Kristen L. Steffens

On the Origins of Sudden Adhesion Loss at a Critical Relative Humidity: Examination of Bulk and Interfacial Contributions

The origins for abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces have been explored using a model poly(methyl methacrylate) (PMMA) film on glass. The interfacial and bulk water concentrations within the polymer film as a function of D 2O partial pressure were quantified using neutron reflectivity. Adhesion strength of these PMMA/SiO 2 interfaces under the same conditions was quantified using a shaft loaded blister test. A drop in adhesion strength was observed at a critical RH, and at this same RH, a discontinuity in the bulk moisture concentration occurred. The moisture concentration near the interface was higher than that in the bulk PMMA, and at the critical RH, the breadth of the interfacial water concentration distribution as a function of distance from the SiO 2/PMMA interface increased dramatically. We propose a mechanism for loss of adhesion at a critical RH based upon the interplay between bulk swelling induced stress and weakening of the interfacial bond by moisture accumulation at the PMMA/SiO 2 interface.

Planar laser-induced fluorescence of CF2 in O2/CF4 and O2/C2F6 chamber-cleaning plasmas: Spatial uniformity and comparison to electrical measurements

Planar laser-induced fluorescence (PLIF) measurements were made to determine two-dimensional spatial maps of CF2 density as an indicator of chemical uniformity in 92% CF4/O2 and 50% C2F6/O2 chamber-cleaning plasmas at pressures between 13.3 Pa (100 mTorr) and 133.3 Pa (1000 mTorr). Measurements were also made of broadband optical emission and of discharge current, voltage and power. All measurements were made in a Gaseous Electronics Conference Reference Cell, a capacitively coupled, parallel-plate platform designed to facilitate comparison of results among laboratories. The CF2 PLIF and emission results were found to correlate with discharge current and voltage measurements. Together, these optical and electrical measurements provide insight into the optimization of chamber-cleaning processes and reactors, suggest new methods of monitoring plasma uniformity, and identify important spatial effects which should be included in computer simulations.

Electrical control of the spatial uniformity of reactive species in plasmas

We report a new method for controlling the spatial distribution of reactive chemical species in a parallel-plate plasma reactor, by means of a variable-impedance load placed between the unpowered electrode and ground. The technique was demonstrated in 89% CF4/11% O2 and 51% C2F6/49% O2 chamber-cleaning plasmas at 13.3–133 Pa (0.1–1.0 Torr) in a Gaseous Electronics Conference Reference Cell. The rf current and voltage at both electrodes were measured, and plasma spatial characteristics were observed using two-dimensional (2D) planar laser-induced fluorescence of the CF2 radical and 2D broadband optical emission measurements. By adjusting the load impedance to cancel the impedance of stray capacitances in parallel with the load, or the sheath capacitance in series with the load, the rf current at the load electrode could be made higher or lower than the current received when the electrode is grounded. When the rf current at the load electrode was minimized, regions of intense optical emission and high CF2 d...

Elastic, adhesive, and charge transport properties of a metal-molecule-metal junction: the role of molecular orientation, order, and coverage

The elastic, adhesive, and charge transport properties of a metal-molecule-metal junction were studied via conducting-probe atomic force microscopy (AFM) and correlated with molecular structure by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The junctions consisted of Co-Cr-coated AFM tips in contact with methyl-terminated alkanethiols (CH(3)(CH(2))(n-1)SH, denoted by C(n), where n is the number of carbons in the molecular chain) on Au substrates. AFM contact data were analyzed with the Derjaguin-Muller-Toporov contact model, modified by a first-order elastic perturbation method to account for substrate effects, and a parabolic tunneling model, appropriate for a metal-insulator-metal junction in which the thickness of the insulator is comparable to the Fermi wavelength of the conducting electrons. NEXAFS carbon K-edge spectra were used to compute the dichroic ratio R(I) for each film, which provided a quantitative measure of the molecular structure as a function of n. As n decreased from 18 to 5, there was a change in the molecular phase from crystalline to amorphous (R(I) --> 0) and loss of surface coverage, and as a result, the work of adhesion w increased from 82.8 mJ m(-2) to 168.3 mJ m(-2), the Youngs modulus of the film E(film) decreased from 1.0 to 0.15 GPa, and the tunneling barrier height phi(0) - E(F) decreased from 2.4 to 2.1 eV. For all n, the barrier thickness t decreased for small applied loads F and remained constant at approximately 2.2 nm for large F. The change in behavior was explained by the presence of two insulating layers: an oxide layer on the Co-Cr tip, and the alkanethiol monolayer on the Au surface. X-ray photoelectron spectroscopy confirmed the presence of an oxide layer on the Co-Cr tip, and by performing high-resolution region scans through the film, the thickness of the oxide layer t(oxide) was found to be between 1.9 and 3.9 nm. Finally, it was shown that phi(0) - E(F) is strain-dependent, and the strain at which the film is completely displaced from under the tip is -0.17 for all values of n.

