Synthesis of ammonium silicon fluoride cryptocrystals on silicon by dry etching

Cryptocrystal layers of ammonium silicon fluoride (NH4)2SiF6 were synthesized on silicon wafers by dry etching method using vapor of the mixture of HF and HNO3 solutions at room temperature. Crystalline layers having thicknesses of up to 8 mm have been produced at growth rates of around 1 m/h. The crystallinity was analyzed by X-ray diffraction that indicates an isometric hexoctahedral system (4/m 32/m) with Fm3m space grouping of (NH4)2SiF6 cryptohalite crystals. These results have been confirmed by the presence of vibrational absorption bands of (NH4)2SiF6 species by Fourier transform infrared (FTIR) spectroscopic measurements. Strong absorption bands were observed in the infrared at 480, 725, 1433 and 3327 cm 1 and assigned to N–H and Si–F related vibrational modes of (NH4)2SiF6. Annealing above 150 8C leads to the formation of individual crystals with sizes up to 20 mm on the surface, thus indicating the possibility of forming solid compound layers with fine grain sizes on silicon.