Stephen R. Cain

On single-photon ultraviolet ablation of polymeric materials

The nature of uv ablation of organic polymers is discussed in terms of a pseudo‐zeroth‐order rate law of the form dx/dt = k0e−(Eact/kT), where Eact is assumed to be the strength of the weakest bonds in the polymer and T is the local temperature increase from the incident laser pulse. Equations derived from previous models that assumed nonthermal photodecomposition were duplicated from this photothermal model. Even for the simple case of single‐photon absorption, nonideal behavior is affected by radiationless decay, pulse length, and thermal diffusion. These effects were probed. Results indicated that thermal diffusion may have a significant effect on the threshold fluence and to some degree on the shape of the etch depth versus fluence curve. Absorption dynamics (saturation and radiationless decay) appear to be the dominant factor in determining the functional dependence of etch depth on fluence. As a result of competition between absorption saturation and radiationless decay, the penetration depth is int...

Photothermal description of polymer ablation: Absorption behavior and degradation time scales

A photothermal model of ablation is used to investigate the time scales for polymer degradation by UV laser light. In the absence of a significant incubation effect, strong absorbers (e.g., polyimide) are expected to decompose roughly three orders of magnitude faster than weak absorbers (e.g., polymethylmethacrylate), owing to the higher surface temperature attained during the absorption. This difference in the time scales reflects processes taking place at significantly different temperatures and should extrapolate to the overall ablation process. The very short calculated time scales (femtoseconds for polyimide and picoseconds for polymethylmethacrylate) indicate that polymer decomposition occurs rapidly compared to the actual ejection of material. For multipulsed ablation experiments, significant incubation modification tends to increase the absorbance of weak absorbers, making the effect less marked than in the ideal case. Incubation effects are attributed to nonablative decomposition that occurs at c...

The effect of pulse repetition rate on laser ablation of polyimide and polymethylmethacrylate-based polymers

The effect of pulse repetition rate on polymer ablation was studied experimentally and theoretically for polyimide and 8230 photoresist, a polymethylmethacrylate (PMMA)-based polymer. Both the experimental data and the theoretical model showed a distinct tendency for increased repetition rate to decrease the ablation threshold, without substantially altering the absorption coefficient. This was attributed to heating of the sample sequentially by the laser pulses and the magnitude of the effect is proportional to the square of the absorption coefficient. Polyimide, with an absorption coefficient of , is expected to exhibit the repetition rate effect in the range of tens to hundreds of kilohertz; hence the effect has not heretofore been observed. The PMMA-based photoresist, on the other hand, exhibits the effect at a much lower repetition rate, owing to the smaller effective absorption coefficient.

A diffusional model for transient liquid phase bonding

Abstract Transient Liquid Phase (TLP) bonding for the Au/Sn/Au system is discussed in terms of a numerical diffusional model. Results suggest that the bonding process is sensitive to metal thicknesses, the heating rate, and the age of the sample prior to bonding. Simulations for rapidly heating a fresh sample show multiple peaks in the liquid depth vs time curve, each corresponding to a different solid phase composition. These peaks disappear sequentially as either the heating rate is decreased, or the sample age is increased. Results of this study have practical implications regarding the application of TLP bonding in manufacturing.

Acid-base properties of copper oxides and fluorides by means of band calculations: Relevance to metal-polymer adhesion

Classification of acids and bases in the Lewis sense implies electronic participation and it is extendable to non-protonic solvent interactions as well as solventless reactions. In many microelectronic applications, interfacial interactions between a polymer and a metal occur and contribute to the resulting adhesion of the bond. In the practical sense, metal surfaces are either oxidized or can be chemically altered by process conditions; therefore, the interaction is through a modified surface. The present study shows how tight binding calculations of the extended Huckel type can be used to estimate the relative acid strengths of copper, its oxides and fluorides. The trends obtained from bulk and surface layers in each case were related to the Fermi level of the crystal and the number of empty states derived from the copper 3d orbitals. Our results show the relative acid strengths follow the trend: CuF2 > CuO > CuF ~ Cu2O > Cu The methods used here can be directly applicable to classify the relative acid ...

Modeling of chemical interactions at Cr/Cu interfaces

Abstract Band calculations of the Extended Huckel type were performed to investigate the nature of chromium-copper interfaces. Model systems were comprised of a chromium or copper atom in close proximity to a three- or four-layered slab of substrate material. The substrates considered were metallic chromium, metallic copper, Cr 2 O 3 , and Cu 2 O. Preferred geometries of attack were inferred from the total energy and projected Density of States analyses. Based on the total energy, metal-metal bonds were found to form preferentially.

On the interpretation of polyoxymethylene valence XPS spectra using band calculations

Abstract Tight binding calculations of the extended Huckel type were performed on two conformations of polyoxymethylene. Valence XPS spectra were calculated and compared to previously reported theoretical spectra derived from ab initio calculations. Good agreement in the qualitative features of the spectra suggest that band calculations, even at a crude level of approximation, may be useful for interpreting XPS valence band spectra. Differences in the calculated spectra of the two conformations are attributed to the packing efficiency of the more stable conformer. Because the more stable conformer has a smaller unit cell, the bands are dispersed over a wider energy range.

Quantum-mechanical approach to understanding acid-base interactions at metal-polymer interfaces

Molecular orbital theory is presented as a means to understanding metal-polymer interfacial chemistry. While the systems used in molecular orbital studies tend to be rather idealized, much insight may be gained from such studies. Three different investigations are cited as examples of the applicability of molecular orbital theory in describing how functional groups of an adsorbate bond to a metal surface as well as how quantum-mechanical calculations may be used to interpret experimental data, e.g. photoelectron spectra. Experimental techniques (nucleophilic displacement, isoelectric points, flow microcalorimetry, and temperature-programmed desorption) for determining acid-base interactions are also discussed briefly.

How formaldehyde interacts and reacts with chromium and chromium oxide surfaces: A model for chromium-polyimide binding

Abstract Chemical interactions of chromium and chromium oxide with carbonyl functional groups of a polyimide chain were investigated theoretically. Band calculations of the extended Huckel type were performed on model systems. The chromium (chromium oxide) surface was represented by a three-layered slab of material, and for computational economy, the polyimide carbonyls were represented by the formaldehyde molecule. Results suggested that while formaldehyde binds to Cr 2 O 3 via its oxygen lone pair electrons, it binds to metallic Cr in a π or η 2 (dihapto) fashion. Further, the calculations indicated that formaldehyde was susceptible to decomposition when bound to clean Cr, but was not susceptible to decomposition when bound to Cr 2 O 3 . These differences were attributed to the relatively lower Fermi energy of Cr 2 O 3 . By analogy, the imide groups in polyimide may be substantially decomposed at the polyimide/Cr interface, with strong covalent bonding between the metal and the fragmented polymer.

Charge iterated parameters for extended Hückel calculations on polytetrafluoroethylene: relevance to X-ray photoelectron spectroscopy

Abstract Tight-binding band calculations of the extended Huckel type were performed on polytetrafluoroethylene in order to probe the effect of the atomic orbital parameters on the bonding in the polymer. Comparison was made with previous extended Huckel and ab initio calculations, as well as with experimental data. It was found that the original parameterization of Hoffmann makes the CF bond too polar and should be corrected, as prescribed by Gray. With these corrections, the Hoffmann formulation for extended Huckel calculations allows a reasonable, albeit approximate, simulation of the X-ray photoelectron spectrum.