A technique for temperature mapping in fluorocarbon plasmas using planar laser-induced fluorescence of CF

Planar laser-induced fluorescence measurements of CF A 2Σ+–X 2Π(1,0) were used to determine two-dimensional maps of rotational temperature in CF4 plasmas. Measured rotational temperatures are expected to be in equilibrium with the gas temperature due to the long chemical lifetime of CF relative to the collision rate. Experiments were performed in the capacitively coupled Gaseous Electronics Conference rf reference cell at pressures from 26.7 Pa (200 mTorr) to 107 Pa (800 mTorr) and powers of 10 to 30 W deposited in the plasma. Temperatures, which ranged from 273±15 K to 480±15 K, were fairly axially symmetric and increased with pressure and power. All plasmas were coolest near the electrodes, which provided a substantial sink for heat in the plasma. Highest temperatures were found at a radial position near the edge of the electrodes. The strong temperature gradients observed in the plasmas can have serious effects on density measurements that probe a single rotational level, as well as on reaction rate co...

Fundamentals of Adhesion Failure for a Model Adhesive (PMMA/Glass) Joint in Humid Environments

The origins for the abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces were explored using a poly(methyl methacrylate) (PMMA) film on silicon oxide as a model system. The interfacial and bulk water concentrations within the polymer film were quantified as a function of D2O partial pressure using neutron reflectivity. The adhesive fracture energies of these PMMA/SiO2 interfaces at the same conditions were determined using a shaft-loaded blister test. Discontinuities in the adhesive fracture energy, bulk moisture solubility, and the width of the interfacial moisture excess near the interface were observed at the critical RH. A mechanism based on the coupling of bulk swelling-induced stresses with the decreased cohesive strength due to moisture accumulation at the interface is proposed and is consistent with all experimental observations.

Superconformal Ni Electrodeposition Using 2-Mercaptobenzimidazole

Ni superconformal filling of sub-micrometer trenches is demonstrated using a Watts bath containing an inhibitor 2-mercaptobenzimidazole (MBI). Hysteretic voltammetry marks the breakdown of the MBI-induced passive-state coincident with the onset of Ni deposition. Chronoamperometry reveals that disruption of MBI inhibition is a strong function of the immersion conditions and potential conditioning. The passive to active transition involves a competition between potential-dependent MBI adsorption and its subsequent deactivation and/or consumption during Ni deposition. Breakdown of the MBI-passivating layer results in sulfide formation and its incorporation into the Ni film. The current efficiency of Ni deposition on the activated surface is close to that for the additive-free electrolyte. For recessed surface features, such as trenches, inhibition-breakdown initiates preferentially at the bottom corners. Scanning and transmission electron microscopy (SEM/TEM) studies show the growth front within the trenches initially develops as a v-notch shape while negligible deposition occurs on the free surface. This is followed by the onset of Ni deposition over the entire surface profile that results in trench filling by geometrical leveling. Depending on the specimen immersion process a distinct microstructural transition marks the change in growth mode from a superconformal deposition mode to geometrical leveling during trench filling.

Electrical control of plasma spatial uniformity investigated by planar laser-induced fluorescence

Planar laser-induced fluorescence was performed in the parallel-plate gaseous electronics conference reference cell to determine two-dimensional maps of the CF/sub 2/ radical in 89% CF/sub 4//11% O/sub 2/ chamber-cleaning plasmas. The spatial characteristics of the CF/sub 2/ density and of broadband optical emission were controlled by varying the current at the upper electrode by adjusting the impedance between the upper electrode and ground. The results suggest that electrical control of the current paths through the plasma could be used to control the spatial distribution of reactive chemical species, aiding in the optimization of chamber-cleaning plasmas.

Effect of changing the electrode gap on the spatial and electrical properties of O2/CF4 plasmas

Planar laser-induced fluorescence (PLIF) measurements were made to determine two-dimensional spatial maps of CF2 density as a chemical marker of plasma uniformity in 9% O2/91% CF4 chamber-cleaning plasmas. Broadband optical emission and discharge current and voltage measurements were also made and compared to the PLIF results. Measurements were made in a capacitively coupled Gaseous Electronics Conference Reference Cell as pressure was varied from 13 (100 mTorr) to 133 Pa (1000 mTorr) and electrode gap was varied from 2.25 to 0.5 cm. Smaller gaps resulted in increased radial uniformity and extended the pressure range over which CF2 density and electrical properties remain insensitive to pressure. These effects are explained by a decrease in the bulk plasma resistance at narrower electrode gaps, which changes the path that rf current takes through the discharge, thus affecting where electron heating and CF2 production occur. These results provide insight into the optimization of chamber-cleaning processes ...

Time-resolved surface infrared spectroscopy during atomic layer deposition of TiO2 using tetrakis(dimethylamido)titanium and water

Atomic layer deposition of titanium dioxide using tetrakis(dimethylamido)titanium (TDMAT) and water vapor is studied by reflection-absorption infrared spectroscopy (RAIRS) with a time resolution of 120 ms. At 190 °C and 240 °C, a decrease in the absorption from adsorbed TDMAT is observed without any evidence of an adsorbed product. Ex situ measurements indicate that this behavior is not associated with an increase in the impurity concentration or a dramatic change in the growth rate. A desorbing decomposition product is consistent with these observations. RAIRS also indicates that dehydroxylation of the growth surface occurs only among one type of surface hydroxyl groups. Molecular water is observed to remain on the surface and participates in reactions even at a relatively high temperature (110 °C) and with long purge times (30 s).