Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

2C/2002S2-002

Crystallization in HfO2 gate insulators with in situ annealingstudied by valence-band photoemission and x-ray absorption spectroscopy
Satoshi TOYODA1, Jun OKABAYASHI1, Hiroshi KUMIGASHIRA1, Masaharu OSHIMA1,
1
2
2
2
Koichi YAMASHITA , Masaaki NIWA , Koji USUDA and Guo-Lin LIU
1The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
2STARC, Kohoku-ku, Kanagawa 222-0033, Japan
Introduction
In the downscaling of complementary metal-oxidesemiconductor (CMOS) field effect transistor dimensions
using a gate oxide thickness of less than 2 nm,
conventional thermal SiO2 is not applicable because ofexcess direct tunneling leakage current. HfO2 has a highpotential for the next-generation high-k gate insulator
using merits such as considerably large energy-band gap,
high dielectric constant and compatibility withconventional CMOS processes because of the thermalstability. However, the crystallization of the amorphous
HfO2 layer by annealing is one of the most serious
problems during the CMOS device processing causing ina leakage current through the high-k insulators [1]. We
have found that valence-band photoemission and x-ray
absorption spectroscopy (XAS) are applicable forinvestigating the crystallization for HfO2 gate insulator
films including the interfacial layer since the electronic
states are strongly affected by the crystal structure.

sharp features and additional peaks caused by annealing
are clearly observed is well correlated with the splitting in
the valence band spectra. In order to elucidate therelationship between the change in spectral line shapes
and crystallization, cross-sectional TEM images wereobserved after the annealing in UHV at 900 °C. As shownin Fig 1 (c), the amorphous structure at the as-grownstage [2] changed to the different images and some parts
showed the crystallization in the HfO2 top layer although
the images in the interfacial layers remain unchanged as
the amorphous structure. The TEM image reveals that thechanges in both valence-band and O K-edge absorption
spectra are related to the crystallization of the HfO2 layer.
Valence-band photoemission and x-ray absorption
spectroscopy can be utilized to qualitatively investigate
the crystallization features in high-k gate insulators [3].
Valence Bandhν = 800 eV

Experimental
Photoemission measurements were carried out at anundulator beam line BL-2C of the Photon Factory inHigh-Energy Accelerator Research Organization (KEK),
where a high performance photoelectron analyzer(GAMMADATA-SCIENTA SES100) was equipped. The
Hf-metal layer and the HfO2 layer were deposited by DC
sputtering using a Hf-metal target on the Si substrates atroom temperature. We measured the sample having aHfO2 layer of 3.9 nm and an interlayer of 1.7 nm asshown in the insets of Fig. 1 (a) [2].

(a)

HfO2

3.9 nmHf-silicate

1.7 nmO K-edge XAS(b)900 ºC

Si (001)
Hf-metal predeposition
x3

1000 ºC

800 ºCx 10

as-grown

900 ºC
x 10

520

530

540

550

glue

Results and Discussion
Figures 1 (a) and (b) show annealing-temperaturedependence in valence-band spectra and XAS spectra in
HfO2/Hf-silicate/Si structure. Insets also show schematic
structures of the sample. Since the total oxide thicknesses
in this experiment were 5.6 nm, the valence-band
intensities from Si substrates between -1 eV and 3 eV ofbinding energy were considerably small. After theannealing at 800 ºC and 900 ºC, two components appear
with the splitting of 3.0 eV around the binding energy of
5-9 eV as indicated by the solid triangles. In the XAS
spectra, the absorption edge at 533 eV became sharp andadditional peaks (solid triangles) appeared by theannealing at 800 °C and 900 °C in the higher energy
region than the photon energy of 540 eV. The fact thatUsers' Report(c)800 ºCx 10

as-grown
14 12 10Si864205 nm

Binding Energy (eV)Fig. 1. Annealing-temperature dependence in valence-band
spectra (a) and XAS spectra (b) are shown. (c) Cross-sectional
TEM image after the annealing in UHV at 900 °C is also shown.

[1] H. Kim et al., Appl. Phys. Lett. 82, 106 (2003).
[2] K. Yamamoto et al., Jpn. J. Appl. Phys. 42, 1835
(2002).
[3] S. Toyoda et al., J. Appl. Phys. 97, 104507 (2005).
1

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

62560

Photon Energy (eV)