Secondary-electron and negative-ion emission from Al: Effect of oxygen coverage J. C. Tucek, S. G. Walton, and R. L. Champion Department of Physics, College of William and Mary, Williamsburg, Virginia 23187 Received 27 November 1995; revised manuscript received 12 February 1996 Absolute values for secondary-electron and negative-ion yields resulting from positive ions impacting an aluminum surface have been measured as a function of oxygen coverage on the surface. The experiments have been performed with positive sodium ions at collision energies below 500 eV. The dominant sputtered negative ion is observed to be O - for the entire range of impact energies studied. The yields for electrons and O - , as secondary species, exhibit similar and marked dependencies on impact energy and oxygen coverage of the surface. Using recently calculated potential parameters for the Al/O system, a specific model for the sputtering of O - is presented. Additionally, a mechanism for the secondary emission of electrons involving the formation of O 1 S  by the neutralization of the impacting Na + directly with O - residing on the surface is proposed as the source of secondary electrons. S0163-18299607421-8 I. INTRODUCTION A complete understanding of secondary-electron and negative-ion emission resulting from low energy, ion-surface collisions is of fundamental interest to the field of surface physics. While the various aspects of secondary emission at high impact energies have been studied extensively, 1,2 little data is available for ultrahigh vacuum prepared metallic sur- faces for impact energies below 500 eV. Previous investiga- tions, however, clearly have distinguished the ‘‘kinetic’’ and ‘‘potential’’ emission processes of particle induced electron emission at low impact energies. 3,4 Kinetic emission requires a substantial momentum transfer to a conduction electron which is only likely to occur for impact energies in the keV range, where the velocity of the incoming particle is similar to that of a conduction electron with the Fermi energy. In a potential emission process, the neutralization of the incident positive ion via electron tunneling will release energy which may be transferred to another electron in the solid. That sec- ond electron can be emitted into the vacuum if the energy gained from neutralization is greater than twice the surface’s work function. 5 While kinetic electron emission from clean metal surfaces at low collision energies is very improbable, 6–9 such is not the case for an adsorbate-covered surface where large secondary-electron emission coefficients are routinely observed. 10,11 The reason for the enhancement of emission is not known. Since all functional devices are inevitably gas covered, the effect of the adsorbate on electron and negative-ion emission is of great practical interest, and it remains an active area of inquiry. 12 The specific purpose of these experiments is to investigate the role of the oxygen coverage on secondary-electron and negative-ion emission due to collisions of positive sodium ions Na +  with an aluminum Al surface. Na + was chosen in the present experiments to prevent potential emission from occurring since the ionization potential of Na 5.1 eV is much less than twice the work function of Al 4.2 eV. Previous studies illustrated the effect of alkali-metal cover- age on the work function and secondary-electron and negative-ion emission; 10,13,14 however, in the present work, care was taken to not alter the work function appreciably with the Na + beam. The effect of the change in the work function, induced by Na, is the subject of another study. In these experiments, the primary beam is incident at a fixed angle of 60° with respect to the surface normal of a poly- crystaline Al ribbon. The absolute yields of secondary elec- trons and negative ions are determined for variable oxygen coverage of the Al ranging from none up to complete cover- age. The yields are defined as the ratio of electrons or nega- tive ions exiting the surface per incident ion. The Na + im- pact energy is varied from below the observed threshold the minimum impact energy for the production of secondary negative products up to 500 eV. Ion-induced electron and negative-ion emission from sur- faces play a critical role in the sheath thickness, equilibrium concentrations, and ion-energy distributions in plasmas, and these results should be relevant to many plasma based phe- nomena including plasma etching, 15,16 glow discharges, 17 and GEC Gaseous Electronics Conference reference cell experiments. 18,19 Additionally, secondary yields are impor- tant in understanding ion source performance, 20 the future development of particle detectors, 21–23 and the erosion of satellite materials in low earth orbits. 24 In the following, we present a complete description of the experimental procedure and the results for oxygen covered Al. Also, using recent calculations for the Al/O system, 25 a model specifically for O - emission the dominant negative-ion observed along with a suggested mechanism for secondary-electron emission will be presented. II. EXPERIMENTAL METHOD The experiment is conducted in a fully bakeable ultrahigh vacuum chamber with a base pressure less than 210 -10 Torr and is monitored by a residual gas analyzer and a stan- dard ion gauge. The vacuum in the main chamber is main- tained by a turbomolecular drag pump, five sputter ion pumps, and a titanium sublimation pump, with another sput- ter ion pump on the quadrupole mass analyzer housing. Ex- ternal to the chamber are a turbomolecular drag pumped dif- ferential line for the argon ion gun and a gas handling system which provides both the gas for the argon ion gun and for PHYSICAL REVIEW B 1 JUNE 1996-I VOLUME 53, NUMBER 21 53 0163-1829/96/5321/141278/$10.00 14 127 © 1996 The American Physical Society