Infrared Study of Room Temperature Atomic Layer Deposition of SnO₂ Using Sn(CH₃)₄ and Plasma Excited Humidified Argon

Abstract

Tin oxide (SnO2) is an oxide semiconductor material with an excellent optical transparency, where it is used as transparent conducting films. It is also used as a channel material for gas sensors. Recently atomic layer deposition (ALD) of SnO2 has been studied intensively since ALD allows the conformal deposition of the oxide film with an atomic level precision. However, the high process temperature in excess of 100 °C in the conventional SnO2 ALD is a major problem for its application for flexible electronics. We developed a room temperature deposition technique with a precursor of tetramethyl tin (Sn(CH3)4, TMT) and plasma excited humidified argon. The surface reaction of the precursor adsorption and its oxidization were monitored by infrared absorption spectroscopy (IRAS) with a multiple-internal-reflection (MIR) geometry. A p-type Si(100) substrate with a resistivity of 8-12 Ωcm was used as a sample. Prior to the experiment, the sample was cleaned with buffered HF acid and sulfuric acid-hydrogen peroxide mixture solutions. The Sn precursor of TMT was introduced with a variable leak valve. A plasma excited humidified argon source was installed to the ALD chamber as shown in Fig. 1. The source gas for OH-radical is a mixture of H2O vapor and Ar made by a water bubbler. The plasma was generated in a glass tube with an induction coil with a frequency of 13.56MHz and a RF power of 30 W. Before the ALD process, the substrate surface was treated with the plasma excited humidified argon for 5 min to be terminated with Si-OH. For the SnO2 deposition, we repeated the cycle of TMT saturation and plasma excited humidified argon at RT. The condition of TMT saturation and oxidization were determined by the MIR-IRAS experiment. In this experiment, the TMT saturation exposure and oxidization time were 2×105 Langmuir and 10 min, respectively. The oxidation state of the SnO2 film was evaluated by X-ray photoelectron spectroscopy (XPS). The thickness of the grown film was measured by spectroscopic ellipsometry. In Fig. 2, we show Sn 3d XPS spectra obtained from the RT grown SnO2 film, where we confirm the evolution of full oxidized state peak according to the increase in the ALD cycle number. We calculated the growth per cycle as 0.02 nm/cycle. In the conference, we present the results of RT ALD of SnO2 and discuss the surface-reaction mechanism based on the IRAS observation. Acknowledgement; This work was partly supported by JST-CREST, JSPS KAKENHI Grant Numbers 15H03536 and 15K13299. Figure 1