THE GROWTH OF SUBMICRON FILMS OF TPA-SILICALITE-1 ON SINGLE-CRYSTAL SILICON-WAFERS FROM LOW-TEMPERATURE CLEAR SOLUTIONS

The direct synthesis of thin films of crystalline silicalite-1 upon si ngle crystal silicon wafers at a crystallization temperature of 100 de grees C has been investigated by varying the composition of the clear tetrapropylammonium (TPA) silicate synthesis solutions. Synthesis mixt ure compositions known to yield monodisperse colloidal crystals of TPA -silicalite-1 upon hydrothermal treatment as well as those reported to yield silicalite-1 films at higher temperatures have been found not s uitable for the preparation of silicalite-1 films at 100 degrees C. Lo wer crystal growth rates and smaller thicknesses of the gel film that forms on the wafer at this temperature decrease the tolerance to alkal inity, resulting in etching via the consumption of the gel layer befor e the growing crystals succeed in forming a closed film followed by th e removal of the protective silicon oxide film on the wafer. Thin orie nted silicalite-1 films with thicknesses in the range of 180 nm to 1 m u m have been obtained by varying the alkalinity and water, the TPA, a nd the silica contents of the reaction mixture. Lower alkalinities and higher silica concentrations favor the formation of a thicker amorpho us gel layer. Although increased TPA(+) concentrations at constant alk alinity increase the number of nuclei that form on this layer, higher TPA(+) concentrations have been observed to be required at higher alka linities to achieve similar rates of nucleation. Rinsing the wafer sur faces initially with a 0.025 M TPAOH solution before rinsing with wate r and acetone produces cleaner surfaces free of post-treatment artifac ts. (C) Elsevier Science Inc. 1997.