Ion-surface interactions on c-Si(001) at the radiofrequency-powered electrode in low-pressure plasmas: Ex situ spectroscopic ellipsometry and Monte Carlo simulation study

Abstract

We use variable-angle spectroscopic ellipsometry (VASE) to investigate oxide and interface formation during plasma-oxidation of monocrystalline Si(001) at the radiofrequency (rf) powered electrode of a plasma-enhanced chemical vapor deposition reactor. HF-etched c-Si(001) wafers were exposed to an oxygen plasma under conditions similar to those used in optical coatings deposition in order to ascertain the effects of plasma-bulk interactions, and to gauge to what depth O2+ and O+ ions interact with and alter the structure and composition of the target in the presence of negative self-bias, VB. From VASE analyses, modifications are best described using a two-layer model: A top layer consisting of SiO2 and a defective interfacial layer (DL) composed of a mixture of c-Si, a-Si, and SiO2. The saturation value of the modification depth (oxide and DL thickness) increases from 3.4±0.4to9.6±0.4nm, for VB ranging from −60to−600V, respectively, and scales with Emax1∕2, where Emax is the maximum energy of ions from an rf discharge. These results are in agreement with nuclear ion-bulk interactions leading to atomic displacements and defect accumulation. The interfacial layer broadens with increasing ∣VB∣ while the fraction of a-Si detected increases from ∼1% up to ∼55% over the investigated VB range, indicative of ballistic and thus depth-dependent oxygen transport to the SiO2–Si interface. Monte Carlo simulations in the binary collision approximation predict significant surface recession due to sputtering, therefore resulting in an apparent self-limiting oxidation mechanism. The surface layers reach their steady-state thicknesses within the first 2min of plasma exposure and subsequently move into the bulk of the c-Si substrate as a result of oxide sputtering and oxygen transport.