8. 5  Advanced Technology in the Nano-World
- Atomic-Scale Control of Oxidation Reactions
 


Fig. 8-9 Arrangement of atoms on a Si(001) surface

Buckled dimers of Si atoms (1st layer) have a different lattice structure and chemical reactivity compared with those of Si atoms beneath the second layer.


Fig. 8-10 Saturated oxygen content on a Si(001) surface as a function of the O2 beam energy Et

The oxygen content depends on Et. In region I, oxygen atoms are chemisorbed on the first Si layer, while in regions II and III, the O2 beam reacts with the lower bonds of the first and second Si layer to form oxide layers, respectively.


Fig. 8-11 Si-2p photoemission spectra of a Si(001) surface which has been irradiated with O2 beams

The oxidation number of Si atoms can be determined from the spectral pattern. Increases in the Si4+ component and in the oxide layer thickness Tox show the change of elemental Si atoms into SiO2.


Field effect transistors (FET's) play an important role in silicon-based LSI technology. With the rapidly decreasing scale of device integration, the thickness of silicon-oxide layers as gate insulators in FET's will be required to be as small as 1 nm in the near future. Therefore, precise control of oxidation effects on Si(001) surfaces is an important subject in the development of ULSI's as well as for surface science.
The surface reaction analysis apparatus (SUREAC2000), which has been installed on a soft X-ray beamline at SPring-8, employs beams of O2 molecules with controlled kinetic energy Et. Irradiation with energetic O2 beams, i.e., having a speed ~10 times larger than that in air, has resulted in the formation of ultra-thin oxide layers with maximum thickness 0.5 nm on Si(001) surfaces at room temperature (Fig. 8-9), and the maximum thickness has been found to be dependent on Et. The kinetic energy threshold in the oxidation process predicted by a first-principles molecular dynamics calculation has been verified experimentally for the first time (Fig. 8-10). Using photoemission spectroscopy with high energy resolution synchrotron radiation, it has been also revealed that the degree of oxidation increases with an increase in Et (Fig. 8-11).
Photoemission spectroscopy using intense synchrotron radiation has been successfully applied to real-time in-situ analyses of the state of oxidation during the reaction. Irradiation with supersonic O2 beams has been found to be an effective means to control the oxidation process on the Si(001) surface. It is expected that this method will be useful for the control of oxidation on metal surfaces as well.


Reference
Y. Teraoka et al., Commissioning of Surface Chemistry End-Station in BL23SU of SPring-8, Appl. Surf. Sci., 169-170, 738(2000).

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Persistent Quest - Research Activities 2001
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