Etching and Surface Smoothing with Gas-Cluster Ion Beams
- PDF / 1,505,773 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 46 Downloads / 278 Views
the Joule-Tompson cooling is sufficient to cause nucleation and condensation into droplets even for source gases at 300 K. Useful concentrations of these droplets, or clusters, requires that the exit of the nozzle be in the form of a tight cone (ca. 100 ) that temporarily sustains sufficient gas density such that the nuclei can grow by aggregation into sizes that are thermodynamically stable against complete re-evaporation, i.e., greater than their critical radius. The vapor pressure of the liquid state is much greater than the vacuum and hence a small portion of each cluster will evaporate and thereby cool the cluster and reduce its vapor pressure. Especially for argon, this is expected to result in solidification of the clusters. The high source-gas pressures and narrow nozzle exit cone also result in the gas moving at high velocity in a hypersonic directed jet. Since this cluster-formation process of nucleation, growth, freezing and dispersal into the vacuum happens quite rapidly and before equilibrium is reached, there is a wide distribution in cluster sizes formed. Typically, for argon clusters the most probable size is about 2000 atoms per cluster (diameter -5 nm), but the distribution has a tail extending to 10,000 or more. The cluster jet is ionized by electron impact and knock off, thereby forming a beam of positive ions with considerable momentum along the direction of the neutral cluster jet. Typical GCIB systems utilize only the minimum electron energy required and the cluster ions are found to be just singly ionized. Thus, the charge-to-mass ratio for these clusters is -1,000 times smaller than for corresponding monomer ions. The Frank-Condon effect predicts that electron impact will not dissociate clusters even for very weakly-bound argon (-80 meV/atom). It is known that the electronic structure of solid argon (van der Waals bonded) has a filled valence band with the lowest lying excitation being an exciton near the conduction band [6]. Thus, the ionized argon clusters are modeled as being solid nanoparticles with a hole in the valence band (positive ion), and a uniform, delocalized charge cloud at the surface of the spherical cluster. Bombarding surfaces with gas clusters accelerated to energies of -100 keV or more results in some surface damage and roughening. Below about 5-7 keV, an energy threshold, it is found that cluster ions do not etch surfaces. However, over the range of 10-30 keV where the energy per atom in a cluster is of order 10 eV, both etching and smoothing of surfaces are observed. Interestingly, the etching rate is reported to be linear with argon cluster energy over this range [3-5] unlike monomer-ion etching which is proportional to the square root of energy. The latter effect is ascribed to momentum conservation in the atom-to-atom collisions of these small ions with surfaces. Toyoda et al [7] have proposed that the observed cluster sputtering yield, and hence etch rates follow a total energy-conservation mechanism. This may be interpreted as an indication that clusters undergo hig
Data Loading...
