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A high sensitivity molecular flow detector for direct measurements of capture coefficients
Abstracts
of the papers presented
at the Thirteenth
National
adsorption on polycrystalline molybdenum than on single crystal nickel. Some models are offered which may account for the observations described. David Lichtman, R B McQuistan and T R Kist, HoneyweN Corporate
Vacuum Symposium
A high sensitivity molecular flow detector for direct measurements of capture coefficients An instrument designed to detect molecular flow rates in the presence of the relatively high background gas noise of a space environment chamber is described. The technique of molecular beam modulation is employed. Molecular beam intensities of about 1 x lOlo molecules/sec/cmZ are distinguishable from noise at a background total pressure of 1 x lo-’ torr which corresponds to a random gas intensity of 4x 1Or3 molecules/sec/cmz. The detector performance for Ne, argon, He, and CO, is reported. This high sensitivity makes possible the use of molecular beam techniques for direct measurements of gas condensation and adsorption rates on solid surfaces. The application of this detector in an experimental programme to measure capture coefficients for various gas-surface systems is discussed. J H H&d, Jr, Aerospace Environmental Facility, ARO, Znc, Arnold
Structure determined electrical transport properties of bismuth thin films Thin films of bismuth ranging in thickness from 200 A to one micron have been grown by condensation from the vapour in vacuum at pressures of IO-’ torr. Measurements of conductivity, Hall effect, magneto-resistivity and Seebeck effect have been made on these films in situ both during and immediately following deposition. Electron micrographs of the films at their final thickness show them to consist of an aggregate of crystallites ranging in average linear size of from one to four times their thickness. The carrier mean free path of 2 to 4 p at room temperature in bulk single crystal bismuth is much greater than the average distance between grain boundaries in the film. For this reason, it is proposed that the electrical transport properties of thin polycrystalline bismuth films are determined by crystallite size rather than film thickness. The experimental data verifies this hypothesis. The average crystallite size, and hence the measured properties were found to be dependent on the film thickness, deposition rate and substrate temperature. There was also a slight dependence on the surface energy of the substrate material and the ambient pressure. Data will be presented showing the effects of varying each of these parameters while holding all others constant. Michael R Nemnan and Wen H Ko, Engineering Design Center, Case
Engineering
Institute
Research
Center, Hopkins,
Development
Minnesota.
Center, AroId
Air Force Station,
Tennessee.
Adsorption of simple gases on evaporated horon films The adsorption behaviour of several simple gases on evaporated films of amorphous baron has been studied at 300°K. Evidence is presented for believing that H,, O2 and CO chemisorb readily on these films at room temperature. This evidence is derived from several sources: the rapid absorption of large amounts of gas on the film, the apparent irreversibility of the gas adsorption, and observations of significant reductions in the surface areas of the films after exposure to the gases. These criteria are mutually consistent. Nitrogen is not observed to chemisorb to any significant extent on the boron films at 300°K. P E McElligott and R W Roberts, G E Research and Development Center, PO Box 8, Schenectady,
New
York.
A correlation of experimental and theoretical isotherms for helium on alnminnrn Experimental data for helium adsorption on an aluminium cryopanel, for pressures from 1O-4 to lo-ia torr, have been correlated with a physical adsorption isotherm theory. The statistical theory was developed from a triangular site model and used in this programme to compute isotherms for temperatures from 4” to 20°K. Stepwise isotherms caused by adsorbed particle interactions were predicted by theory and observed experimentally. A sensitive quartz piezoelectric crystal oscillator was constructed to measure the amount of adsorbed material with a sensitivity of 1O-1og/cm”. Special techniques such as crystal cuts, extended coaxial leads, and uhv measurement techniques are described. Values were determined for binding energy, interlayer spacing, and other parameters relating to cryosorption. Both the computer programme and the experimental equipment are useful for predicting cryopanel operation. P J Bryant, C M Gosselin and L H Taylor, Midwest Research Institute, 425 Volker Blvd, Kansas
Crystal
growth
City, Missouri
64110.
and epitaxy
Effects of size and nucleation on the melting point of small islands The equations of Thomson-Lacmann for the melting point reduction of small islands and of Turnbull for supercooling are compared with experimental results. Although both mathematical descriptions are based on nucleation theory, they yield a different dependence of AT,,, on island size. This is shown to be caused by the different phenomena involved, with reversible phase transitions possible only for island diameters below 1000 A. In addition. melting and solidification are affected by the residual gas pressure dur&g deposition. Small crystallites display the equilibrium form, and good agreement between the radius of a model with 116 atoms and the nucleus of critical size of 0.07 Tme6 is obtained for numerous fee metals. This fact provides an explanation for the liquid condensation initially found above 0.67 T,,,, corresponding to a much larger reduction of the melting point than is possible in supercooling. Klaus H Behmdt, Bell Telephone Laboratories Incorporated, Murray HiIl, NJ. 170
of Technology,
Cleveland,
Ohio.
