Local-density approximation calculations have been used to study the electronic structure of the Al-14/n-fold SiO2 ring/Si-15 cluster system for n=3, 4, and 6. The sixfold ring placed between aluminum and silicon has an insulator character with the fundamental band gap of 4 eV and the energetic barrier of 3.5 eV. The fourfold ring has a band gap of 2 eV, however its insulator properties are doubtful because of the very small distance between the Fermi level and the first unoccupied state (0.2 eV). The threefold SiO2 ring has not a fundamental gap at all. Considering the Al-14/n-fold SiO2 ring/<(Si15H)over bar(18)> system as a model of the metal-oxide-semiconductor device, we can predict that the thin silica film (d approximate to 7 Angstrom) can be used as an insulator layer. However, if the thickness is less than 5 Angstrom, the insulator properties of silica disappear.