This study examines modulated electron cyclotron resonance (ECR) plasma discharge occurring within a few tens of μs. It can control the generation of reactive species in plasmas. Reactive species are measured by an actinometric optical emission spectroscopy in the pulsed plasma. Good correlation is found between the density ratio of CF2 radicals and F atoms in the CHF3 plasma, and the combination of the pulse duration and pulse intervals. These characteristics are explained by the dependence of reactive species generation in ECR plasma on a time within a few tens of μs. This method provides for controlling the polymerization during SiO2 etching.
The influence of fluorocarbon film composition on oxide etching selectivity have been studied in a CHF3/CO mixture-gas plasma, using a magnetron-enhanced reactive ion etching system. The Fluorocarbon-film deposition rate dependence on CO concentration was measured. The Sputtering rate and carbon concentration of fluorocarbon films, which were deposited under various CO concentrations, were investigated. With CO addition, the carbon concentration in the fluorocarbon film increases, and the sputtering rate of the deposited film is reduced. High carbon concentration in a deposited polymer can enhance the selectivity of etching by protecting the Si surface from ion bombardment. In addition, an adequate CO mixing can yield a high selectivity of polymer deposition on a Si surface as compared to an oxide surface, which, in turn, achieves a high selectivity in oxide etching on Si.
In this letter, we report the relationships between polymerization on Si substrates and the generation ratio for reactive species in fluorocarbon plasma, and reveal essential points for SiO2 etching selectivity to underlying Si. The SiO2 etching selectivity to underlying Si is determined by both the fluoropolymer deposition rate and the sputtering rate on underlying Si. The sputtering rate strongly depends on the F atom concentration in the polymer. Conversely, the deposition rate depends on the CF2 radical density in the plasma. Therefore, control of the generation ratio of CF2 radicals to F atoms is needed to realize highly selective SiO2 etching.
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