Understanding the oxidation of silicon has been an ongoing challenge for many decades. Ozone has recently received considerable attention as an alternative oxidant in the low temperature, damage-free oxidation of silicon. The ozone-grown oxide was also found to exhibit improved interface and electrical characteristics over a conventionally dioxygen-grown oxide. In this review article, we summarize the key findings about this alternative oxidation process. We discuss the different methods of O(3) generation, and the advantages of the ozone-grown Si/SiO(2) interface. An understanding of the growth characteristics is of utmost importance for obtaining control over this alternative oxidation process.
We have investigated ultrathin silicon oxide film growth by highly concentrated ozone at atmospheric pressure. Oxide film Ͼ2 nm was grown on as-received Si͑100͒ even at room temperature. The etching rate by dilute hydrofluoric acid solution of oxide fabricated on Si͑100͒ at 350°C by this method was almost the same as that of thermally grown oxide so that film density is equivalent to that of thermally grown oxide. The etching rate of this film also shows no dependence on the film depth. This is indicating that the transition layer due to the lattice mismatch of substrate and oxide is limited within a thinner region than that of thermally grown oxide. It also indicates that an oxide film with higher film density can be synthesized on the surface with preoxide film already formed to protect bare substrate surfaces.
We examined the structure around the interface of SiO2 and Si using medium-energy ion scattering spectroscopy (MEIS) to investigate the interfacial Si displacement of an ultrathin silicon dioxide formed by oxidation of a Si(100) substrate with atmospheric-pressure ozone at a substrate temperature of 375 °C. A thermally grown oxide with the same thickness as an ozone-formed oxide was also measured with MEIS for comparison. The ozone-formed oxide exhibited considerably less Si displacement in the oxide layers near the interface than a thermally grown oxide, which indicates that an ozone oxide is homogenous. These results explain well our previous findings that an ozone oxide exhibits a constant HF etching rate of silicon dioxide while a thermally grown oxide slows the etching rate near the interface.
Initial oxidation by high purity ozone and molecular oxygen of Si(111)7×7 was investigated by second harmonic generation (SHG) with a 1.064 μm Nd:YAG laser. Decrease of second harmonics (SH) intensity to almost zero after 5 L ozone gas exposure, in spite of the fact that molecular oxygen kept SH intensity for the same amount of exposure, indicated that ozone is inserted into the Si–Si backbond in the subsurface layers more effectively than molecular oxygen. In the initial exposure, rates of rapid decrease in SH intensity for both ozone and oxygen adsorption were in the same order of magnitude, although O 1s x-ray photoelectron spectroscopy (XPS) intensity showed high reactivity of ozone. This is because of a difference in the information depth between SHG and XPS so that oxygen species in the subsurface layers are not effective in decreasing SH intensity. This indicates that the process of attacking backbonds is underway even with an initial exposure of <5 L.
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