Clean Aluminum Surface Research Articles

Hydrogen adsorption on alkali modified aluminum

Coadsorption experiments of alkali metals and hydrogen on Al(100) are reported. The probability for H-H (D-D) dissociation and subsequent atomic adsorption is less than 1:10 4 at all alkali coverages. As a consequence, each overlayer had to be prepared by exposing the sodium or potassium modified surface to a beam of hydrogen (deuterium) atoms. We observed an attractive alkali-hydrogen interaction at all coverages. This interaction is best described as the formation of an alkali hydride in the presence of excess alkali atoms. The formation of the hydride shifts the recombination and desorption temperature of hydrogen adatoms on Al(100) from around 350 to around 500 K. While no isotope effect was detected on the clean aluminum surface (diffusion limited recombination), a significant 10–15 K shift was observed for both alkali hydrides (bond breaking). Furthermore, both alkali hydrides observed intense vibrational losses, thus revealing the ionic character of the metal-hydrogen bond. Electron energy loss spectra of an annealed monolayer of sodium hydride (deuteride) showed metal-hydrogen stretching bands at 1850 (1350) cm −1 and 1715 (1250) cm −1, a deformation band at 800 (600) cm −1, and a metal-metal band at 200 (200) cm −1. Corresponding peak positions for potassium hydride (deutride) were 1650 (1200) cm −1, 1500 (1100) cm −1, and 775 (585) cm −1. The metal-metal vibration could not be separated from the elastic peak because of the larger mass of potassium compared to sodium.

Improved coupling coefficient of laser radiation to aluminum by means of absorptive polymer coatings

The application of a polymer coating to aluminum is shown to provide a convenient energy-absorbing coating which increases the absorptance of the surface in a theoretically predictable way that is independent of the applied coating thickness. The dynamic absorptance of the coated surface for continuous-wave CO2 laser radiation was measured and found to agree with a theoretical model. Above temperatures of 400–500 °C, the polymer coating was completely removed, leaving a clean aluminum surface. It is shown that up to this temperature the absorptance of the surface was approximately 0.30 and also dependent on the incident laser intensity.

Evidence for a multipole surface plasmon at a clean aluminum surface from microscopic theory of photoyield spectra

We establish a relation between pole structures of Feibelman's surface response function d ⊥ (ω) and the existence of “multipole” surface modes in addition to the regular “monopole” surface plasmons. Due to this relation the frequency dependence of both real and imaginary part of d ⊥ (ω) taken from Feibelman's microscopic theory, which is in excellent agreement with photoyield spectra of Al, strongly evidences the existence of such extra modes at a clean smooth Al surface. This confirms previous conjectures based on hydrodynamic model calculations.

Origin and prevention of high contact resistance in multilevel metal-polyimide structures

When polyimide is used as the insulating dielectric in multilevel-metal structures, a high contact resistance can result within the interconnecting vias. This paper examines the particular case of oxygen plasma patterning of the polyimide using a photoresist mask. Auger analysis in combination with compositional depth profiling was employed on a series of samples to measure surface composition of etched vias in polyimide. Results show two effects which, together, can account for high contact resistance: first, there is a thicker than normal aluminum oxide layer on the first level metal surface (due to exposure to the oxygen plasma); second, there is a thin, etch-resistant carbonaceous film (due to redeposition of organic material during plasma etching) that prevents oxide thinning through chemical means. It was found that by lowering the plasma pressure to 50 mTorr near the end of the etch, the organic film can be removed. In the absence of the carbonaceous layer, the oxide can then be chemically thinned to produce clean aluminum surfaces within the vias.

Ion-induced Auger-electron emission from aluminum

We have observed $\mathrm{Al} {L}_{2,3}$ Auger electrons from clean aluminum surfaces bombarded by mass-analyzed beams of noble-gas ions at 45\ifmmode^\circ\else\textdegree\fi{} incidence, and in the energy range 600 eV - 15 keV. The electron energy spectra show a wide structure similar, though not identical, to that observed under electron impact, and sharp lines. These lines are identified as resulting from the Auger decay of $2p$-excited Al and ${\mathrm{Al}}^{+}$. The decay occurs in vacuum from sputtered particles, as suggested by the asymmetric Doppler broadening of the Auger lines. The Auger yields can be scaled when plotted against the maximum energy transfer in a projectile-Al collision, indicating that the excitation occurs mainly in symmetric collisions between target atoms. Exceptions are the cases of impact with ${\mathrm{He}}^{+}$ and low-energy ${\mathrm{Ne}}^{+}$ ions where projectile-Al collisions are responsible for the excitation, possibly by two-electron transitions.

