Control of surface reactions in high-performance SiO2 etching

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The relation between SiO2 etch rates and incident fluxes of reactive species in a dual-frequency (27 MHz and 800 kHz) parallel-plate system was evaluated by using various in situ measurements tools. C4F8/Ar/O2 was used for etching gases. The steady-state thickness TC–F of a fluorocarbon polymer layer on the etched SiO2 surface was also measured. The SiO2 etch rate could be related to total F atom flux ΓF-total, which depends on both the incident fluxes of C–F reactive species and the surface reaction probability s. The s is a function of the net energy on the reactive layer (Vnet). This energy is determined by the incident ion energy and the energy loss at the C–F polymer on the etched surface. A change in Vnet from 500 to 1450 V was estimated to correspond to a change in s from 0.01 to 0.1. The steady-state thickness of the C–F polymer TC–F increased when excess C–F species were supplied to the etched surface. A thick polymer (TC–F>1 nm) decreases the ion energy and slows or stops the etching in fine holes. A polymer 5 nm thick can decrease the ion energy by about 750 V. The TC–F must therefore be controlled when high-aspect contact holes are etched.