Alternative Inline Analysis of Acidic Etching Baths

A process control system was developed which continuously monitors the composition of acidic etching baths. The simple physical parameters, sound velocity, conductivity, and refractive index are verified for suitability to determine the concentrations of hydrofluoric acid, nitric acid, and hexafluorosilicic acid. In pre-tests the characteristics of the individual concentrations were observed with the measuring devices and the effect of the formation of nitrous gases on the physical measuring devices is observed. In contrast to the conductivity electrode, the sound velocity and refractive index sensors show highly reproducible results. The formation of nitrous gases has no significant effect on the physical measurement methods. The influence of different concentrations of a mixture on the physical parameters is determined building up a design of experiment. With aid of multiple linear regression model equations were created which describe the quantitative relationship between the concentration and the physical variables. The model equations which lead to a quantitative correlation between the concentration of the acids as a function of the parameters sound velocity, conductivity, refractive index and temperature are compared and finally validated. The model equation predicts the concentration of HF with a recovery rate of 96.40%, HNO3 with 96.38% and Si in H2SiF6 with 78.74%.