Chemisorption of Organic Compounds on a Clean Aluminum Surface Prepared by Cutting Under High Vacuum

The chemisorption of organic compounds as model compounds of lubrication additives has been studied at room temperature on a clean aluminum surface which was prepared by cutting under high-vacuum conditions. Hydrocarbons such as n-hexane, butene, and cyclohexene did not adsorb, but alkyl halides, organic acid, and alcohols adsorbed on the clean aluminum surface. The adsorption activity was monitored with a quadrupole mass spectrometer and was affected by cutting conditions. The rate of adsorption was proportional to the cutting speed. The chemisorption took place not only during but also after the cutting. From the kinetic considerations, it has been found that the activity was mainly due to the newly formed aluminum surface. The reactivity sequence of the organic compounds was as follows; alkyl halides > alcohols > organic acid >> alkane, alkene.

Adsorption of hydrazine and ammonia on aluminium

The adsorption and decomposition of hydrazine and ammonia by clean polycrystalline aluminium surfaces have been studied by X-ray photoelectron spectroscopy. At 85 K both ammonia and hydrazine are adsorbed molecularly, with N(1 s) peaks at 400.5 eV. At 290 K hydrazine is initially adsorbed to give an N(1 s) peak at about 399 eV, but with time (and further exposure) the position of the peak maximum drift to lower N(1 s) values, finally approaching 397 eV after heating the ad-layer to 390 K. These observations are interpreted in terms of a slow dissociative chemisorption process: N 2H 4(a) → NH 2(a) → NH(a) → N(a). There is no doubt that the NN bond in hydrazine is broken and that hydrogen ad-atoms formed inhibit the subsequent adsorption of N 2H 4 at 290 K. Ammonia dissociates more slowly than hydrazine to give mainly amine (NH 2) species at 290 K.

An X-ray photoelectron spectroscopic study of the interaction of oxygen and nitric oxide with aluminium

The interaction of oxygen and nitric oxide with clean aluminium surfaces has been investigated by X-ray photoelectron spectroscopy. Studies have been in the main confined to the temperature range 80–290 K and pressure range 10 -6 –10 -2 Pa. The Al(2p) level is shown to exhibit a shifted component at a binding energy of ~ 74.5 eV after oxygen interaction at 80 K. A curve-fitting and deconvolution analysis of the shifted peak indicates that it is made up of two components, one at a binding energy of 74.0 eV and the other at 75.3 eV. The lower binding energy component (designated α ) develops preferentially at 80 K while the higher binding energy one ( β ), assigned to Al 2 O 3 , is dominant at 290 K. We suggest that α is an incorporated oxygen structure which is a precursor to the formation of Al 2 O 3 . The initial sticking probability of oxygen at 80 K is 0.07 while at 290 K it is 0.02. The plasmon-loss features associated with the Al(2s) peak are shown to be sensitive to oxygen adsorption and therefore useful in confirming the surface monolayer. At 290 K and an oxygen pressure of 10 4 Pa the oxide thickness is estimated to be about 0.9 nm. When NO was adsorbed at 80 K three distinct N(1s) peaks were observed at binding energies of about 397, 403 and 407 eV. We assign the 397 eV peak to N δ- ads arising from dissociation of the molecule and the two higher binding energy peaks to N δ- ads and N 2 O ads . The N(1s) peak characteristic of NO δ- ads is close to the lower binding energy peak of the two N(1s) peaks associated with N 2 O ads while the O(1s) of NO δ- ads overlaps with the O(1s) peak of the surface oxide at a binding energy of about 532 eV. Mass spectroscopic analysis of the gas phase indicated that on warming to 85 K, the adlayer formed at 80 K, N 2 O was desorbed, confirming our assignment of the core-level spectra. The NO δ- and N 2 O species are not observed at 290 K while at 80 K exposure of the adlayer to water vapour results in the complete removal of weakly adsorbed N 2 O. By monitoring the intensities of the Al(2p), N(1s) and O(1s) peaks, estimates were made of the absolute and relative concentrations of the various species and various molecular processes delineated.

Effect of pressure and temperature on changes in the work function of aluminum during interaction with oxygen

Changes in the work function of clean bulk aluminum surfaces at room temperature and 250°C have been determined during interaction with oxygen in the pressure range of 10 −9 torr to 20 torr. At low oxygen pressure (≈ 10 −7 torr), the initial rate of work function decrease at 250°C was approximately four times faster than the rate at 26°C, indicating that the initial oxidation of aluminum is a thermally activated process with an activation energy of about 2 kcal mole −1. At oxygen pressures below 10 −7 torr, the initial rate of work function decrease was proportional to P O 2 0.95±0.10 at 26°C and to P O 2 0.75±0.10 at 250°C. A model based on the contributions of outer surface and incorporated oxygen is proposed to explain the effect of pressure and temperature on the limiting work function change over the entire range of experimental conditions investigated.

