Synthesis of ammonium silicon fluoride cryptocrystals on silicon by dry   etching
  Cryptocrystal layers of ammonium silicon fluoride (NH4)2SiF6 were grown on silicon wafers by dry etching using the vapors of HF:HNO3 solution at room temperature. As-grown layers are composed of white granular crystalline film with thicknesses of up to 8 micrometer which were synthesized with growth rates of around 1 micron/hour. The crystallinity was analysed by X-ray diffraction which 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 the vibrational absorption bands of (NH4)2SiF6 species by FTIR transmission. Strong absorption bands were observed in the infrared at 480cm-1, 725cm-1, 1433cm-1 and 3327cm-1 and assigned to N-H and Si-F related vibrational modes of (NH4)2SiF6. Annealing above 150oC leads to the formation of individual crystals with sizes up to 20 micrometer on the surface, thus indicating the posibility of forming solid compound layers with fine grain sizes on silicon. 
Seref Kalem