Synthesis of ammonium silicon fluoride cryptocrystals on silicon by dry etching

摘要

Cryptocrystal layers of ammonium silicon fluoride (NH4)2SiF6 were grown onsilicon wafers by dry etching using the vapors of HF:HNO3 solution at roomtemperature. As-grown layers are composed of white granular crystalline filmwith thicknesses of up to 8 micrometer which were synthesized with growth ratesof around 1 micron/hour. The crystallinity was analysed by X-ray diffractionwhich indicates an isometric hexoctahedral system(4/m -32/m) with Fm3m spacegrouping of (NH4)2SiF6 cryptohalite crystals. These results have been confirmedby the presence of the vibrational absorption bands of (NH4)2SiF6 species byFTIR transmission. Strong absorption bands were observed in the infrared at480cm-1, 725cm-1, 1433cm-1 and 3327cm-1 and assigned to N-H and Si-F relatedvibrational modes of (NH4)2SiF6. Annealing above 150oC leads to the formationof individual crystals with sizes up to 20 micrometer on the surface, thusindicating the posibility of forming solid compound layers with fine grainsizes on silicon.