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Computer simulation of surface atom ejection by ion impact
Classified
abstracts
high bias levels (-2 capable of producing
G J Kominiak and J
1935-l
941
kV) and at low gas prcssurcs (- IO /cm Hg), is a surface free of added impurities. E Uhl, J C’nc Sci Tcclr~tol, 13 (I ). 1976, I70- 174. 31
1935. Computer simulation of surface atom ejection by ion impact. (USA) The mechanisms for ejecting atoms from a solid surface subjected to ion bombardment are generally very complicated. Howcvcr. the most energetic sputtered atoms must result from incident ions which enter the solid and, after relatively few collisions, return to impact with a surface atom. The velocity spectrum of scattered and ejected atoms has been calculated using a three-dimensional computer simulation. The trajectories, taken as a series of binary encounters. were computed rapidly by using a parametrized analytic form relating impact parameter, energy, and dcllection angle. The parameters were obtained from a representative sample of trajectories computed by numerical integration of the dynamical equations using an 8-4 interaction potential. The trajectories were followed fat 1500-eV Ne+ ions entering a (I 1 I) surface of a gold crystal at 45’ to the surface normal. To simplify the calculations, ions passing to a depth greater than the third atomic plane were considered lost. For ions returning to the surface, the recoil velocities which they produce in the surface atoms were computed and sorted to produce the desired ejection-velocity spectrum. The velocities of ions emerging from the surface were treated similarly to obtain the spectrum of scattered velocities. H F Helbig, J Vuc Sri Trclt~ol, 13 (I), 1976, 368-373. 31
1936. Redeposition of sputtered species’ by the electric fields of an incident ion beam and sputtered ion. (USA) To explain the recently observed redeposition of sputtered Au, Ag, and Cu, it is shown here that low energy ions sputtered from a plane metallic surface by an obliquely incident ion beam can be returned to the surface by the electrostatic fields of the beam and of the image charges of the sputtered ions. Trajectories for sputtered Au ions were obtained by numerical integration of the equations of motion. The ion image force dominates the motion near the plane (
31 1937. Effect of secondary electrons and negative ions on sputtering of films. (USA) Experiments have been carried out to determine quantitatively the effects of the bombardment of a growing film by secondary electrons and negative ions emitted at a target. The secondary electrons have been found to depress deposition rates and to cause changes in composition of the films. This effect takes place apparently by biasing the substrate negatively by secondary electrons and subsequent resputtering by Ar+ ions. Negative ions such as halide ions have been found not only to depress the deposition rates to zero, but also to etch the substrates. This latter experiment demonstrates sputtering of the anode by negative ions. J J Hanak and J P Pellicane, J Vuc Sci Tecl~~~o/,13 (l), 1976,406-409. 31
1938. Vacuum microbalance for determining sputtering yields operable in the vicinity of a magnetically confined, low-pressure arc discharge. (US.4 A vacuum
microbalance
is described
for
use in studying
sputtering
yields of materials. The balance is constructed so as lo bc operable in the vicinity of a magncticafly confined, low-pressure arc discharge which is used as the source of the sputtering ions. The balance is a modified Cahn RG Electrobalancc mounted within the vacuum system in common with the sputtering apparatus. A secondary balance arm is used as a means of biasing the sputtered target. Damping of vibrations caused by the clcctrostatic and mechanical forces is achieved by the USC of a viscous-drag damping system. The performance of the balance is illustrated by data taken on the sputtering yields of Ge under bombardment by Ar+ ions. R G Hart and C B Cooper, J Voc Sci Teclrrrol, 13 (I ), 1976. 553-555.
33. NUCLEONICS
33
1939. Ion-solid interaction in fusion reactors The impact of energetic ions and atoms from the plasma on the first wall causes serious problems in achieving and maintaining a thermonuclear plasma. Simple model calculations show that sputtering by working gas particles is the main source of impurities in the plasma. Wall erosion is estimated to be up to 4.1 mm/yr. Recent experiments show that blistering due to ions and atoms in the mean energy range of the plasma particles (l-20 keV) does not contribute to wall erosion and may be completely avoided. The charge and energy distributions of working gas particles backscattered from the first wall is largely responsible for the impact of high energy resonant charge exchange neutrals on the wall. The energy distribution of backscattered neutrals dialers significantly from that of ions. High amounts of gas may be trapped in the first wall, but a reemission factor of one will be attained for most materials after a short starting period. High trapping efficiences for hydrogen even at very high doses are found in materials like Ti and Zr. This may be helpful in increasing the pumping efficiency of a divertor. B M U Scherzer, J Vuc Sci Tec/lr~o/, 13 (I), 1976, 420-428.
