American Vacuum Society

High-resolution electron energy loss spectroscopy has been applied to study the adsorption of acetylene and ethylene on Pt (111) and the surface reactions of these adsorbates as a function of temperature and in the presence of chemisorbed hydrogen. Making use of the surface selection rule and the observed frequencies a rather detailed picture is developed about the chemical nature of the adsorption states and reaction intermediates. Below room temperature, acetylene rehybridizes so as to form one strong di/σ-like bond to the surface and an additional bond to the surface with the remaining π orbital. Ethylene is essentially just di/σ bonded. Around room temperature, ethylene converts to a new chemical species which we identify to be ethylidene (CH3–CH=). Ethylidene is also formed from acetylene when the surface is first exposed to hydrogen, then to acetylene at low temperatures, and subsequently annealed to 350 K. The same result was obtained by exposing adsorbed acetylene to atomic hydrogen at 350 K. We further find that the hydrogenation of adsorbed acetylene to ethylidene proceeds via the intermediate –CH2–C≡ (2-ethyl–1-ylidine).

Paracyanogenlike films – (CN)n– are prepared by reactive rf sputtering of carbon in nitrogen. The material is deposited on quartz, silicon, metals, sapphire, and mica from 20° to 450°C. Above 450°C, there is no film accumulation on the substrates. The sputtering process yields a deposition rate greater than ten times the best known commercial method for paracyanogen film preparation. Also described are the results of sputtering carbide targets i.e. HfC, TiC, SiC, in the presence of nitrogen. Due to the stability of the C≡N bond, for example, stoichiometric Si3N4 films unexpectedly were deposited from a SiC target sputtered in N2. Similar results are described for HfC and TiC targets.

Thin films of α-A12O3 were prepared for in situ TEM investigations of the nucleation and growth characteristics of various vapor-deposited materials. In this work, gold was deposited at 1.5×10−3nm s−1 at a substrate temperature of 650 °C until the average particle size had reached 8 nm. The nucleation and growth kinetics of these particles were recorded in situ in 0.5-s intervals. Coalescence and secondary growth were observed during the entire deposition period. The azimuthal orientation of the deposit particles under the stated deposition conditions is random. A small area of the specimen, about 20μm diameter, was then locally heated with the electron beam until reevaporation of the gold particles occurred. The characteristics of the gold deposit in the 10-μm-wide transition range between the cooler, nonirradiated area, and the electron-beam-heated area were analyzed in terms of the degree of epitaxy, particle area coverage, number density, and particle shape. A very distinct epitaxial alignment of the gold particles was found. Even in the relatively cool portion of the transition zone, well before the temperature was high enough to induce noticeable changes of particle shape, coalescence, or reevaporation, almost perfect epitaxy was observed. These results suggest that epitaxy on many refractory substrate materials could be achieved by short-duration annealing at high temperatures of randomly oriented deposits.

Flash filament techniques and mass spectrometry have been used to make sputtering measurements for nitrogen which is chemisorbed on a tungsten surface and bombarded with Ar+ ions. The sputtering rate for a given energy between 20 eV and 300 eV is described by the equation dN/dt=−S(N/)N0)ni, where N is the surface coverage, N0 the saturation surface coverage, ni the ion flux, and S the sputtering ratio measured at saturation. The sputtering ratio for nitrogen was found to be ∼0.02 at 17 eV, indicating a low threshold energy for sputtering of the strongly bound nitrogen atom. This result suggests that the well-known experimental relationship between sublimation energy (∼7 eV/nitrogen atom) and threshold energy may not hold for chemisorbed gases. For all energies between 100 and 300 eV the measured sputtering ratio for nitrogen was approximately twice that found for tungsten from atomically clean tungsten. This is particularly surprising when one considers that nitrogen covers less than 20% of the geometrical surface area. This result may, indicate a large sputtering ratio even for gases with large binding energies.

