Ion Beam Oxidation of Surfaces

When a molecule strikes a surface, the chemical nature of the incident particle and the electronic structure of the surface are critical in determining the outcome of the collision. Under thermal conditions, desorption and decomposition often compete with surface chemical reactions, leaving the experimentalist with limited control over the distribution of products. In contrast, the introduction of nonthermal energies into the molecule/surface system can activate chemical reactions that go unobserved under thermal conditions. The figure below identifies some of the more common events that can occur when a molecular projectile strikes a surface target. For a given molecule/surface system, the translational energy of the incident molecule largely determines the branching ratio between the various reaction pathways. The reaction mechanisms that operate predominantly in the hyperthermal energy range are nonadiabatic electron transfer, dissociative scattering, abstraction, and processes related to ion beam deposition. These fundamental mechanisms are discussed below.

A Si(001) wafer, exposed to 60 eV O⁺ ions, develops a thin oxide film less than 30 Å thick. Prolonged exposure to the ion beam does not result in a thicker oxide film, because the surface is being eroded at the same rate that it is oxidized. Scattering experiments performed within this steady-state regime revealed the complex behavior associated with ion beam oxidation. Isotopic labeling techniques helped to define the origin of each oxygen atom in the scattered products. Initially, an isotopically pure Si¹⁶O_x film was grown using a mass-selected ¹⁶O⁺ beam. Following the oxide growth phase, scattering commenced with a mass-selected ¹⁸O⁺ beam.

The scattering of O⁺ on SiO_x has been conducted at collision energies varying from 5 eV to 150 eV. At collision energies above 16 eV, an incident O⁺ ion can neutralize on approach to the surface, scatter from the lattice, and pick up an oxygen atom on the outbound trajectory (scattering mediated abstraction). The resulting O₂^- ion scatters in the forward direction with approximately 15% of the initial O⁺ kinetic energy. At collision energies above 33 eV, a second reaction channel for O₂^- emergence opens up. Here, the incident projectile creates an O-atom recoil upon impact. As the oxygen recoil leaves the surface, it abstracts a neighboring oxygen atom from the lattice to form scattered O₂^- (recoil abstraction). These two reaction pathways demonstrate some of the ways in which a hyperthermal O⁺ beam will etch away a silicon oxide layer. Notwithstanding, the same O⁺ beam is also responsible for further oxidizing silicon in the 30 Å oxide layer.

The reaction of 5 eV O⁺ ions with silicon oxide is an important benchmark for simulating how spacecraft materials will stand up in the harsh LEO environment. Initially, an oxide layer is grown on a Si(001) substrate using a pure ¹⁶O⁺ ion beam. The energy of the oxygen ions deposited during the growth phase was varied to alter the properties of the thin Si¹⁶O_x film. Next, 5 eV ¹⁸O⁺ ions were directed at the Si¹⁶O_x surface, and the isotopic composition of the topmost layer was monitored as a function of ¹⁸O⁺ dose. As seen in the figure on the right, the cross section for incorporating 5 eV ¹⁸O⁺ into the top layer of the film depends on the deposition energy under which the film was initially grown. The cross section reflects the ease with which incident 5 eV ¹⁸O⁺ is able to replace an oxygen atom in the film. The lowest measured value for the cross section corresponds to films grown at 20 eV. This presumably indicates that films grown with 20 eV O⁺ are most free of defects. At deposition energies lower than 20 eV, the incident ions are not energetic enough to locally anneal out vacancies developed in the film. At higher deposition energies, the incident ions generate new defects as they impulsively displace atoms within the lattice.

For more information, see:

• Quinteros, C. L., Tzvetkov, T., Qin, X., and Jacobs, D. C. (2001) "Reactive Scattering of O⁺ on oxidized Si(001) " Nuclear Instruments and Methods in Physics Research B, 182:187-192.
  
  
• Maazouz, M., Quinteros, C. L., Tzvetkov, T., Maazouz, P. L., Qin, X., Barstis, T. L. O. and Jacobs, D. C. (2001) "Reactive Collisions of Hyperthermal Ions with Oxide Surfaces " in Applications of Accelerators in Research and Industry, edited by J.L. Duggan and I. L. Morgan, American Institute of Physics, 576:122-125.
  
  
• Tzvetkov, T., Qin, X., and Jacobs, D. C. (2002) "Dynamical Study of Ion Beam Oxidation: Incorporation of Hyperthermal Oxygen Ions into Silicon Oxide Thin Films" submitted to Physical Review B.
  
  
• Jacobs, D. C. (2002) "Reactive Collisions of Hyperthermal Energy Molecular Ions with Solid Surfaces" in Annual Review of Physical Chemistry, edited by Steven R. Leone, Annual Reviews, 53:379-407.

The complex dynamics associated with ion beam oxidation(IBO) of Si(001) by 5 – 100 eV O⁺ and O₂⁺ is studied in situ. Room-temperature oxidation of silicon under ultrahigh vacuum conditions is accomplished with a mass-selected, monoenergetic, oxygen ion beam. Initially, a thin Si¹⁶O_x film is prepared by bombarding clean Si(001) with hyperthermal energy ¹⁶O⁺ . Switching the incident ion flux to ¹⁸O⁺ or ³⁶O₂⁺ creates an isotopically labeled tracer for monitoring the rate at which subsequent incident oxygen ions are incorporated into the topmost layer of the growing silicon oxide film. The cross section for oxygen incorporation is found to depend strongly on: (1) the conditions under which the underlying oxide layer was grown, (2) the kinetic energy of the incorporating ion, and (3) whether the incident ion is atomic or molecular oxygen.

