RT Atomic Layer Deposition of Al2O3 By Using Remote Plasma Excited Water Vapor

Aluminum oxide (Al₂O₃) is well known as an oxide coating material. Al₂O₃ is also used as a barrier layer preventing water moisture and oxygen for organic light emitting diodes (OLEDs) [1]. Al₂O₃ has been also studied as an anticorrosion coating material [2]. On the other hand, Al₂O₃ has been studied as a passivation layer for Si solar cells [3]. The growth temperature for the Al₂O₃ deposition has been desired to be decreased to room temperature (RT) because the devices mentioned above have been demonstrated to be fabricated on plastic or flexible substrates for organic electronics. In this work, we developed RT atomic layer deposition of Al₂O₃ with trimethylaluminum (TMA) and a remote-plasma excited water vapor.

  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 a buffered HF acid solution and a sulfuric acid-hydrogen peroxide mixture solution. TMA as the aluminum precursor was introduced with a mass flow controller (MFC). A remote plasma system generating oxidizing gas was installed to the ALD chamber as shown in Fig.1. The source gas for the oxidizing gas is a mixture of H₂O vapor and Ar gas. 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 water vapor for 5 min to be terminated with OH groups. For the Al₂O₃ deposition, we repeated the cycle of TMA saturation and the oxidizing gas treatment at RT. The TMA exposure was set to 1.67×10^-3 Torr×15 s. The saturation of TMA on the hydroxylated surface has been confirmed by the IR absorption spectroscopy. The plasma excited oxidizing gas was injected for 2 min. The oxidization state of the Al₂O₃ film was evaluated by X-ray photoelectron spectroscopy (XPS). The thickness of the grown film was measured by spectroscopic ellipsometry.

  Fig.2 shows Al2p spectra obtained from Al₂O₃ on Si grown by the present ALD. According to the increase in the ALD cycles, the Al2p peak at around 74.4 eV evolves. The peak position of 74.4 eV indicates the Al₂O₃ growth on Si [4]. Fig.3 shows the Al₂O₃ thicknesses as a function of the ALD cycle. The linear increase of Al₂O₃ thickness with the ALD cycle indicates that the Al₂O₃ is grown in the ALD mode. The growth rate is estimated to be 0.15 nm/cycle. This value is comparable with those released in the previous RT ALD studies [4-6].

  We checked the composition of Al and O on the ALD grown Al₂O₃ film with XPS. The ratio of Al and O was calculated to be 1:1.83. The refractive index of our Al₂O₃ film was measured to be 1.56 at 633 nm that is the same value as that of RT grown Al₂O₃ reported elsewhere [7].

  To conclude, we developed the non-heating, RT ALD of Al₂O₃ with the remote plasma source. We are now investigating the gas barrier property. The RT grown Al₂O₃ films might be applicable for the barrier layer of water moisture and oxygen on flexible and soft materials.

References

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