K. Behnke

Metal/polymer interfaces with designed morphologies

The morphology of a metal/polymer interface is important for many properties, e.g. its adhesional strength. Starting from the basic processes occurring in the initial stages of metal/polymer interface formation, it is possible to obtain different morphologies by variation of the preparation conditions. In this report we present selected examples from our own work of how metal/polymer interfaces with different morphologies can be prepared by evaporating noble metals (Au, Ag, Cu) onto chemically different polymers, i.e. bisphenol-trimethyl cyclohexane polycarbonate (TMC-PC), pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide (PI), and on Teflon AF 1601. The interfaces were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The combination of these techniques allows one to determine morphological parameters such as the concentration and distribution of metal clusters at the surface and in the near-surface region. Using low deposition rates and elevated temperatures, spread-out metal/polymer interfaces can be formed, whereas the use of high deposition rates and moderate temperatures results in relatively sharp interfaces. Another approach to obtain a defined morphology is to form large metal clusters of 10-30 nm diameter on the polymer surface and embed them into the polymer in a controlled manner by a subsequent annealing process. First experiments on the macroscopic adhesion of Au and Cu on TMC-PC showed that the initially low peel strength could be increased substantially by subsequent annealing above the glass transition temperature.

Condensation coefficients and initial stages of growth for noble metals deposited onto chemically different polymer surfaces

The initial stages of growth of noble metals deposited onto untreated as well as Ar+ beam-treated polymer surfaces were investigated by means of X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and radiotracer methods. Condensation coefficients C of the metals on the polymer surfaces were determined by comparison of the XPS signals of the metals with the nominal thicknesses measured by a quartz microbalance during deposition. These measurements were combined with radiotracer measurements, which allow to determine C independently with high accuracy and sensitivity. C was found to depend strongly on the chemical composition of the polymer surface, e.g., at room temperature, C varies between 0.95 for PMDA-ODA polyimide and 0.002 for Teflon AF. By ion beam treatment of the latter, C was raised considerably. During nucleation of the metal on the polymer surface, C increases strongly with metal coverage. C also depends on the deposition rate of the metal and decreases strongly at elevated temperatures.

Formation of metal-polymer interfaces by metal evaporation: influence of deposition parameters and defects

Abstract Metal–polymer interfaces with different but well defined morphologies were prepared by evaporating noble metals (Au, Ag, Cu) onto chemically different polymers, i.e. bisphenol-trimethyl cyclohexane polycarbonate (TMC-PC), pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide (PI), polystyrene (PS) and the low-k dielectric Teflon AF 1601. The interfaces were characterised using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The combination of these techniques allowed one to determine morphological parameters such as concentration and distribution of metal clusters at the surface and in the near-surface region. In addition, radiotracer measurements yielded exact metal condensation coefficients C and was used to determine the extent of diffusion of metal atoms into the polymers. First experiments on the macroscopic adhesion of Cu on TMC-PC showed that the initially low peel strength can be increased substantially by subsequent annealing above the polymer glass transition temperature, T g .

Tailoring the morphology of metal/polymer interfaces

Based on a good knowledge of the basic processes occurring in the initial stages of metal/polymer interface formation the preparation conditions can be selected to obtain a desired interfacial morphology. Very sharp or spread-out metal/ polymer interfaces can thus be formed, the cluster density on the surface can be adjusted, or metal clusters can be embedded into the polymer in a controlled manner. This may have a strong impact on the macroscopic adhesion of metal films evaporated onto polymers.

