Graded etching of silicon dioxide layer and its application to schottky diodes

Abstract

Reproducible tapered windows in thermally grown silicon dioxide layer have been produced by depositing a thin layer of silicafilm on the thermal silicon dioxide layer before chemical etching. As densification temperature of silicafilm is varied from 175℃ to 1,150℃, taper angles from 3˚ to 40˚ are obtained. Analytical model of this graded etching process has been presented and etched profile equations of the silicon dioxide layer have been derived using Fermat``s principle of least time. Etched contours for two limiting cases are discussed in correlation with the physical behavior of the etching process. Profiles obtained from scanning electron microscope analysis show good agreement with the theoretical profiles. As an application of the graded etching process, Schottky diodes with a tapered window have been discussed by numerical analysis and experimental results. Potential and field distribution within the Schottky diode have been obtained by two-dimensional numerical analysis. The result of calculation shows two peaks of the electric field, one at the contact edge and the other at the edge of the metal electrode. The result also shows that as the taper angle decreases, the principal peak of the electric field at the contact edge diminishes, while the secondary peak at the metal electrode remains nearly unchanged. The magnitude of the two electric field peaks become equal to each other at the taper angle of about 6˚. Hence, for the taper angle of 6˚ or less, field peak at the edge of the metal electrode is responsible for the breakdown. Also, for the taper angle of 6˚ and above, field peak at the contact edge is the cause of the breakdown. Aluminum-silicon Schottky diodes with a tapered window have been fabricated using the graded etching process with silicafilm. The breakdown voltage of the Schottky diode with the taper angle of 3˚ has been drastically increased to 200V in comparison with 60V for the normal metal-overlap diode. Tapered sidewall Schottky di...