For O₂⁺ ions incident on silicon oxide, both scattered O^- and O₂^- product channels were detected. Isotopic labeling techniques helped to define the origin of each oxygen atom in the scattered products. Initially, an isotopically pure Si¹⁶O_x film was grown using a mass-selected ¹⁶O⁺ beam. Following the oxide growth phase, scattering commenced with a mass-selected ³⁶O₂⁺ beam. Both ¹⁶O^- and ¹⁸O^- product ions emerged from ³⁶O₂⁺ bombardment, consistent with sputtering and dissociative scattering pathways, respectively. The O₂^- products comprised ³²O₂^-, ³⁴O₂^- and ³⁶O₂^- signals. Whereas the ³²O₂^- and ³⁶O₂^- species could be assigned to sputtering and scattering processes, respectively, the ³⁴O₂^- signal must correspond with a substitution reaction at the surface.

Figure (b) shows the angle-resolved energy distribution of ³⁴O₂^- ions produced in the scattering of 60 eV ³⁶O₂⁺ on Si¹⁶O_x. The ³⁴O₂^- products emerge in the specular direction with a mean energy of 7.8 eV. The forward scattering behavior is consistent with ³⁴O₂^- being formed in a direct collision with the surface. In contrast, if the projectile ion had encountered multiple bounces on the surface, memory of its momentum direction would be lost, and the ³⁴O₂^- product would emerge at an angle close to the surface normal. It is interesting to compare the angle-resolved energy distribution for the ³⁴O₂^- substitution product with that for the directly scattered parent, ³⁶O₂^- (a). The shapes of the two angular distributions are similar, but the mean kinetic energy of ³⁴O₂^- (b) is 70% of that for the directly scattered ³⁶O₂^- species (a). It is conceivable that ³⁴O₂^- could be formed in either a stepwise or concerted fashion. A stepwise approach would first involve dissociation of the incident ³⁶O₂⁺ projectile into ¹⁸O fragments, which then go on to abstract ¹⁶O from the surface. As a point of comparison, scattering 30 eV ¹⁸O⁺ projectiles (same incident velocity as ³⁶O₂⁺ projectiles) on the identical Si¹⁶O_x surface yielded ³⁴O₂^- abstraction products (c) with significantly less kinetic energy than that observed for the substituion channel (b). Consequently, both of the O-atoms in the ³⁶O₂⁺ projectile must be involved in the transition state for the substitution reaction. The ³⁴O₂^- product is formed by a direct concerted substitution of ¹⁸O, from the ³⁶O₂⁺ projectile ion, with ¹⁶O from the Si¹⁶O_x surface. In addition to a substitution of atoms on the molecular projectile, isotopic analysis of the topmost layer of the surface oxide reveals facile replacement of ¹⁶O-atoms in the lattice with ¹⁸O as a result of ³⁶O₂⁺ bombardment.

For more information, see:

• Tzvetkov, T., Qin, X., and Jacobs, D. C. (2002) "Dynamical Study of Ion Beam Oxidation: Incorporation of Hyperthermal Oxygen Ions into Silicon Oxide Thin Films" submitted to Physical Review B.
  
  
• Jacobs, D. C. (2002) "Reactive Collisions of Hyperthermal Energy Molecular Ions with Solid Surfaces" in Annual Review of Physical Chemistry, edited by Steven R. Leone, Annual Reviews, 53:379-407.

A beam of 5 eV O⁺ reactively etches a decanethiolate/Au(111) self-assembled monolayer (SAM). Ion-induced modifications of the SAM are monitored in situ by X-ray photoelectron spectroscopy at different stages during the O⁺ dose. Approximately 3.8 carbon atoms are removed when 10 O⁺ ions strike a decanethiolate molecule; furthermore, 18% of the incident oxygen ions become trapped in the SAM. This facile reactivity is directly compared to that observed for thermal O(3P) incident on an alkanethiolate SAM and to hyperthermal O(³P) bombarding various polymeric films. The scattered products are observed with mass-, energy-, and angle-resolved detection. This system serves as a model for understanding the mechansims by which polymeric materials degrade in the low-earth orbit environment.

For more information, see:

• Qin, X., Tzvetkov, T., and Jacobs, D. C. (2002) "Reaction of 5 eV O⁺ with a Decanethiolate/Au(111) Self-Assembled Monolayer" submitted to Nuclear Instrumental Methods B.

The scattering of 25 – 120 eV Ne⁺ on single-domain and mixed-domain Si(001)-(2x1) is studied experimentally and through classical trajectory calculations. Scattering behavior on single-domain Si(001)-(2x1) is contrasted for Ne⁺ projectiles that are directed parallel versus perpendicular to the dimer rows, and for Ne+ directed up versus down a step edge. The ions, that scatter from the surface without undergoing neutralization, exhibit angle-resolved energy distributions consistent with predominantly a single collision between the projectile ion and a silicon atom at the surface. Notwithstanding, multiple collisions become more prevalent when Ne⁺ approaches the surface at an energy below 50 eV. An efficient method is presented for empirically optimizing the set of charge transfer parameters incorporated within the classical trajectory calculations.

For more information, see:

• Tzvetkov, T., Quinteros, C., Qin, X., Camden, J., and Jacobs, D. C. (2002) "Inelastic Scattering and Charge Transfer: Hyperthermal Energy Ne⁺ on Si(001)-(2x1)" submitted to Journal of Chemical Physics.
  
  
• Jacobs, D. C. (2000) "Dynamics of Hypervelocity Gas/Surface Collisions," in Chemical Dynamics in Extreme Environments, edited by R. Dressler, Advanced Series in Physical Chemistry, World Scientific Publishers, 349-389.

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