IMPORTANCE OF THE MOLECULAR IDENTITY OF ION SPECIES IN REACTIVE ION ETCHING AT LOW ENERGIES

Surface reactions on Si and SiO2 induced by CF+ and CF2+ from a mass-separated ion-beam system were studied with x-ray photoelectron spectroscopy. The bombardment energies were varied from 100 to 2 +/- 0.6 eV. We found that the surface reactions induced by fluorocarbon molecular ion bombardment largely depended on the molecular identity of the ion species, its degree of molecular dissociation upon impact, and the reactivity of the substrate surface. Four examples are shown to illustrate this new approach in altering reaction pathways. (1) CF+ bombardment at 2 eV: This led to molecular adsorption on both Si and SiO2 and to SiF(x) formation on Si but little reaction with SiO2. (2) CF+/20 eV. This led to a higher probability of molecular dissociation upon impact and thereby opened the channel for CF disproportionation. The energy carried by the fragment was sufficiently high that the reaction channels with SiO2 Was also opened. (3) CF+/100 eV: This resulted in efficient molecular dissociation and incorporation of atomic fragments rather deeply into the substrate. The channel for fluorocarbon formation was closed and those for SiC and SiF formation were opened. A silicon compound layer was formed on both substrates. (4) CF2+/100 eV: This resulted in molecular dissociation but the average energy per fragment was low. The channel for deep penetration was closed and most reactants were confined in the very top surface. CF disproportionation became a viable channel on both Si and SiO2 and a new channel was opened for etching SiO2 due to the availability of a sufficiently high local concentration of fluorine.