HIGHLY SELECTIVE SIO2/SI REACTIVE ION-BEAM ETCHING WITH LOW-ENERGY FLUOROCARBON IONS

Highly selective SiO2/Si reactive ion beam etching was achieved from a new specific ion gun, the electrostatic reflex ion source operated with either CHF3 or CF4. An extensive fragmentation of the neutrals may be obtained. The beam energy dependence of SiO2 and silicon etch rates as studied using profilometry and on-line variations of SiF4 partial pressure. Surface modifications of single-crystal silicon were characterized by Auger sputter profiling and metal-Si contact electrical evaluation. It is shown that both etch rates decrease with decreasing energy, whereas the SiO2:Si selectivity increases. The latter becomes infinite as the silicon etch rate vanishes at a particular energy W(0), about 150 eV. A highly selective etching may thus be achieved with low energy fluorocarbon ions. At 200 eV, the selectivity and SiO2 anisotropic etch rate are respectively 55 and 105 nm min-1 mA-1 cm-2 with CHF3, and 30 and 80 nm min-1 mA-1 cm-2 with CF4. Similarly, the conditions that minimize both the residue contamination and the single-crystal damage are fulfilled at W(0). Therefore a convenient trade-off between etch rates, selectivity and damage may be achieved by a proper choice of the ion energy slightly above W(0).