Reactive ion etching related Si surface residues and subsurface damage: Their relationship to fundamental etching mechanisms

Abstract

The role of silicon near-surface modifications caused by reactive ion etching in obtaining oxide-to-silicon etch selectivity has been studied by combining Si etch rate and plasma optical emission spectroscopy data with results from the analysis of etched Si surfaces. The nature of the near-surface modifications was established from the study of Si crystals subjected to overetching during CF4/x% H2 selective reactive ion etching of SiO2 on Si. The experimental techniques used included x-ray photoemission spectroscopy, He ion channeling, nuclear reaction techniques, and ellipsometry. Impurity penetration has been verified by nondestructive depth profiling using nuclear reaction and angular dependent core level photoemission techniques. Fluorocarbon film deposition, a highly disordered near-surface region (∼30 Å), and etching gas related impurity penetration into the Si lattice are among the Si substrate modifications observed. By nuclear reaction analysis, hydrogen at concentrations of the order 0.1 at. % has been detected at a depth of several 100 Å from the Si surface. By substituting deuterium for hydrogen in reactive ion etching and conducting secondary ion mass spectroscopy, deuterium at concentrations greater than 1017 atoms/cm3 has been detected more than 2000 Å from the Si surface after 1 min plasma exposure. Direct observation of silicon–carbon bonds and silicon–hydrogen bonding in the modified Si layer was possible by photoemission and Raman scattering. The function of the Si near-surface modifications in the reactive ion etching process, specifically their role in achieving oxide-to-Si etch selectivity, has been elucidated. The etch rate of silicon is controlled by the thickness of the fluorocarbon layer on the silicon surface. It is consistent with a diffusion limited supply of F across the C,F film to the Si/C,F film interface and/or outdiffusion of etch product. The Si subsurface modifications, e.g., lattice disorder and impurities, do not contribute to the decrease of the Si etch rate; reactive ion etching (RIE) process modifications which minimize damage to the Si substrate are therefore discussed. Our results also demonstrate directly the recombinant/passivant model of etch anisotropy for the CF4/H2 RIE-Si etching system. Anisotropic etching of Si in a CF4/H2 plasma occurs via ion bombardment mediated C,F-layer thickness control. Energetic ion bombardment minimizes the thickness of the C,F layer and Si etching can proceed. Areas exposed to little ion bombardment (sidewalls) are covered by thicker C,F layers and no Si etching occurs.