Chemical analysis of acidic silicon etch solutions

The processing of silicon in microelectronics and photovoltaics involves the isotropic chemical etching using HF-HNO3 mixtures to clean the surface from contaminations, to remove the saw damage, as well as to polish or to texture the wafer surface. Key element of an effective etch process control is the knowledge of the actual etch bath composition in order to maintain a certain etch rate by replenishment of the consumed acids. The present paper describes a methods for the total analysis of the etch bath constituents HF, HNO3, and H2SiF6 by ion chromatography. First step is the measurement of the total fluoride and nitrate content in the analyte. In a second step, H2SiF6 is precipitated as K2SiF6. After careful filtration of the precipitate, the fluoride concentration in the filtrate is measured and the content of free HF is calculated therefrom. The K2SiF6 is dissolved again and the fluoride content measured and recalculated as H2SiF6. The results obtained with the presented method are discussed with respect to the results from two other, previously published methods, based on a titration using methanolic cyclohexylamine solution as titrant and based on a method using a fluoride ion selective electrode (F-ISE). An evaluation with respect to the needs for an industrial application is given.