Surface Studies by ESCA: Importance to Environmental Research

Abstract X-ray photoelectron spectroscopy (ESCA) is a technique which samples the first ca. 20 A of a surface. This important characteristic has direct relevance for environmental studies. The present paper reports some recent studies in our laboratory which indicate the potential of ESCA for examining surfaces of interest to environmental research. When an adhesive is stripped from a clean aluminum surface a very thin (ca. 20 A) layer of adhesive remains after stripping. Reactions on surfaces can be monitored such as organic functional groups attached to glass, leaching of metals from glass surfaces or the oxidation of lead sulfide. Quantitative analysis of species on surfaces is possible. By use of fiberglass mats containing chelating groups on their surfaces it is possible to scavenge trace metals from solution and to measure them quantitatively. currently this technique permits detection of 20 ppb lead in solution.

Work function changes during low pressure oxidation of aluminum at room temperature

The oxidation kinetics of clean bulk aluminum surfaces exposed to low oxygen pressures (10 −8 to 2 × 10 −6 torr) at room temperature were followed by measurement of work function changes. The interaction of oxygen with aluminum resulted in a decrease in the work function, indicating that the chemisorbed surface oxygen was unstable and incorporation into the subsurface region took place even at very small surface coverages. A “stable layer” of constant work function was finally formed on the aluminum surface. The limiting work functio nchange (work function change to form the “stable layer”) became less negative with increase in oxygen pressure. It was not possible to explain these results on the basis of previously published oxygen uptake models. A new model based on a pressure dependent limiting amount of outer surface and incorporated oxygen is proposed to explain the pressure dependence of the limiting work function change.

Interaction of oxygen with clean aluminum surfaces by measurement of work function changes

Work function changes were used to study the interaction of clean bulk aluminum surfaces with oxygen in the low pressure region (10 −6 to 10 −8torr). The effect of temperature and pressure on these work function changes indicated that the amount of oxygen adsorbed on the outer surface of the aluminum at these pressures was very small. Low outer surface adsorption was due to the high rate of incorporation of oxygen into the aluminum subsurface. The chemisorbed oxygen led either to a structure of high work function or to a structure of low work function relative to the work function of the clean aluminum depending on experimental conditions. Comparison of results of this work with results of previous studies of adsorption of water on aluminum showed that the lowered work function was definitely a result of oxygen interaction instead of water contamination as proposed by other investigators.

The growth of thin nickel films on a (111) aluminium surface

Nickel films 35 Å thick were deposited in UHV on to clean (111) aluminium surfaces at various temperatures between 70° and 330 °C, to determine the effect of substrate temperature on the structure and crystallographic orientation of the deposited film. At 70 °C the Ni formed an epitaxial layer containing small epitaxial particles of AlNi. At 250 °C a continuous layer of Al 3Ni was produced with no preferred orientation. At 330 °C an island structure was obtained with the islands elongated in the Al 〈1 1 0〉 directions: the islands were unstable at 330 °C and tended to become spherical to minimise their surface area. Comparison with deposits on NaCl indicated that Ni atoms are more mobile on Al at about 300 °C than they are on NaCl at the same temperature.

Synthesis of amodel system for the primary energy conversion reactions in photosynthesis.

A model system is constructed which, like the photosynthetic apparatus of green plants, can convert light to chemical free energy through pigment-sensitized photooxidation of water. The system has two light-harvesting subunits connected electrically in series. Each subunit is made of a multimolecular layer of Zn(II)-tetraphenylporphyrin deposited on a clean aluminum surface and immersed in an aqueous mixture of potassium ferri- and ferrocyanide. Upon illumination by amber light, charge transfer takes place across more than 70 molecular layers of the pigment with a photoelectromotive force of 1.1 to 1.3 volts per subunit. With NADP as the electron acceptor and NADP-reductase as a mediator, the system can photooxidize water to oxygen gas. With these model experiments as a guide, a molecular mechanism for the primary energy conversion reactions in photosynthesis is formulated that offers a unified interpretation of most of the relevant observations reported in the literature.

Bestimmung der mittleren Laufwege von Sekund�relektronen in einer Polymerisatschicht

The secondary electron yield of an aluminum surface was measured in a vacuum of 10−4 tor. It depends on the time for which the surface has been exposed to the primary electron beam and decreases from the value corresponding to a clean aluminum surface to that of the polymerized hydrocarbon layer formed under electron bombardment in poor vacuum. After the yield had decreased to a constant value, the measurement was interrupted and the thickness of the polymerized layer was determined. From the descent of the yield curve, a mean path of the secondary electrons in the polymerized hydrocarbon layer could be estimated to be $$\lambda = 95 \pm 20{\AA}.$$