Etching mechanism of vitreous silicon dioxide in HF-based solutions

A reaction mechanism is proposed for the dissolution process of silicon dioxide networks in aqueous HE-based solutions. Etch experiments with thermally grown silicon dioxide were used to create a model for the etch process. Literature data on the etching of other vitreous silicon dioxide materials were used to refine the model. A new method, using a quartz microbalance, is used to monitor the etch rate in situ and to establish the reactive species. The first reaction step determines the rate of the etch process. It is the substitution of a surface SiOH group, which is bonded to three bridging oxygen atoms, by an SiF group. Due to an acid/base equilibrium reaction of the silanol groups on the surface with its protonated and deprotonated forms, the substitution reaction rate is pH dependent. At low pH (<1.5) water is eliminated from the protonated silanol group and an HF2- ion or an H2F2 molecule supplies an F- that binds to the positively charged silicon atom. At higher pH values (>1.5), the elimination of an OH- group from the SiO2 surface becomes the major reaction route. Once the OH- group is eliminated, an HF2- molecule supplies an F- ion. The pK(a), value of the deprotonation reaction increases due to the buildup of surface charge at pH > 4. Consequently, the SiOH surface concentration and the etch rate are higher than expected from a simple acid/base equilibrium reaction. All subsequent reaction steps to remove the Si-F unit from the SiO2 matrix are fast reaction steps (18-20 times faster) involving HF2- addition reactions on FxSi-O bonds. Using this reaction model,published etch rate data of multicomponent glasses can be understood. Metal ions in glass break up the SiO2 network and create Si atoms bonded to less than four bridging oxygen atoms. The nonbridging oxygen atoms are terminated by a metal ion, and the silicon bonded to these oxygen atoms etches as fast as the Si-F units in vitreous silicon dioxide. Therefore, the etch rates of multicomponent glasses are higher than that of vitreous silicon dioxide.