Spectrometric analysis of process etching solutions of the photovoltaic industry—Determination of HNO3, HF, and H2SiF6 using high-resolution continuum source absorption spectrometry of diatomic molecules and atoms

The surface of raw multicrystalline silicon wafers is treated with HF–HNO3 mixtures in order to remove the saw damage and to obtain a well-like structured surface of low reflectivity, the so-called texture. The industrial production of solar cells requires a consistent level of texturization for tens of thousands of wafers. Therefore, knowing the actual composition of the etch bath is a key element in process control in order to maintain a certain etch rate through replenishment of the consumed acids. esent paper describes a novel approach to quantify nitric acid (HNO3), hydrofluoric acid (HF), and hexafluosilicic acid (H2SiF6) using a high-resolution continuum source graphite furnace absorption spectrometer. The concentrations of Si (via Si atom absorption at the wavelength 251.611 nm, m0,Si = 130 pg), of nitrate (via molecular absorption of NO at the wavelength 214.803 nm, m 0 , N O 3 − = 20   ng ), and of total fluoride (via molecular absorption of AlF at the wavelength 227.46 nm, m 0 , F = 13   pg ) were measured against aqueous standard solutions. The concentrations of H2SiF6 and HNO3 are directly obtained from the measurements. The HF concentration is calculated from the difference between the total fluoride content, and the amount of fluoride bound as H2SiF6. H2SiF6 and HNO3 can be determined with a relative uncertainty of less than 5% and recoveries of 97–103% and 96–105%, respectively. With regards to HF, acceptable results in terms of recovery and uncertainty are obtained for HF concentrations that are typical for the photovoltaic industry. The presented procedure has the unique advantage that the concentration of both, acids and metal impurities in etch solutions, can be routinely determined by a single analytical instrument.