Dynamics of ion bombardment-induced modifications of Si(001) at the radio-frequency-biased electrode in low-pressure oxygen plasmas: In situ spectroscopic ellipsometry and Monte Carlo study

Low-pressure O ₂ plasma exposures were performed on c-Si(001) at a radio frequency (rf)-powered electrode in the presence of substrate self-biasing (V _B) from V _B=-60 to -600V, in order to evaluate ion-surface interactions at the growth surface under ion bombardment conditions suitable for the fabrication of high quality optical coatings. The plasma-surface interactions were monitored in situ using real-time spectroscopic ellipsometry (RTSE), which reveals time- and ion-fluence-resolved information about depth-dependent modifications, such as damage and oxidation below the c-Si substrate surface. RTSE analysis indicates almost immediate damage formation (≪1 s) to a depth of a few nanometers below the surface after exposure to a low oxygen ion fluence (∼5 × 10 ¹⁴ O cm ^-2). Oxide growth is detected at intermediate fluence (∼10 ¹⁵-10 ¹⁶ O cm ^-2) and is attributed to O subplantation (shallow implantation); it forms near the surface of the target on top of an O-deficient interfacial damage layer (DL). Both layers experience a self-limiting growth behavior at high fluence (> 10 ¹⁷ cm ^-2) as oxide and DL thicknesses reach bias-dependent steady-state values, determined by the maximum ion penetration depth, which increases from ∼3.6 to 9.5 nm for V _B=-60 to -600 V. The in situ experimental study was complemented by Monte Carlo TRIDYN simulations based on the binary collision approximation, which were modified to calculate dynamic changes in the composition of a target exposed to a broad-energy ion source (rf plasma source) at high fluence. Simulation results are found to agree exceptionally well with experiment. In addition, they reveal that the 1.2-3.5-nm-thick DL formed in the steady-state regime is a result of (1) damage formation due to the presence of a small number of high energy O ⁺ ions in the plasma environment, capable of penetrating and damaging up to 3 nm deeper than the majority ion population (O ₂ ⁺), and (2) because of important surface motion resulting from oxidation-induced swelling (at low fluence) and sputtering-induced recession (at high fluence). Surface motion in general is found to inhibit oxygen incorporation at high depth in the substrate, thus forming the O-deficient DL. We discuss the implications of these findings on optical coatings deposition and propose a growth mechanism for coatings subjected to intense ion bombardment.