C. Burleigh Cooper

MASS SPECTROMETRIC STUDY OF NEUTRAL PARTICLES SPUTTERED FROM Cu BY 0- TO 100-ev Ar IONS

A low‐pressure magnetically confined argon arc discharge was used in a mass spectrometer ion source to study low‐energy (0–100 eV) sputtering of polycrystalline copper. Target bombarding ion current densities ranged from 60–200 μA/cm2. Neutral particles were studied. Cu atoms and Cu2 molecules were detected. The mass ratio of analyzed Cu2 molecules to Cu atoms increased with bombarding ion energies to about 5½% at ion energies of 100 eV. Target voltages for appearance of Cu atoms and Cu2 molecules were − 19 and − 50 V, respectively. No Cu3 molecules were detected; if they were present, it was estimated that the ratio 189Cu3 to 63Cu is less than 0.09%. The method has been found to be promising for the study of neutral particles in low‐energy sputtering. Yield curves agree well with results of other observers; sensitivities of 7×10−4 atoms/ion were attained, and this figure can be improved.

Energies of Cu1+ ions sputtered from Cu by very low energy (50 eV < E < 1 keV) Inert gas ions☆

He+, Ne+ and Ar+ ions with energies E1 between 50 and 1000 eV were used to bombard a polycrystalline Cu target at an angle of 45°. The energies, E2, of the Cu1+ ions sputtered at 90° to the primary beam were investigated using a UHV magnetic sector mass spectrometer. The maxima of the energy distributions as measured by the instrument, were at values of E2 of about 4 eV (±1 eV), nearly independent of e1 and primary ion mass. Plots of log N(E2) versus log E2 displayed limited linear portions over which the functional dependence of N(E2) is proportional to E2−0.5. Plots of the average secondary ion energy, Ē2, versus the energy transferred by the primary ion to a Cu atom in a direct collision ([4m1m2(m1 + m2)2]E1), indicate that Ē2 increases linearly with transfer energy up to a transferred energy of about 200 eV, independent of primary ion mass. Above about 200 eV transferred energy, Ē2 asymptotically approaches values which depend upon primary ion mass. At transferred energies below about 200 eV, the collision kinematics in the fust few collisions appears to dominate the emission process.

Energy distributions of neutral atoms sputtered by very low energy heavy ions

Abstract Polycrystalline Cu was sputtered by normally incident, very low energy Ar + ions ( E 0 = 40–1000 eV). The kinetic energy ( E ) distributions of the neutral Cu atoms sputtered normally from the Cu surface were measured, using secondary neutral mass spectrometry. For values of E 0 above approximately 600 eV, the observed energy distributions agreed closely with the Thompson-Sigmund theory. For values of E 0 less than about 600 eV the distributions fell off faster than predicted by the Thompson-Sigmund theory, and the peak value of the distribution shifted to somewhat lower energies. Both these effects were exaggerated as E 0 was further lowered. The average kinetic energy of the sputtered neutral Cu atoms increased with increasing E 0 . The rate of this increase was less at higher values of E 0 .

Sputtering Yields of Several Semiconducting Compounds under Argon Ion Bombardment

Sputtering yields of single crystals of SiC, InSb, GaAs, GaP, CdS, and PbTe under argon ion bombardment have been measured. The energy of the argon ions, obtained from a magnetically confined argon arc, ranged from 75 to 600 eV. The sputtering yields were determined from target weight losses measured by an in situ quartz spring microbalance. Yields at 500‐eV argon ion energy ranged from 0.40 for SiC to 1.30 for PbTe.

Measurements of energy distributions and yields of neutral Cu2 and Cu3 species sputtered from Cu by low energy Ar+ ions

Abstract Polycrystalline copper was bombarded by normally incident, very low energy argon ions (E0 = 100–1000 eV). The translational kinetic energy distributions and relative yields of neutral Cu2 and Cu3 species sputtered normally from the copper surface were measured using a secondary neutral mass spectrometer. The energy distribution of a given species (Cu2 or Cu3) was found to be approximately constant in shape for 600 E ) were in the order: E (Cu 1 ) > E (Cu 2 ) > E (Cu 3 ) . E (Cu 2 ) increased with increasing bombarding ion energy. The relat depend on the total sputtering yield of copper squared and the relative yield of Cu3 on the total yield cubed. The measured polyatomic energy distributions and relative yields indicate that a multiple-collision process is responsible for sputtered neutral metallic cluster formation in this system.

