the order: Ti » Al₂O₃ (thin films) > Cu > W » Mo. For highly adherent films, delamination procedures were carried out under controlled conditions in order to preserve the integrity of the interfacial species. The exposed interfaces were characterized by X-ray photoelectron spectroscopy (XPS), scanning Auger microscopy (SAM), scanning electron microscopy (SEM) and Raman microprobe spectroscopy. We find that substantial interfacial reaction layers exist at all interfaces except in the diamond-copper system and are composed of both oxides and carbides of the native substrate. Variations in the relative concentration of these species and the distribution throughout the reaction layer also were observed for the different substrates. We believe that both the chemical composition and morphology of the interface influence the adhesion properties of the diamond coating. Correlated investigations of the interfacial surfaces reveal that fracture of the diamond-metal interface occurs discretely at the diamond nucleation plane or within a reaction layer near the diamond interface. We discuss each of these findings in light of qualitative observations of adhesion and suggest avenues for improving the adhesion of diamond films.     

Analysis of surface structure with regard to interfacial delamination in polyester films with incompatible polymers

Poly(ethylene terephthalate) (PET) films containing incompatible polymer particles were analyzed, with particular reference to the relationship between the PET particle interfacial tension and the microvoids, or the protrusion that were formed when the composite material was stretched at 90°C. A model was developed to simulate void formation and surface protrusion due to interfacial delamination between PET and three types of dispersed incompatible polymers, poly(4‐methyl‐1‐pentene), polypropylene, and polystyrene. The numerical results, obtained with the finite element method, were compared with experimental data of the blends for both the internal and subsurface regions. The experimental measurements showed that the increase in the difference in the surface tension between PET and the added incompatible polymer was associated with the formation of larger voids. The protrusions were also generated in the stretching and delamination between PET and the incompatible polymers, but a decrease in the interfacial tension agreed with the formation of a larger protrusion. Modeling studies showed that increasing the interfacial tension between the two components in a blend causes a decrease in the critical stress for delamination. Interfacial tension values related qualitatively to the critical stress for void formation and protrusion calculated with the numerical analysis. A concavity was also necessary for understanding the surface structure of the films, along with protrusion. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1243–1251, 2004     

Studies on the adhesion of polyimide coatings on copper foil

The peel strength and the color of the copper foil peeled at 90 degrees from five different polyimide films were studied. The interfacial surfaces of copper foil and polyimide were examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy dispersion analysis by X-ray (EDAX). There is a correlation between peel strength, and the color of the interfacial side copper caused by oxygen diffusion. Study of the imidization process carried out in vacuum indicates that the geometric arrangements of the atoms of polyimide also play a very important role in peel strength.     

Controlled energy dissipation in fibrous composites I. Controlled delamination

Delamination mechanisms in continuous fiber reinforced composites were investigated. The concept of controlled interlaminar bonding (CIB) is proposed as a guideline for preparing fiber-epoxy composite laminates with enhanced fracture toughness without significant degradation in strength properties. The interlaminar bonding was manipulated by several specialized techniques including insertion of delamination promotors and surface modification of laminae. Results indicated that the plane-strain fracture toughness of E-glass-epoxy laminates could be improved by inserting perforated interlaminar films of aluminum, paper, polyester and polyimide, and fabrics. Such interlayers were used to promote delamination which dissipate strain energy by blunting and diverting a propagating crack. The fracture resistance of a laminate was found to be dependent on the degree of delamination. The competition between the growth of delamination cracks and the propagation of a main crack is controlled by the relative magnitude of the interlaminar bonding strength and the lamina cohesive strength. The interlaminar bonding is controlled by the degree of interlayer perforation and the adhesion between interlayer and lamina. The loading direction was found to be very important in dictating the failure processes. Experimental results from several composite systems are presented and discussed along with post-failure analysis data.     

The Non-destructive Testing of Adhesively Bonded Structure: A Review

The types of defect encountered in adhesive joints and the non-destructive testing techniques available to detect them are reviewed. Three types of defect: complete voids or dis-bonds, poor cohesive strength of the adhesive layer and poor adhesion between the adhesive layer and adherend are commonly present. It is shown that a variety of techniques is available for dis-bond and void detection, ultrasonics and sonic vibration being the most commonly used. The detection of poor cohesive and adhesive properties, however, is much more difficult than void and dis-bond detection and is the subject of current research. At present there is only one commercially available instrument which claims to predict cohesive strength. There is no reliable non-destructive test to detect poor adhesion.     

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.     

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.     

