Low-temperature silicon oxidation, in which the $Si/SiO_{2}$ interface has a good quality, is needed to improve the performance of polycrystalline Si thin-film transistors (TFTs). Plasma oxidation methods were studied to reduce the temperature of silicon oxidation. Planar-type inductively coupled plasma (ICP) is a high-density plasma and the simple structure of the ICP system enables the use of a glass substrate with a large area for oxidation of silicon. In this study, we investigated the effects of low-temperature ICP oxidation on the characteristics of $Si/SiO_{2}$ interface and the device performance of TFTs.
(100) Si substrate was oxidized at 400℃ in inductively coupled oxygen plasma. Interstitial oxygen was found at the surface of Si substrate in the initial stage of oxidation by FTIR measurements. An x-ray rocking curve of Si substrates showed a lower peak intensity due to lattice distortion by the interstitial oxygen. The interstitial oxygen was found by plasma oxidation using $N_{2}O$ gas, in addition to $O_{2}$ gas. The ICP oxidation using $N_{2}O$ gas was performed by atomic oxygen, not by molecular oxygen, indicating that atomic oxygen in plasma is responsible for the incorporation of interstitial oxygen into Si surface. Interstitial oxygen incorporation during plasma enhanced atomic layer depostion (PEALD) of $Al_{2}O_{3}$ film was also investigated. $Al_{2}O_{3}$ films were grown on p-type (100) silicon wafers at 100℃ using PEALD. Methylpyrrolidine alane ($C_{5}H_{14}NAl$) and capacitively coupled $O_{2}$ plasma were used as the sources of Al and O, respectively. The interstitial oxygen in the Si substrate was found by FTIR spectroscopy and the amount of interstitial oxygen increased with increasing the thickness of $Al_{2}O_{3}$.
The structure of silicon dioxide on polycrystalline Si substrate was studied using FTIR spectroscopy. Polycrystalline Si and single crystal Si were oxidized at 400℃ in inductively coupled oxygen plasma. The positions of Si-O stretching band on a single crystal Si and a polycrystalline Si were $1062cm^{-1}$ and $1087cm^{-1}$, respectively. XPS Si 2p spectra of ICP oxide revealed that the silicon atoms were fully oxidized by ICP oxidation. This indicates that the low Si-O stretching band position of $SiO_{2}$ on a single silicon is due to the compressive stress, not by oxygen deficiency. The high Si-O stretching band position of polyoxide is probably due to the relaxation of compressive stress through grain boundary reconstruction. The $SiO_{2}$ on polycrystalline Si has the more ordered and less stressed structure.
Polycrystalline Si film was oxidized at 400℃ in inductively coupled oxygen plasma and the oxide was subsequently etched away. Silicon dioxide was found in the polycrystalline Si film by FTIR measurements. The amount of the incorporated oxygen increased with increasing the oxidation time. Oxygen was found at Si grain boundaries by energy dispersive x-ray spectroscopy, which showed the formation of intergrain silicon dioxide. By the oxidation through grain boundaries, the electron mobility in TFTs was improved due to grain boundary defect passivation.
ICP oxidation with $N_{2}O/O_{2}$ mixed gas has been utilized to grow silicon oxynitride at 400℃ and the silicon oxynitride was then applied to polycrystalline Si TFTs. The 17.6-nm thick oxynitride with nitrogen at $Si/SiO_{2}$ interface enhanced the field effect mobility of a polycrystalline Si TFTs due to the reduced interface trap at the Si/SiO2 interface and due to the reduced trap density at the Si grain boundaries by nitrogen passivation.
