Ultrathin Silicon Dioxide Layers Research Articles

Selective thermal decomposition of ultrathin silicon oxide layers induced by electron-stimulated oxygen desorption

The mechanism of electron-beam-induced selective thermal decomposition of ultrathin oxide layers on Si surfaces was studied by scanning reflection electron microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. We found that the change in the oxide layer composition caused by electron-stimulated oxygen desorption accounted for the selective thermal decomposition, where nanometer-scale voids were densely generated at a low heating temperature (720 °C). This implies that oxygen desorption from the oxide layers promotes the formation of a volatile oxide (SiO), and generates void nucleation sites.

Stability of plasma oxidized ultrathin SiO2 layers in NTP conditions

The changes of the thicknesses of plasma oxidized ultrathin silicon dioxide layers have been studied after they have been kept in room air. The thickness increases faster in the beginning and saturates then, so that the total increasement is typically no more than about 1 nm. The surface quality, produced by the oxidation process, is likely to be the main factor which affects the amount of increase. To produce very stable layers with nonreactive surfaces one has to adjust the oxidation temperature and the native oxide removal very carefully.

Fabrication of ultrathin silicon dioxide layers in ultra high vacuum

Silicon dioxide layers of thickness 1–10 nm have been manufactured by the plasma oxidation of silicon in ultra high vacuum (UHV). UHV chamber is a very clean environment, temperature and molecular fluxes are easily controllable and in situ device processing is possible. In the temperature range 700–850°C the oxidation is very slow in the beginning. After an uniform layer is developed on the surface, the growth rate increases. Increasing temperature also clearly increases the growth rate, as the temperature dependence of the initial growth phase is probably less. Our measurements predict, that to get a good quality layer, the processing temperature should be at least 700°C.

Application and properties of sub-monomolecular layers of silicon dioxide deposited under mild conditions

Ultra-thin layers of silicon dioxide (< 1 nm) can be applied via the UVO3CVD method whereby a gas mixture of O 3 O 2 and TMOS N 2 (TMOS = tetramethyl orthosilicate) is decomposed with UV light (254 nm) to form silicon dioxide at room temperature and atmospheric pressure. A reaction chamber is designed and the parameters that influence the deposition rate are optimized. The deposition rate is independent of the TMOS concentration, but depends on the ozone concentration. Probably, the reaction kinetics is pseudo-first-order in the ozone concentration. With increased distance between the UV lamp and the substrate the deposited silicon dioxide contains more organic groups. Silicon dioxide layers deposited on metal substrates are analyzed with static-SIMS, XPS, RBS, SEM, and glancing incidence asymmetric Bragg diffraction (GIABD). Both with XPS and with RBS the maximum deposition rate of the present equipment is determined to be 1 Å/min. According to the XPS measurements the O Si ratio in the deposit is 2.2 and the carbon content is near zero. With static-SIMS characteristic fragment ions of silicon dioxide and the metal substrate are detected. The relative peak intensities of these fragments linearly relate with the silicon dioxide layer thickness when the layer thickness is smaller than 2 nm. With SEm ultra-thin silicon-dioxide layers can be visualized, because the secondary electron emission of a coated and an uncoated metal surface is different. This is probably attributable to a difference in surface potential. With GIABD a 29 nm thick layer was found to be crystalline, but no known diffraction pattern of SiO x fitted the measured pattern. Metals can be coated with ultra-thin silicon dioxide. It was found that these ultra-thin layers inhibit metal corrosion. A metal surface coated with ultra-thin silicon dioxide can be further modified with known coupling agents, such as silanes, as if the metal was silicon dioxide.

A heat‐resisting acid catalyst: Thermal stability and acidity of a thin silica layer on alumina calcined at 1493 K

AbstractThe chemical vapor deposition of a thin layer of silica on alumina is shown to improve the thermal stability of alumina with respect to sintering and to reduce the loss of surface area of alumina during heating. Benzaldehyde‐ammonia titration showed that samples with high stability were almost completely covered by silica. These observations suggest that an ultrathin silica layer, probably a monolayer, thermally stabilizes the surface. Brønsted acidity was observed even after calcination at 1493 K: it was initially diminished by this calcination, but readily recovered on exposure to a humid atmosphere, whereas conventional solid‐acid catalysts are irreversibly deactivated by complete sintering.

Influence of ionizing radiation on the conduction properties of ultra-thin silica layers

The influence of X-ray irradiation on the conduction properties and breakdown of silica layers less than 10 nm thick has been investigated. It has been found that these properties are not drastically affected, and that the X-ray irradiation induces a decrease of the defectiveness, as for much thicker layers.

Mechanism of leakage current through the nanoscale SiO2 layer

We clarify the mechanism of leakage current through the nanoscale ultrathin silicon dioxide (SiO2) layer in a metal-insulator-semiconductor structure based on the multiple scattering theory when technologically important phosphorus doped polycrystalline silicon is adopted as the gate electrode. We also derive an analytic expression for the direct tunneling current, and show that its measurement presents an excellent opportunity to determine the effective mass of an electron in the SiO2.

Generation of acid sites by SiO 2 deposition on groups IVB metal oxides

An ultra thin silica layer was prepared by chemical vapor deposition of Si(OCH 3) 4 onto titania and zirconia. The relationship between surface structure and acidity was studied by measurement of coverage, IR spectroscopy of probe molecules, and test reactions. Silica was deposited to form a 1 : 1 complex with the oxide cation (-MSi-), until more than 90% of monolayer coverage was obtained. The silica overlayer showed weak Brønsted acidity only sufficient to catalyze the isomerization of 1-butene and the dehydration of tert-butyl alcohol. The acidity could not be correlated with the difference in coordination number or valence of cations but seemed to correlate with the interaction between silica and metal oxide.

Surface potential transients of ultrathin SiO 2Si structures

The surface potential of ultrathin (1–3 nm) silicon-dioxide layers upon silicon substrate has been measured by vibrating capacitor (Kelvin) method. By illuminating the surface of the ultrathin SiO 2Si system, a change of some hundreds of millivolts in the work function difference has been observed. After switching off the light the original (dark) value of the work function is reached through a transient process. The time constant of this transient process increases very strongly with increasing oxide thicknesses. Analysing the transient process the native oxide growth can be observed. Other oxides (e.g. that has been prepared in boiling nitric acid and thermally grown oxides) have also been investigated.