Metal cluster sputtering under reactive ion bombardment investigated by TOF-SNMS-laser-system
The sputtering of metal surface clusters under bombardment with polyatomic and monatomic projectiles was investigated experimentally by means of timeof-flight mass spectrometry (TOF-MS) characterizing the composition of the sputtered flux. In order to obtain information about the relative abundance of clusters among the flux of sputtered particles independent of their charge state, mass spectra of both secondary ions and sputtered neutral particles are recorded under comparable experimental conditions. The neutral species are post-ionized prior to mass analysis by means of photo-ionization using a intense of UV laser at a wavelength of 193 nm. As a first step, the formation of sputtered indium clusters under bombardment with SF+ 5 (m = 1, ..., 5) and Ar+ projectiles is investigated. In these experiments, a photon energy of the ionizing laser (6.4 eV) is larger than the ionization energy of indium atoms (5.79 eV) and all indium clusters Inn. Therefore, the photo-ionization of all neutral species is achieved by absorption of a single photon (SPI) and, hence, a high ionization eﬃciency and low fragmentation rates have been achieved. In addition, the nuclearity and fluorine content of the projectile is systematically varied. Such conditions give us the possibility to unravel the behavior of the partial sputtering yields and ionization probabilities as a function of the fluorine content and nuclearity of the projectile. The investigations demonstrated that the measured secondary ion signals increase much more than those of the corresponding neutral particles if SF+ m projectiles are used instead of Ar+ ions, indicating that the ionization probability under bombardment with SF+ m is enhanced by a chemical matrix eﬀect induced by fluorine incorporation into the surface. Interestingly, the largest values of the ionization probability are observed for SF+ 3 projectiles. The total sputtering yield is found to be larger for SF+ m compared to Ar+ projectiles and to increase linearly with increasing m. Both findings are shown to be understandable in the framework of linear cascade sputtering theory. The partial sputtering yields of Inn clusters exhibit a stronger enhancement than the sputtered monomers, the magnitude of the eﬀect increasing with increasing cluster size and projectile nuclearity. A second step, the formation of sputtered silver clusters under bombardment with Xe+ and SF+ 5 bombardment is investigated. It is found that measured Ag+ n signals increase significantly if SF+ 5 projectiles are used instead of rare gas Xe+ ions of the same kinetic impact energy. On other hand, the signals of neutral Ag atoms and Agn clusters exhibit only a relatively small increase, thus indicating that the enhancement observed for secondary ions is predominantly caused by an increased ionization probability of sputtered particles under SF+ 5 bombardment rather than by enhanced partial sputtering yield. The mass distribution of sputtered particles does not change in favor of large clusters between SF+ 5 and Xe+ projectiles This finding shows that the use of polyatomic SF+ 5 projectiles does not lead to a higher eﬃciency in producing sputtered clusters. Expanding on the comparison between Xe+ and SF+ 5 projectiles, we have investigated the influence of fluorine projectile nuclearity on sputtering of silver by bombarding the target surface under SF+ m (m = 1, ..., 5) projectiles. The results demonstrated that the total sputtering yield produced by a set of isoenergetic SF+ m projectiles is increased linearly with increasing projectile nuclearity m. This increase can be almost fully explained by linear cascade sputtering theory. For SF+ 5 projectiles the obtained total sputtering yield is slightly larger than that estimated from linear cascade theory. The finding may refer to small contribution of nonlinear cascades to the sputtering process (spike regime). Finally, the experiments are repeated in a diﬀerent ultrahigh vacuum system containing an X-ray photoelectron spectrometer in order to obtain more information about the surface chemistry of silver subjected to SF+ m bombardment and to determine the concentration of fluorine at the bombarded surface as a function of projectile nuclearity. The results show that the concentration of fluorine atoms at the bombarded surface increases directly with increasing fluorine nuclearity in the projectile. In addition, the silver peaks of photoelectron spectrum are shifted to higher kinetic energy ranging from 0.2 to 0.5 eV depending on the fluorine nuclearity m . This indicates the existence of silver-fluoride (AgF). These results are largely consistent with those obtained by secondary neutral time-of-flight mass spectrometry (TOF-SNMS). However, the physical reason for the observed, extraordinarily high ionization probability in the case of SF+ 3 bombardment remains unclear.