Low Temperature Synthesis of Nanocrystalline Silicon and Silicon Oxide Films by Plasma Chemical Vapor Deposition

Abstract

Nanocrystalline silicon (nc-Si) and SiO₂ films have been synthesized at a low substrate temperature using inductively coupled plasma chemical vapor deposition (ICP-CVD) methods with internal low-inductance antenna (LIA) units. The synthesis of these materials was performed in the separate vacuum containers where LIA units were installed so that the induced electric field from an antenna could be used effectively. Radio frequency (13.56 MHz) power was supplied through the matching circuit units. H₂ and SiH₄ gases were used for nc-Si synthesis, and O₂ and SiH₄ gases were used for SiO₂ deposition. The gas flow ratios were 15 for H₂/SiH₄ and 4.0 for O₂/SiH₄. A substrate temperature, gas pressure, RF power and process time were varied in order to investigate optimum conditions for nc-Si synthesis. Silicon oxide films were deposited under conditions of 300°C, 0.2 Pa and 24 mW/cm³. A sample was prepared by SiO₂ deposition and subsequent nc-Si synthesis after removing the natural oxide on silicon substrate by buffered 1%-HF (BHF) solution. In some cases, plasma treatments were inserted before or after nc-Si synthesis. The diameter and number density of nc-Si were determined by a high-resolution transmission electron microscopy (HR-TEM). Plan-view TEM images of nc-Si showed that spatially isolated nc-Si was synthesized and that the diameter and the standard deviation of nc-Si could be controlled not only with a substrate temperature, gas pressure, RF power and process time but also with pre/post plasma treatments. The resultant trend suggests that radical precursors and reactive nucleation sites on the SiO₂ surface have an important role in the synthesis of nc-Si. The diameter of almost all nc-Si under the present conditions was less than 10 nm. In particular, under conditions of the substrate temperature of 200°C and 4.0 Pa with oxygen plasma pretreatment and hydrogen plasma posttreatment, the mean diameter and number density of nc-Si were 2.7 ± 0.5 nm and 6.5 × 10¹¹ cm⁻², respectively. This result is suitable for quantum effect device applications. In addition, electronic properties of a single SiO₂ film were examined with the fabricated metal oxide semiconductor (MOS) capacitor. Breakdown voltage was 7.5 MV/cm at 1.0 × 10⁻⁶ A/cm² and leakage current was 1.0 × 10⁻⁹ A/cm² at 2.0 MV/cm for a SiO₂ film with a thickness of 12 nm. This result clearly supports the present SiO₂ film is capable of the thin dielectric layer of nc-Si devices.