Vapor HF cann't react with SiO2 at temperature of above 100℃.With the help of some polar compounds or superacids

which are called etching accelerators

the etching reaction may occur at the interface between SiO2 layer and photocrosslinked polymer film.The accelerators can be categorized into three groups:(1) tertiary amine compounds

which can form ammonium with HF to generate a high concentration of active fluorine anions.(2) organic compounds containing carbonyl

nitro

amino formacyl.These dipolar aprotic accelerators can activate HF to become an active necleophilic attacking reagent and make the etching reaction possible.(3) superacids

such as HBF4

HPF6 and HAsF6

which can be generated through the photolysis of photosensitive acid generators.Superacids can activate the leaving group and act as catalyst of the reaction.Polymer film can prevent the small molecular accelerator added in it from escaping.And the polymer carrying accelerating groups itself

which is called polymer accelerator

can act as film former and accelerator simultaneously.By using photochemical reaction

the area-selective etching reaction under polymer can be realized.