3.3 Nitric Acid Oxidation

The continuous decrease in semiconductor device sizes demands a reduction in oxidation times for high-temperature oxidation because high temperatures influence the distribution of impurities in the silicon bulk and at the Si-SiO$_2$ interface. The movement of impurities affects device size and its electrical properties. Therefore, alternatives to high temperature oxidation have been sought out to grow thin oxides which have good electrical properties found in thermally grown oxides [78]. Similarly, the fabrication of Thin Film Transistors (TFT) for flat-panel displays requires low temperatures because of the presence of glass substrates [78]. The use of Rapid Thermal Annealing (RTA) and high pressure thermal oxidation can reduce the amount of time during which a high temperature is applied. However, processes which perform oxidation at low temperature ($<$ 600 $^\textrm {o}$C) are preferred. Plasma oxidation of silicon [155] started to gain at traction, because it can be performed at low temperatures. In plasma oxidation, oxygen ions O$^-$ are the responsible species and the reaction which takes place is

$$Si+O^{-}\rightarrow SiO_{2}+2e^{-},$$ (93)

where $e^-$ represents a single electron [21].

Plasma-assisted oxidation of silicon has been performed in microwave, RF, and DC plasmas. It has been shown that, compared to thermal oxidation, film growth rates are accelerated by plasma-enhanced generation of the reactive species (O$^-$). The primary limitation in the use of plasma-grown oxides in ultra large scale integration is the inability to control oxide properties such as the oxide charge density [21]. Although some researchers achieved good dielectric properties for plasma-grown oxides, this was only possible with post-oxidation high-temperature treatment. Even after such treatments, low field leakage currents were found, thought to be due to oxide damage caused by the plasma radiation [78]. A proposed alternative involves the growth of a thin oxide using low-power discharge followed by a CVD deposition of additional oxide, which adds complexity to the growth process and an additional interface in the oxide [78]. Metal-enhanced oxidation and UV ozone oxidation have also been used to generate thin oxide layers, but uniformity, controllability, and good electrical characteristics of SiO$_2$ layers have not been achieved with these methods [103].

More recently, Nitric Acid Oxidation (NAOS) was suggested by Asuha et al. [88], [3], [89], [103], [104]. NAOS is developed as a process to grow gate oxide layers for TFTs, which require good chemical properties on very thin films. These films require the electrical properties of thermally grown films, but because they are grown on a glass substrate cannot be exposed to a high temperature environment. However, when the films are deposited using CVD or HPCVD, the electrical properties of the film are not sufficient for TFT applications. Several experimental results regarding the growth of oxide on silicon using NAOS in azeotropic [3] and vapor [90] environments have been published [103], [105]. In addition, a two-step process [152] involving a combination of NAOS processes is suggested to enable the growth of SiO$_2$ layers with thicknesses larger than 10nm with good electrical properties.