A STUDY OF THE DISSOCIATION STATE AND THE SIO2 ETCHING REACTION FOR HF SOLUTIONS OF EXTREMELY LOW CONCENTRATION

The conductivities of HF, KF, and KHF2 solutions have been measured at very low concentrations. The concentration of HF2- ion, a dominant ion in the SiO2 etching process, has been calculated from the measured F- concentration in HF and KHF2 solutions. The dissociation state of HF in HF solution has been classified into three regions as a function of initial HF concentration. Etching caused by a high concentration of F- ions was followed along with the conductivity of KF solution for a thermal SiO2 film. Further, with HF and KHF2 solutions, the etching rate for a thermal SiO2 film was a function of the HF2- concentration. The activation energy for the etching reaction in HF solution is minimal in a certain range of HF concentration. Conductivity measurements can be.used to monitor the etching rate of SiO2 in HF solutions accurately when the etching rate is relatively slow (around 1 angstrom/min). Furthermore, the relationship between conductivity and etching rate is independent of the temperature of the HF solution (in the range of 24 to 45-degrees-C) when the HF concentration is between 0.005 and 0.04 mol/kg.