Simultaneous determination of mineral acids, fluoride, and silicate in etching baths
Dual Detection by Ion Chromatography
The ion chromatographic method presented here is used for the simultaneous determination of HF, HNO3, H2SO4, short-chain organic acids, and H2SiF6 in acidic texturing baths that are used in the wet chemical etching process of solar cell production.
Setup of the ion chromatographic device (Pictures: Metrohm)
Energy generation from renewable sources such as biomass, biogas, biofuels, water, wind, and solar power is becoming increasingly important in our energy-hungry society. Particular attention is given to solar energy, which by human criteria is inexhaustible. Solar cells used in photovoltaic units convert the energy in sunlight directly into electricity.
Exact analysis saves costs
Solar cells are manufactured from ultrapure multicrystalline or monocrystalline silicon wafers whose surface has been treated in acid etching baths (also known as texturing baths) before being spiked with foreign atoms (P, B). The etching solutions consist of various acids, which act as oxidizing agent (HNO3), complexing agent (HF), stabilizer and wetting agent (CH3COOH), or buffers (H3PO4, CH3COOH) and determine the surface structure and thus the efficiency of the solar cells. The replenishment of components used up in the etching process extends the bath life and saves costs, though it does require knowledge of the exact composition of the bath, especially the concentration of silicon and hexafluorosilicate. Use of titration and ion chromatography (IC) permits quick and precise determination of the key components. This ion chromatographic method separates all relevant components in the bath on an anion-exchange column and identifies them by dual detection in a single run. After suppressed conductivity detection of the acid anions, the undissociated silicic acid is derivatized in a post-column reaction (PCR) to form molybdosilicic acid, which is determined spectrophotometrically at 410 nm. The concentrations of fluoride and hexafluorosilicate are determined by way of a simple stoichiometric calculation that is performed by the chromatography software.
Constant etching rate
Fig. 2: Conductivity chromatogram of a simulated etching bath with 25 mg/L fluoride, 20 mg/L acetate, and 10 mg/L nitrate. The undissociated orthosilicic acid is not recorded in the conductivity detector.
In the wet chemical etching of silicon surfaces, nitric acid is used to oxidize silicon to form silicon dioxide which is further etched by hydrofluoric acid.
3 Si + 4 HNO3 + 18 HF -> 3 H2SiF6 + 4 NO + 8 H2O
During the etching process, the concentrations of HF and HNO3 in the etching bath decrease and the concentrations of water and hexafluorosilicate increase. To ensure a constant etching rate and surface properties, the etching bath can be regenerated several times by subsequent replenishment of spent acids. The increasing concentration of H2SiF6, however, limits the number of possible regeneration cycles. This requires semi-continuous monitoring of the bath components, which can be achieved conveniently by automated ion chromatography.
Dual detection by conductivity and UV/Vis
The acid anions present in the etching bath – mainly fluoride and nitrate, though sometimes also sulfate and acetate – are separated under alkaline elution conditions and determined by conductivity detection. In the alkaline eluent, the hexafluorosilicate is converted into orthosilicic acid which is undissociated and therefore ‘invisible’ in the conductivity detector.
Na2SiF6 + 4 NaOH -> Si(OH)4 + 6 NaF
The undissociated orthosilicic acid is determined by way of post-column reaction with an acid molybdate solution and subsequent UV/Vis detection at 410 nm.
H4SiO4 + 12 MoO42- + 24 H+ -> H4[Si(Mo3O10)4] + 12 H2O
The injection of SiF62- produces a fluoride peak in the conductivity detector and a silicate peak in the UV/Vis detector. The mass balance derived from the respective peak areas confirms that the concentration of SiF62- results stoichiometrically from the determined fluoride and silicate concentrations, provided there are no other sources of fluoride or silicate present. Thus the concentration of free HF can be calculated as the difference between the total fluoride concentrations and fluoride concentration from hexafluorosilicate:
[HF] = [F]total – [F]hexafluorosilicate
Analysis of baths and validation
Fig. 3: UV/VIS chromatogram of a 10 mg/L silicic acid standard. Silicic acid is derivatized to molybdosilicic acid which is then detected spectrophotometrically
After being diluted at ratios between 1:1000 and 1:5000, four samples from different texturing baths are analyzed for their constituents by using the above IC method with dual detection. The figure shows the chromatograms of etching bath sample 1 obtained by conductivity (a) and UV/VIS (b) detection. The Table provides an overview of the concentrations of the relevant bath components determined by ion chromatography with dual detection. For comparison, the concentrations obtained by titration are also shown. The potentiometric determination of acid concentrations and H2SiF6 content was carried out using aqueous acid-base titration with 1 mol/L NaOH solution.
Knowledge of etching bath composition assures constant wafer quality
Ion chromatography with dual detection allows determining the concentration of all relevant constituents of texturing baths for solar cell production in less than 30 minutes. The acids consumed in the texturing process can be replenished subsequently in a targeted way. This extends the life of the etching baths, guarantees clean and reproducible wafer surfaces, cuts costs, and protects the environment.
Fig. 4: (a - this picture) Conductivity and (b) UV/VIS chromatograms of the sample of etching bath 1 with 1:2000 dilution. The chromatographic parameters are the same as those of the prior chromatograms
Fig. 4: (a) Conductivity and (b - this picture) UV/VIS chromatograms of the sample of etching bath 1 with 1:2000 dilution. The chromatographic parameters are the same as those of the prior chromatograms