After low pressure fluorine plasma ecthing and oxygen ion passivation, a crystallized layer composed of SiO2 nano-crystal grains is observed in an amorphous fused silica surface. The depth of crystallized layer is at least several hundreds nanometers. Fluorine and carbon ion are generated from Ar/CF4 by the method of electron cyclotron resonance (ECR). F ion breaks Si-O band of initial silica surface layer and releases O ion. Carbon ion combines with oxygen ion, and turns into CO2, and SiF4 is generated from fluorine and silicon. After initial surface layer is removed, unsaturated Si atom remains. Si dangling bond recombines with new O ion and then creates crystallized -cristobalite nano-crystal grains under a high temperature.