Reactive ion beam etching of silicon with a new plasma ion source operated with CF4 : SiO2 over Si selectivity and Si surface modification

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Abstract
Reactive Ion Beam Etching is obtained from a new specific ion gun, the Electrostatic Reflex Ion Source (Maxi-ERIS), which is operated with pure CF4 gas. The reported results concern both silicon dioxide and single-crystal silicon. They show that the operation of the source discharge down to its minimum pressure which implies an extensive fragmentation of the injected neutrals, provides a very convenient process for selective etching of SiO2 over Si, a basic problem in semiconductor technology. From the characteristic performances which are achieved, this process appears as a fair alternative solution to the standard reactive ion etching process with CF4/H2 or CHF3 (in a plasma environment). It is known that these latter ones lead to deep lying modifications of the Si single-crystal, which are attributed to hydrogen-induced extended defects. For the proposed RIBE process with a 500 eV beam at normal incidence the main features are : i) selectivity SiO2/Si : 19/1 ; ii) etch rates : 130 nm/min and 7 nm/min, respectively for SiO2 and Si, data normalized to a 1 mA cm-2 current density ; iii) the blocking carbonaceous film which is formed over the silicon and insures the slow-down of the etch rate may be removed by a simple dip for 60 s in concentrated hydrofluoric acid (50 %) ; iv) such a post-etching treatment — without further plasma oxidation or thermal annealing - leaves a clean Si substrate, the electrical properties of which are only slightly altered as compared to a control sample. Informations about the kinetics and mechanisms of the formation of both the overlayer and the near-surface damage are obtained from ellipsometry, Auger electron spectroscopy, Auger sputter profiling and metal-silicon contact electrical measurements. They are reported and discussed with a special emphasis on the effect of both the ion exposure dose and the operation pressure of the ion gun