Engineering surface reactions with polyatomic ions

Abstract

Mass-separated polyatomic ions, including CH 3 +, CF +, CF 2 + and OH +, have been delivered to an ultrahigh vacuum chamber in the energy range of 2–100 eV for the studies of surface reaction mechanisms induced by such ion arrivals as a function of impact energy, with a particular reference to reactive ion etching of semiconductors for CH 3 +, CF +, and CF 2 +, and surface modification of polymers for OH +. It was found that at the low end of the energy range, these polyatomic ions could survive from molecular dissociation at impact and react with a target surface as hyperthermal molecules. For example, 3 eV CH 3 + was successfully used for etching InP in a reversed organometallic chemical vapor deposition approach, and 10 eV OH + for adding a single functionality of COH to polystyrene. However, when the ion arrival energy is higher than 10–20 eV, impact dissociation of the arriving reactant switches the reaction to those associated with the dissociation fragments instead of the parent reactant. At even higher arrival energies, the atomic fragments from the impact dissociation possess enough energy to penetrate into the sub-surface region, and reactions involving these atomic species are driven by the relevant local chemical potentials inside the substrate. For example, 20 eV CH 3 +/InP showed no effective etching of InP but deposition of a carbon film, and 20 eV OH +/ polystyrene gave a mixture of oxygen containing functionalities. In the study of reactive ion etching of Si and SiO 2, it was found that CF + arrivals on Si gave simple CF accumulation at an arrival energy of 2 eV, a mixture of CF, CF 2 and CF 3, species at 20 eV, and no fluorocarbon but silicon carbide at 100 eV. Etching of SiO 2 was observed with 100 eV CF 2 +, the mechanism of which can be attributed to the release of a sufficiently high concentration of fluorine in the near surface region. Hence, unlike the CH 3 +/InP reaction system, the CF x +/SiO 2 system switches from overlayer deposition to etching when the impact energy is high enough for effective impact dissociation.