orphous substance at least comprising defects such as strained Si-Si bonds, vacancies of oxygen in the form Si-Si and superoxided Si-O-O-Si bonds. This indicates the importance of the local stoichiometry. Indeed, oxygen intervenes in all aspects of the behavior of holes: trapping and transport (valence band edge), polaron regime (deformation of the Si-O-Si bonds by polarization), trapping (for example on oxygen vacancy Si- Si). Fig. 57 shows an example of defect calculation using quantum chemistry codes. Such work allows understanding the configurations of defects (bond lengths and position) and energies levels associated with charge states. 2,28 65 2.47.0.62 a-SiO2 no defect neutral positively Oxygen charged Vacancy Oxygen Vacancy Figure 57. Atomistic calculation of oxygen vacancy by the method of clusters using Density-Functional Theory (DMOL and Fast-Structure codes) [Cour-99]. The relaxation of bond lengths is clearly seen after trapping a positive charge (all dimensions in Angstroms). Beyond this oxygen vcancy, there exists a number of defects, intrinsic or extrinsic (Si-OH for example), more or less well known or measurable [Chu-90], [Warr-92], [Pail-95c]. If the process is not well controlled, the hardness level has is likely to be unstable (irreproducible) at one time. This is due of course to variations of the microstructure of the oxide and its interface with the silicon. Indeed, quantities at stake are very weak (with respect to the total number of bonds, the number of precursors NT is on the order of 10-4 in surface and 10-6 in volume). Moreover, these defect ratios are very sensitive to manufacturing variations, some of which are known (temperatures, anneal atmospheres), and others, probably unknown. It appears often during the change of machines, modifications of process (plasmas, rampings, various deposits sometimes well after the end of oxidation), and that these modifications apparently III-69 without influence on electrical transistor parameters or capacitors have an influence on the hardness le