Etch rate control in a 27 MHz reactive ion etching system for ultralarge scale integrated circuit processing

Abstract

The etch rates of SiO2, photoresist, Si, and SiN in a 27 MHz reactive ion etching system at constant ion flux of 6×1016 cm−2 s−1 and ion energy of 1450 V were studied. Typical incident flux densities of CF2 and CF+ were on the order of 1017 and 1016 cm−2 s−1, respectively. The SiO2 etch rate was determined by the balance of the energy supplied by the total ion flux and the amount of the C–F reactive species supplied by radicals and ions. When we roughly assumed the surface reaction probabilities of F, CF, CF2 and CF3 to be 0.1, 0.1, 0.1, and 0.5, the SiO2 etch rate could be expressed well as a function of the total number of F in the net radical fluxes. To clarify the dominant flux including radicals and ions, however, further research on surface reaction probabilities on the actual etched surface must be conducted because the incident fluxes strongly depend on these constants of the surface reaction probability. Lowering the total ion flux or ion energy decreased the etch rate of SiO2. A higher ion flux or higher ion energy is required to obtain higher etch yields. When excess C–F reactive species exist on the etched surface, they disturb the etching reaction by wasting the energy of incident ions. Under these conditions, a reactive species is no longer an “etchant,” but an “inhibitor.” Therefore, it is important to control the amount of total reactive species according to the ion conditions. Oxygen contributed to the removal of these excess C–F species, resulting in a higher etch yield. In contrast, the etch rates of a photoresist, Si, and SiN did not depend on flux of the C–F reactive species, but on the oxygen concentration. It is concluded that a process with high selectivity requires low oxygen concentration, high ion flux, and optimized flux of C–F reactive species.