Ion bombardment effects in low-pressure plasmas: In situ spectroscopic ellipsometry and Monte-Carlo simulation study

Plasma-enhanced chemical vapor deposition (PECVD) is a very versatile, yet highly complex process which has attracted the attention of the optical coatings community for its ability to synthesize thin film materials with a wide and continuous range of optical properties.

In this work, we investigate the effects of ion-surface interactions in the case of hyperthermal ions (10⁰ to 10³ eV) accelerated at the RF-biased electrode of a PECVD reactor, in order to better understand their effect beneath the substrate surface, on growing films, and on interface formation. We apply in situ real-time spectroscopic ellipsometry (RTSE): (1) to monitor modifications at the surface of model c-Si(001) substrates exposed to low-pressure O₂ plasma at the RF-powered electrode as a function of substrate bias voltage (V_B), (2) to determine interface broadening during the initial stages of TiO ₂ deposition on SiO₂, and (3) to monitor the Ar plasma treatment of the interface between porous and dense Si₃N₄ films, and its effect on the growth of multilayer dense/porous stacks.

The first part of this thesis focuses on the modifications of a c-Si substrate resulting from an exposure to an O₂ plasma at the RF-powered electrode by using ex situ variable angle spectroscopic ellipsometry (VASE). The study demonstrates the presence of significant sub-surface modifications, giving rise to a top layer oxide (SiO ₂) and an interfacial damage layer on c-Si(001). The depth of modifications was found to scale with ∼|V_B|^½ , increasing from ∼3.4 nm up to ∼9.6 nm for V _B ranging between -60 and -600 V after 10 minutes of plasma exposure. Static Monte-Carlo TRIM simulations confirmed that the modifications and scaling can be explained on the basis of depth-dependent O transport by ion implantation.

In the second part of this work, we studied the dynamical effects of plasma-surface interactions by using in situ RTSE in combination with TRIDYN (a dynamical version of TRIM) simulations. TRIDYN simulations without any fitting parameter were found to be in excellent quantitative agreement with RTSE results. The modifications of c-Si are observed immediately following plasma ignition (< 1 s), with significant damage first observed after a fluence of ∼5×10¹⁴ O cm^-2; the onset of oxidation is observed at a slightly higher fluence of ∼5×10 ¹⁵ O cm^-2. Modifications saturate at high fluence (∼10 ¹⁷ O cm^-2) where the depth of modifications converges towards the maximum ion penetration depth, leading to a steady-state modification structure as a result of the self-limiting oxide growth behavior. (Abstract shortened by UMI.)