Etching mechanism of vitreous silicon dioxide in HF-based solutions

A reaction mechanism is proposed for the dissolution process of silicon dio xide networks in aqueous HE-based solutions. Etch experiments with thermall y grown silicon dioxide were used to create a model for the etch process. L iterature data on the etching of other vitreous silicon dioxide materials w ere 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 specie s. The first reaction step determines the rate of the etch process. It is t he 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 form s, the substitution reaction rate is pH dependent. At low pH (<1.5) water i s eliminated from the protonated silanol group and an HF2- ion or an H2F2 m olecule supplies an F- that binds to the positively charged silicon atom. A t higher pH values (>1.5), the elimination of an OH- group from the SiO2 su rface becomes the major reaction route. Once the OH- group is eliminated, a n HF2- molecule supplies an F- ion. The pK(a), value of the deprotonation r eaction increases due to the buildup of surface charge at pH > 4. Consequen tly, the SiOH surface concentration and the etch rate are higher than expec ted from a simple acid/base equilibrium reaction. All subsequent reaction s teps 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. Usin g this reaction model,published etch rate data of multicomponent glasses ca n be understood. Metal ions in glass break up the SiO2 network and create S i atoms bonded to less than four bridging oxygen atoms. The nonbridging oxy gen atoms are terminated by a metal ion, and the silicon bonded to these ox ygen atoms etches as fast as the Si-F units in vitreous silicon dioxide. Th erefore, the etch rates of multicomponent glasses are higher than that of v itreous silicon dioxide.