Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

2C/2002S2-002

Annealing-temperature dependence: Mechanism of Hf silicidation in HfO2 gateinsulators on Si by core-level photoemission spectroscopy
Satoshi TOYODA1*, Jun OKABAYASHI1, Hiroshi KUMIGASHIRA1, Masaharu OSHIMA1,
2
2
2
Masaaki NIWA , Koji USUDA , Guo-Lin LIU
1The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
2STARC, Kohoku-ku, Kanagawa 222-0033, JapanIntroduction
HfO2 has a high potential for the next-generation ultralarge scale integration device applications using merits
such as considerably large energy-band gap, highdielectric constant (high-k) and compatibility withconventional complementary metal–oxide-semiconductor(CMOS) processes. However, there remain serious
problems to be solved such as low-dielectric-constant Hfsilicate layer formations at the interface, the formation ofmetallic Hf silicide (HfSi2), and the crystallization in
HfO2 layers after annealing for the activation. In order tounderstand the silicidation mechanism, the changes inchemical states by annealing have to be clarified
explicitly.
Experimental
HfO2 layers were deposited in Ar/O2 mixed
atmosphere by dc sputtering using a Hf-metal target atroom temperature. Photoemission spectroscopy usingsynchrotron radiation was performed at the undulator
beam line BL-2C of the Photon Factory in High-Energy
Accelerator Organization (KEK). Annealing wasperformed in ultra-high vacuum by the direct current
flowing method through the samples. Using the sampleposition dependence in annealing-temperature ranges of
10 ºC step, we performed the narrow range control of theannealing temperature around 920 ºC.
Results and Discussion
Figure 1 shows Si 2p and Hf 4f core-level spectra withthe annealing-temperature dependence. Spectra at eachpanel are normalized at maxima of the peak height. Curve
fitting for Hf 4f spectra was carried out by a leastsquares-fitting procedure. By the annealing at 900 ºC,
cross-sectional TEM images suggest that the layer
structure remains unchanged [1,2]. With increasing theannealing temperature from 900 to 920 ºC, the intensityof Si 2p oxide components decreases drasticallycompared with that of the bulk components, suggesting
the reduction of interfacial SiO2-layer thickness. In Hf 4fspectra, the Hf-silicide component also increases
compared with the oxide one. In addition, oxidecomponents in Si 2p and Hf 4f spectra are shifted byabout 1 eV toward lower binding energy. Annealing up to940 ºC promotes the silicidation reaction. On the otherhand, the oxide components in Si 2p and Hf 4f spectra areshifted by about 0.2 eV toward higher binding energy
than those by the 920 ºC annealing. The oxide
components in both Si 2p and Hf 4f spectra disappear
completely by annealing up to 1000 ºC.

By the annealing at 900 ºC, it is considered that theHf-silicide formations occur by the following two-stepreactions,(1)
SiO2 + Si → 2SiO↑,
HfO2 + 6SiO↑ → HfSi2 + 4SiO2.
(2)First, at the interface between the SiO2 layer and the Sisubstrate, a SiO gas is generated by the reduction of theSiO2 layer. The formation of the SiO gas by the reaction
between SiO2 and the Si substrate during annealing at
higher temperature than 600 ºC in ultra-high vacuum, i.e.,
oxygen free ambient, is well known. Second, thediffusion of the SiO gas into the HfO2 layer triggers thereduction of the HfO2 top layer. According to the reaction(2), both HfSi2 and SiO2 are generated as products. By thereaction (1), the interfacial-layer thickness decreases andthe HfO2 layer is directly contacted with the Si substrateby the annealing at 920 ºC as shown in schematic viewsin Fig. 1.

Fig. 1: Annealing-temperature dependence of (a) Si 2p and(b) Hf 4f core-level photoemission spectra. Hatched areas in(a) show the SiO2 peak positions for several SiO2 layer
thicknesses [3].

Reference[1] K. Yamamoto et al., Jpn. J. Appl. Phys. Part I 42, 1835
(2002).
[2] S. Toyoda et al., J. Appl. Phys. 97, 104507 (2005).
[3] T. Eickhoff et al., J. Electron Spect. Rel. Phenom. 137-140,
85 (2004).
*

toyoda@sr.t.u-tokyo.ac.jp

Users' Report45