THE INFLUENCE OF NH4F ON THE ETCH RATES OF UNDOPED SIO2 IN BUFFERED OXIDE ETCH

The dependence of etch rates of thermal oxide and undoped vapor-deposited silicon dioxides as a function of the composition of buffered HF was examined. The composition of the buffered oxide etch (BOE) was varied from 0-30 weight percent (w/o) ammonium fluoride (NH4F) with 2-15 w/o hydrofluoric acid (HF). The etch rates (V(E)) of the silicon dioxides run through a maximum. The position of the maximum increases linearly to higher NH4F concentrations with higher HF content. For the reaction of dissolution, two different paths are discussed. In BOE with NH4F concentration lower than 15 w/o silicon dioxides react mainly with HF2-. With higher ammonium fluoride concentration the reaction with HF becomes increasingly dominant. In BOE with 6 w/o HF, the activation energy for the reaction of dissolution was determined. Diagrams of etch rates as a function of HF and HF2- concentration are presented.