A silicon oxynitride film (SiO0.8N0.2) was etched with anhydrous HF/pyridine mixtures dissolved in supercritical carbon dioxide at 50-75 degrees C and 160 +/- 10 bar to achieve concentrations of 2.8-22.4 mM, spanning the range from kinetically limited to transport-limited etching. The SiF4 and NH3 etching products reacted further, forming an insoluble salt layer chemically identified as (NH4)(2)SiF6 using FTIR and quantitative XPS and imaged using SEM and AFM. The etching reaction was first-order below 11 mM because the surface was open to the fluid phase between salt crystals, and etching rates were approximately 1 nm/min. In this regime, salt crystals did not hinder etching and formed primarily by the addition of fluorosilicates to the bottom interface with the silicon oxynitride film. The reaction order was less than one above 11 mM HF because lateral growth of salt crystals became significant and hindered transport of the fluid to the film. Between 50 and 70 degrees C, the salt formed cubic crystals, whereas at 75 degrees C a powder-like product formed. Partial removal of the (NH4)2SiF6 salt was accomplished by sublimation under ultrahigh vacuum conditions at room temperature.