Thin-film epitaxial relationships between phases of the system Au-AI Diffusion effects occurring at gold contacts bonded to aluminium film metallization, and associated bond weakening, have long been a source of concern to manufacturers of silicon devices. As yet, the kinetics of diffusion and the interfacial stresses generated between the participating metals and the intermediate phases generated have received little systematic investigation. In the present study, epitaxial film couples were interdiffused by thermal annealing in the hot stage of an electron microscope. Using electron diffraction data obtained from epitaxial Au-Al film products deposited in separate experiments, the identities, mutual orientations and interfacial misfits of the phases Al, AuAl,, AuAl, Au,Al,, Au,Al, and Au were established. With most of these phases simple parallel mutual orientations are adopted, the misfit varying from zero to about 7.4%. The roles of these orientations and misfits in influencing diffusion rates and interfacial stress (and hence bond weakening) are discussed. A J Noreika and M H Francomhe, Westinghouse Research Laborutories, Pittsburgh,
Pennsylvania
15235.
Effect of applied dc fields on the orientation of evaporated NaCl films The effect of a dc field on the orientation characteristics of evaporated NaCl has been measured using x-ray diffractometer techniques. The fields (O-250 V/cm) were applied in the plane of the target fused silica substrates. For a deposition temperature of 25°C the effect of applied voltage is quite apparent. With zero applied field the NaCl is primarily a mixture of 220 and 200 orientations. With 250 V/cm the NaCl is essentially all 220. Even with gradients as low as 10 V/em the voltage effect is easily noted. At a deposition temperature of 150°C no field effect is noted even at 250 V/cm. Orientation effects resulting from interruption of the dc field during deposition have been measured at room temperature and indicate that the applied field is effective primarily in the initial stages of deposition. These results are discussed. W Robert Sinclair and D W Stillinger, Bell Telephone Laboratories, Inc, Murray
Hill, New Jersey.
Deposition of cesium on a rhenium substrate Cesium metal has been deposited on a large grained rhenium substrate and both the growth of solid films and the wetting of substrate has been observed to vary strongly from grain to grain. It was found that island growth appeared on some grains and epitaxial crystallites on others. The experiments were done in a sealed-off vacuum tube which had a residual gas pressure of 1O-B torr before cesium was admitted. The mode of film growth appeared dependent upon monolayer amounts of impurities. When the substrate was above the melting temperature of cesium, the liquid cesium drops deposited showed variation of contact angle on the various grams indicating
of the papers presented
at the Thirteenth
National
adsorption on polycrystalline molybdenum than on single crystal nickel. Some models are offered which may account for the observations described. David Lichtman, R B McQuistan and T R Kist, HoneyweN Corporate
Vacuum Symposium
A high sensitivity molecular flow detector for direct measurements of capture coefficients An instrument designed to detect molecular flow rates in the presence of the relatively high background gas noise of a space environment chamber is described. The technique of molecular beam modulation is employed. Molecular beam intensities of about 1 x lOlo molecules/sec/cmZ are distinguishable from noise at a background total pressure of 1 x lo-’ torr which corresponds to a random gas intensity of 4x 1Or3 molecules/sec/cmz. The detector performance for Ne, argon, He, and CO, is reported. This high sensitivity makes possible the use of molecular beam techniques for direct measurements of gas condensation and adsorption rates on solid surfaces. The application of this detector in an experimental programme to measure capture coefficients for various gas-surface systems is discussed. J H H&d, Jr, Aerospace Environmental Facility, ARO, Znc, Arnold
Structure determined electrical transport properties of bismuth thin films Thin films of bismuth ranging in thickness from 200 A to one micron have been grown by condensation from the vapour in vacuum at pressures of IO-’ torr. Measurements of conductivity, Hall effect, magneto-resistivity and Seebeck effect have been made on these films in situ both during and immediately following deposition. Electron micrographs of the films at their final thickness show them to consist of an aggregate of crystallites ranging in average linear size of from one to four times their thickness. The carrier mean free path of 2 to 4 p at room temperature in bulk single crystal bismuth is much greater than the average distance between grain boundaries in the film. For this reason, it is proposed that the electrical transport properties of thin polycrystalline bismuth films are determined by crystallite size rather than film thickness. The experimental data verifies this hypothesis. The average crystallite size, and hence the measured properties were found to be dependent on the film thickness, deposition rate and substrate temperature. There was also a slight dependence on the surface energy of the substrate material and the ambient pressure. Data will be presented showing the effects of varying each of these parameters while holding all others constant. Michael R Nemnan and Wen H Ko, Engineering Design Center, Case
Engineering
Institute
Research
Center, Hopkins,
Development
Minnesota.
Center, AroId
Air Force Station,
Tennessee.