33 1440. Measurement and modification of first-wall surface composition in the Oak Ridge tokamak (ORMAK.) (USA) Impurities coming into the plasma from the walls of present-day toroidal plasma confinement devices modify plasma behaviour substantially. Small fractions of high-Z ions in the plasma greatly decrease plasma temperatures and increase plasma energy losses. We are attempting to identify and control the sources of impurities from the “first-wall” in ORMAK. Auger electron spectroscopy, soft x-ray appearance-potential spectroscopy, and other surface-sensitive techniques have been used to characterize the surface composition of the first wall and to develop methods to remove carbon and oxygen. Oxygen glow-discharge cleaning has been shown, in the laboratory, to be an effective way of removing carbon from gold films (simulated ORMAK liner material) and the use of oxygendischarge cleaning in ORMAK has resulted in a decrease in plasma contamination, a 50% increase in plasma current, and an accompanying increase in plasma temperature. In spite of these improvements the walls of ORMAK are far from clean. Substantial amounts of carbon, oxygen, iron, and other elements remain. R E Clausing et al, J Vuc Sci Techrrol, 13 (I), 1976, 437-442.
33 1941. Graphite surface erosion and blistering. (USA) Bulk reactor-grade graphite and WCA graphite cloth were exposed to an atom and ion beam produced in a Hall accelerator. Samples exposed to hydrogen received a total dose of 10zO atoms/cm* of 580-eV mean energy Ho and 5 x’ lO’8 ions/cm2 of 875-eV mean energy H+. Samples exposed to helium received a total dose of 10” atoms/cm2 of HE0 and 5 x IO’* ions/cm2 of He+. During irradiation, the pressure in the sample region was governed by the partial pressure of the source gas (H,or He) and was at 47 mPa. The samples were then examined with a scanning electron microscope and an optical interference microscope to determine the surface damage. Upon exposure to Ho and H+, the bulk graphite and the graphite cloth samples showed substantial surface erosion. The amount of erosion was found to be strongly dependent on the sample surface temperature. Upon exposure to He+, blistering was observed on the graphite cloth sample. No measurable surface erosion of the samples exposed to He was observed with the scanning electron microscope.
B Feinberg and R S Post, J Vuc Sci Technol. 13 (I), 1976,44346.
abstracts
high bias levels (-2 capable of producing
G J Kominiak and J
1935-l
941
kV) and at low gas prcssurcs (- IO /cm Hg), is a surface free of added impurities. E Uhl, J C’nc Sci Tcclr~tol, 13 (I ). 1976, I70- 174. 31
1935. Computer simulation of surface atom ejection by ion impact. (USA) The mechanisms for ejecting atoms from a solid surface subjected to ion bombardment are generally very complicated. Howcvcr. the most energetic sputtered atoms must result from incident ions which enter the solid and, after relatively few collisions, return to impact with a surface atom. The velocity spectrum of scattered and ejected atoms has been calculated using a three-dimensional computer simulation. The trajectories, taken as a series of binary encounters. were computed rapidly by using a parametrized analytic form relating impact parameter, energy, and dcllection angle. The parameters were obtained from a representative sample of trajectories computed by numerical integration of the dynamical equations using an 8-4 interaction potential. The trajectories were followed fat 1500-eV Ne+ ions entering a (I 1 I) surface of a gold crystal at 45’ to the surface normal. To simplify the calculations, ions passing to a depth greater than the third atomic plane were considered lost. For ions returning to the surface, the recoil velocities which they produce in the surface atoms were computed and sorted to produce the desired ejection-velocity spectrum. The velocities of ions emerging from the surface were treated similarly to obtain the spectrum of scattered velocities. H F Helbig, J Vuc Sri Trclt~ol, 13 (I), 1976, 368-373. 31
1936. Redeposition of sputtered species’ by the electric fields of an incident ion beam and sputtered ion. (USA) To explain the recently observed redeposition of sputtered Au, Ag, and Cu, it is shown here that low energy ions sputtered from a plane metallic surface by an obliquely incident ion beam can be returned to the surface by the electrostatic fields of the beam and of the image charges of the sputtered ions. Trajectories for sputtered Au ions were obtained by numerical integration of the equations of motion. The ion image force dominates the motion near the plane (
31 1937. Effect of secondary electrons and negative ions on sputtering of films. (USA) Experiments have been carried out to determine quantitatively the effects of the bombardment of a growing film by secondary electrons and negative ions emitted at a target. The secondary electrons have been found to depress deposition rates and to cause changes in composition of the films. This effect takes place apparently by biasing the substrate negatively by secondary electrons and subsequent resputtering by Ar+ ions. Negative ions such as halide ions have been found not only to depress the deposition rates to zero, but also to etch the substrates. This latter experiment demonstrates sputtering of the anode by negative ions. J J Hanak and J P Pellicane, J Vuc Sci Tecl~~~o/,13 (l), 1976,406-409. 31
1938. Vacuum microbalance for determining sputtering yields operable in the vicinity of a magnetically confined, low-pressure arc discharge. (US.4 A vacuum
microbalance
is described
for
use in studying
sputtering
yields of materials. The balance is constructed so as lo bc operable in the vicinity of a magncticafly confined, low-pressure arc discharge which is used as the source of the sputtering ions. The balance is a modified Cahn RG Electrobalancc mounted within the vacuum system in common with the sputtering apparatus. A secondary balance arm is used as a means of biasing the sputtered target. Damping of vibrations caused by the clcctrostatic and mechanical forces is achieved by the USC of a viscous-drag damping system. The performance of the balance is illustrated by data taken on the sputtering yields of Ge under bombardment by Ar+ ions. R G Hart and C B Cooper, J Voc Sci Teclrrrol, 13 (I ), 1976. 553-555.