Condensation coefficients of noble metals on polymers: a novel method of determination by x-ray photoelectron spectroscopy

We developed a method to determine condensation coefficients, C, of noble metals on polymer surfaces and to determine cluster sizes by means of XPS only, or in combination with transmission electron microscopy (TEM). The surface concentration of the adatoms was determined using a mathematical correction of the XPS intensity that takes into account cluster growth of the noble metals on the polymer surfaces. We establish our method with experimental results of C for Cu and Ag on pyromellitic dianhydride (PMDA)–4,4′-oxydianiline (ODA) polyimide, bisphenol-A polycarbonate, trimethylcyclohexane bisphenol-A polycarbonate, polystyrene and Teflon AF 1601. Values of C for Ag determined with the XPS method agree well with those of radiotracer measurements. The condensation coefficient varies by three orders of magnitude from 1 to 0.002, depending on the deposition parameters and properties of the adsorbents. The method can be applied in general to investigate the deposition of non-wetting metals on solid surfaces. Copyright

Determination of condensation coefficients of metals on polymer surfaces

We used X-ray photoelectron spectroscopy (XPS) in combination with transmission electron microscopy to characterize the nucleation and growth of noble metals deposited onto polymers and to determine the condensation coefficients of the metals. The surface concentration of the adatoms was determined using a mathematical correction of the XPS intensity to take into account the cluster formation of noble metals on polymer surfaces. Condensation coefficients for Cu and Ag depend strongly on the chemical composition of the polymer and on temperature and can vary by more than three orders of magnitude already at room temperature. The influence of the deposition parameters and the metal/polymer morphology on the condensation coefficient will be discussed.

Nucleation, growth, interdiffusion, and adhesion of metal films on polymers

Various investigations, involving surface spectroscopy, electron microscopy, ion scattering, direct radiotracer measurements, Monte Carlo simulations, and a novel technique to determine condensation coefficients have contributed considerably to our understanding of metal-polymer interface formation. A strong correlation was established between the reactivity of a metal and its diffusion behavior. Reactive metals like Cr and Ti form relatively sharp interfaces with polymers. Metals of lower reactivity, on the other hand, diffuse into polymers at elevated temperatures but have a very strong tendency to be immobilized by aggregation. The critical cluster consists of only one atom. Diffusion into the polymer increases strongly at low deposition rates, where a large fraction of isolated metal atoms is able to diffuse into the polymer. The extent of diffusion is determined by the early stage of the deposition process. Here condensation coefficients may deviate markedly from unity. No significant diffusion is ex...

Diffusion Of Metals In Polymers And During Metal/Polymer Interface Formation

The increasing application of polymers in microelectronics, particularly their potential use as low dielectric constant materials in IC applications has aroused much interest in diffusion of metals in polymers. Various investigations, involving surface spectroscopy, electron microscopy, ion scattering, direct radiotracer measurements, and Monte Carlo simulations have contributed considerably to our understanding of the diffusion behavior. Metal diffusion in polymers proved to differ markedly from ordinary diffusion because of the sharply contrasting properties of the materials. A strong correlation has been established between the reactivity of a metal and its diffusion behavior. Reactive metals like Cr and Ti form relatively sharp interfaces with polymers. Metals of lower reactivity, on the other hand, diffuse into polymers at elevated temperatures but have a very strong tendency to be immobilized by aggregation. Diffusion into the polymer increases strongly at low deposition rates, where a large fraction of isolated metal atoms is able to diffuse into the polymer before being trapped by other atoms at or near the surface. The extent of diffusion appears to be determined by the early stage of the deposition process. Here, condensation coefficients may deviate markedly from unity, thus keeping the initial metal concentration at the surface very low. No significant diffusion is expected from a continuous metal film as a consequence of the high cohesive energy of the metal, unless chemical interaction involves the formation of metal ions. Metal ions are highly mobile and do not aggregate due to electrostatic repulsion. The model emerging from these observations allows us to predict the salient features of diffusion and interface formation between metals and polymers in general, and particularly with respect to the new low dielectric constant polymers.

Comparative analysis of the nucleation and growth of copper on different low-k polymers

In this work we present investigations of the nucleation and growth of evaporated copper on several low-k polymers. The evolving interfaces were characterized using transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The results were compared between the PMDA/ODA polyimide, Teflon AF 1601 and Silk ® . A diffusion coefficient for copper atoms in Silk ® determined by low energy ion-beam depth profiling in conjunction with XPS is reported.