Investigação interferométrica da reação HF-Sílica (SIO2) em presença de campos elétricos

Abstract

Etching of silica (SiO2) with hydrofluoric acid (HF) is an important chemical reaction, used in fabrication of integrated circuits, whose mechanism is not fully understood. One curios aspect of this reaction is that its velocity can be changed by electric fields in the acid– silica interface. A theoretical model of the effect is based on the hypothesis that the chemical reaction can only take place if the HF molecule approaches the silica surface with the F atom pointing towards the silica surface so that the molecular axis forms a small angle with the surface normal vector. This way a partial orientation of the molecules in an electric field modifies the etching velocity. This model predicts a non-linear relation between etching velocity and electric field. In literature the model is used in order to measure electric fields recorded in silica samples by an electro-thermal poling process. In this application, one trusts the theoretic model in the non-linear regime although the model had been tested only in the liner region where the electric fields are much smaller that in the poled glass samples. In the present woke one describes measurements made to investigate the effect in the non-linear regime. In order to detect the non-linearity one has to measure the etching velocity with extremely high precision and in the presence of high electric fields. This was done with an interferometric method using an expended laser beam and a sophisticated analysis of fringe images. With these methods, one obtained first non-linear signals. A comparison of experimental values with theoretical predictions indicated that some corrections of the theoretical model had to be made. With these corrections theoretical values coincided the the experimental data within experimental uncertainties. In literature, theoretic studies of the HF- silica reaction use the hypothesis that only one HF molecule participates in the primary step of the reaction. But the results of the present work indicate that the etching reaction initiates with two HF molecules acting at the same time.