33. NUCLEONICS
33
1939. Ion-solid interaction in fusion reactors The impact of energetic ions and atoms from the plasma on the first wall causes serious problems in achieving and maintaining a thermonuclear plasma. Simple model calculations show that sputtering by working gas particles is the main source of impurities in the plasma. Wall erosion is estimated to be up to 4.1 mm/yr. Recent experiments show that blistering due to ions and atoms in the mean energy range of the plasma particles (l-20 keV) does not contribute to wall erosion and may be completely avoided. The charge and energy distributions of working gas particles backscattered from the first wall is largely responsible for the impact of high energy resonant charge exchange neutrals on the wall. The energy distribution of backscattered neutrals dialers significantly from that of ions. High amounts of gas may be trapped in the first wall, but a reemission factor of one will be attained for most materials after a short starting period. High trapping efficiences for hydrogen even at very high doses are found in materials like Ti and Zr. This may be helpful in increasing the pumping efficiency of a divertor. B M U Scherzer, J Vuc Sci Tec/lr~o/, 13 (I), 1976, 420-428.
33 1440. Measurement and modification of first-wall surface composition in the Oak Ridge tokamak (ORMAK.) (USA) Impurities coming into the plasma from the walls of present-day toroidal plasma confinement devices modify plasma behaviour substantially. Small fractions of high-Z ions in the plasma greatly decrease plasma temperatures and increase plasma energy losses. We are attempting to identify and control the sources of impurities from the “first-wall” in ORMAK. Auger electron spectroscopy, soft x-ray appearance-potential spectroscopy, and other surface-sensitive techniques have been used to characterize the surface composition of the first wall and to develop methods to remove carbon and oxygen. Oxygen glow-discharge cleaning has been shown, in the laboratory, to be an effective way of removing carbon from gold films (simulated ORMAK liner material) and the use of oxygendischarge cleaning in ORMAK has resulted in a decrease in plasma contamination, a 50% increase in plasma current, and an accompanying increase in plasma temperature. In spite of these improvements the walls of ORMAK are far from clean. Substantial amounts of carbon, oxygen, iron, and other elements remain. R E Clausing et al, J Vuc Sci Techrrol, 13 (I), 1976, 437-442.
33 1941. Graphite surface erosion and blistering. (USA) Bulk reactor-grade graphite and WCA graphite cloth were exposed to an atom and ion beam produced in a Hall accelerator. Samples exposed to hydrogen received a total dose of 10zO atoms/cm* of 580-eV mean energy Ho and 5 x’ lO’8 ions/cm2 of 875-eV mean energy H+. Samples exposed to helium received a total dose of 10” atoms/cm2 of HE0 and 5 x IO’* ions/cm2 of He+. During irradiation, the pressure in the sample region was governed by the partial pressure of the source gas (H,or He) and was at 47 mPa. The samples were then examined with a scanning electron microscope and an optical interference microscope to determine the surface damage. Upon exposure to Ho and H+, the bulk graphite and the graphite cloth samples showed substantial surface erosion. The amount of erosion was found to be strongly dependent on the sample surface temperature. Upon exposure to He+, blistering was observed on the graphite cloth sample. No measurable surface erosion of the samples exposed to He was observed with the scanning electron microscope.
B Feinberg and R S Post, J Vuc Sci Technol. 13 (I), 1976,44346.