It is shown that continuous or intermittent ion bombardment during deposition can change the growth mode of thick deposits formed by sputtering or evaporation. The columnar morphology commonly found when deposits exceed 1 μ in thickness can be disrupted and the deposit densified by negatively biasing the deposit and subjecting it to ion bombardment during growth. This effect is found to occur in a number of metal systems and may be attributed to resputtering during growth. In the case of sputter deposited tantalum, the density of 6-μ thick films can be increased from 14 g/cm3 at 0 deposit bias to 16.3 g/cm3 at −500-V bias. The macroscopic stress in the deposited film is also shown to be a function of the deposit bias. It is proposed that this growth habit modification may also be obtained in sputter deposited nonmetallic materials and chemical vapor deposits by ion bombarding the deposit during growth.

It is shown that sputter deposited gold films containing atomic ratios of helium-to-gold of up to 0.4 may be formed in a helium gas dc diode discharge. Gas content is primarily a function of substrate temperature and bias, gas pressure, and target voltage. Data indicate that high-energy neutral helium atoms originating at the target are important in the gas incorporation at low gas pressures and positive substrate biases. Maximum helium incorporation occurs at about −80 V bias, where the sticking coefficient is expected to approach unity. Deposition conditions are influential in the subsequent thermal release of the incorporated gas. Gas analysis was performed by heating and flash evaporation of the gas-containing film and then determining the pressure rise in a known volume. It is proposed that this technique is applicable to the formation of many nonequilibrium gas–metal/ceramic systems.

Energetic ion bombardment of metal and ceramic deposits during deposition is shown to affect the morphology, structure, stochiometry, and physical properties of the resulting deposit. The bombardment of the growing deposit tends to eliminate the columnar growth morphology normally encountered in thick-crystalline deposits. Generally, in metals it is found that internal stress, density, and gas content increase with increasing ion bombardment energy, but in some cases there is a decrease at high energies, possibly due to heating. In the case of sputter-deposited glass films, it is found that ion bombardment affects the stoichiometry, the coefficient of thermal expansion, and the strain point of the glass. Specific data are presented for thick (> 1 μ) rf sputter-deposited Corning 1720 glass and also sputter-deposited and electron-beam ion-plated chromium deposits with both a dc and a rf substrate bias.

Silver coatings of 15–50-μ thickness were deposited from a hollow cathode source with substrate bias voltage between 0 and −80 V, and deposition rates varied from 2 to 30 μ/min. A study of the relationship between the substrate bias voltage and the coating characteristics was made. The crystallographic orientation of the coating varied with bias voltage. The microstructure and microhardness of the coatings were also examined as a function of the bias voltage. The microstructure of the coatings resembled that of sputtered coatings; grain structure was changed from columnar to equiaxed by increasing the bias voltage. With the changes in the morphology of the microstructure, the microhardness of the coating also varied by a factor of 2. The coating fractured in a ductile mode and showed excellent adhesion to its substrate.

Thick [1–10 mil (25.4–254.0 μm)] OFHC copper coatings were deposited on copper, tantalum, and stainless-steel substrates maintained at temperatures (T) in the 50 °–950 °C range, at rates of from 200 to 18 000 Å/min, using primarily hollow and also post-type cathode sputtering apparatuses at argon pressures of 1 and 30 mTorr. Coating structures were examined by preparing metallographic cross sections. Surface topographies and fracture cross sections were examined by scanning electron microscopy. Crystallographic orientations were determined by x-ray diffraction. No significant deposition rate influence was found on the low-temperature structure zones reported previously [J. A. Thornton, J. Vac. Sci. Technol. 11, 666 (1974)] or on the columnar nature of coatings formed at elevated T. Truly equiaxed grain structures were generally not observed with hollow cathodes. Annealing twins were found within the grains for T≳350 °C. Evidence of extensive recrystallization and grain growth was seen for T∠900 °C. Coatings deposited at low rates and T≳400 °C on Cu substrates exhibited epitaxial effects; those on dissimilar substrates showed evidence of agglomeration.