AVS 56th International Symposium & Exhibition
    Thin Film Tuesday Sessions
       Session TF2-TuA

Paper TF2-TuA7
In situ X-ray Photoelectron Spectroscopy for the Study of Initial Stages of TiO2 ALD on Silicon

Tuesday, November 10, 2009, 4:00 pm, Room B4

Session: ALD/CVD: Oxides and Barriers
Presenter: R. Methaapanon, Stanford University
Correspondent: Click to Email
Titanium dioxide (TiO2) is one of the widely studied atomic layer deposition (ALD) systems due to its outstanding electrical and optical properties that are suitable for variety of applications. The properties of the deposited TiO2, especially at the interface with the substrate, become more important as the size of required devices in each application approaches the nano-scale. Due to different chemical properties of the substrate and the deposited material, nucleation at the interface can be different from the steady state growth of the bulk.

In this work, TiO2 ALD is carried out in an integrated ALD reactor/UHV chamber that allows for X-ray photoelectron spectroscopy (XPS) analysis after each precursor pulse without vacuum break. Titanium tetrachloride (TiCl4) and water (H2O) are selected as precursors due to their molecular simplicity and broad operating temperature range that result in several achievable TiO2 phases. The initial growth at 100oC on two substrates − chemical oxide on silicon prepared by piranha treatment, and hydrogen-terminated silicon prepared by HF etch − is compared. The intensities and binding energies of characteristic peaks from the XPS spectra are used to analyze the elemental compositions and chemical state of each species as the deposition progresses. TiO2 growth on both SiO2 and H-Si surfaces exhibits linear behavior, as normally achieved by ALD, but the TiO2 growth rate is lower on hydrogen-terminated surface than on silicon dioxide surface. Interestingly, no incubation period is observed on either surface.

The chemical shifts of the Si 2p, O 1s and Ti 2p XPS peaks after TiO2 deposition on the SiO2 substrate suggest bond formation between titanium and silicon-bound oxygen at the interface. The data also suggest that some chlorine is trapped at the SiO2/TiO2 interface and that the titanium oxide right at the interface is sub-stoichiometric. The results on the hydrogen-terminated Si surface show different interfacial properties. There is no detectable amount of oxidized silicon species on hydrogen-terminated silicon after deposition under vacuum. Together with the results of ex situ studies, it can be concluded that interfacial silicon dioxide grows after air exposure, not during ALD reactions. The absence of silicon oxide and a shift in the Si 2p binding energy in the as-grown samples suggest the possibility of an ALD mechanism which involves direct bonding between titanium and silicon on the surface. The differences between the two substrates will be discussed.