Synthesis of ammonium silicon fluoride cryptocrystals on silicon by dry etching

TL;DR

This paper reports the dry etching synthesis of ammonium silicon fluoride cryptocrystals on silicon wafers at room temperature, analyzing their structure and growth characteristics.

Contribution

It introduces a dry etching method to grow crystalline ammonium silicon fluoride layers on silicon, with detailed structural and vibrational analysis.

Findings

Crystalline layers up to 8 micrometers thick were grown at ~1 micron/hour.

X-ray diffraction confirmed hexoctahedral crystal structure.

Annealing produces larger crystals up to 20 micrometers.

Abstract

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.