The effect of residual stresses on adhesion measurements

The adhesion of films and coatings is often measured by determining the load required to separate them from their substrate. If there are residual stresses that are relaxed upon delamination, then an additional contribution to the energy-release rate will affect the measurements. These residual stresses may also cause a shift in the mode-mixedness of the interface crack which, in turn, can affect the interfacial toughness. To ensure an accurate interpretation of adhesion measurements, therefore, the effects of these stresses must be considered. These effects are discussed with particular reference to two commonly used test geometries: the blister test and the peel test.     

Synthesis and Microstructural Characterization of Unsupported Nanocrystalline Zirconia Thin Films

A polymeric precursor spin-coating technique is illustrated in which yttrium-stabilized zirconia (YSZ) thin films are produced on Si, Al₂O₃, and NaCl at temperatures less than 350°C. High-resolution transmission electron microscopy (HRTEM) examinations show that the YSZ films are nanocrystalline (grain size of less than 5 nm), fully dense, and have a stabilized cubic fluorite structure. Using the polymeric precursor spin coating method, unsupported nanocrystalline thin films of YSZ with thicknesses ranging from 30 to 1000 nm are prepared by transferring the films from a host substrate to metallic TEM grids with unsupported areas exceeding 1 mm².     

Adhesion behavior of CuCr alloy/polyimide films under 350°C and 85° C /85 % RH environments*

CuCr alloys with varying Cr content were sputter-deposited onto polyimide films and the metal/polyimide films were exposed to a 350°C/N₂ environment for up to 10 h or to 85°C/85% relative humidity (RH) (T/H) conditions for up to 840 h for reliability measurements. The Cr contents of the alloy layers (x) prepared were 0, 2.5, 8.5, 17, 25, 34, and 100 atomic %. Before exposure to hostile environments, the peel strength increased proportionally with the Cr content in the alloy layer up to x = 17 and saturated around 550 J/m², and failure occurred within the polyimide near the metal/polyimide interface, except for the specimen with no Cr (x = 0). After exposure to 350°C, the peel strength dropped for all specimens, but most drastically for the specimens with lower Cr contents (x = 8.5) which failed along the Cr-oxide/polyimide interface. The general trend was the same in the case of the T/H treatment, where interfacial failure along the CuCr-oxide/polyimide interface was found for the specimens with lower Cr content (x 17). The extent of interfacial failure over the peeled metal surfaces was found to increase with the T/H treatment time and was inversely related to the peel strength.     

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     

Effect of Substrate Orientation, Roughness, and Film Deposition Mode on the Tensile Strength and Toughness of Niobium–Sapphire Interfaces

The strength and toughness of interfaces between sputter-deposited polycrystalline niobium films and sapphire substrates with basal and prismatic orientations were measured. The effect of deposition parameters and substrate roughness on these interface properties also was investigated. Substrates of polycrystalline alumina with two different surface morphologies were chosen for studying the effect of interface roughness. The interface strength was measured using a previously developed laser spallation experiment in which a laser-generated compressive stress pulse in the substrate, upon reflection into a tensile stress pulse from the coating's free surface, pulls the interface apart. The interface toughness was obtained using a controlled delamination technique, in which a residually stressed loading layer was used to buckle the underlying test layer from its substrate. The energy balance in the prebuckeled and postbuckled states provided a direct measure of the interface toughness. These values were independently obtained by another experiment in which well-characterized, artificially generated, interfacial flaws were loaded using a stress pulse in the laser spallation assembly. The coating's free surface velocity upon crack initiation was related to the critical energy release rate via a numerical simulation. The results of the two toughness experiments conformed to each other and related fairly well to the independently obtained strength measurements.     

Stresses associated with diffusion in polyimide and polyacrylics films

When a constrained polymeric thin film is immersed in a liquid medium, its state of stress changes because the diffusion of the liquid into the film causes it to swell. Polyimide and polyacrylics films are used in the laminated structures of inkjet printheads. Swelling behavior of the films in various ink component solutions are of great interest because they are closely related to the delamination problem. Swelling stress at constant strain was measured using an environmental tensile tester at both ambient and elevated temperatures. Two kinds of information were extracted. One is the extent of stress relaxation, which is the amount of stress decay between initial stress and equilibrium stress. The difference in the extent of stress relaxation could result in delamination if two different materials with different swelling characteristics are bonded together and exposed to a swelling agent. The other piece of information was the diffusion coefficients of the liquid agents into the polymer films. The magnitude of the diffusion coefficient indicates the rate of proceeding by the penetrant, i.e., the rate of swelling of the films. The results show that the extent of stress relaxation and diffusivity are different for two films. In addition, the diffusion in polyimide film is Fickian and the diffusion in polyacrylics film is pseudo‐Fickian. The changes in mechanical properties after swelling also differ. Among all the ink solutions tested, one component was identified as the most influential and detrimental agent. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006