Nitric Acid Oxidation Method to Form a Gate Oxide Layer in Sub-Micrometer TFT

Abstract

We have succeeded in fabrication of sub-micrometer TFT which can be operated at 1.5 V, having stack gate oxide structure formed by the nitric acid oxidation of Si (NAOS) method. The ultrathin NAOS SiO 2 layer possesses a low leakage current density, and consequently, the gate oxide layer afterward deposited by the CVD method can be made as thin as 20 nm. Because of the thin gate oxide, miniaturization of TFT with the 0.6 µm gate length is achieved. The operation voltage of the TFTs can be set at as low as 1.5 V because of the low threshold voltages (i.e., −0.5 V for Pch TFT and 0.5 V for N-ch TFT). The sub-threshold swing value is ~80 mV/decade and the on/off ratio is ~10 9 for both the P-ch and N-ch TFTs. The channel mobility is ~100 cm 2 /V·s for the P-ch TFT and ~200 cm 2 /V·s for the N-ch TFT.

Improved Interface and Electrical Properties by Inserting an Ultrathin SiO2 Buffer Layer in the Al2O3/Si Heterojunction

“…The unexpectedly formed low‐density SiO x layer in the ALD‐only sample was expected to show increased leakage current density, because the leakage current can flow via the interface states. However, the ultrathin SiO 2 buffer layer produced by the NAOS method was expected to block the leakage current due to its high atomic density and low defect state density . The leakage current density of the Al 2 O 3 /Si sample was estimated to be 8.1 × 10 −9 A cm −2 ; as expected, NAOS treatment decreased this value by two orders of magnitude to 1.1 × 10 −11 A cm −2 , indicating that the growth of an ultrathin SiO 2 buffer layer between the Al 2 O 3 layer and the Si substrate did indeed improve its insulation properties.…”

“…In the case of the Al 2 O 3 /Si sample, the EOT increased to 2.5 nm after PMA treatment, suggesting that the density of defect states such as dangling silicon bonds at the interface were reduced, and its electrical insulation properties were improved . However, the EOT of the Al 2 O 3 /SiO 2 /Si structure decreased to 3.2 nm after PMA treatment, suggesting that incomplete SiO x diffused to and reacted with the Al 2 O 3 film to form Al–SiO x during the PMA annealing …”

“…However, high density SiO 2 layers cannot be fully fabricated using these deposition methods because incomplete suboxides are formed during the process . In contrast, direct oxidation methods can allow the formation of a high density SiO 2 layer . Widely used thermal oxidation processes are not suitable, as they require heating at temperatures above 800 °C, which generates interfacial stress.…”

“…Kobayashi and co‐workers reported the formation of ultrathin (≈2 nm) SiO 2 with high density and low suboxide content by nitric acid oxidation of Si (NAOS) method at 120 °C . In addition, uniform thickness SiO 2 layers can be fabricated even on poly‐Si substrates with rough surfaces through NAOS methods . In this study, we have fabricated a high density ultrathin SiO 2 buffer layer in order to improve the interface properties between the Al 2 O 3 dielectric layer and the Si substrate.…”