Adsorption of simple gases on evaporated horon films The adsorption behaviour of several simple gases on evaporated films of amorphous baron has been studied at 300°K. Evidence is presented for believing that H,, O2 and CO chemisorb readily on these films at room temperature. This evidence is derived from several sources: the rapid absorption of large amounts of gas on the film, the apparent irreversibility of the gas adsorption, and observations of significant reductions in the surface areas of the films after exposure to the gases. These criteria are mutually consistent. Nitrogen is not observed to chemisorb to any significant extent on the boron films at 300°K. P E McElligott and R W Roberts, G E Research and Development Center, PO Box 8, Schenectady,
New
York.
A correlation of experimental and theoretical isotherms for helium on alnminnrn Experimental data for helium adsorption on an aluminium cryopanel, for pressures from 1O-4 to lo-ia torr, have been correlated with a physical adsorption isotherm theory. The statistical theory was developed from a triangular site model and used in this programme to compute isotherms for temperatures from 4” to 20°K. Stepwise isotherms caused by adsorbed particle interactions were predicted by theory and observed experimentally. A sensitive quartz piezoelectric crystal oscillator was constructed to measure the amount of adsorbed material with a sensitivity of 1O-1og/cm”. Special techniques such as crystal cuts, extended coaxial leads, and uhv measurement techniques are described. Values were determined for binding energy, interlayer spacing, and other parameters relating to cryosorption. Both the computer programme and the experimental equipment are useful for predicting cryopanel operation. P J Bryant, C M Gosselin and L H Taylor, Midwest Research Institute, 425 Volker Blvd, Kansas
Crystal
growth
City, Missouri
64110.
and epitaxy
Effects of size and nucleation on the melting point of small islands The equations of Thomson-Lacmann for the melting point reduction of small islands and of Turnbull for supercooling are compared with experimental results. Although both mathematical descriptions are based on nucleation theory, they yield a different dependence of AT,,, on island size. This is shown to be caused by the different phenomena involved, with reversible phase transitions possible only for island diameters below 1000 A. In addition. melting and solidification are affected by the residual gas pressure dur&g deposition. Small crystallites display the equilibrium form, and good agreement between the radius of a model with 116 atoms and the nucleus of critical size of 0.07 Tme6 is obtained for numerous fee metals. This fact provides an explanation for the liquid condensation initially found above 0.67 T,,,, corresponding to a much larger reduction of the melting point than is possible in supercooling. Klaus H Behmdt, Bell Telephone Laboratories Incorporated, Murray HiIl, NJ. 170
of Technology,
Cleveland,
Ohio.
Thin-film epitaxial relationships between phases of the system Au-AI Diffusion effects occurring at gold contacts bonded to aluminium film metallization, and associated bond weakening, have long been a source of concern to manufacturers of silicon devices. As yet, the kinetics of diffusion and the interfacial stresses generated between the participating metals and the intermediate phases generated have received little systematic investigation. In the present study, epitaxial film couples were interdiffused by thermal annealing in the hot stage of an electron microscope. Using electron diffraction data obtained from epitaxial Au-Al film products deposited in separate experiments, the identities, mutual orientations and interfacial misfits of the phases Al, AuAl,, AuAl, Au,Al,, Au,Al, and Au were established. With most of these phases simple parallel mutual orientations are adopted, the misfit varying from zero to about 7.4%. The roles of these orientations and misfits in influencing diffusion rates and interfacial stress (and hence bond weakening) are discussed. A J Noreika and M H Francomhe, Westinghouse Research Laborutories, Pittsburgh,
Pennsylvania
15235.
Effect of applied dc fields on the orientation of evaporated NaCl films The effect of a dc field on the orientation characteristics of evaporated NaCl has been measured using x-ray diffractometer techniques. The fields (O-250 V/cm) were applied in the plane of the target fused silica substrates. For a deposition temperature of 25°C the effect of applied voltage is quite apparent. With zero applied field the NaCl is primarily a mixture of 220 and 200 orientations. With 250 V/cm the NaCl is essentially all 220. Even with gradients as low as 10 V/em the voltage effect is easily noted. At a deposition temperature of 150°C no field effect is noted even at 250 V/cm. Orientation effects resulting from interruption of the dc field during deposition have been measured at room temperature and indicate that the applied field is effective primarily in the initial stages of deposition. These results are discussed. W Robert Sinclair and D W Stillinger, Bell Telephone Laboratories, Inc, Murray
Hill, New Jersey.
Deposition of cesium on a rhenium substrate Cesium metal has been deposited on a large grained rhenium substrate and both the growth of solid films and the wetting of substrate has been observed to vary strongly from grain to grain. It was found that island growth appeared on some grains and epitaxial crystallites on others. The experiments were done in a sealed-off vacuum tube which had a residual gas pressure of 1O-B torr before cesium was admitted. The mode of film growth appeared dependent upon monolayer amounts of impurities. When the substrate was above the melting temperature of cesium, the liquid cesium drops deposited showed variation of contact angle on the various grams indicating