Rutherford backscattering studies of plasma-etched silicon

Abstract

Near-surface modifications of Si induced by reactive ion etching (RIE), such as surface residues and a disordered Si near-surface region, have been studied by He ion channeling. The dependence of the surface residue layer thickness/composition and the Si surface disorder intensity on the etching gas (mixture) used, the plasma exposure time, and the applied rf power density were investigated. For the case of SiO2/Si selective etching (Si etch rate low), thicker surface residue layers were formed and the Si near-surface disorder was more intensive than for a nonselective RIE process. In selective SiO2/Si etching, both Si substrate disorder and surface residue layer thickness increase initially with plasma exposure time of the substrate. The intensity of Si substrate disorder was found to increase with greater self-bias voltage (caused by more rf power) applied during etching. A thermal annealing study showed that the observed Si disorder must be due to several distinct silicon defect states and is not equivalent to amorphized Si. The deposited C,F-layer thickness was found to depend on the ion bombardment possible during reactive ion etching; above a certain ion energy threshold C,F-layer deposition is suppressed. Various approaches to recover high-quality Si surfaces following RIE were evaluated. Due to the complexity of the near-surface modifications caused by plasma exposure of a Si specimen, simple thermal annealing is ineffective in restoring the surface properties. Promising recovery techniques for device-quality surfaces were found to be O2 RIE and O2 annealing post-RIE treatments. Possible difficulties of the channeling method in the characterization of dry-etched Si were demonstrated by the observation of ion-beam-induced surface impurity desorption and surface-roughness-related Si surface peak areal density changes.