On liquid-phase deposition of silicon dioxide by boric acid addition

The current status of liquid-phase deposition (LPD) of SiO2 by adding boric acid (H3BO3) to a hydrofluosilicic acid (H2SiF6) is reviewed and compared with some new results from the authors' laboratory. Large discrepancies exist in the literature concerning the effects of Various processing parameters on deposition rate. We have shown that much confusion arises from the misconception of using an SiO2 additive to ''saturate'' and usng water to ''dilute'' the growth solution. In this paper the role of H2O as a reagent and the detrimental effect of an SiO2 additive as seeds for depleting the nutrient in the growth solution if added after the H2O addition are emphasized. Despite the variations between different investigators, the following characteristics of the LPD process are in general agreement: (i) the deposition process is surface controlled, (ii) the deposition rate depends approximately linearly on temperature, (iii) the deposition rate is constant (independent of time) over a wide range of experimental conditions, and (iv) the deposition rate increases with increasing boric acid concentration.