Experiments on the sputtering of neutral Cu2 dimers from Cu by Ar+ ions (60–300 eV)

Abstract A planar polycrystalline copper target was bombarded normally by Ar + ions with energies ranging from 60 to 300 eV. in the source of a mass spectrometer. The neutral particles sputtered normally from the target were post-ionized and analyzed in the mass spectrometer. A thermionically sustained, magnetically confined low pressure arc plasma was used to supply the bombarding ions, and for post-ionization of the neutral sputtered species. It was found that the relative yields of the sputtered neutral post-ionized Cu 2 dimers are linearly proportional to S 2 , where S is the total sputtering yield for Ar + -Cu. The results support the recombination model for the formation of neutral dimers in sputtering.

Mass Spectrometric Study of Sputtering of KB by Low‐Energy Ar+ and Xe+ Ions

A KBr target was bombarded normally with Ar+ and Xe+ ions, with energies ranging from 25 to 180 eV, in the source of a mass spectrometer capable of detecting neutral and negative sputtered species. A relatively strong signal of Br− ions sputtered from the target was detected, and relative sputtering yield curves were obtained. The yield was greater for Ar+ than for Xe+ ion bombardment at all ion energies. No neutral K0 or Br0 atoms, nor KBr0 molecules, were detected as having been sputtered from the target during bombardment.

Mass spectrometric study of the binary approximation in scattering of low energy ions from solid surfaces

Abstract The validity of the binary collision approximation (in a “single scattering” event) when low energy inert gas ions are scattered from the surface of a copper target was investigated. He and Ne ions of energies 20–1000 ev were incident on the surface of a polycrystalline target at 45°; ions scattered at 90° with respect to the incident beam direction were analyzed using a magnetic sector mass spectrometer. The scattered ions were identified and their energies measured. The binary collision approximation was found to be valid for primary ion energies as low as 40 ev for the HeCu system, and 20 ev for the NeCu system. No evidence of an increased “effective mass” of the target atoms was found.

Angular Distribution of Sublimed and Sputtered Particles from Ag Single Crystals

When silver (100) and (110) single crystals were sputtered by 160–200‐eV argon ions from an arc the well‐known spot patterns appeared when the sputtered particles were collected as a thin film on glass plates. To determine whether similar spot patterns would result from sublimation, the same crystals were sublimed at temperatures between 642° and 815°C under high‐vacuum conditions. Optical transmission densitometer measurements on the resulting thin films indicated that during sublimation the emission was approximately cosine, and no evidence of preferential ejection of silver particles during sublimation was detected. It is estimated that if preferential ejection is taking place along the close‐packed directions of the crystal during sublimation in this temperature range, this emission produced a thickness change of the collected silver films such that the transmitted light intensity varied from a cosinelike curve by less than 5% at those angles.

Low energy sputtering of eutectic Ag/Cu alloys

Abstract Experiments were carried out on the sputtering of a eutectic Ag/Cu (NAg/NCu = 1.5) alloy by normally incident 110 eV Ar+ ions. The collected sputtered flux was analyzed by Rutherford backscattering (RBS). Material sputtered normal to the surface at a fluence of 7 × 1019 Ar/cm2 was stoichiometric (NAg/NCu ≈ 1.5), but became increasingly Cu rich at larger angles of ejection up to 75° at which NAg/NCu was about 1.09. The overall ejection was slightly Cu rich (NAg/NCu ≈ 1.3). RBS analysis of the sputtered alloy surfaces indicated they were Cu rich to a depth of about 1500 A, with the greatest Cu content at about 450 A depth (up to 56% Cu), the effect depending on the fluence of the bombarding Ar+ ions. Scanning electron micrographs of the sputtered alloy surfaces showed dense microtopographic formations.