The growth kinetics and electrical properties of $SiO_2$ film grow by Si oxidation in an inductively coupled $N_2O$ plasma system have been studied and the plasma oxide has been applied as a gate insulator of poly-silicon thin film transistors (TFTs).
The growth of oxide film by plasma oxidation has been extensively investigated to lower the process temperature in integrated circuits fabrication. But plasma oxides suffer from the slow oxidation rate at low temperature and the poor oxide quality due to ion bombardment. However, inductively coupled plasma (ICP) has a high ion density and a small sheath voltage, which can enhance oxidation kinetics and reduce oxide damage. In addition, the ICP system with a planar configuration has a simple structure with low aspect ratio (height/diameter) so that it is suitable to low-temperature plasma oxidation of large-area poly-Si substrate.
(100) Si wafers were oxidized in an ICP oxidation system with $N_2O$, $O_2$, Ar gases at the RF power from 0.5 kW to 2.0 kW, in the pressure from 1 mtorr to 100 mtorr. The maximum ion density in the $N_2O$ and $O_2$ plasma were $1.1\times10^{11}$ and $1.3\times10^{11}/cm^3$, respectively, at 2 kW RF power in 5 mtorr pressure. The Ar plasma showed the maximum ion density of $2\times10^{12}/cm^3$ at 2 kW in 100 mtorr. The plasma density by $N_2O$ supply was slightly lower than that by $O_2$ supply.
The oxidation growth rate was much retarded by ICP $N_2O$ plasma oxidation compared to ICP $O_2$ plasma oxidation. By the ICP $N_2O$ plasma oxidation, nitrogens were strongly accumulated at the $Si/SiO_2$ interface. The incorporated nitrogen atoms tightly bound to silicon atoms at the interface with the N(1s) electron energy of 397.4 eV, which is equivalent to the bonding energy of silicon nitride $(Si_3N_4)$. The accumulation of nitrogen at the $Si/SiO_2$ in the form of Si-N bonding retarded the diffusion of oxygen to the $Si/SiO_2$ interface, reducing the oxidation rate.
The breakdown field of the $SiO_2$ film grown on Si wafer by the ICP $N_2O$ plasma was 10~16 MV/cm, that of the $SiO_2$ film grown on Si wafer by ICP $O_2$ plasma was 6~7 MV/cm. The C-V measurements showed that the fixed-state charge densities at the $Si/SiO_2$ interface by the ICP $N_2O$ plasma and by the ICP $O_2$ plasma were $-9.29\times10^{11}$ and $5.25\times10^{12}/cm^2$, respectively. Note that the presence of nitrogen at the $Si/SiO_2$ interface induces the high breakdown field and the negative fixed-state charge which was slightly reduced by annealing at 600℃ for 1h in Ar ambient.
N-channel poly-Si TFTs with a (ICP oxide + LPCVD oxide) double layer as a gate dielectric were fabricated. The application of the double layer gate oxide either by $N_2O$ plasma oxidation or by $O_2$ plasma oxidation effectively reduced the interface fixed-state and fast-state charges at the $Si/SiO_2$ interface, compared to the single-layer gate oxide only with LPCVD oxide. The reduction of interface charges improved the performance of TFTs. By $N_2O$ plasma oxidation, the Si-dangling bonds at the $Si/SiO_2$ interface was substituted by Si-N and Si-O bonding. The nitrogen in the ICP $N_2O$ oxide moved into the interface after 600℃ 48h annealing. Therefore, the TFT with the ICP $N_2O$ oxide showed the higher field-effect mobility, lower threshold voltage, and lower off-current than that with the ICP $O_2$ oxide. The field-effect mobilities of TFTs with (5.4 nm ICP $N_2O$ oxide + 90nm LPCVD oxide), (72 nm ICP $O_2$ oxide + 900nm LPCVD oxide), and only 100 nm LPCVD oxide were 22.4, 17.7, and 11.1 ㎠/Vs, respectively. It is believed that the ICP $N_2O$ plasma more effectively passivated the interface defect states at the $Si/SiO_2$ interface than ICP $O_2$ plasma.
