HF-(NH₄)₂S₂O₈-HCl Mixtures for HNO₃- and NO_x-free Etching of Diamond Wire- and SiC-Slurry-Sawn Silicon Wafers: Reactivity Studies, Surface Chemistry, and Unexpected Pyramidal Surface Morphologies

Abstract

The wet-chemical treatment of silicon wafers is an important production step in photovoltaic and semiconductor industries. Solutions containing hydrofluoric acid, ammonium peroxodisulfate, and hydrochloric acid were investigated as novel acidic, NOx-free etching mixtures for texturization and polishing of monocrystalline silicon wafers. Etching rates as well as generated surface morphologies and properties are discussed in terms of the composition of the etching mixture. The solutions were analyzed with Raman and UV/vis spectroscopy as well as ion chromatography (IC). The silicon surfaces were investigated by scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), diffuse reflection infrared spectroscopy (DRIFT), and X-ray photoelectron spectroscopy (XPS). Surprisingly, pyramidal surface structures were found after etching SiC-slurry as well as diamond wire-sawn monocrystalline Si(100) wafers with hydrochloric acid-rich HF-(NH4)2S2O8-HCl mixtures. Acidic etching solutions are generally not known for anisotropic etching. Thus, the HNO3-free mixtures might allow to replace KOH/i-propanol and similar alkaline solutions for texturization of monosilicon wafers at room temperature with less surface contamination. Besides, common HNO3-based etching mixtures may be replaced by the nitrate-free system, leading to significant economic and ecological advantages.

On The Mechanism of the Anisotropic Dissolution of Silicon in Chlorine Containing Hydrofluoric Acid Solutions

“…As also shown in previous works, anisotropic pyramidal 14 and inverted pyramidal 18,20 structures are generated on monocrystalline silicon (100) surfaces, when treating silicon wafers in HF-HCl mixtures with an added oxidizing agent. Figure 9 shows a SEM image of a silicon wafer surface after etching in an HF-HCl-Cl 2 mixture.…”

Etching Silicon with Aqueous Acidic Ozone Solutions: Reactivity Studies and Surface Investigations

“…Especially, no vibrational band around 780 cm −1 (representing S−O 2 −H fragments) was noticeable. 48 Obviously, S 2 O 8 2− did not oxidize the silicon surface in spite of the high standard redox potential of 2.01 V (pH = 0), as presented by Stapf et al 49 We conclude that the titrimetrically determined amount of oxidizing agents is traced back to O 3 (aq) . The high O 3 (aq) concentration caused a slight blue coloration of the mixture (photo see Supporting Information Figure S3).…”

Etching SiC‐slurry and diamond wire‐sawn silicon wafers with HF‐HCl‐Cl2 mixtures: Parameter influences on etch rates and surface structures

“…But because of the lower pH‐value in the higher concentrated hydrochloric acid, the Si–H bonds become more stable . Since the oxidation of silicon by chlorine is based on the attack on the Si–H bonds, the reactivity is lower .…”

“…We recently examined acidic mixtures based on hydrofluoric and hydrochloric acid with added oxidants for the generation of pyramidal structures on monocrystalline (100) wafers. The investigated oxidizing agents include ammonium persulfate ((NH 4 ) 2 S 2 O 8 ) , hydrogen peroxide (H 2 O 2 ) , and ozone (O 3 ) . In these mixtures, chlorine (Cl 2 ) and trichloride ions (Cl 3 − ) are formed by the oxidation of the chloride ions contained in the hydrochloric acid by the respective oxidizing agent (Eqs.…”